REACTIONS OF DIMETHOXYCARBENE AND FLUOROMETHOXYCARBENE WITH HYDROXYL COMPOUNDS - ABSOLUTE RATE CONSTANTS AND THE HEAT OF FORMATION OF DIMETHOXYCARBENE

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 × 104 s-1 (ethanol) to 6.7 × 108 s-1 (hexafluoroisopropyl alcohol). A BrΦnsted correlation (α = -0.66) was obtained between log kψ for the decay of (MeO)2C in 1 M ROH/CH3CN and the pKa of ROH (in water). FCOMe was much less reactive toward ROH than(MeO)2C; only with hexafluoroisopropyl alcohol (kψ, ~9 × 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 ∆Hf ~ -61 kcal/mol for (MeO)2C; 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. © 1990, American Chemical Society. All rights reserved.