RESUMEN
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.
RESUMEN
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.
RESUMEN
The selenocysteinyl radical 1 has been generated for the first time by laser flash photolysis (lambda(exc) = 266 nm) of dimethyl bis(N-tert-butoxycarbonyl)-l-selenocystine 2 and of [(9-fluorenylideneamino)oxycarbonyl]methyl(N-tert-butoxycarbonyl)-l-selenocysteine 3 in acetonitrile and characterized by time-resolved (TR) UV/Vis, Fourier transform infrared (FTIR), and electron paramagnetic spectroscopy in combination with theoretical methods. A detailed product study was conducted using gas chromatography and one- and two-dimensional NMR spectroscopy. In the case of [(9-fluorenylideneamino)oxycarbonyl]methyl(N-tert-butoxycarbonyl)-l-selenocysteine 3, the (9-fluorenylideneamino)oxycarbonyl moiety serves as a photolabile protection group providing a "caged selenocysteinyl radical" suitable for biophysical applications. Cleavage of the diselenide bridge or the selenium-carbonyl bond by irradiation is possible in high quantum yields. Because of the lack of a good IR chromophore in the mid-IR region, the selenocysteinyl radical 1 cannot be monitored directly by TR FTIR spectroscopy. TR UV/Vis spectroscopy revealed the formation of the selenocysteinyl radical 1 from both precursors. The selenocysteinyl radical 1 has a lifetime tau approximately 63 mus and exhibits a strong band located at lambda(max) = 335 nm. Calculated UV absorptions of the selenocysteinyl radical (UB3LYP/6-311G(d,p)) are in good agreement with the experimental results. The use of TR UV/Vis spectroscopy permitted the determination of the decay rates of the selenocysteinyl radical in the presence of two quenchers. The product studies demonstrated the reversible photoreaction of dimethyl bis(N-tert-butoxycarbonyl)-l-selenocystine 2. Products of the photolysis of the "caged selenocysteinyl radical" precursor 3 are dimethyl bis(N-tert-butoxycarbonyl)-l-selenocystine 2, carbon dioxide, and some further smaller fragments. In addition, the photodecomposition of the (9-fluorenylideneamino)oxycarbonyl moiety produced 9-fluorenone-oxime 4, 9-fluoren-imine 5, and 6 and 7 as products of the dimerization of two 9-fluorenoneiminoxy radicals 8.