Are RR′C–PR′′(BH3)2 ‘electron poor’ phosphorus ylides?— an ab initio–NMR study

Abstract

The parent RR′C–PR′′(BH₃)₂, 3 (R, R′, R′′ = H), has no minimum geometry with ylide structure. In contrast to ‘normal’ ylides, RR′C–PR′′₃, which are destabilised when the substituents (R, R′) have π donor character, the investigated RR′C–PR′′(BH₃)₂ require at least one amino group to form a stable ylidic structure (e.g.4NH with R = NH₂, R′ = H, and R′′ = H). Without π donor substituents the molecules lack a tautomerisation barrier for the hydroborylation (RR′CPH(BH₃)–BH₂H′→(RR′H′C–PH(BH₃)BH₂). Compound 6, cyclo-1-(C–PH(BH₃)₂)-2,5-(NH)₂-3,4-(CH)₂, is a model for the ylide recently obtained by Arduengo. Analysis of the electronic structure confirms his suggestion that a considerable delocalisation from the carbene moiety into the ‘electron poor’ PR′′(BH₃)₂ group occurs. This electron delocalisation is reflected by the partial charge of the ylide group (−0.50 in 4HN, −0.51 in 4NN, and −0.85 in 6) which is negative (positive in ‘normal’ ylides). The term ‘inverse ylides’ could express this special bonding situation.