Substituent effects in benz[a]anthracene carbocations: a stable ion, electrophilic substitution (nitration, bromination), and DFT study

A series of novel carbocations were generated from isomeric monoalkylated and dialkylated benz[a]anthracenes (BAs) by low-temperature protonation in FSO(3)H/SO(2)ClF. 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 major carbocation) 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% protonation at 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-protonated carbocations were initially formed (C-7/C-12), rearranging in time to give the C-12 protonated carbocations exclusively (16aH+, 17aH+, and 18aH+). Protonation outcomes are compared with the computed relative energies by DFT. Charge delocalization paths in the resulting carbocations were deduced based on the magnitude of Deltadelta13C values. For the thermodynamically more stable C-12 protonated carbocations, the charge delocalization path is analogous to those derived based on computed NPA charges for the benzylic carbocations 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 resulted in isolation and characterization of several novel derivatives. Excellent agreement is found between low-temperature protonation selectivities and the regioselectivities observed in model substitution reactions.