Enolboration 2. Dicyclohexylchloroborane/Triethylamine as a Convenient Reagent

摘要

A smooth, rapid, regio- and stereoselective enolboration of representativeclasses of ketones is achieved with dicyclohexylchloroborane in the presence of triethylamine in simple solvents such as methylene chloride, diethyl ether, carbon tetrachloride and hexane. All classes of ketones, methyl, ethyl, alpha, beta-unsaturated, cyclic, bicyclic, heterocyclic, and aromatic ketones, except the sterically hindered isopropyl ketones, and related ketones containing two sec-alkyl groups attached to the carbonyl group, undergo enolboration successfully. The enolborinates, generated instantaneously with concurrent formation and precipitation of Et3NHCl, react readily with aldehydes at temperatures as low as -78 C, comparable to the behavior of the organoboron triflates previously utilized for such enolborations. The remarkable reactivity, regioselectivity, stereoselectivity, ease of preparation and handling, and the greater stability of dicyclohexylchloro-borane, all make it a preferred reagent for enolboration.