ENOLBORATION .1. DICYCLOHEXYLCHLOROBORANE TRIETHYLAMINE AS A CONVENIENT REAGENT FOR ENOLBORATION OF KETONES AND OTHER CARBONYL DERIVATIVES

A facile enolization of ketones and other carbonyl derivatives is achieved with dicyclohexylchloroborane in the presence of triethylamine in simple solvents such as methylene chloride, ethyl ether, carbon tetrachloride, and hexane. A number of R2BCl reagents have been examined using 2-butanone and 3-pentanone as model ketones to explore their effect on the regioselectivity and reactivity of enolization: (1) B-chloro-9-borabicyclo[3.3.1]nonane (B-Cl-9-BBN); (2) bis(exo-norbornyl)chloroborane (exo-Nrb2BCl); (3) dicyclooctylchloroborane (Coc2BCl); (4) dicyclohexylchloroborane (Chx2BCl); (5) disiamylchloroborane (Sia2BCl); and (6) bis(2,5-dimethylcyclohexyl)chloroborane (2,5-Me2Chx2BCl). Reagents 2-6 achieve rapid, regioselective, and quantitative enolization for the unhindered methyl ketone, 2-butanone. Reagent 1 achieves enolization but fails to give a regioselective product. On the other hand, reagents 1-4 achieve quantitative enolization for the more hindered diethyl ketone. However, the more hindered reagents, 5 and 6, achieve only very slow enolization of this ketone. Consequently, the moderately sterically hindered reagent 4, Chx2BCl, was screened for the enolization of both simple ketones and many other carbonyl derivatives, such as aldehydes, carboxylic acids, anhydrides, acid chlorides, esters, tertiary amides, and thioesters. Finally, a bifunctional derivative, a keto ester, was also examined. It was observed that with the exception of acid chlorides, esters, and amides, all of these classes of carbonyl compounds were easily and rapidly converted into enolborinates in greater-than-or-equal-to 94% conversion with concurrent formation and precipitation of Et3NHCl. The visual observation of the formation of Et3NHCl as a white precipitate as the enolization progresses is an added advantage for this new reagent providing a convenient guide to the course of the reaction, in contrast to the behavior of Et3NHOTf, which does not precipitate. These enolborinates react readily with aldehydes at temperatures as low as -78-degrees-C, comparable to the reaction with aldehydes of the enolborinates produced by the organoboron triflates previously introduced and used in organic synthesis. The impressive regioselectivity and reactivity of Chx2BCl, together with its greater stability and ease of formation and handling, indicate it to be the reagent of choice for enolboration. Consequently, this reagent was emphasized in this exploratory study.