Theoretical Study on the Mechanism of Ni‐Catalyzed Alkyl–Alkyl Suzuki Cross‐Coupling |
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Authors: | Dr. Zhe Li Yuan‐Ye Jiang Prof. Dr. Yao Fu |
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Affiliation: | Department of Chemistry, University of Science and Technology of China, 230026 Hefei (China), Fax: (+86)?551‐3606689 |
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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 NiI–NiIII catalytic cycle with three main steps: transmetalation of [NiI( L1 )X] (X=Cl, Br) with 9‐borabicyclo[3.3.1]nonane (9‐BBN)R1 to produce [NiI( L1 )(R1)], oxidative addition of R2X with [NiI( L1 )(R1)] to produce [NiIII( L1 )(R1)(R2)X] through a radical pathway, and C? C reductive elimination to generate the product and [NiI( 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 Ni0–NiII catalytic cycle is not favored in the present systems because reductive elimination from both singlet and triplet [NiII( L1 )(R1)(R2)] is very difficult. |
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Keywords: | boranes cross‐coupling density functional calculations homogeneous catalysis reaction mechanisms |
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