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.