Quantum Dot-Catalyzed Photoreductive Removal of Sulfonyl-Based Protecting Groups

Abstract

This Communication describes the use of CuInS₂ /ZnS quantum dots (QDs) as photocatalysts for the reductive deprotection of aryl sulfonyl-protected phenols. For a series of aryl sulfonates with electron-withdrawing substituents, the rate of deprotection for the corresponding phenyl aryl sulfonates increases with decreasing electrochemical potential for the two electron transfers within the catalytic cycle. The rate of deprotection for a substrate that contains a carboxylic acid, a known QD-binding group, is accelerated by more than a factor of ten from that expected from the electrochemical potential for the transformation, a result that suggests that formation of metastable electron donor-acceptor complexes provides a significant kinetic advantage. This deprotection method does not perturb the common NHBoc or toluenesulfonyl protecting groups and, as demonstrated with an estrone substrate, does not perturb proximate ketones, which are generally vulnerable to many chemical reduction methods used for this class of reactions.

Keywords: copper indium sulfide; deprotection; phenylsulfonates; photocatalysis; quantum dots.

Figure 1.

Reaction conditions and proposed catalytic cycle for the photocatalyzed 2-e⁻ deprotection of aryl sulfonates by CuInS₂/ZnS QDs. Ar₁ and Ar₂ are aryl groups on the protected alcohol and the protecting group, respectively, see Table 1. Sacrificial triethylammonium (TEA⁺) decomposes into diethyl amine and acetaldehyde, see Figure S1 of the SI.

Figure 2.

(A) The concentration of phenol product in reaction mixtures with 10 μM CuInS₂/ZnS QDs, 0.001 M substrate (labeled as in Table 1), and 0.1 M TEA in DMSO-d₆ as function of time of illumination with a 4.5-mW, 532-nm laser diode. The potentials listed in the legend correspond to the measured E_p,c[S/S^2-] value for each substrate. The lines are the fits used to acquire the initial rates of the reactions. (B) The rate constants, as determined by the method of initial rates, for the deprotection reactions as a function of the E_p,c[S/S^2-] of the substrates, listed in Table 1.

Figure 3.

The yield of phenol product after the photocatalyzed deprotections of substrate 6 (black squares) and substrate 11 (blue circles) using 1 mol % CuInS₂ QDs (filled symbols) and 9.7 mol % fac-Ir(ppy)₃ (open symbols) upon illumination from a 5-mW, 405-nm laser. 6 has a carboxylic acid substituent and 11 has no substituent with reported affinity for the QD surface.