Liquid-phase photolysis. Part XII. Evidence from nuclear magnetic resonance for stereospecific electronic ground-state interaction between benzene and maleic anhydride and some maleimides

Abstract

N.m.r. chemical shifts of the ethylenic protons of maleic anhydride, N-n-butyl, N-phenyl-, and N-2,6-xylylmaleimide have been compared in carbon tetrachloride and benzene solutions. Large upfield chemical shifts are induced by the benzene solvent, and these are attributed to a 1 : 1 exo-stereospecific association of the solute and solvent molecules. In the case of the maleic anhydride–benzene complex, a model in which the hydrogen atoms of the exo-oriented maleic anhydride are located at a mean position 2·55 Å above the plane of the benzene ring provides the best fit with the experimental results.

The relevance of these findings to the stereochemistry of corresponding photoaddition reactions is discussed.