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

Abstract

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 FSO₃H–SO₂ClF to form monoarenium ions. The Δcompounds 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 methyl group and occur in the A-ring for the A-ring methylated derivatives (8, 9).

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 ethoxy (17) and propoxy (18) derivatives are more reactive, forming a mixture of mono- and dication (with 15 and 17) or give mostly a carboxonium-arenium dication (with 18).

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 ¹³C NMR chemical shifts were computed by the GIAO method and their overall charge delocalization paths were deduced via differences in the NPA 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.

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