Selenides, aryl coupling reactions

In this concluding section, we will recall that a-selanylvinylmetal derivatives can be produced by a-deprotonation of vinylic aryl selenides 11, cleavage of ketene selenoacetals 12 or bromine/lithium exchange of a-bromovinyl selenides 13 (Scheme 49). These strategies involve vinylic selenides whose syntheses are not simple. More recent works have used organocuprate additions, hydrometala-tions (M = Sn, Zr) or hydroborations of readily available acetylenic selenides 14 (Scheme 49). The process can be regio- and stereocontrolled and produce the a-selanylvinylmetal derivatives under mild conditions. The latter can be considered as vinyl dianion equivalents and are very useful intermediates for cross-coupling reactions. 

Hydroboration of acetylenic selenides with 9-BBN led to the regio- and stereoselective formation of a-selanylalkenyl boranes which were then converted into Z-a, -disubstituted vinyl selenides by cross-coupling reaction with aryl bromides [80] (Scheme 58). With unsubstituted acetylenic selenides, an inversion of regioselectivity during the hydrozirconation was observed [81,82]. 

Among the applications of allyl silanes reported this year their reaction with phenylselenyl chloride and subsequent oxidation of the resulting allyl selenides to allylic alcoholshas been used in the sequence outlined in Scheme IS. 3-Bromoallyltrimethylsilane therefore behaves as a hydroxypropenyl synthon. Nitroalkenes react with allylsilanes to give, after hydrolysis, y,S-unsaturated ketones, and a synthesis of substituted cyclopentenones based on this procedure has been described.Allyl trimethylsilyl ethers undergo a palladium-promoted coupling with aryl iodides to give /3-aryl-a,/3-unsaturated ketones, and palladium also catalyses the reaction of various aryl bromides with trimethylsilyl acetylene to produce ethynylated aromatics. 

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