Hence, the calculations are consistent with the experiments and predict that the reaction of 1b with 2a should be under kinetic control and highly stereoselective at room temperature (G ‡ (298K) = 3.6 kcal mol -1 ). Comparison of TS-1, TS-2, and TS-3 explains the origin of syn-selectivity. TS-1 is a four-membered transition state, and its structure determines the syn-addition to the least hindered face of the alkene. The geometry of TS-1 is asymmetric with one short Mg 1 -C 1 (2.48 Å) and one long Mg 2 -C 2 (3.08 Å) bond. TS-2 is similar. NBO calculations are also consistent with asynchronous bond formation occurring in TS-1, as evidenced by analysis of the NPA charges, which develop asymmetrically for both pairs of magnesium and carbon atoms (Figure ). The data indicate that the Mg 1 -C 1 bond is formed earlier than the Mg 2 -C 2 bond. In contrast, TS-3 is more symmetric, with almost no difference in two Mg-C bond distances (2.62 Å and 2.67 Å). This closer approach of the Mg-Mg reagent, combined with the required antarafacial bond formation to the -system strongly disfavour the anti-addition by TS-3. Further comparison of TS-1 and TS-2 can be used to rationalise the facial selectivity. QTAIM calculations combined with non-covalent interaction (NCI) analysis for reveals that TS-1 is stabilised by a greater number of (π---H-C) noncovalent interactions than TS-2 (Figure ). While we have not quantified these, it is likely a balance between these attractive dispersion interactions, and steric repulsion that favours addition to the face with the methyl substituent.