1,720,992 research outputs found
Asymmetric electrochemical synthesis in flow
Full text linkWe are currently experiencing a resurgence in the realm of electrochemical organic synthesis, driven by the transformative potential of conducting redox chemistry under mild conditions through the simple use of electrons, thereby circumventing the use of harmful reductants and oxidants. This renaissance is further bolstered by the fusion of electrochemistry with flow chemistry, which not only grants precise control over reaction parameters but also promotes sustainability and heightened reproducibility. Despite these promising advancements, the application of flow electrochemistry to steer asymmetric processes remains in its nascent stage. This perspective delves into the limited contributions to date, shedding light on critical challenges and presenting prospective solutions that are essential for fully unleashing the untapped potential of this field. Graphical abstract: [Figure not available: see fulltext.]
Building C–C bridges with oxalic acid as a traceless keystone
No full textThe construction of C–C bonds from underused feedstock is one of the “holy grails” in organic synthesis. In this issue of Chem, Wu, Lu, and co-workers report a method that makes use of photocatalysis and oxalic acid to promote the reductive cross-coupling of alkenes in a two-step procedure
Homogeneous catalytic C(sp3)-H functionalization of gaseous alkanes
Full text linkThe conversion of light alkanes into bulk chemicals is becoming an important challenge as it effectively avoids the use of prefunctionalized alkylating reagents. The implementation of such processes is, however, hampered by their gaseous nature and low solubility, as well as the low reactivity of the C-H bonds. Efforts have been made to enable both polar and radical processes to activate these inert compounds. In addition, these methodologies also benefit significantly from the development of a suitable reactor technology that intensifies gas-liquid mass transfer. In this review, we critically highlight these developments, both from a conceptual and a practical point of view. The recent expansion of these mechanistically-different methods have enabled the use of various gaseous alkanes for the development of different bond-forming reactions, including C-C, C-B, C-N, C-Si and C-S bonds. This journal i
Asymmetric Photocatalytic C-H Functionalization of Toluene and Derivatives
No full textReported herein is a visible-light-mediated organocatalytic direct C-H functionalization of toluene derivatives to afford enantioenriched β-benzylated aldehydes from the corresponding enals. The process combines the oxidative power of a chiral excited-state iminium ion and the basic nature of its counteranion to trigger the generation of benzylic radicals by means of a sequential multisite proton-coupled electron transfer mechanism. This study shows that feedstock chemicals generally used as solvents, such as toluene and xylene derivatives, can be used as substrates for making chiral molecules with high enantioselectivity
Synthetic Methods Driven by the Photoactivity of Electron Donor-Acceptor Complexes
Full text linkThe association of an electron-rich substrate with an electron-accepting molecule can generate a new molecular aggregate in the ground state, called an electron donor-acceptor (EDA) complex. Even when the two precursors do not absorb visible light, the resulting EDA complex often does. In 1952, Mulliken proposed a quantum-mechanical theory to rationalize the formation of such colored EDA complexes. However, and besides a few pioneering studies in the 20th century, it is only in the past few years that the EDA complex photochemistry has been recognized as a powerful strategy for expanding the potential of visible-light-driven radical synthetic chemistry. Here, we explain why this photochemical synthetic approach was overlooked for so long. We critically discuss the historical context, scientific reasons, serendipitous observations, and landmark discoveries that were essential for progress in the field. We also outline future directions and identify the key advances that are needed to fully exploit the potential of the EDA complex photochemistry
Scale-Up of Photochemical Reactions: Transitioning from Lab Scale to Industrial Production
Full text linkIn the past two decades, we have witnessed a rapid emergence of new and powerful photochemical and photocatalytic synthetic methods. Although these methods have been used mostly on a small scale, there is a growing need for efficient scale-up of photochemistry in the chemical industry. This review summarizes and contextualizes the advancements made in the past decade regarding the scale-up of photo-mediated synthetic transformations. Simple scale-up concepts and important fundamental photochemical laws have been provided along with a discussion concerning suitable reactor designs that should facilitate scale-up of this challenging class of organic reactions
Photocatalytic (3 + 2) dipolar cycloadditions of aziridines driven by visible-light
No full textHerein, we document the design and development of a novel (3 + 2) cycloaddition reaction aided by the activity of an organic photocatalyst and visible light. The process is extremely fast, taking place in a few minutes, with virtually complete atom economy. A large variety of structurally diverse aziridines were used as masked ylides in the presence of different types of dipolarophiles (28 examples with up to 94% yield and >95 : 5 dr). Mechanistic insights obtained from photophysical, electrochemical and experimental studies highlight that the chemistry is driven by the in situ generation of the reactive ylide through two consecutive electron-transfer processes. We also report an aerobic cascade process, where an additional oxidation step grants access to a vast array of pyrrole derivatives (19 examples with up to 95% yield). Interestingly, the extended aromatic core exhibits a distinctive absorption and emission profile, which can be easily used to tag the effectiveness of this covalent linkage
Photochemical Organocatalytic Borylation of Alkyl Chlorides, Bromides, and Sulfonates
Full text linkReported herein is a photochemical strategy for the borylation of alkyl halides using bis(catecholato)diboron as the boron source. This method exploits the ability of a nucleophilic dithiocarbonyl anion organocatalyst to generate radicals via an SN2-based photochemical catalytic mechanism, which is not reliant on the redox properties of the substrates. Therefore, it grants access to alkyl boronic esters from readily available but difficult-to-reduce electrophiles, including benzylic and allylic chlorides, bromides, and mesylates, which were inert to or unsuitable for previously reported metal-free borylation protocols
Photochemical generation of acyl and carbamoyl radicals using a nucleophilic organic catalyst: Applications and mechanism thereof
No full textWe detail a strategy that uses a commercially available nucleophilic organic catalyst to generate acyl and carbamoyl radicals upon activation of the corresponding chlorides and anhydrides via a nucleophilic acyl substitution path. The resulting nucleophilic radicals are then intercepted by a variety of electron-poor olefins in a Giese-type addition process. The chemistry requires low-energy photons (blue LEDs) to activate acyl and carbamoyl radical precursors, which, due to their high reduction potential, are not readily prone to redox-based activation mechanisms. To elucidate the key mechanistic aspects of this catalytic photochemical radical generation strategy, we used a combination of transient absorption spectroscopy investigations, electrochemical studies, quantum yield measurements, and the characterization of key intermediates. We identified a variety of off-the-cycle intermediates that engage in a light-regulated equilibrium with reactive radicals. These regulated equilibriums cooperate to control the overall concentrations of the radicals, contributing to the efficiency of the overall catalytic process and facilitating the turnover of the catalyst. This journal i
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