Reactions and lifetime of the 6,6-dimethyl-3-phenylbenzenium ion in aqueous solution. Comparison with the 6-imino-3-phenylbenzenium ion (biphenyl-4-ylnitrenium ion)

Abstract

The title cation (1) has been generated from three precursors in aqueous solution containing 20% acetonitrile; in the solvolysis of 6,6-dimethyl-3-phenylcyclohexa-2,4-dien-1-yl acetate (9) and from the isomeric alcohols 4,4-dimethyl-1-phenylcyclohexa-2,5-dien-1-ol (7) and 6,6-dimethyl-3-phenylcyclohexa-2,4-dien-1-ol (8) under acidic conditions. In addition, 8 reacts by way of 1 in weakly acidic solutions containing sodium azide to form an equilibrium mixture with an azide adduct, tentatively assigned as 6-azido-5,5-dimethyl-2-phenylcyclohexa-1,3-diene (12). This experiment provides a novel method for obtaining the k_az∶k_w ratio, which with the application of the azide clock method, supplies absolute rate constants. The overall analysis shows that 1 reacts in 20% acetonitrile forming 8 (90%, k = 6.2 × 10⁶ s^-1), 7 (8%, k = 5.5 × 10⁵ s^-1) plus a small amount of the rearrangement product 3,4-dimethylbiphenyl (10) (2%, k = 1.6 × 10⁵ s^-1). The pK_R values are -5.3 for 1 ⇌ 8 + H⁺ and -2.9 for 1 ⇌ 7 + H⁺, and the equilibrium constant 7 ⇌ 8 is 2.9 × 10². The conjugated alcohol 8 is thus both the kinetic product and the thermodynamic product of the addition of water to 1.These results are compared with ones obtained previously for the biphenyl-4-ylnitrenium ion (2), a cation which can be regarded in terms of carbenium resonance contributors as the 6-imino-3-phenylbenzenium ion. The lifetimes in aqueous solution of the two cations are similar (ca. 150 ns for 1 and 300 ns for 2), as is also the case for the rate constants for addition of water at the carbon that bears the phenyl group, the reaction that results in unconjugated alcohols. The difference is that the kinetic product with 2 is this unconjugated alcohol; an AM1 calculation suggests that the conjugated isomer would be the thermodynamic product, as is the case with 1. It is suggested that the electronegative nitrogen of 2 results in a greater fraction of the positive charge being localized at the remote para carbon, so that is the site where water preferentially reacts.