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Dimethyl(tert-butyl)-, di(tert-butyl)methyl- and tri(tert-butyl)methylenephosphorane (CH 3 ) n [(CH 3 ) 3 C] 3-n P=CH 2 , 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  1 H and  13 C NMR spectroscopy for 3 c in toluene solution. Traces of protic species cause a rapid intermolecular H 2 C(P) hydrogen exchange.

zeitschrift für naturforschung b

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