Stable ion study of protonated cyclopenta[a]phenanthrenes. Structure–reactivity relationships and charge delocalization in the carbocations
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Abstract
Protonation studies are reported for a series of cyclopenta[a]phenanthrenes Cp[a]P in superacid media. compounds 3, 6 are protonated at the D-ring double-bond to form stable α-phenanthrene-substituted carbocations. The 17-keto derivatives 2, 5, 8, 9, 19, 20 are CO-protonated in FSO3H–SO2ClF to form carboxonium ions. Carboxonium ions derived from 8 and 20 undergo ring fluorosulfonation in the biologically important A-ring under thermodynamic control (higher temperatures and prolonged reaction times). Low temperature protonation of 8 and 9 with FSO3H·SbF5 (4∶1)–SO2ClF gives their corresponding carboxonium-arenium dications (protonation of 2 with FSO3H·SbF5 (1∶1)–SO2ClF gave a mixture of mono- and dications), where ring protonation sites are controlled by the position of the
Whereas the 11-methoxy derivative (16) forms a carboxonium ion in FSO3H–SO2ClF analogous to the 11-Me derivative (5), the 11-phenol derivative (15), the
Substituent effects observed under stable ion conditions emphasize relative carbocation stability and relief of peri-strain. Under thermodynamic control, carboxonium ions undergo fluorosulfonation in the biologically important A-ring. Charge delocalizations in the resulting mono- and dications (deduced primarily based on magnitude of Δδ13C) are discussed and compared. In an effort to further enhance the NMR assignments and for comparison, mono-arenium ions 1H++, 4H++, 6H++, 7H++ and their neutral precursors were calculated at the B3LYP/6-31G(d,p) level of ab initio theory; their 1H and
Stable ion studies of CP[a]P provide useful insights into the contrasting regioselectivities observed in chemical and biological activiation.
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