PHOSPHOLES WITH REDUCED PYRAMIDAL CHARACTER FROM STERIC CROWDING .1. SYNTHESIS AND NMR CHARACTERIZATION OF ,4-DI-TERT-BUTYL-6-METHYLPHENYL)-3-METHYLPHOSPHOLE

The sterically crowded ,4-di-tert-butyl-6-methylphenyl)-3-methylphosph ole was synthesized by dehydrohalogenation of the corresponding 3,4-di bromophospholane, in order to probe the possibility that the steric co ngestion would cause some flattening of the phosphorus pyramid and an increase in electron delocalization. The phosphole was a recrystalliza ble solid with P-31 NMR delta 1.8. Semiempirical calculations indicate d that the pyramidal shape was retained but was noticeably flatter tha n in 1-phenylphosphole. In the low energy conformation, the phosphole and phenyl ring planes are approximately orthogonal, with the 2-tert-b utyl group in the less crowded position that is syn to the lone pair o n phosphorus. The 6-methyl group is positioned under the phosphole rin g. This conformational prediction was amply confirmed by several chemi cal shift and coupling effects in the C-13 NMR spectrum. The H-1 NMR s pectrum displayed an unusually large four-bond coupling (6 Hz) of P-31 to the m-phenyl proton syn to the lone pair (and none to the anti-met a proton), consistent with the orthogonal conformation. The oxide of t he phosphole showed more stability than that of less crowded phosphole s and gave a P-31 NMR signal that was detectable over a several hour p eriod at room temperature. The oxide proceeded to give the usual Diels -Alder dimer and also formed a cycloadduct with N-phenylmaleimide. The phosphoryl group of the latter was reduced with trichlorosilane to gi ve the phosphine. This new 7-phosphanorbornene derivative gave the mos t downfield P-31 NMR shift (delta 153.3) of any member of this family, all of which are characterized by remarkable deshielding in the syn i somer.