Flash vacuum pyrolysis of stabilised phosphorus ylides. Part 3. Preparation of o-methoxyphenyl- and o-methylsulfanylphenyl-alkynes and their cyclisation to benzofurans and benzothiophenes

Abstract

Fourteen new β-oxo phosphorus ylides 1–14 bearing o-methoxybenzoyl or o-(methylsulfanyl)benzoyl groups have been prepared and their pyrolytic behaviour studied. While flash vacuum pyrolysis (FVP) at 700 °C brings about extrusion of Ph3PO to give the expected alkynes 16, this is accompanied at 850 °C by loss of Me· and cyclisation of the resulting radicals to afford 2-substituted benzofurans or benzothiophenes 17–24. Where the substituent R1 on the ylidic carbon of the starting material is phenyl, this is incorporated unchanged into the heterocyclic products. Where R1 is Et or Pri the vinyl products are formed by intramolecular abstraction of a β-hydrogen atom following cyclisation. For R1= Me, Pr, Bu and C5H11 the cyclisation is followed by hydrogen atom abstraction from the alkyl group leading to its fragmentation and giving products with 2-methyl, ethyl and vinyl substituents. In these cases the products can be accounted for by a radical chain reaction involving the unusual homolytic substitution of a carbon radical at a saturated carbon atom.