Methyl substituent effects on electrophilic additions to some benzobicyclooctadienes

Abstract

Methoxyiodination and epoxidation, as weak electrophilic additions to a series of benzobicyclooctadienes, were studied. Under investigation was the interaction between the aryl $\pi$-system and that of the carbon-carbon double bond of 2-methyl-5,8-dimethoxy-1,4-dihydro-1,4-ethanonaphthalene in its reactions with weak electrophiles. Previous investigations by L. Paquette et al. and W. B. Smith et al. were left with divergent conclusions. The investigation by Paquette et al. concluded that the interaction was unimportant based upon experimental studies with the above olefin and MINDO/3 calculations with 5,8-dimethoxy-1,4-dihydro-1,4-ethanonaphthalene. Based largely on reactions with the latter compound, Smith et al. concluded that the interaction gained considerable importance with weaker electrophiles. Methyl hypoiodite (prepared in situ from iodine plus iodic acid in methanol) was added to 2-methyl-5,8-dimethoxy-1,4-dihydro-1,4-ethanonaphthalene. Four products were isolated, and their structures were established by 2-D NMR techniques. No aryl participation was evident from the structure of these products--a result in marked contrast to those previously obtained by Smith et al. with 5,8-dimethoxy-1,4-dihydro-1,4-ethanonaphthalene. The epoxidation reactions of 2-methyl-5,8-dimethoxy- and 5,8-dimethoxy-1,4-dihydro-1,4-ethanonaphthalene (and their 5,8-diacetoxy analogs) were studied in ether and in methylene chloride using p-peranisic acid, perbenzoic acid and p-nitroperbenzoic acid. The ratio of anti- to syn-epoxide and the amount of unreacted olefin were determined by proton NMR. The amount of addition anti to the aryl ring increased systematically with a decrease in the electrophilic strength of the reagent system used. This result is consistent with the increasing importance of a homoconjugative stabilization of the transition state with weaker electrophiles. These results stand in contrast to earlier predictions based upon MINDO/3 calculations.