Theoretical Study On The Reaction Mechanism Of Nickel Complexes Catalyzed Reactions

Due to relatively low toxicity and low price of nickel compared with palladium,the nickel complex catalyzed reactions have attracted extensive attentions of chemical researchers in the past decades.Initially,the nickel catalysts were considered as the low-cost alternatives to the palladium catalysts.However,with an account of further researches,the nickel catalysts even show better catalytic performance than palladium catalysts,especially in activation of some inert C?O,C?C,C?S,and C?H bonds.Because of the weak electronegativity and easily providing its d electrons to the?acceptors,the nickel complexes show good catalytic performance for cross coupling,isomerization and related reactions.Although there are a number of experimental researches on nickel complex catalyzed reduction of inert C?S bonds,[3+2]cycloaddition and olefin isomerization,few theoretical studies on the reaction mechanism have been found.Therefore,herein,the detailed reaction mechanisms of these three reactions have been studied systematically by using theoretical methods.The details are as follows:?1?Reaction mechanism of“ligandless”Ni-catalyzed hydrodesulfurization of aryl sulfideThe reaction mechanism of Ni?cod?₂-catalyzed hydrodesulfurization of aryl sulfide PhSMe with HSiMe₃ as the reducing agent has been studied by using density functional theory methods.Both PhSMe-coordinated pathway and“ligandless”pathway have been identified and compared.It is found that these two reaction pathways are kinetically competitive and the?-complex assisted metathesis??-CAM?transition state is the highest point on each profile for both pathways.Moreover,both the singlet and triplet reaction pathways of ligand substitutions have been compared and found that both singlet and triplet reaction mechanisms are competitive for the ligand substitution of cod with PhSMe on PhSMe-coordinated pathway while the triplet mechanism holds a distinct advantage over singlet one for that of cod with HSiMe₃ on“ligandless”pathway.?2?Reaction mechanism of Ni?cod?₂-catalyzed[3+2]cycloaddtion of methyleneaziridines with diynesThe detailed reaction mechanism of Ni?cod?₂-catalyzed cycloaddition of methyleneaziridines with diynes has been studied by using density functional theory methods.The catalytic cycle proposed in experiments have been first examined involving oxidative coupling,?-C elimination and redunctive elimination steps.Then,other possible reaction mechanisms of the cycloaddition reaction have been considered and calculated.It's found that C?C oxidative addition of methyleneaziridine to Ni center is preferred to the oxidative coupling process proposed in experiments.The calculated preferred reaction pathway involves C?C oxidative addition,insertion,reductive elimination,C?H oxidative addition,isomerization and reductive elimination steps.Moreover,the reaction mechanism of dimerization of diynes has been considered.In addition,for different diynes,the energy barriers of cycloaddition and dimerization have been calculated and compared.The theoretical yields have been calculated and found close to these listed in experiments.?3?Reaction mechanism of Ni?PPh₃?₂-catalyzed isomerization of N-allylamides to generate N-propenylamidesReaction mechanism of Ni?PPh₃?₂-catalyzed isomerization of N-allylamides to generate N-propenylamides has been studied theoretically in detail by using density functional theory methods.The C?H bond activation,isomerization,and reductive elimination to form new C?H bond steps have been involved.For C?H bond activation and isomerization steps,Ni?PPh₃?₂ and Ni?PPh₃?with only one PPh₃ ligand have been considered and found that the former is more active than the later for these two steps.Both?-allyl and?-allyl mechanisms have been calculated for isomerization and found that the?-allyl mechanism is preferred kinetically to the?-allyl one.The rate-determining energy barrier to generate the E isomer of product is 141.8 kJ/mol,close to that of 141.1 kJ/mol to generate the Z isomer,in agreement with the experimental result that E/Z=56/44.Considering Pd?PPh₃?₂ as the catalyst active species,it is found that the rate-determining energy barriers for the formation of E and Z isomers are more than 175 kJ/mol,consistent with the experimental observation that Pd?PPh₃?₂ showed no reactivity.The difference of reactivity between Ni?PPh₃?₂ and Pd?PPh₃?₂ can be understood from the more strong back-donation of d electrons from Ni to?*anti-bonding orbital of allyl anion comparing with Pd.In addition,the influence of substituent in reactants on E/Z selectivity has been analyzed and found that the steric repulsion between substituent and Ph of PPh₃ in the rate-determining transition states to generate E and Z isomers induces the difference of E/Z selectivity.