Ab Initio Calculations on Potential Intermediates in Metal Reductions of Carbonyl Compounds. Stereochemical Considerations

Conformational preferences of ketyls, hydroxyalkyl radicals, and hydroxyalkyl carbanions derived from propionaldehyde and cyclohexanone have been studied with ab initio molecular orbital calculations. Each species is significantly pyramidalized toward a tetrahedral geometry, and there is staggering about the single bonds to the pyramidal center. For the 1-hydroxypropyl radical, there is little conformational preference about the C₁-C₂ bond. For both propionaldehyde ketyl and the 1-hydroxypropyl anion, the conformations with the methyl group situated gauche to the C-O bond and anti to the nonbonding orbital (anti conformation) are considerably less stable than the conformations with the methyl group gauche to both the C-O bond and the nonbonding orbital (inside conformation). The third conformations with the methyl group anti to the C-O bond are least stable (outside conformation). Cyclohexanone ketyl, the 1 -hydroxycyclohexyl radical, and the 1-hydroxycyclohexyl anion in equatorial conformations are 2.4, 0.8, and 3.9 kcal/mol respectively, more stable than in axial conformations at the MP2/6-31+G* level. The high equatorial preferences of the cyclic species dramatically contrast with the conformational preferences of the acyclic species. The mechanism and stereochemistry of metal reductions are also discussed.