Conformer-Specific [1,2]h-Tunneling 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-2oxoacetate 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-tunneling reaction through a 33.3 kcal/mol barrier (the highest penetrated barrier of all H-tunneling 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-tunneling process to date. During the tunneling 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 tunneling rate via vibrational activation. Push-pull stabilization of hydroxycarbenes through s-and p-withdrawing groups therefore is even more stabilizing than push-push substitution.

Hydrogen Bonding Shuts Down Tunneling in Hydroxycarbenes: A Gas-Phase Study by Tandem-Mass Spectrometry, Infrared Ion Spectroscopy, and Theory

“…Note that we also considered various options of H-bonding to and from the hydroxyl group of the carbene 7-H, as well as alternative hydroxyl-hydrogen conformers (see Figure S30, SI). 15 None of the IR spectra computed for such species matches the experimental one better than 7-H 5r or 7-H 7r (see Figures S29−S34, SI). Finally, we considered intramolecular carbene insertion reactions into one of the C−H bonds of 7-H to form covalently bound cyclic ion structures, although this is a spin-selective process that is limited to triplet carbenes.…”