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Rhodium-catalyzed allylation reactions are well known for their unique selectivity and reactivity due to the high memory effect of Rh as compared to other metals. These reactions involve the substitution of allylic rhodium intermediates with a diverse range of different nucleophiles, leading to C–C and C–heteroatom bond formation. Modern organic chemists are, however, interested in atom-economical protocols under greener pathways and following recent increased understanding of mechanistic aspects of Rh-catalyzed allylation via the hydrofunctionalization of allenes or alkynes, great strides have made in the design and development of new atom-economical protocols. In this article, we review this field from its beginning to current state.1 Introduction2 Rhodium-Catalyzed Allylic Substitution3 Rhodium-Catalyzed Allylation with Allenes4 Rhodium-Catalyzed Allylation with Alkynes5 Rhodium-Catalyzed Allylation with Dienes6 Rhodium-Catalyzed Allylation by ARO of Oxabicyclic Alkenes7 Rhodium-Catalyzed Enantioselective Allylation in Natural Product and Drug Synthesis8 Conclusion

ortho-Alkyl aromatic nitro compounds as vinylogous nucleophiles in catalytic asymmetric synthesis have rarely been reported. Herein an asymmetric vinylogous Michael addition of 5-methyl 4-nitroisoxazoles with α,β-unsaturated 2-acyl imidazoles catalyzed by a chiral-at-metal rhodium(III) complex has been developed. The corresponding adducts were obtained in good yields (80–96%) with excellent enantioselectivities (up to 97%). The reaction can be conducted on a gram scale with a low catalyst loading (0.25 mol%) without impacting its efficiency.

A highly efficient enantioselective C2-nucleophilic functionalization of 3-aminobenzofurans with α,β-unsaturated carbonyl compounds has been realized under catalysis of chiral-at-metal rhodium(III) complexes, affording a large array of potentially bioactive C2-substituted benzofuran derivatives with high yields (76–99%) and enantioselectivities (up to 98% ee).

An asymmetric [3 + 2]-cycloaddition reaction of α,β-unsaturated 2-acyl imidazoles with spirovinylcyclopropanyl-2-oxindoles catalyzed synergistically by an achiral palladium(0) catalyst and a chiral-at-metal rhodium(III) complex has been developed. A series of biologically important 3-spirocyclopentane-2-oxindoles with four contiguous stereocenters were synthesized in high yields (up to 99%) with excellent stereoselectivities (up to 99% ee, 20:1 dr).

Due to energy-efficient and green processes of photocatalysis, the visible-light-driven photocatalytic organic synthesis has developed greatly in the last decade. The well-known photocatalysts, RuII-polypyridyl complexes, have been extensively exploited in the synthesis of various chemicals. However, homogeneity of RuII-polypyridyl complexes makes their recycling difficult. Here, we show an effective strategy for boosting photocatalysis by incorporating [RuII(bpy)3] into a radical metal-organic framework (FJI-Y2). Recyclable photocatalyst FJI-Y2 can efficiently catalyze the cross-dehydrogenative coupling reactions of N-phenyltetrahydroisoquinoline derivatives with phosphite esters, forming medicinal α-aminoquinoline phosphonates. More importantly, the photocatalytic efficiency of FJI-Y2 is actually higher than its homogeneous counterpart [RuII(bpy)3]Cl2, because multiple intermolecular interactions between 1,4,5,8-naphthalenediimide radicals and [RuII(bpy)3] induce optimal photoinduced electron transfer process to expedite chemical conversion and recycling of catalysts. It is envisioned that MOFs containing radicals can be ideal platforms to incorporate more noble-metal photocatalysts for promoting photocatalysis.

An enantioselective three-component cascade reduction-Michael addition reaction catalyzed by chiral-at-metal Rh(III) complexes has been developed. With a Hantzsch ester as the hydride source, a number of malononitrile derivatives were prepared in good yields and excellent enantioselectivities. A model that accounts for the origin and influence factors of the stereoselectivity has been proposed based on experiments.

An efficient asymmetric 1,3-dipolar cycloaddition of α,β-unsaturated 2-acyl imidazoles with N,N′-cyclic azomethine imines catalyzed by a chiral-at-metal rhodium complex is reported. The corresponding N,N′-bicyclic pyrazolidine derivatives with three contiguous tertiary stereocenters were obtained in good yields (up to 99%) with excellent stereoselectivities (>20:1 dr and >99% ee). Remarkably, as little as 0.5 mol % of a chiral Rh(III) complex can promote a gram-scale reaction with excellent yield and enantioselectivity.

An efficient enantioselective conjugate addition of pyrazolones with α,β-unsaturated 2-acyl imidazoles catalyzed by chiral-at-metal rhodium complex is reported. The corresponding adducts were obtained in good yields (85%-96%) with excellent enantioselectivities (up to >99%). This protocol exhibits extraordinary reactivity, because of the fact that a complex with as little as 0.05 mol % Rh(III) can catalyze the title reaction on a gram-scale with excellent enantioselectivity.

The asymmetric Michael addition of diphenylbutazone and its analogues to α,β-unsaturated 2-acyl imidazoles has been developed with a chiral-at-metal Rh(III) complex as a catalyst. The corresponding adducts were obtained in good yields (89–98%) with excellent enantioselectivities (up to >99%). The reaction can be conducted on a gram-scale with a low catalyst loading (0.04 mol%) without impacting its efficiency. Moreover, the adducts can be converted into other useful synthetic building blocks.

An enantioselective Michael addition of α,β-unsaturated 2-acyl imidazoles with cyclic ketones catalyzed synergistically by a chiral-at-metal Rh(III) complex and a secondary amine has been developed, affording the corresponding adducts with up to 98% yield and 99% ee. The diastereomeric ratios are in the range from 2 : 1 to 10 : 1. In these reactions, the combination of a chiral-at-metal complex and a secondary amine catalyst activates an electrophile and nucleophile separately.

A highly regio-, diastereo- and enantioselective addition of 2-acyl imidazoles or 2-acyl pyridines with allenes promoted by Rh/Lewis acid synergistically catalytic system is described. This atom economic approach leads to the formation of the branched allylic alkylated products including acyclic quaternary all-carbon stereogenic centres in good yields with good to excellent diastereo- and enantioselectivities. Kinetic studies reveal that the rate-determining step in this process is the oxidative addition of Rh(I) with C-H bond.

Chiral-at-metal Rh(III) complexes catalyzed highly efficient enantioselective Mannich reaction of 2-acyl imidazoles with 1,3,5-triazinanes is developed, affording the corresponding adducts in 81-99% yields with up to >99% enantioselectivities. This protocol performs with 0.1 mol % of Rh(III) complex on gram scale without loss in enantioselectivity.

A newly prepared chiral-at-metal Rh(III) complex catalyzed highly efficient asymmetric conjugate addition of para-vinylanilines with α,β-unsaturated 2-acyl imidazoles is developed, affording the corresponding adducts in 67-95% yields with 86-95% ee. Remarkably, employing as low as 0.05 mol % of Rh(III) complex as catalyst, a gram-scale reaction still affords the desired product in 81% yield with 92% ee.

Herein we describe an efficient protocol for the regioselective addition of 1,3-dicarbonyl compounds to internal alkynes catalyzed by rhodium/Lewis acid catalysts. The corresponding branched/linear allylic alkylation products could be selectively obtained in good yields. Rh–H species were considered to be generated by direct C–H oxidative addition of 1,3-dicarbonyl compounds with rhodium catalyst with the assistance of Lewis acid. Moreover, a retro-allylic alkylation process was observed in this transformation.

An efficient and catalytic α-chlorination of 2-acyl imidazoles with readily available tosyl chloride catalyzed by an octahedral iridium complex under mild condition was reported. A range of 2-acyl imidazoles were converted to their corresponding α-chlorination products in good to excellent yields.

A catalytic enantioselective hydroxyamination of α,β-unsaturated 2-acyl imidazoles with N-protected hydroxyl amines catalyzed by a chiral-at-metal Rh(III) complex (0.5–2 mol%) has been developed. The approach affords valuable N-Boc protected β-amino acid derivatives in moderate to good yields with high enantioselectivity (up to 94% yield, 99.5% ee).

Chiral Rh(III) complex catalyzed highly efficient enantioselective cyclopropanation of α,β-unsaturated 2-acyl imidazoles or pyridine with 2-bromomalonate is developed to generate corresponding multisubstituted cyclopropanes in 70-99% yields with 93→99% enantioselectivity. The proficiency of the process is also demonstrated in gram scale reaction maintaining same reactivity and selectivity level in lower catalyst loading. Moreover, the developed methodology is applicable in challenging synthesis of biscyclopropane scaffold with 97% ee in a single operation.

A synergistic catalysis combination of chiral-at-metal rhodium complex and amine catalyst was developed for enantioselective alkylation of aldehydes with α,β-unsaturated 2-acyl imidazoles. The corresponding adducts were obtained in good yields with excellent enantioselectivities (up to 99% ee).

An asymmetric [3+2] cycloaddition of C,N-cyclic azomethine imines with α,β-unsaturated 2-acyl imidazoles catalyzed by a chiral-at-metal rhodium complex has been developed. The corresponding C-1-substituted tetrahydroisoquinoline derivatives were obtained in high yields (>90%) with excellent stereoselectivities (up to 99% ee and >20 : 1 dr). The reaction can be conducted on a gram-scale using a low catalyst loading (0.5 mol%) with high yield and selectivity.

Nanocrystals of Rh, an important member of the noble metal catalyst family, have wide applications in heterogeneous catalytic reactions. Controlling the morphology of these noble metal nanocrystals has become an effective strategy for improving their catalytic activity and durability. In this work, well-defined Rh nanodendrites with very thin triangular branches as subunits are synthesized using a facile diethylene glycol reduction method, assisted by polyethyleneimine as a complex-forming agent and surfactant. For the first time, the methanol oxidation reaction (MOR) on Rh nanocrystals with a well-defined morphology is investigated using various electrochemical techniques in an alkaline medium. Unexpectedly, the as-prepared Rh nanodendrites, with ultrathin nanosheet subunits, exhibit superior electrocatalytic activity and durability during the MOR in an alkaline medium, indicating that Rh nanocrystals with specific morphology may be highly promising alternatives to Pt electrocatalysts in the MOR in an alkaline medium.

A direct and highly stereoselective approach for the synthesis of Z-alkenyl-pyranoindoles had been developed by utilizing Rh(II)-catalyzed intramolecular cyclization of N-sulfonyl-1,2,3-triazoles with indole derivatives. A variety of pyranoindoles were obtained in 44–93% yields. Moreover, a more convenient synthesis of pyranoindoles starting from terminal alkyne was realized via a Cu–Rh sequentially catalyzed one-pot cascade reaction.

An efficient approach for the synthesis of polycyclic spiroindolines has been developed by utilizing Rh(II)-catalyzed dearomatizing annulation of sulfonyl triazoles. An enantioselective variant of the process was also looked into briefly to achieve polycyclic spiroindolines in good yield with a promising 73.3: 26.7 enantiomeric ratio, demonstrating the potential for asymmetric catalysis.

The hollow noble metal nanostructures have attracted wide attention in catalysis/electrocatalysis. Here a two-step procedure for constructing hollow Rh nanospheres (Rh H-NSs) with clean surface is described. By selectively removing the surfactant and Au core of Au-core@Rh-shell nanostructures (Au@Rh NSs), the surface-cleaned Rh H-NSs are obtained, which contain abundant porous channels and large specific surface area. The as-prepared Rh H-NSs exhibit enhanced inherent activity for the methanol oxidation reaction (MOR) compared to state-of-the-art Pt nanoparticles in alkaline media. Further electrochemical experiments show that Rh H-NSs also have high activity for the electrooxidation of formaldehyde and formate (intermediate species in the course of the MOR) in alkaline media. Unfortunately, Rh H-NSs have low electrocatalytic activity for the ethanol and 1-propanol oxidation reactions in alkaline media. All electrochemical results indicate that the order of electrocatalytic activity of Rh H-NSs for alcohol oxidation reaction is methanol (C1 ) > ethanol (C2 ) > 1-propanol (C3 ). This work highlights the synthesis route of Rh hollow nanostructures, and indicates the promising application of Rh nanostructures in alkaline direct methanol fuel cells.

The rhodium complex Λ-Rh1 containing chiral pinene-modified pyridine ligands is prepared through a two-step synthetic procedure; it exhibits excellent reactivity and enantiocontrol towards the enantioselective Mukaiyama-Michael reaction of α,β-unsaturated 2-acyl imidazoles with silyl enol ethers, affording enantioenriched 1,5-dicarbonyl compounds in good yields (up to 99 %) with excellent enantioselectivities (up to 99 % ee).

A sequentially catalyzed cascade approach for the synthesis of benzofuran derivatives (up to 88% yield) from 1-sulfonyl-1,2,3-triazoles has been developed. The cascade reaction involves sequential rhodium-catalyzed C(sp3)H insertion and copper-catalyzed aerobic oxidation. The method was made more convenient towards the synthesis of benzofurans starting from terminal alkynes via a one-pot sequential copper-catalyzed alkyne-azide cycloaddition (CuAAC)/CH insertion/dehydrogenation cascade reaction. Moreover, the utility of the cascade approach was demonstrated in the synthesis of the indole framework.

An efficient approach for palladium(II) acetate catalyzed allylic amination of homoallylic alcohols with various amines via sequential retro-allylation and amination was developed, which afforded the corresponding allylic amines in up to 98% yield.

The catalyst with high activity, 100% selectivity and good durability is highly desirable in developing a hydrous hydrazine-based hydrogen generation system. Herein we report the synthesis of nickel–platinum (Ni–Pt) bimetallic nanoparticles supported on mesoporous alumina (MA) using a simple one-pot evaporation-induced self-assembly method. The Ni–Pt/MA catalyst exhibits excellent activity and satisfactory stability for selectively catalyzing the decomposition of hydrous hydrazine to generate hydrogen at mild temperatures, which compares favorably with the performance of the catalysts developed to date. The facile synthesis of high-performance and cost-effective Ni-based catalyst is of clear significance for the development of hydrous hydrazine as a viable hydrogen carrier.

We aimed to describe an efficient CoCl2-catalyzed direct C–C bond formation of tetrahydrofuran (THF) with various alkynes in the presence of tert-butyl hydroperoxide and catalytic amount of acid to obtain vinyl-substituted THFs. Mono- and di-substituted alkynes were suitable for this transformation.

We report an efficient enantioselective conjugate addition of photogenerated α-amino radicals to Michael acceptors catalyzed by a newly prepared chiral-at-metal rhodium complex. This protocol shows that a single Rh(III) complex can serve not only as a Lewis acid but also as a photoredox catalyst to control the stereoselectivity during the bond formation.

A newly prepared chiral-at-metal Rh(III) complex-catalyzed, highly efficient enantioselective decarboxylative Michael addition of β-keto acids with α,β-unsaturated 2-acyl imidazoles or pyridine has been developed, affording the corresponding adducts in 94–98% yield with up to 96% enantioselectivity. This protocol exhibits remarkable reactivity, as the complex with a Rh(III) loading as low as 0.05 mol % can catalyze the decarboxylative Michael addition on a gram scale without loss of enantioselectivity.

A highly regio- and stereoselective Rh-catalyzed intermolecular head-to-head [2 + 2] cycloaddition of allenamides was developed. The intermolecular cycloadducts, trans-dimethylenecyclobutane-1,2-diamine derivatives, were achieved in good yields with high regioselectivity and stereoselectivity.

Catalyst study is a central issue in the development of sodium borohydride (NaBH 4 )-based hydrogen generation systems. Here, we report the preparation, characterization and utilization of a novel cobalt–molybdenum–boron (Co–Mo–B) catalyst. Our study found that the Co–Mo–B catalyst prepared by chemical reduction method using an ethylene glycol solution of cobalt chloride exhibits remarkably higher catalytic activity than the Co–B and Co–Mo–B catalysts prepared in aqueous solution. On the basis of the phase, microstructure and elemental chemical state analyses, the mechanistic reasons for the improved performance of the newly prepared Co–Mo–B catalyst were discussed. Our study demonstrated that the combined usage of concentrated NaBH 4 solution, Co–Mo–B catalyst and a small amount of Al powder constitutes a high-performance hydrogen generation system, which can yield >6 wt% hydrogen with fast kinetics and high fuel conversion.

A direct and highly stereoselective approach for the synthesis of Z-alkenyl-pyranoindoles had been developed by utilizing Rh(II)-catalyzed intramolecular cyclization of N-sulfonyl-1,2,3-triazoles with indole derivatives. A variety of pyranoindoles were obtained in 44–93% yields. Moreover, a more convenient synthesis of pyranoindoles starting from terminal alkyne was realized via a Cu–Rh sequentially catalyzed one-pot cascade reaction.

A highly regio- and stereoselective Rh-catalyzed intermolecular head-to-head [2 + 2] cycloaddition of allenamides was developed. The intermolecular cycloadducts, trans-dimethylenecyclobutane-1,2-diamine derivatives, were achieved in good yields with high regioselectivity and stereoselectivity.

An efficient approach for the synthesis of polycyclic spiroindolines has been developed by utilizing Rh(II)-catalyzed dearomatizing annulation of sulfonyl triazoles. An enantioselective variant of the process was also looked into briefly to achieve polycyclic spiroindolines in good yield with a promising 73.3: 26.7 enantiomeric ratio, demonstrating the potential for asymmetric catalysis.

An efficient approach for palladium(II) acetate catalyzed allylic amination of homoallylic alcohols with various amines via sequential retro-allylation and amination was developed, which afforded the corresponding allylic amines in up to 98% yield.

Benzofurane derivatives are prepared from triazole substrates via a tandem insertion/aerobic oxidation sequence.

Similar, but different: Possessing almost the same but not identical structures, the recently discovered natural products hopeahainol A (1) and hopeanol (2) exhibit important but differing biological properties (see structures). Their first total synthesis has now been achieved through a series of novel cascade reactions and skeletal rearrangements.

Slowly does it! By adding the substrate by a syringe pump, a highly efficient Friedel-Crafts reaction of 4,7-dihydroindoles with nitroolefins was realized with 0.5 mol % of a chiral phosphoric acid. The Friedel-Crafts alkylation, together with a subsequent oxidation of the product, led to 2-substituted indoles in excellent enantiomeric excesses, which can be easily transformed to enantioenriched tetrahydro-gamma-carbolines.

Chiral phosphoric acid-catalyzed enantioselective Friedel–Crafts reaction of indoles with ethyl glyoxylate imines was developed. With 10 mol % of the catalyst, the Friedel–Crafts reactions between a wide range of indoles and imines have been carried out, affording optically active (3-indolyl)glycine derivatives with excellent yields and high enantioselectivities (up to 87% ee).

Asymmetric synthesis of trans-alkenyl α-amino esters was realized by chiral phosphoric acid catalyzed transfer hydrogenation of β,γ-alkynyl α-imino esters. Utilizing Hantzsch esters as the hydrogen donor, both the alkyne and imine moieties of β,γ-alkynyl α-imino esters were reduced to afford trans-alkenyl α-amino esters with up to 96% ee.

The asymmetric Friedel–Crafts (AFC) alkylation reaction is one of the most important approaches to the synthesis of optically pure organic compounds. Various catalytic systems, including well-defined organometallic complexes (Zn, Cu, Sc, etc.) and small organic molecules (chiral imidazolidinones, thioureas, prolinols, BINOL-derived phosphoric acids, etc.), have been successfully developed and applied in asymmetric Friedel–Crafts alkylation reactions. Remarkable levels of enantioselectivity have been achieved for electron-rich arenes such as indoles, pyrroles, and naphthols, and electrophiles such as carbonyl compounds, imines, α,β-unsaturated carbonyl compounds, and π-allyl metal species.

We aimed to describe an efficient CoCl2-catalyzed direct C–C bond formation of tetrahydrofuran (THF) with various alkynes in the presence of tert-butyl hydroperoxide and catalytic amount of acid to obtain vinyl-substituted THFs. Mono- and di-substituted alkynes were suitable for this transformation.

Allyl esters are prepared via sequential retro-allylation and esterification of secondary allyl alcohols with acids.

Precise control of the morphology, composition and structure of metal nanostructures not only effectively improves their catalytic activity and durability but also enhances their range of applications. In this work, bimetallic Au@Rh core–shell nanodendrites are synthesized by a facile one-pot hydrothermal method. Physical characterizations show that the dendritic Rh consists of two-dimensional (2D) ultrathin Rh nanoplates with a thickness of approximately 1.2 nm. For the first time, Au@Rh core–shell nanostructures are used as a catalyst for the hydrogen generation reaction from aqueous hydrazine solution (N2H4=N2+2H2, HGR-N2H4). Bimetallic Au@Rh core–shell nanodendrites exhibit improved catalytic activity and durability for the HGR-N2H4 compared with commercial Rh nanocrystals, which can be attributed to the atomically ultrathin structure of 2D Rh nanoplates and the interconnected structure of nanodendrites, respectively. Under light irradiation, bimetallic Au@Rh core–shell nanodendrites show light-enhanced catalytic activity for the HGR-N2H4, originating from the distinctive localized surface plasmon resonance of Au icosahedron cores. An easy way to make bimetallic nanoparticles with ultrathin branch-like structures that can enhance hydrogen production has been developed. Nanomaterials are useful for a wide range of applications, from targeted drug delivery to catalysts for chemical processes. In most cases, their effectiveness depends critically on their composition and shape. Now, using a one-pot hydrothermal process, Yu Chen and colleagues from Shaanxi Normal University in Xi'an, China, have synthesized bimetallic nanoparticles with branch-like structures, or dendrites, on their surfaces. These structures consist of an icosahedron-shaped core of gold surrounded by a shell of rhodium plates. When the researchers used these bimetallic core–shell nanodendrites as a catalyst for generating hydrogen from a hydrazine solution, they achieved a higher catalytic activity and durability than commercial rhodium nanocrystals. The bimetallic Au@Rh core–shell nanodendrites with ultrathin Rh nanoplate shells show the light-enhanced catalytic activity for the hydrogen generation reaction from aqueous hydrazine solution.

Palladium-catalyzed allylic esterifications of secondary homoallyl alcohols with acids via sequential retro-allylation and esterification are demonstrated, affording the corresponding allyl ester in up to 99% yield. The electron effect of the substituent of the secondary alcohol was found to be crucial to the selective C–C bond cleavage.