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

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 (Cd(0)) deposition. The Cd(0), 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.