963-82-6Relevant academic research and scientific papers
Sterically Induced Methoxyl Migration on Acid-Catalyzed Dehydration of K-Region trans-Dihydrodiol Monomethyl Ethers
Nashed, Nashaat T.,Rao, Tata Venkata S.,Jerina, Donald M.
, p. 6344 - 6348 (2007/10/02)
The regioisomers of the trans-dihydrodiol monomethyl esters (DME) at the K-regions of 4- and 7-methyl- and 7,12-dimethylbenzanthracene, which possess a ring methyl substituent peri to the methoxyl group, react with BF3*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 benzanthracene 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 methoxy 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.
Solvolysis of K-region arene oxides: Substituent effects on reactions of benz[a]anthracene 5,6-oxide
Nashed, Nashaat T.,Balani, Suresh K.,Loncharich, Richard J.,Sayer, Jane M.,Shipley, David Y.,Mohan, Ram S.,Whalen, Dale L.,Jerina, Donald M.
, p. 3910 - 3919 (2007/10/02)
The solvolytic reactivity and products formed from benz[a]anthracene 5,6-oxide (BA-O) on substitution of a methyl group at positions 1 (1-MBA-O), 4 (4-MBA-O), 7 (7-MBA-O), 11 (11-MBA-O), and 12 (12-MBA-O), on 7,12-dimethyl substitution (7,12-DMBA-O), and on 7-bromo substitution in 1:9 dioxane-water and in methanol at 25°C are reported. These substitutions result in > 150-fold differences in their rates of acid-catalyzed solvolysis and cause marked changes in the distribution of solvent adducts and phenols resulting from isomerization. Optically pure BA-O, 7-MBA-O, 12-MBA-O, and 7,12-DMBA-O as well as their optically pure trans dihydrodiols were utilized to determine the point of attack by water in the hydrolysis reactions. In general, the reactions in aqueous dioxane (0.1 M NaClO4) obeyed the rate equation kobsd = kH[H+] + k0, where kH is the second-order rate constant for acid-catalyzed reaction and k0 is the first-order rate constant for spontaneous reaction, to provide biphasic pH-rate profiles. When ionic strength was maintained with 0.5 M KCl, however, more complex pH-rate profiles were observed for some of the arene oxides due to attack of chloride on the neutral epoxide to produce steady-state concentrations of chlorohydrins. Rate enhancement on methyl substitution is largest (kH, ca. 5-fold) when the methyl group is present in the hindered bay region (C1 or C12) or adjacent to the epoxide at C7. The combined effect of two methyl groups (7,12-DMBA-O) is additive (ca. 25-fold). Theoretical calculations (molecular mechanics by PCMODEL-PI and ab initio by GAUSSIAN 86 and 88 programs) of carbocation stability indicate the importance of steric factors in determining relative reactivity and types of products formed from substituted benz[a]anthracene 5,6-oxides.
