Synthesis of Phosphorus Ylides Bearing a PH Bond from a Kinetically Stabilized 1,3,6‐Triphosphafulvene

In mixing 2,4,6‐tris(2,4,6‐tri‐ tert ‐butylphenyl)‐1,3,6‐triphosphafulvene with alkyllithium compounds and acetic acid, both of nucleophilic alkylation and electrophilic protonation occurred at the exo sp 2 ‐phosphorus atoms to afford [2,4‐bis(2,4,6‐tri‐ tert ‐butylphenyl)‐1,3‐diphosphacyclopentadienylidene](alkyl)(2,4,6‐tri‐ tert ‐butylphenyl)phosphoranes which are phosphorus ylides that bear a PH bond. A phosphorus ylide bearing both PH and PF bonds was obtained by reaction of 2,4,6‐tris(2,4,6‐tri‐ tert ‐butylphenyl)‐1,3,6‐triphosphafulvene with hydrogen tetrafluoroborate, and the structure was determined by X‐ray crystallography. Both PC double bond and P + C − zwitterionic character was indicated by the metric parameters. The isolated phosphorus ylide bearing a PH bond, [2,4‐bis(2,4,6‐tri‐ tert ‐butylphenyl)‐1,3‐diphosphacyclopentadienylidene](2,4,6‐tri‐ tert ‐butylphenyl)phosphorane, showed no isomerization by H‐migration to the corresponding phosphinodiphospholes, probably due to the π‐accepting ability of the unsaturated PC bonds and aromaticity of the C 3 P 2 ring. The ylide structure and aromaticity of 2,4‐diphosphacyclopenta‐2,4‐dienylidenephosphorane was characterized by theoretical calculations. In addition, the regioselective protonation of the lithiated phosphinodiphospholes generated from the 1,3,6‐triphosphafulvene is discussed.