Propargylzinc reagents

Scheme 4.53 Pd-catalyzed cross-coupling of allenyl-/propargylzinc reagents [193].

Organozinc reagents are insuffieiently reactive to react with carbonyl derivatives and double bonds, exeept allyl and propargylzinc reagents that react with carbonyl derivatives, nitriles and alkynes. 

The organozinc reagents 123, which were generated from 1-phenyl-l-alkyne 122 with nBuLi in the presence of 1.5 mol% of HgCl2 followed by addition of 1.5 equiv. of ZnBr2, exists as a tautomeric mixture of an allenylzinc and a propargylzinc species (Scheme 3.62) [103]. 

Interestingly, a second allenyl adduct, isomeric with the major product, was also formed in the latter reaction. This isomer was surmised to arise by a retro reaction of the initial allenyl adduct to form a TMS propargylzinc bromide reagent, which subsequently adds to the liberated aldehyde (equation 12). The regiochemistry of this addition is also thought to be directed by steric effects, again involving the methyl and TMS substituents. 

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