Selectivity in the reactions of less stable carbocations with magnesium halides in aprotic solvents. A theoretical approach to the constant selectivity principle

• Published in: Tetrahedron 1995-06, Vol.51 (25), p.7077-7084
• Main Authors: Martínez, A.García, Alvarez, R.Martínez, Teso Vilar, E., García Fraile, A., Osío Barcina, J., Subramanían, L.R.
• Format: Article
• Language: English
• ISSN: 0040-4020; 1464-5416
• 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 MgI 2 and MgBr 2 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 halides. The reaction with MgI 2 was found to be second-order and is about 50 times more reactive than MgBr 2. The selectivities of the intermediate cations 5–8 toward the nucleophiles MgI 2 and MgBr 2 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.