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Computational insight into the metal-free cleavage of the carbon-carbon triple bond in alkynes

Cleavage of the C-C triple bond is one of the most important processes in modern organic synthesis. Majority of the existing methods rely on the oxidative cleavage with toxic organometallic catalysts under harsh reaction conditions [1], which makes their application less desirable. Recently, Yanada et al. [2] reported a metal-free cleavage of internal alkynes to nitriles using trimethylsilylazide (TMSN3) as the nitrogen source. Inspired by these experimental advances, in this work, we used DFT computations at M06-2X/6-31+G(d) level of theory to clarify the mechanism of the mentioned reaction. After inspecting several mechanistic possibilities, our results show that the cleavage of the carbon-carbon triple bond in symmetrical internal alkynes 1 is a downhill process (∆rG between -191.8 and -195.3 kcal mol-1), which occurs in six steps. In the first step, a simultaneous addition of azide from TMSN3 and iodine from N-iodosuccinimide (NIS) to the C-C triple bond, leads to the formation of iodo vinyl azide 2. The intermediate 2 undergoes internal rearrangement to obtain 2- iodo-2H-azirine 3, accompanied by the release of the nitrogen gas under the thermal condition. Addition of TMSN3 to the double bond in 3 with a simultaneous loss of iodine, generates azide azirine 4. The final cyano compounds 5 are obtained in the last, rate- limiting step (ΔG‡ = 33.5 kcal mol-1) which belongs to the second part of the synthetic route that occurs under temperatures over 70 ⁰C. The computations indicate that alkynes with electron-donating moieties tend to give nitriles easily, which is experimentally confirmed by their higher yields [2]. We believe that obtained results could be of significant value for the future experiments with different reagents and solvents. [1] G. Yan, Y. Zhang , J. Wang, Adv. Synth. Catal. 2017, 359, 4068-4105. [2] N. Okamoto, M. Ishikura, R. Yanada, Org. Lett. 2013, 15, 2571-2573.

Cleavage of C-C triple bond ; reaction mechanism ; computational chemistry

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