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

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 Ph(3)PO upon FVP to give the 1.3-enynes 12 and 13 in moderate yield. At 500 degrees C there is little double bond isomerisation. but at 700 degrees 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 fall 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 C-13 NMR spectra for 20 enynes are reported. One example of the isomeric beta,gamma-unsaturated-delta-oxo ylides, 14 has been prepared and is found to undergo loss of Ph(3)P on FVP at 700 degrees C to give indene and benzene in low yield.