Stable ion study of protonated cyclopenta[a]phenanthrenes. Structure-reactivity relationships and charge delocalization in the carbocations

Protonation studies are reported for a series of cyclopenta[a]phenanthrenes C-p[a]P in superacid media. Hydrocarbons 1, 4, 7, are ring protonated in F SO3H-SO2ClF to form monoarenium ions. The Delta(16,17) compounds 3, 6 are p rotonated at the D-ring double-bond to form stable alpha-phenanthrene-subst ituted carbocations. The 17-keto derivatives 2, 5, 8, 9, 19, 20 are CO-protonated in FSO3H-SO2Cl F to form carboxonium ions. Carboxonium ions derived from 8 and 20 undergo ring fluorosulfonation in the biologically important A-ring under thermodyn amic control (higher temperatures and prolonged reaction times). Low temper ature protonation of 8 and 9 with FSO3H . SbF5 (4 : 1)-SO2ClF gives their c orresponding carboxonium-arenium dications (protonation of 2 with FSO3H . S bF5 (1 : 1)-SO2ClF gave a mixture of mono- and dications), where ring proto nation sites are controlled by the position of the methyl group and occur i n the A-ring for the A-ring methylated derivatives (8, 9). Whereas the 11-methoxy derivative (16) forms a carboxonium ion in FSO3H-SO2 ClF analogous to the 11-Me derivative (5), the 11-phenol derivative (15), t he ethoxy (17) and propoxy (18) derivatives are more reactive, forming a mi xture of mono- and dication (with 15 and 17) or give mostly a carboxonium-a renium dication (with 18). Substituent effects observed under stable ion conditions emphasize relative carbocation stability and relief of peri-strain. Under thermodynamic contr ol, carboxonium ions undergo fluorosulfonation in the biologically importan t A-ring. Charge delocalizations in the resulting mono- and dications (dedu ced primarily based on magnitude of Delta delta(13)C) are discussed and com pared. In an effort to further enhance the NMR assignments and for comparis on, mono-arenium ions 1H(+), 4H(+), 6H(+), 7H(+) and their neutral precurso rs were calculated at the B3LYP/6-31G(d,p) level of ab initio theory; their H-1 and C-13 NMR chemical shifts were computed by the GIAO method and thei r overall charge delocalization paths were deduced via differences in the N PA charges (cation minus neutral). The results are compared and discussed. Stable ion studies of C-P[a]P provide useful insights into the contrasting regioselectivities observed in chemical and biological activiation.