The photochemistry of 9‐fluorenone oxime phenylglyoxylate (1) in tetrachloromethane was investigated by a variety of time‐resolved methods [STEP‐SCAN TRIR and time‐resolved EPR (TREPR)]. Photolysis of 1 yields the benzoyl radical, carbon dioxide and the fluorene‐9‐iminyl radical on pulsed irradiation (λ = 355 nm). All radicals have 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. The benzoyl radical shows a strong absorption ($(\tilde \nu$C=O = 1824 cm−) in the IR spectrum. Upon purging the solution with oxygen, the benzoyl radical is quenched to yield the benzoylperoxy radical ($(\tilde \nu$C=O = 1814 cm−), which has a lifetime that is several microseconds longer than that of the benzoyl radical. Owing to the lack of a good IR chromophore, the fluorene‐9‐iminyl radical is not detected by IR spectroscopy. It can, however, be detected by TREPR spectroscopy, which allows the detection of a 1 : 1 : 1 triplet of the fluorene‐9‐iminyl radical at 3456 G and a 1 : 2 : 1 triplet of the benzoyl radical at 3460 G. Copyright © 2004 John Wiley & Sons, Ltd.
A series of alkoxycarbonyl radicals has been generated by laser flash photolysis (355 nm) of fluorenone oxime alkyl oxalates in carbon tetrachloride and characterized by time-resolved infrared spectroscopy using the step-scan technique. The alkoxycarbonyl radicals (ν C=O = 1802 cm −1 for R = ethyl) generally have a lifetime of several microseconds, decaying by reaction with the solvent to yield esters of chloroformic acid. In some cases, decarboxylation yielding alkyl radicals has also been observed. Thus, photolysis of fluorenone oxime tert-butyl oxalate results in the formation of tert-butoxycarbonyl radicals, which subsequently decay, mainly yielding CO 2 and tert-butyl radicals. The benzyloxycarbonyl radical and the acetoneiminoxycarbonyl radical both decarboxylate too rapidly to be detected with our spectrometer (25 ns risetime). Upon purging the solution with oxygen, the alkoxycar- [a]
A series of alkoxycarbonyl radicals has been generated by laser flash photolysis (355 nm) of fluorenone oxime alkyl oxalates in carbon tetrachloride and characterized by time‐resolved infrared spectroscopy using the step‐scan technique. The alkoxycarbonyl radicals ($\tilde {\nu}$C=O = 1802 cm−1 for R = ethyl) generally have a lifetime of several microseconds, decaying by reaction with the solvent to yield esters of chloroformic acid. In some cases, decarboxylation yielding alkyl radicals has also been observed. Thus, photolysis of fluorenone oxime tert‐butyl oxalate results in the formation of tert‐butoxycarbonyl radicals, which subsequently decay, mainly yielding CO2 and tert‐butyl radicals. The benzyloxycarbonyl radical and the acetoneiminoxycarbonyl radical both decarboxylate too rapidly to be detected with our spectrometer (25 ns rise‐time). Upon purging the solution with oxygen, the alkoxycarbonyl radicals were efficiently quenched, to yield alkoxycarbonylperoxy radicals ($\tilde {\nu}$C=O = 1845 cm−1 for R = ethyl), which again had a lifetime of the order of several microseconds. A short‐lived transient ($\tilde {\nu}$ = 1768 cm−1, τ ≈︁ 200 ns) is assigned as the carbonyloxy radical 4a on the basis of comparison with time‐resolved UV/Vis data. A further product of the photolysis of fluorenone oxime oxalates can be tentatively assigned as the 9‐fluorenylideneiminoxy radical 3 ($\tilde {\nu}$ = 1670 cm−1), which according to our DFT calculations should show a very intense $\tilde {\nu}$C=N−O,as. = 1665 cm−1. Fluorenone oxime oxalates are compounds well suited as precursors for alkoxycarbonyl radicals, since they are easily synthesized as crystalline solids, show a convenient absorption at λ = 355 nm, and exhibit a high degree of thermal stability.
This note describes the photochemistry of O-chlorooxalyl- and O-fluoroformyl-9-fluorenone oxime. The solution photochemistry of both precursors was investigated by time-resolved step/scan FTIR spectroscopy. Experiments on O-chlorooxalyl-9-fluorenone oxime only allowed for detection of CO2 and bleaching of the precursor, indicating predominant N-O cleavage. The chlorocarbonyl radical, ClCO*, was not detected. In contrast, TRIR investigations on fluoroformyl oxime 2 gave evidence for formation of the fluoroformyl radical FCO* (3), which rapidly adds to the solvent acetonitrile, yielding a fluoroformyl-functionalized iminyl radical 4. Its reaction with triplet molecular oxygen, on the other hand, is impeded by an activation enthalpy that has been calculated as deltaH# = 3.2 kcal/mol.
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.
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