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.

References

1. Part 9, R. A. Aitken and N. Karodia, Liebigs Ann. Recl., 1997, 779.
2. R. A. Aitken, C. Boeters and J. J. Morrison, J. Chem. Soc., Perkin Trans. 1, 1994, 2473.
3. J. Becker, C. Wentrup, E. Katz and K.-P. Zeller, J. Am. Chem. Soc., 1980, 102, 5110.
4. Preliminary communication, R. A. Aitken, C. Boeters and J. J. Morrison, Tetrahedron Lett., 1995, 36, 1303.
5. D. W. Jones, R. S. Matthews and K. D. Bartle, Spectrochim. Acta, 1972, 28A, 2053.
6. C. Graebe, Ber. Dtsch. Chem. Ges., 1894, 27, 952.
7. M. E. Gurskii, I. D. Gridnev, Yu. V. Il'ichev, A. V. Ignatenko and Yu. N. Bubnov, Angew. Chem., Int. Ed. Eng., 1992, 31, 781; H. Jiao and P. von R. Schleyer, Angew. Chem., Int. Ed. Engl., 1993, 32, 1763.
8. H. Heimgartner, J. Zsindely, H.-J. Hansen and H. Schmid, Helv. Chim. Acta, 1973, 56, 2924.
9. R. F. C. Brown and G. L. McMullen, Aust. J. Chem., 1974, 27, 2385.
10. M. Koller, M. Karpf and A. S. Dreiding, Helv. Chim. Acta, 1986, 69, 560.
11. A. I. Meyers, R. J. Himmelsbach and M. Reuman, J. Org. Chem., 1983, 48, 4053.
12. A. R. Katritzky, G. J. Hitchings, R. W. King and D. W. Zhu, Magn. Reson. Chem., 1991, 29, 2.
13. R. A. Aitken and G. Burns, J. Chem. Soc., Perkin Trans. 1, 1994, 2455.
14. W. C. Young, Ber. Dtsch. Chem. Ges., 1892, 25, 2102.
15. H. W. Gschwend and A. Hamdan, J. Org. Chem., 1975, 40, 2008.
16. W. Borsche, P. Hofmann and H. Kühn, Liebigs Ann. Chem., 1943, 554, 23.
17. G. B. Arrowsmith, G. H. Jeffery and A. I. Vogel, J. Chem. Soc., 1965, 2072.
18. R. A. Aitken and J. I. Atherton, J. Chem. Soc., Perkin Trans. 1, 1994, 1281.