TEMPO in Chemical Transformations: From Homogeneous to

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TEMPO in Chemical Transformations: From Homogeneous to Heterogeneous Hazi Ahmad Beejapur, Qi Zhang, Kecheng Hu, Li Zhu, Jianli Wang, and Zhibin Ye ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b05001 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 14, 2019

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TEMPO in Chemical Transformations: From Homogeneous to Heterogeneous Hazi Ahmad Beejapur,a Qi Zhang,a Kecheng Hu,a Li Zhu,a Jianli Wang,a,* and Zhibin Ye b,*

a

State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang Province Key Laboratory of Biofuel, Biodiesel Laboratory of China Petroleum and Chemical Industry Federation, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, P. R. China

b

Department of Chemical and Materials Engineering, Concordia University, Montreal, Quebec H3G 1M8, Canada

*

Corresponding

authors;

Emails:

[email protected]

(J.

W.);

[email protected] (Z. Y.)

ACS Paragon Plus Environment

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ABSTRACT:

The organic nitroxyl radical, TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl), finds a variety of industrial applications for chemical transformations. Due to the economic and environmental concerns, the recovery and reuse of TEMPO with maintained high activity are utmost important. In this critical review, we summarize the most important advances made by the scientific community in TEMPO immobilization on various organic and inorganic support materials for recovery and reuse, and discuss the activity and stability, as well as the procedures. Also summarized is the wide range of applications of TEMPO in both homogeneous and heterogeneous forms in chemical transformations, beginning from methodology tuning in synthetic chemistry to the use in polymer chemistry.

KEYWORDS: TEMPO, immobilization, recovery, chemical transformations, catalysis, applications


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1

Introduction

Cyclic nitroxides, best represented by TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) and its derivatives (see Scheme 1), are stable radicals that show valuable redox behavior.1 As a low molecular weight free radical, TEMPO has been extensively used as a reagent and/or catalyst in a broad range of laboratory and industrial processes. Some notable applications in various research areas include: (a) in synthetic chemistry, TEMPO has been the very effective catalyst for the transformations of functional groups, such as oxidations,2,3 CC and CN bond formations, and natural product synthesis;4 (b) in polymer chemistry, TEMPO has been commonly used in nitroxide mediated polymerization (NMP), enabling the synthesis of well-defined functional polymers of complex macromolecular architectures;4,5 (c) in electrochemistry, especially for organic radical batteries, TEMPO bounded polymers have recently been established for electrochemical energy storage with excellent electrochemical properties (i.e., favorable stability and fast redox kinetics);6 (d) in medicinal chemistry field, TEMPO can act as


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anti-oxidant7 and plays a role as the drug carrying agent.8 In addition, TEMPO related spin probes have been effectively used in magnetic resonance imaging (MRI) as well as for electron spin resonance experiments (ESR).9 Furthermore, several recent articles also mentioned the applicability in fluorescent probes and food chemistry.10

N O TEMPO 1

OH

NH2

O

O

N O

N O

N O

N O

2

3

4

5 O OH P O OH

O

O

HN

O

O O S O

N O

N O

N O

N O

N O

6

7

8

9

10

O

OH

Scheme 1. TEMPO and its common derivatives.

This review is focused on the applications of TEMPO in both homogeneous and heterogeneous forms in synthetic chemistry and polymer chemistry areas. The structure, properties, and synthesis of TEMPO and its derivatives have been summarized in a recent book1 as well as reviews4,11 on nitroxides and are thus not included herein. Readers are referred to these existing references for related information.


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Despite the high activity and selectivity of TEMPO in synthetic applications, its recovery and reuse has been considered as one of the key factors for sustainable technologies. TEMPO is considerably expensive and consequently its recovery and reuse is highly desirable. To mitigate these circumstances, immobilization of TEMPO on various support materials can facilitate its convenient separation and reuse, and thus minimize the stinging economical barriers. In recent years, immobilization of organic catalysts on suitable support materials has attracted growing interest for academic and industrial purposes.12 Additional advantages are that the immobilization strategy can minimize catalyst traces in the product and improve handling, process control and the possibility to screen various reactions quickly by performing high-throughput measurements.13 Importantly, in some cases, immobilized catalysts are even more active or selective than their non-immobilized counterparts.14

Herein, the first part of this critical review is focused on the immobilization of TEMPO and its derivatives on a variety of organic/inorganic/molecular supports, and, more importantly, to discuss the catalytic performance (e.g., activity and reusability) of the


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immobilized TEMPO catalysts towards different reactions. This part differs from a most recent review paper by Megiel15 on surface modification using TEMPO and its derivatives, which has a particular focus on the strategies for surface immobilization with TEMPO.

The second part of the review covers a comprehensive range of applications of TEMPO and its derivatives in both homogeneous and heterogeneous forms in synthetic chemistry and polymer chemistry fields. Though the majority of these applications in these fields thus far employs the homogeneous ones, recyclable heterogeneous TEMPO shows strong potential in these applications. Meanwhile, mechanistic aspects have also been discussed in this part for alcohol oxidation using various TEMPO catalytic systems.

2

TEMPO Immobilization

The main focus of catalysis research in the past days was to enhance catalytic activity and selectivity. Recovery of the catalyst was not really a serious concern. Nonetheless, in “green chemistry” approaches for catalytic reactions, the recovery and reuse of the


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catalysts have become an important factor because of stringent ecological and economic demands for sustainability.16 Homogeneous catalysts have the advantage that they are well defined on a molecular level and readily soluble in the reaction medium. Such single-site catalysts are highly accessible to the substrate molecules and often show high catalytic activity and selectivity. However, removing them from the reaction mixture to avoid contamination of the product requires expensive and tedious purification steps.17 Therefore, despite their intrinsic advantages, homogeneous catalysts are used in less than 20% of the industrially relevant processes.18 The advantages of heterogeneous catalysts over homogeneous ones are numerous. Traditional heterogeneous catalysts can be recycled after the reaction or used continuously in a packed bed reactor and the separation of the catalyst from the reaction mixture is simple.19 On the other hand, there are often different catalytically active sites with different activities and selectivity in the bulk material of heterogeneous catalysts, which are challenging to probe on a molecular level.20 Heterogenization of homogeneous catalysts with maintained activity and selectivity along with stability and


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reusability is thus an important issue in the sustainable and large-scale production of fine chemicals.

TEMPO and its derivatives have been immobilized on a variety of supports. The support materials for TEMPO immobilization can be categorized mainly into two sub-sections depending on the nature of the support, which are (i) inorganic supports and (ii) organic supports. In the first case, TEMPO immobilization on amorphous and mesoporous (SBA-15 and MCM-41) inorganic silica materials, magnetic nanoparticles (e.g., Fe3O4 or Co) as such or covered with silica, and carbon nanomaterials (Fullerenes [C60], carbon nanotubes and graphene oxide) will be summarized. On the other hand, the second sub-section illustrates the usefulness of organic small-molecule supports such as greener ionic liquids and fluorous tags. Moreover, it also covers the implementation of various soluble and non-soluble polymers as the support materials for TEMPO, including polyethylene glycol (PEG), polystyrene (PS), polyetherketone (PEK), polyethersulfone (PES), and other polymeric substrates. While briefly introducing the


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synthetic strategy, this section also summarizes and compares the catalytic performance the prepared supported catalytic materials.

2.1

Inorganic Supports

2.1.1 Silicas Silica is an easy and economically available support material for immobilization of TEMPO derivatives. Silica supported TEMPO organic-inorganic hybrid materials are commonly used as heterogeneous catalysts.21 One of the most common supports used for heterogeneous TEMPO catalysts is mesoporous silica. On the other hand, amorphous silica is sometimes used due to its high surface area and low cost, but the irregularity of the surface and pore structure can be detrimental in some applications. Moreover, microporous materials such as zeolites can be difficult to be functionalized and the small pore size (< 2 nm) limits the scope of catalytic reactions whereas the large pore size (~210 nm) reduces mass transfer limitations and allow even large reactant molecules to enter the pores.22 Apparently, mesoporous silica (pore diameter


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250 nm), such as SBA-1522 and MCM-41,23 are easy for functionalization by either a direct synthesis or post-synthetic grafting procedure.24

In a direct sol-gel synthesis (one-pot synthesis), a silica precursor is polymerized in the presence of functional organosilanes in a single step. Using this synthetic route, functionalized silica materials can achieve higher loadings of functional groups and these groups can be well distributed within the silica matrix. Post-synthetic modification (grafting) involves covalent attachment of organosilanes to the surface silanols of a premade silica material. In general, a more reactive silane will lead to higher organic loadings, but it could give poor distribution of the functional groups.16

The surface of silica support can play an important role in the catalytic activity of heterogeneous catalysts. The weakly acidic silanol groups can form hydrogen bonds to reactants or transition states, leading to cooperative catalysis with surface organic groups.25 In 1998, Bobbitt26 published a landmark article on perchlorate oxoammonium salt of TEMPO for the metal-free oxidation reaction of alcohols. In this article, silica was used to increase the rate of the reaction and also for better recovery of the catalyst. In


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addition, various other solid materials were examined such as alumina, florisil, charcoal and powdered sucrose (a chiral material) in order to increase the rate of the reaction. Silica and alumina showed similar results. Since both alumina (weakly basic) and silica gel (weakly acidic) could enhance the reaction, it is not likely that the reaction was acid catalyzed. It is probable that the enhancement arose from the surface concentration of the reactants.27

Post synthetic modification is most common for TEMPO immobilization on silicas. The surface of silica supports is commonly functionalized by reaction with surface silanol groups to create desired surface anchoring sites. This is followed with the subsequent reaction with TEMPO derivatives bearing a complimentary reactive group for their targeted covalent surface tethering. Compared to this common immobilization procedure, the sol-gel technology offers several advantages in the preparation of heterogeneous catalysts. Inorganic sol-gel supports for TEMPO derivatives are indeed superior in their thermal stability, inertness, protectability towards entrapped molecules, as well as their porosity and high surface areas.21


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In the most recent review by Megiel,15 some of the earlier reports on heterogeneous TEMPO catalysts immobilized on various silicas, including ultrafine silica particles,28 commercial aminopropyl-functionalized silica,29 sol-gel silica,30,31 MCM-41,32 and SBA15,33 have been well summarized. These heterogeneous TEMPO powder catalysts were applied for alcohol oxidation and were readily recovered from the reaction mixture by centrifugation, which facilitated their convenient recycling and reuse. In particular, the ultrafine silica particles supported TEMPO catalyst was reused for 45 reaction cycles28 and SBA-15 supported TEMPO catalyst was recycled for 14 reaction cycles without significant loss in its activity.33

Built upon these earlier studies summarized already, further advancements with silicasupported TEMPO catalysts have evolved. In one case, Karimi et al.34 have expanded to immobilize another nitroxyl radical, 3-oxo-9-azabicyclo [3.3.1]nonane-N-oxyl (3-oxoABNO), on SBA-15. The supported 3-oxo-ABNO catalyst exhibited performance comparable its homogeneous analogue for the metal-free aerobic oxidation of 39 alcohols under identical conditions and much superior catalytic activity relative to


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TEMPO. It was also found that the heterogeneous catalyst could be conveniently recovered and reused at least 12 times without significant effect on its catalytic efficiency.

Karimi et al.35 have also demonstrated that the confinement of the ionic liquid (IL) [bmim]Br inside the mesopores of SBA-15 functionalized TEMPO resulted in a highly recyclable catalytic system for the selective aerobic oxidation of alcohols at normal oxygen pressures. The catalyst can be recovered and reused for at least 11 reaction cycles. Notably, N2 adsorption-desorption analysis of the recovered catalyst demonstrated that the leaching of [bmim]Br was negligible in the recovered catalyst. Particularly,

this

heterogeneous

IL@SBA-15-TEMPO

catalyst

system

showed

remarkably much higher selectivity and efficiencies in the oxidation of allylic alcohols in comparison to SBA-15-TEMPO (without IL).35


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OH O NO2

NO

Si Br O N N N

Br Br2

O N

Si

11

Scheme 2. TEMPO@PMO-IL-Br (11). Reproduced from Ref. 36. Copyright 2016 Royal Society of Chemistry.

The same group has also introduced a bifunctional catalyst, TEMPO@PMO-IL-Br (11) composed of TEMPO anchored in the nanospaces of a periodic mesoporous organosilica (PMO) having an imidazolium bromide network (Scheme 2).36 Due to the close proximity of TEMPO and the bromide functionalities in the same solid, the catalyst can induce a synergistic effect, leading to effective activity of the catalyst for aerobic oxidation of alcohols. This strategy allows simultaneous recovery of both IL and


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TEMPO. This unprecedented cooperative effect resulted in much superior activity of 11 as compared to that of either TEMPO@SBA-15 not bearing IL or the one with individual catalytic functionalities (PMO-IL/TEMPO@SBA-15).36

Fernandes et al.37 have further developed a simple strategy for the controlled assembly of bifunctional heterogeneous TEMPO catalysts (12a-e) bearing covalently grafted TEMPO as well as pyridyltriazole (pyta) ligand capable of complexing with Cu. The key feature of this approach relies on the immobilization of both functionalities on a single azide-functionalized mesoporous silica platform with a mixture of alkynes by copper(I)catalyzed alkyne-azide cycloaddition (CuAAC) “click” reaction. Significantly, modifying the relative proportion of the different alkynes in the grafting solution allowed to achieve an accurate control over surface composition (Scheme 3). They have demonstrated the influence of surface composition of the functional materials on the catalytic activity. These bifunctional TEMPO catalysts were evaluated for aerobic oxidation of alcohols in the presence of Cu metal, with good to quantitative yields achieved. However, the recycling experiment with the TEMPO and pyta ligand/Cu bifunctional catalyst 12c.Cu


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(see Scheme 3) showed a significant decrease in activity after each run. Atomic absorption spectroscopy analysis of used catalyst after 3 runs confirmed a minor Cu loss of ca. 10%.37 Later, the same group expanded the catalytic system to molecular engineered, supported trifunctional pyta-Cu/TEMPO/NMI complexes (13a-d, Scheme 4) by adding the third N-methylimidazole (NMI) functionality.38,39,40 The catalytic reactivity for the oxidation of benzyl alcohol has been increased by designing NMI into the trifunctional catalysts as compared to bifunctional catalyst. Notably, a fine-tuning of the linker length on imidazolium site together with the optimization of surface composition allowed for efficient catalysis, along with the reduced Cu loss to ca. 4% even after 5 reaction cycles.


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N CuI, Et3N DMF, 50 oC

Silica Si

N N N

N

Cu

(CuCH3)4OTf CH3CN

N

O

O Silica Si

N3

CuI, Et3N DMF, 50 oC

Si CuI, Et3N DMF, 50 oC

Silica Si

N N N

O

N O

O N O

Silica Si N N N

N N N

(CuCH3)4OTf CH3CN

12a.Cu 12b.Cu 12c.Cu 12d.Cu 12e.Cu

N

12a, pyta:TEMPO = 75/25 12b, pyta:TEMPO = 50/50 12c, pyta:TEMPO = 25/75 12d, pyta:TEMPO = 12.5/87.5 12e, pyta:TEMPO = 6.25/93.75

Scheme 3. Synthesis of silica-supported mono- and bifunctional heterogeneous catalysts. Reproduced from Ref. 37. Copyright 2016 Royal Society of Chemistry.


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O N

N

N

N O

Si Silica

N

Si

Si

13a, n = 0 13b, n = 2 13c, n = 4 13d, n = 6

N N N

O

N n

N N

N

Cu N I

Scheme 4. Silica-supported trifunctional pyta-Cu/TEMPO/NMI complexes. Reproduced from Ref. 38. Copyright 2016 Wiley-VCH.

Recently, Hearn et al.41 synthesized TEMPO radical polymer-grafted silicas by using both “grafting-from” and “grafting-to” methods with reversible addition-fragmentation chain transfer (RAFT) polymerization of the monomer, 2,2,6,6-tetramethylpiperidine methacrylate (TMA). This class of solid-state catalysts was evaluated for their activity towards oxidation of alcohols with good to quantitative yields achieved. The catalysts were recycled for at least 5 consecutive cycles without any significant reduction in the activity. More recently, Wang et al. have successfully synthesized a series of novel silica nanoparticles (SNs) functionalized with multiple TEMPO groups, SN-g-(PGMA-


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TEMPO), via the surface-initiated atom transfer radical polymerization (SI-ATRP) of glycidyl methacrylate (GMA) and efficient “Click” chemistry for TEMPO tethering. The number of TEMPO groups, as well as functionalities of amino and bromoisobutyl groups, was well tuned on silica nanoparticles using this procedure. The silica grafted heterogeneous catalyst was found to have promising application in the oxidation reaction of alcohols to their respective carbonyl compounds with excellent catalytic activity, efficiency, stability, and recyclability.42

Table 1 summarizes the various silica-supported TEMPO catalysts discussed above and compares their performance towards oxidation of benzyl alcohol as a representative substrate to benzaldehyde. In general, the TEMPO content on the silicasupported TEMPO catalysts is within the range of 0.070.87 mmol g-1 given the monolayer tethering of TEMPO on silica surface in most cases, except the high content of 1.71 mmol g-1 achieved with the TEMPO polymer grafted silica.41 In particular, the latter catalyst showed the highest activity with a TOF of 1140 h-1, which is about an order of magnitude higher than those (TOF: 1240 h-1) of other silica-supported TEMPO


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catalysts shown in the table. Despite the different reaction conditions employed for the various catalysts, which may affect the TOF values in some degree, this distinctively higher TOF underscores the superior activity of the TEMPO polymer grafted silica.41 This may possibly result from the good dispersion/solubilization of the grafted TEMPO polymer chains in the reaction media, which improves the catalytic performance. For most of the silica-supported TEMPO catalysts, their reusability was generally demonstrated only through 414 reuses though with the possibility for further extended reuses. The exception is the ultrafine silica-supported TEMPO catalyst that was reported for 45 cycles of reuse though with no details provided,28 underscoring its remarkable reusability.


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ACS Catalysis

Table 1. Summary of various silica-supported TEMPO catalysts and comparison of their performance towards oxidation of benzyl alcohol to benzaldehyde

Ref.

Silica supports

Supported TEMPO catalysts

TEMPO contenta [mmol/g]

Time

TEMPOb

(h)

[mol%]

Oxidant

Yield

Selectivity

[%]

[%]

TON

TOF [h-1]

CuCl2

88.1

44

0.92

Number of reuse

28

Ultrafine silica

Silica-TEMPO

0.2

48

2

Cu(NO3)2 ·3H2O

90.1

45

0.94

45

29ac

Silica

SG-TMP-OH

0.87

0.5

1

NaClO

92

92

184

10

29bd

Silica

Silica-supported TEMPO 7

0.5

1

0.58

NaClO

75

129

129

10

29ce

Silica

Silica supported TEMPO

2

20

O2

92

4.60

2.3

5

30f

Sol–gel silica

Sol–gel entrapped TEMPO

0.23

4

5

NaClO

100

20

5

4

31ag

Sol–gel silica

SG-TEMPO-2

0.27

0.25

1

NaClO

60

60

240

7

31bh

Sol-gel silica

SG-TEMPO-2

0.067

14

10

NaClO

64

6.4

0.46

7

32a

MCM-41

TEMPO-ether-MCM-41

0.6

48

1.5

O2

35

>99

23

0.49

32bi

SBA-15

SBA-15/DICB/TEMPO

0.369

24

0.92

TBHP

89

90

96.7

4.03

2

32bj

MCM-41

MCM-41/DICB/TEMPO

0.143

24

0.36

TBHP

86

80

239

9.95

2

33

SBA-15

SBA-15-supported TEMPO

0.33

O2

100

99.8

100

66.67

14

Air

100

99.1

100

39.64

34k

APSBA-15

SBA-15-ABNO

0.34

1.5

0.6

O2

>99

167

111

12

35

SBA-15

IL@SBA-15-TEMPO

0.33

3.5

1

O2

>99

100

29

11

36

PMO-IL-AMP

TEMPO@PMO-IL-Br (11)

0.25

1

1.5

O2

>99

67

67

8

pyta-Cu/TEMPO (12a.Cu)

0.07

5

O2

53

10.6

2.12

37

Azidefunctionalized silica

pyta-Cu/TEMPO (12b.Cu)

0.16

5

O2

83

16.6

3.32

pyta-Cu/TEMPO (12c.Cu)

0.19

5

O2

93

18.6

3.72

1.5 2.5

1

5

3


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38l

39

Azidefunctionalized silica

Silica

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pyta-Cu/TEMPO (12d.Cu)

0.24

4

O2

100

20

5

pyta-Cu/TEMPO (12e.Cu)

0.26

5

O2

75

15

3

78

15.6

5.2

88

17.6

5.87

Trifunctional PytaCu/TEMPO/NMI (13a)

3

Trifunctional PytaCu/TEMPO/NMI (13b)

3

Trifunctional PytaCu/TEMPO/NMI (13c)

2

95

19

9.5

Trifunctional PytaCu/TEMPO/NMI (13d)

1.5

95

19

12.67

Pyta-Cu/TEMPO/Imidazole 1

2

73


0.83

83

Pyta-Cu/TEMPO/Imidazole 3a

1

Pyta-Cu/TEMPO/Imidazole 3b

1


2

59


2

5

3

5

O2

96

19.2

6.4

0.17

0.5

NaClO

95

190

1140

10

O2

100

10

5

40

NMI

NMI-TEMPO-1

41

Silica

TEMPO polymer-grafted silica (4-2)

1.71

42

Silica nanoparticles

SN-g-(PGMA-TEMPO)

0.75

5

O2

O2

100 100

5 4

TEMPO content in the supported TEMPO catalyst. b TEMPO loading in the reaction relative to the substrate. c Reusability test with nonan-1-ol as the substrate. Reusability test with the silica-supported catalyst 8. e Reusability test with -methylbenzyl alcohol as the substrate. f Methyl -D-glucopyranoside as the substrate to the oxidized product of uronic acid. g Reusability test with 1-nonanol as the substrate. h Oxidation of chloramphenicol as the substrate to chloramamphenicol carboxylic ester. i Reusability test with isoborneol as the substrate. j Reusability test with isoborneol as the substrate. k ABNO was immobilized on SBA-15. l Reusability test with catalyst 8 in Ref. 38.

a

d


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2.1.2 Magnetic Nanoparticles (MNPs)

Nanoparticles have recently emerged as efficient alternatives for the immobilization of homogeneous catalysts.43 The large specific surface area of nanoparticles refers to that the high loading of catalytically active sites are guaranteed and diffusion in the pores will no longer limit the kinetics.44 Thus, there is plenty of room on the surface of these nanoparticles for the heterogenization of various homogeneous catalysts. Unlike conventional micrometer-sized particles, nanoparticles can be easily dispersed in a liquid medium to form stable suspensions. Nevertheless, particles with a diameter of less than 100 nm are difficult to separate by filtration techniques. In such cases, expensive ultracentrifugation is often the only way to separate the product and the catalyst. This drawback can be overcome by using magnetic nanoparticles, which can be easily removed from the reaction mixture by magnetic separation.45

Megiel15 has summarized in her review some of the earlier reports on heterogeneous TEMPO

catalysts

immobilized

on

MNPs,

including

graphene-coated

cobalt


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Page 24 of 227

nanoparticles46 and iron oxide superparamagnetic nanoparticles,47,48 as well as their catalytic performance and recyclability by magnetic separation.

Apparently, pristine iron oxide MNPs will aggregate rapidly into large clusters and thus will lose their unique properties associated with the presence of single domains. The nanoparticles have to be coated with organic or inorganic surfactants to prevent irreversible aggregation and retain their nanoscale properties.49 Since most of the MNPs derived from metals or metal alloys are susceptible to easy oxidation upon exposure to air or even solvated oxygen species, core-shell structures have usually been employed in subsequent works to protect these materials and retain their nanoscale properties.50

Coating the surface of nanoparticles with polymers is an alternative approach to stabilize the surface of MNPs.51,52 Recently, Wang et al.53 have synthesized magnetite (Fe3O4)-encapsulated, cross-linked polystyrene nanoparticles containing pendant benzyl chloride functionalities through a miniemulsion polymerization technique. So prepared composite nanoparticles were used to immobilize TEMPO moieties by tethering onto the benzyl chloride groups (see Scheme 5). The activity of the magnetic


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polymer latex immobilized TEMPO catalyst (14) was evaluated for the oxidation of alcohols to their corresponding carbonyl compounds. Furthermore, the novel catalyst was simply recovered using external magnetic field, without loss in its catalytic activity and selectivity for 20 consecutive cycles.53

Di vinylbenzene Cross-linked Polystyrene

Cl Magnetic Fluid

Miniemulsion Polymerization

Cl MPN (1.0 mmol Cl g-1)

NaOH Bu4NI THF/H2O

OH N O

O

PS

N O

TEMPO/MPNs (0.68 mmol radical g-1) 14

Scheme 5. TEMPO immobilization on MPN-encapsulated, cross-linked polystyrene nanoparticles bearing benzyl chloride functionalities. Reproduced from Ref. 53. Copyright 2013 Wiley-VCH.


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Inorganic oxide coatings such as silica on MNP surface have offered unique advantages for applications compared to organic surfactant-protected MNPs. Iron oxides themselves are well known catalysts. It is necessary to build up a strong barrier between the magnetic core and the molecular catalysts to circumvent unwanted interactions with molecular catalysts bound to the surface of the nanoparticles. Coating MNPs with silica avoids unfavorable contacts with the core and prevents particle aggregation. Furthermore, the presence of silanol groups on silica coating makes the MNPs more hydrophilic and thus more biocompatible than those stabilized with other protectors.45b,54

Karimi and Farhangi have developed a highly efficient silica coated magnetic core-shell nanoparticle-supported TEMPO (MNST) catalyst with high TEMPO loadings (Scheme 6).55a The heterogeneous catalytic system was employed for the water-mediated aerobic oxidation of a wide range of alcohols under transition metal- and halogen-free conditions. In particular, the system was featured with the advantages of easy separation and excellent reusability of up to 20 cycles (typical of heterogeneous


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catalysts) as well as high activity and reproducibility typical of homogeneous catalysts. These characters have made the MNST system a prominent catalyst for practical and large-scale applications. The same group has also used the MNST catalyst for a novel domino oxidative Passarini three-component reaction (OP-3CR).55b The catalyst showed excellent selectivity and good to excellent yields for the synthesis of a range of α-acyloxy carboxamides using different combinations of alcohols, carboxylic acids, and isocyanides. Moreover, the catalyst was used in 14 subsequent OP-3CR runs upon recycling, with only slight decreases in catalytic activity.55b

N

O Fe3O4 SiO2

Scheme

6.

Silica-coated

O Si O

magnetic

O

N H

core-shell

nanoparticle-supported

TEMPO.

Reproduced from Ref. 55a. Copyright 2011 Wiley-VCH.

Recently, Lu et al.56 have designed and synthesized a magnetic silica supported bifunctional hybrid-type ionic liquid TEMPO catalytic system (15 in Scheme 7) bearing TEMPO and H5PV2Mo10O40-based polyoxometalate (POM) anion to the ionic liquid tag.


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Herein, silica protected MNPs (SMNP) were employed to support both the POM-tagged ionic liquid and TEMPO catalyst in a non-covalent manner. The catalytic system (IL/TEMPO/POM/SMNP) was proven to be efficient for selective oxidation of a wide set of alcohols with good to excellent yields. Notably, it underwent a pseudo-homogeneous reaction process. After the reaction, the catalyst could be recovered by using an external magnet.56b

O N H 2N Fe3O4

SiO2 NH4OH

Fe3O4

N

O O

H5PV2Mo10O40

Cl-

H 2N

N

O O

Fe3O4

15

N

O

5

PV2Mo10O40

Scheme 7. Non-covalently supported TEMPO-IL on SMNPs. Reproduced from Ref. 56b. Copyright 2012 Royal Society of Chemistry.

Most recently, Wang et al.57 have further synthesized successfully polymeric TEMPO containing Fe3O4@SiO2@PTMA nanohybrids (PTMA, poly(TEMPO methacrylate)) using a simple and efficient distillation precipitation polymerization method. Using this polymerization method, the particle size, surface hydrophilicity and TEMPO loadings


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(1.02.4 mmol g-1) of the Pickering interfacial catalyst (PIC) system were finely tuned. The catalyst showed superior activity as well as selectivity and it was recovered easily by external magnetic field.

Table 2 summarizes the various MNP-supported TEMPO catalysts and compares their performance towards the oxidation of benzyl alcohol to benzaldehyde. The two MNPcontaining nanohybrid TEMPO catalysts designed by Wang et al.53,57 show the high catalytic performance, with high TOF values. In particular, the MNP-encapsulated crosslinked polystyrene nanoparticle catalyst (14) shows a high TOF of 1200 h-1 along with a reusability

of

20

cycles

and

the

magnetic

polymeric

TEMPO

containing

Fe3O4@SiO2@PTMA nanohybrid catalyst shows an even higher TOF of 12874 h-1 with NaOCl as the oxidant. Other MNP-supported TEMPO catalysts instead show the TOF values in the range of 0.388125 h-1 under O2 or air as the oxidant.


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Page 30 of 227

Table 2. Summary of various magnetic nanoparticle supported TEMPO catalysts and comparison of their performance towards oxidation of benzyl alcohol to benzaldehyde Ref.

MNP supports


TEMPO contenta

Time

[mmol/g]

TEMPOb

[h]

[mol%]

Oxidant

Yield

Selectivity

[%]

[%]

TON

TOF [h-1]

Number of reuse

46

Magnetic C/CoNanoparticles

CoNP–TEMPO

0.1

1

2.5

NaClO

85

>98

34

34

6c

47

Fe3O4 SPNs

TEMPO-coated SPN catalyst 5b

0.48

1

5

Air

85

>99

17

17

23

53

MNPs

TEMPO/MPNs (14)

0.68

0.083

1

NaClO

>99

>99

100

1200

20

55a

AMNP

MNST

0.3

4

0.2

O2

100

100

500

125

20

56b

SMNP

IL/SMNP (15)

0.064

5

50

O2

97

>99

1.94

0.388

10

57

magnetopolymeric nanohybrids

Fe3O4@SiO2@PT MA

1.7

0.008

0.85

NaClO

91

>99

107

12847

4

TEMPO content in the supported TEMPO catalyst. b TEMPO loading in the reaction relative to the substrate. c Reusability test with 4-methylbenzyl alcohol as the substrate.

a


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ACS Catalysis

2.1.3 Fullerenes [C60]

In 1985, Kroto et al.58 discovered that fullerene C60 has a soccer-ball-like structure consisting of 12 pentagons and 20 hexagons facing symmetrically. C60 is a multifunctional molecule with outstanding electronic, topological and photophysical properties.59 Following the success in the multigram production of C60, this exotic allotrope of carbon has been studied intensively to find suitable applications. Some of the potential applications of C60 derivatives are in the field of organic photovoltaics,60 nanomedicine,61 and smart materials.62

In the past few years, a range of fullerene C60-functionalized TEMPO derivatives has been synthesized by various groups.63-68 Very few studies, however, dealt with the use of fullerenes in catalysis, which were mainly focused on the preparation of metal-based heterogeneous catalysts for organic transformations.69 Recently, Gruttadauria et al. have reported fullerene C60-TEMPO based organocatalysts.70 Therein, fullerene has been used as a molecular platform to support a controlled number of TEMPO moieties. As shown in Scheme 8, the synthetic route starts from the formation of TEMPO-


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bismalonate from malonyl chloride and 4-hydroxy TEMPO. The Bingel reaction has been applied to the synthesize three TEMPO/fullerene conjugates, mono-adduct C60-T2 (16), bis adduct C60-T4 (17) and hexakis adduct C60-T12 (18) in a regio-controlled manner by altering the stoichiometry of TEMPO-bis malonate. These catalysts were tested for the oxidation of a series of different alcohols to their respective carbonyl compounds using three different co-oxidants, including [bis-(acetoxy)iodo]benzene (BAIB), sodium hypochlorite (NaOCl), and oxygen, with good to quantitative yields obtained. Notably, the twelve TEMPO-contained [6:0] hexakis adduct 18 showed much higher activity than the other two catalysts in the case with BAIB as the co-oxidant. The catalyst (18) was recovered by simple filtration with little silica pad and was recycled up to 7 reaction cycles with no significant loss in activity.70a


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ACS Catalysis

OH O Cl

O

O

DCE / Py Cl

O

N O

N

N

O

O

N O

O

N

O

N

O

O

N

O

O N

O

17

16

N O

O

OO

OO

O N

18

O N O

O O O

O O O

O N

O

O

O

O

O N

O N

O

O

O

O C60, CBr4 DBU, PhCl rt, 72 h. O N

C60, CBr4 DBU, PhCl rt, 24 h.

O

O

O O

0 oC to rt

O

O

N

O

O O

O

O O

O O

N O N O

O

N O

O N O

N O

Scheme 8. Synthesis of C60-TEMPO derivatives: monoadduct C60-TEMPO2 (16); bisadduct C60-TEMPO4 (17); hexakis adduct C60-TEMPO12 (18). Reproduced from Ref. 70a. Copyright 2014 Wiley-VCH.

Very recently, a novel fullerene hetero [5:1] hexakis adduct bearing two TEMPO radicals and ten 1-propyl-3-methylimidazolium bromides has been synthesized in good yield.71 So prepared C60IL10TEMPO2 hybrid catalyst 19 (Scheme 9) has been successfully employed for selective oxidation of a wide series of alcohols. It was highly active at just 0.1 mol% loading of TEMPO, with a high TOF of 1357 h-1 towards the oxidation of benzyl alcohol to benzaldehyde (see Table 1). Moreover, it could be recovered by adsorption onto a multilayered covalently-linked SILP phase (mlc-SILP)


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Page 34 of 227

support through the “release and catch” approach and reused for 12 cycles without loss in efficiency. Interestingly, a catalytic synergistic effect has been observed, due to the close proximity of TEMPO and imidazolium bromide moieties.

O N Br N

Br N

N

N

N

O O

O Br N

O OO

OO

O

O O O

O N Br N

N Br

O O O

O O

N N Br

O

O O

O O

Br N

O

O N

N Br N

N N

N

Br N

Br N N

19

Scheme 9. IL and TEMPO functionalized fullerene C60-IL10TEMPO2. Reproduced from Ref. 71. Copyright 2015 Wiley-VCH.

Table 3 summarizes various C60-supported TEMPO catalysts and compares their performance towards the oxidation of benzyl alcohol to benzaldehyde. The performance of hybrid C60IL10TEMPO2 hybrid catalyst 19 is comparable to those of some best


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supported TEMPO catalysts, in terms of activity and reusability. However, the use of expensive precursors (C60 and IL) makes the catalyst infeasible for practical applications despite its scientific value.


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Page 36 of 227

Table 3. Summary of C60-supported TEMPO catalysts and comparison of their performance towards oxidation of benzyl alcohol to benzaldehyde

Ref.

Catalyst


Time [h]

TEMPOb [mol%]

Oxidant

Yield [%]

TON

TOF [h-1]

70a

C60-TEMPO12 (18)

0.31

0.5

1

NaClO

>95

95

190

70b

C60TEMPO10@Au

O2

96

71

C60IL10TEMPO2 (19)

16

Number of reuse 5c 5d

12e 0.7

0.1

BAIB

>95

950

1357 4f

TEMPO content in the supported TEMPO catalyst. b TEMPO loading in the reaction relative to the substrate. c Reusability test with 1-phenylethanol as the substrate. d Reusability test with decanol as the substrate. e Reusability test with 1-phenylethanol as the substrate with 1 mol% TEMPO loading. f Reusability test with 1-phenylethanol as the substrate with 0.1 mol% TEMPO loading.

a


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2.1.4 Carbon Nanotubes (CNTs) and Graphene Oxide (GO)

The discovery of CNTs by Iijima has opened up a new era in material science and nanochemistry.72 There are two distinct types of CNTs. The so-called single-walled carbon nanotube (SWCNT or graphene tube) is made of one layer of a graphene sheet, while the multiwalled carbon nanotube (i.e., MWCNT or graphitic tube) consists more than one layer. CNTs have high chemical stability, large surface area, and outstanding mechanical properties, but very few literature has reported their use as a support for covalently bonded homogeneous catalysts.73 Due to the limited solubility of these supported catalysts in the reaction mediums, these reported processes often focused on heterogeneous catalysis. It has been proposed that the functionalization of carbon nanotubes with organic molecules can improve the solubility of the nanotubes in aqueous or organic solvents.74 Inspired by high solubility of functionalized carbon nanotubes,75 Wang et al. have immobilized TEMPO on multiwalled carbon nanotubes and prepared soluble MWCNT-TEMPO catalyst (20 in Scheme 10; TEMPO content, 0.74 mmol g-1) and evaluated its catalytic activity for the oxidation reaction of 11 primary


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Page 38 of 227

and secondary alcohols to carbonyl compounds using the Anelli condition.76 The catalyst was readily soluble in water (3.7 mg ml-1) and CH2Cl2 (0.23 mg ml-1) to form approximately homogeneous solution. The catalyst could be easily recovered by filtration and recycled for 7 times without any significant loss in catalytic activity.

Graphene does not easily exfoliate to monolayer graphene sheets from graphite whereas GO can be exfoliated easily by graphite oxide nanosheets using different ultrasonic methods. In addition, GO is the practical precursor of graphene and efficiently used for the functionalization of most organic and inorganic materials as compared to the reduced form of GO or chemically modified graphene materials.77 There are mainly two reproducible methods for the functionalization of GO, covalent and non-covalent functionalization. Covalent functionalization takes advantages of the carbon surface chemistry by forming carboxyl and hydroxyl bonds to GO. On the other hand, noncovalent functionalization is based on pi-pi stacking or van der Waals interactions.77

Very recently, Ionita et al.78 have covalently grafted 4-amino TEMPO (3) onto GO through an amide bond to synthesize heterogeneous GO-TEMPO catalysts (21 in


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Scheme 10; TEMPO content up to 1.1 mmol g-1). It was found that the catalysts can be successfully used as easily recoverable solid catalysts for selective oxidation of five alcohols (benzyl alcohol, 1-phenylethanol, diphenylmethanol, furfuryl alcohol, and 1octanol) into corresponding aldehydes or ketones using NOx as co-catalyst and oxygen as final oxidant under very mild conditions. A turnover number (TON) of 67 and a turnover frequency (TOF) of 3.3 h-1 were achieved in the case with benzyl alcohol as the most active reactant.

Hou et al. have used TEMPO as a co-oxidant along with GO for the selective aerobic oxidation of 5-hydroxymethylfural (HMF) into 2,5-diformylfuran (DFF).79 Enhanced catalytic activity with full conversion of HMF and nearly 100% selectivity was found upon the use of GO under optimized conditions. It was shown that oxygen functionalities in GO had a crucial effect on the catalytic oxidation of HMF. The enhanced activity was proposed to result from the synergistic effect of the carboxylic acid groups and unpaired electrons at GO edge defects.


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O N N

O

O ON

O

O

O

O N

O

O

O O

O

20

O

21

O

O

NO

N O

Scheme 10. MWCNT-TEMPO (20; reproduced from Ref. 76; Copyright 2012 Royal Society of Chemistry) and GO-TEMPO (21; reproduced from Ref. 78; Copyright 2016 Elsevier).

In recent study by Yuan et al.,80 4-amino-TEMPO (3) was grafted to the surface of GO and electrochemically reduced GO modified glassy carbon electrode by a simple, rapid and green electrografting method. The calculated surface coverage for 3 was up to 1.55 × 10-9 mol cm-2. The modified electroactive interface exhibited excellent electrocatalytic activity towards the electro-oxidation of reduced glutathione and hydrogen peroxide.

Table 4 summarizes the various CNT- and GO-supported TEMPO catalysts and compares their catalytic performance towards the oxidation of benzyl alcohol to


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ACS Catalysis

benzaldehyde. The MWCNT-TEMPO catalyst (20) shows a high TOF of 1200 h-1 and good reusability.

Besides the above classes of inorganic supports, TEMPO has also been immobilized on other metal supports,15 such as gold and silver nanoparticles.81-83 Since rarely employed for catalytic applications, these additional supported TEMPO derivatives are not discussed herein.


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Table 4. Summary of CNT or GO supported TEMPO catalysts and comparison of their performance towards oxidation of benzyl alcohol to benzaldehyde

Ref.

Supports



Time

TEMPOb

[h]

[mol%]

76

MWCNTs

MWCNTTEMPO (20)

0.74

0.083

1

GO-TEMPO (21) 78

1.1

GO 0.6

NaClO

Yield

Selectivity

[%]

[%]

100

>99

TOF [h-1]

Number of reuse

100

1200

7c

42 d

15.27

0.76

57 e

20.73

1.04

46 d

30.67

1.53

99 e

66

3.3

TON

2.75 20

iGO-TEMPO

Oxidant

O2 1.5

TEMPO content in the supported TEMPO catalyst. b TEMPO loading in the reaction relative to the substrate. c pH = 8.6, reaction time = 0.7 min. d Co-catalyst, (20 mol% NaNO2). e Co-catalyst (NO2). a


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ACS Catalysis

2.2

Organic Supports

2.2.1 Small Molecular Supports

(1)

Ionic Liquids (ILs)

ILs are organic salts with low melting points (≤100 oC) and very low vapor pressures.84 Their non-volatile nature is one of the main motives to explore them as an alternative for volatile organic solvents.85 Depending on their composition, ILs can dissolve or reject organic compounds. ILs are ionic and polar to hold charged and polar catalyst species like transition-metal complexes,86 homo or hetero organo-catalytic species.87 Ionic liquids are greener solvents but they are expensive compared to other commercial organic solvents. In addition, the high viscosity of ILs can cause mass transfer limitations during the reaction and work up processes. In order to circumvent these drawbacks, an ionic-tag strategy has been introduced.88 In this strategy, ionic liquid moieties are attached to homogeneous catalysts in the covalent or non-covalent manner and the whole catalytic system adopts characteristics of both moieties. These systems can impart the active site with improved performance during the reaction


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Page 44 of 227

relative to its precursor, as well as high recyclability.14,89 The recovery of the tagged ionic liquids is illustrated in Scheme 11, in which the catalyst anchored onto an ionic liquid moiety is soluble in polar organic solvents and can undergo liquid-phase reaction. After completion of the reaction and evaporation of the solvent, the excess reagents and products or by-products can be removed by washing with a less polar organic solvent and/or an aqueous solution in which the ionic-liquid-anchored catalyst is insoluble. Eventually, the remained catalyst can be dried and re-used for next reaction cycle.

excess reagents Substrate

Substrate

liquid-phase

org. solvent

reaction

aq. phase

Product Product

detachment

org.solvent

phase separation

inorganics

Scheme 11. Recovery of IL tags. Reproduced from Ref. 88a. Copyright 2006 Thieme Publishing Group.

Earlier to the ionic tag strategy, Ragauskas and Jiang reported the use of a catalytic amount of 4-acetamido TEMPO (7) in [bmim]PF6 ionic liquid solvent for efficient oxidation of alcohols.90 After the reaction, 4-acetamido TEMPO was recovered by the


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ACS Catalysis

conventional method of extraction with the use of Et2O solvent since it is soluble in [bmim]PF6 but not in Et2O. Consequently, acetamido-TEMPO/[bmim]PF6 system was recovered effectively. The recovered system was reused for two consecutive reactions, with no contamination of the reactions by the earlier product.90a The same group also reported the recovery of a three-component system acetamido TEMPO/Cu(ClO4)2/N,N'dimethylaminopyridine

(DMAP)

in

1-butyl-4-methylpyridiniumhexafluorophosphate

([bmpy]PF6) ionic liquid and its reuse for five runs without any significant loss of catalytic activity.91

Gao et al.92 synthesized a TEMPO-derived task-specific ionic liquid (22, Scheme 12) for the first time in a simple three-step process. The imidazolium PF6 ionic liquid-supported radical was proved to be an efficient, selective, and recoverable catalyst, with similar catalytic activity and selectivity compared to non-supported TEMPO. The catalyst was recovered up to five catalytic cycles without observing any significant decrease in catalytic activity.92,93


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Page 46 of 227

He et al.94a developed a three-component catalytic system, comprising of TEMPO immobilized imidazolium salt ([Imim-TEMPO]+X-) along with carboxylic acid substituted imidazolium salt and sodium nitrite, for aerobic oxidation of alcohols in a greener pathway. The homogeneous catalytic system showed high selectivity and successfully reused at least four times. Later, the same group had employed the [bmim-TEMPO]Cl catalyst (23) and replaced the use of the conventional acids by self-neutralizing acid system CO2/H2O in order to avoid tedious synthetic procedure for immobilized acids.94b

A novel bifunctional ionic liquid supported iodoarene-TEMPO catalysts bearing two catalytic sites, the iodoarene and TEMPO moieties 25 and 26 were synthesized by Zhdankin et al..95 They demonstrated these catalysts can be useful for the efficient and environmentally benign oxidation of various alcohols to their corresponding carbonyl compounds. Moreover, the catalysts were recovered easily and reused with no loss of its activity.

Fall et al. introduced click chemistry for efficient synthesis of a recyclable IL-CLICKTEMPO catalyst (27). The catalyst was used for the oxidation of alcohols to aldehydes


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ACS Catalysis

and ketones in ionic liquid. The catalyst was recycled for at least four cycles without the loss of efficiency.96

In addition, in 2013, bis(imidazolium)-tagged TEMPO catalysts (28a,b) were synthesized.97 The synthetic route is a quite simple two-step process, starting from 1,3,5-tris(bromomethyl)benzene compound through the immobilization of imidazolium tags to 4-hydroxy TEMPO derivative (Scheme 13).

O O N O

N

N X

22; if X = PF6 23; if X = Cl 24; if X = BF4

I 25

O

N

N

O O S

O

N O

N N N

N BF4

27 O N

N O

O R

26

I N

O O S O

N

N O

N N

R R

28a; R = H 28b; R = CH3

N N

Scheme 12. IL tagged TEMPO catalysts.


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O N

O N Br R

R

4-Hydroxy-TEMPO, Bu4Br, o

Br

R

Br

50% NaOH, Tolune, 70 C

R Br

R R

O

Methylimidazole

O

R

R

CHCl3, 60 oC

Br

N Br

N

R

N N

Br

28a; if R = H 28b; if R = CH3

Scheme 13. Schematic synthesis of bis-IL tagged TEMPO catalysts. Reproduced from Ref. 97. Copyright 2013 Wiley-VCH.

Recently, many groups have been employing supported ionic liquid phase (SILP) catalyst system for efficient utilization of the IL and catalyst. In one system, an IL film is immobilized on a high surface area porous solid (e.g., silica) and the homogeneous catalyst is dissolved in the IL layer. The resulting catalyst is solid with the active species being solubilized in the IL phase and behaving as a homogeneous catalyst. Typically, there is no direct interaction between the support’s surface and the homogeneous catalyst. SILP catalysis combines the most attractive features of homogeneous catalysis like high activity and selectivity with benefits of heterogeneous catalysts such as large interfacial reaction areas and ease of product separation.98 An evolution of SILP involves the use of covalently attached ionic liquid moieties affording a supported ionic-


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ACS Catalysis

liquid-like phase (SILLP). In some cases, additional IL can be absorbed on the SILLP, giving rise to the formation of several layers of free IL on the support even if some leaching of IL may occur.89,99

Gruttadauria et al.100 introduced a new approach employing a new class of multilayered covalently supported ionic liquid phase (mlc-SILP) materials (29 in Scheme 14). These materials involve the covalent linking of ILs on a solid support (e.g., silica), which are formed by radical polymerization of divinyl ionic liquid derivatives in the presence of the thiol-modified silica to incur thiol-ene coupling.100a In addition, a similar approach has also been applied for the preparation magnetic particles entrapped into highly crosslinked imidazolium salts (30 in Scheme 14).100b

The introduction of ionic tags onto TEMPO is quite strategic by increasing the affinity of TEMPO towards an IL-modified silica gel or other supports. The bis(imidazolium)-tagged TEMPO catalysts (28a,b) in combination with mlc-SILP were employed for oxidation of a series of different alcohols to their corresponding carbonyl compounds. The catalysts were easily recycled up to 13 consecutive cycles by simple filtration.71,97 The function of


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Page 50 of 227

the catalytic system achieves via a “release and catch” mechanism. The catalyst is released during the course of the reaction from non-covalently adsorbed support + catalyst system. After the completion of the reaction, the catalyst is recaptured on the surface of the support materials when the solvent s removed. Notably, such a “catalystsponge like” catalytic system allows one to combine the benefits and characteristics of homogeneous and heterogeneous catalysis.101

SiO2

S N

N Br

N

N

Br

N Br

30 N Br

N

N

= Fe2O3 =

Br N N

NBr N

29

Scheme 14. mlc-SILP (29) and magnetic nanoparticle-entrapped mlc-SILP (30). Reproduced from Ref. 97. Copyright 2013 Wiley-VCH.

Table 5 summarizes the various IL-supported TEMPO catalysts and compares their performance towards the oxidation of benzyl alchol to benzaldehyde. Most of the ILsupported TEMPO catalysts show relatively low or average TOF values (2120 h-1)


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ACS Catalysis

except 22 that shows a TOF value of 1128 h-1 in the oxidation reaction with NaClO as the oxidant.92 The use of expensive ILs in the catalyst synthesis and in the reaction for catalyst recycling is a major restriction of these IL-supported TEMPO catalysts.


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Page 52 of 227

Table 5. Summary of ionic liquid tagged TEMPO catalysts and comparison of their performance towards oxidation of benzyl alcohol to benzaldehyde

Ref.

ILs

90a

None

91

None

92

[bmim]PF6

93

[bmim]PF6

94a

Imidazolium salt

IL-tagged TEMPO catalysts

Time

TEMPOa

[h]

[mol%]

2

3

5

TEMPO-IL (22)

Oxidant

Yield

Selectivity

TOF [h-1]

Number of reuse

30.67

15.34

4

18.4

3.68

5b

94

1128

3

TON

[%]

[%]

H2O2

92

98

5

O2

92

0.083

1

NaClO

94

TEMPO-IL (22)

0.5

20

TBA-OX

90

4.5

9

5c

[Imim-TEMPO]+Cl- (23)

0.5

5

>99

20

40

4d

49

9.8

39.2

AcetamidoTEMPO/[bmim]PF6 AcetamidoTEMPO/[bmpy]PF6

>99

O2

[Imim-TEMPO]+BF4― (24)

0.25

5

94b

Imidazolium salt

[Imim-TEMPO][Cl] (23)

8

5

O2

97

19.4

2.43

3e

96

[HMIM][BF4]

IL-CLICK-TEMPO (27)

0.083

10

BAIB

99

9.9

118.8

5f

TEMPO loading in the reaction relative to the substrate. b 4-Methoxybenzyl alcohol as the substrate in reusability testing. c 4-Methoxybenzyl alcohol as the substrate in reusability testing. d 2-Nitrobenzyl alcohol as the substrate in reusability testing. e The reaction time = 2 h in the reusability test. f p-Methoxybenzyl alcohol as the substrate in reusability testing. a


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ACS Catalysis

(2)

Fluorous Tags

Fluorous techniques have been recognized as increasingly useful methods for highthroughput synthesis of small organic molecules or for catalyst recovery.102 The fluorous techniques are attractive for strategic separation of reaction mixtures because fluoroustagged catalysts can be quickly separated from untagged compounds in binary liquid/liquid and solid/liquid extractions. Fluorous/organic liquid extractions are commonly

used

for

separation

and

recovery

but

in

some

cases

fluorous/organic/aqueous liquid extraction is also used. Curan demonstrated a solid/liquid extraction over fluorous reverse-phase column.103

Many reports have demonstrated the usage of fluorous tag strategy for the immobilization of TEMPO moieties. In 2005, for the first time, Pozzi et al.104 synthesized several perfluoroalkyl substituted fluorous tagged-TEMPO catalysts (31a,b and 32a,b in Scheme 15) with fluorine content about 50%. These catalysts were used for the oxidation of alcohols to carbonyl compounds in conventional organic solvents under homogeneous as well as in liquid/liquid aqueous-organic conditions. Quantitative yields


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Page 54 of 227

were obtained with high fluorinated TEMPO catalyst 33 and it was recycled for six times for the oxidation of 1-octanol showing only minor loss of catalytic activity. Notably, the catalysts were isolated from reaction products by convenient liquid/liquid as well as solid/liquid extractions.104

O O

O RF

O

C8F17 C8F17

RF N

N O

N O

31a: RF = n-C7F15 32a: RF = n-C7F15 31b: RF = -CH2CH2(n-C8F17) 32b: RF = -CH2CH2(n-C8F17)

O F3C(F2C)5

N O 33

O N N O 34

(CF2)5CF3

F3C(F2C)3

N

(CF2)3CF3

N O 35

O N O 36

N

N N C8F17

Scheme 15. Fluorous tagged TEMPO catalysts.


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ACS Catalysis

Dobbs et al.105 prepared two light fluorous-TEMPO catalysts (34 and 35) and utilized them for the oxidation of a series of alcohols to their corresponding carbonyl compounds, as well as employed them as a radical spin trapping agent. Fluorous solidphase was used to recover these low molecular weight catalysts.

Reiser et al.106 have developed a facile strategy for the synthesis of a new fluorous tagged immobilized TEMPO catalyst by using the CuAAC “click” reaction. The perfluorinated F17-CLICK-TEMPO catalysts (36 in Scheme 15) was proved to be very stable, highly effective in the chemoselective oxidation of alcohols. The catalysts were easily recovered using silica gel and recycled in four cycles without loss of activity. The same group further investigated the combination of a higher number of perfluorinated alkyl chains and triazole moieties for more active and easily recoverable TEMPO catalysts (37, 38 and 39 in Scheme 16).107 The catalysts can be readily synthesized as shown in Scheme 16. The synthetic route starts with the formation of benzyl chloride derivative prepared from commercially available benzoic acid derivative in a three-step procedure. The fluorous tagged azide can be prepared by chloro azide exchange with


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Page 56 of 227

azide compound. Eventually, the F17-click-TEMPO catalysts can be obtained by reacting with TEMPO-alkyne derivative. So prepared catalysts were evaluated for oxidation of alcohols using Anelli-Montanary and Minisci’s conditions and the catalysts were easily recovered by liquid/emulsion filtration through sintered glass funnel.107

N3 CO2Et HO

R1

OH

R1 = H R1 = OH

1) 2)

1) R

Cl

Br

N3

CuI, DIPEA THF, rt, 24 h

LAH O

3) PPh3, CCl4

O N N N

O

R1

2) NaN3, DMSO 60 oC, 24 h

R1 = H (45%) R1 = OCH2C CH (35%)

R

O N

73%

N N

N

N N R

R1

O

N

N N

37 (Fluorine content: 45%)

N O

N O

O

C8F17 O

O

A: R1 = H, R = CH2C8F17 (68%) B-F: R1 = OCH2(Triazole)CH2CH2C8F17, R = CH2C8F17 (93%) B-Ph: R1 = OCH2(Triazole)Bn, R = Ph (55%)

N N N

Cu(I) (6 mol%)

R1

N N N

C8F17

O

N O

B-F or B-Ph Cu(I) (6 mol%) R

NN N

O

O O N N N

N

N N

R

R 38: (84%: R1 = CH2C8F17; Fluorine content: 49%) 39: (68%: R1 = Ph)


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ACS Catalysis

Scheme 16. Synthesis of TEMPO catalysts with multiple triazole and perfluoroalkyl moieties. Reproduced from Ref. 107. Copyright 2008 American Chemical Society.

Table 6 summarizes the various fluorous tagged TEMPO catalysts and compares their performance towards the oxidation of benzyl alcohol to benzaldehyde. The catalytic activity of these systems is generally low or average, with TOF in the range of 10198 h1.

Though conceptually elegant, the synthesis of these fluorous tagged TEMPO

catalysts requires the use of special expensive fluorous precursors, which undermines their value for practical applications.


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Page 58 of 227

Table 6. Summary of fluorous tagged TEMPO catalysts and comparison of their performance towards oxidation of benzyl alcohol to benzaldehyde

Ref.

Fluorous tagged TEMPO Catalyst

Time [h]

TEMPOa

104b

fluorous-tagged TEMPO radical (31a)

2

5

1 105a

a d

Yield

Selectivity

[%]

[%]

BAIB

>99

>99

3

NaClO

10 1

[mol%]

TOF [h-1]

Number of reuse

20

10

5

89

29.67

29.67

Oxone

69

6.9

NaClO

99

Oxidant

TON

Lighter Rf-TEMPO (35)

106

F17-CLICK-TEMPO (36)

107

3-ponytails perfluorinated TEMPO (38)

0.5

>98

99

198

4c 4d

2

1

O2

89

89

44.5 3e

TEMPO loading in the reaction relative to the substrate. b 1-Octanol as the substrate in the tests. c 4-Bromobenzyl alcohol as the substrate in the reusability test. 4-Methylbenzyl alcohol as the substrate in the reusability test. e 4-Bromobenzyl alcohol as the substrate in the reusability test.


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ACS Catalysis

(3)

Other Molecular Supports

Charette et al.108 used tetraarylphosphonium (TAP) salts to support TEMPO moiety and synthesized TAP-supported-TEMPO catalyst (40). The preparation of the catalyst is simple (see Scheme 17). The catalyst was tested for oxidation of primary and secondary alcohols under Annelli’s conditions using bleach as terminal oxidant, with excellent yields obtained. The carbonyl products were easily separated from the TAPsupported TEMPO by precipitation/filtration. The catalyst was also recycled at least for three consecutive cycles with little loss in its activity.

O OH N O

i) Cl3OC COCl3 DCM, -20 oC ClO4 ii) Ph3P

ClO4 Ph3P

OH

O

O O

N O

40

Scheme 17. Synthesis of TAP-supported-TEMPO. Reproduced from Ref. 108. Copyright 2009 American Chemical Society.


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Page 60 of 227

O N

H N

N N

N O

O N N

N

N

N

N

N

N N

N

H N

O

41

Scheme 18. CHIMAS-SORB 966. Reproduced from Ref. 109. Copyright 2003 American Chemical Society.

Minisci et al.109 utilized a macrocyclic material containing four immobilized TEMPO moieties (41 in Scheme 18, synthesized by Ciba specialty chemicals company) as the catalyst for oxidation reaction using H2O2 and metal nitrate oxidants (Mn, Co, or Cu). The catalyst showed effective activity along with Mn(II)/Co(II) couple, but with poor results when with the other Mn(II)/Cu(II) or Co(II)/Cu(II) couples. Eventually, the catalyst was recovered and recycled for 5 reaction times with no significant loss in its activity.

2.2.2 Polymeric Supports


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ACS Catalysis

Organic polymers have extensive uses in solid phase synthesis as well as in various types of purifications. The main objective for catalyst immobilization is to simplify the work up procedures and to facilitate product separation. The cross-linked polymers are insoluble before, during, and after the reaction, thus making the immobilized catalyst separable at any phase of the reaction since it is attached to an insoluble support. On the other hand, in the case of soluble polymer supports with the reaction to be carried out in the homogeneous conditions, the catalysts can be separated after the reaction by precipitation.110

Solvent precipitation is the most general way to perform a solid/liquid separation of a soluble polymer bounded catalyst and product after a homogeneous catalytic reaction. Nevertheless, all liquid/liquid separations rely on a gravity-based separation of two liquid phases to recover and separate the catalysts. Moreover, all useful liquid/liquid separations require a soluble polymer-bound catalyst to have very high phase selective solubility.110a,111 For the preparation of soluble polymer supported organocatalysts, noncross-linked PS and PEG are the most useful supports.


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(1)

Page 62 of 227

PEG

PEG is a linear polymer formed from the polymerization of ethylene oxide. Along with polystyrene, it is one of the first soluble polymers used to facilitate catalysis and synthesis. Various strategies have been applied to attach TEMPO catalyst onto PEG. Pozzi et al.112 immobilized TEMPO onto PEG using a trimethyleneoxy benzyl ether as a linker. So prepared PEG-TEMPO (42, Scheme 19) was proved to be an effective catalyst for oxidation of primary and secondary alcohols to aldehydes and ketones. With NaOCl as the terminal oxidant at 1 mol% of the catalyst, typical yields were obtained at >90% and the reactions were completed within 30 min.112 PEG is insoluble in ethers, such as diethyl ether and tert-butyl methyl ether (TBME), which facilitate their use as the solvents for the catalyst recovery by precipitation following reactions. It is true with other soluble polymers too that the polymer precipitation and the consequent heterogeneous reaction conditions often significantly decrease reaction rates.113 Benaglia’s group also noted that the catalyst’s reactivity was not enhanced in acetic acid despite the remarkable accelerating effect this solvent showed in reactions with free TEMPO.114 This may reflect the known propensity of the multiple ether oxygen groups present in


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ACS Catalysis

the polymer backbone of PEG that can cause hydrogen bonding to -CO2H groups of acetic acid.115 Catalyst 42 was easily recovered by solvent precipitation with diethyl ether and was recycled six times with no loss of catalytic activity.112

O N O

O O

O

R2

N O

nO

N O

43

H 3C O

O n O

O

O

O

O

O N O

44 =

O N

N O N

O N O O O

O O PEG 46

O

O

O

O

O

nO O

O

O N O

N O

MeO-(CH2-CH2O)n-CH2CH2Mw 5000 Daltons 42

O N

45

O

O N N N

N N PEG N O

O

O

O N

O

PEG

N

47

N Cl N

48

Scheme 19. PEG-immobilized TEMPO derivatives.

Tsang et al. also tethered TEMPO directly onto PEG using SN2 chemistry and synthesized PEG-bound TEMPO derivative 43 with a single TEMPO group by directly reacting PEGs with 4-hydroxy-TEMPO (2).116 In their work, they used a series of PEG samples with varying average molecular weights (ranging from 164 to 10,000 g mol-1) and studied the reactivity of the products. Among them, PEG10,000-bound TEMPO (R2 =


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Page 64 of 227

TEMPO in 43) showed exceptional results with a TOF of 0.218 s-1. Recovery of the catalysts was done by using solvent precipitation with diethyl ether, with moderate to good results obtained if the PEG molecular weight was higher than 5000 g mol-1. The authors successfully recycled the PEG10,000-bound TEMPO for four reaction cycles. A slight decrease in the catalytic activity was observed and it was attributed to mechanical losses of the catalyst during the filtration process.

In extension of their initial studies, Tsang et al. further investigated branched PEGsupported TEMPO catalysts 44 and 45.117 These catalysts contained two or four TEMPO groups per polymer and possessed several advantages over unbranched TEMPO catalysts attached to the polymer 43. The rate of oxidation of primary alcohols by 45 increased by 45 fold compared to 43. This higher activity was attributed to intramolecular re-oxidation of the intermediate hydroxylamine by a neighboring oxoammonium ion. The catalysts could also be recovered by solvent precipitation with diethyl ether twice without any change in TOF.


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ACS Catalysis

Pozzi et al. investigated the effect of the trimethyleneoxy benzyl ether linker unit between PEG and TEMPO on aerobic oxidation of alcohols to carbonyl compounds by using 5 mol% of 42 or 43. It was observed that catalyst 42 which had the linker unit showed higher activity than 43 without the linker unit.112,118 Matsumoto et al.119 synthesized several PEG3400-bound TEMPO catalysts by simple esterification or click chemistry. Consisting of two TEMPO moieties, these catalysts were employed for oxidation of alcohols using oxone as a terminal oxidant. Among those catalysts, PEG-supported TEMPO bearing a succinate spacer between the PEG and TEMPO parts (46) was found to have the highest reactivity and product selectivity, as compared to a triazolelinked PEG-bound TEMPO (47), other non-supported, and insoluble polymer supported TEMPO. Catalyst 46 was recycled for 10 consecutive cycles without significant loss in its activity.119

Recently, Lu et al. have developed an efficient and environmentally friendly method for oxidation of alcohols catalyzed by gold(III) and an ionic liquid functionalized, PEG1000-


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Page 66 of 227

immobilized TEMPO (48). The so prepared Imim-PEG1000-TEMPO catalyst showed high activity and substrate compatibility.120

Table 7 summarizes the various PEG-supported TEMPO catalysts and their performance towards the oxidation of benzyl alcohol to benzaldehyde. High TOF values (up to 6000 h-1) are observed when used with NaOCl as the oxidant.


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ACS Catalysis

Table 7. Summary of PEG-supported TEMPO catalysts and comparison of their performance towards oxidation of benzyl alcohol to benzaldehyde Ref.

PEG-supported TEMPO Catalyst


112c

Time

TEMPOb

[h]

[mol%]

0.5

1

Oxidant

Selectivity

[%]

[%]

TOF [h-1]

Number of reuse

＞98

>196

7

TON

PEG5000-TEMPO (42)

0.18

MeO-PEG164-TEMPO (43)

2.67

>99

>99

100

1200

TEMPO-PEG2000-TEMPO(43)

0.86

>99

>99

100

1200


0.19

>99

>99

100

1200

TEMPO-PEG6000-TEMPO (43)

0.29

87

>99

87

1044


0.19

65

>99

65

780

5


0.19

0.083

5

PEG5000-2TEMPO (44)

0.36

0.025

2TEMPO-PEG6000-2TEMPO (45)

0.54

0.017


0.18


0.20

119


0.19

120

Imim-PEG1000-TEMPO (48)

116

117

118

0.083

3 3 8 6

1

1

NaClO

Yield

NaClO

NaClO

5

O2

10

Oxone

1

O2

＞98

81

81

972

91.6

91.6

3664

>99

100

6000

3

>99

>99

20

6.67

6d

>99

>99

20

6.67

6e

70

98

7

2.33

85f

0

8.5

1.06

84

14

84h

10g

TEMPO content in the supported catalysts. b TEMPO loading in the reaction relative to the substrate. c 1-Octanol as the substrate. d 4-Bromobenzyl alcohol as the substrate. e 4-Bromobenzyl alcohol as the substrate. f Additional DMF after 3 h. g 2-Decanol as the substrate. Reaction time=24 h. h Imim-PEG1000-TEMPO (S/C=1)/AuCl3 (S/C=1), T=60°C. a


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(2)

Page 68 of 227

PS

PS is an attractive linear soluble polymer for supporting catalysts. Immobilization on PS has been extensively developed because of the importance of such supports in peptide synthesis and in combinatorial chemistry during the past 30 years.94 In the case of linear polystyrene, catalysts can be introduced directly during polymerization using an appropriate co-monomer or post-polymerization by modification of the polymer. Control of the amount of functionality can be achieved by varying the amount of co-monomer during polymerization or by controlling reaction conditions in post-polymerization modifications. In addition, analysis of the amount of functionality of the catalyst can often be carried out by solution state 1H NMR. PS-bound catalysts can be recovered and recycled by the same type of solvent precipitation process used for PEG bound ones. Some substituted polystyrenes can be recovered using a liquid/liquid separation too.121

The immobilization of TEMPO was first carried out in 1985 on both cross-linked and linear PS to give 49 and 50 (Scheme 20).122 Those polymers presented high TEMPO


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ACS Catalysis

loadings, with molar content ranging from 40 to 100 mol%. These resins were used to efficiently catalyze the oxidation of benzyl alcohol to benzaldehyde at room temperature using potassium ferricyanide or cupric chloride as the terminal oxidant with 4 mol% TEMPO catalyst loadings. The resins with lower TEMPO loadings showed better performance for this reaction. The introduction of additional functionality on the polymeric backbone tends to increase its hydrophilicity, leading to a clearly positive effect on the catalytic activity of the resulting resins 49 and 50. This can be associated with the fact that the reaction was carried out in water/organic solvent mixtures.

CH3

O Desmodu 25

75 N O

25

75 NH2

O

N 49

O

N 50

O N H

N H

N O

O

PS

PU - TEMPO 0.453 mmol/g TEMPO loading 51

O

O

Y

O

O PS

HN

N

PS - TEMPO 1.59 mmol/g TEMPO loading 52

N O

53 NO

O

O

O

PS

N O n N Cl n = 4, 6 and 10 54

Scheme 20. PS-supported TEMPO catalysts.

Baucherel et al.123 demonstrated PS-supported TEMPO was an efficient catalyst for the efficient oxidation of alcohols using sodium hypochlorite, molecular oxygen, and air as


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Page 70 of 227

the terminal oxidant. Under these conditions, PS-supported TEMPO catalyst showed higher activity than silica-supported TEMPO under the same conditions. The PSTEMPO catalyst was recycled for up to 20 reaction runs while retaining its catalytic activity and selectivity.

Eilbracht et al.124 chose commercially available isocyanate functionalized polymers to prepare polyurethane (PU) and PS based TEMPO catalysts in a simple one-step process. The obtained catalysts have been employed in the oxidative reaction in biphasic [H2O/dichloromethane (DCM)] and monophasic media (DCM, toluene etc.) depending on co-oxidants used. The PU-TEMPO 51 and PS-TEMPO 52 showed comparable catalytic activities as the parent TEMPO compound and these catalysts were easily recovered and recycled for 5 cycles with a very slight decrease in conversion. The PU-TEMPO was carried out in a continuously operated chemical membrane reactor for 30 residence times, with steady conversions up to 92% achieved.124


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McQuade et al.125 presented a novel method for preparing site-isolated polymeric catalysts for easy separation and enhanced activity relative to the traditional encapsulation of polymer-supported catalysts. The prepared TEMPO triazole in PSmicrocapsule showed 2.5 times higher activity for oxidation of alcohols than TEMPO triazole in polyacrylate microcapsule and TEMPO triazole immobilized on Merrifield resin. The catalyst gave an isolated yield of 96% and did not show any signs of capsule breakage even under fast stirring, allowing the catalyst contained PS microcapsule to be recycled three times.

Wang’s group has made significant contributions on PS bead-supported TEMPO catalysts.126,127 They immobilized TEMPO on PS beads through either an ether bond or ionic liquid bridge and prepared catalysts 53 and 54, respectively. In particular, catalyst 54 exhibited high thermal stability in different atmospheres as well as biphasic oxidation conditions and was recycled 30 times without significant loss in its catalytic activity.126 Moreover, with catalyst 54 as a Pickering emulsifier, they developed a new micellecatalyzed biphasic system for Anelli alcohol oxidation reaction. The activity of the


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Page 72 of 227

system was significantly enhanced due to the dual accelerating effect of both Pickering emulsification and miceller catalysis.127

Liu et al.128 used cross-linked PS microspheres for the preparation of TEMPO polymer brushes on the surface of PS microspheres via surface-initiated ARGET (activators regenerated by electron transfer) ATRP (atom transfer radical polymerization). As shown in Scheme 21, the synthetic route starts by supporting ATRP initiator on the surface of the surface-hydroxylated cross-linked PS microspheres. Using these microspheres, the homopolymer brush of PTMA [poly(2,2,6,6-tetramethyl-4-piperidyl methacrylate) and block copolymer brush of P(MMA-b-TMA) [poly(methyl methacrylate-

b-2,2,6,6-tetramethyl-4-piperidyl methacrylate)] were synthesized and the grafting density was obtained 3.3 and 3.8 chains nm-2, respectively. So prepared homo and copolymer brushes were further oxidized with m-chloroperoxybenzoic acid (m-CPBA) to yield nitroxide polymer brushes of and P(MMA-b-TMA) [poly(methyl methacrylate-bTEMPO methacrylate), 55] with poly TEMPO groups. The catalytic performance of polymer brushes was evaluated for the oxidation of benzyl alcohol to benzaldehyde,


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with excellent catalytic properties shown. The P(MMA-b-TMA) brushes demonstrated an equivalent catalytic performance with the non-supported TEMPO. The prepared catalyst was recycled for 5 reaction cycles without much significant loss in its activity. Later, the same group has provided a new way of preparing TEMPO polymer brushes at high TEMPO loadings.129 They synthesized both homopolymer brush of PGMA containing GMA and block copolymer brush of P(MMA-b-GMA) consisting GMA and MMA via surface-initiated ARGET ATRP. These polymer brushes were used to make nitroxide polymer brushes of P(MMA-b-GMA)-TEMPO (56 in Scheme 22). The polymer brush catalyst was successfully employed for the synthesis of epoxidized soybean oil acrylate and showed a good inhibiting effect with easy recyclability for up to 6 cycles without loss in its activity.


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Br O

Surfactant-free emulsion polymerization

O OH

O

O

Br

O O

N O

Br

O

ARGET ATRP MMA

O O

NH

O

Br

O

O

mCPBA O

O

Microsphere initiator

O

Br

O O

Br

P(St-co-HEMS) microsphere

PS microsphere

Styrene

OH

O

CH2Cl2, 0 oC

ARGET ATRP O

TMPM

P(MMA-b-TMPM) brushes

P(MMA-b-TMA) brushes 55

O

O

PMMA brushes

Scheme 21. Synthesis route for the preparation of P(MMA-b-TMA) brushes on polystyrene microspheres. Reproduced from Ref. 128. Copyright 2014 Express Polymer Letters.

O

Br

O O

OH O

O O

N O

O 56

Scheme 22. TEMPO polymer brushes PS-P(MMA-b-GMA). Reproduced from Ref. 129. Copyright 2015 American Chemical Society.


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Table 8 summarizes the various PS-supported TEMPO catalysts and compares their performance towards the oxidation of benzyl alcohol to benzaldehyde. High TOF values are noted with these catalysts. In particular, the magnetic micelle-catalyzed biphasic system developed by Wang et al.127b show outstanding catalytic activity with TOF up to 55260 h-1, along with convenient magnetic separation, and high reusability.


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Table 8. Summary of PS-supported TEMPO catalysts and comparison of their performance towards oxidation of benzyl alcohol to benzaldehyde

Ref.

122

123

PS supports

1.61

Copolymer

Polymer (P-12) (50)

2

Acid functionalised FibreCatTM

polystyrene Chloromethylated polystyrene beads

Fe3O4/chloromethyl PS NPs

Polymer 128 a

[mmol/g]

Polymer (P-11) (49)

124f

127b

TEMPO contenta

Copolymer

polyurethane

126

PS-supported TEMPO catalysts

PS microsphere

PS-TEMPO

2

Time

TEMPOb

Oxidant

Yield

[h]

[mol%]

[%]

20

4

K3[Fe(CN)6]

0.17

1.67

NaClO

99

2

10

O2

3

8

Selectivity [%]

TOF

TON

[h-1]

Number of reuse

31

7.75

0.39

55

13.75

0.69

99

59.28

355.69

20c

>99

>99

10

5

5c

O2

99

99

12.38

4.13

PU-TEMPO (51)

0.453

0.5

5

NaClO

>99

>19.8

>39.6

5

PU-TEMPO (51)

0.453

0.083

5

BAIB

>99

>19.8

>237.6

5

PS-TEMPO (52)

1.59

0.5

5

NaClO

>99

>19.8

>39.6

5

PS-TEMPO (52)

1.59

0.083

5

BAIB

>99

>19.8

>237.6

5

100

20

13.33

40

100

20

13.33

7

PS-IL-TEMPO (54)

1.5

PS-TEMPO (53) Fe3O4/PS[imC4TEMPO]Cl

1.15

Fe3O4/PS-TEMPO

1.02

Fe3O4/PS[imC4TEMPO]Cl

1.15


1.09


0.97

PTMA

3.1

P(MMA-b-TMA) brush (55)

3.6

0.0042

5 5

NaClO

1

NaClO 0.017

0.05

0.1

1

NaClO

>99.9

>99

100

24000

50.0

>99

500

30000

81.2

>99

812

48720

92.1

>99

921

55260

87.2

>99

872

52320

91

100

91

1820

96

100

96

1920

10

5

TEMPO content in the supported catalysts. b TEMPO loading in the reaction relative to the substrate. c reusability test with 1-octanol as the substrate.


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(3)

Other Polymer Supports

A soluble, commercially available hindered amine oligomer, Chimassorb 944, was used to prepare the polyamine immobilized TEMPO.130 This resulting catalyst was effective at low TEMPO loadings (1 mol%) for the selective oxidation of alcohols using bleach or cuprous chloride/ air as terminal oxidants with short reaction times, giving 8099% yields. The catalyst was recovered by precipitation with TBME, which allowed its reuse at least twice. It was found to be more active than other supported TEMPO systems, including those developed from silica and other inorganic supports.28,29b Later, Toy et al. showed a JandaJel supported TEMPO could also act as an efficient catalyst (at 1 mol%) for the oxidation of alcohols to aldehydes and ketones in a multipolymer system in which PS-supported diacetoxyiodosobenzene was the terminal oxidant.131 An oxoammonium resin, the expected oxidation product from polymer-supported TEMPO, has been used as a stoichiometric reagent for the efficient oxidation of alcohols.132

Lee et al.133 synthesized poly TEMPO radical grafted polythiophene (PEBBT-g-PTMA) polymer (57 in Scheme 23) by oxidative polymerization and ATRP. Its electrochemical


77

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properties in organic radical batteries have been studied. In addition, the group confirmed that PEBBT-g-PTMA thin-film electrode has a better electrochemical performance than the PTMA thin-film electrode by cell performance studies. However, its catalytic performance was not evaluated.

Br m

O S

O

n

O

O

O

57 O S O

N H

O

N O

N

O









O

HN N

O 60

58

O

X PES-im-TEMPO (TEMPO: 0.2 mmol g-1) X:Y1:Y2 = 0.85:0.08:0.07

O

O S O

O Y1

O S O

O Y2

N

N

N

N 4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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O 59

ON

Scheme 23. Other polymer supported TEMPO catalysts: PEBBT-g-PTMA (57), TEMPO/HBPEH (58), PES-im-TEMPO (59), and PPO-TEMPO (60).

Recently, Nabae and his group134 used a hyperbranched aromatic poly(ether ketone) (HBPEK) polymer for the immobilization of TEMPO and prepared a novel


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TEMPO/HBPEK homogeneous catalyst (58, TEMPO loading of 2 mmol/g) for the aerobic oxidation of alcohols. These homogeneous catalysts showed TON number of 46 as compared to commercial TEMPO with a TON of 60. They also synthesized recyclable heterogeneous catalysts (TEMPO/HBPEK/CB and TEMPO/HBPEK/PI) by grafting TEMPO/HBPEK onto insoluble supports including carbon black (CB) and polyimide (PI) nanoparticles, respectively. Such heterogeneous catalysts showed TON number of 27 and 22, respectively, while PS-immobilized TEMPO catalyst only showed a TON >1.

More recently, Wang et al.135 have further employed PES for immobilization of TEMPO and prepared PES-im-TEMPO PIC (59) by anchoring on imidazolium groups. This welldesigned PIC catalyst self-assembled to nanoaggregates in DCM and was used as a Pickering emulsifier in the Anelli oxidation system. De-emulsification of the Pickering emulsion was achieved by adding a little amount of acid, resulting in high catalyst recycling after centrifugation. The catalyst exhibited high catalytic activity for the oxidation of benzyl alcohol.


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Porous polymerized organocatalysts (PPOs) with a high surface area, large pore volume, hierarchical porosity and superior chemical stability have been rationally synthesized via a mild solvothermal polymerization of the corresponding vinylfunctionalized organocatalyst monomer.136 Meng et al.136c have employed this novel synthetic method to prepare heterogeneous porous polymerized TEMPO (PPOTEMPO) catalyst (60, Scheme 23). This organocatalyst showed high catalytic activity and excellent recyclability (5 times) in selective oxidation of a variety of alcohols to the corresponding aldehydes or ketones.

For the first time, Tanyeli et al. used polynorbornene to support TEMPO as recyclable catalysts and synthesized three sorts of TEMPO based polymeric catalysts (61a-c, Scheme

24)

by

ring-opening

metathesis

polymerization

(ROMP).137

These

homogeneous catalysts were employed for the oxidation of alcohols using bleach as a co-oxidant and showed efficient activity to produce respective carbonyl compounds. The polymeric TEMPO catalysts were recovered up to 3 times by solvent precipitation and filtration. No drastic decrease in activity was observed after each cycle.


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ACS Catalysis

Ph 50

RO

O O O N OH

61a: R = TEMPO 61b: R = H 61c: R = Me

Scheme 24. Polynorbornene supported TEMPO moieties. Reproduced from Ref. 137a. Copyright 2003 Elsevier.

The various supported TEMPO catalysts on other macromolecular supports are summarized in Table 9, along with a comparison of their catalytic performance.


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Table 9. Summary of TEMPO catalysts immobilized on other macromolecular supports and comparison of their performance towards oxidation of benzyl alcohol to benzaldehyde Ref.

Polymer supports

Supported TEMPO Catalysts


Time [h]

TEMPOb

130a

Chimassorb944

PIPO

3.2

0.33

1

131

JandaJel (JJ) Resins

JJ-TEMPO

2

5

1

PS

TEMPO/PS

1

HBPEK

TEMPO/HBPEK (58)

2.04

HBPEK/CB

TEMPO/HBPEK/CB

HBPEK/PI

134

135

136c

Yield

Selectivity

[%]

[%]

NaClO

> 99

> 99

PSDIB

Number of reuse

100

300

3

80

80

16

3c

1.23

99

98

100

2000

5

88

85

88

1060

> 99

98

100

2000

> 99

98

100

2000

70

70

70

67

67

67

69

69

69

TEMPO content in the supported catalysts. b TEMPO loading in the reaction relative to the substrate. c Reusability test with 2-Phenylethanol as the substrate.


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2.3

Summative Discussion on Various Supported TEMPO Systems for Catalysis Applications

Among the various supported TEMPO systems summarized above, only cost-effective supported TEMPO catalysts with superior catalytic performance coupled with convenient inexpensive synthesis and recovery procedures are promising for practical industrial applications with stringent requirements on process economics. Though showing some unique catalytic properties with scientifically intriguing design, IL-, C60-, CNT-, and GO-immobilized TEMPO catalysts, in our opinion, are not well suited for practical catalytic applications due to the high costs of these support materials and/or the requirements of special expensive solvents (such as ILs in the case IL-tagged TEMPO catalysts). On the contrary, TEMPO catalysts immobilized on low-cost abundant supports, such as silica, magnetic nanoparticles, PEG, and PS, are highly promising for large-scale applications. Among them, the recovery of supported TEMPO catalysts on inorganic silica and MNPs should be even more convenient than those supported on soluble organic polymer supports since the latter will often require precipitation with the use of an additional solvent that could represent extra costs. In this regard, MNP-encapsulated supported TEMPO catalysts, particularly, the hybrid magnetite (Fe3O4)-encapsulated, TEMPO-bound polymer nanoparticle catalysts reported by Wang et al.,53,57,127b are most attractive, with their convenient low-cost synthesis, superior catalytic activity, high reusability, and easy fast recovery by simple magnetic separation. This represents the direction of further developments for practical applications. 3

TEMPO in Chemical Transformation


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In this part, the applications of TEMPO in chemical transformations, including synthetic chemistry and polymer chemistry, are summarized. A particular focus is on the oxidation reactions catalyzed with TEMPO and its derivatives, both homogeneous and heterogeneous. We have made an effort to cover the advances made by various groups up to date using different economic and environmental-benign stoichiometric oxidants including NaOCl, BAIB, O2, transition metal and metal-free oxidants, as well as electrochemical approaches along with TEMPO. Moreover, mechanistic aspects are also being taken into consideration for a better understanding of the reactivity of the reagents utilized for the reaction with TEMPO. Simultaneously, other organic functional group transformations, like C=O, CC, and CN bond forming reactions are also briefly commented. Furthermore, other applications of TEMPO in catalytic sulfoxide formation, coupling reactions, H-abstraction, N-alkoxyamine formation and homolysis, multicomponent Passarini reaction, and natural product synthesis are also discussed. Lastly, a subsection is also dedicated to discuss recent developments in NMP with TEMPO for the synthesis of macromolecular polymeric architectures.


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3.1

Organic Transformations

3.1.1 Oxidation of Alcohols The oxidation of alcohols to their carbonyl compounds138 or their full oxidants139 are among the central reactions in synthetic organic chemistry.140 The transformation is useful for the development of environmentally benign processes,141 preparation of novel materials,142 and for energy sources.143 Playing a pivotal role in industrial fields, this transformation has been continuing to attract a great attention for disclosing new catalysts,144 substrates, oxidants with peculiar features and for many applications.145

However, concerning the oxidants, stoichiometric oxidations with classical transition metal compounds or sulfoxides are still in common use despite the formation of a large number of undesired products. The most commonly used oxidants include small organic molecule-based catalysts, such as Dess-Martin periodinane,146 swern oxidation,147 Ley,148 or metal-based systems (e.g., Jones, Collins, Oppenauer reagents, pyridinium chlorochromate, pyridinium dichromate, barium permanganate, manganese dioxide, ruthenium tetroxide and silver carbonate),149 2-iodoxybenzoic acid,150 Moffatt oxidants, Corey-Kim oxidants, SO3/pyridine, as well as some moisture-sensitive and expensive


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ones (e.g., N,N’-dicyclohexylcarbodiimide, oxalyl chloride).138 These oxidants provide a plethora of options for the generation of desired carbonyl compounds from the starting alcohols. Nevertheless, in the interest of mitigating the toxic effects of these oxidations, alternative catalysts have been sorted out in recent years. To this end, TEMPO has been found to be highly attractive species because it is environmentally benign, userfriendly with extremely mild conditions needed.150b,151 In this part, a brief attention has been paid to the TEMPO-mediated alcohol oxidation using various co-oxidants.

3.1.1.1

(1)

Oxidation with NaOCl/TEMPO

Anelli Condition

A wide range of alcohols has successfully been oxidized with the TEMPO/NaOCl oxidizing system. Usually, the oxidation kinetics of primary alcohols is much faster than that of secondary alcohols. The oxidation system can be used in water alone for the oxidation of water-soluble alcohols. Glycerol can be oxidized to the sodium salt of ketomalonic acid in high yields (Scheme 25).30,152


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ACS Catalysis

HO

O

TEMPO, NaOCl, NaBr

OH OH

aq. NaOH, pH 10

Glycerol

NaO

ONa

O O Salt of ketomalonic acid

Scheme 25. Formation of the sodium salt of ketomalonic acid from glycerol by oxidation with TEMPO/NaOCl. Reproduced from Ref. 152. Copyright 2003 Wiley.

Oxidation of carbohydrates are mostly catalyzed by TEMPO/NaOCl system. A series of methyl 2-deoxy-2-acetamido-D-glycopyranosyl-1-azide uronates was prepared as shown in Scheme 26.153,154

OH HO HO

O N3 NHAc

TEMPO, NaOCl KBr, NaHCO3, H2O, rt

NaO2C HO HO

O N3 NHAc

Scheme 26. Oxidation of carbohydrates with TEMPO/NaOCl system. Reproduced from Ref. 153. Copyright 1995 Elsevier.

Wang’s group155 has recently reported a universal strategy to enhance biphasic Anelli oxidation reaction by forming a controllable Pickering emulsion with surface tunable magnetic nanoparticles along with free TEMPO. During the emulsification of the


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biphasic system, the unstable milli-meter scale dispersion of droplets were broken into a stable micro-meter scale emulsion, which significantly increased the overall reaction rate.

Moreover, large scale reactions156 and continuous process oxidations have also been devised using Anelli conditions.157 Nitroxide-catalyzed bleach reactions have been considered and are likely used commercially on a large-scale production.158

As shown in Scheme 27, the catalytic cycle in Anelli oxidation involves oxidation of alcohols by the oxoammonium cation and regeneration of the latter can be possible by the reaction of TEMPOH with the secondary oxidant. Herein, HOBr acts as a secondary oxidant, which is generated from KBr and HOCl. The absence of KBr in the reaction can strongly affect the rate of the reaction.159


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ACS Catalysis

Primary oxidant R-CH2OH

N O

Br

HOBr

R-CHO

N OH

HOCl

Cl

BrO Secondary oxidant

Scheme 27. Proposed mechanism for Anelli oxidation condition. Reproduced from Ref. 159. Copyright 1994 Wiley-VCH.

(2)

Zhao’s Condition

Zhao et al. modified Anelli’s procedure.160 In Zhao’s modified procedure, side reactions induced by the presence of sodium hypochlorite were lessened by reducing its use from stoichiometric quantities to the catalytic amounts. In this modified procedure, sodium chlorite (NaClO2) was employed as stoichiometric oxidant, which served both to regenerate NaOCl and to operate as the primary oxidant for the transformation of intermediate aldehyde into carboxylic acid.161


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Tschaen et al. reported the conversion of acid using Zhao’s protocol.162 The reaction was carried out in aqueous acetonitrile, thus avoiding the biphase system of the bleach oxidations (Scheme 28). The reaction took 610 h to reach completion at 35 C. The oxidation of alcohol to aldehyde occurred in the first step and aldehyde was converted to acid by NaClO2 in the second step.

R-CH2OH

NaClO2, cat. NaClO, cat. TEMPO MeCN/ sodium phosphate buffer

R-CO2H

Scheme 28. Preparation of acid using Zhao’s oxidation condition. Reproduced from Ref. 162. Copyright 2005 Wiley.

Huang et al.163 utilized these reaction conditions for the oxidation of different primary alcohols and carboxylic acid. Several glycosaminoglycans including hyaluronic acid, chondroitin, and heparin oligosaccharides were prepared in high yields.

Similar to Anelli’s catalytic cycle, the oxidation in Zhao’s procedure starts with the oxoammonium salt, which acts as a catalytic primary oxidant. Stoichiometric sodium chlorite (NaClO2) helps to generate sodium hypochlorite (NaClO) and later acts as a


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ACS Catalysis

secondary oxidant to regenerate primary oxidant from a reductive form of TEMPOH (Scheme 29).

RCHO

RCH2OH

OH R C OH H

OH R C O Cl H O

OH R

O

H

Cl

O

N OH

N X O

NaCl

Catalytic NaOCl secondary oxidant

NaClO2 NaOCl Stoichiometric oxidant

Scheme 29. Proposed mechanism for Zhao’s oxidation condition. Reproduced from Ref. 160. Copyright 1999 American Chemical Society.

(3)

BAIB/TEMPO Condition

BAIB/TEMPO catalytic system has a number of advantages. The oxidation reactions can be carried out with substrates that bear other functional groups such as carboncarbon double bonds, sulfides, epoxides, selenides, and electron-rich aromatic rings. Most importantly, primary alcohols can be selectively oxidized in the presence of


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Page 92 of 227

other secondary alcohols. Rozners and Xu selectively oxidized primary alcohols to their corresponding acid derivatives in the presence of secondary alcohol on the nucleoside derivative (Scheme 30).164

N3

HO O HN O

O

0.18 eq. TEMPO, 2.2 eq. PhI(OAc)2 HN MeCN/water (1:1), r.t., 27 h.

N3 CO2H

HO O

OH

O

O

Scheme 30. Selective oxidation of p-alcohols to acids with BAIB/TEMPO condition. Reproduced from Ref. 164. Copyright 2003 American Chemical Society.

In addition, the BAIB/TEMPO is a very good reagent for the conversion of 1,4- and 1,5diols to lactones.165 Gruttadauria’s group has reported oxidation of alcohols using BAIB/TEMPO catalytic system. The catalytic system effectively oxidized a series of various alcohols to their carbonyl compounds only with 1 mol% of the catalyst and it was used for 13 reaction runs for different primary and secondary alcohols.70a,97 Later, the same group has communicated that BAIB could produce acylated hypobromite


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(AcOBr) in the reaction mixture, which could aid to increase the rate of the reaction. Therefore, the catalyst was employed only at 0.1 mol% to check the scope of the catalytic system. It facilitated quantitative oxidization of the substrates to corresponding carbonyl compounds with no observation of any over oxidized products.71,166 In general, only the aldehyde was isolated, except that in one case a primary alcohol was oxidized completely to a carboxylic acid in the presence of water.167 Apparently, the main byproducts are iodobenzene and acetic acid. The reaction rate can be increased with a small amount of acetic acid114 and the iodobenzene can be removed by silica gel chromatography or by vacuum distillation.168

The catalytic cycle for the BAIB/TEMPO system differs from the normal nitroxide incorporated oxidation systems. The BAIB reagent does not oxidize TEMPO directly to oxoammonium salt and the initial oxoammonium salt is formed by the acid catalyzed disproportionation of the nitroxide (Scheme 31). The acetic acid which is generated from BAIB converts all TEMPO to its reactive analog by disproportionation and after the


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oxidation reaction the resulting hydroxylamine is again oxidized to primary oxidant by PhI(OAc)2.138

RCH2OH Primary Oxidant

X

RCHO

N OH

N O

N O

AcO

I

OAc Secondary oxidant

I 2 AcOH

TEMPO

Scheme 31. Proposed mechanism for BAIB/TEMPO catalytic system. Reproduced from Ref. 138. Copyright 2006 Springer.

The reaction is carried out in the slightly acidic medium due to the acetic acid formation. The conversion of primary alcohols with oxoammonium salts can work either through a linear transition state A (Scheme 32) or via a compact five-membered transition state B under basic conditions. The more compactness of five-membered transition state under basic conditions would lead to a faster reaction rate and greater selectivity for oxidation of primary alcohols.138


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ACS Catalysis

R

R

N

O

HO R 1 H R

R N O R1 O H

B Basic condition

R

B: H N O 1R R OH R A Acidic condition

N

O OH

R R1

Scheme 32. Reactivity of oxoammonium salt with TEMPO under acidic and basic conditions. Reproduced from Ref. 138. Copyright 2006 Springer.

3.1.1.2 Aerobic Oxidations with Metal Oxidants Oxygen or air is among the cheapest and least polluting stoichiometric oxidants and only produces water as the sole by-product. The implementation of transition metalbased catalysts in combination with oxygen represents an emerging alternative to the traditional oxidations.169 This part aims to give an overview of aerobic oxidations catalyzed by various transition metals along with TEMPO moieties.

(1)

Ru/TEMPO Catalytic System

Ruthenium compounds have been extensively studied as catalysts for the aerobic oxidation of alcohols, due to its widest range of oxidation states.140b,169


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Kobayashi et al.170 developed a polymer-incarcerated ruthenium catalyst (PI Ru) for the oxidation of alcohols along with TEMPO catalyst. PI Ru was synthesized from a polystyrene based copolymer with ruthenium chloride hydrate as the metal source. The PI Ru/TEMPO catalyst showed wide applicability to various alcohols with high reusability. In specific, 4-methoxybenzyl alcohol was oxidized in the presence of 5 mol% of PI Ru and 15 mol% of TEMPO to afford 98% yield of corresponding aldehyde (Scheme 33).

OH MeO

O2 (0.1 MPa), PI Ru (5 mol%) TEMPO (15 mol%) 80 oC, time (h), 1,2-dichloroethane

O MeO 98%

Scheme 33. Selective oxidation of 4-methoxy benzaldehyde. Reproduced from Ref. 170a. Copyright 2007 Wiley.

Although, Ru-TEMPO has been proved to be an effective catalytic system for the aerobic oxidation of alcohols, further improvements such as lowered catalyst loading and mild conditions for the reaction need to be addressed.


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According to the mechanistic study by Sheldon et al.,3a,171 Ru/TEMPO-catalyzed aerobic oxidation of alcohols proceeds via a hydridometal mechanism, involving a “RuH2(PPh3)” moiety as the active catalyst (Scheme 34).

R1 OH

RuCl2L3

R2 H

2 HCl+ R1 R2

R1

R2

O

O

RuH2L3 (L= PPh3)

2

N O

-H elimination

H RuL3 R1R2CHO D

H L3Ru O N R1 OH

N OH

R2 H

N OH inert atm.

H 2O

1/2 O2

C N H TEMPH

Scheme 34. Proposed mechanistic aspects of Ru/TEMPO catalytic system. Reproduced from Ref. 171. Copyright 2001 American Chemical Society.

This ruthenium dihydride complex reacts with two molecules of TEMPO to form a complex C (Scheme 34) and one molecule of TEMPOH. Later, proton transferred from the alcohol to the piperidinyloxy ligand in complex C affords complex D (Scheme 34) and another molecule of TEMPOH. Under the aerobic condition, the two molecules of


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TEMPOH are oxidized by oxygen to generate two molecules of TEMPO. On the other hand, under anaerobic conditions, in the Ru-catalyzed reaction of alcohol with TEMPO, TEMPH is formed by disproportionation of TEMPOH. In both cases, complex D undergoes normal β-hydrogen elimination to produce the ketone/aldehyde and the active ruthenium dihydride species.

(2)

Fe/TEMPO Catalytic System

Aerobic oxidation of alcohols has been performed by using a number of different Fe/TEMPO catalytic systems. Unlike other metal/TEMPO (e.g., Ru, Cu) systems, Fe/ TEMPO catalysts do not require any base or organic ligands (such as PPh3 or 2,2’bipyridine (bpy)) and therefore they are sterically less hindered compared to other metal/TEMPO systems. Recently, Hayton et al.172 have prepared the complex of MCl3(ɳ1-TEMPO) by coordinating TEMPO with MCl3 (M = Fe, Al) and employed the complex for the oxidation of alcohols.

Nevertheless, most of the catalytic studies have been utilizing Fe(NO3)3 as the iron salt or NaNO2 as an additive along with catalytic amounts of TEMPO. Liang et al.173 used


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the FeCl3/NaNO2/TEMPO catalytic system for aerobic oxidation in trifluorotoluene at roo Ent ry

4-Substituted TEMPO

Time [h]

Conv. [%]

Select. [%]

Ent ry

4-Substituted

Time

Conv.

Select.

TEMPO

[h]

[%]

[%]

m

temperature. This system worked well for primary and secondary benzylic alcohols as well as cinnamyl alcohols. Later, the same group employed a variety of 4-substituted TEMPO derivatives instead of TEMPO to develop a more efficient catalytic system for aerobic oxidations (Table 10).174 Based on the screening results of 4-substituted TEMPO derivatives, the catalyst comprised of 4-acetamidoTEMPO, iron chloride and sodium nitrite has been developed for the highly efficient oxidation of a wide range of primary alcohols including primary aliphatic alcohols to the corresponding aldehydes under mild conditions.

Table 10. Catalytic aerobic oxidation of 2-ethyl hexanol in the presence of the 4substituted TEMPOs. Reproduced from Ref. 174. Copyright 2010 Wiley-VCH.


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O

N

1

O

O

O

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2

81

90

10

2

86

91

11

Cl

2

90

91

12

Cl

O

N

O O

2

59

98

2

85

92

2

82

94

2

79

91

2

86

91

2

71

90

2

81

92

2

92

98

2

49

99

O N

2

O

O

N

O

O

NO2

O

O

3

N O 2N

O

O

NO2

O

N

Cl

N

O

4

O

O

N

O

2

81

92

13

O

O Cl

N CF3

5

O

O

N O Cl

O

2

85

91

O

14

N

O

F3C

O

O

N O

6

N

2

81

92

15

O

O O 2N

Cl

O

O

7

N

N O

2

Cl

89

90

O

16

Cl

NO2 O O S O

8 9

N O

O O

O

O

N O

2

87

96

17

2

75

85

18

N

O N H

N

O

HO

O


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Zhang et al.175 developed a task-specific bimagnetic imidazolium salt [Imim-TEMPO] [FeCl4] containing cooperative functionalities. The catalyst was proved to be highly active for a variety of aromatic alcohols under mild conditions without utilization of any organic solvents (Scheme 35). In addition, it is worth to mention that the reaction could be carried out even with air as an oxidant instead of pure oxygen. However, the exact role of NaCl is not clear in the reaction mechanism.

OH R1

R2

5 mol% [Imim-TEMPO][FeCl4] 5 mol% NaNO2 o

O R

1

R2

0.2 MPa O2, H2O, 30-100 C

Scheme 35. Oxidation of alcohols using air as an oxidant. Reproduced from Ref. 175. Copyright 2011 Royal Society of Chemistry.

Gao et al.176 have recently prepared a catalytic system through covalently bonding TEMPO on silica. This system can be efficient for the oxidation of benzyl alcohols using


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only 0.01 mol% of heterogeneous radical along with 8 mol% of FeCl36H2O and 10 mol% of NaNO2 (Scheme 36). After the reaction, the heterogeneous catalytic system was recycled with co-oxidant for 5 runs and 10 runs with freshly added co-oxidants.

OH R1

R2

SBA-15-TEMPO (0.01 mol%), FeCl3 6H2O (8 mol%) NaNO2, (10 mol%), toluene, 1 atm O2, 25 oC

O R1

R2

Scheme 36. Oxidation of alcohols using FeCl3/NaNO2/TEMPO catalytic system. Reproduced from Ref. 176. Copyright 2013 Elsevier.

Recently, Zhang et al.177 attempted to immobilize TEMPO on deep eutectic solvents (DESs)178 and synthesized novel DES-TEMPO catalysts. The catalyst consisted of N,Ndimethyl-(4-(2,2,6,6-tetramethyl-1-oxyl-4-piperidinyl)butyl)dodecyl

ammonium

salt

([quaternium-TEMPO]+Br-) and urea as a H-donor to make the H-bond with halide anion of DES moiety. The catalytic activity was evaluated for the aerobic oxidation of alcohols with Fe(NO3)3 as a co-oxidant under solvent-free condition (Scheme 37). The catalytic system DES-TEMPO/Fe(NO3)3 showed excellent catalytic activity for standard model alcohols and the catalyst was recycled for up to 5 consecutive cycles without significant


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loss in activity. After the 6th cycle, the activity started to decrease and 27% of yield was obtained after the 8th cycle, which might be attributed to the accumulation of the solid substances derived from Fe(NO3)3 during the recycling process.

DES-TEMPO

OH R

1

R

2

Fe(NO3)3 9H2O, O2

O R

1

R2

Scheme 37. Aerobic oxidation using Fe(NO3)3/TEMPO catalytic system. Reproduced from Ref. 177. Copyright 2014 Royal Society of Chemistry.

More recently, Zhang et al.179 reported that amino acids could greatly enhance the activity of the TEMPO catalyst. A catalytic system of Fe(NO3)39H2O/TEMPO/KCl was developed for the efficient oxidation of alcohols to carboxylic acids. Importantly, the total synthesis of naturally occurring phloemic acid was accomplished for the first time using this system.180,181

The detailed mechanism of the Fe/TEMPO catalytic system is still not clear. According to Ma et al.,180 Fe3+ plays a crucial role in the catalytic cycle during the oxidation reaction (Scheme 38). The proposed catalytic cycle starts with TEMPO by coupling with


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Fe3+ to afford intermediate E, which would react with the alcohols to form intermediate F by releasing H+. Later, the intermediate F would undergo β-H elimination and reductive elimination to produce the aldehyde or ketone, Fe2+, and TEMPOH.180 TEMPO may be regenerated from TEMPOH by reacting with Fe3+.182 In the presence of a proton, Fe2+ can be re-oxidized to Fe3+ by reacting with NO2 at the same time to form NO and H2O. NO2 was first generated from NO3- and would be regenerated by the reaction of NO with O2. Although the role of NaCl was not quite clear, it was believed that Cl- may behave as a unique electron-donating ligand to supply electrons to Fe3+ d orbitals183 to facilitate the semi-oxidative addition-type coupling with TEMPO. In the absence of NaCl, this coupling reaction of TEMPO with Fe3+ would be very slow due to its +3 oxidation state.

N O

Fe3+ E

Fe2+ H+

H+

Fe3+ Fe2+ N OH

R1 R

Fe2+ 2 H+

2

N OFe3+ R1 OH R2 H

N O H

Fe2+ R1 O R2 F NO Fe3+

O

NO2

H 2O

O2


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Scheme 38. Proposed mechanism for Fe/TEMPO catalytic system. Reproduced from Ref. 180a. Copyright 2011 Wiley VCH.

(3)

Cu/TEMPO Catalytic System

Most well-known catalytic system for aerobic alcohol oxidation is copper combined with TEMPO radical. The first example using this Cu/TEMPO catalytic system was reported by Semmelhach et al. in 1984.184 Subsequently, Sheldon et al. developed a system, which consisted of CuBr2, TEMPO, bpy as a ligand with t-BuOK base (Scheme 39).185 This Cu led catalytic system converted several primary alcohols to aldehydes without over oxidation of carboxylic acid in CH3CN/H2O solvent mixture. In addition, excellent conversions were obtained even with inactivated primary alcohols (1-octanol) using this mild reaction procedure with air instead of pure oxygen.

R

OH

5 mol% CuBr2, 5 mol%

N

N

5 mol% TEMPO, 5 mol% t-BuOK CH3CN/H2O (2:1); Air, 25 oC

R

O

Scheme 39. Aerobic oxidation of alcohols using Cu/bpy/TEMPO catalytic system. Reproduced from Ref. 185b. Copyright 2004 Wiley.


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Sekar and Mannam developed enantiopure (R)-(+)-1,1'-bi(2-naphthylamine) ((R)BINAM) and Cu(OTf)2 complex.186 The complex was employed in the synthesis of enantiomerically pure oxidized amino carbonyl derivatives along with TEMPO through an oxidative kinetic resolution method. Under these catalytic conditions both ortho-and

para-substituted benzylic amino alcohols were resolved with good to excellent enantiomeric excesses (ee) (Scheme 40).

OH H 2N

Cu(OTf)2 (5 mol%) (R)-BINAM (10 mol%) TEMPO (5 mol%) Toluene, 80 oC, O2

O H 2N

OH H 2N



Scheme 40. Enantiomerically pure synthesis of aminocarbonyl derivative with Cu/(R)BINAM/TEMPO. Reproduced from Ref. 186. Copyright 2009 Elsevier.

Stahl et al. carefully analyzed a series of catalysts/ligands/bases/solvents for the aerobic oxidation of alcohols and found that CuI salts combined with bpy were superior to CuII salts using NMI base in CH3CN solvent.187 The (bpy)CuI/TEMPO showed excellent functional group tolerance and efficient selectivity for primary and secondary


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ACS Catalysis

alcohols. Later, the same group used the Cu(I)/TEMPO catalytic system for alcohol oxidation under continuous flow conditions.188

Gao et al.189 have developed a novel bio-inspired bifunctional molecule incorporating both the TEMPO moiety and a bipyridine ligand, which was used as a metal-binding site for forming Cu(I) metal-ligand complex. The bpy(CuI)-TEMPO catalytic system exhibited high catalytic activity for the oxidation of inactivated primary aliphatic alcohols as well as activated alcohols and obtained good to excellent yields.

Kerton and Hu190 have noted that the ligands have crucial importance in maintaining high yields during aerobic oxidation under Cu/TEMPO catalyzed systems and have investigated a series of tetradentate pyridyl-imine terminated Schiff-base ligands. In this case, a polydimethylsiloxane derived pyridyl-imine ligand and other tetradentate ligands were studied in combination with a CuBr2-TEMPO catalytic system for the conversion of a variety of alcohols. The catalytic systems showed catalytic activity as good as 2,2’bipyridine based systems (Scheme 41).


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H

H N

N

OH

Si

O

Si

N

20

N

5 mol% CuBr2, 2.5 mol% tetradentate N-donor ligand

O

5 mol% TEMPO, 5 mol% t-BuOK, CH3CN/H2O; Air; 25 oC; 2 h

Scheme 41. CuBr2/tetradentate ligand/TEMPO catalyzed the aerobic oxidation of benzyl alcohol. Reproduced from Ref. 190. Copyright 2012 Elsevier.

Afterward, a highly efficient, selective and green protocol for open-air oxidation of primary benzylic alcohols in distilled water was reported by Repo and his group.191 The catalytic system incorporated CuBr2 and N-(isopropyl)-3,5-di-tertbutylsalicylaldimine ligand with TEMPO as a co-catalyst. It was highly active in water for open air oxidation of alcohols. The catalyst system does not need an additional base to be active but one of the salicylaldimine ligands can abstract proton from the alcohol. This proton transfer generates an anticipated alkoxide complex and hence initiates facile catalytic oxidation. The catalyst system is selective and oxidize various -activated alcohols using low loading of both TEMPO (3 mol%) and catalyst (0.3 mol%).


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Several other groups used various solvents in order to obtain efficient recycling and simple recovery of the Cu/TEMPO catalytic system. Jiang and Ragauska91,192 used a pyridyl based ionic liquid [bmpy]PF6 for aerobic oxidation of primary alcohols catalyzed by a three-component system acetamido-TEMPO/Cu-(ClO4)/DMAP. The ionic liquid allowed the recovery and reuse of catalyst up to five runs without loss of activity.

Later on, the same group reported a similar three component system acetamidoTEMPO/CuBr/4-pyrrolidinopyridine under solvent-free conditions and simply recovered the catalytic system by adding a non-polar solvent.193

In order to improve catalytic recovery and recyclability, Knochel et al. used a biphasic solvent system such as chlorobenzene/perfluoro acetyl bromide and a pyridine ligand containing fluorinated ponytails for a CuBr-Me2S-TEMPO catalytic system.194 The fluorous layer contained catalytic system was recovered up to eight times with little loss of activity. Moreover, a multinuclear copper(II) complex in combination with TEMPO was used for the aerobic oxidation of benzyl alcohols using sole water as a solvent without the need of any organic or alternative solvents.195


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Yang et al.196 supported copper-manganese oxide mixture on the surface of the active carbon and prepared a heterogeneous co-catalytic system. Such Cu-Mn oxide/C oxidant along with TEMPO was utilized for aerobic oxidation for various alcohol oxidation under neutral conditions.196,197

Recently, Chen et al.198a presented a series of in situ generated Cu-NHC-TEMPO catalysts (NHC: N-heterocyclic carbene), which was employed for aerobic oxidation to evaluate the activity of the catalysts. Nonetheless, the structural changes of the catalytic system were unclear and also the isolation of catalytic system from the reaction mixture was a difficult procedure, due to the low loading of the catalyst and good solubility in chlorobenzene solvent.198

Recently, bipyridine copper(I) complex immobilized on MCM-41 was used for alcohol oxidation under air in combination with TEMPO (Scheme 42).199,200 The heterogeneous catalytic system [MCM-41-bpy-CuI] was recycled for 10 reaction runs with no significant leaching of copper.


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5 mol% MCM-41-bpy-CuI, 5 mol% TEMPO R

OH

aqueous ammonia (1.0 equiv), EtOH, Air, 50 oC

O O

R

O

OEt Si

OSiMe3 O Si O OEt

N H H N

N

CuI

N

MCM-41-bpy-CuI

Scheme 42. Alcohol oxidation using immobilized bpy-CuI/TEMPO. Reproduced from Ref. 199b. Copyright 2015 Elsevier.

Stahl et al.187a,201 recently investigated a simplified mechanism for CuI/bpy and CuII/bpy complexes along with a base (e.g., NMI or 1,8-diazabicyclo[5,4,0]undec7-ene) and TEMPO. The catalytic cycle of CuIOTf/bpy/NMI/TEMPO system involved four mechanistic steps (Scheme 43). In the first step, LnCuI is oxidized with oxygen to form [LnCu]2(O2) complex, which is later oxidized in the presence of TEMPOH to render CuII metal complex and TEMPO radical (step 2). Further, oxidation of alcohol is initiated by formation of a CuII-alkoxide (step 3), followed by TEMPO-mediated H-atom abstraction to produce the aldehyde and TEMPOH, and to regenerate LnCuI (step 4). On the other


111

ACS Catalysis

hand, the catalytic cycle of CuIIOTf2/bpy/NMI/TEMPO system proceeds similarly as the CuI catalytic cycle. As illustrated in Scheme 43, LnCuII-OH can form reversible bis-µhydroxy CuII dimer G, which is a more efficient catalyst precursor than other CuII sources and this dimer enter the catalytic cycle after dissociation of the dimer into monomeric (bpy)CuII(OH)(OTf). Moreover, they also ascribed that the change in reaction rate and kinetics is associated with NMI basic additive and a CuII-alkoxide intermediate formed through CuIILn-X species. Eventually, the catalytic cycle was completed by H-abstraction with TEMPO to produce aldehyde and reduced TEMPOH.

R

TEMPOH

1/2 O2

LnCuI

Ln = bpy, B: (e.g., NMI, DBU), CH3CN

H H

BH X

R

1/2 [LnCu]2(O2)

p2

LnCuII O

3

H 2O

Ste

+ -

1

B:, RCH2OH

ep

TEMPO

St

St ep

4

O

ep St

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 112 of 227

TEMPOH

LnCuII-OH R CuIILn

OH

TEMPO

+ -

X

BH X

B:, H2O

1/2 LnCuII

H O II O Cu Ln H G

Scheme 43. Proposed mechanism for Cu/TEMPO catalytic system. Reproduced from Ref. 201a. Copyright 2013 American Chemical Society.


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ACS Catalysis

Other Transition Metals/TEMPO Catalytic System

Cobalt and manganese metals have also been used as a co-catalyst along with low loadings of TEMPO or its derivatives for aerobic oxidation of alcohols.202,109,203

The polyoxometalate H5PV2Mo10O40 can be used for the aerobic oxidations with TEMPO.56,204 In this case, the polyoxometalate oxidizes TEMPO to form the oxoammonium salt, which further electrooxidizes alcohols to their corresponding carbonyl compounds. Vanadium can also be used for TEMPO catalyzed aerobic oxidations, but the VOSO4/TEMPO system did not perform well with secondary aliphatic alcohols even with extended reaction times.205 Very recently, gold was immobilized on the surface of silica supported TEMPO and the heterogeneous catalytic system was employed for aerobic alcohol oxidations.206 More recently, double-supported silicametal-organic framework palladium nanocatalyst was utilized for the aerobic oxidation of alcohols.207

(5)

Transition Metal Free NOx Oxidants/TEMPO


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Transition metal-free aerobic oxidation has a number of potential advantages over transition metal catalyzed oxidations. Transition metals like copper, ruthenium, and iron can conjugate with TEMPO under aerobic condition and may have the possibility to contaminate the product, as a result it will hinder the development of the total catalytic system.208 Moreover, these metal catalytic systems can show the chelating ability to heteroatoms (such as nitrogen, oxygen, and sulfur) and can promote competing oxidation of other functional groups as well as hinder the rate of the reaction.209 There are a number of reports, as shown below, for transition metal free aerobic systems using NOx as co-oxidant along with TEMPO. In those systems, oxoammonium salt acts as a primary oxidant and directly converts alcohols to carbonyl compounds, while NOx helps to regenerate primary oxidant in the presence of terminal oxidant (molecular oxygen, O2).

Hu et al.210 utilized NaNO2 as a NOx source for the first time and employed metal free TEMPO/NaNO2/Br2 oxidizing system for aerobic oxidation using DCM as solvent at elevated temperature. Later, they introduced 1,3-dibromo-5,5-dimethylhydantoin by


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replacing with bromine, due to the hazardous nature of bromine albeit in catalytic amounts. Furthermore, they identified tert-butyl nitrite (TBN) as an efficient NO equivalent for the activation of molecular oxygen and employed a robust triplecomponent catalytic system TEMPO/HBr/TBN for various alcohol oxidations (Scheme 44).211

R1 R2

TEMPO/HBr/tert-Butyl nitrite OH o

O2, 80 C

R1

O R2 Yield 40-98 % TON up to 16000

Scheme 44. Aerobic oxidation of alcohols using TEMPO/t-butyl nitrite. Reproduced from Ref. 211a. Copyright 2007 American Chemical Society.

Liang et al.212 showed TEMPO/HCl/NaNO2 catalytic system permitted the selective aerobic oxidation of a broad range of alcohols. He et al.94a tried to remove certain toxic and high volatile substances (such as HBr, Br2, HCl) and used immobilized TEMPO along with carboxylic acid on imidazolium ionic liquids. The [Imim-TEMPO]+Cl-/[ImimCOOH]+Cl-/NaNO2 catalytic system was highly selective for aerobic oxidation of various alcohols. Subsequently, in the same year they updated the catalytic system by replacing


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the conventional acids with self-neutralizing acids that form in situ in the CO2/H2O system for the aerobic oxidation.94b This protocol offers simple neutralization, which does not require any waste disposal with an advantage of improved reaction rates (Scheme 45).

OH R1

O

[Imim-TEMPO][Cl]/NaNO2/O2, 373 K

R2

CO2+H2O

R1 = Aryl, R2 = Aryl, or H R1 = Alkyl, R2 = Alkyl or H

R1

R2

H2CO3

Scheme 45. TEMPO/HCl/NaNO2 for aerobic oxidation of alcohols. Reproduced from Ref. 94b. Copyright 2010 American Chemical Society.

Studer and Wertz213 described an innovative metal-free catalytic system, which consisted of TEMPO and aqueous NH2OH to accomplish the aerobic oxidation of a relatively wide range of alcohols with good to excellent yield. Nonetheless, despite those interesting achievements, the TEMPO/NH2OH catalytic system requires a relatively high concentration of non-recyclable and expensive TEMPO (4 mol%) and high oxygen pressure up to 3 bars.


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Karimi et al. employed SBA-15 immobilized TEMPO catalyst with NaNO2/n-Bu4NBr for aerobic oxidation of alcohols in acetic acid.33 This catalytic system is not suitable for acid-sensitive alcohol substrates and allylic alcohols were not oxidized to α, βunsaturated alcohols. Later on, an excellent catalytic performance with high selectivity for allylic alcohols was achieved by utilizing NaNO2/TBN catalytic system at normal oxygen pressures.35,55a,214

Hermans et al.215a used commercially available silica-immobilized TEMPO catalyst for aerobic alcohol oxidations under continuous flow regimes using HNO3 as NOx source at 5 bar O2. Using the system, a renewable substrate, HMF, was also converted in high yields of either 2,5-diformylfuran (DFF) or 2,5-furandicarboxylic acid.215 Han et al.216 used the catalytic amount of bromide-bromate coupling, H2SO4, and NaNO2, together with TEMPO for oxidation in the presence of air. With this catalytic system, cinnamyl alcohol was easily converted in quantitative yield, whereas primary alcohols were less suitable for the catalytic system.


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Recently,

TEMPO/tetrachlorobenzoquinone/TBN/HCl

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catalytic

system

has

been

employed for metal-free and solvent-free aerobic oxidation of a broad range of alcohols under a mild condition with excellent yields.217 Furthermore, NH4NO3/TEMPO/H+ threecomponent catalytic system was proved to be efficient under aerobic conditions and gave moderate to quantitative yields for chemoselective oxidation of a comprehensive range of alcohols including those bearing oxidizable heteroatoms, alkyl, cycloalkyl and allyl-type substituted substrates (Scheme 46).218

OH Ph R

R1

O

Air, NH4NO3 (Cat.), TEMPO, HCl (Cat.) MeCN, 60 oC

Ph R

R1

Scheme 46. Three component catalytic system TEMPO/NH4NO3/H+ for aerobic oxidation. Reproduced from Ref. 218. Copyright 2014 Wiley.

Mechanistic studies on NOx as a co-oxidant along with TEMPO and in the presence of O2 as a terminal oxidant have been undertaken.219 As shown in Scheme 47, the general catalytic cycle starts with the oxidation of nitric oxide (NO) to form nitrogen dioxide (NO2), which can act as a bi-functional molecule by oxidizing TEMPO radical to primary


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oxidant oxoammonium salt and also helping to regenerate the primary oxidant from reduced form of TEMPOH. In the latter process, one molecule of water can be generated. Eventually, oxoammonium salt reacts with alcohols and directly convert them to their respective carbonyl compounds.219b

N O

1/2 O2

NO2

Primary oxidant

N X O

NO

R-CH2OH

Secondary oxidant NO2

H 2O

N OH

R-CHO

Scheme 47. A proposed mechanism using metal free NOᵪ/TEMPO aerobic oxidation. Reproduced from Ref. 211b. Copyright 2009 Wiley-VCH.

3.1.1.3 Oxidations with Miscellaneous Oxidants/TEMPO A number of other secondary oxidants has been employed for TEMPO-catalyzed alcohol oxidations and they are briefly summarized below.


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Hypervalent bromine oxidant has been used as a secondary oxidant, similar to above discussed hypervalent iodine (III) derivative. Particularly, a resin containing diacetoxy bromine (I) derivative was applied with TEMPO for complete conversion of enantiomerically

pure

primary

and

secondary

alcohols

by

preserving

their

enantiopurity.220

m-CPBA was utilized as a secondary oxidant for the nitroxide catalyzed oxidation reactions.221 m-CPBA can also oxidize 2,2,6,6-tetramethylpiperidine to TEMPO, thus the oxidation reaction can be carried out either by using piperidine hydrochloride (Scheme 48)222 or by TEMPO.223

N H2 Cl-, m-CPBA (excess) OH

CH2Cl2, rt, 1.5 h

+ O

N O

Scheme 48. mCPBA/TEMPO catalyzed oxidation of secondary alcohols. Reproduced from Ref. 222. Copyright 1975 American Chemical Society.


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Oxone (2KHSO5KHSO4K2SO4) is a commercially available inorganic salt, which was used as a secondary oxidant in the TEMPO-catalyzed oxidation of secondary alcohols (e.g., Scheme 49).224 Moreover, it has also been used in conjugation with silver carbonate-silica co-catalyst and TEMPO for the oxidation of carbohydrates.225

OH

O TEMPO (cat.), (n-Bu)4NBr (cat.), Oxone CH2Cl2 or toluene, rt

Scheme 49. Alcohol oxidation using oxone/TEMPO catalytic system. Reproduced from Ref. 224a. Copyright 2000 American Chemical Society.

Trichloroisocyanuric acid (TCCA) was employed for the conversion of alcohols to carbonyl compounds in combination with TEMPO and primary alcohols were oxidized selectively over secondary alcohols.226 Nonetheless, the fate of TCCA in the reactions is not stated.

Laccases are a class of enzymes; several reports have been presented with this class of enzymes as a secondary oxidant in combination with TEMPO for alcohol


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oxidation.227,228 Riva et al. used the laccase-TEMPO system for regioselective oxidation of primary alcohols of sugar derivatives.229 Detailed mechanistic studies of this reaction confirmed that oxoammonium ions were the primary oxidants and laccase oxidized TEMPOH to TEMPO in one-electron reaction.227a,230

Elemental iodine was utilized as a secondary oxidant in TEMPO catalyzed oxidations. Iodine molecule was used for the preparation of commercially important Losartan aldehydes from its alcohols in quantitative yields.231 The main reason to use iodine rather chlorine or bromine is that it prevents oxidative degradation of electron-rich and heteroaromatic rings in the substrate.

Periodic acid (H5IO6)232 and sodium metaperiodate (NaIO4)233 have also been utilized as the secondary oxidant. In the latter case, the reactions were slow (8 to 70 h.) and a possible problem in this reaction was that the periodate induced cleavage could occur with 1,2-diols.234

Iron salts have been employed as a secondary oxidant in combination with TEMPO. Particularly, potassium ferricyanide [K3Fe(CN)6] has been used in conjunction with


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monomeric and polymeric nitroxide systems. The monomeric BzO-TEMPO catalytic system was used for oxidation of alcohols in aqueous KOH and acetonitrile.122,235 Furthermore, TEMPO in combination with ferric chloride and sodium nitrite was used for oxidation of various alcohols.236

Oxidation of hydrogen peroxide (H2O2) catalyzed by AcNH-TEMPO was carried out in ionic liquid ([bmim]PF6). In this case, AcNH-TEMPO was more convenient to use than TEMPO itself.90a Kim and Jung used ceric ammonium nitrate as the oxidant with TEMPO for the oxygen-mediated oxidation of a number of primary and secondary alcohols.237

An efficient aerobic alcohol oxidation was induced with visible light (λ > 450 nm) by using dye-sensitized TiO2 and the reaction was conducted with commercially available Alizarin Red as a sensitizer and TEMPO as a co-catalyst in a suspension of TiO2 with trifluorotoluene solvent.238,239 Moreover, it has been found in a recent report79 that GO can also act as an oxidant in aerobic oxidation of 5-hydroxymethylfurfural to 2,5diformylfuran in the presence of TEMPO.


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3.1.1.4 Electrochemical Oxidations The electrochemical oxidation of alcohols is a major focus of energy and chemical conversion efforts. These oxidations have potential applications ranging from fuel cells to

biomass

utilization

and

fine-chemical

synthesis.143b,240

Small

molecular

electrocatalysts for this type of reactions are promising targets for further development.241 Organic nitroxyl, such as TEMPO radical, has been most widely studied as an electrocatalyst in alcohol oxidation due to its redox behavior.

In 1983, Semmelhack et al.242 used TEMPO as an electrocatalyst for the first time to synthesize a series of carbonyl compounds in lithium perchlorate (LiClO4) electrolyte in the presence of 2,6-lutidine. Later on, bromide was also utilized for electrochemical oxidations along with TEMPO derivatives in the biphasic medium.243 Moreover, several TEMPO electrocatalysts were coated on Pt electrode surface by copolymerization with 2,2′-bithiophene for electrochemical alcohol oxidation,244 while many of TEMPO-coated electrodes were prepared on graphite felt using 1,6-hexanediamine cross-linked polyacrylic acid.245


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In continuation of the investigation on catalytic electrodes, TEMPO was immobilized onto Nafion polymer and employed for carbohydrate oxidation in carbonate solution buffered at pH 10.246 Furthermore, a simple method was reported to form a submonolayer of TEMPO radicals chemically tethered to the surface of a graphite electrode through an amide link to the graphite felt electrode.247 In both cases, the catalyst was active and selective for the first electrocatalytic reaction run, but the density of TEMPO decreased rapidly due to catalyst degradation.

Recently, in a short span of time, a number of articles have been published on this emerging area of TEMPO catalyzed electrochemical reactions248 and many researchers have been focusing on electrochemical oxidations rather than chemical oxidations (usage of co-catalysts such as NaOCl, BAIB, O2, NOᵪ, etc.) due to its highly efficient waste-free system.249 In 2012, Brown et al.250 developed an electrocatalytic oxidation system for the conversion of primary and secondary alcohols to their respective carbonyl compounds in a microfluidic electrolytic cell. In this case, they utilized a


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buffered (pH 11.5) aqueous tert-butanol reaction medium along with TEMPO (30 mol%) with 20 mA cell current at ambient temperature.

Recently, Karimi et al.251 have reported that the pH of the buffered aqueous solution greatly influences the electrocatalytic performance of the catalysts towards alcohol oxidation. The same group252 has constructed TEMPO functionalized ordered mesoporous silica (MCM-41) as well-oriented channels on an electrode surface by using an electro-assisted self-assembly process.253 Such a TEMPO grafted mesoporous silica electrode showed superior activity than TEMPO grafted amorphous silica electrode. The electrode was employed successfully on 20 mmol scale in a single run for alcohol oxidation and was reused for several runs. Notably, the system achieved a TOF up to 3070 h-1, which is much superior to all the reported nitroxyl radicals under chemical, electrochemical or aerobic oxidation condition in terms of activity. Furthermore, TEMPO coated gold nanoparticles were also employed for more efficient conversion of benzyl alcohol.81b


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ACS Catalysis

1,3-Dihydroxy acetone is a main active ingredient in all sunless tanning skin care preparations and is currently produced from glycerol by microbial fermentation over

Gluconobacter oxydans.254 A clean one-pot direct conversion of glycerol to 1,3dihydroxy acetone was achieved by applying a small electric potential in the presence of 15 mol% of TEMPO (Scheme 50) and hydro pyruvic acid was obtained by increasing the reaction time.255 Recently, Sigman et al. have demonstrated the complete electrochemical oxidation of the biofuel glycerol to CO2 using both oxalate oxidase enzyme and 4-amino-TEMPO (3) molecular catalyst.256 In order to improve the performance of the electrocatalytic activity of TEMPO for oxidation reaction, they prepared covalently immobilized TEMPO onto linear poly(ethylenimine) (LPEI) and the TEMPO-LPEI moiety was cross-linked onto the surface of a glassy carbon electrode to form a hydrogel electrode.257 This TEMPO-LPEI modified electrode was used as an anode capable of generating currents under neutral pH at 25 C. In addition, they constructed a biofuel cell by combining an enzymatic biocathode along with the TEMPO-LPEI anode and produced less current (0.38 ± 0.04 mA cm-2) in 2 M methanol.


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Afterward,

they

have

examined

the

Page 128 of 227

structure-function

relationships

on

their

electrocatalytic activity of a diverse range of water-soluble nitroxyl radical derivatives.258

OH HO

OH

E (1.1 vs Ag/AgCl) TEMPO, pH 9.1

O HO

O OH

1,3-dihydroxy-acetone

HO

OH

O hydroxypyruvic acid

Scheme 50. Electrochemical oxidation of glycerol to hydro pyruvic acid. Reproduced from Ref. 255. Copyright 2006 Elsevier.

Stahl et al.240c have reported that the catalytic activity of TEMPO derivatives in electrochemical oxidations was much more depending on its nitroxyl/oxoammonium redox potential rather than the steric effects of the electrocatalyst. In this case, they examined the activity of 4-acetamido-TEMPO (7) by comparing with different bicyclic nitroxyl derivatives, such as 9-azabicyclo[3.3.1]nonane N-oxyl (ABNO) and 2azaadamentane-N-oxyl (AZADO), for the oxidation of alcohols and observed that it exhibits higher electrocatalytic activity than the others. Moreover, the mid-point potential (Εmp) of the one-electron nitroxyl/oxoammonium couple varied and was dependent on the nitroxyl structure (i.e., mono- vs bicyclic) and/or substituents. Most recently, this


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ACS Catalysis

group reported that the (bpy)Cu/TEMPO catalytic system proceeded with much faster rates for electrochemical oxidation with lower electrode potential compared with metalfree TEMPO.241a

3.1.2 Oxidation of Sulfides

Selective oxidation of sulfides into the corresponding sulfoxides is one of the most important transformations in organic synthesis259 and this transformation can be mediated by a catalytic amount of TEMPO.

In 1994, Skarżewski260 synthesized mono- or disulfoxides from sulfides, mediated by TEMPO, potassium bromide, and phase transfer catalyst, along with sodium hypochlorite in the biphasic medium. Notably, over-oxidation of sulfides to corresponding sulfones was observed under these conditions. Moreover, N-protected βamino sulfides were readily converted into the diastereoisomeric sulfoxides (R,Ss) and (R, Rs) using the same biphasic conditions (Scheme 51).261

R

NHXR S (R)

PhSSPh, Bu3P, THF Ph

76 oC, 72 h

NHXR S R Ph O (R, Ss)

NHXR S R Ph O (R, Rs)


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Scheme 51. Oxidation of sulfides to sulfoxides. Reproduced from Ref. 261. Copyright 1996 Thieme Publishing Group.

Transition Cu-metal complex with salen was employed for sulfide oxidation, in combination with H2O2 at an ambient temperature. It was observed that addition of 5 mol% of TEMPO enhanced the sulfoxide selectivity and yield. Moreover, TEMPO linked Mn or Fe metalloporphyrin catalysts were utilized for efficient oxidation of sulfides to sulfoxides. Among Fe and Mn complexes, the latter showed efficient selectivity using NaOCl as the oxidant in biphasic conditions.262

Reiser and Chinnusamy263 extended the Minisci oxidation conditions from alcohol oxidations to aerobic sulfide oxidations and employed readily prepared heterogeneous fluorous tagged TEMPO catalyst (38 in Scheme 16) for the conversion of a wide range of sulfide derivatives in good to quantitative yields.

Very recently, selective oxidation of organic sulfides has been achieved by carrying out visible light induced photo redox catalysis of dye-sensitized TiO2 in conjugation with TEMPO as a redox mediator.264 The substrate scope of this system showed good to


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quantitative yields for various thioanisole derivatives in different reaction times. Electronic effects of the substrates were directly affected by the rate of the reaction (Scheme 52). Thioanisoles with electron donating groups typically require shorter reaction times, whereas those with electron withdrawing groups require longer reaction times than thioanisole.264

S R

O2

CH3OH

ARS-TiO2, TEMPO 450 nm

O S R

HCHO

H 2O

Scheme 52. Visible light induced sulfide oxidation with TEMPO. Reproduced from Ref. 264. Copyright 2016 Wiley.

3.1.3

Ketone Formation

In 2005, Belgsir et al.265 utilized TEMPO for the conversion of activated alkenes and dienes into the corresponding alkenones in excellent yields under aqueous acetonitrile conditions. Significantly, oxoammonium cations were regenerated electrochemically from the radical parent TEMPO at a vitreous carbon anode in the presence of water and 2,6-lutidine. Subsequently, Mitsudo et al.266 developed an electrochemical method for


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Page 132 of 227

generating cationic palladium complexes [Pd(CH3CN)4][X]2 (X = BF4, PF6, and ClO4) and then used them for the in situ generation of the reagent for accomplishing the electro oxidative Wacker-type reaction in the presence of TEMPO (30 mol%). In this case, the presence of TEMPO is necessary for the effective oxidation of Pd(0) to Pd2+ in the course of formation of ketone derivatives from various alkene derivatives (Scheme 53), while only a trace amount of product formation was observed without TEMPO.

n-C10H21

Pd(OAc)2 (10 mol%) TEMPO (30 mol%) Et4NOTs (0.05 M)

O n-C10H21

CH3CN/H2O (7/1) room temp. divided cell, (Pt)-(Pt) 5 mA, 3 F/mol

Scheme 53. Ketone formation from activated alkenes using TEMPO. Reproduced from Ref. 266. Copyright 2007 American Chemical Society.

β,β′-Difunctionalized enones are essential and versatile in organic synthesis.267 Vatèle268a developed two mild and environmentally friendly methods for the oxidative rearrangement of tertiary allylic alcohols to their corresponding transposed βdisubstituted enone derivatives in excellent to fair yields by reaction with TEMPO in


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ACS Catalysis

combination with PhIO and Bi(OTf)3 (Scheme 54). Significantly, unsaturated aldehydes were more readily obtained with a better yield from tertiary vinyl carbinols with the TEMPO/PhIO oxidizing process.268

R2 H R1 R

OH 4 3R

TEMPO cat. Method A or B

R2 O

R4

R1 R3

Method A: PhIO, B(OTf)3 cat. or Re2O7 cat., CH2Cl2 Method B: CuCl2 cat., O2, 4A MS, CH3CN

Scheme 54. Oxidative rearrangement of allylic alcohols with TEMPO/PhIO/Bi(OTf)3 system. Reproduced from Ref. 268a. Copyright 2010 Elsevier.

3.1.4 Ester and Lactone Formations Oxidative esterification has been reported as a convenient pathway to both symmetric esters as well as asymmetric esters.269 TEMPO has found use in this ester-forming methodology due to generally mild conditions employed.

In 2004, Bobbitt et al.270 described a dimeric esterification of polyfunctional primary alcohols catalyzed by 4-acetylamino-TEMPO in the presence of pyridine. The ester was


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the predominant product of the reaction, even with the use of alcohols containing βoxygen. In the absence of base, little amounts of product were formed.

Szpilman et al. synthesized symmetric esters through a direct oxidative dimerization of alcohols with TCCA/TEMPO catalytic system.271 Later on, the same group has discovered that the aldehydes were activated for oxidation by a carboxylic acid. This novel concept was developed into an efficient process for the TEMPO catalyzed oxidation of aldehydes to mixed anhydrides with pivalic acid (Scheme 55).272 These mixed anhydrides can be converted in situ into a wealth of esters and amides.

O

O R

H

+

Me HO Me Me pivallic acid

N O Pyridine, MeCN t-BuOCl

O R

O O

Me Me Me

Scheme 55. TEMPO catalyzed oxidation of aldehydes to mixed anhydrides. Reproduced from Ref. 272. Copyright 2013 Royal Society of Chemistry.


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ACS Catalysis

Leadbeater et al.273 have reported a facile and high yielding methodology for the oxidative esterification of various aldehydes in the presence of hexafluoroisopropanol (HFIP)

to

form

correspondent

asymmetric

HFIP

esters.

Recently,

a

TEMPO/CaCl2/oxone catalytic system has been utilized for the effective oxidative symmetrical esterification of primary alcohols under hydrous biphasic conditions.224b

An oxidative lactonization of α,ω-diols has been achieved by TEMPO/BAIB catalytic system in order to synthesize medium-sized lactones (7 or 8 membered).199b,274 Sheldon et al.227b demonstrated the use of Trametes versicolor laccase/TEMPO catalytic system for the efficient oxidative esterification of aliphatic diols (1,4- or 1,5diols) to esters by maintaining their regioselectivity and/or enantiomeric excess (ee) in the case of enantioenriched diols. Moreover, a successful recyclability of the system allowed to prepare laccase-cross-linked enzyme aggregates. The scalability of this process has also been demonstrated by using inexpensive commercially available laccase.227b


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Most recently, Stahl and Xie275 have identified that both Cu/TEMPO and Cu/ABNO catalytic systems can work efficiently for the selective aerobic oxidative lactonization of 1,4-, 1,5- and some 1,6-diols. In addition, for the first time, 3-alkoxyamine lactam derivatives have been synthesized selectively by dual sp3 C-H functionalization at the alpha- and beta- positions of cyclic amines using NaClO2/TEMPO/NaClO catalytic system (Scheme 56).276

NaClO2/TEMPO/ NaOCl CH3CN n N R 0 oC to room temperature P

YO n O

N R P

Y=

N

Scheme 56. SP3 C-H functionalization of cyclic amine using NaClO2/TEMPO/NaClO. Reproduced from Ref. 276. Copyright 2016 American Chemical Society.

3.1.5

Passarini Reaction

In the past few years, an increasing number of multicomponent reactions (MCRs) has been developed for the synthesis of diverse complex molecules through a combination of three or more starting materials in a one-pot operation, which can reduce waste and increase safety.277 Among MCRs, Passarini three-component reaction (P-3CR)278 has


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ACS Catalysis

been utilized for the synthesis of α-acyloxy carboxamide derivatives by mixing three components such as alcohols, isocyanides and carboxylic acids. Recently, the first example of this reaction has been documented in the presence of a catalytic amount of cupric chloride, NaNO2, and TEMPO under an oxygen atmosphere to render a series of α-acyloxy carboxamide derivatives in good to excellent yields (Scheme 57).279

R1CH2OH

R2NC

R3COH

CuCl2 (15 mol%) TEMPO (15 mol%) NaNO2 (15 mol%)

R3 O

O2 balloon, toluene, rt

R1

O NHR2 O

Scheme 57. Passarini three-component reaction with CuCl2/NaNO2/TEMPO system. Reproduced from Ref. 279a. Copyright 2010 American Chemical Society.

Further, a three-component coupling reaction among piperidine, phenylacetylene, and benzaldehyde has been performed, yielding a propargylic amine in quantitative conversions using gold (III) chloride in catalytic amounts (Scheme 58).279,280 The lifetime and recyclability (33 cycles) of AuCl3 was increased by 3,300% with the addition of several equivalents of TEMPO and catalytic amounts of copper(II) chloride.


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O AuCl3 N H

N Au Cl Cl

N

N2, H2O, 40 - 70 oC

N O TEMPO

Scheme 58. Three-component coupling reaction of benzaldehyde, piperidine, and phenylacetylene. Reproduced from Ref. 280. Copyright 2011 Wiley.

3.1.6 Coupling Reactions

In last few years, TEMPO has been involved in various homo and cross-coupling reactions using metal as well as metal-free reaction conditions.

Mitsudo et al.281 described a facile electro oxidative method for synthesizing homocoupled biaryls from aryl boronic acids or aryl boronic esters in the presence of a catalytic amount of Pd(OAc)2/TEMPO. Moreover, the Rh-catalyzed coupling reaction of arenes and heteroarenes with aryl boronic acids via direct C-H arylation was achieved in the presence of TEMPO oxidant.282


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In 2008, Studer et al.283 described a transition-metal-free homocoupling reaction of various aryl, alkenyl and alkynyl combined with magnesium compounds in the presence of TEMPO catalyst and O2 as terminal oxidant. In this case, they found that, even in the absence of TEMPO, alkynyl magnesium compounds underwent homocoupling to provide corresponding dynes in the presence of O2 at higher temperatures. Later on, they performed Sonogashira-type cross-coupling reaction between a various number of

ortho-substituted aryl derivatives and alkenyl Grignard reagents under metal-free conditions with TEMPO.284 Subsequently, they have described a highly efficient one-pot protocol for the cross-coupling reaction between various aliphatic/aromatic nitrones and alkynyl-Grignard reagents using TEMPO under aerobic conditions (Scheme 59).285

O

N

t-Bu

R1 H

+

R2

2 equiv H2O then 2.2 equiv. TEMPO MgCl THF, r.t.

O

N

R1

t-Bu R2

Scheme 59. Coupling reaction between nitrones and alkynyl-Grignard reagents using TEMPO. Reproduced from Ref. 285. Copyright 2011 Wiley.

3.1.7 CC Bond Formation


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The CC bond formation is a pivotal reaction for laboratory research and industrial processes. TEMPO has been utilized as a molecular catalyst for this fundamental transformation.286 Recently, the CC bond formation by cross-coupling of 9,10dihydrouridine with activated nucleophiles was described by Jiao et al..287 In this report, a wide range of 9,10-dihydrouridine derivatives has been synthesized from the coupling of two C(sp3)H bonds under aerobic conditions.

Mancheno and Richter288 contributed to the development of a TEMPO oxoammonium salt mediated dehydrogenative coupling of C(sp3)H bond of N-alkyl anilines with a variety of olefins to synthesize substituted quinoline derivatives. Furthermore, the same group has constructed the CC bond between cyclic benzyl ethers and a variety of carbonyl compounds (Scheme 60).

H R

O

1

R2 R1 R2

O O

T+BF4- / M cat

H R1, R2 = H, alkyl, aryl

H

R2 R1

R

O H N O

O R

O

T+BF4- / M cat R2

O R1 H

R1, R2 = H, alkyl, aryl BF4

+

T BF4-


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Scheme 60. CC bond formation between cyclic benzyl ethers and carbonyl compounds using TEMPO. Reproduced from Ref. 288. Copyright 2010 Wiley.

Liu et al. successfully developed a novel and simple synthetic method for the trimerization of indoles by utilizing TEMPO under aerobic conditions (Scheme 61).289 The use of the Cu/TEMPO system increased the rate of the reaction and also reduced the usage amount of TEMPO.290 Moreover, oxidative trifluoromethylation of indoles has been developed at room temperature by Pd/TEMPO catalyzed system with BAIB.291

O

TEMPO, benzoic acid N H

NH NH

N H

HN

NH NH HOTs

Scheme 61. Trimerization of indoles using TEMPO under aerobic condition. Reproduced from Ref. 289b. Copyright 2012 Royal Society of Chemistry.

Sekar et al. developed an efficient chiral copper catalyzed system [(R)-BINAM-CuClTEMPO] for an asymmetric oxidative coupling of 2-naphthol derivatives to synthesize enantiomerically pure BINOL derivatives.292 Surprisingly, the rate of the reaction was


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drastically increased by adding TEMPO to the (R)-BINAM-CuCl complex and enantiomeric excess (ee) was maintained up to 97% at room temperature.

Jia et al.293 synthesized a series of quinolines derivatives using a domino Csp3-H functionalization of glycine amides and peptide derivatives under radical cation saltinduced

aerobic

conditions.

They

employed

commercially

available

tris(4-

bromophenyl)aminium hexachloroantimonate (TBPA+) radical cation salt and used InCl3 as a co-catalyst to enhance the rate of the reaction. Recently, they reported a novel double Friedel-Crafts alkylation reaction of glycine derivatives in combination with TBAP+ SbCl6- salt under aerobic conditions.294 Furthermore, TEMPO has been utilized as an initiator for polyarene synthesis by alternating radical/anionic chain growth SRN1type polymerization process, which does not require any transition metals.285,295

Metal-free carbonylated benzofurans were synthesized from 5-exo-dig cyclization of phenol-linked 1,6-enynes in the presence of O2, TEMPO, and t-BuONO through a radical process. Notably, this transformation incorporated two oxygen atoms from O2 and TEMPO into benzofuran system. Very recently, Knowles et al. described a novel


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photocatalytic process to produce carbocation intermediates via a mesolytic cleavage of alkoxyamine radical cations under mild and Bronsted-neutral catalytic conditions.296 A variety of cyclic and acyclic benzyl carbocation intermediates was trapped by a diverse range of silylenol ether nucleophiles with moderate to good results. This process would be amenable to the traditional carbocation generation processes.

3.1.8 CN Bond Formations

(1)

CN Bond Formations

In last few years, a number of interesting articles have been published on CN bond forming

reactions

catalyzed

by

TEMPO.

Amongst,

the

catalytic

asymmetric

aminooxygenation of olefins is a very important process due to the significance of the products as building blocks in the synthesis of drugs and natural products.297 Chemler et

al.298

have

done

pioneering

works

on

enantioselective

intramolecular

aminooxygenation of alkenes to synthesize various chiral isoxazolidine, indolines and pyrrolidines derivatives using Cu salt and TEMPO as oxidant and oxygen source.


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Subsequently, this methodology was also implemented for the multigram production of chiral substituted Indoline.299

Cook et al. utilized Cu/TEMPO catalytic system for the synthesis of N-heterocycles such as indoles and quinoline derivatives by aerobic oxidation as a key synthetic step.300 Furthermore, an effective method of constructing the indoline moiety via intramolecular nucleophilic ring closure of a diaryliodonium salt was described (Scheme 62).301

H N

R1

I+ -OTs

Cs2CO3, TEMPO (0.1 equiv) DMF, 80 oC, 1 h

N R1

R2

Scheme 62. Construction of indoline derivatives via intramolecular ring closure of diaryliodonium salts. Reproduced from Ref. 301. Copyright 2012 American Chemical Society.

In addition, the CN bond formation was observed from Cu-catalyzed intermolecular amination between electron-deficient polyfluoroarenes acidic CH bond with an array of primary aromatic amines using molecular oxygen in combination with TEMPO.302 The


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ACS Catalysis

function of TEMPO in this reaction was unclear though its presence improved the product yield. Subsequently, a highly efficient direct amination of non-activated benzoxazoles and 1,3,4-oxadiazoles with secondary amines has been developed under metal-free conditions using catalytic amounts of triflic acid and a readily recyclable TEMPO+BF4- salt (Scheme 63).303

O N

H +

H N

R1 R

A: TfOH (10 mol%) or B: Sc(OTf)3 (2 mol%) MeCN, 60 oC, 6 h

2

O N

N

R1 R2

Scheme 63. CN bond formation between benzoxazoles and secondary amines. Reproduced from Ref. 303. Copyright 2011 Wiley.

Recently, Chen et al. developed a tandem process for the oxidative amidation of benzyl alcohol with amine hydrochloride salts using inexpensive Fe(NO3)39H20-TEMPO as catalyst in the presence of air and aqueous t-butyl hydroperoxide (TBHP) as oxidants.304 In this case, both secondary and tertiary benzamides were prepared under


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mild conditions without using any noxious reagents. Moreover, in the last amination step it was believed that the oxidation of the hemiaminal was performed by Fe(III) and TBHP.

Maiti et al. developed few methods for stereoselective nitration of olefins. At first, they carried out the nitration of a wide range of olefins to selective E nitroolefins using TEMPO and AgNO3 catalytic system.305 Later, they employed Fe(NO3)3 as an economic alternative to replace AgNO3 for stereoselective olefin nitration.306 Subsequently, the group performed the same transformation using a metal-free protocol with TBN and obtained good to excellent yield as well as efficient E selectivity.307 Recently, they also used TBN as a -NO2 precursor for decarboxylative nitration of olefins along with TEMPO (Scheme 64).308

Ar

COOH

t-BuONO 0.2 mmol TEMPO 1 mL CH3CN, air, 50 oC

NO2 Ar E-only isolated

Scheme 64. Decarboxylative nitration of olefins with TEMPO/t-BuONO. Reproduced from Ref. 308b. Copyright 2013 Royal Society of Chemistry.


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(2)

ACS Catalysis

C=N (Imine) Bond

Imines are an important class of nitrogen compounds due to their high reactivity. They have been used as a nitrogen source in different types of transformations that can be further utilized in synthetic, biological, pharmaceutical, and industrial applications.309

Xu et al.310 developed a low-loading Pd-catalyzed aerobic oxidative tandem reaction of alcohols and amines along with TEMPO for the synthesis of a series of imine derivatives in moderate to quantitative yields. Subsequently, they replaced Pd with Cu and employed the Cu/TEMPO catalytic system for synthesis of a series of various imine derivatives in excellent yields.311 In addition, the same group synthesized imines directly from amines without using any alcohols via an efficient Cu/TEMPO catalyzed aerobic amine oxidation method under ambient and neat conditions.312 Later on, they reported that an easily abundant and cheaply available Fe-mediated Fe(NO3)3/TEMPO catalytic system efficiently facilitated imine synthesis through the aerobic oxidation reactions of primary, secondary and benzylamines with anilines and also alcohols with amines.313 However, aliphatic amine substrates were not suitable for the oxidative imine synthesis


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with this catalyst. Wertz and Studer reported a high yielding TEMPO-mediated oxidation of various hydroxylamines and alkoxyamines to the corresponding oximes and oxime ethers (Scheme 65).314 In addition, they have synthesized alkoxyamines directly from benzyl bromides using excess TEMPO (2.2 equivalents).

R1 R

H N 2

X OR3

N O 2.2 equiv. X=H, AcNH BTF, 80 oC

R1 N R

2

OR3

Scheme 65. Synthesis of oximes from alkoxyamines using TEMPO. Reproduced from Ref. 314. Copyright 2012 Wiley.

Han et al. reported the synthesis of various heterocyclic moieties by the TEMPOmediated catalytic process.315-317 In 2008, they synthesized five-membered heterocyclic rings such as benzoxazoles, benzothiazoles, and benzimidazoles by aerobic oxidation of aromatic aldehydes with respective amine derivatives using 4-methoxy-TEMPO as a catalyst (Scheme 66).315 Later, they synthesized six-membered heterocycles such as quinazolines via benzyl C-H bond amination by a one-pot reaction of arylmethanamines


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ACS Catalysis

with 4-hydroxy-TEMPO radical as the catalyst.316 Furthermore, they reported the aerobic oxidative synthesis of 2-substituted quinazolines and 4H-3,1-benzoxazines using a CuCl/1,4-diazabicyclo[2.2.2]octane/4-HO-TEMPO catalytic system and a range of substituted heterocycles was prepared in good to excellent yields.317 Moreover, Chen et al. have developed a three-component cascade reaction for the synthesis of 2substituted quinazolines in moderate to excellent yields using CuCl/bpy/TEMPO catalytic system.318

OH R1

NH2

+ OHC-R2

O

4-Methoxy-TEMPO (5 mol%) o

O2 , Xylene, 120 C

R1

N

R2

Scheme 66. C=N bond formation from aromatic aldehydes and respective amines. Reproduced from Ref. 315. Copyright 2008 Wiley.

In 2012, Cook et al. described the utilization of alcohols as a substrate, which is an attractive route to synthesis of N-heterocyclic moieties because of its easy availability and handling. Therein, they prepared a series of substituted indoles and quinolines using Cu/TEMPO catalytic system (Scheme 67).300 Recently, Mancheño et al. have


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employed TEMPO as a stoichiometric oxidant for the synthesis of oxazinones,319 substituted quinolines and dihydroquinazolines.313a,320,321

OH R NH2 R = OMe, F, Cl

Cu(OTf)2, 2-2'Bipy, TEMPO, DBU, NMI o

3A MS, MeCN, 60 C Flask open to air

R

N H

Scheme 67. Synthesis of indoles using Cu/TEMPO catalytic system. Reproduced from Ref. 300. Copyright 2012 Royal Society of Chemistry.

(3)

C≡N (Nitrile) Bond

Nitriles are vital synthetic intermediates for pharmaceuticals, materials, agricultural, dyes and fine chemicals.322 Recently, in a short span of time, numerous TEMPO mediated methods have been developed for the synthesis of aryl nitriles from easily available alcohols and ammonia.

Tao et al.323 reported a new and efficient method for the synthesis of aryl nitriles directly from the corresponding benzylic alcohols, which were easily converted into nitriles by


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ACS Catalysis

using aqueous ammonia as a nitrogen source and Cu(NO3)2 as catalyst under aerobic condition (Scheme 68).

Ar-CH2OH

Cu(NO3)2: 5 mol%, TEMPO: 5 mol% NH3 (aq.): 3eq., O2 (1 atm), 80 oC

Ar-CN

Scheme 68. Synthesis of aryl nitriles from benzylic alcohols. Reproduced from Ref. 323. Copyright 2013 Royal Society of Chemistry.

Furthermore, one-pot synthesis of primary aryl amides could also be achieved under these conditions. Later on, the synthesis of a series of aromatic and aliphatic nitriles was achieved via a dehydrogenation cascade mediated procedure catalyzed by CuI, bpy and TEMPO in the presence of O2.324 This protocol enables the one-pot synthesis of various pharmaceutically attractive heterocycles.

Cook et al.325a have avoided the use of pure O2 atmosphere and developed a method to use air in open flask or dilute oxygen (8%) mixtures. For the conversion to nitrile, activated aldehydes or alcohols were catalyzed by Cu(OTf)2, bpy and TEMPO in acetonitrile-water as a solvent (Scheme 69). It has been found that, by adding NaOH


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Page 152 of 227

base, the rate of the reaction was increased. Nonetheless, aliphatic alcohols were slow to react with this method. Furthermore, unprotected and protonated amines reacted rapidly with oxoammonium salt TEMPO. In some cases, the reaction involved an oxidation of amines to yield imines, with these imines either hydrolyzed to aldehydes or further oxidized to nitriles.325b,c

R R

or

O

Cu/TEMPO Cat

OH

NH3(aq), air

N R

Scheme 69. Synthesis of nitriles using activated alcohols or aldehydes. Reproduced from Ref. 325a. Copyright 2013 Royal Society of Chemistry.

3.1.9

CO Bond (N-Alkoxyamines)

N-Alkoxyamines, derived from persistent sterically hindered aminoxyl radicals, represent an important and rapidly growing class of organic compounds in natural product synthesis and pharmaceutical agents.326 These alkoxyamines can also be used for controlled radical polymerization,327 as polymer light stabilizers, peroxide substitutes


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ACS Catalysis

(rheology modifiers) or fireproofing agents.328 However, there is ongoing interest in the development of simple, efficient methods for synthesizing N-alkoxyamines.329

Bobbitt and Bailey demonstrated a highly selective synthetic route to synthesize secondary allylic alkoxyamines H in high yields by an ene-like reaction between an oxoammonium tetrafluoroborate salt and tri-substituted alkenes (Scheme 70).330

NHAc

R1 R2

N O BF4

CH3CN rt

O N H

AcHN

R2

Na2CO3

R3

BF4

R3

R1

H

H 2O

N O

AcHN BF4

R1

R2 R3

80 - 95% yield

Scheme 70. Synthetic route to prepare allylic N-alkoxyamines from oxoammonium salt. Reproduced from Ref. 330. Copyright 2006 American Chemical Society.

The sterically hindered N-alkoxy amines were synthesized in good to excellent yields by coupling TEMPO with hydrocarbyl radicals, which were generated in situ by t-BuOOH hydrogen abstraction from hydrocarbons.331 Most Recently, a bpy(Cu)/TBHP catalytic system has been developed for synthesis of a broad range of N-alkoxyamine derivatives in excellent yields.332


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NHC can be applied in synthesis, as an organic electron transfer reagent to activate various organic moieties.333 Studer et al. deployed NHCs catalyzed synthetic pathway to activate aldehydes and to synthesize CO bonded TEMPO ester derivatives.334 Later on, the same group prepared various N-alkoxyamine derivatives in high yields by reaction between B-alkylcatecholbaranes with TEMPO.335 Moreover, they introduced a Pd-catalyzed highly stereoselective carboaminoxylation of indoles or indenes with aryl boronic acids and TEMPO (Scheme 71).336 Recently, the same group reported the high yielding α-aminoxylation of various ketones or enones with chlorocatecholborane along with TEMPO.337 The substrate scope was broad and the products were obtained in good to excellent yields.

N R

Ar

ArB(OH)2 Pd(OAc)2

ArB(OH)2 Pd(OAc)2 TEMPO

N R

TEMPO

R = H, Me

N

O

Ar N R R = Protecting Group

Scheme 71. Pd-catalyzed carboaminoxylation of indoles with TEMPO and aryl boronic acids. Reproduced from Ref. 336a. Copyright 2009 Wiley.


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ACS Catalysis

Maruoka et al. developed a metal-free direct aminoxylation reaction of aldehydes with an oxoammonium salt catalyzed by a novel binaphthyl-based amine (S).338 This method represents a rare example of the catalytic and highly enantioselective synthesis of bench-stable α-aminoxy aldehydes.339

Crich et al. showed that S-sialosyl xanthate was photolyzed in dichloroethane at room temperature in the presence of TEMPO, which led to TEMPO glycoside as a separable 1:2 mixture of α- and β-anomers.340,341

Jang et al.342 presented a Cu(OAc)2-catalyzed reaction of propargyl alcohols and TEMPO to afford a number of TEMPO incorporated α, β-unsaturated carbonyl compounds (vinylic alkoxyamines) (Scheme 72a). In this system, regardless of the nature of alkyne substitutes (aromatic and aliphatic groups) and the alcohols type (1o and 2o), the desired vinylic alkoxyamines were obtained in good yields. Furthermore, a highly efficient and straightforward aminoxylation of titanium(IV) enolates has been developed from oxazolidine-2-one derivatives with TEMPO (Scheme 72b). In this case, a wide array of functional groups on the acyl moiety, including alkyl and aryl


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Page 156 of 227

substituents, olefins, esters or α-cyclopropyl as well as α-trifluoromethyl groups, is well tolerated.343

a)

Ph

O b)

O

Cu(OAc)2 (5 mol%) TEMPO (2 equiv)

O N Bn

R

O

TiCl4, iPr2NEt CH2Cl2, 0 oC

Ph N

Air, toluene, 50 oC

O O

Cl4 Ti O N R

O

O

O TEMPO

CH2Cl2, 0 oC, 1 h Bn iPr2NHEt

O

O N Bn O

R N

Scheme 72. (a) Synthesis of vinylic alkoxyamines (reproduced from Ref. 342; Copyright 2014 American Chemical Society) and (b) aminoxylation of oxazolidine-2-one derivatives with TEMPO (reproduced from Ref. 343b; Copyright 2014 Wiley).

3.1.10 CO Bond (N-Alkoxyamine) Homolysis The CO bond in N-alkoxyamines are generally weak and efficient homolysis can happen at lower temperatures (>90oC).344 Notably, CO bond homolysis of alkoxyamines is a reversible process and, during course of the reaction, reactive radicals are generated continuously because of the persistent radical effect (PRE).345


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ACS Catalysis

PRE is a general principle that explains the highly specific formation of the crosscoupling product (R1R2) between two radicals R1 and R2. PRE has already been used in various chemical systems and also in NMP.346

If thermal CO bond homolysis of N-alkoxyamines is conducted in the presence of olefin derivatives, a thermal radical carboaminoxylation reaction (alkoxyamine isomerization reaction) can occur.4a,347 Studer et al. conducted the pioneering work on this kind of reactions by conducteding long time (22 h) heating of N-alkoxyamine derivatives in the presence of camphor sulphonic acid at 130 C to yield a mixture of four alkoxyamine isomers.348

O EtO P EtO N

O N O

R1

MW

R2 2 min

O EtO P EtO

O N R

HWE R3CHO

1

R2

MW, 6 min

R3

O 1 N R

R2

Scheme 73. Synthesis of α, β-unsaturated oxindoles from alkoxyamines. Reproduced from Ref. 349. Copyright 2004 American Chemical Society.


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Moreover,

one-pot

PRE-mediated

intramolecular

Page 158 of 227

homolytic

substitutions

were

conducted on readily prepared alkoxyamine derivatives to synthesize a small library of α, β-unsaturated oxindoles (Scheme 73).349 In this first process, oxindole derivatives were prepared upon simple heating of alkoxyamine. Later, the synthesis was improved by ionic Horner-Wadsworth-Emmons type olefination reaction under microwave (MW) induced heating to generate α, β-unsaturated oxindoles. Interestingly, using MW irradiation, a 430-fold of acceleration was achieved upon switching from classical heating to MW-induced heating.350 Later on, steric effects in the nitroxide moiety on the outcome of the PRE-mediated radical alkoxyamine isomerization and intermolecular addition reaction were demonstrated.351

Gigmes et al.352 developed a method to synthesize highly valuable alkoxyamines, such as bicyclic, spiro and eight-membered, via CO bond homolysis as well as radical cyclization. Like other reactions, this conversion was also triggered by thermal initiation (Scheme 74). Besides olefins, carbon monoxide353 and isonitrile354 were also shown to be suitable for the PRE-mediated thermal carboaminoxylation reaction. In the case of


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ACS Catalysis

reaction with isonitrile, the carboaminoxylation comprises of an initial CO bond homolysis of N-alkoxyamine, isonitrile trapping followed by cyclization, and, eventually, a homolytic aromatic substitution, leading to substituted dihydroquinoline. Furthermore, for the first-time, olefin-terminated self-assembled monolayers were functionalized using this intermolecular carboxy amination reaction to form CC bond through CO bond homolysis (Scheme 75). With the use of this novel method, various functional groups can be successfully attached to Si wafers.355

 

O



H

O

* **

SG1

H

O

O

*

70%

* SG1

O

O 



SG1

O O

t-BuOH

*

* O * * SG1

N2, 110 oC, 12 h

60%

O

* SG1 

O

O

*

70% *

SG1*

SG1-Alkoxyamine

Scheme 74. Preparation of bicyclo, spiro, and eight-membered alkoxyamines via CO bond homolysis. Reproduced from Ref. 352a. Copyright 2005 Elsevier.


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R2 R1 O N

O3Si

n

O3Si

n

R1

R1 X

OH

O3Si X= O N

R2

R2

n

X O3Si

n

OH

Scheme 75. Functionalization of self-assembled monolayers by intermolecular carboxy amination. Reproduced from Ref. 355. Copyright 2007 American Chemical Society.

3.1.11 H Abstraction Sterically hindered TEMPO radical is generally considered to be both kinetically and thermodynamically stable and it does not typically undergo hydrogen atom abstraction reaction with hydrocarbon substrates. But, there are some early reports on thermally initiated hydrogen-atom-abstraction reactions by persistent TEMPO in the presence of excess substrate as a solvent.356 In 2004, Pastor et al.357 reported a CH abstraction reaction between cyclohexene and 2 molar equivalents of TEMPO and proposed a mechanistic low-energy pathway by experimental and computational evidence. As shown in Scheme 76, the reaction pathway involves the abstraction of α-hydrogen by


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ACS Catalysis

TEMPO radical from cyclohexene to generate stable cyclohexene radical I, which further reacts with another molecule of TEMPO to generate N-alkoxy derivative J. Coseri and Ingold358 distinguished between abstraction and addition as the first step in the reaction, revealing that the first step proceeds through initial hydrogen abstraction (~80%) and to a lesser extent (~20%) by an initial addition to the double band. As already discussed above (Section (3) in 3.1.1.2), α-H atoms in CuII alcoholates can be abstracted by TEMPO to give the corresponding carbonyl compounds (see Scheme 43).187a,201a

OH 70 oC N O

I OH

OH

H

O

N O

N OH N OH

OH N O

O N

J

OH


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Page 162 of 227

Scheme 76. Proposed reaction pathway for the H-abstraction by TEMPO from cyclohexene. Reproduced from Ref. 357. Copyright 2002 American Chemical Society.

Scaiano et al.359 synthesized several prefluorescent TEMPO probes, which were used to study the H-transfer reactions of various phenols and polyphenols (Scheme 77). The H-transfer to TEMPO radical could be extremely fast and the resulting TEMPO-H derivative showed strong fluorescence intensity.

OH N

O N O

O

O OH Ar-OH N

O OH

Slow

Ar-O N

CH3

Non-fluorescent

CH3

Fluorescent

Scheme 77. H-abstraction reaction using TEMPO derivative. Reproduced from Ref. 359a. Copyright 2003 American Chemical Society.


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ACS Catalysis

Lucarini et al.360 reported that TEMPO can abstract the H-atom from group 14 hydrides such as Bu3SnH and Ph3GeH to render TEMPOH and the corresponding stannyl or germyl radicals at 80 C. Eventually, these radicals ended up with dimerized Bu3SnSnBu3 or Ph3GeGePh3 products. Moreover, TEMPO can also abstract the H-atom from aliphatic solvents in a photochemical pathway to afford hydroxylamine and aminoethers.356c

TEMPO can abstract the proton from the terminal dangling bonds on silicon surfaces such as H-Si(100) and H-Si(111). Thus, the generated radical on the surface of the silicon, subsequently coupled with another molecule of TEMPO.361 Similar kind of reactivity of TEMPO has been observed with group 13 metal trihydride-Lewis base adducts.172,362 TEMPO reacted with this metal complex by a formal homolytic substitution and generated quinuclidine-complexed TEMPOMH2 along with dihydrogen. In addition, two equivalents of TEMPO produced [(TEMPO)2AlH(quin)] by reacting with [AlH3(quin)] (Scheme 78).


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N O

i

H M H quin

M = Al M = Ga

MH3 (quin) ii quin =

N

quin N O M H O N

Scheme 78. TEMPO reaction with metal complexes by a formal homolytic substitution. Reproduced from Ref. 362a. Copyright 2007 Royal Society of Chemistry.

TEMPO also reacted with a dimer of aryl zinc alkoxide through an associative mechanism.363 Besides group 13 and 14 metal complexes, transition metal hydrides (Rh, Ir) can also participate in the H-atom transfer reaction with TEMPO, rendering reduced form of TEMPOH and other by-products of metalloid radicals.364,365

3.1.12 Total Synthesis of Natural Products

TEMPO catalyzed oxidative reactions have played a pivotal role in various complex macromolecular level synthesis, particularly in natural product synthesis.366


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Carreira et al.367 employed TEMPO-mediated catalytic system for the selective oxidation of an alcohol of an intermediate K to L (Scheme 79), in the synthesis of 35-deoxy Amphotericin B aglycone that has a great importance in medicine. Moreover, the same group demonstrated a chemoselective two-step oxidation for the synthesis of erythronolide A seco acid.368 Furthermore, TEMPO was also utilized for the oxidation of alcohols in the multi-step synthesis of Azadirachtin369 and (_)-Exiguolide370 natural products.

Ph OH O

Ph

O

N

RO

OH HO

1) TEMPO, NaClO

O

O

N

OH

HO2C

2) NaClO2, 0 oC

RO

HO L

K

Scheme 79. Selective oxidation of Amphotericin B aglycone intermediate by TEMPO. Reproduced from Ref. 367. Copyright 2008 Wiley.

Nicolaou et al.371 have provided Monorhizopodin and 16-epi-Monorhizopodin natural products to the scientific community with a rich source of novel molecular architectures, many of which have become important therapeutics for clinical use.372 Nonetheless,


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throughout the synthesis of these natural products, a selective oxidation of the primary hydroxy group of diol M led to aldehyde N (Scheme 80) using TEMPO as an oxidant.

OH OH OMeOTBDPS

M

O

TEMPO Ph(OAc)2

OH OMeOTBDPS

H N

Scheme 80. Selective oxidation of primary hydroxy group in diol to the aldehyde using TEMPO. Reproduced from Ref. 371. Copyright 2011 Wiley.

Aphanorphine alkaloid is isolated from the freshwater blue-green algae Aphanizomenon flos-aquae,373 which has attracted considerable attention in the scientific community owing to its structural similarities to natural and non-natural analgesics.374 In the synthesis of this alkaloid by Grainger and Welsh,375 the carbonsulfur bond of intermediate O (Scheme 81) was replaced with TEMPO, resulting in a carbonoxygen bond-incorporated TEMPO adduct P. Subsequently, the adduct P was directly oxidized to ketone Q using mCPBA (Scheme 81). Furthermore, Tamoxifen is a most important anti-breast cancer drug in clinical use and it has the potential to be used as a chemopreventive breast cancer agent.376 Studer et al.377 implemented a novel Pd-


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catalyzed oxidative Heck arylation method for the preparation of Tamoxifen-type (Z/E = 13:1) tetrasubstituted olefins in the presence of TEMPO moiety.

O Et2N

S N Me H

S O

hv (125 W Hg arc lamp) 0.05 M, toluene, RT 25 min, quartz (4 equiv.)

N O

O N

O mCPBA

O

O

N Me H P

N Me H Q

Scheme 81. Replacement of CS bond with CO bond by TEMPO. Reproduced from Ref. 375. Copyright 2007 Wiley.

Theodorakis et al.378 reported a concise and protecting group-free total synthesis of (_)Fusarisetin A, which is an inhibitor of cancer metastasis.379 The synthetic route involves TEMPO-mediated one-pot oxidative radical cyclization for the formation of a bond between C1C6 in cyclized TEMPO isomers R and S (Scheme 82). It has been predicted that TEMPO indeed acts as a reactive oxygen species to form the five-membered C ring in (_)-Fusarisetin A.


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H H H O

1

LiHMDS, -78 oC then 0 oC, TEMPO

OEt

Page 168 of 227

H

O

Fe+PF6-

O

N O HH 5 6 H

O

O

O O

12 H

OEt

PhMe, 90 oC O 4-DMAP

OMe OH

H

OEt

N O HH 5 6 O O 1 N H O

OMe OH

N

OEt

1 H O

N O HH 5 H H

O

N

5 6

H

One-pot radical cyclization and aminolysis

H

1

H O H N

N

O

H

O

O O N

OMe OH

S

R

Scheme 82. TEMPO-mediated one-pot oxidative radical cyclization reaction for TEMPO product synthesis. Reproduced from Ref. 378a. Copyright 2012 Royal Society of Chemistry.

As already discussed in Section 3.1.4, BAIB/TEMPO catalytic system has been extensively used for oxidative lactonization of diols. Using this methodology, Sasaki et al.165b,380 constructed B-, D- and E-rings from corresponding 1,6-diols for the synthesis


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of (_)-Brevenal, which is a pentacyclic polyether natural product with intriguing biological activities. Most recently, BAIB/TEMPO-mediated tandem oxidative lactonization process has also been employed for the stereoselective synthesis of lactone intermediate T (Scheme 83) of Vittarilide A,381 which has promising antioxidant property. Furthermore, Rao et al.382 successfully synthesized bioactive isatine scaffolds from different oxindole and indole derivatives by metal-free BAIB/TEMPO-mediated C(sp3), C(sp2) CH bond oxidation.

TBSO TBSO

Ph(OAc)2 TEMPO

OMOM OBn OH OMOM

TBSO TBSO

CH2Cl2, rt, 3 h, 80%

O

O OMOM

MOMO T

Scheme 83. Oxidative lactonization of Vittarilide A intermediate using TEMPO/BAIB system. Reproduced from Ref. 381. Copyright 2015 Elsevier.

3.1.13 Miscellaneous Reactions

Studer and Hartmann383 have developed a new method for transition-metal-free oxyarylation of alkenes with aryl and TEMPO radicals, which were readily generated


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from aryl diazonium salts and TEMPONa, respectively. Recently, the same group has described a radical azido oxygenation of various alkene derivatives.384

Shimizu et al.385 developed a copper-catalyzed intermolecular three component oxyarylation of allenes to yield allylic alcohol derivatives using aryl boronic acids as a carbon source and TEMPO as an oxygen source (Scheme 84). This reaction proceeded under mild conditions with high regio- and stereoselectivity and functional group tolerance.

R4

HO

R3

B

OH

Cu(OTf)2 (10 mol%) t-BuBox (10 mol%)

+ R2

R1

TEMPO (1.2 equiv) MnO2 (1 equiv) DMF, rt

R4 OTEMP R3

R2 R1

Scheme 84. Oxyarylation of allenes with aryl boronic acid and TEMPO. Reproduced from Ref. 385. Copyright 2014 American Chemical Society.

Wang and Jiao disclosed a TEMPO-catalyzed aerobic oxygenation and nitrogenation of hydrocarbons via C=C double bond cleavage and generated a series of oxo nitrile compounds using trimethylsilyl azide (TMSN3) (Scheme 85).386 These molecules can be


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used as precursors for the synthesis of isoquinolines, α-hydroxyketones, and alkene nitriles. Furthermore, benzylic ethers and other related ArCH2OR substrates were oxidatively

cleaved

by

4-acetamido-2,2,6,6-tetramethylpiperidine-1-oxoammonium

tetrafluoroborate in wet CH3CN at room temperature to afford corresponding aldehyde and alcohols.387

H R

TEMPO (15 mol%), O2 (1 atm) TMSN3 (1.5 eq) MeCN, 80 oC

N O

R = H, Ar

Scheme 85. TEMPO catalyzed aerobic oxygenation and nitrogenation of hydrocarbons

via C=C double bond cleavage. Reproduced from Ref. 386. Copyright 2013 American Chemical Society.

Recently, Cao and Ding388 have developed an efficient oxidative deoximation system for the conversion of a range of aldoximes to the corresponding aldehydes in the presence of FeCl3/TEMPO catalytic system under aerobic condition. Notably, an active species nitric oxide (NO) was generated in situ from the cleavage of oxime derivative.


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In addition, the SS bond formation during the disulfide derivative preparation was efficiently done by aerobic oxidative coupling of 2-mercaptobenzothiazole with TEMPO as the catalyst (Scheme 86).389

N H 2O

SH

N O

S

N

O2

S

N OH

S S

S

2 S

S

N

N

S

N

S

Scheme 86. Proposed mechanistic pathway in the preparation of 2,2’-disbenzothiazole disulfide. Reproduced from Ref. 389. Copyright 2012 Wiley.

3.2

Polymer Synthesis by Nitroxide-Mediated Polymerization

Controlled/living radical polymerization (CLRP) of various monomers has mainly been achieved by three revolutionary radical polymerization processes, which are (i) NMP,5,327a,390,391 (ii) ATRP,392 (iii) RAFT polymerization.393 Each of the three methods is useful for the design of well-defined functional and complex macromolecular


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architectures. Among these CLRP technologies, NMP is historically the first and perhaps easiest method applied for the synthesis of different polymers with adjustable molecular weights and low polydispersities below the theoretical limit (polydispersity index, PDI < 1.5). Besides the thermal radical carboaminoxylation discussed in Section 3.1.10, NMP processes are also controlled by PRE.327b,394 As a result, the concentration of the polymeric radicals in the polymerization, remains low. Thus, termination of polymeric radicals by combination or disproportionation is inhibited to a large extent, resulting in a controlled polymerization.

The most frequently used nitroxide radical for NMP is TEMPO, which has generally been employed for CLRP of styrene and styrene derivatives by thermal heating5a,395 as well as by MW irradiation.396 Nevertheless, the control over the polymerization under MW conditions is not perfect. In this section, we will briefly discuss the recent developments in the growing area of TEMPO-mediated polymerization (TMP) processes.


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Frequently, TEMPO is employed in a reversible coupling strategy termed atom transfer nitroxide radical coupling (ATNRC).397 Using this methodology, Monteiro et al.398 synthesized various multi-arm star polymers by reaction between linear PS with a bromine terminal group prepared by ATRP and a core entity bearing multiple TEMPO moieties in the presence of CuBr/PMDETA or CuBr/Me6TREN.399 Furthermore, the TEMPO core entity was also utilized to synthesize branched macromolecular architectures, whose construction through divergent, convergent or parallel sequence was modulated by the copper catalyst.400 Besides, polyhedral oligomeric silsesquioxane nanoparticles were also employed as a core entity in order to prepare star block copolymers through TEMPO mediated polymerization.401

A new strategy for one-pot synthesis of ABC type triblock copolymer via a combination of “click chemistry” and ATNRC reaction was developed by Huang et al..397b,402 In this case, end-functionalized polymer precursors bearing terminal azide (block A), alkyne and bromide (B block), and TEMPO (C block) groups, respectively, were synthesized by living radical polymerization. The click reaction of a terminal azide on block A with the


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terminal alkyne group on B block, accompanied by simultaneous ATNRC reaction of the TEMPO group on block C with the halide end moiety on block B, was successfully conducted to render triblock copolymers with high efficiencies. Similarly, Tunca et al.403 reported an attractive strategy that employed triple click reactions, including CuAAC, nitroxy radical coupling, and Diels-Alder, in a one pot-method to synthesize linear tetra block quaterpolymers. Later on, the same group prepared a series of miktoarm star terpolymers through ROMP404 and nitroxide radical coupling reaction.

Graft copolymers (or polymer brushes) were synthesized mainly through two different strategies defined as “grafting-onto” and “grafting-from” strategies (Scheme 87).394b

a

b

grafting to

grafting from


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Scheme 87. “Grafting-onto” and “grafting-from” strategies for polymer brushes. Reproduced from Ref. 394b. Copyright 2015 American Chemical Society.

In “grafting-onto” strategy, the polymerization is conducted prior to the attachment onto the substrate. With this strategy, Tunca et al. have synthesized graft polymers by combining ROMP and nitroxide radical coupling.405 TEMPO terminated poly(ethylene glycol) (PEG11-TEMPO) or poly(ε-caprolactone) (PCL23-TEMPO) was grafted as side chains onto a ROMP-generated polyoxanorbornene (PONB) main backbone with bromide pendant groups (PONB20-Br) to yield graft polymer (Scheme 88).405 Moreover, post-functionalization of polyoxanorbornene backbone through the combination of bromination and TEMPO radical coupling reaction has recently been reported.406


176

O

O N

O O

Br

O Grubbs catalyst

O

N

O

O

Br

O

O PONB20-Br Cu(0), Cu(I) PMDETA in DMF at rt

O

O N

20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20

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O

O O

O

O

N O

O N

graft polymer

O

O O

O 11

or O

O

H 23

Scheme 88. TEMPO polymer side chains grafted onto polyoxanorbornene backbone. Reproduced from Ref. 405. Copyright 2011 Wiley.

Huang et al.407 synthesized a series of graft copolymers with the ATNRC chemistry. In this

regard,

they

reported

the

tetramethylpiperidine-1-oxyl-co-ethylene

synthesis

of

poly(4-glycidyloxy-2,2,6,6-

oxide)-graft-polystyrene

or

poly(tert-butyl

acrylate) [poly(GTEMPO-co-EO)-g-PS/PtBA] by reacting a readily prepared linear precursor copolymer of [Poly(GTEMPO-co-EO)] with bromide end group-contained PS


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or poly(tert-butyl acrylate) (PtBA) in the presence of 2-bromoisobutyrate as an initiator and CuBr/PMDETA as a catalyst (Scheme 89). Later on, well-defined amphiphilic graft copolymers consisting of hydrophilic poly(acrylic acid) backbone and hydrophobic TEMPO combined poly(propylene oxide) side chains were synthesized by sequential RAFT polymerization and the ATNRC chemistry, followed by selective hydrolysis of PtBA backbone.390a,408 Nonetheless, in this grafting-onto strategy, steric reasons may hinder the effective polymer grafting. As such, the resulting graft polymers or brushes generally show relatively low grafting densities or low film thicknesses.

n

Br

CuBr / PMDETA

n CuBr2 bromide end group-contained PS or PtBA O x

O x

O

y

O

y

O n

N O Poly(GTEMPO-co-EO)

O

N O Alkoxyamine derivative

poly(GTEMPO-co-EO)-g-PS/PtBA


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Scheme 89. ATNRC reaction between TEMPO polymer and bromide end group. Reproduced from Ref. 407a. Copyright 2008 American Chemical Society.

Another one is “grafting-from” approach, which is mainly based on surface-initiated polymerization. The polymer chains are grown from the polymerization initiators that are primarily installed on the substrate surface. For example, alkoxyamine initiators were first attached to the oxidized Si wafer by self-assembly and the corresponding thicker brushes were obtained by surface-initiated nitroxide-mediated polymerization (SINMP).409 Moreover, with this “grafting-from” strategy, mixed styrene and MMA polymer brushes were grown by surface-initiated ATRP and subsequent surface-initiated NMP on Si wafers.410 In addition, structured polymer brushes were obtained by a bottom-up method such as Langmuir-Blodgett lithography411 as well as atomic force microscopy lithography.412 Both lithographic techniques mainly relied on the “grafting-from” approach and TEMPO was utilized as aminoxyl regulator for the preparation of patterned polymer brushes by surface-initiated polymerization. Besides Si wafers, chemical modification of silica particles (SiO2) can also be done by the grafting-from


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method. In this context, Guerra et al.413 utilized a TEMPO bromide salt to functionalize the SiO2 surface with nitroxyl moieties. Such functionalization reaction took place within 48 h under mild conditions. Subsequently, styrene-maleic anhydride copolymers (Scheme 90) were grown from the TEMPO functionalized silica surface by heating it in the presence of the monomers.

HO HO HO

OH

Br N O OH

OH

HO HO HO

OH OH

Et3N, CH2Cl2 25 oC, 48 h.

OH

OH O O N

OH

Et3NHBr

OH

TEMPO functionalized silica

O o

126 C

Free Polymer

O

N O

m

O

O

O 2h

HO O HO

OH

O

OH OH

OH

OH

n

styrene-maleic anhydride copolymer brushes on silica

Scheme 90. Functionalization of silica with TEMPO moiety as a surface initiator for “graft-from” synthesis of polymer brushes. Reproduced from Ref. 413a. Copyright 2007 Wiley.


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Various nanoparticles such as CdSe, titanium oxide and magnetic Fe2O3 nanoparticles were successfully modified chemically by surface initiated NMP using the “grafting-from” approach.52,414 TEMPO related alkoxyamines were employed to prepare polymercoated carbon nanotubes415 or steel materials416 and to functionalize the inner surface of mesoporous silica MCM-41.417.

TEMPO derived nitroxide exchange can be possible between two alkoxyamine polymers comprising of two different nitroxide moieties and such prepared polymers are called “dynamic covalent polymers”.418 The exchange approach was efficiently employed for installing a chromophore at a terminus of a polymeric alkoxyamine prepared by NMP.419 Likewise, TEMPO exchange reaction was utilized for the chemical modification of alkoxyamine-terminated polymer brushes420 and also for the preparation of a molecular library.421 Moreover, a thermodynamic polymer cross-linking system based on radically exchangeable covalent bonds was demonstrated by Takahara et al..422 In this case, poly(methacrylic esters) containing alkoxyamine units afforded crosslinked product via the nitroxide exchange reaction upon heating. More recently, Lutz et


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al. have reported a novel and facile orthogonal iterative strategy for the synthesis of digitally encoded TEMPO-mediated polymers.423

4

Conclusions

In this review, we have overviewed the wide range of applications of TEMPO radical in both homogeneous and heterogeneous forms for chemical transformations. We have shown how chemists have given priority to this environmentally friendly, robust organic radical in the fields from synthetic chemistry to polymer chemistry. From the green chemistry point of view, some traditional, stoichiometric quantities of hazardous metals have been replaced with TEMPO in the industry level. Immobilization of TEMPO on various supports, including organic, inorganic small molecules to polymers and combined hybrid materials through covalent or non-covalent bonds, has reduced stinging economical barriers. Various simple and easy synthetic strategies have been successfully developed to prepare supported and functionalized TEMPO catalysts. The performance (activity, recovery, and reusability) of such supported TEMPO catalysts for


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various organic functional group transformations, natural product synthesis, and polymerization has been summarized in this review.

Besides the immobilization and applicability, this review also clarifies the key challenges that lie ahead. For examples, the efficient regeneration of the oxoammonium salt from hydroxylamine

is

still

a

challenging

issue.

Among

other

functional

group

transformations, TEMPO has been utilized extensively for the oxidation of alcohols under batch and continuous flow regimes188,424 and this reaction plays a pivotal role in industrial processes. Recent developments show that these oxidations can be conducted in the absence of any chemical oxidants (NaOCl or BAIB or O2, NOᵪ, etc.) by using electrochemical potential. The electrochemical oxidation method is flourishing because of its highly efficient waste-free system and the best is yet to come in near future. Along with the alcohol oxidations, many other interesting reactions (Sections 3.1.23.1.13) have been documented in this review by using TEMPO as an oxidant or catalyst. Such transformations are only reported at the batch conditions since rare articles dealt with continuous flow conditions. In addition, the metal-mediated reactions


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using chiral TEMPO has not been extensively investigated up to date. Furthermore, the oxidative transition of metal-free CC and CN bond forming reactions have been mainly performed using TEMPO as stoichiometric amounts, but the usage of TEMPO as the oxidant and also catalytic variants for these transformations have not been explored.

TEMPO has also found wide applications in radical chemistry and reversible thermal homolysis of alkoxyamines for TEMPO-mediated radical polymerization. This approach has been employed for the preparation of complex molecular architectures, such as block, comb, and star polymers. We believe that future applications of this polymerization technique in material chemistry will be a major research focus. Moreover, many new developments are going on by using TEMPO and its derivatives, with a broader applicability in many more fields (e.g. membrane chemistry and material science) and exciting results yet to come.

Corresponding Authors

* Email: [email protected] (J.W.), [email protected] (Z.Y.); Tel.: +86-57188320917 (J.W.), +1-514-8482424 ext. 5611 (Z.Y.)


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Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT

Financial support from National Natural Science Foundation of China (Grant No. 21374103) and the Natural Science Foundation of Zhejiang Province of China (LY18B040004) are greatly acknowledged.

ABBREVIATIONS

ABNO, 9-azabicyclo [3.3.1]nonane-N-oxyl; ARGET, activators regenerated by electron transfer; ATNRC, atom transfer nitroxide radical coupling; ATRP, atom transfer radical polymerization; AZADO, 2-azaadamentane-N-oxyl; BAIB, [bis-(acetoxy)iodo]benzene; BINAM, bi(2-naphthylamine); [bmim]Br, butylmethylimidazolium bromide; [bmpy]PF6,1-


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butyl-4-methylpyridiniumhexafluorophosphate; bpy, 2,2’-bipyridine; CB, carbon black; CLRP, controlled/living radical polymerization; CNTs, carbon nanotubes; CuAAC, cucatalyzed azide-alkyne cycloaddition; DCM, dichloromethane; DESs, deep eutective solvents; DFF, 2,5-diformylfuran; DMAP, N, N'-dimethylaminopyridine; DTBN, di-tertbutyl nitroxide; ee, enantiomeric excesses; ESR, eletron spin resonance; GMA, glycidyl methacrylate; GO, graphene oxide; HBPEK, hyperbranched aromatic poly(ether ketone);

HFIP,

hexafluoroisopropanol;

HMF,

1,1,4,7,10,10-hexamethyltriethylenetetramine;

ILs,

5-hydroxymethylfural; ionic

liquids;

HMTETA,

LPEI,

linear

poly(ethylenimine); m-CPBA, meta-chloroperbenzoic acid; MCRs, multicomponent reactions; mlc-SILP, multilayered covalently supported ionic liquid phase; MMA, methyl methacrylate; MNPs, magnetic nanoparticles; MNST, magnetic core-shell nanoparticlesupported TEMPO; MRI, magnetic resonance imaging; MW, microwave; MWCNT, multiwalled carbon nanotube; NHC, N-heterocyclic carbene; NMI, N-methylimidazole; NMP, nitroxide-mediated polymerization; NMR, nuclear magnetic resonance; OP-3CR, oxidative Passarini three component reaction; 3-oxo-ABNO, 3-oxo-9-azabicyclo [3.3.1]nonane-N-oxyl; PCL, poly(ε-caprolactone); P-3CR, Passarini three component


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reaction;

ACS Catalysis

PDI,

polydispersity

index;

PEBBT,

2,5-poly(3-[1-ethyl-2-(2-

bromoisobutyrate)]thiophene; PEG, poly(ethylene glycol); PEK, poly(ether ketone); PES, poly(ether sulfone); PGMA, poly(glycidyl methacrylate); PI, polyimide; PIC, pickering interfacial catalyst; PI Ru, polymer-incarcerated ruthenium catalyst; PMDETA, N,N,N',N",N‴-pentamethyldiethylenetriamine; PMO, periodic mesoporous organosilica; POM, polyoxometalate; PONB, polyoxanorbornene; PPOs, porous polymerized organocatalysts; PRE, persistent radical effect; PS, polystyrene; PtBA, poly(tert-butyl acrylate); PTMA, poly(TEMPO methacrylate); PU, polyurethane; pyta, pyridyl triazole; RAFT, reversible addition-fragmentation chain transfer; ROMP, ring-opening metathesis polymerization; SI-ATRP, surface-initiated atom transfer radical polymerization; SILLP, supported

ionic-liquid-like

phase;

SI-NMP,

surface-initiated

nitroxide-mediated

polymerization; SMNP, silica coated MNPs; SN, silica nanoparticle; SWCNT, single walled carbon nanotube; TAP, tetraarylphosphonium; TBHP, tert-butyl hydroperoxide; TBME, tert-methyl ether; TBN, tert-butyl nitrite; TBPA, tris(4-bromophenyl)aminium hexachloroantimonate;

TCCA,

trichloroisocyanuric

acid;

TEMPO,

2,2,6,6-


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tetramethylpiperidine-N-oxyl; TMA, TEMPO methacrylate; TMSN3, trimethylsilylazide; TOF, turnover frequency; TON, turnover number.

5

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