Density functional computations of alkynylation of nitrones catalyzed by chiral zinc(II)-complexes

Abstract

The alkynylation of nitrones catalyzed by chiral zinc(II)-complexes was studied by means of the density functional theory (DFT). All the intermediates and transition states were optimized completely at B3LYP/6-31G(d,p) level. Calculation results confirm that this alkynylation of nitrones was endothermic and the total absorbed energy was about 14 kJ/mol. The formation of the M5 complexes was the rate-determining step and the chirality-limiting step for this alkynylation. The transition states TS3-re involve a H(8)-O(2)-Zn(6)-C(9)-C(14)-N(13)-O(12) 7-membered ring, and thus the transition states TS3-si involve a Zn(6)-C(9)-C(14)-N(13)-O(12) 5-membered ring. The dominant reaction is the attack of nitrones from the re-surface of M4. The reactivity of anti-nitrones was stronger than that of syn-nitrones for zinc-catalyzed alkynylation of nitrones. The dominant reaction channel was cata ? TS1b ? M1b ? M2 ? M3b ? TS2b ? M4b ? TS3b1-antire ? M5b1-anti-re ? Pro-R. The dominant products predicted theoretically were of R-chirality, (2R)-N-hydroxy-N-methylpent-3-yn-2-amine.