| Abstract
| - A series of novel carbocations were generated from isomeric monoalkylated and dialkylated benz[a]anthracenes (BAs) by low-temperature protonation in FSO3H/SO2ClF. With the monoalkyl derivatives(5-methyl, 6-methyl, 7-methyl, and 7-ethyl) as well as the D-ring methylated analogues (9-methyl, 10-methyl, and 11-methyl), the C-7 or the C-12 protonated carbocations were observed (as the sole or majorcarbocation) in all cases. Protonation of the 12-methyl derivative (9) gave the C-7 protonated carbocation(9H+) as the kinetic species and the ipso-protonated carbocation (9aH+) as the thermodynamic cation.With the 12-ethyl derivative (10), relief of steric strain in the bay-region greatly favors ipso-protonation(10aH+). With 3,9-dimethyl (14), C-7 protonation (14H+) is strongly favored (with <10% protonationat C-12), and with 1,12-dimethyl (15) the sole species observed is the C-7 protonated carbocation (15H+).For 7-methyl-12-ethyl, 7-ethyl-12-methyl, and 7,12-diethyl derivatives (16, 17, and 18), two ipso-protonatedcarbocations were initially formed (C-7/C-12), rearranging in time to give the C-12 protonated carbocationsexclusively (16aH+, 17aH+, and 18aH+). Protonation outcomes are compared with the computed relativeenergies by DFT. Charge delocalization paths in the resulting carbocations were deduced based on themagnitude of Δδ13C values. For the thermodynamically more stable C-12 protonated carbocations, thecharge delocalization path is analogous to those derived based on computed NPA charges for the benzyliccarbocations formed by 1,2-epoxide (bay-region) and 5,6-epoxide (K-region) ring opening. Nitration(and bromination) of the 4-methyl, 7-methyl, 7-ethyl, 3,9-dimethyl, and 1,12-dimethyl derivatives resultedin isolation and characterization of several novel derivatives. Excellent agreement is found between low-temperature protonation selectivities and the regioselectivities observed in model substitution reactions.
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