Selective Reductive Displacement of Alkyl Halides and Sulfonate Esters with Cyanoborohydride Reagents in Hexamethylphosphoramide

Abstract

The combination of sodium or tetrabutylammonium cyanoborohydride, sodium or potassium 9-cyano-9-hydrido-9-borabicyclo[3.3.1]nonane (9-BBNCN), or polymeric cyanoborane in hexamethylphosphoramide furnishes a convenient, efficient, and mild system for the reduction of alkyl halides and sulfonate esters. The reagents are exceptionally selective in that most other functional groups including ester, carboxylic acid, amido, cyano, alkene, nitro, sulfone, ketone, aldehyde, and epoxide are essentially inert under the reaction conditions and thus the reductive procedure is attractive for synthetic schemes where minimum damage to other sensitive portions of the molecule is demanded. The displacement by hydride occurs predominantly with inversion of configuration and, in general, the leaving ability pattern follows the order I > Cl ≫ F, as expected for an Sn2 process. The method is less successful for vinylic, aromatic, and certain tertiary halides. © 1977, American Chemical Society. All rights reserved.