Kk. Laali et al., Stable ion study of protonated cyclopenta[a]phenanthrenes. Structure-reactivity relationships and charge delocalization in the carbocations, J CHEM S P2, (2), 2000, pp. 211-220
Citations number
35
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
JOURNAL OF THE CHEMICAL SOCIETY-PERKIN TRANSACTIONS 2
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.