Pii: S0040-4039(02)00328-3

Palladium-catalyzed homocoupling reaction of aryl boronic acids has been developed using a protocol similar to the well-documented crosscoupling reaction. -Halocarbonyl compounds are applied to initiate the reaction via oxidative addition to a palladium(0) species. The resulting palladium enolate halide can promote the double transmetallation. Reductive elimination generates the desire homocoupling product. © 2002 Elsevier Science Ltd. All rights reserved. Biaryls form the central core in a large number of natural products and is an important pharmacophore in a variety of biologically active compounds. Pdand Ni-catalyzed crosscoupling reactions have been developed to construct the biaryl compounds. Compared with the enormous literature on crosscoupling reactions, there have been fewer studies carried out on Pd-catalyzed homocoupling reactions. Generally, crosscoupling reactions are initiated by oxidative addition, followed by transmetallation, and reductive elimination to give the coupling products (Scheme 1, path a). If a homocoupling reaction is expected to proceed in a similar manner, a second transmetallation between R–M and intermediate I for forming intermediate II is required (Scheme 1, path b). To our knowledge, there are no reports that the intermediate R–M–X generated by oxidative addition of RX to M can undergo double transmetallation to form intermediate II, which can produce the homocoupling product via reductive elimination. In the course of studying asymmetric -arylation of ketones or esters, we serendipitously found that -halocarbonyl compounds can serve as this type of reagent. The -halocarbonyl compounds can undergo oxidative addition with a palladium(0) species and mediate the aforementioned double transmetallation. Homocoupling products are generated in this process. Herein, we would like to present our results in this area. The -arylation of ketones has been explored by the Hartwig and Buchwald groups, and its asymmetric version has also been achieved by Buchwald et al. However, because the strong basic conditions are necessary for the formation of palladium enolate, the reaction is limited to the formation of chiral quaternary carbon centers. Racemization of tertiary carbon centers adjacent to a carbonyl can occur under the strong basic conditions. In order to overcome this difficulty, we envision that -halocarbonyl compounds can undergo oxidative addition to a palladium(0) species and a palladium enolate can be formed without use of a base. Transmetallation of the palladium enolate with an organometallic compound and reductive elimination can produce the -arylation product. Under this design, we examined a palladium-catalyzed reaction between methyl -bromophenylacetate 1 and 3,5-dimethyl Scheme 1. * Corresponding author. 0040-4039/02/$ see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII: S0040 -4039 (02 )00328 -3