C−H Bond Activation Facilitated by Nickel(II) Complexes Having Mighty Claws

Streamline synthesis of cyclic alcohols has been an enduring goal in synthetic chemistry. In this regard, a series of aminoquinoline‐based pincer ligands (L1(H)=2‐(dimethylamino)‐N‐(quinolin‐8‐yl)acetamide, L2(H)=2‐(diethylamino)‐N‐(quinoline‐8‐yl)acetamide, L3(H)=2‐(dipropylamino)‐N‐(quinoline‐8‐yl)acetamide and L4(H)=2‐(dibutylamino)‐N‐(quinolin‐8‐yl)acetamide)) and corresponding nickel(II) complexes of the type [Ni(L)Cl] (1–4) were synthesized and characterized using modern analytical techniques. Further, the catalytic activity of 1–4 was investigated for the oxidation of cyclohexane in the presence of different oxidants. All the catalysts showed substantial activity in the presence of m‐CPBA as the oxidant in DCM‐CH3CN (v/v, 3 : 1) solvent mixture at 60 ° C to yield cyclohexanol as the major product and cyclohexanone and caprolactone as the minor products. Under the optimised reaction conditions, catalytic activity was in the order 1 (936)>2 (892)>3 (835)>4 (785), which can be rationalized by the steric‐electronic properties of the respective ligands. Notably, with catalyst 1, we could achieve the highest TON (936) reported so far for cyclohexane oxidation using nickel(II) complexes having square planar geometry. The best catalyst, 1 was further used for the oxidation of other cycloalkanes and showed appreciable activity. The formation of chlorinated products in the reaction and product distribution pattern revealed that the oxidation is mediated majorly by 3‐chlorobenzoyloxy radical. Nickel(II) complexes of aminoquinoline‐based pincer ligands were prepared and employed as the catalyst for cyclohexane oxidation using m‐CPBA as the oxidant. All the catalysts facilitated the formation of cyclohexanol as the major product with good selectivity and turnover number. A radical‐mediated oxidation mechanism was identified from the presence of chlorinated products during the oxidation of a series of substrates.