Comprehensive Summary: In the past two decades, the development of asymmetric radical reactions has achieved tremendous progress, which has emerged as a powerful tool for the synthesis of chiral molecules in synthetic chemistry. Among the diverse array of radical processes, the transfer of hydrogen atoms to tertiary carbon radicals offers the potential for constructing chiral tertiary carbon centers in a stereoselective fashion. Notwithstanding the challenges associated with the reactive and evanescent nature of radical species, the use of chiral reagents or mediators has enabled the stereocontrol of the asymmetric hydrogen atom transfer (AHAT), which provides novel avenues for advancing the field of asymmetric synthesis.
Comprehensive Summary: Boronate esters are highly valued in synthetic and pharmaceutical industries for their versatility in creating C—C and C—X bonds. They also find applications as catalysts in chemical transformations as well as stimuli-responsive materials in materials science. Some alkyl boronates themselves also show promising applications in medicinal chemistry. In the past few decades, chemists have been devoted to developing new methods or new starting materials for synthesizing boronate esters. Carboxylic acids and their derivatives are privileged chemical entities due to their readily availability or natural abundance, structural diversity, and chemical stability. Hence, the transformation of carboxylic acid and their derivatives to alkyl/aryl boronate esters has seen its fast development in the past decade. This review summarized the state-to-art development of decarboxylative and decarbonylative borylation of carboxylic acids and their derivatives to aryl and alkyl boronate esters.
Key Scientists: The decarboxylative and decarbonylative borylation of carboxylic acids and their derivatives started only in the past decade. In 2016, the decarbonylative borylation of carboxylic esters and amides was reported by Zhuangzhi Shi and Magnus Reuping’s groups. Then, in 2017, studies on the decarboxylative borylation of redox-active esters such as NHPI esters started to receive increasing attention by Aggarwal, Baran, Fu, Glorius, and Li’s groups. From 2018 to 2023, large numbers of studies on the decarboxylative and decarbonylative borylation of carboxylic acids and their derivatives using transition-metal-catalyst, organo-catalyst, or under photochemical or electrochemical conditions emerged. Due to space limitations, only pictures of scientists who have contributed more than two works in this area are shown herein.
Comprehensive Summary: Electron-rich alkynes, such as ynol and thioynol ethers, have proven to be versatile and appealing partners in catalytic cycloaddition reactions, and thus have raised considerable attentions owing to the practical application in the modular assembly of valuable carbo- and heterocycles. The past decades have witnessed inspiring advances in this emerging field, and an increasing number of related discoveries have been exploited. Divided into two main sections on the basis of substrate type, in each section this comprehensive review will initially summarize their synthetic preparations and subsequently examine their reactivity in every sort of catalytic cycloaddition with emphasis on the methodology development, aimed at providing an access to this burgeoning area and encouraging further innovations in the near future.
Key Scientists: For the cycloaddition of ynol ethers, in 2004, Kozmin et al. firstly developed a silver-catalyzed [2 + 2] cycloaddition of siloxy alkynes with electron-poor olefins. In 2012, Hiyama et al. realized a palladium-catalyzed formal [4 + 2] annulation of alkynyl aryl ethers with internal alkynes. In the same year, Sun et al. discovered an efficient [6 + 2] cyclization between siloxy alkynes and 2-(oxetan-3-yl)benzaldehydes by applying HNTf2 as catalyst. In 2017, Wender et al. first utilized vinylcyclopropanes (VCPs) as coupling partners in the [5 + 2] annulation of ynol ethers. In 2018 and 2020, Ye et al. reported zinc-catalyzed formal [3 + 2] and [4 + 3] cycloaddition, respectively. For the cycloaddition of thioynol ethers, in 2004, Hilt et al. realized a [4 + 2] cycloaddition by employing the alkynyl sulfides and acyclic 1, 3-dienes. In 2006, a ruthenium-catalyzed [2 + 2] cycloaddition of thioynol ethers with bicyclic alkenes was accomplished by Tam. In 2014, Sun et al. reported an elegant iridium-catalyzed click reaction of thioalkynes with azides.
Comprehensive Summary: Cross-electrophile couplings (XEC), a crucial subset of cross-coupling reactions, center on the formation of robust C—C bonds through the union of two electrophiles. Usually, such reactions have primarily been catalyzed by transition metals. However, with the steady advancements in photochemical and electrochemical technologies, XEC reactions have significantly progressed and broadened their scope, allowing for the utilization of a wider array of tolerable functional groups, thus revealing vast application prospects. This review aims to systematically summarize the current prevalent types of electrophiles and delve into their specific application examples within XEC reactions involving electrophiles with identical functional groups. Specifically, XECs between the same type of halides have received considerable attention, whereas carboxylic acids and alcohols are still in the early stages of investigation. Furthermore, certain other common electrophiles remain unexplored in this context. Moreover, this review underscores the remarkable contributions of photochemistry and electrochemistry in the field of XEC reactions, aiming to provide valuable insights and inspiration for researchers. Also, this review hopes to spark further interest in XEC reactions, thereby fueling the continuous development and advancement of this exciting area of research.
Key Scientists: Since the 1960s, advancements in the XEC reaction have been substantial, driven primarily by the application of transition metal catalysts. In this area, many distinguished scientists have contributed their wisdom and efforts. Particularly noteworthy is that, during the systematic study of XEC reactions with the identical functional groups, in 2016, MacMillan achieved a photocatalytic XEC reaction between aryl bromides and alkyl bromides; in 2020, Weix successfully realized a nickel-catalyzed XEC reaction between aryl chlorides and alkyl chlorides. Concurrently, contributions from researchers such as Mei, Wolf, Sevov, Lin, Shen, Browne, Zhang, and Qiu have expanded the scope of XEC reactions to various halides. By 2022, MacMillan and Baran achieved a significant milestone in the XEC between carboxylic acids, further broadening the scope of research in this area. Also, advancements in the XEC of alcohols have been noted, with researchers including Weix, Lian, Tu, and Stahl conducting pioneering work and successfully executing the XEC of protective groups. It is foreseen that the ongoing research endeavors will primarily concentrate on the expansion of diverse electrophiles.
Comprehensive Summary: Tuning electrolyte properties is a widely recognized strategy to enhance activity and selectivity in electrocatalysis, drawing increasing attention in this domain. Despite extensive experimental and theoretical studies, debates persist about how various electrolyte components influence electrocatalytic reactions. We offer a concise review focusing on current discussions, especially the contentious roles of cations. This article further examines how different factors affect the interfacial solvent structure, particularly the hydrogen-bonding network, and delves into the microscopic kinetics of electron and proton-coupled electron transfer. We also discuss the overarching influence of solvents from a kinetic modeling perspective, aiming to develop a robust correlation between electrolyte structure and reactivity. Lastly, we summarize ongoing research challenges and suggest potential directions for future studies on electrolyte effects in electrocatalysis.
Key Scientists: In 1956, Marcus theory was developed to describe the mechanism of outer-sphere electron transfer (OS-ET). In 1992, Nocera et al. directly measured proton-coupled electron transfer (PCET) kinetics for the first time, and their subsequent research in 1995 investigated the effects of proton motion on electron transfer (ET) kinetics. In 1999 and 2000, Hammes-schiffer et al. developed the multistate continuum theory for multiple charge reactions and deduced the rate expressions for nonadiabatic PCET reactions in solution, laying the theoretical foundation for the analysis of PCET kinetics in electrochemical processes. In 2006, Saveant et al. verified the concerted proton and electron transfer (CPET) mechanism in the oxidation of phenols coupled with intramolecular amine-driven proton transfer (PT). Their subsequent work in 2008 reported the pH-dependent pathways of electrochemical oxidation of phenols.
Electrolyte effects in electrocatalysis have gained emphasis in recent years. In 2009, Markovic’s pioneering work proposed non-covalent interactions between hydrated alkaline cations and adsorbed OH species in oxygen reduction reaction (ORR)/hydrogen oxidation reaction (HOR). In 2011, Markovic et al. significantly enhanced hydrogen evolution reaction (HER) activity in alkaline solution by improving water dissociation, which was assumed to dominate the sluggish HER kinetics in such media. In comparation, Yan et al. applied hydrogen binding energy (HBE) theory in 2015 to explain the pH-dependent HER/HOR activity. Cations play a significant role in regulating the selectivity and activity of carbon dioxide reduction (CO2RR). In 2016 and 2017, Karen Chan et al. introduced the electric field generated by solvated cations to explain the cation effects on electrochemical CO2RR. Conversely, in 2021, Koper et al. suggested that short-range electrostatic interactions between partially desolvated metal cations and CO2 stabilized CO2 and promoted CO2RR.
Recent researches have combined the exploration of the electrical double layer (EDL) structure with theoretical analysis of PCET kinetics. In 2019, Huang et al. developed a microscopic Hamiltonian model to quantitatively understand the sluggish hydrogen electrocatalysis in alkaline media. In 2021, two meticulous studies from Shao-Horn’s group analyzed the effects of cations on reorganization energy and the impacts of hydrogen bonds between proton donors and acceptors on proton tunneling kinetics, respectively. Electrolyte effects on proton transport process were researched in recent years. In 2022, Hu et al. and Chen et al. proposed that the cation-induced electric field distribution and pH-dependent hydrogen bonding network connectivity played essential roles in proton transport, separately.
Enzymes are natural treasure troves that hold multiple superiority. Enzymatic catalysis has become a powerful tool for asymmetric synthesis, though it is typically limited to a relatively narrow range of reaction types. By integrating the advantages of enzymatic catalysis with photocatalysis, photoenzymatic catalysis not only expands the catalytic capabilities of enzymes but also provides an effective strategy for the stereo-control of photochemical reactions, thereby emerging as a significant research field. Herein, we focus on new-to-nature photoenzymatic catalysis by repurposing naturally occurring enzymes with visible light. We highlight the seminal work in reshaping various classes of enzymes, emphasizing their catalytic mechanism and synthetic potentials.