Study On Oxazaborolidine Catalysts And The Michanism Of Enantioselective Reduction Reaction

The use of chiral materials is extensive in our life. The enantioselective catalytic reaction is considered widely to be an important way by which chiral compounds are generated. Multifarious catalysts are used in enantioselective catalytic reactions for the sake of obtaining chiral compounds and the chiral oxazaborolidines catalysts were attached high importance to because of their unique characteristics. In the present paper, the enantioselective reduction of prochiral ketone with borane and the enantioselective reduction of prochiral aldehyde with alkynylborane catalyzed by chiral oxazaborolidines are investigated, respectively, by means of the semi-empirical AMI molecular orbital method and the ab initio molecular orbital method. The mechanisms of these enantioselective reductions are discussed in detail.Computations of the enantioselective reduction ofp-methoxyhypnone with boranecatalyzed by CBS catalystsIn this part, the semi-empirical AMI molecular orbital method is employed to study the enantioselective reduction of p-methoxyhypnone with borane catalyzed by chiral pyrrolidino [3,4-c] oxazaborolidine. The catalyst and all the structures of the intermediary and the transition states in this enantioselective reduction are optimized completely and the effects of variations in the structures of the intermediary and the transition states on the enantioselective catalytic reduction are studied. As shown, the catalyst and all the intermediary states have no imaginary frequencies and all the transition states have sole imaginary frequencies. The activating energies of all the transition states are analyzed and the reduction mechanism is explained. The theoretical result is in agreement with the experiment. The controlling-velocity step for the enantioselective reduction is the decomposition of the B-O-B-N 4-numbered ring.Quantum chemical study on the mechanism of enantioselective alknylation of aldehydes catalyzed oxazaborolidinesThe mechanism of the enantioselective reduction of aldehyde with alkynylborane catalyzed by (4s)-oxazaborolidines is studied by means of the DFT method. The catalyst and all the structures of the intermediary and the transition states for this alkynylation are optimized completely at the B3LYP/6-31 G(d) level. The catalyst and all the intermediary states have noimaginary frequencies and all the transition states have sole imaginary frequencies. The changes in the atomic charges are investigated carefully and their effects on the alkynylation are discussed. The natural bond orbital (NBO) analysis for the catalyst-alkynylborane-aldehyde adduct, the catalyst-alkoxyborane adduct and the transition states is performed. It is shown that among the four structures of the catalyst-alkynylborane-aldehyde adduct 5, the n carbonyl bond in 5a is the weakest and is much more easily broken down. In the B-O-B-N 4-membered ring of the adduct 6, the B(2)-OC-o and B2-N(3) bonds are weaker than the N(3)-B(2) and Oc=O-B2 bonds. Among the four structures of the transition state TS3, the B2-C1 bond for TS3a is the weakest and the Cc-o-C l* bond is the strongest. Therefore, the transfer of alkynyl in TS3a is the easiest. During the decomposition of the catalyst-alkoxyborane adduct 6, the B-N and B-O bonds in the B-O-B-N 4-membered ring may be broken down simultaneously.