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Conformer-specific [1,2]H-tunnelling in captodatively-stabilized cyanohydroxycarbene (NC–[C with combining umlaut]–OH)

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Abstract

We report the gas-phase preparation of cyanohydroxycarbene by high-vacuum flash pyrolysis of ethyl 2-cyano-2-oxoacetate and subsequent trapping of the pyrolysate in an inert argon matrix at 3 K. After irradiation of the matrix with green light for a few seconds singlet trans-cyanohydroxycarbene rearranges to its cis-conformer. Prolonged irradiation leads to the formation of cyanoformaldehyde and isomeric isocyanoformaldehyde. Cis- and trans-cyanohydroxycarbene were characterized by matching matrix IR and UV/Vis spectroscopic data with ab initio coupled cluster and TD-DFT computations. Trans-cyanohydroxycarbene undergoes a conformer-specific [1,2]H-tunnelling reaction through a 33.3 kcal mol−1 barrier (the highest penetrated barrier of all H-tunnelling reactions observed to date) to cyanoformaldehyde with a half-life of 23.5 ± 0.5 d; this is the longest half-life reported for an H-tunnelling process to date. During the tunnelling reaction the cis-conformer remains unchanged over the same period of time and the Curtin–Hammett principle does not apply. NIR irradiation of the O–H stretching overtone does not enhance the tunnelling rate via vibrational activation. Push–pull stabilisation of hydroxycarbenes through σ- and π-withdrawing groups therefore is even more stabilizing than push–push substitution.

Graphical abstract: Conformer-specific [1,2]H-tunnelling in captodatively-stabilized cyanohydroxycarbene (NC– [[C with combining umlaut]] –OH)

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Publication details

The article was received on 20 Aug 2018, accepted on 31 Oct 2018 and first published on 31 Oct 2018


Article type: Edge Article
DOI: 10.1039/C8SC03720E
Citation: Chem. Sci., 2019, Advance Article
  • Open access: Creative Commons BY-NC license
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    Conformer-specific [1,2]H-tunnelling in captodatively-stabilized cyanohydroxycarbene (NC–[C with combining umlaut]–OH)

    A. K. Eckhardt, F. R. Erb and P. R. Schreiner, Chem. Sci., 2019, Advance Article , DOI: 10.1039/C8SC03720E

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