Towards Water-Soluble Peroxyl Radial Clocks
Radical Clocks , Peroxyester Synthesis
Peroxyesters undergo homolytic O-O bond cleavage under thermal and photolytic conditions, forming an alkoxyl and acyloxyl radical, the latter of which rapidly decarboxylates to form a carbon centered radical. While peroxyesters are generally decomposed under anaerobic conditions to serve as sources of carbon-centered radicals to initiate radical reactions, it has been shown that β,γ-unsaturated peroxyesters can decompose to yield a delocalized carbon-centered radical which can serve as a precursor to a powerful peroxyl radical clock. This competitive kinetic approach is attractive as it allows the facile determination of rate constants for formal H-atom reactions between peroxyl radicals and reductants (e.g. phenolic and aromatic amine antioxidants) without the need for specialized equipment. Furthermore, this approach allows for the determination of this kinetic data in a wide variety of solvents. Herein, we present our work towards the development of a water-soluble peroxyl radical clock, such that the kinetics of reactions of peroxyl radicals with water-soluble reductants can be determined. To do so, we have designed β,γ-unsaturated peroxyesters analogous to those we have previously developed, but that bear either carboxylate or poly(ethylene glycol) moieties to improve solubility in aqueous media. In addition, we have carried out a preliminary investigation of the use of alkyl and acyl xanthates as precursors to peroxyl radical clocks. These radical precursors offer the advantage that they have a pronounced ultraviolet absorption that tails into the visible, permitting efficient photochemical generation of the same alkyl radicals, but at longer wavelengths than that which are currently employed.