An efficient regioselective method for oxidation of phenols to o-quinones is reported. When this procedure is combined with a subsequent reduction, it proves to be useful for the construction of a variety of catechols.o-Quinones undergo a variety of reactions. For example, these species can be reduced to the corresponding catechol. 1,2 In addition, as a highly reactive 8π-electron system, o-quinones display two 4π components as potential sites for Diels-Alder reactions. 3 The selectivity between sites results from the polarizability of the complementary 2π component. For example, polarized alkenes such as enamines add to the external dione to restore aromaticity and yield a dioxin, 4 while less polarized alkenes such as styrene add to the cyclohexadiene portion to yield a [2.2.2]bicyclooctane. 5 Because o-quinones display four 2π components, there are varieties of [3 + 2] cycloaddition formats as well. The dipolar addition can be controlled so that either a carbonyl or an olefin moiety undergoes reaction. 6 In addition, there are many other reactions where the functional groups of o-quinones can be distinguished. For example, cesium fluorosulfate converts the most electrophilic carbonyl group of an o-quinone to 2,2-difluorocyclohexadienone, 7 while addition of a Wittig reagent can generate a benzopyran-2-one. 8 The ring olefins can be distinguished in nonsymmetric o-quinones: the most nucleophilic olefin is either oxidized or cleaved with peroxyacid 9 and Pb 10 reagents, respectively.Such versatility clearly suggests that unsymmetric o-quinones should be of considerable synthetic use. However, a convenient means to access a range of differently substituted oquinones has been lacking due in part to the difficulty in accessing the appropriately substituted catechol. A thorough survey of the literature reveals a few methods that allow for the regioselective conversion of a phenol to an o-catechol. 11 To the best of our knowledge, there are no previous examples of a regioselective procedure for the direct conversion of a phenol into an o-quinone. Oxidants such as Fremy's radical, 12 MeReO 3 -H 2 O 2 ,13 dimethyldioxirane, 14 and benzeneseleninic anhydride 15 are indiscriminate or favor oxidation of the para position unless blocked with a substituent.* To whom correspondence should be addressed.. † Undergraduate research participant supported by the California Alliance for Minority Participation (CAMP) in science.
Supporting Information Available:A general experimental procedure along with the crude 1 H NMR spectra used to estimate yields for o-quinones 4, 6, 8, 11, 13, 15, 17, 19, and 23 and full spectral characterization of 5, 20, 21, and 24. This material is available free of charge via the Internet at http://pubs.acs.org. (Figure 1).
NIH Public AccessBecause of this unexpected yet pleasing transformation, we paused to investigate the potential of this novel transformation. Table 1 illuminates the scope of this procedure. The oxidation of an array of phenols (0.1 M in a respective solvent) with a suspensi...
