Monohydroboration allenes

Monohydroboration of allenes with 9-BBN places the boron atom exclusively at the terminal position. Thus, 3-methyl-l,2-butadiene gives B-3-methyl-2-butenyl-9-BBN as the only product (Eq. 43)96). However, such a procedure for the preparation of allylic boranes by the hydroboration of conjugated dienes and allenes is not a general... 

Hydroboration ofallenes Monohydroboration of cyclic allenes with catecholborane (1) followed by oxidation results in a single ketone arising from regiospecific attack of boron at the central carbon atom of the allene (equation I). The reaction with diborane or disiamylborane is not selective. 

The reactions of simple, acyclic allenes with borane-THF are usually complicated. However, cyclic al-lenes give cleaner reactions, which can be controlled to give monohydroboration products, involving predominant attack of boron on the central carbon atom, though the intermediate vinylboranes have not generally been isolated. 

The monohydroboration of internal allenes with 9-BBN affords the corresponding allylboranes [1], Unlike the normal allylboranes, oxidation of silylallylbo-ranes with alkaline hydrogen peroxide produces [2] the E- and Z-geometric isomers [3, 4] predominating in E isomers (R = CH, -Bu, i-Pr) of allyl alcohols. Increase in size of the R group, however, results in the reversal of selectivity (Chart 6.11) [4],... 

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