Mechanism and Stereochemistry of Rhodium-Catalyzed [5 + 2 + 1] Cycloaddition of Ene-Vinylcyclopropanes and Carbon Monoxide Revealed by Visual Kinetic Analysis and Quantum Chemical Calculations

Rhodium-Catalyzed [5 + 1 + 2] Reaction of Yne-Vinylcyclopropenes and CO: The Application of Vinylcyclopropenes for Higher-Order Cycloaddition

Transition metal-catalyzed higher-order cycloadditions involving vinylcyclopropenes (VCPEs) have not been realized to synthesize challenging medium-sized rings, partially due to their poor stability and many competing side reactions. We report here a Rh-catalyzed [5 + 1 + 2] reaction of yne-VCPEs and CO for the synthesis of eight-membered carbocycles with trienone moiety, which so far can be accessed by only limited reactions. The key to this higher-order cycloaddition is that once C-C cleavage of VCPE (C5 synthon) to form a six-membered metallacycle is initiated, CO (C1 synthon) insertion happens before alkyne (C2 synthon) insertion, attributing to the special reactivity of the sp2 carbon in the vinylcyclopropene. Quantum chemical calculations have been applied to support this reaction pathway. The present [5 + 1 + 2] reaction has a broad scope, and the C2 synthon can also be extended to alkenes and allene. Of the same importance, the present reaction can be catalyzed by either [Rh(CO)2Cl]2 or a cheaper complex, RhCl3·nH2O.

Mechanistic Insights into Molecular Copper Hydride Catalysis: the Kinetic Stability of CuH Monomers toward Aggregation is a Critical Parameter for Catalyst Performance

The activity of molecular copper hydride (CuH) complexes toward the selective insertion of unsaturated hydrocarbons under mild conditions has contributed significantly to versatile methodologies for upgrading these feedstocks. However, these catalysts are particularly susceptible to deleterious aggregation, leading to the depletion of active CuH species. Little is known about the mechanisms of CuH aggregation, how it influences overall catalyst performance, and how it can be controlled. We address these challenges with mechanistic studies on a model reaction of unactivated alkene hydroboration catalyzed by (IPr*CPh3)CuH (LCuH). We report a comprehensive mechanistic investigation of this system, identifying an aggregation pathway that continuously depletes catalytically active LCuH to form inactive CuH clusters during turnover. Deactivation of LCuH is controlled primarily by the competition between the kinetics of the initial LCuH dimerization step and that of alkene insertion into LCuH. We therefore propose that a comprehensive understanding of CuH catalyst performance must account for the kinetics of the initial LCuH dimerization step, revising a previously explored thermodynamic understanding of CuH aggregation, where the concentration of active species is controlled by equilibria established between CuH clusters and monomers. With a series of (NHC)CuH congeners (NHC = N-heterocyclic carbene), we demonstrate that ostensibly minor structural modifications to the ligand peripheries can drastically affect the LCuH dimerization kinetics, while maintaining reactivity toward on-cycle alkene insertion. We employed a computational approach based on molecular dynamics simulations to provide an in-depth understanding of how specific structural ligand modifications can substantially increase the kinetic stability of monomeric CuH catalysts. Our combined experimental and computational studies suggest strategies for rational ligand design that can be broadly applied to molecular catalyst systems that are susceptible to deactivation via aggregation pathways.

Vinylcyclopropanes as Three-Carbon Synthons in Rhodium-Catalyzed Cycloadditions: Reaction Development, Mechanistic Studies, New Inspirations, and Synthetic Applications

ConspectusCyclic structures are common in natural products and pharmaceuticals, but pose major synthetic challenges. Transition metal-catalyzed cycloadditions provide a direct and efficient route to complex ring systems in a single step. The demand for new transition metal-catalyzed cycloadditions remains high, as these methods enable access to diverse ring systems with unique substituents and stereochemistries that are often unattainable through existing cycloaddition techniques. Vinylcyclopropanes (VCPs) are widely recognized as versatile five-carbon (C5) synthons in various transition metal-catalyzed cycloadditions, including [5 + 1], [5 + 2], and [5 + 2 + 1] reactions. In these reactions, VCP uses its vinyl group to facilitate C-C bond cleavage in the strained cyclopropane, aided by transition metals. In contrast, isolated cyclopropanes typically lack this reactivity. Building on these advantages, we discovered that by altering the connectivity between VCPs and other synthons, such as alkenes, alkynes, allenes, or dienes, VCPs can act as novel three-carbon (C3) synthons, enabling previously unknown cycloadditions. This account outlines these discoveries.By connecting two-carbon (C2) synthons to VCPs at positions 1, 2, or α, we created various substrates, including 2-trans-ene/allene-VCPs, 1-ene/yne/allene-VCPs, and α-ene-VCPs. These substrates undergo [3 + 2] cycloadditions to construct fused bicyclic structures. Notably, 1-ene/yne/allene-VCPs enable the construction of 5/5 fused rings with bridgehead quaternary centers, representing a remarkable synthetic advancement. This reaction has also been extended to its asymmetric variant, marking the first asymmetric [3 + 2] reaction of its kind. Furthermore, 1-ene/yne-VCPs have been adapted for [3 + 2 + 1] cycloadditions, allowing the synthesis of 5/6 and 6/6 fused ring systems with bridged quaternary centers. The utility of this method is demonstrated through its application in the synthesis of several natural products. The success of the [3 + 2 + 1] cycloaddition further inspired the development of a novel [4 + 2] reaction using yne-vinylcyclobutanones (yne-VCBOs). While VCBO has traditionally been used as a six-carbon (C6) synthon, we discovered that it functions as a four-carbon (C4) synthon when alkynes are connected at the 1-position of VCBOs. This [4 + 2] reaction cocatalyzed by Rh and Zn yields 5/6 or 6/6 fused rings with bridgehead quaternary centers, which is the same motif formed via the [3 + 2 + 1] reaction of 1-yne-VCPs and CO.The synthesis of seven-membered rings remains a challenging endeavor. By connecting a diene to the 1-position of VCPs, we developed a Rh-catalyzed [4 + 3] cycloaddition, yielding 5/7 fused ring structures. Additionally, introducing CO into the reaction enabled a [4 + 3]/[4 + 1] cycloaddition, generating 5/7/5 triangular ring scaffolds. Both [4 + 3] and [4 + 3]/[4 + 1] reactions feature an unprecedented endo-oxidative cyclometalation mode, which could be utilized in future cycloaddition design. Further developments may include expanding reaction scopes, applying these methods to natural product synthesis and medicinal chemistry, realizing asymmetric variants, understanding reaction mechanisms, and inventing new synthons and cycloaddition reactions.

Rh-Catalyzed trans-Divinylcyclopropane Rearrangement: An Approach to 1,5-Disubstituted 1,4-Cycloheptadienes

Under mild conditions and without the need of inert gas protection, a Rh-catalyzed rearrangement of trans-divinylcyclopropanes to form 1,5-disubstituted 1,4-cycloheptadienes has been developed and is disclosed here. This reaction can expedite the synthesis of challenging seven-membered carbocycles.

Access to Vinyl Gem-Difluorinated Cyclopropanes Via Photopromoted Palladium-Catalyzed Heck Reaction

A photopromoted Pd-catalyzed Heck reaction of gem-difluorocyclopropyl bromides (DFCBs) with styrenes to deliver vinyl gem-difluorinated cyclopropanes (VDFCs) under mild conditions has been developed. The reaction demonstrates good functional group compatibility while providing high E/Z ratio of the products. Furthermore, the desired VDFCs can be easily transformed into fluorinated cyclic/acyclic architectures, which may broaden its applications in organic synthesis.

IPLOT-VKA: An Integral-Method Powell-pLOT-Enhanced Visual Kinetic Analysis for the Determination of Orders of Reaction

Determination of partial orders of reactions in kinetics is a general entry step for any mechanistic investigation; recently, considerable attention has been given to the benefits of using visual methods to help in this task, such as easy utilization and data-efficiency. In this respect, we here revisit and improve the classic method by Powell and Margerison for kinetic analysis. For the first time, analytical equations for a bimolecular reaction have been worked out for all values of partial orders of reactions, and the solutions have been implemented in a free, open-access and easy-to-use Web-application, named IPLOT-VKA, which also allows estimation of the errors on the kinetic constant, and residual standard error on concentrations. Several examples, taken from reference teaching experiences and from recent literature in catalysis show the efficacy and accuracy of our approach, which also has the advantage of requiring less experimental runs than other visual methods currently available.

Site-selective decarbonylative [4 + 2] annulation of carboxylic acids with terminal alkynes by C-C/C-H activation strategy and cluster catalysis

Cycloaddition and annulation strategies are among the most powerful methods for creating molecular complexity in organic molecules. In this manuscript, we report a highly site-selective palladium-catalyzed decarbonylative [4 + 2] cyclization of carboxylic acids with terminal alkynes by a sequential C-C/C-H bond activation. Most notably, this method represents the first use of carboxylic acids as the ubiquitous and underdeveloped synthons for intramolecular cycloadditions by decarbonylative C-C bond cleavage. The method provides a solution to the long-standing challenge of the regioselective synthesis of substituted naphthalenes by cycloaddition. Mechanistic studies show that this reaction occurs through a sequential process involving the formation of key palladacycle by a sequential C-C/C-H bond activation and highly regioselective alkyne insertion enabled by cluster catalysis. Wide substrate scope for both carboxylic acids and terminal alkynes is demonstrated with high functional group tolerance. Moreover, this reaction is scalable and applicable to the synthesis of functionalized molecules featuring bioactive fragments. This reaction advances the toolbox of redox-neutral carboxylic acid interconversion to cycloaddition processes. We anticipate that this approach will find broad application in organic synthesis, drug discovery and functionalized material research fields.

Total Synthesis of (+)-Kalmanol

Kalmanol, the flagship member of the kalmane diterpene family, possesses a complex and highly oxidized 5/5/8/5 tetracyclic skeleton with nine contiguous stereocenters and exhibits significant analgesic effects and cardiotoxic properties. We have achieved the efficient total synthesis of (+)-kalmanol in 22 steps with 2.3 % yield. The synthesis featured a Rh-catalyzed [5+2+1] cycloaddition reaction to construct 5/5/8 tricyclic skeleton, and a meticulously designed sequence of stereoselective oxidations of the 5/5/8/5 tetracyclic skeleton.

Mechanistic Insights from Density Functional Theory into Rh/Acid-Catalyzed Synthesis of 1,2-Dihydroquinolines via Skeleton-Reorganizing Coupling of Cycloheptatriene and Amines

Density functional theory calculations were conducted to refine our understanding at the molecular level of the synthesis of fused 1,2-dihydroquinolines through Rh- and acid-catalyzed skeleton-reorganizing coupling reactions of cycloheptatriene with amines. The results reveal that the reaction progresses via cascade catalysis, consisting of consecutive steps of Rh-catalyzed intermolecular coupling involving two RhIII-RhI-RhIII transformations with a maximum energy barrier of 27.1 kcal/mol, followed by acid-catalyzed intramolecular skeleton reorganization with a peak energy barrier of 23.3 kcal/mol. The most significant finding of this work is the identification of a new oxidation-reduction mode of the Rh center. This mode is achieved via migration of a proton from the ammonium ion to the metal center and nucleophilic attack-induced intermolecular reductive coupling, distinguishing it from the conventional oxidative addition-reductive elimination process. The acid-catalyzed intramolecular skeleton reorganization necessitates the assistance of a second HOTs molecule, along with its conjugate base, which sequentially facilitates retro-Mannich-type C-C cleavage and the isomerization of the terminal imine to enamine via acid-base catalysis. Our calculations also explain why the azabicyclic tropene byproduct does not compete with the formation of the fused 1,2-dihydroquinoline product. These theoretical insights are expected to provide valuable guidance for further improvements in the efficiency of skeleton-reorganizing coupling reactions between cycloheptatriene and amines.

Palladium catalyzed stereoselective intramolecular [3 + 2] cycloaddition reactions of (E) & (Z)-ene-vinylidenecyclopropanes

A palladium-catalyzed ring-opening cyclization of (E) & (Z)-ene-vinylidenecyclopropanes has been developed via an intramolecular [3 + 2] cycloaddition process in the presence of a sterically bulky biaryl phosphine ligand, stereoselectively affording fused cis- & trans-bicyclo[4.3.0] skeletal products in good yields with a broad substrate scope and good functional tolerance. A plausible reaction mechanism was proposed on the basis of previous work and the DFT calculations.

Mechanistic Studies on Rhodium-Catalyzed Chemoselective Cycloaddition of Ene-Vinylidenecyclopropanes: Water-Assisted Proton Transfer

Rhodium-catalyzed cycloaddition reactions are a powerful tool for the construction of polycyclic compounds. Combined experimental and DFT studies were used to investigate the temperature-controlled chemoselectivity of cationic rhodium-catalyzed intramolecular cycloaddition reactions of ene-vinylidenecyclopropanes. After a series of mechanistic studies, it was found that trace amounts of water in the reaction system play an important role in generating the product with endo double bond located on a five-membered ring and revealed that trace amounts of water in the reaction system, including the rhodium catalyst, substrate and solvent, were sufficient to promote the formation of the product with endo double bond located on a five-membered ring, and additional water could not further accelerate the reaction. DFT calculation results show that the addition of water indeed significantly lowers the energy barrier of the proton transfer step, making the formation of the product with endo double bond located on a five-membered ring more likely to occur and confirming the rationality of water-assisted proton transfer occurring in the selective access to the product with endo double bond located on a five-membered ring.

DFT Study on Mechanism of Ni-Al Bimetallic-Catalyzed C-H Cyclization to Construct Tricyclic Imidazoles: Roles of NHC Ligand and AlMe3

The mechanism of the Ni-Al bimetallic-catalyzed C-H cyclization to construct tricyclic imidazoles is investigated using density functional theory calculations. The calculation result shows that the reaction mechanism involves sequential steps of substrate coordination, ligand-to-ligand hydrogen transfer (LLHT), and C-C reductive elimination to produce the final product tricyclic imidazole. The LLHT step is calculated to be the rate-determining step. The oxidative addition of the benzimidazole C-H bond to the Ni center and the insertion of the alkene into the Ni-H bond occur concertedly in the LLHT step. The effects of N-heterocyclic carbene (NHC) ligands and AlMe3 on the reactivity and regioselectivity were also analyzed. These calculation results shed light on some ambiguous suggestions from experiments.

Rh(I)-Catalyzed [4+3]/[4+1] Cycloaddition of Diene-Vinylcyclopropanes and Carbon Monoxide to Access Angular 5/7/5 Tricycles

Report here is a Rh-catalyzed [4+3]/[4+1] cycloaddition of diene-vinylcyclopropanes (diene-VCPs) and carbon monoxide to access compounds with angular 5/7/5 tricyclic skeleton found in natural products. The reaction has broad scope and further transformation of the [4+3]/[4+1] cycloadduct was also investigated. How this [4+3]/[4+1] reaction occurs and why its competing [4+3] reaction is disfavored have been investigated computationally.

Rhodium-Catalyzed [7 + 1] Cycloaddition of Exocyclic 1,3-Dienylcyclopropanes and Carbon Monoxide

A rhodium-catalyzed [7 + 1] reaction of exocyclic 1,3-dienylcyclopropanes and carbon monoxide has been developed to synthesize eight-membered carbocycle-embedded bicyclic and tricyclic molecules. In addition, ab initio calculations were conducted to reveal the reaction mechanism.

Rh-Catalyzed [4 + 1] Reaction of Cyclopropyl-Capped Dienes (but not Common Dienes) and Carbon Monoxide: Reaction Development and Mechanistic Study

Transition-metal-catalyzed [4 + 1] reaction of dienes and carbon monoxide (CO) is the most straightforward and easily envisioned cyclization for the synthesis of five-membered carbocycles, which are ubiquitously found in natural products and functional molecules. Unfortunately, no test of this reaction was reported, and consequently, chemists do not know whether such kind of reaction works or not. Herein, we report that the [4 + 1] reaction of common dienes and CO cannot work, at least under the catalysis of [Rh(cod)Cl]2. However, using cyclopropyl-capped dienes (also named allylidenecyclopropanes) as substrates, the corresponding [4 + 1] reaction with CO proceeds smoothly in the presence of [Rh(cod)Cl]2. This [4 + 1] reaction, with a broad scope, provides efficient access to five-membered carbocyclic compounds of spiro[2.4]hept-6-en-4-ones. The [4 + 1] cycloadducts can be further transformed into other molecules by using the unique chemistry of cyclopropyl groups present in these molecules. The mechanism of this [4 + 1] reaction has been investigated by quantum chemical calculations, uncovering that cyclopropyl-capped dienes are strained dienes and the oxidative cyclization step in the [4 + 1] catalytic cycle can release this (angular) strain both kinetically and thermodynamically. The strain release in this step then propagates to all followed CO coordination/CO insertion/reductive elimination steps in the [4 + 1] catalytic cycle, helping the realization of this cycloaddition reaction. In contrast, common dienes (including cyclobutyl-capped dienes) do not have such advantages and their [4 + 1] reaction suffers from energy penalty in all steps involved in the [4 + 1] catalytic cycle. The reactivity of ene-allenes for the [4 + 1] reaction with CO is also discussed.

Synthesis of Polycyclic n/5/8 and n/5/5/5 Skeletons Using Rhodium-Catalyzed [5 + 2 + 1] Cycloaddition of Exocyclic-ene-vinylcyclopropanes and Carbon Monoxide

A rhodium-catalyzed [5 + 2 + 1] reaction of exocyclic-ene-vinylcyclopropanes (exo-ene-VCPs) and CO has been realized to access challenging tricyclic n/5/8 skeletons (n = 5, 6, 7), some of which are found in natural products. This reaction can be used to build tetracyclic n/5/5/5 skeletons (n = 5, 6), which are also found in natural products. In addition, 0.2 atm CO can be replaced by (CH₂O)_n as the CO surrogate to achieve the [5 + 2 + 1] reaction with similar efficiency.

Strain-Release-Controlled [4 + 2 + 1] Reaction of Cyclopropyl-Capped Diene-ynes/Diene-enes and Carbon Monoxide Catalyzed by Rhodium

Achieving transition-metal-catalyzed reactions of diene-ynes/diene-enes and carbon monoxide (CO) to deliver [4 + 2 + 1] cycloadducts, rather than the kinetically favored [2 + 2 + 1] products, is challenging. Here, we report that this can be solved by adding a cyclopropyl (CP) cap to the diene moiety of the original substrates. The resulting CP-capped diene-ynes/diene-enes can react with CO under Rh catalysis to give [4 + 2 + 1] cycloadducts exclusively without forming [2 + 2 + 1] products. This reaction has a broad scope and can be used to synthesize useful 5/7 bicycles with a CP moiety. Of the same importance, the CP moiety in the [4 + 2 + 1] cycloadducts can act as an intermediate group for further transformations so that other challenging bicyclic 5/7 and tricyclic 5/7/5, 5/7/6, and 5/7/7 skeletons, some of which are widely found in natural products, can be accessed. The mechanism of this [4 + 2 + 1] reaction has been investigated by quantum chemical calculations, and the role of the CP group in avoiding the possible side [2 + 2 + 1] reaction has been identified, showing that the [4 + 2 + 1] is controlled by releasing the ring strain in the methylenecyclopropyl (MCP) group (about 7 kcal/mol) in the CP-capped dienes.

Computational Study of Mechanisms and Tether Length Effects of Rh-Catalyzed [3+2] and [3+2+1] Reactions of Ene/Yne-Vinylcyclopropanes

DFT calculations have been applied to study the mechanisms of [3+2] and [3+2+1] reactions of ene/yne-vinylcyclopropanes (shorted as ene/yne-VCPs). The [3+2] reactions of ene/yne-VCPs start from C-C cleavage of cyclopropane (CP cleavage) to form six-membered rhodacycle, followed by alkene/alkyne insertion and reductive elimination. The [3+2+1] reactions have two competing pathways, one is the [3+2+1] pathway (CP cleavage, ene/yne insertion, CO insertion and reductive elimination) and the other is the [3+1+2] pathway (CP cleavage, CO insertion, ene/yne insertion and reductive elimination). The length of tether in substrates affects the ene/yne insertion steps in these cycloadditions, making some reactions fail or changing the reaction pathways. The reasons for these tether length effects are discussed.

Rhodium-catalyzed aminoacylation of alkenes via carbonylative C–H activation toward poly(hetero)cyclic alkylarylketones

This work discloses the facile construction of polyheterocyclic alkylarylketones by the rhodium-catalyzed carbonylative aminoacylation of alkenes involving C–H activation, which provides molecules as candidates for the screening of antitumor agents.

Mechanistic Insights into Cobalt-Catalyzed Regioselective C4-Alkenylation of 3-Acetylindole: A Detailed Theoretical Study

A detailed mechanistic study of Co(III)-catalyzed C4-alkenylation of 3-acetylindole (1a) was done based on calculations at density functional theory (DFT) and correlated wave function levels. The whole catalytic cycle consists of four steps: C-H activation, olefin insertion, β-hydride elimination, and regeneration of the catalyst. The theoretical results support olefin insertion as the rate-determining step leading to the experimentally observed regioselectivity of the C4 site over the C2 site. By the analysis of three-dimensional (3D) geometries and the NCl plot, the preference for the C4 site over the C2 site could be attributed to the weaker repulsive interaction between the indole moiety and olefin in the transition states of the olefin insertion step for the former. The reliability of the theoretical mechanistic results is further confirmed through the DFT calculation of other related indole derivatives and olefin substrates.

Why [4 + 2 + 1] but Not [2 + 2 + 1]? Why Allenes? A Mechanistic Study of the Rhodium-Catalyzed [4 + 2 + 1] Cycloaddition of In Situ Generated Ene-Ene-Allenes and Carbon Monoxide

Transition metal-catalyzed [4 + 2 + 1] cycloaddition of in situ generated ene/yne-ene-allenes (from ene/yne-ene propargyl esters) and carbon monoxide (CO) gives the [4 + 2 + 1] cycloadducts rather than [2 + 2 + 1] cycloadducts. Investigating the mechanism of this [4 + 2 + 1] reaction and understanding why the [2 + 2 + 1] reaction does not compete and the role of the allene moiety in the substrates are important. This is also helpful to guide the future design of new [4 + 2 + 1] cycloadditions. Reported here are the kinetic and computed studies of the [4 + 2 + 1] reactions of ene-ene propargyl esters and CO. A quantum chemical study (at the DLPNO-CCSD(T)//BMK level) revealed that the [4 + 2 + 1] reaction includes four key steps, which are 1,3-acyloxy migration (rate-determining step), oxidative cyclization, CO migratory insertion, and reductive elimination. The allene moiety in the substrates is critical for providing additional coordination to the rhodium center in the final step of the catalytic cycle, which in turn favors the reductive elimination transition state in the [4 + 2 + 1] rather than in the [2 + 2 + 1] pathway. The CO insertion step in the [4 + 2 + 1] reaction, which could occur through either the UP (favored here) or DOWN CO insertion pathway, has also been deeply scrutinized, and some guidance from this analysis has been provided to help the future design of new [4 + 2 + 1] reactions. Quantum chemical calculations have also been applied to explain why [4 + 2] and [4 + 1] cycloadditions do not happen and how trienes as side products for some substrates are generated.

Rh(i)-catalyzed dimerization of ene-vinylidenecyclopropanes for the construction of spiro[4,5]decanes and mechanistic studies

Rh(i) complex catalyzed dimerization of ene-vinylidenecyclopropanes took place smoothly to construct a series of products containing spiro[4,5]decane skeletons featuring a simple operation procedure, mild reaction conditions, and good functional group tolerance. In this paper, the combination of experimental and computational studies reveals a counterion-assisted Rh(i)-Rh(iii)-Rh(v)-Rh(iii)-Rh(i) catalytic cycle involving tandem oxidative cyclometallation/reductive elimination/selective oxidative addition/selective reductive elimination/reductive elimination steps; in addition, a pentavalent spiro-rhodium intermediate is identified as the key intermediate in this dimerization reaction upon DFT calculation.

Six-Step Total Synthesis of Isohirsut-4-ene through [5+2+1] Cycloaddition and Transannular Epoxide-Alkene Cyclization

A six-step total synthesis of isohirsut-4-ene with a 5/5/5 tricyclic core has been achieved. The synthesis features a Rh(I)-catalyzed [5+2+1] cycloaddition, a Corey-Chaykovsky reaction, and a transannular epoxide-alkene cyclization that afford the skeleton of the target molecule. This three-step strategy was further utilized to synthesize more 5/5/5 tricyclic analogues with one or two bridgehead quaternary centers.