Mechanism Investigation Of Ni-catalyzed Coupling Reactions

Transition-metal catalyzed coupling reactions represent a powerful tool for theconstruction of C-C bonds and C-X (heteroatom) bonds. At present, tranditionaltransition-metal catalyzed cross-coupling reactions includes Suzuki cross-coupling,Kumada cross-coupling, Negishi cross-coupling, Stille cross-coupling etc. However,these tranditional transition-metal catalyzed cross-coupling reactions needpreprepared carbon nucleophilic reagents that are sensitive to air and moisture.Therefore, direct reductive cross-coupling reaction that does not need prepreparecarbon nucleophilic reagents and direct C-H bonds activation attract considerableattention in recent years. Although several elegant works have been accomplished inthis area, the detailed mechanism investigation has not been explored using theoreticalmethods. Hence, in this essay, we choose Ni-catalyzed reductive coupling reactionsand Ni-catalyzed chelation-assisted C-H bond activation as research objects andexplore their detailed mechanisms using DFT calculation with B3LYP method. Mainworks are listed below:I. Mechanism investigation of Ni-catalyzed reductive coupling reaction for theformation of biaryl. In this progress, singlet/triplet Ni0and NiIcomplexes acting asthe active catalysts were discussed to search the optimal reaction path. Besides that,the effects of solvent and ligand to the reaction were also explored. Based on theobtained results, the triplet Ni0catalyst is favored and the whole catalytic cycleinvolves the following basic steps:1. First oxidative addition (Ni0â†'NiII);2.Reduction (NiIIâ†'NiI);3. Second oxidative addition (NiIâ†'NiIII);4. Reductiveelimination (NiIIIâ†'NiI), and catalyst regeneration (NiIâ†'Ni0). Using the simulationmethod, we also studied the mechanism of Ni-catalyzed reductive cross-coupling toform unsymmetrical biaryl compounds and proved that organozinc reagent did notexist in the reaction system.II. Mechanism investigation of Ni-catalyzed reductive cross-coupling of aryl bromideand secondary alkyl bromide. In this process, several proposal mechanisms including the concerted mechanisms and the single electron transfer mechanisms were explored.Based on the calculated results, there are two mechanisms are feasible for thisreaction system. The first one is a five-step catalytic cycle, which involves radicalproduction (NiIâ†'NiII), reduction (NiIIâ†'Ni0)， oxidative addition (Ni0â†'NiII), radicaladdition (NiIIâ†'NiIII), reductive elimination (NiIIIâ†'NiI), and catalyst regeneration(NiIIâ†'NiI). Another mechanism includes the following basic steps:1. oxidativeaddition (Ni0â†'NiII);2. reduction (NiIIâ†'NiI);3. radical production (NiIâ†'NiII);4.radical addition (NiIIâ†'NiIII);5. reductive elimination (NiIIIâ†'NiI) and catalystregeneration(NiIIâ†'NiI). For both mechanisms, the rate-determinging step is theradical addition process with the energy barrier of10.42kcal/mol.III. Mechanism investigation of Ni-catalyzed chelation-assisted C-H activation. In thisprocess, mono/bis phosphorus ligand and singlet/triplet Ni0complex acting as theactive catalyst were discussed to find the optimized reaction path. Based on theacquired results, the overall catalytic cycle involves the following basic steps:oxidative addition (Ni0â†'NiII), insertion (NiIIâ†'NiII), elimination (NiIIâ†'NiII), insertion(NiIIâ†'NiII) and reductive elimination (NiIIâ†'Ni0). Oxidative addition is therate-limiting step for the overall catalytic cycle with the energy barrier of26.04kcal/mol.