Absolute rate constants for hydrogen abstraction from aldehydes and conformational studies of the corresponding aromatic acyl radicals

Abstract

Absolute rate constants for the reactions of rert-butoxyl radicals with some organic aldehydes have been measured in solution by using laser flash photolysis techniques. At 297 K, the rate constants are 8.9, 6.8, 1.0, and 0.05 X 107 M-1 s-1 for EtCHO, C6H5CHO, Me2NCHO, and EtOCHO, respectively. Arrhenius parameters were determined for benzaldehyde. Polar effects are very important in determining the reactivity of organic aldehydes toward tert-butoxyl radicals. The EPR spectra are described for a number of furoyl, thenoyl, and pyrroyl radicals (ArCO) which were generated photolytically over a wide range of temperatures. Conformational isomers were detected and their structural assignment obtained from the magnitude of the hyperfme splittings of the ring protons, by comparison with the conformers of the parent aldehydes, and from the solvent dependence.

Chemistry of Acyl Radicals

“…45 On the other hand, the spectra of a number of furoyl, thienoyl, and pyrroyl radicals whose parent aldehydes have relatively high rotational barriers decay faster than they interconvert, and so their conformational ratio is mainly determined by that of the parent aldehydes. 46 The larger hfs in these heteroaromatic-substituted acyl radicals is due to the hydrogen which adopts a W-relationship with respect to the SOMO.…”

“…The Arrhenius expression for the hydrogen abstraction from PhCHO by tert-butoxyl radicals was found to be log(k/M -1 s -1 ) ) 8.8 -1.3/θ, where θ ) 2.3RT kcal mol -1 . 46 Relative rate constants for H-atom abstraction from substituted benzaldehydes (p-Me, m-Me, p-Cl, m-Cl, p-Br, m-F, m-CF 3 , and m-CN) have also been reported, and a F + value of -0.32 was rationalized in terms of a polar contribution to the transition state. 101,102 Some rate constants for the reaction of HOO • , RC(O)OO • , and ROO • radicals with a variety of aldehydes have been obtained from studies on the oxidation of aldehydes.…”

“…Similarly, the rate constant for the reaction of tert-butoxy radical with N,N-dimethyl formamide and ethyl formate are reported in Table 9 and discussed in section IV.A.1. 46 The related radicals in which oxygen in acyl radicals is replaced by heteroatoms, such as imidoyl radicals, 3l, 373 are not covered in this review.…”

Sulfate Radical Anion as a New Reagent for Fast Photochemical Oxidation of Proteins

“…Besides hydroxyl radicals and sulfate radical anions, there are several potential peroxy-22 and diazirine23 species that when formed by UV photolysis, generate either other radicals or carbenes, which may be selected for their physical properties or residue specificity. These opportunities make clear another outcome of this study; that is, a method for validating methods using new reactive reagents for protein footprinting.…”

Photochemistry of Diketones: Observation of a Triplet State−Oxygen Adduct

“…These substrates are convenient sources for acyl and the acyl-peroxyl radicals under N 2 and O 2 atmosphere, respectively. [13][14][15][16] The results show that although the absorption spectra due to both radicals are similar to that assigned to the triplet-oxygen adduct, their time evolution differs significantly from that of the latter intermediate (see Supporting Information). These radicals show halflives of at least 4 µs.…”