Dehydroboration process

Other Ketones. Ketones such as acetophenone are reduced rather slowly by THF solutions of Alpine-Borane (eq 9). A competing dehydroboration process leads to reduction via 9-BBN (eq 10). 

However, with slower-reacting and sterically hindered carbonyl compounds, the organoborane reduction may take an alternate pathway involving a prior dehydroboration process (Eqs. 4.18, 4.19) [11]. 

At 65 °C, Alpine-Borane undergoes 50% dehydroboration in 500 min. At rt there is approximately 1-2% dehydroboration per day. Running the reaction neat increases the rate of the favorable bimolecular reduction. Alternatively, high pressure may be used to increase the rate of the bimolecular process and retard the rate of the dehydroboration reaction (eq 11 ). ... 

Isomerization of allylboranes proceeds not by a dehydroboration-rehydroboration mechanism that would involve the intermediacy of allenes, but by an allylic rearrangement or 1,3-metallotropy that is a symmetry-allowed concerted process. The probable HOMO-LUMO interactions suggested for the 1,3-metallotropy are shown in formulas XXI and XXH. 

Similar to the reduction of aldehydes, reduction of ketones with Alpine-Borane also involves two competing reaction pathways, a bimolecular -hydride elimination process (cyclic mechanism) affording optically active product [6], and a dehydroboration-reduction sequence yielding racemic product [2] (Scheme 26.1). 

B-alkyl-9-borabicyclo[3.3.1]nonanes undergo olefin-alkyl group exchange when refluxed with an olefin in THF. Kinetic and competition studies support a dehydroboration-hydroboration process rather than a concerted mechanism. Ab initio M.O. calculations show that the reaction between C2H4 and BH3 proceeds through a two-step process. A loose three-center (C—B—C) tt complex is formed which is then transformed into the product via a four-center transition state in a rate-determining step. 

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