Flash vacuum pyrolysis of some sulphonyl and sulphinyl stabilised phosphorous ylides (1990)

The reaction between alkylidene or arylmethylene triphenyl phosphoranes and sulphonyl fluorides or sulphonic anhydrides has been used to give a variety of α-sulphonyl stabilised phosphorus ylides. Flash Vacuum Pyrolysis (FVP) of alkyl- and arylsulphonyl alkylidene triphenyl phosphoranes leads to two fractions. The first fraction is made up of up to 20 phosphorus containing products, the major one being Ph₃PO. The second fraction is a liquid at low temperature which forms a white, insoluble polymeric material on warming to room temperature. Trapping reactions of the liquid with Diels-Alder dienes or chlorine gives unknown and unstable compounds except with anthracene where an unidentified complex is formed. FVP of benzyl- and substituted-benzylsulphonyl alkylidene and phenylmethylene triphenyl phosphoranes proceeds by loss of Ph₃P followed by intramolecular insertion of the resulting carbene to form episulphones which lose SO₂ to give stilbene or styrene derivatives. In the cases where the phosphorane is an α-sulphonyl ethylidene triphenyl phosphorane, a 1,2-H shift of the carbene can occur to form a vinyl sulphone, which decomposes to vinyl and benzyl radicals and SO₂ with formation of bibenzyl products. Where the phosphorane is an α-sulphonyl ortho-substituted-benzylidene triphenyl phosphorane, a second carbene insertion pathway occurs to give eventually benzofuran or benzothiophene and bibenzyl. The Ph₃P group reacts with the liberated SO2 to give some conversion to a 2:1 mixture of Ph₃PO and Ph₃PS, a previously unrecognised reaction. The initially formed stilbene or styrene products can react to a certain extent to form cross over products under the reaction conditions used. A number of α-sulphinyl alkoxycarbonylmethylene triphenyl phosphoranes have been made from alkoxycarbonylmethylene triphenyl phosphoranes, sulphinyl chlorides and Et₃N. These ylides exhibit broadening in their ¹H and ¹³C NMR spectra, variable temperature NMR studies indicating the presence of cis and trans rotamers. FVP of these compounds gave Ph₃P, Ph₃PO, vinyl or substituted vinyl sulphides, alkyl sulphides and an as yet unidentified carbonyl compound. The vinyl or substituted vinyl sulphides are obtained via loss of Ph₃PO from the starting ylide followed by insertion of the carbene form of the initially formed thia-acetylene and loss of CO₂ from the resulting β-lactone. The other products are obtained from a variety of insertion, rearrangement and elimination reactions of the initially formed carbenes some of which have been elucidated with the help of ¹³C labelling. Isopropylsulphinyl benzylidene triphenyl phosphorane was synthesised and upon FVP gave Ph₃P, isopropyl thiobenzoate and thiobenzoic acid. The formation of isopropyl thiobenzoate is rationalised via an S to C oxygen transfer mechanism of the initially formed carbene, involving the intermediacy of the previously unknown λ⁴-oxathiirene ring system. A β-keto tri-n-butyl phosphorane, benzoyl butylidene tri-n-butyl phosphorane was synthesised and its FVP performed at room temperature, 300°C and 500°C. The almost complete extrusion of Buⁿ₃PO from the ylide at 300°C indicates that much lower temperatures are required for loss of Bun₃PO as compared to loss of Ph₃PO.