Hammett Analysis of a Family of Carbene−Carbene Complex Equilibria

Abstract

p-X-substituted phenylchlorocarbenes (X = NO(2), CF(3), Cl, H, Me, and MeO) form π-type complexes with trimethoxybenzene in pentane. The carbenes and complexes are in equilibrium, and logarithms of the measured equilibrium constants are well correlated by Hammett σ(p) constants with ρ = 2.48. The carbene complexes are characterized by UV-vis spectroscopy, and computational analysis is afforded by DFT calculations.

Visualizing Carbene Equilibria

“…We obtained a good Hammett correlation of log K with σ p , with ρ = +2.48 . As in dihalomethide carbanion formation, Eq.…”

“…The pentafluorophenyl group is more electron withdrawing than the phenyl group as manifested by σ I or σ R + values (, M. Charton, personal communication), and F 5 ‐PhCCl should therefore bind more strongly to TMB than PhCCl.…”

“…(10), where ρ = +3.18, the positive sign indicates that complexation is favored by electron‐withdrawing substituents on phenyl. These substituents destabilize the carbene, but enhance charge transfer in the carbene–TMB complex . Donating substituents (Me, MeO) result in low values of K .…”

“…(10), where q = +3.18, the positive sign indicates that complexation is favored by electron-withdrawing substituents on phenyl. These substituents destabilize the carbene, but enhance charge transfer in the carbene-TMB complex (28). Donating substituents (Me, MeO) result in low values of K. Note that K for F 5 -PhCCl exceeds K for p-O 2 N-PhCCl (Table 2); pentafluorophenyl is a strong electron-withdrawing moiety (25).…”

“…A more general demonstration of the manipulation of K by modulating the electronic character of the phenyl ring of PhCCl can be obtained from a Hammett study of the substituent dependence of the equilibrium constant of Eq. . We examined the π ‐complexation equilibria of p ‐ X ‐PhCCl with TMB, where X = NO 2 , CF 3 , Cl, H, Me and MeO.…”