8-Mercaptoquinoline as a Ligand for Enhancing the Photocatalytic

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8-Mercaptoquinoline as Ligand for Enhancing the Photocatalytic Activity of Pt(II) Coordination Complexes: Reactions and Mechanistic Insights Antonio Casado-Sánchez, Mustafa Uygur, Daniel González-Muñoz, Fernando Aguilar-Galindo, José Luis Nova-Fernández, Judith Arranz-Plaza, Sergio Díaz-Tendero, Silvia Cabrera, Olga Garcia Mancheno, and José Alemán J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b00520 • Publication Date (Web): 18 Apr 2019 Downloaded from http://pubs.acs.org on April 18, 2019

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The Journal of Organic Chemistry

8-Mercaptoquinoline as Ligand for Enhancing the Photocatalytic Activity of

Pt(II) Coordination

Complexes: Reactions and Mechanistic Insights Antonio Casado-Sánchez,a Mustafa Uygur,b Daniel González-Muñoz,a Fernando AguilarGalindo,c José Luis Nova-Fernández,d Judith Arranz-Plaza,d Sergio Díaz-Tendero,c,e,f Silvia Cabrera,*d,f Olga García Mancheño,*b and José Alemán*a,f a

Organic Chemistry Department, Universidad Autónoma de Madrid, 28049 Madrid (Spain)

b

Organic Chemistry Department, University of Münster, 48149 Münster (Germany)

c

Chemistry Department, Universidad Autónoma de Madrid, 28049 Madrid (Spain)

d

Inorganic Chemistry Department, Universidad Autónoma de Madrid, 28049 Madrid (Spain)

e

Condensed Matter Physics (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid (Spain)

f

Institute for Advanced Research in Chemical Sciences (IAdChem), Universidad Autónoma de

Madrid, 28049 Madrid (Spain).

KEYWORDS: Photocatalysis• Platinum complex• Thioquinoline ligand • CASPT2 calculations • Mechanism

ACS Paragon Plus Environment

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ABSTRACT. A family of quinoline-platinum(II) complexes as efficient photocatalysts is presented. Their key characteristic is their easy preparation by coordination of the readily available 8-hydroxy- or 8-thio-quinolines ligands, which are well known for their strong chelating ability to different metal ions. In the different photochemical transformations investigated, such as crossdehydrogenative couplings, oxidation of aryl boronic acids and asymmetric alkylations of aldehydes, the 8-mercaptoquinoline-Pt(II) complex proved to be the most general catalyst. Moreover, quenching experiments showed that, contrary to related methods reported in the literature, these complexes followed an oxidative quenching mechanism in all the transformations studied. Besides, simulations performed with high-level ab initio methods of the complexes have helped to understand their photocatalytic activity.

INTRODUCTION Visible-light photocatalysis has become a powerful tool for the formation of new bonds and molecules, using smooth and mild reaction conditions.1 In recent years, most of the organic synthetic transformations have been carried out using organic dyes such as eosin Y or flavin,2 and transition metals such as Ru(II) and Ir(III) complexes as photocatalysts.3 Despite the rich photochemical and photophysical properties of many metallic complexes, their application as catalysts in light-mediated organic transformations has focused mainly on the aforementioned ruthenium and iridium complexes.4 For example, cyclometalated platinum(II) complexes, which are known to be highly luminescent, have been widely studied as organic-light emitting diodes (OLEDs),5 but scarcely applied as photocatalysts in spite of their rich photophysical properties. In fact, the use of platinum(II) complexes as photocatalysts in organic transformations is reduced to organometallic complexes (I-III, Figure 1).6 These complexes I-III have been used successfully for different reactions, but they present some drawbacks related to their preparation: i) their


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synthesis requires a large number of steps and only low overall yields are obtained; ii) each complex contains two different ligands that are not commercially available (usually 3-5 synthetic step/ligand); iii) Schlenk techniques are required. Therefore, to optimize each reaction different organometallic platinum complexes must be synthesized, which is a lengthy and time-consuming process. It would be highly desirable to find an easy synthesis and a fine-tuning platinum photocatalyst for broader applications. The coordination of simple and commercially available 8-hydroxy- or 8-thio-quinoline ligands to platinum, which are known as non-innocent ligands,7 would modify the photophysical and redox properties of complexes in a very easy manner. Sulfur based ligands have been used with different metals such as Cu, Pd, Pt or Rh, especially in the field of transition metal catalysis for the improvement of their catalytic performances.8 However, this type of ligand has been less explored in the photocatalytic area. In 2016, we reported that platinum(II) complexes (Y= O, Figure 1) bearing 8-hydroxyquinoline ligands9 were excellent photocatalysts for the oxidation of sulfides in both batch and under flow reaction conditions.10 We wondered if the substitution of the hydroxyquinoline by a thioquinoline ligand has any influence in photocatalytic activity of three different reactions. In this work, we present the application of mercapto-quinoline platinum(II) complex (Y= S, Figure 1) to a variety of photocatalytic reactions such as the aerobic CrossDehydrogenative-Coupling (CDC), the oxidation of boronic acids, and the dual-catalyzed asymmetric alkylation of aldehydes. We also analyzed the higher catalytic performance of the thioplatinum(II) complex compared to that of the hydroxyl-analogue. In addition, high-level ab initio calculations of the singlet and triplet excited states of the platinum complexes, and different mechanistic proofs have been performed to explain the better catalytic activity of this sulfurplatinum complex.


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Previously described Pt(II) photocatalysts: Organometallic complexes MeO

OMe

tBu

tBu

N Pt

N

N Pt

N

N Pt Ph2P PPh2

N

F

F

F

F F II

N

III

Multi-Step Synthesis of Ligands and Complexes Difficult Photocatalytic Modulation Schlenk Technique Synthesis

I

This work: Pt(II): Coordination Complex better S than O?

N Y

Pt dmso Cl

Commercially Available Ligands One Step Synthesis Straightforward Photocatalytic Modulation Air Compatible Synthesis

1 (Y= S, O) Reactions Studied

N

Ar

O R

Ar

Nu

Ar B(OH)2

H

N

LIGHT

Nu

S

N Pt dmso Cl

R' Br

Ar OH O R'

H R

Figure 1. Selected platinum(II) photocatalysts previously described and our family of photocatalysts.


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RESULTS AND DISCUSSION Photocatalytic Cross-Dehydrogenative-Coupling. The C-H bond cross-dehydrogenativecoupling reaction represents a potent method to construct C-C and C-heteroatom bonds under oxidative conditions, avoiding pre-functionalization and defunctionalization steps on the substrates.11 In this type of chemistry, the choice of the appropriate oxidant for each targeted reaction is crucial for its success.11,12 During the past few years, the functionalization of C-H bonds using visible light photoredox catalysis has emerged as a powerful approach for the efficient synthesis of organic molecules under mild, atom-economic and environmental friendly conditions.13 In this context, the direct C(sp3)-H functionalization of heterocycles represents an important synthetic transformation towards a broad variety of biologically active compounds.11,14 First, Stephenson reported the oxidative coupling of tetrahydroisoquinolines (THIQs) with nitroalkanes under visible-light catalysis, using an iridium-based catalyst.15 As an alternative, other groups have developed metal-free, visible-light-mediated oxidative C(sp3)-H coupling reactions,16 as well as dual catalytic systems combining photoredox with H-donor and Lewis base catalysis17 or N-heterocyclic carbene catalysis.18 With these precedents in mind we wondered if our family of platinum(II) complexes would be able to effectively catalyze the CDC reaction of THIQs with different nucleophiles. Therefore, the oxidative coupling of N-phenyl THIQ (2a) with dimethylmalonate (3a) in the presence of catalyst 1, blue light (3W blue LED) and air atmosphere was evaluated as a model reaction (Table 1).


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Table 1. Optimization of reaction conditions and catalyst for the photocatalytic CrossDehydrogenative-Coupling of 2a.a

1a (Y= S) 1b (Y= O)

N Y N

Ph

Pt dmso Cl N MeO2C

CO2Me

3a blue LEDs, air solvent, 24 h, rt

2a

MeO2C

Ph

CO2Me

4aa

Entry

Catalyst 1

Solvent

Conversion (%)b

1c

--

MeCN

10

2c

1a

MeCN

>98 (>98)e

3c

1b

MeCN

>98 (80)e

4d

1a

MeCN

>98

5d

1a

EtOH

92

6d

1a

Toluene

92

7d

1a

MeCN:H2O (1:1)

59

8d

1a

Tol:H2O (1:1)

30

9f

1a

MeCN

17

10g

1a

MeCN

98h

a

Conditions: 2a (0.2 mmol), catalyst 1 (1 mol %) and malonate 3a (0.24 mmol, 1.2 eq.) were dissolved in dry acetonitrile (0.1 M). b Conversion measured by 1H-NMR. c Reaction using a glass-finger system open to the air (blue LED, 3W). d Reaction using a vial with a septum open to the air with a needle (blue LEDs, 15W). e Conversion after 18 h in parentheses. f Following conditions d but under N2atmosphere. g Following conditions d but the system was initially bubbled with O2 (5 min) and kept under an O2-atmosphere (1 atm, balloon). h Conversion after 8 h.


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Firstly, the reaction that was carried out in the absence of the catalyst proceeded with only a very low conversion, which confirmed the essential role of the photocatalyst in our reaction conditions (entry 1, Table 1). Conversely, using any of the two platinum(II) complexes 1 the reaction proceeded with full conversion after 24 h (entries 2-3). However, among them, the mercapto-catalyst 1a was the most active after 18 h (see results between parentheses). Having identified 1a as the best catalyst, different solvents in a more powerful irradiation system (15W blue LED) were screened (entries 4-8). Toluene and acetonitrile were the most effective, while the presence of water had a negative effect and decreased the conversion of the reaction. In addition, as anticipated the reaction performed under a nitrogen atmosphere was unsuccessful (entry 9), while the reaction carried out under an O2 atmosphere achieved a full conversion in only 8 h (entry 10). Once the optimal conditions were determined, we proceeded to study the scope of the reaction with different nucleophiles using the catalyst 1a (Table 2). The reaction tolerated the introduction of nitrile, nitro and phosphite derivatives, affording the corresponding products 4ab-4ad in good yields. Moreover, the PMP-protected derivative 2b was also used for the functionalization with methyl and ethyl malonate (3a and 3e) and potassium allyltrifluoroborate (3f) to give 4ba, 4be and 4bf. Lastly, the intramolecular version of this reaction worked well, allowing the synthesis of the fused derivatives 4c and 4d in 58 and 78% yield, respectively.


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Table 2. Scope in the 1a-photocatalyzed Cross-Dehydrogenative-Coupling of THIQs 2.a

N S N

Ar

+ Nu

N

Ar

Nu 4

N

Ph

N

Ph

CN

CO2Me

4aa, 48% Yield

N

Ph PO(OEt)2

MeO2C

4ad, 82% Yieldb

Ph

NO2

4ab, 52% Yield

N

N

N

blue LEDs (15W), O2 8-24 h, CH3CN, rt

3

2

MeO2C

Pt dmso Cl 1a (1 mol%)

4ac, 53% Yield

N

PMP

CO2Me

4ba, 46% Yield

EtO2C

PMP

CO2Et

4be, 61% Yield

N

N

O

O

PMP Cl

4bf, 47% Yield

4c, 58% Yield

4d, 78% Yield

a

Conditions: 2 (0.2 mmol), catalyst 1a (1 mol%) and nucleophile 3 (0.24 mmol, 1.2 equiv.) were dissolved in dry acetonitrile (0.1 M). Reaction was initially bubbled with O2 (5 min) and kept under an O2-atmosphere (1 atm, balloon), using a vial with a septum (blue LEDs, 15W). Isolated yields. b Reaction with (EtO)3P in toluene/H2O (1:1) under air using 5 mol% of 1a.

Photocatalytic Oxidation of Boronic Acids. The oxidation of arylboronic acid derivatives with peroxides under basic conditions is one of the most efficient methodologies to access their corresponding phenols, which are important natural and bioactive compounds.19 However, the use


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of peroxides as oxidants in industrial processes is undesirable due to the safety issues associated with their handling. Therefore, the photo-oxidation of arylboronic acids using atmospheric O2 would be a safer alternative.20 In this section, the oxidation of arylboronic acids using the platinum complexes 1 was evaluated (Tables 3 and 4). The optimization of the reaction with phenylboronic acid began with the investigation of different platinum complexes 1 (Table 3). This revealed that the best photocatalyst was again the mercapto catalyst 1a, which afforded full conversion in 48 h (entries 1-3). Next, the reaction time was shortened to 24 h and the catalyst loading was decreased to 2 mol% for the screening of the solvent (entries 4-9) and amine (entries 5, 10-11). Under these reaction conditions, DMF gave the highest yield (56% yield, entry 4), which was increased to 70% using Et3N instead of DIPEA as the sacrificial electron donor (entry 10). Finally, the evaluation of the irradiation source (entries 12-14) showed that the white LED was the most appropriate, leading to the phenol product (6a) in an excellent yield (91%). Unfortunately, lowering further the amount of catalyst to 1 mol% substantially decreased the yield of the oxidation (entry 15).

Table 3. Optimization of reaction conditions for the oxidation of phenylboronic acid.a

N B(OH)2

5a

Y

1a (Y= S) 1b (Y= O) OH

Pt dmso Cl amine, light, DMF air, rt

6a

Entry

1 (mol%)

Amine (light)

Solvent

Time (h)

Yield (%)b

1

--

DIPEA (CFL)

DMF

24

n.r.

2

1a (5)

DIPEA (CFL)

DMF

48

90


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3

1b (5)

DIPEA (CFL)

DMF

48

41

4

1a (2)

DIPEA (CFL)

DMF

24

56

5

1a (2)

DIPEA (CFL)

MeCN

24

33

6

1a (2)

DIPEA (CFL)

Toluene

24

46

7

1a (2)

DIPEA (CFL)

DCM

24

44

8

1a (2)

DIPEA (CFL)

EtOH

24

5

9

1a (2)

DIPEA (CFL)

EtOH/H2O

24

3

10

1a (2)

Et3N (CFL)

DMF

24

70

11

1a (2)

CH3N(Ph)2 (CFL)

DMF

24

7

12

1a (2)

Et3N (green LED)

DMF

24

34

13

1a (2)

Et3N (blue LED)

DMF

24

74

14

1a (2)

Et3N (white LED)

DMF

24

91

15

1a (1)

Et3N (white LED)

DMF

24

79

a

Conditions: 5a (0.10 mmol), amine (0.11 mmol) and catalyst 1 (x mol%) were dissolved in the corresponding solvent (0.05 M) under the irradiation system indicated. b NMR-yield determined using MeNO2 as the internal standard.

With the optimized reaction conditions established, the scope of the reaction was explored (Table 4). The transformation worked smoothly with arylboronic acids bearing weakly activating groups such as methyl (6b and 6c) or with strong EDG and EWGs such as 4-MeO, CF3 or CN (6d6f), leading to the products in good yields (60-89%). The presence of a chlorine atom at the aryl group (6g) and heteroaryl moieties (6h) was also compatible with the photo-oxidation conditions, obtaining the corresponding phenols in moderate to good yields (48-70%).


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Table 4. Scope of oxidation of arylboronic acids under 1a catalyst.a

N S

Pt dmso Cl 1a

Et3N (1.1 equiv.)

Ar B(OH)2

Ar OH

DMF , white LED air, rt, 24-48 h

5 OH

OH

6 OH

OH

OMe 6a, 91% Yield

b

6b, 60% Yield

F3C

6c, 70% Yield OH

OH

OH

CN 6e, 81% Yield

b

CF3

6f, 88% Yield

b

b

6d, 89% Yield

HO N H

Cl 6g, 70% Yieldb

6h, 48% Yield

a Conditions:

5 (0.10 mmol), Et3N (0.11 mmol) and catalyst 1a (2 mol%) were dissolved in DMF (0.05 M) under white LEDs. Yields determined after flash chromatography. b NMR-yield determined using MeNO2 as internal standard.

Photocatalytic Asymmetric -Alkylation of Aldehydes. In the previous sections, we have shown that platinum(II) complexes can be used in photo-oxidation reactions. In order to demonstrate the general application of our photocatalysts, the platinum(II) complexes were tested in a more complex reaction: the asymmetric -functionalization of aldehydes with bromo derivatives. The asymmetric -alkylation of aldehydes and ketones has become an attractive reaction due to the synthetic utility of the final products obtained. In 2008 MacMillan and coworkers found an extraordinary example in which the combination of an imidazolidinone organocatalyst and the photoredox catalyst Ru(bpy)32+ enabled the asymmetric -alkylation of


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aldehydes, starting from alkyl-bromo derivatives.21 Since then, different catalytic systems that combine a photocatalytic counterpart with an imidazolidinone organocatalyst have also been reported.22 However, to the best of our knowledge this study represents the first platinum(II) cocatalyzed asymmetric -alkylation of aldehydes. To evaluate our photocatalysts in this transformation, the aldehyde 7a, bromomalonate 8a and the MacMillan catalyst 9a were initially used in the presence of photocatalysts 1a-b under visiblelight irradiation (Table 5). Surprisingly, the reaction only proceeded with photocatalyst 1a, obtaining null conversion in the case of the platinum complex 1b (entries 1-2). Moreover, the change to the bulkier amino organocatalyst 9b slightly increased the enantioselectivity to 92% (entry 3). To our delight, the use of a blue LED photoreactor system gave full conversion and an outstanding 96% ee (entry 4), better than either the green or white light (entries 5 and 6). Other solvents such as DMSO or CH3CN provided lower conversions (entries 7-8). As we have previously demonstrated,22d the reaction did not proceed without catalyst 1, being the aminocatalyst 9b unable to promote the -alkylation of aldehydes without an external photocatalyst (entry 9). Under the optimized reaction conditions (entry 4, Table 5), the alkylation reaction was then explored using different aldehydes and organic bromides (Table 6). As a result, various aldehydes 7 bearing pendant alkylic, arylic and heterocyclic groups were efficiently -alkylated with diethyl bromomalonate (8a) and bromoacetophenone (8b), leading to the products 10a-d in good to excellent yields (47-91%) and enantioselectivities (84-96% ee). This study validates the compatibility of the platinum(II) photocatalyst with organocatalysis and more importantly, its broad application in photoredox processes.


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Table 5. Optimization of reaction conditions for the -alkylation of aldehydes.a O H

+

EtO2C

CO2Et

2,6-lutidine, Solvent, Light N2 atmosphere, rt, 24h

Br

5

7a

O

1a-b (2 mol%), 9a-b (20 mol%) H

CO2Et 5

8a

10a O

N S

Pt

Me

N dmso

Cl 1a

CO2Et

O

Pt

dmso

Cl 1b

O

N tBu tBu

N H

9a

N N H

Ph

9b

Entry

1

9

Solvent

Light

Conversion (%)b

ee (%)c

1

1a

9a

DMF

CFL (15W)

42

90

2

1b

9a

DMF

CFL (15W)

n.r.

-

3

1a

9b

DMF

CFL (15W)

28

92

4

1a

9b

DMF

Blue LED

>98

96

5

1a

9b

DMF

Green LED

95

95

6

1a

9b

DMF

White LED

60

94

7

1a

9b

DMSO

Blue LED

50

95

8

1a

9b

MeCN

Blue LED

62

96

9

-

9b

DMF

Blue LED

8-Mercaptoquinoline as a Ligand for Enhancing the Photocatalytic

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