ON THE ORIGIN OF PI-FACIAL DIASTEREOSELECTIVITY IN NUCLEOPHILIC ADDITIONS TO CHIRAL CARBONYL-COMPOUNDS .2. CALCULATED TRANSITION-STATE STRUCTURES FOR THE ADDITION OF NUCLEOPHILES TO PROPIONALDEHYDE-1, CHLOROACETYLDEHYDE-2, AND 2-CHLOROPROPIONALDEHYDE-3

The transition state structure for addition of LiH to 1, 2, and 3 are computed and analyzed at MP2/6-31G(d)//HF/6-31G(d). Three factors are found to be important for the relative energies of the transition state structures; (i) the conformational energy of the aldehyde; (ii) the interactions between the nucleophile and the aldehyde; (iii) the counter ion effect. The relative energies of the lowest lying transition state structures for addition of LiH and CN- to 3 leading to the major and minor product are not determined by the interactions between the attacking nucleophile and the aldehyde, but by the conformational energies of the aldehyde in the two transition states. The interaction between LiH and 3 is in fact more favorable in the transition state leading to the minor isomer.