CONICET | Buscador de Institutos y Recursos Humanos

Photochemistry of aryl benzoates in micro-heterogeneous mediaGastón Siano, Sergio M. Bonesi Departamento de Química Orgánica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, CIHIDECAR-CONICET, Pab. II, 3er piso, Ciudad Universitaria, 1428, Buenos Aires, Argentina. The photo-Fries rearrangement was discovered by Anderson and Reese in 1960[1] and the photophysical and photochemical aspects of the photoreaction have been studied.[2] Successful application of the photoreaction in organic synthesis has also been attempted, for example, in the total synthesis of murrayacine, daunomicine and tetracyclines.[2] However, the photo-Fries rearrangement of aryl benzoates has scarcely been investigated.[2] Interestingly, 2-hydroxybenzophenone derivatives can be obtained from the photo-Fries rearrangement reaction of aryl benzoates. These benzophenone derivatives can be used as photochemical filters in solar creams, as possible actinometers due to their photochemical stability and also, in the synthesis of building blocks of natural products. The structure of the esters and the photochemical reaction are shown in Scheme 1. Herein, we present the results obtained during the irradiation of aryl benzoates in cyclohexane and micro-heterogeneous media by means of steady-state and time-resolved spectroscopies. First of all, a series of p-substituted phenyl benzoates were prepared and fully characterized by physical and spectroscopical methods (pf, NMR). Then, preparative photochemical reactions of the aryl esters were carried out with UV light (254 nm) in cyclohexane leading to isolate from the reaction mixture and fully characterize by NMR spectroscopy the 2-hydroxybenzophenone derivatives. Three different surfactants were used to build up the micellar solutions of the aryl esters: SDS (anionic surfactant), Brij-P35 (neutral surfactant) and CTAC (cationic surfactant), and the micellar solutions were also irradiated at 254 nm. A high regio selectivity of the photoreaction was observed in these media favoring the formation of 2-hydroxybenzophenone derivatives up to 98 % yield. Quantum yields of consumption of the aryl esters (r) were also measured in both media affording higher values of r in micellar solutions than in cyclohexane. Likewise, the constants of binding (Kb) were measured in presence of the surfactants giving values between 70 and 1700 M-1 depending on the nature of the substituent attached to the aryl moiety of the esters as well as the kind of surfactant used. Finally, 2D NMR spectroscopy (NOESY experiments) of an aqueous solution of the aryl benzoates and each surfactant was performed in order to show that the aryl benzoates are located within the core of the micelles. Finally, irradiation of some representative esters in cyclohexane and micellar solutions with a laser pulse of 266 nm led to the characterization of the transient p-substituted phenoxyl radicals as well as the 2-benzoylcyclohexadienones which are the responsible for the observed product distribution of the photoreaction.[1] J. C. Anderson, C. B. Reese, Proc. Chem. Soc., London, 1960, 217.[2] M. A. Miranda and F. Galindo, Photo-Fries Reaction and Related Processes, in Handbook of Organic Photochemistry and Photobiology, ed. W. M. Horspool and F. Lenci, CRC Press, Boca Ratón, Fl., 2004, Ch. 42.