Conformer-specific [1,2]<i>H</i>-tunnelling in captodatively-stabilized cyanohydroxycarbene (NC–C–OH)

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 <i>trans</i>-cyanohydroxycarbene rearranges to its <i>cis</i>-conformer. Prolonged irradiation leads to the formation of cyanoformaldehyde and isomeric isocyanoformaldehyde. <i>Cis</i>- and <i>trans</i>-cyanohydroxycarbene were characterized by matching matrix IR and UV/Vis spectroscopic data with <i>ab initio</i> coupled cluster and TD-DFT computations. <i>Trans</i>-cyanohydroxycarbene undergoes a conformer-specific [1,2]<i>H</i>-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 <i>cis</i>-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 <i>via</i> vibrational activation. Push-pull stabilisation of hydroxycarbenes through σ- and π-withdrawing groups therefore is even more stabilizing than push-push substitution.