DIASTEREOSELECTIVITY OF ENOLATE ANION PROTONATION - REACTION OF HCN WITH THE ENOLATE OF 3-FLUOROBUTANOIC ACID - AN AB-INITIO MOLECULAR-ORBITAL STUDY

The protonation of the enolate of 3-fluorobutanoic acid by hydrogen cy anide was chosen as a suitable computational model for the H/D exchang e reaction of ethyl 3-ethoxybutanoate in ethanol-d. A diastereomeric e xcess of 82% is found in the experimental system, compared to calculat ed selectivities which range from 84 to 91%, dependent on the level of theory used. Both cis and trans enolates yield similar diastereomeric ratios. In the lowest energy transition state for each diastereomeric pathway the C-F bond is oriented anti to the incipient C-H bond. Thes e two transition states are differentiated by steric forces, the highe r energy diastereomer having a gauche interaction between the CH3 and CO2H groups. The orientation of the C-F bond in these two transition s tates is rationalized as a stabilizing orbital interaction between the electron rich a orbital of the enolate-HCN bond and the low-lying sig ma orbital of the C-F bond, an interaction also proposed by Anh to ex plain the selectivity of nucleophilic addition to chiral carbonyl comp ounds. Alternatively, an electrostatic argument can account for the da ta. When the C-F bond is anti to the incipient C-H bond, the dipole mo ment, and hence the electrostatic energy, is at a minimum.