Theoretical study of structure and reactions of metalated oximes and oxime ethers
The potential energy surfaces of acetaldoxime carb- anion and its ion pairs formed with lithium and sodium cations ions have been explored with ab fnftfo methods to model and study the regiochemistry of metalated oxime ethers. Planar structures of the carbanions produced by deprotonating acetal- doxime are minima on the potential energy surface. The syn- isomer is 2.6 Kcal/mole more stable than the anti. This dif- ference is not a manifestation of cyclic conjugation but more likely is a result of electrostatic effects. Two chiral and almost isoenergetic minima have been located for the ion pairs formed by either of the isomeric carbanions with Li+ or Na'. The gegenion engages either in face coordination or bridges the NO-bond in a r12-fashion. In oxime ethers face coordination is expected to become dominant for sterlc reasons. LiC-con- tacts are surprisingly long in all of the ion pairs. Bonding to the metals in the ion pairs is predominantly ionic. Ion pair formation increases the syn preference energy compared to the free anions, and the syn preference energy is greater for Na+ than for Li+. Reactions with electrophiles via the syn- coordinated metal permits prior coordination and ion pair for- mation in the Product.