REACTIVITY OF METHYLENETRIPHENYLPHOSPHORANES HAVING TWO PHENYL GROUPS CONSTRAINED WITH ETHANO OR ETHENO BRIDGE

Abstract

Methylenetriphenylphosphoranes having two phenyl groups tied together with ethano or etheno bridge were prepared by conventional ways in order to suppress delocalization of an ylide-carbanion by deforming geometrical arrangement of phenyl groups from that of the diarylmethylene derivative and hence to elevate reactivities of diarylmethylenephosphoranes. These P-ylides were, however, still unreactive with usual aldehydes and ketones. The reasons are deliberated by molecular orbital calculations and the 31P-NMR data. The HOMO orbitals of ethano-bridged and etheno-bridged diarylmethylenephosphoranes are located on a monoarylcarbanion and a carbanion, respectively. Although these tendencies are auspicious to enhance the Wittig reactivity of them, the electron densities of the ylidic carbons are still insufficient, 31P-NMR chemical shifts of the ylides revealed that the objective P-ylides do not have a large extent of the ylide-contribution in the ylide-yllene resonance. Meanwhile, they reacted readily with tetrahalo o-quinones to give 1,3-dioxoles in good yields. Thermo-chromic behavior of one of the precursors, dibenzosuberenylphosphonium salt, was observed at 80 °C in acetonitrile in the presence of trace perchloric acid. The colored species is elucidated as dibenzo[a,e]tropylium cation.