Hydroboration. 86. Convenient conversion of aldehydes and ketones into the corresponding alkenes via hydroboration of their enamines. A remarkably simple synthesis of either (Z)- or (E)-alkenes

Abstract

Aldehydes and ketones are converted into the corresponding alkenes via hydroboration of their enamines. Hydroboration of aldehyde enamines by 9-borabicyclo [3.3.1] nonane (9-BBN), followed by methanolysis, affords the corresponding terminal alkenes in 75-90% yields. Unsaturated aldehyde enamines produce the corresponding dienes under these conditions. Enamines derived from substituted cyclic ketones and heterocyclic ketones are readily accommodated in this reaction to afford the corresponding alkenes in very good yields. The synthesis of pure (Z)- or (E)-alkenes is readily achieved from the same acyclic ketone enamine by modification of the hydroboration-elimination procedure: (A) hydroboration by 9-BBN followed by methanolysis or (B) hydroboration by borane methyl sulfide (BMS) followed by methanolysis and hydrogen peroxide oxidation. Mechanistic rationale is provided