Density Functional Studies On Mechanisms Of Nickel(0) Catalyzed Carbon-Carbon Bond Cleavage Reactions

Catalysis plays a pivotal role in almost all strategies for the establishment of energy-and atom-efficient sustainable chemical technologies.Efficient and predictable creation of carbon-carbon/carbon-sulfur bonds draws much attention over the last few decades.N ickel catalysis is one of the most promising approaches for the development of new sustainable synthetic methods.This ab undant and nontoxic metal has been shown to perform well in many important reactions that had been traditionally dominated by noble-metal-based complexes,especially palladium.Computational tools have taken on significant roles in homogeneous catalysis,thanks to ever-increasing computer power and molecular modeling methods that balance cost and accuracy.Reaction mechanisms revealed by combined theoretical and experimental studies can support the rational design of new catalytic systems.In the thesis,we perform theoretical mechanism analysis on two recently developed N i(0)catalyzed carbon-carbon bond cleavage reactions.i)N i-Catalyzed 1,2-acyl migration reactions triggered by C-C bond activation of ketones.The process of 1,2-acyl migration followed by olefin isomerization provides a convenient access to α,β-unsaturated ketones.The mechanism includes a fast oxidative addition step followed by β-hydride elimination,N i-hydride insertion and reductive elimination.Free energy profile along the reaction path shows that the rate-determining step of the catalytic cycle is the β-hydride elimination step.ii)A controlled Ni-catalyzed decarbonylation process of α-ketothioesters.Mono-and double-decarbonylations,which gave thioesters and thioethers,respectively,were selectively achieved by changing the ligand.Computations support that the catalyst is Ni L(CO),where L represents a ligand.The P(Ph)3(L1)and N-heterocyclic carbine(NHC)ligand IPrMe(L2)are chosen in this study according to their excellent reaction selectivity.Irrespective of the ligands,the reaction path of mono-decarbonylation is composed of oxidative addition of the C-S bond,dissociation of CO,migratory extrusion of carbon monoxide and reductive elimination.If instead of the red uctive elimination,a CO ligand is released followed by a second migratory extrusion of carbon monoxide and reductive elimination,the product thioether is obtained.In the ligand L1 case,the rate-determining intermediates(RDIs)of the catalytic cycles are both the species after the first migratory extrusion of carbon monoxide,however,the rate-determining transition state(RDTS)is for the oxidative addition in the mono-decarbonylation process and the reductive elimination step in the double-decarbonylation process.In the ligand L2 case,the RDIs are both the coordination complex of the substrate and the catalyst while the RDTSs are both for the reductive elimination step.The reaction selectivity is due to the differences in the energy spans of each process where the one for double-decarbonylation with L1 is the highest.The thesis demonstrates how the detailed dynamics of the Ni(0)catalyzed homogeneous reactions could be obtained through DFT computations.The information is not only in line with experimental observations,but also could provide insights on further catalytic system development.