Synthesis of a Persistent 1σ3,3σ3‐Diphosphaallyl Cation Featuring a Localized Phosphorus−Carbon Double Bond

Abstract

Abstracttert‐Butyl thiol, 2,6‐dimethylthiophenol, 2,6‐dimethylphenol, and 2,6‐bis(tert‐butyl)‐4‐methylphenol cleanly add to the 1σ4,3σ2‐diphosphacumulene 1 to afford the corresponding C‐phosphanyl phosphorus ylides 2a−d. Treatment of 2a with gallium trichloride quantitatively leads to the C‐phosphonio phosphaalkene 3, resulting from the1,3‐migration of the sulfido group. Under the same experimental conditions, cleavage of the arylthio and aryloxy groups of 2b and 2c leading to the C‐phosphonio phosphaalkene 4 is observed. In the case of the sterically most crowded derivative, 2d, abstraction of the chloride ion affords the 1σ3,3σ3‐diphosphaallyl cation 5, which is stable for several weeks in dichloromethane at −20 °C, but decomposes after a few minutes at above 0 °C. Compound 5 reacts at low temperatures with an excess of acetonitrile or benzaldehyde to afford quantitatively and regioselectively the corresponding five‐membered heterocycles 6 and 7. The NMR spectroscopy data and reactivity suggest that 5 features a localized double bond with a planar diamino‐substituted phosphorus center, and a single bond with a pyramidalized (amino)(aryloxy)phosphanyl group. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)