Energy transfer (EnT) photocatalysis enabled by gold-N-heterocyclic carbene (NHC) complexes

Light-enabled intramolecular [2 + 2] cycloaddition via photoactivation of simple alkenylboronic esters

The photoactivation of organic molecules via energy transfer (EnT) catalysis is often limited to conjugated systems that have low-energy triplet excited states, with simple alkenes remaining an intractable challenge. The ability to address this limitation, using high energy sensitizers, would represent an attractive platform for future reaction design. Here, we disclose the photoactivation of simple alkenylboronic esters established using alkene scrambling as a rapid reaction probe to identify a suitable catalyst and boron motif. Cyclic voltammetry, UV-vis analysis, and control reactions support sensitization, enabling an intramolecular [2 + 2] cycloaddition to be realized accessing 3D bicyclic fragments containing a boron handle.

Visible light-driven dearomative meta-cycloadditions of 2-acetonaphthalenes via triplet energy transfer cascade

Dearomative cycloadditions (DACs) with ortho- and para-variants have been well documented under visible light-mediated triplet-triplet energy transfer (VLEnT) catalysis. The prospective [3 + 2] or the meta-DACs propelled via VLEnT catalysis remains elusive. Classically, meta-DACs are known under harsher UV irradiations and are symmetry allowed in the excited singlet potential energy surface. Herein, we report formal meta-DACs of 2-acetonaphthalenes propagated via a two-step VLEnT cascade circumventing the attainment of energetically higher singlet excited states. The photosensitizer selectively promotes the [4 + 2] cycloaddition followed by a contra-thermodynamic di-π-methane type skeleton rearrangement cascade. The DFT studies in conjugation with electrochemical, photoluminescence, kinetic, quadratic dependency, and control experiments support the VLEnT cascade. The described protocol delivers highly sp3-rich polycyclic frameworks in high yields with wide functional group tolerance. The inclusion of bioactive molecules and the establishment of a wide array of post-synthetic derivatizations further underscores the adaptability of the methodology for generating complex three-dimensional molecules.

Cobalt-based Photocatalysis: From Fundamental Principles to Applications in the Generation of C-X (X=C, O, N, H, Si) Bond

Over the past few decades, the merger of photocatalysis and transition metal-based catalysis or self-photoexcitation of transition metals has emerged as a useful tool in organic transformations. In this context, cobalt-based systems have attracted significant attention as sustainable alternatives to the widely explored platinum group heavy metals (iridium, rhodium, ruthenium) for photocatalytic chemical transformations. This review encompasses the basic types of cobalt-based homogeneous photocatalytic systems, their working principles, and the recent developments (2018-2024) in C-X (X=C, N, O, H, Si) bond formations. Noteworthy to mention that cobalt-based heterogeneous photocatalysis is beyond the scope of the present review. An elaborate presentation on the mechanistic intricacies of cobalt-based photocatalysis, without any external photocatalyst, and cobalt-based dual organophotoredox catalysis have been provided in this comprehensive review, excluding the dual-metal photoredox catalysis. To the best of our knowledge, this is the only contemporary review encompassing the aforementioned two major types of cobalt-based photocatalysis, in general synthetic chemistry, covering all types of C-X bond formations spanning a range of the last six years.

Mechanism and Origins of Regio- and Stereoselectivities of NHC-Catalyzed Dearomative Annulation of Benzoazoles and Cinnamaldehydes from DFT

A theoretical study on the mechanism, regioselectivity, and enantioselectivity of NHC-catalyzed dearomatizing annulation of benzoxazoles with enals has been conducted using density functional theory calculations. Our calculated results indicate that the favored mechanism occurs through eight reaction steps: initial binding of the NHC to enals, followed by formation of the Breslow intermediate via proton transfer. Subsequent oxidation generates the α,β-unsaturated acylazolium intermediate, which can undergo Michael addition with benzoxazoles. Sequential protonation/deprotonation/cyclization produces the six-membered cyclic intermediate that undergoes catalyst elimination, leading to the final product. DABCO·H+ was found to play important roles in proton transfer and cyclization. Without DABCO·H+, the energy barrier up to 44.2 kcal/mol for step 2 is too high to be accessible. With DABCO·H+, the corresponding value is lowered to 18.6 kcal/mol. The energy barrier for cyclization can be lowered by 7.4 kcal/mol by using DABCO·H+. The Michael addition step determines both the enantioselectivity and the regioselectivity. According to NCI analysis, the enantioselectivity is controlled by the strong interactions (such as C-H···O, C-H···N, and π···π) between the α,β-unsaturated acylazolium intermediate and benzoxazoles. We also discuss the solvent and substituent effects on the enantioselectivity and the role of the NHC. The mechanistic insights obtained in the present study would help improving current reaction systems or designing new synthetic routes.

The influential IPr: 25 years after its discovery

N-Heterocyclic carbenes (NHCs) have emerged as a privileged ligand family in organometallic chemistry, widely recognized for their unique steric and electronic properties. Among them, the 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene (IPr) ligand has become a cornerstone of NHC chemistry for its remarkable versatility, stability, and broad use. Since its discovery by the Nolan group in 1999, IPr has played a pivotal role in advancing catalytic transformations and facilitating the utilization of NHC ligands in various domains. This article highlights major contributions where IPr has helped shape modern organometallic chemistry, with a focus on its influence in transition metal catalysis and ligand design. Twenty five years after its discovery, the IPr ligand continues to be a benchmark ligand, inspiring and driving innovation.

Luminescent cyclometalated gold(iii) complexes covalently linked to metal-organic frameworks for heterogeneous photocatalysis

Phosphorescent gold(iii) complexes possess long-lived emissive excited states, making them ideal for use as molecular sensors and photosensitizers for organic transformations. Literature reports indicate that gold(iii) emitters exhibit good catalytic activity in homogeneous photochemical reactions. Heterogeneous metal-organic framework (MOF)-supported gold(iii) photocatalysts are considered to show high recyclability in photochemical reactions and potentially provide new selectivities. Here, we report the design and development of visible-light-absorbing MOF-covalently linked gold(iii) photocatalysts. A MOF-supported gold(iii) complex exhibits a longer phosphorescence lifetime than its homogeneous counterpart, reaching approximately 110 μs under argon and 12 μs under air when suspended in acetonitrile. This is attributed to the localization of the gold(iii) complex within the MOF nano-cages. The MOF-derived gold(iii) photosensitizer exhibits good catalytic performance in intramolecular and intermolecular [2 + 2] cycloaddition reactions to construct functionalized cyclobutanes and azetidines, both of which are important building blocks for pharmaceuticals. These photochemical [2 + 2] cycloaddition reactions catalyzed by the gold(iii)-MOF are governed by an energy transfer mechanism and do not require redox modulators. Under similar reaction conditions, the crossed [2 + 2] photocycloaddition reaction of two activated alkenes proceeds smoothly, and the cyclobutane product is obtained within 3 hours of irradiation under dilute conditions (0.2 mol dm-3 of alkene).

NHC-Au-xanthate complexes

We report the synthesis, isolation, and comprehensive characterization of N-heterocyclic carbene gold xanthate (NHC-Au-X, X - xanthate) complexes. These easily accessible complexes demonstrate significant versatility as photocatalysts, facilitating [2+2]-cycloadditions, and as π-catalysts in the intramolecular hydroxylation of allenes and hydrohydrazination of alkynes.

The Golden Age of Thermally Activated Delayed Fluorescence Materials: Design and Exploitation

Since the seminal report by Adachi and co-workers in 2012, there has been a veritable explosion of interest in the design of thermally activated delayed fluorescence (TADF) compounds, particularly as emitters for organic light-emitting diodes (OLEDs). With rapid advancements and innovation in materials design, the efficiencies of TADF OLEDs for each of the primary color points as well as for white devices now rival those of state-of-the-art phosphorescent emitters. Beyond electroluminescent devices, TADF compounds have also found increasing utility and applications in numerous related fields, from photocatalysis, to sensing, to imaging and beyond. Following from our previous review in 2017 ( Adv. Mater. 2017, 1605444), we here comprehensively document subsequent advances made in TADF materials design and their uses from 2017-2022. Correlations highlighted between structure and properties as well as detailed comparisons and analyses should assist future TADF materials development. The necessarily broadened breadth and scope of this review attests to the bustling activity in this field. We note that the rapidly expanding and accelerating research activity in TADF material development is indicative of a field that has reached adolescence, with an exciting maturity still yet to come.

Simple synthesis of [Au(NHC)X] complexes utilizing aqueous ammonia: revisiting the weak base route mechanism

Aqueous ammonia has been examined as a new weak base for the synthesis of [Au(NHC)Cl] complexes, as well as for the activation of C-H, S-H, and N-H bonds. Its low cost and mild operational conditions (in air and using technical grade solvents) make it an attractive alternative for producing gold-NHC complexes. Synthetic pathways have been investigated in silico, assessing the role of the deprotonation and metalation steps within the reaction mechanisms.

Gold-Mediated Cyclization of N-Acrylamides via Energy Transfer Photocatalysis

We disclose a gold-mediated photocyclization method for the efficient synthesis of acrylamide-derived β-lactams and 3,4-dihydroquinolin-2-ones, two privileged bioactive scaffolds. The protocol proceeds with ease, providing good yields of product while exhibiting excellent functional group tolerance at low catalyst loadings in sustainable solvents. Additionally, reaction times are significantly reduced compared with prior-art methods. Notably, the present gold system exhibits superior performance in activating poorly conjugated substrates, which are typically unreactive with other photocatalysts, highlighting the exceptional position of this organogold system among modern photocatalysts.

Efficient photoredox catalysis in C-C cross-coupling reactions by two-coordinated Au(I) complex

Photocatalysis provides a versatile approach to redox activation of various organic substrates for synthetic applications. To broaden the scope of photoredox catalysis, developing catalysts with strong oxidizing or reducing power in the excited state is imperative. Catalysts that feature highly cathodic oxidation potentials and long lifetimes in their excited states are particularly in demand. In this research, we demonstrate the catalytic utility of two-coordinate Au(I) complex photocatalysts that exhibit an exclusive ligand-to-ligand charge-transfer (LLCT) transition in C-C cross-coupling reactions between N-heterocycles and (hetero)aryl halides, including redox-resistant (hetero)aryl chlorides. Our photocatalysis system can steer reactions under visible-light irradiation at a catalyst loading as low as 0.1 mol% and exhibits a broad substrate scope with high chemo- and regioselectivity. Our mechanistic investigations provide direct spectroscopic evidence for each step in the catalysis cycle and demonstrate that the LLCT-active Au(I) complex catalysts offer several benefits, including strong visible-light absorption, a 210 ns-long excited-state lifetime without short-lived components, and a 91% yield in the production of free-radical intermediates. Given the wide structural versatility of the proposed catalysts, we envision that our research will provide useful insights into the future utilization of the LLCT-active Au(I) complex for organic transformations.

Synthesis and reactivity of N-heterocyclic carbene (NHC) gold-fluoroalkoxide complexes

We disclose a novel series of N-heterocyclic carbene (NHC) gold complexes with varied steric and electronic properties, bearing fluorinated alkoxide anions. Early reactivity studies involving these synthons, lead to the synthesis of various complexes of relevance to gold chemistry and catalysis.

Valence Tautomerism Induced Proton Coupled Electron Transfer:X-H Bond Oxidation with a Dinuclear Au(II) Hydroxide Complex

We report the preparation and characterization of the dinuclear AuII hydroxide complex AuII 2(L)2(OH)2 (L=N,N'-bis (2,6-dimethyl) phenylformamidinate) and study its reactivity towards weak X-H bonds. Through the interplay of kinetic analysis and computational studies, we demonstrate that the oxidation of cyclohexadiene follows a concerted proton-coupled electron transfer (cPCET) mechanism, a rare type of reactivity for Au complexes. We find that the Au-Au σ-bond undergoes polarization in the PCET event leading to an adjustment of oxidation levels for both Au centers prior to C(sp3)-H bond cleavage. We thus describe the oxidation event as a valence tautomerism-induced PCET where the basicity of one reduced Au-OH unit provides a proton acceptor and the second more oxidized Au center serves as an electron acceptor. The coordination of these events allows for unprecedented radical-type reactivity by a closed shell AuII complex.

Synthesis of cyclobutane-fused chromanones via gold-mediated photocatalysis

Energy transfer (EnT) photocatalysis has emerged as a valuable tool for constructing complex organic scaffolds via [2 + 2]-cycloaddition reactions. Herein, we present the use of [Au(SIPr)(Cbz)] as a sensitizer for the [2 + 2]-cycloaddition of coumarins and unactivated alkenes. Widely used in EnT catalysis, iridium and organic sensitizers proved less efficient under the examined catalytic conditions. The developed protocol permits the synthesis of cyclobutane-fused chromanones from readily available starting materials. A wide range of alkenes and substituted coumarins, including naturally occurring compounds, were reacted under mild conditions leading to structurally complex products with good functional group tolerance. Mechanistic studies reveal a previously overlooked reaction pathway for energy transfer catalysis involving coumarins.

Synthesis of cyclohepta[b]indoles via gold mediated energy transfer photocatalysis

Photocatalysis involving energy transfer (EnT) has become a valuable technique for building intricate organic frameworks mostly through [2+2]-cycloaddition reactions. Herein, we report a synthetic method leading to functionalized cyclohepta[b]indoles, an important structural motif in natural products and pharmaceuticals, using gold-mediated energy transfer photocatalysis. The scope of this operationally simple and atom-economical strategy is presented. Density functional theory studies were employed in order to gain insights into the mechanism of formation of the cyclohepta[b]indole core.

Electrochemical Synthesis of Gold-N-Heterocyclic Carbene Complexes

An electrochemical synthesis of gold(I)-N-heterocyclic carbene (Au-NHC) complexes has been developed. The electrochemical methodology uses only imidazolium salts, gold metal electrodes, and electricity to produce these complexes with hydrogen gas as the only by-product. This high-yielding and operationally simple procedure has been used to produce eight mononuclear and three dinuclear Au-NHC complexes. The electrochemical procedure facilitates a clean reaction with no by-products. As such, Au-NHC complexes can be directly transferred to catalytic reactions without work-up or purification. The Au-NHC complexes were produced on-demand and tested as catalysts in a vinylcyclopropanation reaction. All mononuclear Au-NHC complexes performed similarly to or better than the isolated complexes.

Rejuvenation of dearomative cycloaddition reactions via visible light energy transfer catalysis

Dearomative cycloaddition is a powerful technique to access sp3-rich three-dimensional structural motifs from simple flat, aromatic feedstock. The building-up of unprecedentedly diverse polycyclic scaffolds with increased saturation and stereochemical information having various applications ranging from pharmaceutical to material sciences, is an essential goal in organic chemistry. However, the requirement of large energy inputs to disrupt the aromaticity of an arene moiety necessitates harsh reaction conditions for ground state dearomative cycloaddition. The photochemical requirement encompasses use of ultraviolet (UV) light to enable the reaction on an excited potential energy surface. The microscopic reversibility under thermal conditions and the use of high energy harmful UV irradiation in photochemical manoeuvres, however, constrain their widespread use from a synthetic point of view. In this context, the recent renaissance of visible light energy transfer (EnT) catalysis has become a powerful tool to initiate dearomative cycloaddition as a greener and more sustainable approach. The excited triplet state population is achieved by triplet energy transfer from the appropriate photosensitizer to the substrate. While employing mild visible light energy as fuel, the process leverages an enormous potential of excited state reactivity. The discovery of an impressive portfolio of organic and inorganic photosensitizers with a range of triplet energies facilitates visible light photosensitized dearomative cycloaddition of various substrates to form sp3-rich fused polycyclic architectures with diverse applications. The tutorial review comprehensively surveys the reawakening of dearomative cycloadditions via visible light-mediated energy transfer catalysis in the past five years. The progress ranges from intra- and intermolecular [2π + 2π] to [4π + 2π], and ends at intermolecular [2π + 2σ] cycloadditions. Furthermore, the review provides potential possibilities for future growth in the growing field of visible light energy transfer catalysis.

Gold(I)-N-Heterocyclic Carbene Synthons in Organometallic Synthesis

The prominent role of gold-N-heterocyclic carbene (NHC) complexes in numerous research areas such as homogeneous (photo)catalysis, medicinal chemistry and materials science has prompted organometallic chemists to design gold-based synthons that permit access to target complexes through simple synthetic steps under mild conditions. In this review, the main gold-NHC synthons employed in organometallic synthesis are discussed. Mechanistic aspects involved in their synthesis and reactivity as well as applications of gold-NHC synthons as efficient pre-catalysts, antitumor agents and/or photo-emissive materials are presented.

Mechanisms and Stereoselectivities in the NHC-Catalyzed [4 + 2] Annulation of 2-Bromoenal and 6-Methyluracil-5-carbaldehyde

To disclose the reaction mechanism and selectivity in the NHC-catalyzed reaction of 2-bromoenal and 6-methyluracil-5-carbaldehyde, a systematic computational study has been performed. According to DFT computations, the catalytic cycle is divided into eight elementary steps: nucleophilic attack of the NHC on 2-bromoenal, 1,2-proton transfer, C-Br bond dissociation, 1,3-proton transfer, addition to 6-methyluracil-5-carbaldehyde, [2 + 2] cycloaddition, NHC dissociation, and decarboxylation. The Bronsted acid DABCO·H+ plays a crucial role in proton transfer and decarboxylation steps. The addition to 6-methyluracil-5-carbaldehyde determines both chemoselectivity and stereoselectivity, leading to R-configured carbocycle-fused uracil, in agreement with experimental results. NCI analysis indicates that the CH···N, CH···π, and LP···π interactions should be the key factor for determining the stereoselectivity. ELF analysis shows the main role of the NHC in promoting C-Br bond dissociation. The mechanistic insights obtained in the present work may guide the rational design of potential NHC catalysts.

Access to Azetidines via Gold Mediated Energy Transfer Photocatalysis

The area of energy transfer photocatalysis to generate four-membered rings is experiencing an unprecedented level of activity. Here, we report an operationally simple method toward azetidines from 2-isoxasoline-3-carboxylates and alkenes, using [Au(cbz)(NHC)] complexes as photocatalysts. The procedure enables the reaction for a wide range of substrates. Mechanistic studies confirm the energy transfer pathway. This contribution adds to the earlier reported use of these gold catalysts as a potentially versatile tool in energy transfer chemistry and catalysis.

Blue-to-UVB Upconversion, Solvent Sensitization and Challenging Bond Activation Enabled by a Benzene-Based Annihilator

Several energy-demanding photoreactions require harsh UV light from inefficient light sources. The conversion of low-energy visible light to high-energy singlet states via triplet-triplet annihilation upconversion (TTA-UC) could offer a solution for driving such reactions under mild conditions. We present the first annihilator with an emission maximum in the UVB region that, combined with an organic sensitizer, is suitable for blue-to-UVB upconversion. The annihilator singlet was successfully employed as an energy donor in subsequent FRET activations of aliphatic carbonyls. This hitherto unreported UC-FRET reaction sequence was directly monitored using laser spectroscopy and applied to mechanistic irradiation experiments demonstrating the feasibility of Norrish chemistry. Our results provide clear evidence for a novel blue light-driven substrate or solvent activation strategy, which is important in the context of developing more sustainable light-to-chemical energy conversion systems.