Photocatalytic stereoselective N-cyclization of 2,6-diaminopimelic acid into piperidine-2,6-dicarboxylic acid by an aqueous suspension of activated cadmium(II) sulfide particles

Abstract

Photoirradiation at a wavelength of >300 nm of a deaerated suspension of cadmium(II) sulfide (CdS) particles in an aqueous solution of a mixture of stereoisomers of 2,6-diaminopimelic acid (DAP; (S,S)∶(R,S)∶(R,R) = 1∶2∶1) produced piperidine-2,6-dicarboxylic acid (PDC) via a redox-combined mechanism including oxidation and reduction by positive holes and photoexcited electrons, respectively. Both the yield and trans∶cis ratio of PDC markedly depended on the kinds of CdS photocatalysts, their pre-treatment, and the loading of fine platinum (or its oxide) particles. A CdS photocatalyst prepared by heat treatment of a commercial powder in hexagonal (wurtzite) structure at 1023 K in the presence of a small amount of air gave the highest yield and trans∶cis ratio. Analyses of the activated CdS powder by powder X-ray diffraction (XRD), photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and BET surface area measurements revealed the formation of sulfur vacancies due to heat treatment, which promote the photoinduced cadmium metal (Cd⁰) deposition. The Cd⁰, deposited in situ on the CdS surface and detected by reduction of methylviologen, may act as a reduction site for photoexcited electrons toward a cyclic Schiff base intermediate produced by oxidation of DAP with positive holes followed by deamination and intramolecular condensation. Optically pure (R,R)- or (S,S)-PDCs were prepared successfully by photocatalytic reaction with the activated CdS particles using (R,R)- or (S,S)-DAPs, indicating that stereoselective conversion of organic compounds can be achieved via photocatalytic reaction under controlled conditions.