Preparation, structure and reactivity of some new types of stabilised phosphorus ylides

Abstract

Sixteen examples of the previously unknown β,γ,β'-trioxo phosphorus ylides have been prepared and fully characterised. Upon flash vacuum pyrolysis (FVP) these undergo clean loss of Ph₃PO selectively across the central position to afford diacylalkynes in most cases. The pyrolysis results are discussed in the light of the fully assigned ¹³C NMR spectra presented and in particular the values of ²jp-C(C=O). Six examples of the higher homologues, the β,γ,β,γ-tetraoxo ylides have also been prepared and are found, quite unexpectedly, to give poor results upon FVP but to undergo moderately successful Ph₃PO extrusion to afford trioxoalkynes using conventional pyrolysis. The known β,γ-dioxophosphonium salts required as precursors to these have been characterised by NMR for the first time and are shown to exist predominantly as mixtures of E and Z enol forms in solution. By reaction with oxalyl chloride, a range of higher polyoxo bis ylides has been obtained, including three examples of hexaoxo bis ylides which are remarkable in containing an acyclic series of eight contiguous C=X centres. The ¹³C NMR spectra for the oxalyl bis ylides are interesting due to the pattern of coupling observed to both phosphorus atoms. These compounds do not in general give any useful results from FVP presumably due to the thermal instability of the expected products. The reactivity of dioxo, trioxo, tetraoxo and oxalyl bis ylides towards a variety of oxidants has been examined and has given promising preliminary results for the formation of vicinal polycarbonyl compounds. Vicinal triones and tetraones are readily obtained and some evidence for a pentaone was obtained in one case. All the products readily form the molecular hydrates with geminal diol structures and these have been characterised spectroscopically. Reaction of the various ylide types with NO₂ has been examined and a range of different processes is observed including formation of nitriles accompanied in one case by ring nitration of a phenyl group. A total of twenty five β-aminoacyl ylides derived from N-alkoxycarbonyl protected amino acids have been prepared using a carbodiimide coupling method and have been fully characterised. Upon FVP at 600°C these readily lose Ph₃PO to afford protected acetylenic amino acid derivatives and twelve examples of these valuable chiral compounds have been obtained in moderate yield. By standard reactions these products have been transformed into simple chiral analogues of the important neurotransmitter GABA in four cases. Removal of the N-protecting group in the amino acid derived ylides results in a change in pyrolysis behaviour: ethanol is eliminated rather than Ph₃PO to give novel cyclic stabilised ylides related in structure to the tetramic acids. Finally, samples of one trioxo ylide, one tetraoxo ylide, one tetraoxo bis ylide and one hexaoxo bis ylide have been sent for solid state structure determination by X-ray methods. The results obtained provide ready confirmation of the important contribution of phosphonium enolate forms to the structures of these but give surprising results as regards the conformational preference for syn and anti alignments of the adjacent C=X units along the chain.