Photoinduced Aziridination Reaction Sensitized by PbO_x-Modified Zeolite

Abstract

In this work, we report a photoinduced aziridination reaction sensitized by lead oxide-containing zeolite colloids (NaY|PbO x ). When N-aminophthalimide and 3-cinnamoyl-2-oxazolidinone were photolyzed with NaY|PbO x , (2R,3S)- and (2S,3R)-N-phthalimidoaziridines were observed as the major products. Under similar conditions, bare zeolite Y showed an insignificant catalytic effect on this reaction. These results highlight that the contained PbO x is a potential catalyst. According to X-ray photoelectron spectroscopy (XPS) and electrochemical characterizations, the contained lead oxide is firmly deposited on the host and behaves like an n-type semiconductor. The associated conduction band and valence-band edges are located at −0.5 ± 0.2 and 0.9 ± 0.2 V versus a saturated calomel electrode (SCE), respectively. Since the valence-band edge is positive enough to oxidize N-aminophthalimide, N-aminophthalimide radicals can be formed on the illuminated NaY|PbO x and react with 3-cinnamoyl-2-oxazolidinone to produce aziridine products. A 1-h irradiation provided the aziridine products in 3% yield and 93% selectivity. For the low yield, calculations based on the doping density of NaY|PbO x suggest that the penetration depth of light may not be sufficient to excite the majority carriers relative to the thickness of the space-charge layer. Due to this or another reason, such as light blocking by the metallic silver formed on the surface of NaY|PbO x when silver nitrate is used as the sacrificial acceptor, the efficiency in light harvesting may be limited in this case. Although limited by these drawbacks and the low concentration within NaY (<20 μg/g of NaY), the prepared PbO x (NaY) still behaves as an effective aziridination catalyst compared to some other photosensitizers. Noticeably, incorporating appropriate chiral ligands can advance the enantiomeric excess (ee) for the (2S,3R) isomer. X-ray absorption near-edge-structure (XANES) spectral measurements show that the incorporated chiral ligands may function as bidentate agents to interact with PbO x (NaY) and cause the binding energy of Pb (LIII edge) in NaY|PbO x to shift to a higher energy by ∼2.6 eV. Likely due to this chelating effect, an asymmetric oxidation of N-aminophthalimide results. A preferential formation for the (2S,3R) isomer becomes possible.

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