Selective synthesis of cyclic alcohols from cycloalkanes using nickel(ii) complexes of tetradentate amidate ligands

Selective functionalisation of hydrocarbons using transition metal complexes has evoked significant research interest in industrial chemistry. However, selective oxidation of unactivated aliphatic C-H bonds is challenging because of the high bond dissociation energies. Herein, we report the synthesis, characterisation and catalytic activity of nickel(ii) complexes ([Ni(L1-L3)(OH₂)₂](ClO₄)₂ (1-3)) of monoamidate tetradentate ligands [L1: 2-(bis(pyridin-2-ylmethyl)amino)-N-phenylacetamide, L2: 2-(bis(2-pyridin-2-ylmethyl)amino)-N-(naphthalen-1-yl)acetamide, L3: N-benzyl-2-(bis(pyridin-2-ylmethyl)amino)acetamide] in selective oxidation of cycloalkanes using m-CPBA as the oxidant. In cyclohexane oxidation, catalysts showed activity (TON) in the order 1 (654) > 2 (589) > 3 (359) with a high A/(K + L) ratio up to 23.6. Using catalyst 1, the substrate scope of the reaction was broadened by including other cycloalkanes such as cyclopentane, cycloheptane, cyclooctane, adamantane and methylcyclohexane. Further, the Fenton-type reaction in the catalytic cycle was discarded based on the relatively high 3°/2° ratio of 8.6 in adamantane oxidation. Although the formation of chlorinated products during the reactions confirmed the contribution of the 3-chlorobenzoyloxy radical mechanism, the high alcohol selectivity obtained for the reactions indicated the participation of nickel-based oxidants in the oxidation process.