DFT Study On Mechanism Of Transition Metal-Catalyzed Asymmetric Reaction

In present paper, the asymmetric simmons-smith reaction, enantioselective palladium(O)-catalyzed ally lie alkylation with chiral oxazolinylpyridines and asymmetric hydrogenation of enamides with [Rh(BisP*)]+ catalyst are investigated, respectively, by means of the nonlocal density functional method (B3LYP). The mechanisms of these enantioselective reactions are discussed in detail.Density Functional Studies on Asymmetric Simmons-Smith ReactionThe asymmetric reactions of dichloromethane and diiodomethane with (3Z,2S)pentan-3-enyl-2-ol catalyzed by zinc are studied by means of the density functional theory. As is shown, the reactions are exothermic. The key step determining the chirality of the products is the reaction leading to cyclopropane. The reaction of diiodomethane with (3Z,2S)pentan-3-enyl-2-ol is of high enantioselective selectivity and is much faster than the reaction of dichloromethane with (3Z,2S)pentan-3-enyl-2-ol.The Mechanism of Enantioselective Palladium(O)-Catalyzed Allylic Alkylation with Chiral Oxazolinylpyridines: A DFT StudyThe density functional computations of the asymmetric allylic alkylation of (?tans-l,3-dimethylallyl formate, 2, with malonaldehyde catalyzed by chiral Pd-oxazolinylpyridine are performed. All the structures are optimized completely at the B3LYP/LANL2DZ+P level. As illustrated, this allylic alkylation is endothermic and goes mainly through association of Pd-oxazolinylpyridine with trans- 1,3-dimethylallyl formate, oxidative addition of HCOO' in 2 to Pd, nucleophilic addition of malonaldehyde anion to the -allyl cation complex, and dissociation of the Pd-oxazolinylpyridine-product complex to generate the product with regeneration of the catalyst. The turnover-limiting step for this reaction is the nucleophilic addition of malonaldehyde anion. The main substitution product predicted theoretically is (S)-trans-1,3-dimethylally malonaldehyde. The transition states for the oxidative addition and the nucleophilic addition involve a twisted papilionaceous Pd-C-C-C four-membered ring.The Mechanism of Asymmetric Hydrogenation of Enamides with [Rh(BisP*)]+ Catalyst: A Model DFT StudyThe potential energy profile for the [Rh(R,R)-Et-BisP*]+-catalyzed asymmetric hydrogenation of a prochiral enamide, a-acetamidoacrylonitrile, has been studied using a nonlocal density functional method (B3LYP). All the structures are optimized completely at the B3LYP/6-31G(d,p) level. As illustrated, this hydrogenation is endothermic and goes mainly through the association of [Rh(R,R)-Et-BisP*]+ with a-acetamidoacrylonitrile, oxidative addition of hydrogen to give stable six-coordinate pseudo-octahedral dihydride complex, migratory insertion of an olefin carbon into a Rh-H bond to form a five-coordinate alkyl hydride, reductive elimination of C-H from the alkyl hydride to produce the alkane product coordinated to the catalyst, and dissociation of product-catalyst adduct to generate the product with regeneration of the catalyst. The turnover-limiting step for this reaction is the oxidative addition of hydrogen. The main substitution product predicted theoretically is R-conformation. Our results are consistent with available empirical data for rhodium-catalyzed asymmetric hydrogenation. The less stable amide-catalyst adduct has a considerably smaller barrier for reaction with hydrogen than the more stable adduct, reproducing the "anti-lock-and-key" behavior common in asymmetric hydrogenation.