Short-lived intermediates in the photoreaction mechanism of a-ketoamides

C. PALACIOS; H VENNEKATE; S BERTOLLOTTI; D. M. A. VERA

Florianopolis

Congreso; 10 Congreso Latinoamericano de Fisicoquímica Orgánica (CLAFQO10); 2009

p { margin-bottom: 0.21cm; } Short-lived intermediates in the photoreaction mechanism of -ketoamides Cecilia Palacios,1 Hendrik Vennekate,2 Sonia Bertollotti,1 D. Mariano Vera,3 Adriana Pierini,3 Carlos Previtali,1 Jorg Schroeder, 2 Dirk Schwarzer2 and Carlos Chesta1 1) Departamento de Química, Universidad Nacional de Río Cuarto, Río Cuarto, Argentina. 2) Abteilung Spektroskopie und Photochemische, Max-Planck Institut fur Biophysikalische Chemie, Gottingen, Germany. 3) INFIQC, Departamento de Química Orgánica, Universidad Nacional de Córdoba, Córdoba, Argentina. Most -ketoamides (K) photoreact quite efficiently (Q.Y. > 0.3) to yield β-lactams (A) and oxazolidin-4-ones (O). These amides react from their singlet excited states via an intricate mechanism that involves biradical (D) and zwitterionic (Z) species. The participation of D and Z as reaction intermediates was inferred from the analysis of the substituents (R), solvent, temperature and isotope effects on photoproduct selectivity. We report here a series time-resolved IR studies aimed to characterize spectroscopically these reaction intermediates and to analyze their mechanism of formation and reaction. Photoproduct selectivity upon photolysis of ketoamides 1-4 was studied in acetonitrile, dichloro-methane, chloroform and pentane. The photoproducts were characterized by H1 NMR, IR and mass spectroscopy. As expected, 1K is unreactive. 2K and 3K produce mostly the corresponding O in all solvents inspected. 4K yields exclusively 4A in pentane and a mixture of 4A, 4O and N-benzylmandelamide in the more polar media. After femptosecond UV excitation (266 nm) and mid-infrared broadband probe pulses of a solution of 1K in acetonitrile, an instantaneous bleaching of the characteristic IR bands of the ketoamide is observed. This bleaching recovers completely after  50 ps. Similar experiments carried-out using 2K and 3K (in all the solvents studied) showed the same initial bleaching followed by a partial recovery of the absorption before  25 ps. However, the recovery is accompanied by the appearance of five new IR bands between 1450 and 1650 cm-1. None of these bands could be ascribed to the formation of the photoproducts (A, O or other possible intermediates). Pulsed photolysis of 4K in acetonitrile showed a similar behavior but in this case, the built-up of a new band at 1780 cm-1, which indicated the formation of 4A, was also observed. When the same experiment was carried-out in pentane only the formation of 4A was detected. The intensity of the five bands between 1450-1650 cm-1 remain unchanged after 12 ns, indicating that the species responsible of these absorptions are stable in this scale of time. Theoretical MO (DFT) calculations were performed to estimate the relative stabilities of the ketone reactants, photoproducts and reaction intermediates (D and Z), to explore possible routes of reaction and to simulate the corresponding IR spectra. Quite interestingly, the simulated IR spectra pf the Z species match very well the experimental bands observed in the 1450 and 1650 cm-1 spectral range. The results above suggests that upon excitation the ketones undergo a fast intramolecular H transfer to form D. D can either give Z or A. Finally, Z yields O in a much longer scale of time. Apparently, the transformation of D into Z or A seems to be controlled by the substituents (R) and on the properties (polarity, viscosity, etc) of the reaction medium.