Enantioselective conjugate additions of α-amino radicals via cooperative photoredox and Lewis acid catalysis.

We report the highly enantioselective addition of photogenerated α-amino radicals to Michael acceptors. This method features a dual-catalyst protocol that combines transition metal photoredox catalysis with chiral Lewis acid catalysis. The combination of these two powerful modes of catalysis provides an effective, general strategy to generate and control the reactivity of photogenerated reactive intermediates.

Brønsted acid cocatalysts in photocatalytic radical addition of α-amino C-H bonds across Michael acceptors.

In marked contrast to the variety of strategies available for oxidation and nucleophilic functionalization of methylene groups adjacent to amines, relatively few approaches for modification of this position with electrophilic reaction partners have been reported. In the course of an investigation of the reactions of photogenerated α-amino radicals with electrophiles, we made the surprising observation that the efficiency of radical photoredox functionalization of N-aryl tetrahydroisoquinolines is dramatically increased in the presence of a Brønsted acid cocatalyst. Optimized conditions provide high yields and efficient conversion to radical addition products for a range of structurally modified tetrahydroisoquinolines and enones using convenient household light sources and commercially available Ru(bpy)3Cl2 as a photocatalyst. Our investigations into the origins of this unexpected additive effect have demonstrated that the carbon-carbon bond-forming step is accelerated by TFA and is a rare example of Brønsted acid catalysis in radical addition reactions. Moreover, a significant conclusion arising from these studies is the finding that product formation is dominated by radical chain processes and not by photocatalyst turnover. Together, these findings have important implications for the future design and mechanistic evaluation of photocatalytic radical processses.

Electrochemical determination of chromium(VI) using metallic nanoparticle-modified carbon screen-printed electrodes.

Carbon screen-printed electrodes (CSPEs) modified with metal nanoparticles present an interesting alternative in the determination of chromium(VI) by differential pulse voltammetry (DPV). Metallic silver and gold nanoparticle deposits have been obtained by electrochemical deposition. Scanning electron microscopy measurements show that the electrochemically synthesized silver and gold nanoparticles are deposited in aggregated form. The detection limit for the analytical procedures developed in this work were 8.5 x 10(-7) and 4.0 x 10(-7)M for silver and gold nanoparticle-modified CSPE, respectively. In terms of reproducibility, the precision of the above-mentioned method was calculated at 6.7% in %R.S.D. values for silver and 3.21% for gold nanoparticle CSPE.