ABSTRACT
A more comprehensive representation of the Schlenk equilibrium is shown in Scheme 1 . Ab initio molecular orbital calculations for R = Me and X = CI and Br were performed to determine which species in Scheme 1 were thermodynamically preferred [ 1 4] . In the absence of solvents, the various (MeMgCI}z-bridged dimers were all significantly more stable than the corresponding monomers. For the reaction
2MeMgCI �(MeMgClh
the tiRO ranged from - 24.8 to - 37. 1 kcal/mol, and tiGo ranged from - 1 5 .8 to - 33.0 kcal/mol. On the other hand, in the presence of the solvent dimethyl ether, calculations showed that the dimerization process was nearly thermoneutral; tiGo was - 5 . 1 kcal/mol. The authors suggested that the relatively small negative tiGo value indicated that the position of the eqUilibrium could easily be shifted in Scheme 1 to favor the presence of monomers by altering the halide, the alkyl group, or the solvent. The tiRO and tiGo for
Scheme 1
the reaction
2MeMgCI :!:+(MehMg + MgCl2
was calculated to be + 5 .7 and + 4.5 kcal/mol, respectively. Substitution of Br for CI yielded corresponding ,1Ho and ,1Go values 1 .3 kcal/mol less endothermic. Calculations that included dimethyl ether solvation also indicated a slightly more favorable reaction.
