Redox-Active Ligand-Mediated Oxidative Addition and Reductive Elimination at Square Planar Cobalt(III): Multielectron Reactions for Cross-Coupling

Square planar cobalt(III) complexes with redox-active amidophenolate ligands are strong nucleophiles that react with alkyl halides, including CH2Cl2, under gentle conditions to generate stable square pyramidal alkylcobalt(III) complexes. The net electrophilic addition reactions formally require 2e− oxidation of the metal fragment, but there is no change in metal oxidation state because the reaction proceeds with 1e− oxidation of each amidophenolate ligand. Although the four-coordinate complexes are very strong nucleophiles, they are mild outer-sphere reductants. Accordingly, addition of alkyl- or phenylzinc halides to the five-coordinate organometallic complexes regenerates the square planar starting materials and extrudes C−C coupling products. The net 2e− reductive elimination reaction also occurs without a oxidation state change at the cobalt(III) center. Together these reactions comprise a complete, well-defined cycle for cobalt Negishi-like cross-coupling of alkyl halides with organozinc reagents.

携带氧化还原活性酰胺酚配体的平面正方形钴(III)配合物属于强亲核试剂，可在温和条件下与包括二氯甲烷(CH₂Cl₂)在内的卤代烷烃发生反应，生成稳定的四方锥型烷基金属钴(III)配合物。该净亲电加成反应形式上需要金属片段经历二电子氧化，但钴中心的氧化态并未发生改变——这是由于反应通过每个酰胺酚配体的单电子氧化完成。尽管这类四配位配合物具备极强的亲核性，它们同时也是温和的外层还原剂。据此，向该五配位有机金属配合物中加入烷基或芳基锌卤化物，可再生得到平面正方形起始原料，并脱除碳碳偶联产物。该净二电子还原消除反应同样未使钴(III)中心的氧化态发生变化。上述两类反应共同构成了一套完整且定义明确的钴基类根岸(Negishi)交叉偶联循环体系，可实现卤代烷烃与有机锌试剂的交叉偶联。