Superelectrophiles with hydrogen

Superelectrophilic activation has also been proposed to be involved, based upon the reactivity of carbocations with molecular hydrogen (a a-donor).16 This chemistry is probably even involved in an enzymatic system that converts CO2 to methane. It was found that A. A -menthyl tetrahy-dromethanopterin (11) undergoes an enzyme-catalyzed reaction with H2 by hydride transfer to the pro-R position and releases a proton to give the reduced product 12 (eq 15). Despite the low nucleophilicity of H2, cations like the tert-butyl cation (13) are sufficiently electrophilic to react with H2 via 2 electron-3 center bond interaction (eq 16). However, due to stabilization (and thus delocalization) by adjacent nitrogen atoms, cations like the guanidinium ion system (14) do not react with H2 (eq 17). 

A comprehensive series of ionic hydrogenation reactions have been studied by Koltunov, Repinskaya, and co-workers, and superelectrophilic intermediates have been proposed.34 Some of these intermediates have been characterized by and 13C NMR (Table 4). Many of these dicationic intermediates have been shown to react with cyclohexane by hydride abstraction, indicating superelectrophilic character. 

Interestingly, the comparable monocation (190) is not reactive towards benzene or cyclohexane. This is an indication of the superelectrophilic character of dication 188. The isomeric hydroxyquinolines and 5-hydroxy-isoquinoline react with 5-7 molar excess of aluminum chloride and cyclohexane at 90°C to give ionic hydrogenation products, and the corresponding distonic superelectrophiles (191-193) are proposed as intermediates. 

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