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.
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