Selectivity in the Photo-Fries Rearrangement of Some Aryl Benzoates in Green and Sustainable Media. Preparative and Mechanistic Studies

Irradiation of a series of p-substituted aryl benzoates under N 2 atmosphere in homogeneous and micellar media was investigated by means of steady-state condition and of time-resolved spectroscopy.A notable selectivity in favor of the 2-hydroxybenzophenone derivatives was observed in micellar media.The benzophenone derivatives were the main photoproduct.On the other hand, in homogeneous media (cyclohexane, acetonitrile, and methanol) the observed product distribution was entirely different, viz.substituted 2hydroxybenzophenones, p-substituted phenols, benzyl and benzoic acid were found.The binding constants in the surfactant were also measured and NOESY experiments showed that the aryl benzoates were located in the hydrophobic core of the micelle.Laser flash photolysis experiments led to the characterization of both p-substituted phenoxy radical and substituted 2-benzoylcyclohexadienone transients in homogeneous and micellar environment.50 lifetime in the restricted hydrophobic core of the micelle.For 51 example, geminate radical pairs that are produced photochemi-52 cally within the micellar core, have their rotational and 53 translational mobility constrained inside the micelles. 2Indeed, 54 the mobility restricted within the hydrophobic cores of radicals, 55 radical cations, or other reactive intermediates limits unwanted 56 reactions (e.g., radical-radical self-quenching reactions) and the 57 access of adventitious reagents (e.g., water and oxygen) that 58 would cause their collapse in solution.Therefore, micellar 59 solution can induce a product distribution and a relative 60 chemical yield that can be significantly different when compared 61 with homogeneous conditions. 1,262 There have been many studies on the control of the reactivity 63 of radical species generated within the hydrophobic core of the 64 micelle, and some physical parameters (e.g., electrostatic, 65 polarity, hydrophobic interactions, viscosity, as well as hydro-66 gen-bonding solvation) may be involved in determining their 67 reactivities. 3In addition, several studies directed to analyze and 68 quantify the reactivity, selectivity, and efficiency of micellar cage 69 on photochemical reactions in water have been carried out.70 Among the vast number of photochemical transformations, a 71 particular example is represented by the photo-Fries rearrange-72 ment.Anderson and Reese 4 have discovered the photoreaction 73 where a homolytic fragmentation of a carbon-heteroatom bond