Alkenylzinc reagents cross-coupling

The coupling of aryl- and alkenylzinc reagents with various halides has widespread use in the cross-coupling of aromatic rings [101]. The reactions of zinc derivatives of aromatic and heteroaromatic compounds with aryl and heterocyclic halides have wide synthetic applications [103,104],... 

At present, (Z)-/3-substituted alkenylmetals containing other metals are less readily accessible than those mentioned above, even though the Zr-catalyzed carboalumination of ethyne has been shown to produce (Z)-/S-substituted alkenylalanesJ They are generally prepared via metallation-transmetallation of (Z)-/S-substimted hahdes. Despite this drawback, (Z)-/3-substituted alkenylzincs generated by this procedure have been shown to be superior reagents in the subsequent Pd-catalyzed cross-coupling reactiont (Table 11). 

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]. 

Scheme 4.52 a-Selective Pd-catalyzed cross-coupling of alkenylzinc reagents with allylic halides [190]. 

Scheme 3.7 Cross-couplings of a-bromo nitriles with aryl- and alkenylzinc reagents.

C generates in situ the corresponding alkenylzinc reagent 9. Its Pd-catalyzed cross-coupling reaction provides an alternate and complementary route to conjugated enynes and oligoenynes. Thus, for example, a difunctional trienyne 10 suitable for the synthesis of 6,7-dehydrostipiamide (11) has been synthesized from... 

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