Photorearrangement of aryl benzoates in homogeneous and micro-heterogeneous media

GASTON SIANO; BONESI S. M.

Villa Giardino

Encuentro; XIII ELAFOT; 2017

XIII ELAFOT

Photorearrangement of aryl benzoates in homogeneous and micro-heterogeneous mediaGastón Sianoa and Sergio M. Bonesia Departamento de Química Orgánica, CIHIDECAR-CONICET, 3er Piso, Pabellón 2, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Cdad. Universitaria, 1428, CABA, Argentina; E-mail: smbonesi@qo.fcen.uba.ar. E-mail (Presenting Author): gaston.siano@gmail.com 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] The application of the photoreaction in organic synthesis has been also studied, for example, in the total synthesis of murrayacine, daunomicine and tetracyclines.[2] However, the photo-Fries rearrangement of aryl benzoates has been scarcely studied.[2] The photoreaction of aryl benzoates provides the preparation of 2?-hydroxybenzophenone derivatives that are used in solar creams, as possible actinometers and also, in the synthesis of building blocks of natural products. The esters and the photochemical reaction are shown in Scheme 1. Herein, we present the results obtained during the irradiation of aryl benzoates in homogeneous and heterogeneous media by means of steady-state and time-resolved spectroscopies. A representative series of p-substituted phenyl benzoates were prepared and fully characterized by physical and spectroscopical methods (pf, NMR). Preparative photochemical reaction led to isolate and fully characterized the benzophenone derivatives. Quantum yields of consumption (r) were also measured in both media affording higher values of r in heterogeneous than in homogeneous media. The constants of binding (Kb) were measured in presence of SDS (0.10 M) giving values between 70 and 1700 M-1 depending on the nature of the substituents. A high regio selectivity of the photoreaction was observed in heterogeneous media, favoring the exclusive formation of 2?-hydroxy benzophenones as the photoproducts.Laser flash photolysis (266 nm) of some representative esters in homogeneous media led to characterize the transients p-substituted phenoxyl radicals and adducts 2-benzoyl cyclohexadienones. These adducts finally evolve to 2-hydroxy benzophenone derivatives which are the main photoproducts of the photoreaction. Figure 1 shows the transient absorption and decay traces obtained for a solution of p-methoxyphenyl benzoate in SDS 0.10 M. (a) (b) Figure 1. Transient absorption and decay traces of a solution of p-methoxyphenyl benzoate.[1] J. C. Anderson, C. B. Reee, 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.