Silica Nanoparticles Decorated with Polymeric Sulfonic Acids Trigger

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Silica Nanoparticles Decorated with Polymeric Sulfonic Acids Trigger Selective Oxidation of Benzylic Methylenes to Aldehydic and Ketonic Carbonyls Emanuele Paris, Claudio Oldani, Antonino S. Arico, Claudia D'Urso, Franca Bigi, Giovanni Maestri, Francesco Pancrazzi, and Raimondo Maggi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05845 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019

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ACS Sustainable Chemistry & Engineering

Silica Nanoparticles Decorated with Polymeric Sulfonic Acids Trigger Selective Oxidation of Benzylic Methylenes to Aldehydic and Ketonic Carbonyls Emanuele Paris,a Claudio Oldani,b Antonino S. Aricò,c Claudia D'Urso,c Franca Bigi,a,d Giovanni Maestri,a Francesco Pancrazzia and Raimondo Maggi*a

a Dipartimento SCVSA, Università di Parma; 17/A Parco Area delle Scienze, 43124 Parma (Italy); [email protected]

b Solvay Specialty Polymers Italy S.p.A., Viale Lombardia 20, 20021 Bollate (MI) (Italy);

c CNR-ITAE, Istituto Tecnologie Avanzate per l’Energia, Via Salita Santa Lucia sopra Contesse 5, 98126 Messina (Italy);

d IMEM-CNR, 37/A Parco Area delle Scienze, 43124 Parma (Italy).

ACS Paragon Plus Environment

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Heterogeneous Catalysts, Nanocatalysts, Benzylic Oxidation, Metal-free Oxidation, Hydrogen Peroxide.

ABSTRACT: The controlled dispersion of polymeric sulfonic acids on suitable heterogeneous supports paves the way to a new catalytic method for the oxidation of a variety of benzylic methylenes to the corresponding carbonyls by using hydrogen peroxide; narrowly-dispersed silica nanoparticles covered by fluorinated sulfonic acids ionomers allow selective oxidation of toluene to benzaldehyde, minimizing undesired side-reactions and formation of wastes.

INTRODUCTION Oxidation processes are among the most important classes of reactions in organic synthesis, and their synthetic scope and utility have advanced significantly over the past few decades.1-3


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When considering industrial applications, direct C-H oxidations are often recognized as essential among other oxidation methods given their relevance within molecular economy and greenness.4,5 In particular, benzylic oxidation is one of the most useful tools to generate valuable building blocks that can serve to access many drugs, agrochemicals, and various natural products.6 Numerous C-H oxidation methods have been therefore developed to date.7-10 Toluene is the prototypical aromatic hydrocarbon that possesses C-H benzylic bonds. It can be oxidized to several oxygenates,11 namely benzyl alcohol, benzaldehyde and benzoic acid, which are all useful chemical intermediates. Among these, benzaldehyde is the most desirable product, both for its immense importance in our daily life and for the intrinsic chemical challenge to hamper its overoxidation. Benzaldehyde is usually produced in a multistep process starting from toluene chlorination.12 The sustainability of the process suffers from the amount of generated wastes. There are several elegant reports on the oxidation of toluene to benzaldehyde. This is achieved using a metal-based catalyst and H2O2 or molecular O2, which are clean oxidants, although with either low yields or poor selectivity.13-20 On the other hand, such limitations can be efficiently countered by using proper catalysts in combination with molar


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excesses of hypervalent iodine reagents or organic peroxides,21-24 which ultimately affect however the environmental cost of the whole process. Most of these methods require the use of transition-metal catalysts too.25,26 We are unaware of reports on the direct oxidation of inactivated27,28 benzylic methylenes to the corresponding carbonyls mediated by an organic acid catalyst. Herein we report a simple and practical double benzylic sp3 CH oxidation in the presence of hydrogen peroxide using solid sulfonic acid catalysts. This approach can be exploited for the synthesis of aromatic aldehydes and ketones, showing robustness and tolerance to a fairly wide variety of functional groups. The reaction uses a green oxidant and does not require any other organics, minimizing wastes. Perfluorinated sulfonic acids heterogeneized on narrowly dispersed silica nanoparticles are very efficient and active catalysts for this type of reaction, becoming an alternative to classic metal-based methodologies that either are limited to few turnovers or require organics as oxidant or solvent (Scheme 1).


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H

O O

simple

H

X

Ar

H Z

+ 2 H2O2

H

X

O H H

robust

O H

safe H

H H

metal-free O

Heterogeneous catalyst Fluorinated sulf onic acid resin on silica nanoparticles

Ar

Z

+ 3H2O

Z = H, Alk, Ar', Cl, CN ...

Scheme 1. Competitive oxidation routes for toluene and present work.

RESULTS AND DISCUSSION Our investigation began while we were screening proper conditions for Friedel-Crafts acylations. We observed unexpectedly the formation of traces of benzaldehyde (2a) employing non-degassed solutions in combination with strong Brønsted acid catalysts. Warming an aerated toluene solution in the presence of Amberlyst IRA-400 at 110 °C for 24 hours delivered 1% of 2a. Under these conditions, toluene oxidation remained largely unpractical, but intrigued by this responsiveness we decided to optimize the reaction. We investigated various catalysts and conditions, replacing air with hydrogen peroxide as oxidant (Table 1, further details in SI).


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Table 1. Survey of reaction conditions.

H

O

H H

+ 2 H 2O 2

Catalyst

H

N2, 110 °C, 24 h 2a

1a

Yield of 2a

Sel. to

(%)b, [TON]

2a (%)b

Nafion NR50 (0.08)

6, [4]

78

Aquivion PW66S (0.15)

6, [2]

91

-(C6H4)-SO3H@SiO2

3, [2]

92

6, [6]

94

13, [12]

97

Entrya

Catalyst (mmol –SO3H)

1 2 3

4

5

(0.065) -(CH2)3-SO3H@SiO2 (0.051) -(CF2)3-SO3H@SiO2 (0.055)

6

Aquivion@SiO2 (0.026)

20, [38]

95

7c


3, [6]

91

8d


21, [40]

58

9e


47, [45]

99

10e,f


10, [19]

81

11e,g


64, [31]

97

12e,g

Nafion@SiO2 (0.022)

8, [18]

98

96%).


9


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Traces of both benzyl alcohol and benzoic acid appeared from the beginning and did not increase further. This observation suggests that the formation of these side-products is due to mechanisms different than that leading to 2a. Stability and reusability of the catalyst were assessed recovering the solid acid by filtration at the end of the reaction and repeating the oxidation test (Table 2). The catalyst exhibited a constant activity until the 7th run (61% yield) before reaching out to progressive inactivation (56%, 46% and 22% respectively, entry 8-10 in Table 2). In all cases, selectivity remained excellent (>96%). The recovered material seemed still macroscopically identical, but titration showed that the used material presented a tiny residual amount of Brønsted acid sites. We thus treated the inactivated material with an excess of H2SO4. This replenished its original acidity (0.267 mmol –SO3H/g) by restoring –SO3H groups from –SO3M (M = Metal) ones and delivered 2a in 62% yield repeating our model reaction (entry 12). This result further suggests that sulfonic acid groups of the heterogeneous material do not easily leach and are the active sites of the catalyst. Although the reactivated catalyst showed a more rapid deactivation than the fresh one (entry 12-16), present material is chemically stable and


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can easily trigger the selective oxidation of hydrocarbons for more than 300 turnovers (Table 2), outmatching most of their metal-based peers.13-25

Table 2. Catalyst reuse.

Yield 2a

Sel. 2a

Yield 2a

Sel. 2a

(%)a

(%)a

(%)a

(%)a

1

64

98

9

46

95

2

63

97

10

22

93

3

63

97

11b

62

96

4

62

97

12

59

96

5

61

97

13

55

96

6

60

97

14

48

97

7

61

97

15

41

96

8

56

96

16

34

96

Entry

a

Entry

By GC. b Reactivated by treating with an excess of 1 M sol. of H2SO4 in EtOH.

From a synthetic point of view, in all these runs upon recovery of the solid material, the organic phase is essentially a toluene solution of 2a, that could be easily isolated as almost pure compound (> 90%) through a simple toluene evaporation under reduced


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pressure. Although single-stage conversion might appear low, the highly recyclable nature of the catalyst coupled with the observed high selectivity and the simple product recovery grant wide synthetic viability to the present method. The oxidant is always completely consumed during the course of the reaction, as witnessed by the absence of residual peroxides, resulting in a value of 64% of hydrogen peroxide efficiency.31 Taken together, these aspects clearly witness the potential of this approach, which is inherently safe and minimizes formation of wastes. This is clearly confirmed by the evaluation of green metrics parameters32 such as the overall atom economy (AE),33 overall E-factor3437

and intensity (PMI).38 These were compared to those obtained by using other reported

procedures, which suffers from the use of either other organics for reactions or more complex purifications of crudes (Table 3).

Table 3. Calculated green metrics.

Entry

Procedure

Yield (%)

E factor

PMI

AE

1

this work

61

1.76

2.76

0.66

(distillation)a


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2

this work

64

39.30

40.30

0.66

(silica pad)b

a

3

A. S. Roy26

65

24.50

25.50

0.66

4

L. D. S. Yadav28

29

90.08

91.08

0.85

5

F. Wang13

2

44.00

45.00

0.85

6

Y. Wang15

11

85.35

86.35

0.66

See ESI; b proxy for non-volatile products, such as 2n.

Intrigued by the chemical stability of this catalyst, we performed various physical analyses on the material to gain insights on its properties. The white solid displayed a modest surface area (224 m2/g by BET). Furthermore, this result is at odds with usual features of efficient heterogeneous catalysts, as low surface areas limit both substrate adsorption and product desorption. We thus performed Transmission Electron Microscopy (TEM) analyses on the catalyst (Figure 1, top-left).


13


30

25

20

Frequency / %

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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15

10

5

0 0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Particle size / nm

Figure 1. TEM image and measured particle size distribution of Aquivion-based catalyst (top) and TEM images of Nafion-based catalyst (bottom).

Surprisingly, the hybrid material is formed by agglomerates of silica nanoparticles evenly decorated by the perfluorinated polymeric resin. This implied that the sol-gel preparation in the presence of the perfluorosulfonate ionomer inhibited the formation of a mesoporous inorganic scaffold while favouring agglomeration of nanoparticles. Thanks to the ammonium template however, nucleation of silica proceeded in a controlled fashion. This delivered a narrow dispersion of the nanoparticles themselves (3-7 nm, Figure 1, topright). EDX analysis on the material further confirmed that the inorganic and the organic phases were evenly dispersed. The method thus enables for the preparation of


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perfluorosulfonate ionomer-coated narrowly-dispersed silica nanoparticles. Despite the limited surface area of the synthesized material, this proved to work efficiently and selectively as heterogeneous oxidation catalyst. Likely, it is actually the particular morphology at the nanometric scale that grants this catalyst high chemical stability, as displayed during recycling experiments. We then performed TEM analyses on the nearly inactive Nafion-based catalyst too (8 vs 64% yields, Table 1, entries 11 and 12 respectively). In contrast to the best catalyst, this material presents two different morphologies. Together with zones in which Nafion is evenly dispersed among silica (Figure 1, bottom left) as happens for the Aquivion-based catalyst, large areas feature undispersed agglomerated polymer (Figure 1, bottom-right). We reasoned that these differences caused its reduced activity. We then investigated the general validity of this reaction (Scheme 2).


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H

H Z

Ar

Aquivion @SiO2 2.1 mol% -SO3H 5 mmol H2O2 N2, 110 °C, time

O

O Z

Ar

O

2a-n, 11-73%

O

O

O

2a, 63% 2b, 58% 54% on 50 mmols O

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2c, 55%

2d, 13%

O

O OH

Cl

Cl 2e, 5%

2f, 11%

O

4a, 41%

O

2g, 22%

O

O

CN 2h, 48%

2i, 40% O

O 2l, 70%

2j, 43% O

2k, 67% O

S 2m, 73%

O 2n, 70%

Scheme 2. Reaction scope, conditions as Table 1, entry 11; isolated yields.

First of all we scaled up the model reaction (100 and 50 mmols of toluene and hydrogen peroxide, respectively) and we isolated benzaldehyde in 54% yield. Concerning other substrates, toluene could be replaced by xylenes, which were selectively mono-oxidized to the corresponding products 2b-c (Scheme 2) in good yields (58-55%). Mesitylene 1d (Scheme 2) was the least reactive of the series, probably for its steric hindrance, and gave selective monofunctionalization (2d, 13% yield). Halogenated toluenes, such as 1e, afforded products in low yields (2e, 5%); para-nitrotoluene did not react. Ethylbenzene


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gave exclusively acetophenone (2f), albeit in 11% yield. In all cases, selectivity remained almost complete (>95%). The reaction of benzyl alcohol 3a provided benzoic acid (4a, 41%) as main product. We did not detect 2a as intermediate in this reaction. This suggests that present formal 4electrons oxidation of benzylic methylenes to carbonyls does not proceed through two separate sequential reactions.39,40 To confirm this hypothesis we tested other substituted benzylic methylenes. Almost beyond our expectations, benzyl chloride gave indeed benzoyl chloride (2g, 22%). The nitrile group is similarly tolerated, as witnessed by retrieving 48% of 2h. Most surprisingly, selectivity remained excellent in both cases (89% and 92% respectively). Other carbosubstituted benzylic derivatives followed suite, delivering the corresponding ketones with interesting yields (2i-k, 40-67%). Polyheterocycles could be similarly accessed (2l-m, ca 70%); by using 10 mmol of H2O2, 9,10-dihydroanthracene gave 9,10-anthraquinone (2n, 70%). Taken together (2g-2n), these results suggest that benzyl radicals can be initially formed.41


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During the reaction, we did not detect any intermediate of toluene oxidation so we performed further experiments to get insights on the reaction mechanism (Scheme 3).

Scheme 3. Mechanism probes.

We exclude any significant leaching from the solid catalyst by trying the Sheldon test and not observing any further conversion of the filtrate.42 Moreover, X-Ray Fluorescence (XRD) on crude did not find the presence of sulfur from the catalyst –SO3H (detection limit: 4 ppm). We carried out Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) analysis on a freshly prepared material to quantify transition-metal traces


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commonly able to catalyze redox processes (Co, Cr, Cu, Fe, Mn, Ni, Zn). For all quoted metals, the obtained results showed values lower than 10 ppm, except for iron (19 ppm, Table S2 in SI). Analysis on the barely inactive material upon nine recycles (Table 2, entry 10) showed slightly higher values for all of them. This is likely due to concentration of reagent contaminants, which is exacerbated upon several recycles. To exclude the involvement of these metal traces in catalysis, we treated therefore our fresh material with an excess of aqueous NaOH solution. The resulting solid did not exhibit any sulfonic acid sites and ICP-OES analysis showed metal contents close to previous ones. However, the material with –SO3Na groups was completely inactive (Scheme 3, top). This shows that toluene oxidation to 2a is triggered by sulfonic acid groups. If these acid groups underwent salification, the material becomes no longer active. Toluene conversion is inhibited in presence of 0.05 equiv. of 2,6-di-tert-butylphenol. Using 0.05 and 0.1 equiv. of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxyl), conversion of 1a was 27% and 13% respectively. Reactivity was completely quenched with 0.5 equiv. These results strongly suggest that present oxidation proceeds through a radical mechanism.


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Warming the mixture with TEMPO for 9 hours confirmed this hypothesis. We were indeed able to isolate recombination product 5 (6%). This confirmed that benzyl radicals are intermediates of these reactions. On the basis of the aforementioned observations, we propose a possible reaction mechanism in Scheme 4. The sulfonic acid I could be oxidized to the corresponding peracid II by hydrogen peroxide.43 Homolytic severance of the labile O-O bond would then yield a hydroxyl radical together with a sulfoxyl one (III), which is likely stabilized by its perfluorinated backbone.44 The hydroxyl radical subtracts a hydrogen atom from 1 to yield a water molecule and a benzylic radical.45 We expect that the latter recombines with III on the surface of the heterogeneous material. A second H2O2 molecule would then react with the resulting diamagnetic species IV providing V. The former is more electron rich than II thanks to its benzyl fragment. This in turn should make this peroxidation step easier, which might represent the key of the observed selectivity for this sequence. V eventually liberates 2 and regenerates the acid site I, either through a radical or an ionic rearrangement.


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O R-CF 2-S-O-H O

O Z

H 2O2 H 2O

I

2

O H R-CF 2-S O Z O O

O R-CF 2-S-O-O-H O II

O H R-CF 2-S O Z O O H

radical or ionic rearrangement

1

O H Z R-CF 2-S-O O O

V H 2O H 2O2

H Z

O R-CF 2-S-O O OH III

H 2O

O R-CF 2-S-O O

O R-CF 2-S-O + O H

H Z

Z

IV

Scheme 4. Possible reaction mechanism.

CONCLUSIONS We have reported a metal-free catalytic method for the selective oxidation of hydrocarbons featuring benzylic methylenes to the corresponding carbonyls. Narrowly dispersed silica nanoparticles decorated with polymeric perfluorinated sulfonic acids


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proved robust catalysts to trigger this formal 4-electrons oxidation selectively over a broad range of substrates. This combines with the use of a sustainable oxidant and the absence of any other additives to provide an interesting alternative to the synthetic toolbox. Further mechanistic investigations are currently underway.

ASSOCIATED CONTENT Supporting

Information.

The

following

files

are

available

free

of

charge.

Experimental procedures and details, synthesis of the heterogeneous catalysts, catalytic reactions, catalyst characterization files, copies of all NMR spectra (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENT We thank MIUR and UniPR for the financial support. GM warmly acknowledges support from SIR project AROMA-TriP (Grant No.: RBSI14NKFL).


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For table of contents use only graphics:

H Ar

H Z + H2O2

solvent-reagent

-[CFn]m-SO3H @nanoSiO2

O Ar

Z

no metal

Synopsis: Silica nanoparticles bearing sulfonic acid moieties allow the metal-free selective oxidation of benzylic methylenes to carbonyls with hydrogen peroxide.


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Silica Nanoparticles Decorated with Polymeric Sulfonic Acids Trigger

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