Energetics of Cation-Olefin Addition in the Gas Phase and in Solution

The cation-olefin addition of 2-methyl-2-propyl cation and 2-methylpropene has been investigated in the gas phase and in solution with ab initio calculations and Monte Carlo statistical mechanics simulations. The reaction is exothermic by ca. 20 kcal/mol and proceeds without activation energy in the gas phase. However, solvation introduces a 3-4 kcal/mol barrier at a C-C separation near 5 Å in methylene chloride, THF, and methanol. An intermediate in a shallow energy well is found near 3-Å separation. Implications for sterol biosynthesis are discussed.