Linkage isomerism leading to contrasting carboboration chemistry: Access to three constitutional isomers of a borylated phosphaalkene

We describe the reactivity of two linkage isomers of a boryl-phosphaethynolate, [B]OCP and [B]PCO (where [B] = N,N'-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaboryl), towards tris- (pentafluorophenyl)borane (BCF). These reactions afforded three constitutional isomers all of which contain a phosphaalkene core. [B]OCP reacts with BCF via a 1,2 carboboration reaction to afford a novel phosphaalkene, E-[B]O{(C6F5)2B}C=P(C6F5), which subsequently undergoes a rearrangement process involving migration of both the boryloxy and pentafluorophenyl substituents to afford Z-{(C6F5)2B}(C6F5)C=PO[B]. By contrast, [B]PCO undergoes a 1,3-carboboration process accompanied by migration of the N,N'-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaboryl to the carbon centre.