Article pubs.acs.org/Organometallics
Rhodium-Catalyzed Cross-Coupling Reactions of Carboxylate and Organoboron Compounds via Chelation-Assisted C−C Bond Activation Jingjing Wang, Bowen Liu, Haitao Zhao,* and Jianhui Wang* Department of Chemistry, College of Science, Tianjin University, Tianjin 300072, China S Supporting Information *
ABSTRACT: A new rhodium-catalyzed decarbonylated coupling reaction of ethyl benzo[h]quinoline-10-carboxylate and organoboron compounds that occurs through chelation-assisted sp2 C− COOEt bond activation was described. In this system CuCl played a very important role, and a five-membered rhodacycle was also involved as a key intermediate. Various functionalities were compatible in the reaction, and the desired products were obtained in good to excellent yields. DFT calculations on the mechanisms of this reaction using a Rh(I) model catalyst have also been carried out.
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through chelation-assisted sp3 C−COOEt activation.12 However, activation of an sp2 C−COOEt of this type remains a contemporary challenge since the sp2 C−COOEt bond is known to be stronger than the sp3 C−COOEt bond. Considering the ubiquity of esters in organic molecules and the inherently neutral conditions of the decarboxylative reaction, developing cross-coupling reactions that use esters as the counterpart is very attractive. Since benzo[h]quinoline is a good reagent for metal-directed reactions, many research groups have developed its coupling reactions with haloarenes,13 arylenes,14 substituted benzoyl peroxides,15c substituted benzoyl chlorides,15a substituted benzoic anhydrides,15d and phenylmagnesium bromides15b,e to obtain 10-substituted benzo[h]quinoline derivatives through catalyzed C−H bond activations (Scheme 1). Herein, we describe a new rhodium-catalyzed decarbonylated coupling reaction of benzo[h]quinoline-10-carboxylic acid ethyl ester and arylboronic acid that occurs through chelation-assisted sp2 C−COOEt bond activation (Scheme 1).
INTRODUCTION Carbon−carbon bond cleavage reactions catalyzed by transition metals have attracted much attention in recent years1 since they allow chemists to build complex molecules through nontraditional retrosynthetic disconnections.2 Traditionally, C−C bond cleavage has been achieved by the oxidative cyclometalation of ring-strained three- or four-membered-ring compounds to form more stable metallacyclic complexes.3 In these reactions, the released energy helps to overcome the high kinetic barriers in the activation step. Directing a metal to a particular C−C bond using functional groups is another strategy that is commonly used for unstrained C−C bond activation.4 Many important reaction models based on chelation-assisted C−C activation have also been developed recently.5 The discovery of these C− C activation reactions has greatly expanded the methods available for the construction of complex molecules. A wide variety of efficient methods based on C−C bond activations is highly desirable in order to facilitate the use of unstrained C−C σ bonds as versatile synthons. This is currently an area of intense focus. The intermediates that form after the C−C bond cleavage of carbon−nitrile (C−CN) bonds,6 the C−C bond cleavage of tertiary alcohols (C−CR2OH),7 the decarbonylation of aldehydes (C−CHO),8 and the decarboxylation of acids (C− COOH)9 and anhydrides (C−COOCOR)10 are important synthons for the construction of new compounds. These in situ generated intermediates have been used for transition-metalcatalyzed C−C bond formation reactions using arylboronic acids,6e,9b,e,10 haloarenes,7a−d and arylsilanes.6a However, reports on cross-coupling reactions using metal intermediates generated via the decarboxylation of the carbon carbonyl group (C−COOR) of an ester are few.11 Some active esters, for example, 4-nitrophenyl esters11b and isopropenyl esters, have been used in decarbonylative Heck reactions.11c When there is a directing group located at an appropriate position, an sp3 C− COOEt undergoes a coupling reaction with arylboronic acid © 2012 American Chemical Society
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RESULTS AND DISCUSSION In order to probe the possibility of a transition-metal-catalyzed decarbonylated coupling reaction via sp2 C−COOEt bond activation, ethyl benzo[h]quinoline-10-carboxylate (1a) and phenylboronic acid (2a) were chosen as a model system, and the catalyst activity in the presence of different transition metal catalysts under various reaction conditions was examined. The results are summarized in Table 1. Since rhodium complexes are known to activate C−C bonds through the directing group, rhodium complexes were tried first. However, when RhCl3·3H2O, Rh2(COD)2Cl2, or (PPh3)3RhCl was used as the catalyst, the cross-coupling reactions of 1a and 2a Received: October 22, 2012 Published: December 12, 2012 8598
dx.doi.org/10.1021/om300994j | Organometallics 2012, 31, 8598−8607
Organometallics
Article
Scheme 1. Cross-Coupling Reactions through C−H and C−C Activations
improved the yield of the reaction to 68% in 30 h (Table 1, entry 5). When 0.5 or 1.0 equiv of CuBr was used instead of CuI, the reaction of 1a and 2a gave the product 3aa in 60% and 87% yields in 30 h, respectively. CuCl was also found to accelerate the reaction (Table 1, entries 8 and 9). The highest yield (97%) occurred for the reaction using (PPh3)3RhCl as the catalyst with 1.0 equiv of CuCl as the additive (Table 1, entry 9). Other compounds that are often used as additives in the cross-coupling reactions of arylboronic acids, such as Cu(OAc)2·H2O, (NH4)2S2O8, K2S2O8, and Ag2O, were found to prevent the desired reaction (Table 1, entries 11−14). Some other transition metal complexes such as Ni and Pd complexes were also tested for the activation of the sp2 C−COOEt bond, but failed to give any of the desired product (Table 1, entries 15−18) under similar reaction conditions. It should be noted that the cross-coupling reaction of 1a and 2a did not proceed when only CuCl was used as the catalyst. Thus, (PPh3)3RhCl as the catalyst in combination with CuCl (1.0 equiv) was selected as the optimal catalytic system for the current transformation. With the optimized catalytic system in hand, the effects of various arylboronic acids on this cross-coupling reaction were investigated, and the results are shown in Table 2. The reaction of 1a with phenylboronic acid or sodium tetraphenylborate proceeded smoothly to give the corresponding products 3aa in 95% or 91% yield, respectively (3aa, Table 2). Other esters, such as phenyl benzo[h]quinoline-10-carboxylate 1h also reacted with phenylboronic acid to give the desired product 3aa in 81% yield. Phenylboronic acids with a methyl group at the para- or meta-positions also showed good reactivity with 1a, giving the desired coupling product in 75% or 72% yields, respectively (3ac, 3ad, Table 2). However, when there was a methyl group at the ortho-position, the yield of the reaction of the substituted phenylboronic acid with 1a decreased dramatically to 38% (3ab, Table 2). These results suggest that the steric effect of the arylboronic acid is a key factor for controlling reactivity. Unsurprisingly, 3,5-dimethylphenylboronic acid also reacted with 1a to give the desired product 3ae in 81% yield. Phenylboronic acids with a MeO group at the meta- or parapositions reacted smoothly with 1a to give the desired coupling
Table 1. Optimization of the Reaction Conditions for CrossCoupling of Phenylboronic Acid and 1aa
entry
catalyst
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
RhCl3·3H2O Rh2(COD)2C12 (PPh3)3RhCl (PPh3)3RhCl (PPh3)3RhCl (PPh3)3RhCl (PPh3)3RhCl (PPh3)3RhCl (PPh3)3RhCl (PPh3)3RhCl (PPh3)3RhCl (PPh3)3RhCl (PPh3)3RhCl (PPh3)3RhCl NiCl2(dppe) Pd(OAc)2 Pd(OAc)2 Pd(PPh3)4
additive (equiv)
time (h)
yield (%)a,b
Cul (0.5) Cul (1.0) CuBr (0.5) CuBr (1.0) CuCl (0.5) CuCl (1.0) Cul (1.0) Cu(OAc)2 (1.0) (NH4)2S2O8 (1.0) K2S2O8 (1.0) Ag2O (1.0) CuCl (1.0) CuCl (1.0) CuCl (1.0) CuCl (1.0) CuCl (1.0)
48 48 48 30 30 30 30 30 30 48 48 48 48 48 48 48 48 48 48
15 31 40 50 68 60 87 76 97 95
