Investigation of origin of stereo-selectivity of BF3·Et2O-promoted allylboration of aldehydes in the presence of (R)-pinanediol by computational method

The effect of position of BF3 on the BF3·Et2O-promoted allylboration of aldehydes in the presence of (R)-pinanediol have been investigated. The results showed that the energy differences between TS-b2 and TS-a which has been regarded as two competing TS structures is very low, and TS-b2 is favorable only by about 0.9 and 2.5 kcal/mol in the gas phase and the solvent, respectively. Lack of 1,3-syn-pentane interaction in the TS-b2 case, and the withdrawing effect of BF3 as a Lewis acid could explain this phenomenon. NBO analysis showed that the coordination site is crucial in the amount of interaction between the O atom of aldehyde moiety with the B atom of chiral ligand. Attaching BF3 to the aldehyde moiety causes less interaction between the aforementioned two atoms. When BF3 is attached to the chiral ligand this interaction is increased. In this study acetaldehyde was used as a model instead of Ph(CH2)2CHO which was used in experimental work by Chen and Roush [Org. Lett. 12 (2010) 2706–2709].