Visible-light-driven reversible shuttle vicinal dihalogenation using lead halide perovskite quantum dot catalysts

Abstract

Dihalogenation of alkenes to the high-added value vicinal dihalides is a prominent process in modern synthetic chemistry. However, their effective conversion still requires the use of expensive and hazardous agents, sacrificial half-reaction coupling or primary energy input. Here, we show a photocatalytically assisted shuttle (p-shuttle) strategy for redox-neutral and reversible vicinal dihalogenation using low-cost and stable 1,2-dihaloethane under visible light illumination. Energetic hot electrons from metal-halide perovskite QDs enable the challenging photocatalytic reactions. Ultrafast laser transient absorption spectroscopy have unveiled the energy matching of the hot electrons with the high reduction potential of 1,2-dihaloethane, via two consecutive photoexcitation process. Powered by the sustainable energy as the only energy input, our new catalytic system using metal-halide perovskite QDs for dibromination, dichlorination and even unexplored hetero-dihalogenation, shows good tolerance with a wide range of alkenes at room temperature. In contrast to homogeneous photocatalysts, chalcogenide QDs and other semiconductor catalysts, perovskite QDs deliver previously unattainable performance in photoredox shuttle vicinal dihalogenation with the turnover number over 120,000. This work provides new opportunities in visible-light-driven heterogeneous catalysis for unlocking novel chemical transformations.

Fig. 1. Vicinal dihalogenation of alkenes.

a Historical methods for vicinal dihalogenations. b Electrocatalysis- vs. photocatalysis-enabled redox-neutral shuttle reactions. c Energy limitations for photoelectric conversion catalysis and challenges in the development of photocatalytic transfer difunctionalization. d P-shuttle reactions on perovskite QD surface.

Fig. 2. Hot electron-induced shuttle dibromination reactions.

a Visible-light-driven dibromination reactions. b CsPbBr₃ QD photocatalyst characterization. Schematic carrier relaxation, excitation (green lines), emission spectra (blue lines), bandwidth, and TEM image (scale bar: 20 nm) of CsPbBr₃ QDs. c Photocatalytic performance and band structure of various photocatalysts. d Transient dynamics of CsPbBr₃ QDs with and without DBE. e Time-resolved transient absorption spectra of CsPbBr₃ QD catalysts. f Transient dynamics of the CsPbBr₃ QD PA₁ at 480 nm and PA₂ at 525 nm.

Fig. 3. Scope of alkene of the photocatalytic shuttle dibromination reactions.

All yields are isolated yields of the products unless otherwise noted. *GC yield with n-dodecane as the internal standard. ^†1H-NMR yield. Reaction conditions: 0.1 mmol alkene, 0.5/0.75 mmol DBE, 4 mg CsPbBr₃ QD catalyst, 1 mL of DCM and 10 W white LED irradiation with continuous stirring at 25 °C. ^#0.5 mmol DBE, 4 mg Cu-CsPbBr₃ QD catalyst and 1 mL of MeCN. ^$0.75/1 mmol 1,1,2-tribromoethane.

Fig. 4. P-shuttle dichlorination reactions via metal-controlled hot electron generation.

a Schematic representation of metal-controlled p-shuttle dichlorination. b Slow hot electron cooling in metal-doped QDs. After using metal dopants, the ultrafast capture of holes by the intragap states leads to the suppression of the hot electron cooling process. c Scope of alkene of the photocatalytic vicinal dichlorination. All yields are GC yields of the products unless otherwise noted. *¹H-NMR yield. Reaction conditions: 0.1 mmol alkene compound, 1.35 mmol TCE, 4 mg Cu-CsPbBr₃ QD catalyst, 1 mL of MeCN, and irradiation using a 10 W white LED at 25 °C. ^†1.93 mmol TCE as the donor.^#4 mL of MeCN as solvent. d Emission and absorption spectrum for CsPbBr₃ QDs, treated with CuBr₂. e, f Transient dynamics of the CsPbBr₃ and Cu-CsPbBr₃ PB at 510 nm (e) and PA at 475 nm (f).

Fig. 5. Hetero-dihalogenation reactions via selective p-shuttle catalysis.

a Possible product distribution. Four different products were generated via hetero-dihalogenation shuttle catalysis reactions. b Screen out different metals and solvents for optimizing the hetero-dihalogenation reaction conditions. c Scope of alkene of the photocatalytic hetero-dihalogenation. All yields are GC yields of the products unless otherwise noted. *¹H-NMR yield. Reaction conditions: 0.1 mmol alkene compound, 0.17 mmol DBE, 0.95 mmol TCE, 5 mg Cu-CsPbBr₃ QD catalyst, 1 mL of MeCN and irradiation using a 10 W white LED at 25 °C. ^†0.2 mmol alkene compound as the substrate. d Proposed mechanism of the generation of hetero-dihalogenation via metal-controlled hot electron reduction.