An official website of the United States government
Abstract We report herein a rare example of enantiodivergent aldehyde addition with β‐alkenyl allylic boronates via chiral Brønsted acid catalysis. 2,6‐Di‐9‐anthracenyl‐substituted chiral phosphoric acid‐catalyzed asymmetric allylation using β‐vinyl substituted allylic boronate gave alcohols withRabsolute configuration. The sense of asymmetric induction of the catalyst in these reactions is opposite to those in prior reports. Moreover, in the presence of the same acid catalyst, the reactions with β‐2‐propenyl substituted allylic boronate generated homoallylic alcohol products withSabsolute configuration. Unusual substrate‐catalyst C−H⋅⋅⋅π interactions in the favoured reaction transition state were identified as the origins of observed enantiodivergence through DFT computational studies.
more »
« less
Asymmetric Photocatalysis Enabled by Chiral Organocatalysts
Abstract Visible‐light photocatalysis has advanced as a versatile tool in organic synthesis. However, attaining precise stereocontrol in photocatalytic reactions has been a longstanding challenge due to undesired photochemical background reactions and the involvement of highly reactive radicals or radical ion intermediates generated under photocatalytic conditions. To address this problem and expand the synthetic utility of photocatalytic reactions, a number of innovative strategies, including mono‐ and dual‐catalytic approaches, have recently emerged. Of these, exploiting chiral organocatalysis, such as enamine catalysis, iminium‐ion catalysis, Brønsted acid/base catalysis, andN‐heterocyclic carbene catalysis, to induce chirality transfer of photocatalytic reactions has been widely explored. This Review aims to provide a current, comprehensive overview of asymmetric photocatalytic reactions enabled by chiral organocatalysts published through June 2021. The substrate scope, advantages, limitations, and proposed reaction mechanisms of each reaction are discussed. This review should serve as a reference for the development of visible‐light‐induced asymmetric photocatalysis and promote the improvement of the chemical reactivity and stereoselectivity of these reactions.
more »
« less
- Award ID(s):
- 1848463
- PAR ID:
- 10303411
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- ChemCatChem
- Volume:
- 14
- Issue:
- 1
- ISSN:
- 1867-3880
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Efficient utilization of sunlight in photocatalysis is widely recognized as a promising solution for addressing the growing energy demand and environmental issues resulting from fossil fuel consumption. Recently, there have been significant developments in various near‐infrared (NIR) light‐harvesting systems for artificial photosynthesis and photocatalytic environmental remediation. This review provides an overview of the most recent advancements in the utilization of NIR light through the creation of novel nanostructured materials and molecular photosensitizers, as well as modulating strategies to enhance the photocatalytic processes. A special focus is given to the emerging two‐photon excitation NIR photocatalysis. The unique features and limitations of different systems are critically evaluated. In particular, it highlights the advantages of utilizing NIR light and two‐photon excitation compared to UV–visible irradiation and one‐photon excitation. Ongoing challenges and potential solutions for the future exploration of NIR light‐responsive materials are also discussed.more » « less
-
Abstract Visible‐light‐induced halide‐exchange between halide perovskite and organohalide solvents has been studied in which photoinduced electron transfer from CsPbBr3nanocrystals (NCs) to dihalomethane solvent molecules produces halide anions via reductive dissociation, followed by a spontaneous anion‐exchange. Photogenerated holes in this process are less focused. Here, for CsPbBr3in dibromomethane (DBM), we discover that Br radical (Br⋅) is a key intermediate resulting from the hole oxidation. We successfully trapped Br⋅ with reported methods and found that Br⋅ is continuously generated in DBM under visible light irradiation, hence imperative for catalytic reaction design. Continuous Br⋅ formation within this halide‐exchange process is active for photocatalytic [3+2] cycloaddition for vinylcyclopentane synthesis, a privileged scaffold in medicinal chemistry, with good yield and rationalized diastereoselectivity. The NC photocatalyst is highly recyclable due to Br‐based self‐healing, leading to a particularly economic and neat heterogeneous reaction where the solvent DBM also acts as a co‐catalyst in perovskite photocatalysis. Halide perovskites, notable for efficient solar energy conversion, are demonstrated as exceptional photocatalysts for Br radical‐mediated [3+2] cycloaddition. We envisage such perovskite‐induced Br radical strategy may serve as a powerful chemical tool for developing valuable halogen radical‐involved reactions.more » « less
-
Abstract Hydrogen gas is a prominent focus in pursuing renewable and clean alternative energy sources. The quest for maximizing hydrogen production yield involves the exploration of an ideal photocatalyst and the development of a simple, cost‐effective technique for its generation. Iron titanate has garnered attention in this context due to its photocatalytic properties, affordability, and non‐toxic nature. Over the years, different synthesis routes, different morphologies, and some modifications of iron titanate have been carried out to improve its photocatalytic performance by enhancing light absorption in the visible region, boosting charge carrier transfer, and decreasing recombination of electrons and holes. The use of iron titanate photocatalyst for hydrogen evolution reaction has seen an upward trend in recent times, and based on available findings, more can be done to improve the performance. This review paper provides a comprehensive overview of the fundamental principles of photocatalysis for hydrogen generation, encompassing the synthesis, morphology, and application of iron titanate‐based photocatalysts. The discussion delves into the limitations of current methodologies and present and future perspectives for advancing iron titanate photocatalysts. By addressing these limitations and contemplating future directions, the aim is to enhance the properties of materials fabricated for photocatalytic water splitting.more » « less
-
null (Ed.)Due to its clean and sustainable nature, solar energy has been widely recognized as a green energy source in driving a variety of reactions, ranging from small molecule activation and organic transformation to biomass valorization. Within this context, organic reactions coupled with H 2 evolution via semiconductor-based photocatalytic systems under visible light irradiation have gained increasing attention in recent years, which utilize both excited electrons and holes generated on semiconductors and produce two types of value-added products, organics and H 2 , simultaneously. Based on the nature of the organic reactions, in this review article we classify semiconductor-based photocatalytic organic transformations and H 2 evolution into three categories: (i) photocatalytic organic oxidation reactions coupled with H 2 production, including oxidative upgrading of alcohols and biomass-derived intermediate compounds; (ii) photocatalytic oxidative coupling reactions integrated with H 2 generation, such as C–C, C–N, and S–S coupling reactions; and (iii) photo-reforming reactions together with H 2 formation using organic plastics, pollutants, and biomass as the substrates. Representative heterogeneous photocatalytic systems will be highlighted. Specific emphasis will be placed on their synthesis, characterization, and photocatalytic mechanism, as well as the organic reaction scope and practical application.more » « less
