Flash vacuum pyrolysis of stabilised phosphorus ylides. Part 6. Pyrolysis of substituted cinnamoyl ylides as a route to conjugated enynes

Abstract

The substituted cinnamoyl ylides 6 and 7, readily prepared in one step from the quaternary phosphonium salts 8 and cinnamoyl chlorides 9, undergo extrusion of Ph₃PO upon FVP to give the 1,3-enynes 12 and 13 in moderate yield. At 500 °C there is little double bond isomerisation, but at 700 °C this does occur to give almost 1:1 mixtures of E and Z isomers. In a few cases, including those with a nitrophenyl group present, the yields are poor or the reactions fail completely. An attempt to improve upon the poor yield of 1-phenylbutenyne by use of an ester stabilised ylide was only partly successful since the severe conditions required for loss of the ester group resulted in significant isomerisation to naphthalene. The fully assigned ¹³C NMR spectra for 20 enynes are reported. One example of the isomeric β,γ-unsaturated-δ-oxo ylides, 14 has been prepared and is found to undergo loss of Ph₃P on FVP at 700 °C to give indene and benzene in low yield.