1,2,3-Triazoles, computed stability

Other computational studies involving NHC-Cu species considered the formation of phenylisocyanates from nitrobenzene, and the development of [3+2] cycloaddition reactions for the formation of 1,2,3-triazoles. In the latter case the use of NHCs allowed the direct use of copper(i) catalysts, whereas copper(ii) precursors were predominant before. With [(NHC)CuBr] the reaction could be run on water and was successful even for internal alkynes, an unusual observation because the intermediacy of Cu-acetylides had previously been assumed. Calculations showed that the [(SIMes)Cu] fragment was ideally set up to bind internal alkynes in an i] -fashion and hence activate them towards cycloaddition. With terminal alkynes the acetylide route may still be operative. Other computational studies on the catalytic activity of [(NHC)Cu] complexes in which the NHC has no particular role but to stabilize the metal by strong o-donation and offer steric protection have been reported, including the activation of CO2 by [(NHC)Cu(EPh3)] (E = Si, Ge, Sn) and the carboxylation of the C-H bond of heteroarenes. The barriers of the two steps of the catalytic cycle of the [(NHC)Cu ]-catalyzed hydrosilylation of ketones have been computed, yet it was shown that the nature of the NHC was not a controlling factor. While the barrier of the hydrocupration step is determined by the nature of the ketone, that of the o-bond metathesis step occurs mainly under electronic control. 

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