Selectivity in the reaction of triplet phenyl cations.

A DFT study of the reaction of phenyl cation and some 4-substituted derivatives (cyano, methyl, methoxy, amino) with a pi nucleophile (ethylene) as well as with representative n nucleophiles (NH(3), MeOH, and MeCN) reveals a multiform behavior depending on both the cation multiplicity and the trap used. A straightforward addition takes place with the singlet (pi(6)sigma(0) structure) both with ethylene, where a spiro[2,5]octa-4,7-dienyl (phenonium) cation is formed, and with n nucleophiles, where the corresponding onium cations result. On the contrary, with the triplet (pi(5)sigma(1) structure) the reaction depends on the nature of the nucleophile, as indicated by MO correlation diagrams. Thus, with ethylene a bonding interaction occurs between the singly occupied sigma(sp(2)) orbital of the cation and the alkene pi orbital and leads to a planar distonic diradical cation. On the contrary, no addition takes place with n nucleophiles, which interact only with the phenyl cations pi MO, leading to weakly bonded, face-to-face complexes. An electron-withdrawing substituent such as CN allows the formation of a stabilized adduct cation also from the triplet, but only with a good nucleophile, such as ammonia. The spin-dependent dichotomy in the chemical behavior rationalizes recent experimental findings and fits with the prediction formulated by Taft 45 years ago. The unusual combination of a carbocation nature and of triplet multiplicity originates the peculiar chemistry of phenyl cations that appear to be promising intermediates in synthesis.