The origin of diastereofacial control in allylboration reactions using tartrate ester derived allylboronates: attractive interactions between the Lewis acid coordinated aldehyde carbonyl group and an ester carbonyl oxygen
Transition-state structures for the allylboration reaction between the tartrate ester and tartramide modified allylboronates and acetaldehyde are located at the B3LYP/6-31G* level of theory. An attractive interaction between the boron-activated aldehyde and the ester or amide carbonyl oxygen lone pair is found to play a major role in the favored transition states 11a and 13. This attractive interaction appears to be electrostatic in origin. However, an n --> pi* charge-transfer type of interaction has not been ruled out. The distance (2.77 A) between the aldehydic hydrogen and the carbonyl oxygen in transition state 13 is beyond the sum of van der Waals radii. The formyl C-H...O bond angle (109 degrees) in this transition structure deviates far from linearity. Therefore, hydrogen-bonding interactions between the formyl C-H and the amide carbonyl oxygen are considered negligible. The distance (3.81 A) between the aldehydic oxygen and the amide carbonyl oxygen in the diastereomeric, disfavored transition state 14 is also beyond the van der Waals radii, which suggests that n/n electronic repulsion plays a lesser role in stereodifferentiation in the allylboration reaction than originally proposed.