Intramolecular hydroxycarbene C-H-insertion: The curious case of (o-methoxyphenyl)hydroxycarbene

The first C-H insertion of a hydroxycarbene species in the gas phase has been observed experimentally by means of high vacuum flash pyrolysis (HVFP) and subsequent matrix isolation: (o-Methoxyphenyl) glyoxylic acid gives non-isolable (o-methoxyphenyl) hydroxycarbene upon pyrolysis at 600 degrees C, which rapidly inserts into the methyl C-H bond. The insertion product, 2,3-dihydrobenzofuran-3-ol, was trapped in an excess of Ar at 11 K and characterized by infrared spectroscopy. The insertion process kinetically outruns the alternative [1,2]H-tunneling reaction to o-anisaldehyde, a type of reaction observed for other hydroxycarbenes. Traces of the dehydration product, benzo[b]furan, were also detected. The potential energy hypersurface including the insertion and hydrogen migration processes was computed at the all-electron coupled-cluster level of theory encompassing single and double substitutions and perturbatively included triple excitations [AE-CCSD(T)] in conjunction with a correlation-consistent double-zeta basis set (cc-pVDZ) by utilizing density functional theory (DFT) optimized geometries (M06-2X/ccpVDZ) with zero-point vibrational energy (ZPVE) corrections. Exchange of the methoxy for a trifluoromethoxy group successfully prevents insertion and (o-trifluoromethoxy) benzaldehyde is produced instead; however, the carbene cannot be observed under these conditions. Thermal decomposition of (o-methoxyphenyl) glyoxylic acid in refluxing xylenes does not give the insertion product but yields o-anisaldehyde. This unanticipated outcome can be rationalized by protonation of the hydroxycarbene intermediate leading to the tautomeric formyl group. Thermochemical computations at M06-2X/cc-pVDZ in conjunction with a self-consistent solvent reaction field model support this suggested reaction pathway.