Oxidation of Catecholboron Enolates with TEMPO

摘要

The persistent nitroxide radical 2,2,6,6-tetramethylpiperi-dine-N-oxyl (TEMPO) has found widespread application in organic synthesis. Along with its use as a catalyst for the oxidation of alcohols, TEMPO has been shown to oxidize various organometallic compounds leading to efficient C-H arylation and homocoupling reactions. However, alkyl organometallic species of Li, Mg, Zn, Cu, Sm, and Ti do not provide the corresponding homocoupling products upon exposure to TEMPO. In these processes, one equivalent of the nitroxide is used to oxidize the organometallic species to the corresponding C-centered radical, which is then trapped by a second equivalent of TEMPO to give the alkoxyamine. Along these lines, we have shown that B-alkylcatecholbor-anes 1 react similarly: formal homolytic substitution at boron in 1 with TEMPO leads to the boric ester 2 and the corresponding C-centered radical, which is irreversibly trapped by TEMPO (Scheme 1).