Transmetallation boron-zinc exchange reactions

An example for the use of the boron-zinc exchange reaction for copper-mediated SN2 -substitutions of allylic electrophiles is the hydroboration of nitroolefin 130 with diethylborane, followed by successive transmetallation of the borane 131 with diethylzinc and CuCN-2LiCl, and final trapping with allyl bromide to give the product 133 with 83% yield over four steps (Scheme 34).34,34a This transformation again demonstrates the tolerance of the method towards functional groups and acidic hydrogen atoms. 

Preparation of alkenylzinc reagents by boron-zinc transmetalation from stable organoboronic esters and dialkylzinc is hampered by the instability of the evolved alkenylzinc mixtures of the two reagents turned black and the reaction of the evolved alkenylzinc on remaining pinacol alkenylboronic ester was speculated on the basis of the structure of by-products. Conversely, this boron to zinc exchange can lead to good results if the evolved alkenylzinc is trapped in situ by an electrophile. The reaction of an alkenylboronic ester... 

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