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Direct Transformation of Alkynes to Nitriles: Quantum-Chemical Study of the Reaction Mechanism

Cleavage of carbon-carbon triple bond is considered to be one of the most challenging targets in modern organic synthesis. Most of the existing methods rely on the oxidative cleavage with toxic organometallic catalysts under harsh reaction conditions, which makes their application less desirable. Recently, Yanada et al. reported a metal-free direct cleavage of internal alkynes to nitriles using trimethylsilylazide (TMSN3) as the nitrogen source. Inspired by these experimental advances, in this work, we used density functional theory computations to clarify the mechanism of the mentioned transformation. All structures were fully optimized by the M06- 2X/6-31+G(d) model. To account for the solvent effects, Gibbs energies of solvation with acetonitrile were determined using the SMD implicit continuum model at the same level of theory. After inspecting several mechanistic possibilities, our results show that the cleavage of the C-C triple bond in alkyne 1 is a downhill process (∆rG ≈ -193 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 C≡C, leads to the formation of iodo vinyl azide 2. The intermediate 2 undergoes internal rearrangement to obtain 2-iodo-2H-azirine 3 which is 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-determining step (ΔG# = 33.5 kcal mol-1) which includes breaking the bond in azide moiety and the fragmentation of the unsaturated heterocycle in azide azirine 4. The obtained free-energy profiles are in agreement with Yanada's experimental results, though further investigations on various internal alkynes in solvents with different polarity are in progress.

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

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