Sterically induced methoxyl migration on acid-catalyzed dehydration of K-region trans-dihydrodiol monomethyl ethers

Abstract

The regioisomers of the trans-dihydrodiol monomethyl ethers (DME) at the K-regions of 4- and 7-methyl- and 7,12-dimethylbenz[a] anthracene, which possess a ring methyl substituent peri to the methoxyl group, react with BF 3 -etherate to form a single phenol and two regioisomeric phenol methyl ethers, one of which arises by migration of the methoxyl group. In contrast, for DME of benz[a]anthracene and its 1-, 4-, 7-, 11- and 12-methyl- and 7,12-dimethyl-substituted derivatives where there is no peri methyl group, methoxyl migration does not occur, and thus only the phenol methyl ether resulting from loss of water is formed. These results are consistent with a mechanism in which the initially formed carbocation with a pseudoaxial methoxyl group must undergo either conformational change to align the bond of the leaving proton with the empty p-orbital prior to proton loss or migration of the methoxyl group to the adjacent carbocation via a cyclic oxonium ion. In the absence of a ring substituent peri to the methoxyl group, conformational change is faster than formation of the cyclic oxonium ion, and therefore migration of the methoxyl group does not occur. A methyl substituent peri to the methoxyl group raises the activation energy barrier for conformational isomerization due to adverse steric interaction between the two groups. Consequently, formation of the cyclic oxonium ion becomes competitive with conformational change. The resulting oxonium ion opens to the regioisomeric carbocation resulting in rearrangement. Formation of the cyclic oxonium ion in these reactions is analogous to the rapid internal return of the hydroxy carbocation intermediate to protonated epoxide that is thought to occur in the reactions of peri-methyl-substituted K-region arene oxides