A TRIR, TREPR and Computational Study on the Reactivity and Structure of the 2,2,2-Trifluoroethoxycarbonyl Radical†

Abstract

The 2,2,2-trifluoroethoxycarbonyl radical, 3b, has been generated by pulsed irradiation of 9-fluorenone oxime 2,2,2-trifluoroethyl oxalate 1b in carbon tetrachloride and acetonitrile solution. It was characterized by time-resolved electron paramagnetic resonance spectroscopy (EPR) and infrared spectroscopy. The radical has a lifetime in the range of microseconds and can be detected within the rise time of our time-resolved equipment before undergoing recombination or reactions with the solvent. No decarbonylation or decarboxylation was observed. In the presence of oxygen, the radical is quenched to yield the 2,2,2-trifluoroethoxycarbonylperoxy radical 4b, which has again a lifetime in the range of several microseconds. Time-resolved electron paramagnetic resonance spectroscopy (TREPR) allowed for the detection of a 1 : 1 : 1 triplet of the fluorene-9-iminyl radical 7 at g = 2.0032 and a 1 : 3 : 3 : 1 quartet with additional hyperfine splitting (HFS) due to proton coupling at g = 2.001 for the trifluoroethoxycarbonyl radical 3b. Calculations indicate that alkoxycarbonyl radicals can exist in conformations that are s-trans or s-cis with respect to the R-O-C(O) x dihedral. A comparison of experimental TREPR spectra with simulations indicates that the s-trans conformer is observed in the case of the ethoxycarbonyl radical, 3a. In the case of the trifluorethoxycarbonyl radical, 3b, however, the additional proton HFS observed shows that it is the s-cis conformer that is formed. As calculations give evidence for a fairly high activation enthalpy for s-cis-s-trans interconversion of alkoxycarbonyl radicals, this discrepancy is likely due to differing conformational preferences of the precursor molecules.

Epoxidation and 1,2-Dihydroxylation of Alkenes by a Nonheme Iron Model System − DFT Supports the Mechanism Proposed by Experiment

“…Species along this reaction path have been trapped and spectroscopically characterized. It has been shown that the reaction with O 2 (the formation of peroxy radical compounds) enhances the stability of the radical intermediate, and this is essential for the autoxidation process in our reaction . A normal transition-state search to locate tsIV was unsuccessful.…”

Time-Resolved Infrared Study on the Photochemistry of O-Fluoroformyl- and O-Chlorooxalyl-9-fluorenone Oxime: The Reactivity of the Fluoroformyl Radical in Acetonitrile Solution

“…We have now recalculated this activation enthalpy at the UCCSD(T) level of theory 17b and have found very similar values of Δ H # = 3.2 kcal/mol ( E ) or 3.4 kcal/mol ( Z ), with reaction enthalpies of Δ H = −35.7 kcal/mol ( E ) or −34.5 kcal/mol ( Z ). Given these data, we can conclude that unlike other acyl radicals, ,, 3 will not react with 3 O 2 at the diffusion-controlled limit. Of importance, however, could be reactions of FCO • with the solvent, acetonitrile.…”

“…In comparison to other free-radical sources, such as esters derived from N -hydroxy-pyridine-2-thione (“Barton-esters”), their main advantage lies in their much higher thermal stability. In particular, esters of 9-fluorenone oxime, with their yellow color and a corresponding convenient long-wavelength absorption, have recently received attention as photochemical sources of a variety of free radicals, among them the benzoyl radical, alkoxycarbonyl radicals, aroyloxy radicals, the selenocysteinyl radical, and even triplet nitrenes . To generate additional novel acyl radical species by photolysis of esters of 9-fluorenone oxime, we embarked on the synthesis and investigation of esters 1 and 2 , which were designed to yield either the fluoroformyloxy radical F−C(O)−O • or the halocarbonyl radicals X−C(O) • (X = F or Cl), by N−O or O−C cleavage.…”

Phototransformation of tetrazoline oxime ethers – part 2: theoretical investigation

“…24 The photodegradation of oxime species may also compete with the photoisomerization mechanism. Experimental analyses 1,3,[10][11][12][13][14][15] and theoretical studies 18 of photodegradation products gave evidence of N-O bond dissociation as one of the main degradation pathway. The photodissociation mechanism of the N-O bond can proceed through conical intersections that arise from the diabatic crossing of excited singlet states that have signicantly different electronic character (p/p* and s NO /s * NO ).…”

“…Both phototransformations are common photochemical processes. 3,[5][6][7][8][9][10][11][12][13][14][15] This fungicide is investigated by Bayer Crop Science (WO2003/ 016303 DaiNippon) because it is very active biologically in green house while much less active in the eld. E does not show fungicide activity.…”