Phenylhydroxycarbene.

Area of Science:

• Physical Organic Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• Carbenes are reactive intermediates crucial in organic synthesis.
• Understanding carbene reaction mechanisms, including tunneling, is key to controlling chemical transformations.

Purpose of the Study:

• To generate and characterize phenylhydroxycarbene (Ph-C-OH), the parent arylhydroxycarbene.
• To investigate the reaction mechanism and kinetics of phenylhydroxycarbene, particularly hydrogen tunneling.
• To compare experimental findings with theoretical predictions for reaction pathways and energy barriers.

Main Methods:

• High-vacuum flash pyrolysis of phenylglyoxylic acid at 600°C to generate phenylhydroxycarbene.
• Matrix isolation in solid Argon at 11 K for spectroscopic characterization (IR, UV-vis).
• High-level ab initio computations (CCSD(T), EOM-CCSD) for vibrational frequencies, excitation energies, and reaction path analysis.

Main Results:

• Phenylhydroxycarbene was successfully generated and spectroscopically identified.
• Observed IR and UV-vis spectra were in excellent agreement with computed frequencies and excitation energies.
• Phenylhydroxycarbene undergoes facile quantum-mechanical hydrogen tunneling to benzaldehyde with a half-life of 2.5 hours at cryogenic temperatures, despite a high activation barrier (28.8 kcal mol⁻¹).
• Deuteration of phenylhydroxycarbene effectively suppressed the tunneling reaction.

Conclusions:

• The parent arylhydroxycarbene, phenylhydroxycarbene, has been synthesized and characterized.
• The study provides compelling evidence for quantum-mechanical hydrogen tunneling as the dominant reaction pathway for phenylhydroxycarbene, even at low temperatures.
• Theoretical calculations accurately predict the experimental observations, validating the computational approach for studying carbene reaction dynamics.