title: Theoretical study on the mechanism of Ni-catalyzed alkyl-alkyl Suzuki cross-coupling.
abstract: Ni-catalyzed cross-coupling of unactivated secondary alkyl halides with alkylboranes provides an efficient way to construct alkyl-alkyl bonds. The mechanism of this reaction with the Ni/L1 (L1=trans-N,N'-dimethyl-1,2-cyclohexanediamine) system was examined for the first time by using theoretical calculations. The feasible mechanism was found to involve a Ni(I)-Ni(III) catalytic cycle with three main steps: transmetalation of [Ni(I)(L1)X] (X=Cl, Br) with 9-borabicyclo[3.3.1]nonane (9-BBN)R(1) to produce [Ni(I)(L1)(R(1))], oxidative addition of R(2) X with [Ni(I)(L1)(R(1))] to produce [Ni(III)(L1)(R(1))(R(2))X] through a radical pathway, and C-C reductive elimination to generate the product and [Ni(I)(L1)X]. The transmetalation step is rate-determining for both primary and secondary alkyl bromides. KOiBu decreases the activation barrier of the transmetalation step by forming a potassium alkyl boronate salt with alkyl borane. Tertiary alkyl halides are not reactive because the activation barrier of reductive elimination is too high (+34.7 kcal mol(-1)). On the other hand, the cross-coupling of alkyl chlorides can be catalyzed by Ni/L2 (L2=trans-N,N'-dimethyl-1,2-diphenylethane-1,2-diamine) because the activation barrier of transmetalation with L2 is lower than that with L1. Importantly, the Ni(0)-Ni(II) catalytic cycle is not favored in the present systems because reductive elimination from both singlet and triplet [Ni(II)(L1)(R(1))(R(2))] is very difficult.
authors.affiliation: Department of Chemistry, University of Science and Technology of China, 230026 Hefei, China.
authors.forename: Zhe
authors.initials: Z
authors.lastname: Li
authors.forename: Yuan-Ye
authors.initials: YY
authors.lastname: Jiang
authors.forename: Yao
authors.initials: Y
authors.lastname: Fu
mesh_terms: D002384:Catalysis; D003432:Cross-Linking Reagents / Q000737:chemistry*; D008024:Ligands; D008958:Models, Molecular; D008962:Models, Theoretical; D015394:Molecular Structure; D009532:Nickel / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*
chemical_list: D003432:Cross-Linking Reagents; D008024:Ligands; D009942:Organometallic Compounds; D009532:Nickel