THEORETICAL-STUDIES OF STEREOSELECTIVITIES OF NUCLEOPHILIC 1,2-ADDITIONS TO CYCLOHEXENONES - TRANSITION STRUCTURES AND FORCE-FIELD MODELS FOR METAL HYDRIDE AND KETENIMINATE ADDITIONS TO KETONES

The transition structures of the reaction of 2-cyclohexenone with lithium hydride have been located with the 3-21G basis set. The transition structure for axial addition is more stable than the transition structure for equatorial addition by 2.1 kcal/mol. The enone moiety is more nearly coplanar in the transition structure of axial addition than in that of equatorial addition, as indicated by the O=C-C=C dihedral angles of 162-degrees and 146-degrees, respectively. The axial transition structure is nearly perfectly staggered, while there is severe eclipsing in the equatorial transition structure. The high axial selectivity for the nucleophilic additions to cyclohexenones is caused mainly by torsional strain and poor orbital overlap in the equatorial transition structure. The transition structure for the reaction of lithium acetonitrile with acetone was located with ab initio molecular orbital calculations. It is a six-membered planar structure. The transition structure for the reaction of potassium acetonitrile has a nonplanar geometry. This difference between the transition structures is believed to cause the difference in stereoselectivity observed experimentally for the reaction of cyclohexanones and cyclohexenones with lithium and potassium propionitrile. A simple modification of the MM2 force field, based on the torsional strain model, qualitatively reproduces observed stereoselectivities.