Metal transmetallation step isolation

The proposed mechanism involves the regioselective c/s-hydroboration of the 4-bromo-l,3-enyne as observed by Zweifel [2] followed by addition of hydride, originating from t-BuLi [3], to initiate a 1,2-metalate rearrangement forging the C-C bond with inversion at the vinylic center (Scheme 2). This key step enables the stereospecific character of the whole process. To circumvent the low reactivity of (Z)-vinylborane toward aldehydes, the corresponding (Z)-vinylzinc was prepared by transmetalation with diethylzinc and reacted successfully with carboxaldehyde. The isolation of aUylic alcohols in high yields was subordinated to a careful selection of the... 

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