Negishi coupling alkenylzinc reagents

An alternative preparation of stereodeflned cw-a,/3-substituted alkenylzinc reagents involves transmetallation of the corresponding trialkenylboranes or alkenyl tellurides with diethylzincJ f The alkenylzincs thus obtained undergo Negishi coupling with aryl iodides in good yields (Scheme 31). 

The range of nucleophiles can be extended to alkenylzinc reagents. These species are subjected to alkyl chlorides, bromides, or iodides in an iron-catalyzed Negishi coupling to efficiently form olefins (Scheme 4-235). Iron(III) chloride fiinctions as a precatalyst in the presence of TMEDA. ... 

Scheme 4-235. Negishi coupling of alkyl halides with alkenylzinc reagents catalyzed by FeCb/TMEDA.

Synthetic applications of the alkenyl-alkenyl Negishi cross-coupling are intimately related to the preparation method of the alkenylzinc nucleophile. Reagents obtained by Zn(0) insertion from the corresponding halide, or by a halogen-hthium or a tin-lithium exchange/transmetallation sequence, are weU suited, as illustrated with the preparation of compounds 202 [170] and 205 (Scheme 4.46) [46]. Further evidence for the applicability of this method is its use in recent total syntheses of natural products, such as xerulinic acid [171], 6,7-dehydrostipiamide [172], epolactaene [173], and the side chains of mycolactones A and B [174]. 

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