Suzuki enynes, stereodefined

A palladium(0)-catalysed cascade cyclization-Suzuki coupling reaction of various 1,6-enynes (72) with ArB(OH)2 has been developed as a new approach to the synthesis of stereodefined a-arylmethylene-y-butyrolactones, lactams, multifunctional tetrahy-drofurans, pyrrolidines, and cyclopentanes (73) (X = O, H2, Y = O, CH2, NR3). A 7r-allylpalladium intermediate and a chair-like transition state were suggested to account for the stereochemistry of this reaction.86... 

The reaction is extended to the synthesis of arylacetylenes and stereodefined enynes. The reactions sequence involves the preparation of thermally stable lithium complexes from B-OMe-9-BBN and alkyllithium in THE at -78 C. These complexes undergo Pd-catalyzed Suzuki coupling to both aromatic and olefinic substrates to produce a variety of alkynyl derivatives and enynes, with complete retention of the double bond geometry (Eq. 31.4) [4a]. 

Soderquist and coworkers [4] have prepared 9-oxa derivatives of B-OMe-9-BBN and submitted its lithium complex to Suzuki cross-coupling to a variety of aryl and vinylbromides. The yields of both arylacetylenes and stereodefined enynes (Eq. 31.5) match or exceed those obtained in the 9-BBN [4a] process (Table 31.7) [4bj. 

In 2003, Zhu and Zhang reported a palladium-catalyzed cyclization/arylation cascade reactions of enynes with arylboronic acids leading to cyclic products with a stereodefined exocyclic double bond [44] (Scheme 6.26). Several types of enynes can be anployed carbon, oxygen, and nitrogen tethers [Y=C(COjMe)j, O, Ts], and alkyl- and arylalkynes are well tolerated. The author proposed a plausible mechanism, probably involving a Jt-allylpalladium complex which is formed from the aUyUc halide, followed by insertion of the alkynes and Suzuki coupling reactions. 

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