Mechanisms Of Alkyne Cyclization Catalyzed By Palladium Complexes And Alkyne Hydrogenation Catalyzed By Manganese Complexes:A Theoretical Study

Organic reactions catalyzed by transition-metals are of great significance for organic synthesis.Theoretical studies about alkyne cyclization and semihydrogenation were carried out to reveal the reaction nature via DFT(density functional theory)study,and the causes of chemo-,regio-and stereoselectivity were clarified in this thesis.The development of theoretical work provides evidence for the experiments and guides the further synthesis on new types of efficient catalytic systems.The results of the above fieleds studied in this paper are summarized as follows:(1)The mechanism of the diarylalkyne cyclization via the selective insertion of isocyanide to form fused tetracyclic heterocycles catalyzed by divalent palladium(Pd)catalysts was studied using DFT method.Novel conclusions and trends can be reflected in the calculation results,the following steps were mentioned: aminopalladiation,H shift,anti-carbopalladiation,isocyanide insertion,“Pd walk” and reductive elimination steps,the isocyanide insertion step is the rate-determining step with an energy barrier of 22.6kcal/mol.The reaction can proceed smoothly on the existence of dinuclear Pd intermediates: the anti-carbopalladation and “Pd walk” process were achieved via dinuclear Pd intermediates.Importantly,an interesting “Pd walk” process involving several multinuclear Pd intermediates was proposed to complete the catalytic cycle,Pd can transfer between several multinuclear Pd intermediates which exists stably due to the non-covalent interactions between two different aromatic heterocycle intermedients.The results of this part is a novel catalyst reaction mode and provides a theoretical guidance for the study of multinuclear metallic systems.(2)The mechanism of alkyne semihydrogenation to Z-alkene catalyzed by manganese complexes with pincer ligands(Mn-PNP)has been investigated via DFT study.The calculated results show that the whole catalytic cycle includes two stages: methanol dehydrogenation and alkyne semihydrogenation.In stage I,methanol dehydrogenation is a more advantageous path than dihydrogen activation and methanol-assisted dihydrogen activation.The Gibbs free energy barrier for methanol dehydrogenation is 6.2 kcal/mol with an outer-sphere mode,hydride transfer step is the rate-determining step of this stage.In stage II,the semihydrogenation of alkyne to Z-alkene catalyzed by dimolecular Mn-PNP complexes with an energy barrier of 17.2 kcal/mol is better than catalyzed by monomolecular Mn-PNP complexes.The hydrogen bond interactions and van der Waals interactions between the functional groups in different catalysts are the main cause for the stable existence of dimolecular Mn-PNP complexes.The above results reveal the nature of the controlled semihydrogenation of asymmetric alkynes catalyzed by manganese complexes with PNP pincer ligands,a novel mechanism for efficient conversion of alkynes by means of a special stacking mode is proposed,which provides a promising direction for novel catalytic system design.