Tri(tert-butyl)methylenphosphoran : Konsequenzen sterischer Hinderung für innermolekulare Beweglichkeit und thermischen Zerfallsmechanismus / Tri(tert-butyl)methylenephosphorane : Consequences of Sterical Hindrance for Intramolecular Motions and Thermal Decomposition Mechanism

Abstract

Dimethyl(tert-butyl)-, di(tert-butyl)methyl- and tri(tert-butyl)methylenephosphorane (CH3)n[(CH3)3C]3-nP=CH2, 3a-c, (n = 2,1, 0) have been synthesized from the corresponding phosphonium bromides, using standard methods, but under mild conditions. 3 c was found to be thermally unstable, undergoing reductive elimination of isobutene already at room temperature, with di(tert-butyl)methylphosphine as a by-product. 3 b is decomposed similarly above 80 °C. A mechanism for this unusual mode of ylide decomposition is proposed, based on the extreme steric crowding of the molecules. Restricted P–C bond rotation has been detected by 1H and 13C NMR spectroscopy for 3 c in toluene solution. Traces of protic species cause a rapid intermolecular H2C(P) hydrogen exchange.