Bora-aromatic systems I. Competition between haloboration and phenylboration in the attempted synthesis of triphenylboracyclopropene

Abstract

A possible tactical approach to the synthesis of triphenylboracyclopropene (triphenylborirene) is described as involving: ( a) the addition of phenylboron dibromide to the triple bond of diphenylacetylene; and ( b) the interaction of the resulting ( cis-2-bromo-1,2-diphenylvinyl)phenylboron bromide with lithium metal in tetrahydrofuran at −77°. Factors determining the competition between BBr bond addition in step “a” and the slower BC 6H 5 bond addition to the triple bond are analyzed. By treatment of aliquots of the reaction product of steps “a” and “b” with alkyl halides, with carbon dioxide and with O-deuterioacetic acid, it is concluded that a stilbene derivative with vicinal cis-carbon-boron bonds has been formed. The nature of the organoborane precursor of cis-stilbene is discussed in terms of structural alternatives, such as triphenylborirene, hexaphenyl-1,4-diboracyclohexadiene or a polymer.