Stereochemistry of the photochemical di-pi-methane molecular rearrangement

Abstract

A study of the stereochemistry of 1,4-diene systems containing structural features which limit the number f reaction pathways available was undertaken using as a model 2-phenylspiro [5.5] undeca-1,7-diene (76). The sole product from direct irradiation of 76 was shown to be tricyclic 11-phenyltricyclo [5.4.0.0?ü? ????] undec-2-ene (77) by a degradation and independent synthetic sequence with the 1-phenyl-5-(4'hydroxylbutyl)-exo-6-hydroxymethylbicyclo [3.1.0]hexane (86) as the fulcrum compound. Determination of the stereochemistry of 86 was based upon nmr spectroscopy using model 6-hydroxymethylbicyclo compounds. This result was interpreted in terms of a concerted reaction pathway involving a transition state orbital geometry having syndisrotatory overlap of the orbitals involved in cyclopropane formation. Discussion of the reasons why spirodiene 76 undergoes rearrangement by a stereochemical pathway different from other studied di-pi methane systems is presented, and centers on the interesting question methane systems is presented, and centers on the interesting question of reactant vs product structure control of excited state processes. Attempts to sensitize rearrangement of 76 using benzophenone or acetophenone gave no detectable products, showing surprisingly that a singlet and not a triplet excited state of 76 undergoes conversion to 77. Interesting implications of these observations in terms of a new postulate on how structural features control the preferred multiplicity of the reacting di-pi-methane excited state, are presented.