PHOTOCHEMICAL REARRANGEMENTS OF SUBSTITUTED INDENES

The indene phototransposition reaction, a skeletal rearrangement involving an interchange of carbons 1 and 2 in the indene skeleton, was studied, using a variety of substituted indenes. For alkyl-indenes, the effects of positional substitution are consistent with previously reported trends. Results of experiments with (+)-1,2-dimethylindene indicate that the net migration of carbon 1 to carbon 3 necessary for transposition occurs with clean inversion of configuration at carbon 1. This result is consistent with either a concerted or stepwise, orbital symmetry allowed process converting starting indene to benzobicyclo {2.1.0} pent-2-ene intermediate. The concerted mechanism was ruled out by the discovery of the acid catalyzed, photochemical endo to exocyclic double bond migration of indenes. This latter reaction effects conversion of 2-alkylindenes to the corresponding 2-alkylideneindanes. It is shown to be related to the transposition reaction by a common intermediate which is converted to exocyclic olefin via protonation and deprotonation. Data are consistent with an intermediate resulting from initial 2 + 2 closure of the reactant indene. Fluorescence emission data along with reaction efficiency data were used to make a detailed analysis of structure-reactivity effects. Most of the structural modifications affected both fluorescence emission and reaction efficiencies. Thus, they affect the initial step in the reaction, the proposed 2 + 2 cycloaddition. Possible explanations for the observed reaction inhibition resulting from 3-alkyl, 6-methoxy, or 5-trifluoromethyl substitution are discussed, in terms of either (1) frontier orbital coefficient effects or (2) charge effects. Photolysis of indenes in acidic methanol results in formation of anti-Markovnikov methanol adducts. A mechanism involving protonation of the intermediate resulting from initial 2 + 2 closure of the reactant is proposed. In an attempt to extend the scope of the exocyclic migration, it was found that 3-methyl-1,2-dihydronaphthalene does indeed undergo the reaction; however, no comparable reaction is noted for 1-phenyl-2-methylpropene. The mechanism of the dihydronaphthalene reaction appears to be similar to that for the indene rearrangement