Direct, Site-Selective and Redox-Neutral α-C−H Bond Functionalization of Tetrahydrofurans via Quantum Dots Photocatalysis
TL;DR: In this article, a semiconductor quantum dot (QD) conjugate of tetrahydrofuran (THF) was demonstrated to activate α-C-H bond of THF via forming QDs/THF conjugates.
Abstract: As one of the most ubiquitous bulk reagents available, the intrinsic chemical inertness of tetrahydrofuran (THF) makes direct and site-selective C(sp 3 )-H bond activation difficult, especially under redox neutral condition. Here, we demonstrate that semiconductor quantum dots (QDs) can activate α-C-H bond of THF via forming QDs/THF conjugates. Under visible light irradiation, the resultant alkoxyalkyl radical directly engages in radical cross-coupling with α-amino radical from amino C-H bonds or radical addition with alkene or phenylacetylene, respectively. In contrast to stoichiometric oxidant or hydrogen atom transfer reagents required in previous studies, the scalable benchtop approach can execute α-C-H bond functionalization of THF only by a QD photocatalyst under redox-neutral condition, thus providing a broad of value added chemicals starting from bulk THFs reagent. The high step- and atom-economy, high efficiency and broad substrate scope make the photocatalysis with QDs and visible light promising in both academic and industrial setting.
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TL;DR: In this article , the most recent advancements in photocatalytic R˙ generation and highlights representative examples in this field are summarized and discussed in each section and a general mechanistic scenario and key r˙-forming steps are presented and discussed.
Abstract: C(sp3) radicals (R˙) are of broad research interest and synthetic utility. This review collects some of the most recent advancements in photocatalytic R˙ generation and highlights representative examples in this field. Based on the key bond cleavages that generate R˙, these contributions are divided into C–H, C–C, and C–X bond cleavages. A general mechanistic scenario and key R˙-forming steps are presented and discussed in each section.
28 citations
TL;DR: In this paper , the ability of CdS nanocrystal quantum dots (QDs) to function as strong photoreductants was investigated and it was shown that Auger recombination, a photophysical phenomenon known to occur in photoexcited anionic QDs, generates transient thermally excited electrons to enable the observed reductions.
Abstract: Strong reducing agents (<-2.0 V vs saturated calomel electrode (SCE)) enable a wide array of useful organic chemistry, but suffer from a variety of limitations. Stoichiometric metallic reductants such as alkali metals and SmI2 are commonly employed for these reactions; however, considerations including expense, ease of use, safety, and waste generation limit the practicality of these methods. Recent approaches utilizing energy from multiple photons or electron-primed photoredox catalysis have accessed reduction potentials equivalent to Li0 and shown how this enables selective transformations of aryl chlorides via aryl radicals. However, in some cases, low stability of catalytic intermediates can limit turnover numbers. Herein, we report the ability of CdS nanocrystal quantum dots (QDs) to function as strong photoreductants and present evidence that a highly reducing electron is generated from two consecutive photoexcitations of CdS QDs with intermediate reductive quenching. Mechanistic experiments suggest that Auger recombination, a photophysical phenomenon known to occur in photoexcited anionic QDs, generates transient thermally excited electrons to enable the observed reductions. Using blue light-emitting diodes (LEDs) and sacrificial amine reductants, aryl chlorides and phosphate esters with reduction potentials up to -3.4 V vs SCE are photoreductively cleaved to afford hydrodefunctionalized or functionalized products. In contrast to small-molecule catalysts, QDs are stable under these conditions and turnover numbers up to 47 500 have been achieved. These conditions can also effect other challenging reductions, such as tosylate protecting group removal from amines, debenzylation of benzyl-protected alcohols, and reductive ring opening of cyclopropane carboxylic acid derivatives.
18 citations
TL;DR: In this article , cost-effective 2D metal-selenides are verified to be potential candidate cocatalysts for photocatalytic H2 evolution, but their cocatalysis activities are limited by the scarce number of unsaturated Se atoms and their...
Abstract: Cost-effective 2D metal-selenides are verified to be potential candidate cocatalysts for photocatalytic H2 evolution. However, their cocatalytic activities are limited by the scarce number of unsaturated Se atoms and their...
16 citations
TL;DR: In this article , the formation of individual Fe atoms on polymeric carbon nitride (CN), that activates O2 to create O2•− for facilitating the reaction of ethylbenzene to form acetophenone, is demonstrated.
Abstract: Selective oxidation of CH bonds is one of the most important reactions in organic synthesis. However, activation of the α‐CH bond of ethylbenzene by use of photocatalysis‐generated superoxide anions (O2•−) remains a challenge. Herein, the formation of individual Fe atoms on polymeric carbon nitride (CN), that activates O2 to create O2•− for facilitating the reaction of ethylbenzene to form acetophenone, is demonstrated. By utilizing density functional theory and materials characterization techniques, it is shown that individual Fe atoms are coordinated to four N atoms of CN and the resultant low‐spin Fe–N4 system (t2g6eg0) is not only a great adsorption site for oxygen molecules, but also allows for fast transfer of electrons generated in the CN framework to adsorbed O2, producing O2•−. The oxidation reaction of ethylbenzene triggered by O2•− ions turns out to have a high conversion rate of 99% as well as an acetophenone selectivity of 99%, which can be ascribed to a novel reaction pathway that is different from the conventional route involving hydroxyl radicals and the production of phenethyl alcohol. Furthermore, it possesses great potential for other CH activation reactions besides ethylbenzene oxidation.
15 citations
TL;DR: In this paper , the first example of introducing reductive carbon-carbon (C-C) coupling reaction to block the competing H 2 evolution in photocatalytic CO 2 reduction in water was described.
Abstract: Colloidal quantum dots (QDs) consisting of precious-metal-free elements show attractive potentials towards solar-driven CO 2 reduction. However, the inhibition of hydrogen (H 2 ) production in aqueous solution remains a challenge. Here, we describe the first example of introducing reductive carbon-carbon (C-C) coupling reaction to block the competing H 2 evolution in photocatalytic CO 2 reduction in water. In a specific system taking ZnSe QDs as photocatalysts, the introduction of furfural, one of the biomass platform molecules, can significantly suppress H 2 evolution, thus leading to CO evolution with a rate of ~5.3 mmol g -1 h -1 and a turnover number (TON) of >7500 under 24 h visible light. Meanwhile, furfural is upgraded to self-coupling product with a yield of 99.8% based on the consumption of furfural. Mechanistic insights show that the reductive furfural coupling reaction occurs on surface Zn-sites to consume electrons and protons originally used for H 2 production, while CO formation pathway at surface anion vacancies remains to simultaneously evolve solar fuel (CO) from CO 2 and valuable chemicals from raw carbon-oxygenates via artificial photosynthesis.
14 citations
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TL;DR: The conversion of these bench stable, benign catalysts to redox-active species upon irradiation with simple household lightbulbs represents a remarkably chemoselective trigger to induce unique and valuable catalytic processes.
Abstract: A fundamental aim in the field of catalysis is the development of new modes of small molecule activation. One approach toward the catalytic activation of organic molecules that has received much attention recently is visible light photoredox catalysis. In a general sense, this approach relies on the ability of metal complexes and organic dyes to engage in single-electron-transfer (SET) processes with organic substrates upon photoexcitation with visible light.
Many of the most commonly employed visible light photocatalysts are polypyridyl complexes of ruthenium and iridium, and are typified by the complex tris(2,2′-bipyridine) ruthenium(II), or Ru(bpy)32+ (Figure 1). These complexes absorb light in the visible region of the electromagnetic spectrum to give stable, long-lived photoexcited states.1,2 The lifetime of the excited species is sufficiently long (1100 ns for Ru(bpy)32+) that it may engage in bimolecular electron-transfer reactions in competition with deactivation pathways.3 Although these species are poor single-electron oxidants and reductants in the ground state, excitation of an electron affords excited states that are very potent single-electron-transfer reagents. Importantly, the conversion of these bench stable, benign catalysts to redox-active species upon irradiation with simple household lightbulbs represents a remarkably chemoselective trigger to induce unique and valuable catalytic processes.
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Figure 1
Ruthenium polypyridyl complexes: versatile visible light photocatalysts.
6,252 citations
TL;DR: This work over the past several years to form carbon-carbon bonds directly from two different C-H bonds under oxidative conditions, cross-dehydrogenative coupling (CDC) is described, which provides an alternative to the separate steps of prefunctionalization and defunctionsalization that have traditionally been part of synthetic design.
Abstract: Synthetic chemists aspire both to develop novel chemical reactions and to improve reaction conditions to maximize resource efficiency, energy efficiency, product selectivity, operational simplicity, and environmental health and safety. Carbon−carbon bond formation is a central part of many chemical syntheses, and innovations in these types of reactions will profoundly improve overall synthetic efficiency. This Account describes our work over the past several years to form carbon−carbon bonds directly from two different C−H bonds under oxidative conditions, cross-dehydrogenative coupling (CDC). We have focused most of our efforts on carbon−carbon bonds formed via the functionalization of sp3 C−H bonds with other C−H bonds. In the presence of simple and cheap catalysts such as copper and iron salts and oxidants such as hydrogen peroxide, dioxygen, tert-butylhydroperoxide, and 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ), we can directly functionalize various sp3 C−H bonds by other C−H bonds without requiring ...
2,308 citations
TL;DR: This Review highlights the recent progress in the field of cross-dehydrogenative C sp 3C formations and provides a comprehensive overview on existing procedures and employed methodologies.
Abstract: Over the last decade, substantial research has led to the introduction of an impressive number of efficient procedures which allow the selective construction of CC bonds by directly connecting two different CH bonds under oxidative conditions. Common to these methodologies is the generation of the reactive intermediates in situ by activation of both CH bonds. This strategy was introduced by the group of Li as cross-dehydrogenative coupling (CDC) and discloses waste-minimized synthetic alternatives to classic coupling procedures which rely on the use of prefunctionalized starting materials. This Review highlights the recent progress in the field of cross-dehydrogenative C sp 3C formations and provides a comprehensive overview on existing procedures and employed methodologies.
1,528 citations
TL;DR: Recent advances in the understanding of the atomic structure and optical properties of semiconductor nanocrystals are discussed and new strategies for band gap and electronic wave function engineering to control the location of charge carriers are discussed.
Abstract: Semiconductor nanocrystals are tiny light-emitting particles on the nanometer scale. Researchers have studied these particles intensely and have developed them for broad applications in solar energy conversion, optoelectronic devices, molecular and cellular imaging, and ultrasensitive detection. A major feature of semiconductor nanocrystals is the quantum confinement effect, which leads to spatial enclosure of the electronic charge carriers within the nanocrystal. Because of this effect, researchers can use the size and shape of these “artificial atoms” to widely and precisely tune the energy of discrete electronic energy states and optical transitions. As a result, researchers can tune the light emission from these particles throughout the ultraviolet, visible, near-infrared, and mid-infrared spectral ranges. These particles also span the transition between small molecules and bulk crystals, instilling novel optical properties such as carrier multiplication, single-particle blinking, and spectral diffusi...
1,497 citations
TL;DR: In this article, the electron spin resonance hyperfine splitting constants of spin adducts of interest in this area are tabulated and a brief comment on the source of the radical trapped is given.
1,487 citations