o-Quinonoid compounds. Part VI. Spiroconjugation in the ethylene acetal of 1,3-diphenylinden-2-one and derivatives of benzo[c]phosphole 2-oxide

Abstract

The ethylene acetals of 1,3-diphenylinden-2-one, 1,2,3-triphenylbenzo[c]phosphole 2-oxide, and 2-methyl-1,3-diphenylbenzo[c]phosphole 2-oxide have been generated by reduction of their dibromides, and characterised by u.v. spectroscopy and trapping experiments. The long-wavelength u.v. absorption and rapid [4 + 2]π dimerisation of these compounds, in comparison with the properties of 2,2-dimethyl-1,3-diphenyl-2H-indene, are attributed to spiroconjugation.