The mechanism of directed Ni(ii)-catalyzed C-H iodination with molecular iodine

The density functional theory method is used to elucidate the elementary steps of Ni(ii)-catalyzed C(sp²)-H iodination with I₂ and substrates bearing N,N'-bidentate directing centers, amide-oxazoline (AO) and 8-aminoquinoline (AQ). The relative stability of the lowest energy high- and low-spin electronic states of the catalyst and intermediates is found to be an important factor for all of the steps in the reaction. As a result, two-state reactivity for these systems is reported, where the reaction is initiated on the triplet surface and generates a high energy singlet nickelacycle. It is shown that the addition of Na₂CO₃ base to the reaction mixture facilitates C-H activation. The presence of I₂ in the reaction provides the much needed driving force for the C-H activation and nickelacycle formation and ultimately reacts to form a new C-I bond through either a redox neutral electrophilic cleavage (EC) pathway or a one-electron reductive cleavage (REC) pathway. The previously proposed Ni(ii)/Ni(iv) and homolytic cleavage pathways are found to be higher in energy. The nature of the substrate is found to have a large impact on the relative stability of the lowest electronic states and on the stability of the nickelacycle resulting from C-H activation.