C-spirophosphorane-substituted phosphorus ylides

Abstract

The methylenephosphinophosphorane 2 was methylated by MeI forming the phosphonium salt 4, which is stable in crystalline form but rearranges into isomer 10 when heated in chloroform solution at 80°C in a sealed tube for 16 h. The isomerization occurs by elimination of N,N′-dimethylcarbodiimide and formation of the methylenephosphonium phosphonate 5. Compound 5 reacts with the carbodiimide via its ylide from 5a to give the ylide 9, which is transformed reversibly into 10. Compound 2 shows unusual properties, such as its reaction with hexafluoroacetone via its ylide form 2a forming the addition product 12. The ylide structure of 12 was confirmed by X-ray analysis. The coordination geometry at P(2) is distorted trigonal bipyramidal, with axial substituents O (of the chelate ring) and N (of the four-membered ring). The bond lengths and angles at the ylidic carbon confirm the partial multiple bond character of the PC bonds. There is evidence for a CH···O hydrogen bond from the carbon atom bearing the CF 3 groups. The ylide 13 was obtained by the elimination of HI from 4 with triethylamine. Compound 2 reacted with CCl 4 to give the P-chloroylide 15, as well as a small amount of 16, which contains two phosphorus atoms of opposite formal charge and different coordination number (λ 4P +, λ 6P −). Compound 16 is isomeric to 3, which we obtained earlier and investigated by X-ray analysis.