Theoretical Study on the Mechanism of Ni-Catalyzed Alkyl-Alkyl Suzuki Cross-Coupling
Theoretical Study on the Mechanism of Ni-Catalyzed Alkyl-Alkyl Suzuki Cross-Coupling
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Ni催化烷基-烷基Suzuki交叉偶联机理的理论研究
DOI：
10.1002/chem.201103882
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发表时间：
2012-04-01
影响因子：
4.3
通讯作者：
Fu, Yao
中科院分区：
文献类型：
--
作者：
Li, Zhe;Jiang, Yuan-Ye;Fu, Yao
Ni催化的未活化的仲烷基卤化物与烷基硼烷的交叉偶联提供了构建烷基烷基键的有效途径。首次用理论计算研究了Ni/L1（L1=trans-N，N '-二甲基-1，2-环己二胺）体系的反应机理。研究表明，NiI（L1）X（X=Cl，Br）与9-硼双环[3.3.1]壬烷（9-BBN）R1发生金属转移反应生成[NiI（L1）（R1）]，R2 X与[NiI（L1）（R1）]通过自由基途径氧化加成生成[NiIII（L1）（R1）（R2）X]，C？C还原消除生成产物和[NiI（L1）X]。对于伯烷基溴和仲烷基溴，金属转移步骤是速率决定性的。KOtBu通过与烷基硼烷形成烷基硼酸钾盐来降低金属转移步骤的活化势垒。叔烷基卤化物不具有反应性，因为还原消除的活化势垒太高（+34.7千卡？mol-1）。另一方面，由于Ni/L2（L2=trans-N，N '-二甲基-1，2-二苯基乙烷-1，2-二胺）催化烷基氯的交叉偶联反应的活化能低于L1，因此Ni/L2催化烷基氯的交叉偶联反应具有良好的催化活性。重要的是，Ni 0 NiII催化循环在本系统中是不利的，因为从单线态和三线态[NiII（L1）（R1）（R2）]的还原消除是非常困难的。
Ni-catalyzed cross-coupling of unactivated secondary alkyl halides with alkylboranes provides an efficient way to construct alkylalkyl 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 NiINiIII 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 Ni0NiII catalytic cycle is not favored in the present systems because reductive elimination from both singlet and triplet [NiII(L1)(R1)(R2)] is very difficult.