Theoretical Study On The Mechanism And Selectivity Of Phosphine Promoted Coupling Reactions

Organnophosphorus compounds play an important role in the asymmetric reactions catalyzed by transition metal complexs or organocatalysts.Chiral phosphine compounds are commonly used as ligands in transition metal catalyzed asymmetric reactions.The volume of phosphine atoms is relatively large,and their polarizable ability is outstanding.Therefore,phosphine compounds can act as catalysts in some reactions.Up to now,more and more phosphine compounds are used as catalysts or ligands in asymmetric catalytic reactions.The research work of this thesis includes the following of six parts:In the first part of this thesis,the mechanism and the origin of enantioselectivity for the phosphine catalyzed γ-addition of 5H-thiazol-4-one to allenoate was investigated by performing DFT calculations.The results indicate that the reaction mechanism is involve to the nucleophilic attack,proton removal,γ addition,hydrogen migration and substrate release.The enantioselectivity of the reaction comes from the repulsion between the phenyl group of the reacting thiazolone and one of the phenyl group in the catalyst.In the second part.We studied the enantioselective and the regioselectivity of phosphine-catalyzed enantioselective C-2-and C-4-selective γ-additions of oxazolones to 2,3-butadienoates.Theoretical studies via DFT calculations suggested that the origin of the regioselectivity was due to the distortion energy that resulted from the interaction between the nucleophilic oxazolide and the electrophilic phosphonium intermediate.The conjugation of the phenyl group leads to the reaction always occur on the carbon that connected with methyl group.In the third part of this thesis,we reported an unprecedented enantioselective α-addition of deconjugated butenolides,as opposed to the well-studied asymmetric γ-addition of deconjugated butenolides.DFT calculations revealed that the observed regioselectivity was due to the distortion energy that resulted from the interaction between the nucleophilic dienolate and the electrophilic ortho-quinone methide.In the fourth part,combination of theoretical and experimental studies were performed to explore the mechanism,regioselectivity,and enantioselectivity of phosphine-catalyzed [3+2] cycloadditions of allenoate to acrylate.Using density functional theory computations we predicted that both the regioselective and enantioselective determining step is the nucleophilic addition of acrylate with the catalyst-activated allenoate.In the key step,we confirmed a model based on a single hydrogen bond formed between the β-amino of phosphine catalyst and reacting acrylate,as confirmed by theoretical and experimental studies.The computational model showed that steric repulsion between the naphthalenyloxy group on the reacting acrylate and the amino acid moiety of the catalyst led to the observed enantioselectivity.The density functional theory calculation is used to investigate the mechanism,chemoselectivity and enantioselectivity for Rhodium-catalyzed desymmetrization of cyclohexadienones,Computational results indicate that the preferred pathway involves transmetallation to form an aryl–rhodium compound,alkyne insertion,intramolecular olefin insertion,and protonation to generate the hydrobenzofuran product.The enantioselectivity is controlled by the intramolecular olefin insertion step,which is ascribed to the steric repulsions between the ligand and substrate.In the last part,The density functional theory method M11 is employed in order to elucidate how to cleave the in active C(methyl)-Si bond.The computational results indicate that oxidative addition/reductive elimination pathway is favored over direct transmetallation in the C(methyl)-Si bond cleavage step.Theoretical calculations show that the hydrolysis of both aryl and vinyl intermediates are inhibited by intramolecularπ-coordinated groups.