Organometallic Aspects of Diboron Chemistry

Abstract

Formally subvalent compounds of boron (B) containing a B–B single bond are intermediate in structural complexity between simple monoboron derivatives and the polyhedral electron-efficient compounds of the element. Synthesis of specific diboron compounds may involve either reductive coupling reactions of monoboron derivatives to form materials containing the B–B bond or reactions of compounds possessing preformed B2 fragments. An extensive metathetical chemistry of diboron compounds has been developed that provides the means for the synthesis of specific derivatives from more commonly available starting materials such as tetrachlorodiborane and tetrakis (dialkylamino) diboron derivatives. A little physical information is available concerning diboron compounds with organic groups bonded to B. Diboron derivatives differ markedly in thermal stability and the mode of decomposition. One of the most interesting and characteristic reactions of the diboron tetrahalides is their ready addition across carbon–carbon (C–C) double and triple bonds. The stereochemical course of the addition of B2Cl4 to olefins and acetylenes has been studied by several groups. Relatively little is known of the reactions of diboron tetrahalides with aromatics apart from a study by Fox and Wartik of the interaction of B2C14 with benzene and naphthalene. There is no clear evidence for the formation of stable bis(boryl) derivatives from the interaction of diboron tetrahalides with molecules containing multiple bonds other than C=C or C–C. The addition of diboron tetrahalides to olefins provides a convenient route to vicinal bis(dihaloboryl) alkanes, which in general show most of the reactions to be expected of organoboron dihalides.