Metal-Catalyzed Formation and Transformations of Carbon-Boron Bonds

Abstract

Our research seeks new methods for functionalizing organic small molecules using organoboronic derivatives as a versatile handle for late-stage manipulations. Metal-catalyzed formation of new carbon-boron bonds and their subsequent transformations are highlighted. Among the myriad of unsaturated substrates for conducting borylation reactions, allenes have received minimal attention. These substrates are uniquely advantageous given that diboration results in the formation of both allylic and vinylic boronates. Orthogonal reactivity of the sp2 and the sp3 C-B bonds can allow for chemoselective transformations. However, oxidation of the carbon-boron bond is one example in which the conditions are unselective. To address this shortcoming, a platinum catalyst was developed for the diboration of 1,1-diaryl allenes with a differentially protected diboron reagent, pinB-Bdan. The reaction proceeds regioselectively in high yields to furnish olefins bearing a vinylic Bpin and an allylic Bdan moiety. The subsequent chemoselective transformation of each boron center was demonstrated. Methods for preparing 1,8-diaminonaphthalene protected vinylboronates conjugated to carbonyl groups are severely limited. A simple and efficient protocol was developed for carrying out an environmentally friendly copper(II)-catalyzed beta-borylation of alkynoates and alkynamides in water and open-to-air. Following the discriminative activation of the more Lewis acidic pinacol protected boron center in pinB-Bdan, a regio-, stereo- and chemoselective beta-borylation of acetylenic substrates delivers (Z)-beta-boryl enoates and primary, secondary, and tertiary enamides under very mild conditions. As an inexpensive and earth abundant metal, catalysts based on copper are highly desirable. An international collaborative project to develop a copper-catalyzed cross-coupling reaction of beta-boryl carbonyl compounds was explored. Preliminary results found these substrates to be either unstable towards or unreactive under the reactions conditions screened.