Flash vacuum pyrolysis of stabilised phosphorus ylides. Part 8. Preparation of symmetrical and unsymmetrical 1,3-diynes from alkynoyl ylides and oxalyl diylides

Abstract

Seven examples of the alkynoyl phosphorus ylides 5 have been prepared and are found, upon flash vacuum pyrolysis (FVP) at 750 °C, to undergo extrusion of Ph₃PO to give the unsymmetrical 1,3-diynes 9 in moderate yield. Six examples of the oxalyl diylides 6 have been prepared and their fully assigned ¹³C NMR spectra are presented. Upon FVP, those with R = aryl undergo two-fold extrusion of Ph₃PO to give symmetrical 1,3-diynes 13 but this requires the unusually severe temperature of 900 °C. In contrast those with R = CO₂Me, CO₂Et or COPh undergo complete reaction at 500 °C but afford only Ph₃PO and fragmentation products.