Atom-Economic, Regiodivergent, and Stereoselective Coupling of Imidazole Derivatives with Terminal Allenes

Rhodium-Catalyzed Enantioselective Cyclization of 2-Allenylbenzoxazoles

The enantioselective rhodium-catalyzed cyclization of 2-allenylbenzoxazoles to the corresponding vinyl-functionalized, fused heterocyclic structures is reported. The presented method offers several advantages, including the use of low catalyst loadings, commercially available catalyst precursors, and mild reaction conditions. Due to its broad scope, scalability, and valuable products, we consider this methodology to be a useful tool for the construction of benzoxazole-containing building blocks.

Functional-Group-Directed Regiodivergent (3 + 2) Annulations of Electronically Distinct 1,3-Dienes and 2-Formyl Arylboronic Acids

Presented herein is a palladium-catalyzed asymmetric (3 + 2) annulation reaction between 1,3-dienes and 2-formylarylboronic acids, proceeding in a cascade vinylogous addition and Suzuki coupling process. Both electron-neutral and electron-deficient 1,3-dienes are compatible under similar catalytic conditions, and distinct regioselectivity is observed via functional-group control of 1,3-diene substrates. A collection of 1-indanols with dense functionalities is constructed stereoselectively.

Rhodium-Catalyzed Allylic Addition as an Atom-Efficient Approach in Total Synthesis

ConspectusFinding efficient synthetic methods for the asymmetric synthesis of complex molecules has always been of interest to organic chemists. Creating and controlling the stereochemistry of stereogenic centers bearing branched allylic moieties in organic molecules using a catalytic process is an attractive and successful method for the synthesis of several natural products and medicinally important compounds. Remarkable progress toward their synthesis has been achieved via transition-metal catalysis, especially in the case of allylic substitution and allylic C-H oxidation chemistry. However, for allylic substitution the preinstallation of a leaving group is essential, and for allylic C-H oxidation, stoichiometric amounts of oxidant are required. Besides that, the control of regioselectivity with these methods is often problematic because the linear product can be produced as a major isomer. Our research group has developed a regioselective, enantioselective, and atom economic route toward the more valuable branched product via a Rh-catalyzed coupling of easily accessible alkynes or the double-bond isomeric allenes with pronucleophiles. It was demonstrated that, using this new approach, it is possible to add different pronucleophiles to alkynes or allenes to form branched allylic moieties through C-C and C-heteroatom bond formation. Since new organic reactions offer new opportunities in chemical synthesis and the benchmark for new synthetic methods is their application in target-oriented synthesis, we have demonstrated several successful syntheses of natural products and medicinally relevant targets. For example, in the total syntheses of Quercuslactones, Helicascolides A-C, Epothilone D, Homolargazole, and Thailandepsin B, the Rh-catalyzed hydro-oxycarbonylation of allenes was used as key step via C-O bond formation. Remarkably, the Rh-catalyzed C2-symmetric dimerization strategy was used to synthesize the complex molecules Clavosolide A and Vermiculine, leading to an extreme increase in structural complexity within a single step. For the total syntheses of Centrolobine, Pitavastatin, and Rosuvastatin, C-O bond formation was achieved through the addition of a hydroxy function to the allene moiety. The potential of the addition of nitrogen pronucleophiles to allenes was demonstrated in the total syntheses of Cusparein, Angusterein, Cermicin C, Senepodin G, Homoproline, Pipecolinol, Coniceine, Coniine, Ruxolitinib, Sitagliptin, Abacavir, Glucokinase activators, and Chaetominine. All of these examples testify to the wide applicability of the Rh-catalyzed addition of pronucleophiles to allenes or alkynes in target-oriented synthesis, and in this Account we summarize our contribution.

Bisphospholane Josiphos-type Ligands in Rhodium Asymmetric Catalysis

Asymmetric catalysis has become a universal and powerful method for constructing chiral compounds. In rhodium asymmetric catalysis, bisphospholane Josiphos-type ligands and their rhodium complexes are receiving increasing attention. This review provides comprehensive information on the bisphospholane Josiphos-type ligands in rhodium asymmetric catalysis. The scope of the literature covers from 2013 to now. The application of bisphospholane Josiphos-type ligands in rhodium asymmetric catalysis is summarized as follows: (i) asymmetric addition to C(sp2 )-C(sp2 ) bonds, (ii) asymmetric addition to C(sp2 )-C(sp) bonds of allenes, (iii) asymmetric hydrogenation of C(sp2 )-N bonds, C(sp2 )-O bonds and pyridinium salts, and (iv) asymmetric silanization of C-H and O-H bonds.

N-Allylation of Azoles with Hydrogen Evolution Enabled by Visible-Light Photocatalysis

Direct N-allylation of azoles with hydrogen evolution has been achieved through the synergistic combination of organic photocatalysis and cobalt catalysis. The protocol bypasses stoichiometric oxidants and prefunctionalization of alkenes and produces hydrogen (H₂) as the byproduct. This transformation highlights high step- and atom-economy, high efficiency, and broad functional group tolerance for further derivatization, which opens a door for C-N bond formation that is valuable in heterocyclic chemistry.

Unveiling the impact of imidazole derivative with mechanistic insights into neutralize interfacial polymerized membranes for improved solute-solute selectivity

Domestic wastewater (DWW) contains a reservoir of nutrients, such as nitrogen, potassium, and phosphorus; however, emerging micropollutants (EMPs) hinder its applications in resource recovery. In this study, a novel class of nanofiltration (NF) membranes was developed; it enabled the efficient removal of harmful EMP constituents while preserving valuable nutrients in the permeate. Neutral (IM-N) and positively charged (IM-P) imidazole derivative compounds have been used to chemically functionalize pristine polyamide (PA) membranes to synchronously inhibit the hydrolysis of residual acyl chloride and promote their amination. Owing to their distinct properties, these IM modifiers can custom-build the membrane physicochemical properties and structures to benefit the NF process in DWW treatment. The electroneutral NF membrane exhibited ultrahigh solute-solute selectivity by minimizing the Donnan effects on ion penetration (K, N, and P ions rejection < 25%) while imposing remarkable size-sieving obstruction against EMPs (rejection ratio > 91%). Moreover, the hydrophilic IM-modifier synergistically led to enhanced water permeance of 9.2 L m^-2 h^-1 bar^-1, reaching a 2-fold higher magnitude than that of the pristine PA membrane, along with excellent antifouling/antibacterial fouling properties. This study may provide a paradigm shift in membrane technology to convert wastewater streams into valuable water and nutrient resources.

Palladium-Catalyzed Asymmetric Sequential Hydroamination of 1,3-Enynes: Enantioselective Syntheses of Chiral Imidazolidinones

Pd-catalyzed sequential hydroamination of readily available 1,3-enynes is reported. The redox-neutral process provides an efficient route to synthesize a broad scope of imidazolidinones, thiadiazolidines, and imidazolidines. Asymmetric sequential hydroamination generates a series of synthetically valuable, enantioenriched imidazolidinones. Mechanistic studies revealed that the transformation occurred via an intermolecular enyne hydroamination pathway to give an allene intermediate. Subsequent intramolecular hydroamination of the allene intermediate proceeded under the Curtin-Hammett principle to provide enantioenriched imidazolidinone products.

Stereodivergent Palladium- and Rhodium-Catalyzed Intramolecular Addition of Tosylureas to Allenes: Diastereoselective Synthesis of Tetrahydropyrimidinones

The intramolecular addition of tosylureas to allenes is highly syn-/anti-diastereoselective when employing a palladium or rhodium-based catalytic system and affords 1,3-cyclic ureas. Under palladium catalysis a range of thermodynamic anti-tetrahydropyrimidinones are accessible, while rhodium catalysis allows synthesis of the kinetic syn-tetrahydropyrimidinones. For a representative scope of substrates both cyclic ureas were obtained in excellent yields and diastereoselectivities. The obtained tetrahydropyrimidinones were shown to be easily deprotected and modified to demonstrate the synthetic value.

Enantioselective Palladium-Catalyzed Hydrophosphinylation of Allenes with Phosphine Oxides: Access to Chiral Allylic Phosphine Oxides

A Pd-catalyzed hydrophosphinylation of alkyl and aryl-oxyallenes with phosphine oxides has been developed for the efficient and rapid construction of a family of chiral allylic phosphine oxides with a diverse range of functional groups. This methodology was further applied in the facile construction of chiral 2H-chromene and later stage functionalization of cholesterol.

Cu-Catalyzed C-H Allylation of Benzimidazoles with Allenes

CuH-catalyzed intramolecular cyclization and intermolecular allylation of benzimidazoles with allenes have been described. The reaction proceeded smoothly with the catalytic system of Cu(OAc)₂/Xantphos and catalytic amount of (MeO)₂MeSiH. This protocol features mild reaction conditions and a good tolerance of substrates bearing electron-withdrawing, electron-donating, or electron-neutral groups. A new catalytic mechanism was proposed for this copper hydride catalytic system.

Synthesis of medicinally relevant oxalylamines via copper/Lewis acid synergistic catalysis

Allylamines have long been recognized as valuable synthons because of their excellent reactivity in organic synthesis. Here, an efficient amination reaction of allenyl ethers via copper/Lewis acid synergistic catalysis has been established, providing straightforward access to diverse functionalized Z-oxalylamines and E -halogenated oxalylamines in good to excellent yields with high regio- and stereoselectivities. The developed method tolerates more than 100 examples that include late-stage functionalization of bioactive molecules, and features gram-scale synthesis of oxalylamines with high turnover number (TON > 1000) under mild and simple conditions. The applicability of the protocol is further demonstrated with the construction of drug molecules.

Orthogonal Regulation of Nucleophilic and Electrophilic Sites in Pd-Catalyzed Regiodivergent Couplings between Indazoles and Isoprene

Depending on the reactant property and reaction mechanism, one major regioisomer can be favored in a reaction that involves multiple active sites. Herein, an orthogonal regulation of nucleophilic and electrophilic sites in the regiodivergent hydroamination of isoprene with indazoles is demonstrated. Under Pd-hydride catalysis, the 1,2- or 4,3-insertion pathway with respect to the electrophilic sites on isoprene could be controlled by the choice of ligands. In terms of the nucleophilic sites on indazoles, the reaction occurs at either the N¹ - or N² -position of indazoles is governed by the acid co-catalysts. Preliminary experimental studies have been performed to rationalize the mechanism and regioselectivity. This study not only contributes a practical tool for selective functionalization of isoprene, but also provides a guide to manipulate the regioselectivity for the N-functionalization of indazoles.

Enantioselective C2-Allylation of Benzimidazoles Using 1,3-Diene Pronucleophiles

Although substituted benzimidazoles are common substructures in bioactive small molecules, synthetic methods for their derivatization are still limited. Previously, several enantioselective allylation reactions of benzimidazoles were reported that functionalize the nucleophilic nitrogen atom. Herein we describe a reversal of this inherent selectivity toward N-allylation by using electrophilic N-OPiv benzimidazoles with readily available 1,3-dienes as nucleophile precursors. This CuH-catalyzed approach utilizes mild reaction conditions, exhibits broad functional-group compatibility, and exclusively forms the C2-allylated product with excellent stereoselectivity.

Asymmetric synthesis of allylic compounds via hydrofunctionalisation and difunctionalisation of dienes, allenes, and alkynes

This review article provides an overview of progress in asymmetric synthesis of allylic compounds via hydrofunctionalisation and difunctionalisation of dienes, allenes, and alkynes.

NHC-catalyzed β-specific addition of N-based nucleophiles to allenoates

N-heterocyclic carbene (NHC) catalyzed reactions of nitrogenous heterocycles or trifluoromethylated acylhydrazone with allenoates gave regiospecific β-adducts. DFT calculations rationalized the origination of regio- and E/Z selectivities.

Preparation of Chiral Allenes through Pd-Catalyzed Intermolecular Hydroamination of Conjugated Enynes: Enantioselective Synthesis Enabled by Catalyst Design

In this study, we establish that conjugated enynes undergo selective 1,4-hydroamination under Pd catalysis to deliver chiral allenes with pendant allylic amines. Several primary and secondary aliphatic and aryl-substituted amines couple with a wide range of mono- and disubstituted enynes in a nonenantioselective reaction where DPEphos serves as the ligand for Pd. Benzophenone imine acts as an ammonia surrogate to afford primary amines in a two-step/one-pot process. Examination of chiral catalysts revealed a high degree of reversibility in the C-N bond formation that negatively impacted enantioselectivity. Consequently, an electron-poor ferrocenyl-PHOX ligand was developed to enable efficient and enantioselective enyne hydroamination.

Cu-Catalyzed SN2' Substitution of Propargylic Phosphates with Vinylarene-Derived Chiral Nucleophiles: Synthesis of Chiral Allenes

A new Cu-catalyzed enantioselective three-component (i.e., styrenes, B₂pin₂, and propargylic phosphates) allenylation via an S_N2' substitution of propargylic electrophiles with vinylarene-derived chiral nucleophiles is presented. This method provides an efficient and enantioselective approach to access a range of optically pure di-(1,1-), tri-, and tetra-substituted allenes with α-central chirality and axial chirality in excellent chemo-, regio-, diastereo-, and enantioselectivities.

Transition metal catalyzed stereodivergent synthesis of syn- and anti-δ-vinyl-lactams: formal total synthesis of (-)-cermizine C and (-)-senepodine G

A stereodivergent and diastereoselective transition-metal-catalyzed intramolecular hydroamidation of allenes and alkynes furnishing δ-vinyl-lactams is reported. Employing a rhodium catalyst allowed for the selective synthesis of the syn-δ-lactam. Conversely, a palladium catalyst led to the formation of the anti-δ-lactam in high selectivity. The new method shows high functional group compatibility and assorted synthetic transformations were demonstrated as well as its utility for the enantioselective formal total syntheses of (-)-cermizine C and (-)-senepodine G.

Metal-catalyzed regiodivergent organic reactions

This review discusses metal-catalysed regiodivergent additions, allylic substitutions, CH-activation, cross-couplings and intra- or intermolecular cyclisations.

Rhodium-catalyzed addition of sulfonyl hydrazides to allenes: regioselective synthesis of branched allylic sulfones

A rhodium-catalyzed regioselective addition of sulfonyl hydrazides to allenes is reported. With Rh(i)/DPEphos/benzoic acid as the catalyst system, branched allylic sulfones can be obtained, in good to excellent yields and regioselectivities.

Enantioselective and regiodivergent allylation of pyrimidines with terminal allenes: an approach to pyrimidine acyclic nucleosides

An atom-economic addition of pyrimidines to allenes has been developed for the diverse synthesis of branched or linear N-allylpyrimidine analogues.

Regio- and Enantioselective Synthesis of Chiral Pyrimidine Acyclic Nucleosides via Rhodium-Catalyzed Asymmetric Allylation of Pyrimidines

A direct route to branched N-allylpyrimidine analogues is herein reported via the highly regio- and enantioselective asymmetric allylation of pyrimidines with racemic allylic carbonates. With [Rh(COD)Cl]₂/chiral diphosphine as the catalyst, a range of chiral pyrimidine acyclic nucleosides could be obtained under neutral conditions in good yields (up to 95% yield) with high levels of regio- and enantioselectivities (15:1 to >40:1 B/L and up to 99% ee). Furthermore, chiral pyrimidine acyclic nucleoside bearing two adjacent chiral centers has been successfully synthesized by asymmetric dihydroxylation.

Enantioselective Terminal Addition to Allenes by Dual Chiral Primary Amine/Palladium Catalysis

We herein describe a synergistic chiral primary amine/achiral palladium catalyzed enantioselective terminal addition to allenes with α-branched β-ketocarbonyls and aldehydes. The reactions afford allylic adducts bearing acyclic all-carbon quaternary centers with high regio- and enantioselectivity. A wide range of allenes including those aliphatic or 1,1'-disubstituted could be employed, thus expanding the scope of typical asymmetric allylic alkylation reactions.

Rhodium-Catalyzed Diastereoselective Cyclization of Allenyl-Sulfonylcarbamates: A Stereodivergent Approach to 1,3-Aminoalcohol Derivatives

A diastereoselective and stereodivergent rhodium-catalyzed intramolecular coupling of sulfonylcarbamates with terminal allenes is described and it provides selective access to 1,3-aminoalcohol derivatives, scaffolds found in bioactive compounds. The reaction is compatible with a large range of different functional groups, thus furnishing products with high diastereoselectivities and yields. Moreover, multigram scale reactions, as well as the application of suitable product transformations were demonstrated.

Branching Out: Rhodium-Catalyzed Allylation with Alkynes and Allenes

We present a new and efficient strategy for the atom-economic transformation of both alkynes and allenes to allylic functionalized structures via a Rh-catalyzed isomerization/addition reaction which has been developed in our working group. Our methodology thus grants access to an important structural class valued in modern organic chemistry for both its versatility for further functionalization and the potential for asymmetric synthesis with the construction of a new stereogenic center. This new methodology, inspired by mechanistic investigations by Werner in the late 1980s and based on preliminary work by Yamamoto and Trost, offers an attractive alternative to other established methods for allylic functionalization such as allylic substitution or allylic oxidation. The main advantage of our methodology consists of the inherent atom economy in comparison to allylic oxidation or substitution, which both produce stoichiometric amounts of waste and, in case of the substitution reaction, require prefunctionalization of the starting material. Starting out with the discovery of a highly branched-selective coupling reaction of carboxylic acids with terminal alkynes using a Rh(I)/DPEphos complex as the catalyst system, over the past 5 years we were able to continuously expand upon this chemistry, introducing various (pro)nucleophiles for the selective C-O, C-S, C-N, and C-C functionalization of both alkynes and the double-bond isomeric allenes by choosing the appropriate rhodium/bidentate phosphine catalyst. Thus, valuable compounds such as branched allylic ethers, sulfones, amines, or γ,δ-unsaturated ketones were successfully synthesized in high yields and with a broad substrate scope. Beyond the branched selectivity inherent to rhodium, many of the presented methodologies display additional degrees of selectivity in regard to regio-, diastereo-, and enantioselective transformations, with one example even proceeding via a dynamic kinetic resolution. Many advances presented in this account were driven by detailed mechanistic investigations including DFT-calculations, ESI-MS and in situ IR experiments and enabled the application of our chemistry for target-oriented syntheses demonstrated by several examples shown herein. In general, this research topic has matured over the past years into a viable option when synthesizing chiral compounds, from small molecules such as quercus lactones to complex target structures such as Homolargazole or Clavosolide A. This demonstrates the importance and utility of these coupling reactions, especially considering the ease with which carbon-heteroatom bonds can be built stereoselectively, with many of the product classes displaying motifs common in modern APIs.

Non-bonding 1,5-S···O interactions govern chemo- and enantioselectivity in isothiourea-catalyzed annulations of benzazoles

Isothiourea-catalyzed annulations between 2-acyl benzazoles and α,β-unsaturated acyl ammonium intermediates are selectively tuned to form either lactam or lactone heterocycles in good yields (up to 95%) and high ee (up to 99%) using benzothiazole or benzoxazole derivatives, respectively. Computation gives insight into the significant role of two 1,5-S···O interactions in controlling the structural preorganization and chemoselectivity observed within the lactam synthesis with benzothiazoles as nucleophiles. When using benzazoles the absence of a second stabilizing non-bonding 1,5-S···O interaction leads to a dominant C-H···O interaction in determining structural preorganization and lactone formation.