Artificial photosynthesis directed toward organic synthesis

Abstract

In nature, plants convert solar energy into chemical energy via water oxidation. Inspired by natural photosynthesis, artificial photosynthesis has been gaining increasing interest in the field of sustainability/green science and technology as a non-natural and thermodynamically endergonic (ΔG° > 0, uphill) solar-energy-driven reaction that uses water as an electron donor and a source material. Among the artificial-photosynthesis processes, inorganic-synthesis reactions via water oxidation, including water splitting and CO₂-to-fuel conversion, have been attracting much attention. In contrast, the synthesis of high-value functionalized organic compounds via artificial photosynthesis, which we have termed artificial photosynthesis directed toward organic synthesis (APOS), remains a great challenge. Herein, we report a synthetically pioneering and meaningful strategy of APOS, where the carbohydroxylation of C = C double bonds is accomplished via a three-component coupling with H₂ evolution using dual functions of semiconductor photocatalysts, i.e., silver-loaded titanium dioxide (Ag/TiO₂) and rhodium-chromium-cobalt-loaded aluminum-doped strontium titanate (RhCrCo/SrTiO₃:Al).

Fig. 1. Overview of this study.

a Natural photosynthesis (left) and artificial photosynthesis directed toward organic synthesis (APOS) (right, this work). b Radical-to-cation crossover-cascade mechanism of the carbohydroxylation of styrene derivatives in a wasteful manner (previous work) and through clean C–H bond activation with H₂ evolution (this work). LG = leaving group, e.g., AcO⁻, N₂ + BF₄⁻, ArI + BF₄⁻; X^• = heteroatom-centered radical, e.g., ^tBuO^•, ⁱPrO^•; e^– = electron.

Fig. 2. Development of a photocatalytic system based on dual semiconductors.

a Our previous achievements in organic synthesis (left) and in water splitting (right). b Optimization study. Reaction conditions: 1a (0.1 mmol), 2a (1 mL, 19.0 mmol), H₂O (100 μL), PC-1 (10.0 mg), PC-2 (10.0 mg), and LiOH (2 μmol) under LED irradiation (λ = 365 nm) and an N₂ atmosphere at room temperature for 24 h. The yields of 3aa, 4, and 5 were determined by ¹H NMR analysis (theoretical yield of 5: 50 μmol); H₂ was quantified by μGC-TCD. ^aConversion of 1a: 100%, selectivity to 3aa: 72%, yield of CO₂: 7 μmol, yield of O₂: <1 μmol. ^bYield of CO₂: 22 μmol, yield of O₂: 24 μmol.

Fig. 3. Substrate scope.

Standard conditions: 1 (0.1 mmol), 2 (1 mL), H₂O (100 μL), Ag (0.5 wt%)/TiO₂ (10.0 mg), RhCrCo/SrTiO₃:Al (10.0 mg), LiOH (2 μmol) under LED irradiation (λ = 365 nm) and an N₂ atmosphere at room temperature for 24 h. Isolated yield of 3. H₂ was quantified by μGC-TCD. Reaction time: ^a36 h, ^b48 h, ^c72 h, ^d96 h. ^e2a (2 mL), H₂O (200 μL), LiOH (4 μmol). ^fAg/TiO₂ (20.0 mg), ^gRhCrCo/SrTiO₃:Al (20.0 mg). ^h1 mmol scale reaction using two Kessil lamps (λ = 370 nm). ⁱDetermined by ¹H NMR analysis of the crude mixture.

Fig. 4. Demonstration of APOS.

Demonstration of the synthetic potential of this transformation: a synthesis of terfenadine and b application for cyclization. Meeting the requirements for artificial photosynthesis: c a solar-induced endergonic reaction with H₂ evolution and d H₂O serving as the oxygen source. e Plausible reaction mechanism; e^– = electron, h⁺ = hole.