Supporting Information

General: All manipulations were performed under an inert atomosphere of argon using standard schlenk techniques. Dry, oxygen-free solvents were employed. 1H, 13C, 31P NMR spectra were recorded on Buruker AC200, WM250, Avances 300 spectrometers. IR spectra were recorded on a Perkin-Elmer 1600. 1H, 13C NMR chemical shift were reported in ppm relative to Me4Si as external standard. 31P NMR downfield chemicalshift are expressed with a positive sign, in ppm, relative to standard of 85 % H3PO4. The phosphonium salts[1] and hexamethylcarbodiphosphorane[2] were prepared by previously reported procedures. Five-membered heterocycle 1a: To a CH2Cl2 solution (5 ml) of phosphonium trifluoromethane sulfonate (1.74 g, 4.9 mmol) was added dropwise a mixture of bis(diisopropylamino)phosphinodiazomethane (2M solution in hexane, 11 ml, 5.5 mmol) and benzonitlile (4.75 ml, 45.8 mmol) in CH2Cl2 (5 ml) at 0 °C. After the solution was warmed slowly to room temperature, the solvent was removed under vacuum, and the residue was washed with Et2O. 1a was recrystallized from a THF/Et2O at-30 °C. (1.59 g, 48 %