Flash vacuum pyrolysis of stabilised phosphorus ylides. Part 15.1 Generation of alkoxycarbonyl(sulfenyl)carbenes and their intramolecular insertion to give alkenyl sulfides

Abstract

A range of 18 alkoxycarbonyl sulfinyl phosphorus ylides 9 have been prepared and their behaviour upon flash vacuum pyrolysis (FVP) at 600 °C examined. For R¹ = H, Me and Et they lose Ph₃PO and in some cases Ph₃P to give mixtures of products including the alkenyl sulfides 10, the sulfides 11, the disulfides 12 and the thioesters 14. The alkenyl sulfides 10 most likely arise from intramolecular insertion of the alkoxycarbonyl sulfenyl carbenes resulting from loss of Ph₃PO to produce β-lactones which then lose CO₂ and this is supported by the results from ¹³C labelled ylides. Possible mechanisms for the formation of 11 and 14 are also presented and the feasibility of various steps has been examined by preparation and pyrolysis of the proposed intermediates. In contrast, pyrolysis of the ylides 9 where R¹ = Ph and the tert-butoxycarbonyl ylides 30 leads mainly to complete fragmentation with loss of Ph₃PO and benzyl alcohol or 2-methylpropan-2-ol and does not give any useful sulfur-containing products. Four alkoxycarbonyl sulfonyl diazo compounds 33 have been prepared and in three cases they give the alkenyl sulfones 34 upon FVP at 400 °C, probably by an intramolecular insertion and decarboxylation process analogous to the formation of 10 from 9. On the other hand the alkoxycarbonyl carbenes produced by FVP of the amino acid-derived diazo compounds 35 undergo alternative processes with no sign of β-lactone formation. Fully assigned ¹³C NMR data are presented for 13 of the ylides.