Density Functional Theory Studies On C-O/N Bond Activation Reactions Catalyzed By Nickel Complexes: Mechanisitc Insight And Tunable Reactivity

In recent years,transition metals have been widely used to catalyze various reactions,which shows strong catalytic activity.Therefore,transition metal catalysis is a promising field.The study of catalytic reaction mechanism is very important for the improvement of catalyst design and optimization of reaction conditions.Thus,many researchers have done a lot of researches on C-O/N activation catalyzed by transition metals.Compared with the experiments,the theoretical study of C-O/N activation catalyzed by transition metals is relatively few,and the relevant reaction mechanism is not clear.Therefore,it is necessary to reveal the hidden micro-mechanism of macro-reaction from molecular level and to promote the development of transition metals in the field of catalysis.In this paper,the mechanism of the reaction and the activation regulation of C-O/N bond catalyzed nickel complexes are studied by density functional theory?DFT?and natural orbital analysis?NBO?.The main research contents and conclusions are summarized as follows:?1?The reduction carboxylation of C-O bond catalyzed by nickel complex includes three steps:C-O bond cleavage,reduction elimination and CO₂insertion.The C-O bond cleavage is the rate-determining step among these steps,which is mainly affected by the substituent group of nickel complex.The smaller substituent group of nickel complex is,the greater catalytic activity of nickel complex is.?2?For substrates with extended?-systems,the oxidative addition is the optimal pathway for C-O bond cleavage.Following this mechanism,we also successfully elucidated that the nickel complex containing phenyl-substituted bidentate ligand?Ni⁰L1?has the highest catalytic reactivity toward oxidative addition of C?sp²?-O bond,whereas the P?Me₃?₂ was computationally shown to be the optimal ligand for C?sp³?-O bond cleavage.In contrast,the radical pathway is preferred for substrates without extended?–systems?including bimetallic species?.In such homolytic cleavage of C-O bond,the hemilabile directing ligand is highly important because of several reasons:?i?it provides an additional site available for coordination of the second Ni⁰?PMe₃?₂ catalyst,thus effectively stabilizing the transition state of homolytic cleavage of C-O bond;?ii?coordination of two Ni⁰?PMe₃?₂ catalysts on the substrates leads to C-O bond dissociation with a smaller oxidation state change of Ni catalyst?Ni⁰/Ni^I?,and thus the lower reaction barrier is anticipated;?iii?the formation of two Ni^I species is the most exergonic,providing the largest thermodynamic driving force for the rate-determining step of C-O bond cleavage?3?For the reaction of Ni?0?-catalyzed denitrogenative transannulation of1,2,3-benzotrazin-4?3H?-ones with alkynes,the computational results revealed that the transformation is composed of four steps:nitrogen extrusion,carbometalation,Ni-C bond insertion and reductive elimination.The alkyne insertion into Ni-C bond was proved to be the rate-determining step.The advantages of substituents on chemical reactivity and regioselectivity originate from multiple reasons:?i?phenyl groups on N atom of benzotriazinone and/or unsymmetrical alkynes mainly account for the high reactivity and regioselectivity via its electronic effect.?ii?The?-?interaction between the phenyl substituent on the alkyne and triazole ring might partially contribute to the high regioselectivity when unsymmetrical alkynes were employed as the substrates.