Flash vacuum pyrolysis of stabilised phosphorus ylides. Part 10.1 Generation of 2-methylstyrylalkynes and their thermal cyclisation to 2-alkenylnaphthalenes

Abstract

A series of nine 2-methylcinnamoyl phosphorus ylides 7 have been prepared and are found upon FVP at 500 °C to undergo loss of Ph₃PO to afford the corresponding styrylalkynes 8 whose fully assigned ¹³C NMR spectra are presented. FVP of the ylides at 900 °C leads to cyclisation to give substituted naphthalenes 9–18; the mechanism of these reactions may proceed either by initial hydrogen atom loss or an initial [1,7]hydrogen shift, but an alternative route involving an initial [1,3]hydrogen shift has been ruled out by examination of a deuterium labelled analogue. For the α-phenyl ylides 7d and 7i a further cyclisation leads to benzo[c]fluorene derivatives and this process has been extended to a thiophene analogue to give fluoreno[3,4-b]thiophene. The formation of 2-ethylnaphthalene as the main product from the α-methoxycarbonyl ylide 7e is due to a secondary thermal reaction of methyl 2-naphthylacetate which may involve a radical chain reaction featuring, as the propagation step, an unusual homolytic substitution at a methoxy carbon by a 2-naphthylmethyl radical.