Allenylzinc, formation

Reaction of the transient zinc intermediates with various electrophiles yielded the acetylenic substitution products and only minor amounts of allenes (Table 9.49). Reactions with aldehydes were non-selective, affording mixtures of stereo- and regioisomeric adducts. However, prior addition of ZnCl2 resulted in the formation of the homopropargylic alcohol adducts with high preference for the anti adduct, as would be expected for an allenylzinc chloride intermediate (Table 9.50). 

In the reaction.s of allenylindium and allenylzinc reagents 392 and 391 derived from (R)-3S9, formation of the anti homopropargylic alcohol 393 is favored and highest levels of diastereoselectivity are obtained with branched aldehydes (e.g. cyclohexane carboxaldehyde. Table 11-22). The anti stereochemistry of adducts 393 indicates that these reactions occur through cyclic transitions state in a manner similar to the reactions of the trichloroallenylstannane reagent 226 (see above, Scheme 11-27). 

Such Pd-Zn interactions were also noticed by Alvarez et at [40] in the formation of nucleophilic allenylzincs from allenylpalladium complexes, and by Oigan et al. [30] in the alkyl-alkyl Negishi coupling catalyzed by (MfC)-Pd complexes. 

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