Selectivity in the Reactions of Less Stable Carbocations with Magnesium Halides in Aprotic Solvents. A Theoretical Approach to the Constant Selectivity Principle.

Article abstract of DOI:10.1016/0040-4020(95)00388-O

The solvolysis of 7,7-bis(trifluoromethylsulfonyloxy)-2-norbornene (1a), 1-trifluoromethylsulfonyloxy-1-phenylethylene (2a), 2-methyl-1-cyclohexen-1-yl triflate (3a) and 4-camphenyl triflate (4a) in carbon disulfide or diethyl ether with added MgI2 and MgBr2 yields a mixture of the corresponding halides 5b-e, 6b-c, 7b-c and 8b-c.The ionization of the triflate group is favoured by coordination with the magnesium halide.The reaction with MgI2 was found to be second-order and is about 50 times more reactive than MgBr2.The selectivities of the intermediate cations 5-8 toward the nucleophiles MgI2 and MgBr2 are nearly constant, although the solvolysis rates differ by more than three powers of ten.A theoretical explanation of the constant selectivity principle is given.