Synthesis and properties of stabilized bis(2-oxo-2H-cyclohepta[b]furan-3-yl)phenylmethyl and bis(1,2-dihydro-2-oxo-N-phenylcyclohepta[b]pyrrol-3-yl)phenylmethyl cations and their derivatives: remarkable substituent effect on the conformation and stability of the cations

Abstract

The reactions of benzaldehyde and 4-substituted benzaldehyde with 2H-cyclohepta[b]furan-2-one 5 and 1,2-dihydro-N-phenylcyclohepta[b]pyrrol-2-one 6 in TFA–CH₂Cl₂ afford two series of bis(2-oxo-2H-cyclohepta[b]furan-3-yl)arylmethanes 7a–e and bis(1,2-dihydro-2-oxo-N-phenylcyclohepta[b]pyrrol-3-yl)arylmethanes 8a–e, respectively. Upon oxidation reaction with DDQ followed by addition of 60% HPF₆, the methane derivatives 7a–e and 8a–e are converted to the corresponding methyl cations 9a–e·PF₆6⁻− and 10a–e·PF₆6⁻−, respectively. The stability of cations 9a–e and 10a–e is expressed by the pK_R⁺ values, which are determined spectrophotometrically, as 12.4 to 7.9 and 13.9 to 11.1, respectively. The electrochemical reductions of 9a–e and 10a–e exhibit reversible waves and low reduction potentials at −0.55 to −0.23 and −0.72 to −0.44 (V vs. Ag/Ag⁺), respectively, upon cyclic voltammetry (CV); the values are lower than those of the related compounds, diazulen-1-ylarylmethyl cations. Good linear correlations between the pK_R⁺ values and E1_red potentials of 9a–e and 10a–e are obtained. These values (pK_R⁺ and E1_red) are dependent on the substituents on the phenyl group; however, they do not correlate with the substituent constants of the Hammett equation. This feature is ascribed to the difference in the most stable conformation of the three aromatic groups in the cations; the phenyl group having a more electron-donating substituent is prone to become more planar to the reference plane. The most stable conformation is deduced on the basis of MO calculations (MOPAC, AM1 method). The ¹H NMR and UV–vis spectral studies also rationalize the conformational change of the three aromatic rings, depending on the substituent on the benzene ring. On the basis of the study, the stabilizing ability of the 4-substituted phenyl group and heteroazulenes for the cations is clarified to be in the order of 4-dimethylaminophenyl > 1,2-dihydro-2-oxo-N-phenylcyclohepta[b]pyrrol-3-yl > 2-oxo-2H-cyclohepta[b]furan-3-yl ≈ 4-methoxyphenyl.