Reactions of dimethoxycarbene and fluoromethoxycarbene with hydroxyl compounds. Absolute rate constants and the heat of formation of dimethoxycarbene

Article abstract of DOI:10.1021/ja00161a042

Dimethoxycarbene [(MeO)₂C] and fluoromethoxycarbene (FCOMe) were photochemically generated from the appropriate diazirines (2a and 2b) and characterized by UV and IR spectroscopy in cold matrices and by UV in solution at 20-25°C. Laser flash photolytic studies determined rate constants for the reactions of the carbenes with ethanol, methanol, chloroethanol, fluoroethanol, trifluoroethanol, hexafluoroisopropyl alcohol, and acetic acid. The reactivity of (MeO)₂C with the alcohols spanned 4 orders of magnitude, with kψ for carbene decay in 1 M ROH/CH₃CN ranging from 3.2 X 10⁴ S^-1 (ethanol) to 6.7 X 10⁸ S^-1 (hexafluoroisopropyl alcohol). A Bronsted correlation (α = 0.66) was obtained between log kψ for the decay of (MeO)₂C in 1 M ROH/CH₃CN and the pK(a) of ROH (in water). FCOMe was much less reactive toward ROH than (MeO)₂C; only with hexafluoroisopropyl alcohol (kψ ~9 X 10³ s^-1 for carbene decay in 1 M ROH/CH₃CN) could quenching be observed. Products of some of the carbene/alcohol reactions were characterized. A photoacoustic calorimetry study of the (MeO)₂/MeOH reaction afforded ΔH(f) ~ -61 kcal/mol for (MeO)₂C;ΔH(f) for FCOMe was estimated at ~ -53 to -56 kcal/mol. The mechanism(s) of the carbene/O-H insertion reactions are discussed. The results of ab initio and semiempirical molecular orbital calculations on FCOMe are presented.