SELECTIVITY IN THE REACTIONS OF LESS STABLE CARBOCATIONS WITH MAGNESIUM HALIDES IN APROTIC-SOLVENTS - A THEORETICAL APPROACH TO THE CONSTANT SELECTIVITY PRINCIPLE

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