Pd-catalyzed decarboxylative arylation of silyl enol

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Pd-catalyzed decarboxylative arylation of silyl enol ester sp3 b-C–H bond under aerobic conditions† Zhengjiang Fu, Shijun Huang, Jian Kan, Weiping Su* and Maochun Hong

Downloaded by Fujian Institute of Research on the Structure of Matter, CAS on 24 November 2010 Published on 23 November 2010 on http://pubs.rsc.org | doi:10.1039/C0DT01234C

Received 14th September 2010, Accepted 11th October 2010 DOI: 10.1039/c0dt01234c

Pd-catalyzed aerobic oxidative coupling of various benzoic acids with silyl enol esters proceeds via a combination of decarboxylation with sp3 b-C–H bond activation to give Heck-type products. Mechanistic studies reveal this coupling involves in situ generation of olefin from aerobic oxidation of silyl enolate, followed by decarboxylative Heck coupling. Transition-metal-catalyzed cross-coupling reactions are among the most commonly used methods for generating C–C bonds.1 Their fundamental importance in synthetic organic chemistry has given a great impetus to the development of a new version of crosscoupling reaction, in which one or both of traditional coupling partners such as electrophilic aryl halide or/and nucleophilic organometallic reagent are replaced with distinct substrates. In this context, the cross-coupling reaction via C–H bond activation step, namely, the direct C–H bond functionalization, is a focus of current catalysis studies,2–4 because such transformations provide the most efficient synthetic pathways. Recently, decarboxylative cross-coupling reactions of aromatic carboxylic acids have emerged, some seminal studies include the Pd-catalyzed Heck-type reaction of aromatic carboxylic acids, Pd/Cu catalyzed arylation of aromatic carboxylic acids with aryl halides and the Pd-catalyzed arylation of heteroaromatic carboxylic acids with aryl halides.5–8 In these reactions, the organometallic intermediates generated from the metal-mediated decarboxylation served as either nucleophiles or electrophiles depending on the nature of the metal salt used for decarboxylation, exhibiting the potential of the aromatic carboxylic acid as a versatile coupling component.5–14 The ready availability of aromatic carboxylic acid renders this synthetic approach particularly attractive. In contrast to arylation reactions of aromatic C–H bond with aryl halides that have recently been attracted considerable attentions, however, few examples of the direct C–H bond functionalization with aromatic carboxylic acids via decarboxylation have been reported.13 Our group recently reported a Pd/Agcatalyzed method for intermolecular decarboxylative direct C2 arylation of indoles with electron-rich benzoic acids and C3 arylation of indoles with electron-deficient benzoic acids.14 We believe that the combination of decarboxylation with activation of C–H bond for cross-coupling reaction would provide a new synthetic access to C–C bond formation. In this paper, we report State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, The Chinese Academy of Sciences, Fuzhou, Fujian, 350002, China. E-mail: [email protected]; Fax: (+86)591-83771575 † Electronic supplementary information (ESI) available: Experimental procedures and spectroscopic characterization of the products. See DOI: 10.1039/c0dt01234c

This journal is © The Royal Society of Chemistry 2010

a Pd-catalyzed method for the synthesis of Heck-type products via the combination of decarboxylation of benzoic acids with the formal activation of sp3 b-C–H bond in silyl enolate of ester, wherein dioxygen is utilized as the oxidant. Traditional Heck coupling commonly employs a,b-unsaturated carbonyl compounds as coupling partners that are usually synthesized through either halogenation-dehalogenation sequences or dehydrosilylation of the corresponding silyl enolates,15–18 whereas our protocol utilizes silyl enolate of ester as starting material to directly form the Hecktype products. Initially, we observed that the reaction of 2,6-dimethoxybenzoic acid 1a with tert-butyldimethylsilyl (TBS) enolate of methyl isobutyrate 2b in a mixed solvents of DMSO (5%, v/v) in DMF in the presence of 20 mol% Pd(TFA)2 (TFA = trifluoroacetate) ˚ molecular sieves (MS) as drying agent and a as catalyst, 4 A stoichiometric Ag2 CO3 as oxidant under nitrogen atmosphere furnished mono- and di-substituted Heck-type products rather than a-arylated ester that can be generated from Pd-catalyzed coupling of aryl bromide with silyl ketene acetal (entry 1, Table 1).19 Interestingly, the reaction conducted under 1 atm of dioxygen proceeded more efficiently than under nitrogen atmosphere (entries 3–4). In our screening of oxidants, we observed that metal salt oxidant was not necessary when the reaction conducted under 1 atm of dioxygen (entry 4 versus 5). Under otherwise identical conditions, 10 mol% Pd(OAc)2 provided much higher yield than 10 mol% Pd(TFA)2 , indicating that Pd(OAc)2 was superior to Pd(TFA)2 for this reaction (entry 7 versus 5). The removal of MS from the reaction system led to a decrease in the yield (entry 6). MS presumably functioned as a water scavenger to avoid hydrolysis of silyl enolate of ester. Trimethylsilyl (TMS) enolate 2a as substrate was less effective than tertbutyldimethylsilyl enolate 2b (entry 7 versus 8). Considering that silyl enolate of esters are, to some degree, sensitive to protic acid, we hypothesized that introducing a Brønsted base into the reaction system would suppress protonolysis of silyl enolates of esters caused by benzoic acids by tempering acidity of reaction medium, and therefore improve the compatibility of silyl enolates of esters with acid condition. Gratifyingly, addition of 0.2 equivalent of weakly coordinating triphenylamine (Ph3 N) to the reaction system markedly improved the reaction, affording desired products in 84% yield (entry 9). Increasing the amount of Ph3 N to 1 equivalent led to slight enhancement of the yield (entry 10). However, the bases such as Et3 N and K2 CO3 that are much stronger than Ph3 N were ineffective to improve the yield (entries 11–13), presumably because strong bases reduced the reactivity of Pd catalyst toward decarboxylation step by coordinating more tightly to Pd, whereas Ph3 N binding strength to Pd was relatively weak. Although it is known that the oxidation of Ph3 N by Cu(II) salt led to formation of tetraphenylbenzidine and other products via Dalton Trans., 2010, 39, 11317–11321 | 11317

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Table 1 Optimization studies for coupling of 2,6-dimethoxybenzoic acid 1a with silyl enolates of methyl isobutyrate 2a

Entry

Pdb (mol %)

Silyl enolate

1e 2e 3 4 5 6g 7 8 9 10 11 12 13

Pd(TFA)2 (20) Pd(TFA)2 (20) Pd(TFA)2 (20) Pd(TFA)2 (20) Pd(TFA)2 (10) Pd(OAc)2 (10) Pd(OAc)2 (10) Pd(OAc)2 (10) Pd(OAc)2 (10) Pd(OAc)2 (10) Pd(OAc)2 (10) Pd(OAc)2 (10) Pd(OAc)2 (10)

2b 2b 2b 2b 2b 2b 2b 2a 2b 2b 2b 2b 2b

Additive(equiv)

Oxidant(equiv)

Isolated yield(%)c

3 : 4d

47 20 : 1, as determined by 1 H NMR. e Reaction conducted under nitrogen atmosphere. f 1 atm O2 . g Reaction run without MS.

a

radical cation step,20–21 we speculated that if Ph3 N participated in the catalytic process via a radical cation under our aerobic oxidative conditions, there would be two possibilities for Ph3 N: 1) conversion of Ph3 N into tetraphenylbenzidine or other compounds; 2) regeneration of Ph3 N from catalytic recycle. For the latter case, the radical cation formed from oxidation of Ph3 N could be trapped by adding nucleophile such as nBu4 NBr.21 However, we found that Ph3 N can be recovered unchanged from reaction system regardless of the presence or absence of silyl enolate of ester, and Ph3 N was isolated by column chromatography after reaction worked up. The isolated Ph3 N was characterized by 1 H NMR, 13 C NMR, mass spectrometry and measurement of its melting point. Therefore, the first possibility can be excluded. Further experiments showed that the reaction of Ph3 N with nBu4 NBr under our aerobic oxidative conditions did not take place, indicating that Ph3 N did not undergo oxidation to generate radical cation under our aerobic oxidative condition (see Electronic Supplementary Information†). Based on these observations, we tend to believe that Ph3 N would act as a very weak base for this reaction rather than an electron-transfer mediator for accelerating aerobic oxidation, and there would exist the interaction between Ph3 N and benzoic acids through hydrogen bond, which may well prevent silyl enolate of ester from decomposition caused by benzoic acids. We next examined the substrate scope of this reaction under the standard conditions of entry 9 in Table 1, and some adjustments were required to the reaction conditions in some cases due to the variations in reactivity toward decarboxylation of electrondeficient benzoic acids. As shown in Table 2, this protocol tolerated alkoxy, alkyl, amino, chloro, nitro and bromo substituents as well as substitution patterns on aromatic rings. Good yields were obtained with electron-rich aromatic carboxylic acids (entries 1– 8, Table 2), including a benzoic acid bearing amino and chloro substituents on the aromatic ring and a heterocyclic carboxylic acid such as 3-methyl-2-benzofuran carboxylic acid (entries 11 and 14), and moderate yields were obtained with less electronrich aromatic carboxylic acids (entries 9–10, 12–13 and 15), 11318 | Dalton Trans., 2010, 39, 11317–11321

the observed correlation between electronic nature of aromatic carboxylic acid and its reactivity reflected the issue associated with the decarboxylation step. Since Ag salts were effective catalysts for protodecarboxylation of a wide range of benzoic acids,11 and we observed that Pd-catalyzed decarboxylation occurred for electron-rich aromatic carboxylic acids, whereas decarboxylation of electron-deficient aromatic carboxylic acids resulted from the contribution of the silver salt in the decarboxylative coupling of benzoic acids with indoles.14 Thus, a stoichiometric amount of Ag2 CO3 was required to replace Ph3 N to promote the decarboxylative coupling of electron-deficient benzoic acid with silyl enol ester 2c, otherwise essentially no desired product could be obtained under the standard conditions (entries 12–13). Similarly to others and our observations,6,14,24 at least one ortho weakly coordinating substitutent on the aromatic ring of aromatic carboxylic acid was necessary for this reaction to occur, the reason was presumably that the ortho weakly coordinating substituent stabilized the transition structure of decarboxylation process by coordinating to palladium center, and thus enhanced the formation rate of aryl palladium intermediate. For example, 2-methoxy-4-methylbenzoic acid (enties 9–10) participated in this reaction while 4-methoxy2-methylbenzoic acid was recovered unchanged from reaction mixtures under the established conditions. Although most of reactions formed two products, two products can be clearly isolated by column chromatography. Di-substituted products have been applied to synthesis of substituted indenes.22 The protocol was not limited to 0.2 mmol scale reaction, which can also gave the same yield when the reaction on 1 mmol scale was run (entry 1, Table 2). Further studies were performed to gain some insights into the reaction mechanism. Firstly, 53% isolated yield of protodecarboxylation product was obtained when 2,6-dimethoxybenzoic acid 1a was conducted under our aerobic oxidative conditios in the absence of silyl enolate of ester 2 (Scheme 1), the observation supported that a catalytic amount of Pd(OAc)2 enabled to promote decarboxylation of aromatic carboxylic acid under aerobic conditions.23 This journal is © The Royal Society of Chemistry 2010

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Table 2 Pd-catalyzed decarboxylative coupling of benzoic acids with silyl enolate of ester 2a

Entry

Ar–COOH

Silyl enolate

3 + 4b

Isolated yield (%)c , d

1e

1a

2b

3ab + 4ab

84 (1 : 1.6)

2 3

1a 1b

2c 2b

3ac + 4ac 3bb + 4bb

65 (1 : 1.3) 80 (1 : 1.3)

4 5

1b 1c

2c 2b

3bc + 4bc 3cb + 4cb

75 (2 : 8 : 1) 81 (1 : 2.5)

6 7

1c 1d

2c 2b

3cc + 4cc 3db + 4db

85 (1 : 3.7) 86 (1 : 1.8)

8 9

1d 1e

2c 2b

3dc + 4dc 3eb + 4eb

77 (1 : 1.4) 36 (1 : 1 : 1)

10 11

1e 1f

2c 2c

3ec 3fc + 4fc

27 (27 : 0) 72 (1 : 1.4)

12f

1g

2c

3gc

35 (35 : 0)

13f

1h

2c

3hc

32 (32 : 0)

14

1i

2b

3ib + 4ib

63 (1 : 8)

15g

1j

2b

3jb + 4jb

37 (1 : 2.4)

˚ MS, 5% DMSO-DMF (2 mL), 1 atm O2 , 120 ◦ C, 24 h, except as Conditions: 1 (0.2 mmol), 2 (0.4 mmol), Pd(OAc)2 (10 mol%), Ph3 N (20 mol%), 4 A noted. b Average of two runs. c The E/Z ratio was > 20 : 1, as determined by 1 H NMR. d the value in parentheses referred to the ratio of 3/4. e 1 mmol 1a was used. f 3 equiv Ag2 CO3 instead of Ph3 N. g Reaction conducted at 90 ◦ C.

a


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Scheme 1 Protodecarboxylation of 2,6-dimethoxybenzoic acid 1a under aerobic oxidative conditions.

Secondly, the GC-MS analysis revealed that under dioxygen heating compound 2b with 10 mol% Pd(OAc)2 in 5% DMSODMF at 120 ◦ C for 4 h dominantly produced methyl methacrylate in 90% conversion and that the presence of Ph3 N decelerated the conversion of 2b into methyl methacrylate, therefore, we concluded that Ph3 N had no beneficial effect on accelerating aerobic oxidation of 2b to methyl methacrylate from this observation. The Pd-catalyzed aerobic oxidation of silyl enol ether into a,b-unsaturated carbonyl compound conducted in DMSO has previously been reported by Larock and co-workers,16 and the similar conversions have also been reported by Saegusa17 and Tsuji18 with other oxidants such as benzoquinone. Moreover, The reaction of compound 1a with methyl methacrylate in the presence of 10 mol% Pd(OAc)2 under dioxygen afforded decarboxylative Heck coupling products in 97% yield (Scheme 2).24

Scheme 2 Pd-catalyzed coupling of 2,6-dimethoxybenzoic acid 1a with methyl methacrylate.

Although the details of this protocol still remain elusive, these observations pointed to a possible mechanism that involved in situ generation of methyl methacrylate from the aerobic oxidation of silyl enolate of ester 2b, followed by decarboxylative olefination of aromatic carboxylic acid (Scheme 3). Initially, electrophilic Pd(II)

catalytic species reacted with aromatic carboxylic acid substrate to form intermediate A, whereafter intermediate A proceeded a decarboxylation to liberate CO2 and generate key reactive Ar–Pd(II) intermediate B. The olefin was generated from Pdcatalyzed aerobic oxidation of silyl enolate of ester 2b, which inserted into Ar–Pd(II) B to give Pd(II)-alkyl intermediate C. C underwent b-hydride elimination to produce the desired product and active Pd(0) species. Finally, peroxopalladium species D was generated from oxygenation of Pd(0), and D subsequently gave the active Pd(II) catalytic species via protonolysis to complete the catalytic cycle, whereas the concomitant formation of H2 O2 rapidly disproportionated into O2 and H2 O under this effective ˚ MS. On catalytic conditions,25 and H2 O was absorbed by 4 A the other hand, as for the entries 12 and 13 of Table 2, we believed that the decarboxylation of electron-deficient benzoic acids resulted from the contribution of Ag2 CO3 rather than Pd catalyst.11,14 Accordingly, formation of di-substituted product could be explained by the possibility that the decarboxylative Heck reaction was much faster than areobic oxidation of silyl enolate of ester 2b into methyl methacrylate, however, it’s frustrated to obtain di-substituted compound as the sole product by adding excessive aromatic carboxylatic acid (relative to silyl enolate of ester 2b) into reaction sytem. Unfortunately, Our attempt to obtain monosubstituted compound as the major product also failed by adding 1a into reaction system after oxidation reaction of silyl enolate of ester 2b was run for 4 h under aerobic conditions, and this onepot sequential method instead led to a decrease in the yield due in part to decomposition of palladium catalyst in the first step reaction. In conclusion, we have established a new protocol for Pdcatalyzed Heck-type coupling of aromatic carboxylic acid with silyl enolate of ester via the combination of sp3 b-C–H bond cleavage and decarboxylation by using dioxygen as the oxidant. Preliminary mechanistic studies disclosed that this reaction involved in situ generation of a,b-unsaturated ester from silyl enolate of ester under aerobic conditions, followed by decarboxylative Heck coupling. Efforts are currently underway to improve substrate

Scheme 3 Proposed catalytic cycle for aerobic oxidative decarboxylative olefination of aromatic carboxylic acid with silyl enolate of ester 2b.

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scope of this reaction further and investigate the mechanism more detailedly. This work was financially supported by the 973 Program (2006CB932903, 2009CB939803), NSFC (20821061, 20925102), “The Distinguished Oversea Scholar Project”, “One Hundred Talent Project” and Key Project from CAS.


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