Chemically Induced Dynamic Nuclear Polarization XI. Intermediary vinyl alcohol during photochemical reactions of acetaldehyde and of acetoin

Abstract

Vinylalcohol, formed by disproportionation of a-hydroxyethyl radicals, is detected by NMR. spectroscopy during photoreactions of acetaldchyde and acetoin in solution and slowly tautomerizes to acctaldehyde.At room temperature, the equilibrium concentration of vinyl alcohol 1 has been estimated to be at least seven orders of magnitude lower than that of the keto isomer, acetaldehyde 2, i.e. to be lower than the detection limit of most physical techniques [2] 131. This has impeded the determination of spectroscopic properties of this most simple enol. 1 2 During photolysis of 2 and of 2-hydroxybutan-3-one (acetoin, 3) in the probe of a modified HA-100 lH-NMR. spectrometer, we detect 1 as thermally unstable radical reaction product, and observe its vinyl proton spectrum highly enhanced by chemically induced dynamic nuclear polarization (CIDNP.) [4] [S] [6]. Fig. 1 shows lH-NMR.-spectra taken at room temperature during irradiation of 2 (a) and 3 (b), 0 . 2~ in benzene, with the beam of a high pressure mercury capillary lamp filtered to transmit radiation of wavelength 300 nm < 2 < 350 nm. During the periods of observation conversion was less than 10%. The same CIDNP. effects were found with benzene-h,, benzene-$, cyclohexane-h,,, cyclohexane-d,,, and octamethyltetrasiloxane as solvents.The Table gives the assignments of the transitions 1 through 10 of Fig. 1 to the various reaction products together with the description of the C1DNP.-effects in the 1)For part X of this series : see ill.

Computational Study on the Effects of Substituents and Functional Groups in the Isomerization of 1‐ and 2‐Substituted Propenes, Acetaldimines, and Aldehydes

“…The simplest enol, vinyl alcohol, was observed in 1967 by Hay and Lyon [10] upon oxidation of dideuterioacetylene in a mass spectrometer. Blank and Fischer [11] confirmed this finding by NMR spectroscopy in 1973. In addition, Saito [12] identified vinyl alcohol in 1976 as a reactive intermediate during gardless of the site of substitution.…”

Interstellar Enols Are Formed in Plasma Discharges of Alcohols

“…As members of the C n H 2n O isomeric family, enols were proposed as transient chemical intermediates by Erlenmeyer (1880), but were not detected experimentally until 1973, when ethenol was directly identified (Blank & Fischer 1973). Recently, ethenol, propenols, and butenols have been observed as common intermediates in hydrocarbon flames (Taatjes et al 2005), and the role of alkene oxidation in enol formation has been discussed ( Taatjes et al 2006).…”

Photochemistry of Pivalaldehyde in Solution Studied by CIDNP

“…This step appears to occur predominantly via a n excited electronic triplet state of pivalaldehyde. In contrast, this mechanism does not appear to be the main route responsible for the CIDNP effects observed in the photochemical reaction of acetaldehyde in solution (3). Finally, as indicated by the chain mechanism discussed above, the occurrence of CIDNP effects does not preclude the possibility of other reaction steps which may be more important in product formation (see also ref.…”