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Mechanistic Aspects of Pincer Nickel(II)-Catalyzed C–H Bond Alkylation of Azoles with Alkyl Halides

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journal contribution
posted on 15.02.2018, 16:48 by Ulhas N. Patel, Shailja Jain, Dilip K. Pandey, Rajesh G. Gonnade, Kumar Vanka, Benudhar Punji
The quinolinyl-based pincer nickel complex, κNNN-{C9H6N-(μ-N)-C6H4–NMe2}­NiCl [(QNNNMe2)­NiCl; (1)] has recently been demonstrated to be an efficient and robust catalyst for the alkylation of azoles with alkyl halides under copper-free conditions. Herein, we report the detailed mechanistic investigation for the alkylation of azoles catalyzed by (QNNNMe2)­NiCl (1), which highlights an iodine-atom transfer (IAT) mechanism for the reaction involving a NiII/NiIII process. Deuterium labeling experiments indicate reversible cleavage of the benzothiazole C–H bond, and kinetic studies underline a fractional negative rate order with the substrate benzothiazole. The involvement of an alkyl radical during the alkylation is validated by radical clock and external additive experiments. An active intermediate species (QNNNMe2)­Ni­(benzothiazolyl) (5a) has been isolated and structurally characterized. The complex (QNNNMe2)­Ni­(benzothiazolyl) (5a) is found to be the resting state of catalyst 1. Kinetic analysis of electronically different intermediates suggests that the step involving the reaction of 5a with alkyl iodide is crucial and a rate-influencing step. DFT calculations strongly support the experimental findings and corroborate an IAT process for the alkylation reaction.