| Chiral materials exist everywhere in the nature. Chirality is in close relationship with life science, material science, biological engineering, and drug chemistry. As the rapidly increasing process of chirality, the enantioselective catalytic reaction is considered widely to be an important way by which chiral compounds are generated and it is attached high importance to. In this work, the enantioselective reduction of prochiral keto oxime ethers with borane catalyzed by cbiral oxazaborolidines is investigated by means of ab initio molecular orbital method and the mechanism of the enantioselective catalytic reduction is discussed in detail. By means of the Hartree-Fock ab initio method, the oxazaborolidine catalyst and all the structures of intermediary states in the enantioselective reduction of prochiral keto-oxime-ether with borane catalyzed by oxazaborolidine catalyst are optimized completely at HF/6-3 1 g* level. The effects of variations in the structures of the intermediary states on the enantioselective catalytic reduction are studied. Oxazaborolidine catalyst 1 is a twisted chair structure. There is a partial bond between the 0(1) and B(2) atoms. The interaction between N(3) of the catalyst 1 and BH3 results in the formation of the oxazaborolidine-borane adduct 2, the formation reaction is exothermic. The formation of the adduct 2 causes the B-H bond in the borane moiety to be weakened greatly. The interaction between the adduct 2 and the keto oxime ether 1 results in the catalyst-borane-keto oxime ether adduct 3. Because the Oe=o and NC=N atoms of 1 can both donate lone pairs to the empty orbital of the B(2) atom, the adduct 3 may exist two types of structures, 3a and 3b, each of which has four stable plausible structures. The formation of the adduct 3a makes the C=0 bond in the 1 moiety weakened greatly and there is a much weak interaction between Cc=o of 1 and HBH3 of borane. The formation of the adduct 3b causes the C=N bond in the 1 moiety weakened and there is a much weak interaction between CC=N of 1 and HBH3 of borane, thus is of great advantage to the hydride transfer from the borane moiety to the carbonyl carbon and the oxime carbon. After a HBH3 transfers from borane moiety to Cc=o in the adduct 3a, the BBH3 atom may coordinate either with the Oc=o atom to form the adduct 4a with a 4-membered ring or with NC=N to form the adduct 4a* with a 7-membered ring. These two formation reactions are both exothermic. From the view point of energy, because of lower total energy and formation energy, the trend of formation of the adduct 4a is stronger than that of the adduct 4a. If the adduct 4a could be formed,in the next stCp, the B(2)-Oc-o bond may be broken down and Ncm coordinates withB(2), Which results in the formation of 5a' with a 7-membered ring. From thecomParison of the adduct 4a* with 5a', it can be found that the bond length andMulliken overlaP population of the C=N bond and the stabilizition interedionenergy E(2) betWeen all other bond orbitals and C=N ff antibond orbitals in adducts4a* and 5a' are almost similar thus imPlies that the formations of adducts 4a* and5a' affect the oxime bond at the same level. But the investigation of theirdominative configurations revels that the chirality of oxime carbon determined bythe dominative configuration of 5a' is opposite to the experemellt, While that'determined by the dominative configuration of 4a* is in agreement with theexperemeni.If a HBH3 transfers from borane moiety to the Ncm in the adduct 3b, the BBH3atom may coordinate either with the Nc=w atom to form the adduct 4b with a4-membered ring or with Ocro to form the... |
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