Ketones, conjugated reaction with borane

Reactions of chiral allylic boranes with carbonyl compounds Reactions of chiral allyl boranes with imines Asymmetric Addition of Carbon Nucleophiles to Ketones Addition of alkyl lithiums to ketones Asymmetric epoxidation with chiral sulfur ylids Asymmetric Nucleophilic Attack by Chiral Alcohols Deracemisation of arylpropionic acids Deracemisation of a-halo acids Asymmetric Conjugate Addition of Nitrogen Nucleophiles An asymmetric synthesis of thienamycin Asymmetric Protonation... 

The conjugated cisoid ketones react satisfactorily with B-alkenyls-9-BBN, whereas transoid conjugated ketones, like cyclopentenone and cyclohexenone, afford a complex mixture of products. The reaction proceeds thermally and does not appear to involve radicals. The alkenyl group of borane is transformed with strict retention of configuration. As the reaction occurs only with cisoid enones, and thus suggesting a cyclic transition state (Chart 7.8) [10]. 

Brown proposed a mechanism where the enolate radical resulting from the radical addition reacts with the trialkylborane to give a boron enolate and a new alkyl radical that can propagate the chain (Scheme 24) [61]. The formation of the intermediate boron enolate was confirmed by H NMR spectroscopy [66,67]. The role of water present in the system is to hydrolyze the boron enolate and to prevent its degradation by undesired free-radical processes. This hydrolysis step is essential when alkynones [68] and acrylonitrile [58] are used as radical traps since the resulting allenes or keteneimines respectively, react readily with radical species. Maillard and Walton have shown by nB NMR, ll NMR und IR spectroscopy, that tri-ethylborane does complex methyl vinyl ketone, acrolein and 3-methylbut-3-en-2-one. They proposed that the reaction of triethylborane with these traps involves complexation of the trap by the Lewis acidic borane prior to conjugate addition [69]. 

An interesting variation in this reaction combined an intermolecular Mukaiyama aldol followed by an intramolecular reaction (a domino Mukaiyama aldol) that gave cyclic conjugated ketone products.Borane derivatives such as C=C—OB(NMe2)2 react with aldehydes to give p-amino ketones. ... 

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