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The Curious Case of Mesityl Azide and Its Reactivity with bpyNiEt2

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journal contribution
posted on 03.11.2014, 00:00 by Brooke M. Otten, Travis M. Figg, Thomas R. Cundari
A DFT analysis of the reaction of bpyNiEt2 with ArN3 was performed for para-tolyl-azide (Ar = pTol), 3,5-dimethyl-phenyl-azide (Ar = mXy) and ortho-tolyl-azide (Ar = oTol), and mesityl-azide (MesN3). Of particular interest were the different products obtained for the latter (ethylene, butane, azomesitylene, mesityl-ethylamine, etc.) versus the other reagents, i.e., (bpyNi­(N­(Ar)­Et)­(Et)). Calculated thermodynamics and kinetics for metal-free reactions did not differentiate MesN3 from the other aryl azides. Once 2bpyNiEt was generated via bond homolysis, formation of ethylene by β-H elimination was facile, as was formation of nickel-imidyl (NR–•) intermediates by reaction of ArN3 with bpyNiEtx (x = 0–2). On the basis of computed energetics, three reactions of bpyNiEt2 were proposed to compete: Ni–C bond homolysis, reductive elimination of butane, and nucleophilic attack (NA) by ArN3. Inspection of their temperature dependence suggested that NA and Ni–Et bond homolysis dominated at lower and higher temperatures, respectively. Calculated Ni–N and Ni–C bond dissociation free energies (BDFEs) suggested the role of radical pathways in discriminating bpyNiEt2/ArN3 reactions, and implied that the concentration of radicals such as aminyl (ArN(Et)), 2bpyNiEt, and Et will be greatest for MesN3.