Reactivities of 9-arylfluoren-9-yl and 1-(4-dimethylaminophenyl)ethyl cations in water and trifluoroethanol studied by laser flash photolysis

Abstract

9-Arylfluoren-9-yl cations have been observed as transient intermediates upon 248 nm laser photolysis of 9-arylfluoren-9-ols in 2,2,2-trifluoroethanol (TFE) and 1:4 (v/v) acetonitrile–water. Quantum yields for the formation of the cations were measured and found to lie in the range 0.5–0.8. Rate constants for the decay of the cations in the two solvents parallel each other, a plot of log k_TFEversus log k_w being linear with a slope of 1.00. These rate constants are approximately two orders of magnitude larger than those observed in the same solvent for the corresponding monosubstituted triarylmethyl cations. Substituent effects in the two series are similar. For both the hydration reaction and the overall equilibrium, a two-parameter treatment dissecting the substituent contributions into resonance and polar effects shows that the former are more important in the triarylmethyl series, while the latter are more important in the fluorenyl series. Twisting of the 9-aryl ring out of coplanarity with the fluorenyl ring is the likely cause of the diminished resonance interaction in this system. The 1-(4-dimethylaminophenyl)ethyl cation has been observed in TFE and 1:1 TFE–W, being generated by photoprotonation of 4-dimethylaminostyrene. Rate constants for decay are about 50 times smaller than those previously estimated by a clock method. The failure of the latter approach is associated with the rate constant for the clock nucleophile becoming significantly less than diffusion control for this highly stable cation. An extended log k_w–pK_R correlation is constructed. This shows a series of reasonably parallel lines associated with different families. In considering cations with the same pK_R′ the order of inherent reactivity is 9-unsubstituted-9-xanthylium ∼ cyclic dialkoxy-carbocation > phenyltropylium > diarylmethyl ∼9-arylxanthylium > triarylmethyl ∼9-arylfluoren-9-yl. The log k_w–pK_R data for each family are reasonably satisfied by a linear relationship, with a slope d(log k_w)/dpK_R of 0.6 ± 0.1. The triarylmethyl and 9-arylfluoren-9-yl cations form a contiguous series covering 26 pK_R units. The data for these indicate that there is probably a gentle downward curvature in the correlation, particularly at the high reactivity end. This is consistent with a Hammond effect on the transition state such that it becomes more cation-like in the hydration direction for more reactive cations.