Mechanistic Insight into the Diastereoselective Rhodium-Catalyzed Pauson–Khand Reaction: Role of Coordination Number in Stereocontrol

A Density Functional Theory Study on the Cobalt-Mediated Intramolecular Pauson–Khand Reaction of Enynes Containing a Vinyl Fluoride Moiety

The Co2(CO)8-mediated intramolecular Pauson–Khand reaction (PKR) is an effective method for constructing polycyclic structures. Recently, our group reported a series of this type of reaction involving fluorinated enynes that proceed with reasonable reaction rates and yields. However, mechanistic studies involving these fluorinated derivatives in intramolecular PKR are scarce. In this study, density functional theory calculations are used to clarify the mechanism and reactivity of enynes containing a vinyl fluoride moiety for this reaction. In agreement with previous studies, alkene insertion is considered to be the rate-determining step for the overall Pauson–Khand reaction of enynes containing a vinyl fluoride moiety. The effect of the substituent on the Co2(CO)8-mediated intramolecular Pauson–Khand reaction has also been investigated. When introducing heteroatoms as tethering units, the fluorinated enynes exhibited lower reactivity than the malonate homologues, whereas the use of a sulfur-based tether was unsuccessful. This computational study provides detailed information about the PKR mechanism and transition-state structures, and the results are validated with previous experimental results.

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

Previously, we developed a rhodium-catalyzed [5 + 2 + 1] cycloaddition of ene-vinylcyclopropanes (ene-VCPs) and carbon monoxide to synthesize eight-membered carbocycles. The efficiency of this reaction can be appreciated from its application in the synthesis of several natural products. Herein we report the results of a 15-year investigation into the mechanism of the [5 + 2 + 1] cycloaddition by applying visual kinetic analysis and high-level quantum chemical calculations at the DLPNO-CCSD(T)//BMK level. According to the kinetic measurements, the resting state of the catalyst possesses a dimeric structure (with two rhodium centers) whereas the active catalytic species is monomeric (with one rhodium center). The catalytic cycle consists of cyclopropane cleavage (the turnover-limiting step), alkene insertion, CO insertion, reductive elimination, and catalyst transfer steps. Other reaction pathways have also been considered but then have been ruled out. The steric origin of the diastereoselectivity (cis versus trans) was revealed by comparing the alkene insertion transition states. In addition, how the double-bond configuration of the VCPs (Z versus E) affects the substrate reactivity and the origins of chemoselectivity ([5 + 2 + 1] versus [5 + 2]) were also investigated. The present study will provide assistance in understanding other carbonylative annulations catalyzed by transition metals.

Asymmetric synthesis of tricyclic 6,5,5-fused polycycles by desymmetric Pauson-Khand reaction

A new rhodium(I)-BINAP-catalyzed enantioselective desymmetric Pauson-Khand (PK) reaction of prochiral 1,6-enynes with CO has been developed. Diverse tricyclic 6,5,5-fused bowl type polycycles with three continuous chiral centers were synthesized in...

Rhodium-Catalyzed Enantioselective and Desymmetrizative Pauson-Khand Reaction: Access to Tricyclo[6.2.1.04,11]undecenes

Rhodium-catalyzed asymmetric desymmetrization Pauson-Khand reaction of C4-alkynyl-tethered cyclohexadienones has been developed as a novel strategy for access to fused 6-5-5 tricycles bearing three consecutive stereogenic centers. An array of chiral tricyclo[6.2.1.0^4,11]undecenes have been synthesized in high yields and enantioselectivities in a single step under mild conditions. This strategy employs readily accessible internal-olefin-containing 1,6-enynes, providing a potentially powerful tool for constructing chiral polycyclic scaffolds of complex molecules containing cyclopentenones and cyclohexenones.

Evolution of Pauson-Khand Reaction: Strategic Applications in Total Syntheses of Architecturally Complex Natural Products (2016–2020)

Metal-mediated cyclizations are important transformations in a natural product total synthesis. The Pauson-Khand reaction, particularly powerful for establishing cyclopentenone-containing structures, is distinguished as one of the most attractive annulation processes routinely employed in synthesis campaigns. This review covers Co, Rh, and Pd catalyzed Pauson-Khand reaction and summarizes its strategic applications in total syntheses of structurally complex natural products in the last five years. Additionally, the hetero-Pauson-Khand reaction in the synthesis of heterocycles will also be discussed. Focusing on the panorama of organic synthesis, this review highlights the strategically developed Pauson-Khand reaction in fulfilling total synthetic tasks and its synthetic attractiveness is aimed to be illustrated.

Synthesis of α,β-unsaturated carbonyl compounds by carbonylation reactions

Carbonylation reactions represent one of the most important tool box for the synthesis of α,β-unsaturated carbonyl compounds which are key building blocks in organic chemistry. This paper summarizes the most important advances in this field.

Synthesis of Aromatic Rings Embedded in Polycyclic Scaffolds by Triyne Cycloaddition: Competition between Carbonylative and Non-Carbonylative Pathways

Cycloadditions have emerged as some of the most useful reactions for the formation of polycyclic compounds. The carbonylative cycloaddition of triynes can lead to carbonylative and non-carbonylative competitive pathways, each leading to the formation of an aromatic ring. We report herein the one-pot synthesis of fully- and unsymmetrically-substituted tetracyclic 6,5,7,5-troponic and 6,5,6,5-benzenoid scaffolds using pre-organized triynes showing the competition between these two pathways.

Stereoselective construction of sterically hindered oxaspirocycles via chiral bidentate directing group-mediated C(sp3)-O bond formation

The systematic investigation of chiral bidentate auxiliaries has resulted in the discovery of a chiral 2,2-dimethyl-1-(pyridin-2-yl)propan-1-amine-derived directing group that enables stereoselective palladium(ii)-catalyzed intramolecular C(sp3)-O bond formation. This new chiral directing group exhibited high reactivity in the activation of methylene C(sp3)-H bonds with excellent levels of stereoselectivity (a diastereomeric ratio of up to 39 : 1), which allowed the construction of a wide range of oxaspirocycles. Mechanistic investigations were also conducted to elucidate the reaction mechanism and understand the origin of the diastereoselectivity. DFT calculations suggest that only modest levels of diastereoselectivity are accomplished at the rate-determining C-H metalation-deprotonation step and the d.r. is further enriched at the reductive elimination step.

Computationally Guided Catalyst Design in the Type I Dynamic Kinetic Asymmetric Pauson-Khand Reaction of Allenyl Acetates

The Rh(I)-catalyzed allenic Pauson-Khand reaction (APKR) is an efficient, redox-neutral method of synthesizing α-acyloxy cyclopentenones. An enantioselective APKR could provide access to chiral, nonracemic α-acyloxy and α-hydroxy cyclopentenones and their corresponding redox derivatives, such as thapsigargin, a cytotoxic natural product with potent antitumor activity. Rapid scrambling of axial chirality of allenyl acetates in the presence of Rh(I) catalysts enables the conversion of racemic allene to enantiopure cyclopentenone product in a dynamic kinetic asymmetric transformation (DyKAT). A combined experimental and computational approach was taken to develop an effective catalytic system to achieve the asymmetric transformation. The optimization of the denticity, and steric and electronic properties of the ancillary ligand (initially (S)-MonoPhos, 58:42 er), afforded a hemilabile bidentate (S)-MonoPhos-alkene-Rh(I) catalyst that provided α-acyloxy cyclopentenone product in up to 14:86 er. Enantioselectivity of the Rh(I)-(S)-MonoPhos-alkene catalyst was rationalized using ligand-substrate steric interactions and distortion energies in the computed transition states. This asymmetric APKR of allenyl acetates is a rare example of a Type I DyKAT reaction of an allene, the first example of DyKAT in a cyclocarbonylation reaction, and the first catalyst-controlled enantioselective APKR.

Mechanism of Rh-Catalyzed Oxidative Cyclizations: Closed versus Open Shell Pathways

A conceptual theory for analyzing and understanding oxidative addition reactions that form the cornerstone of many transition metal mediated catalytic cycles that activate C-C and C-H bonds, for example, was developed. The cleavage of the σ- or π-bond in the organic substrate can be envisioned to follow a closed or an open shell formalism, which is matched by a corresponding electronic structure at the metal center of the catalyst. Whereas the assignment of one or the other mechanistic scenario appears formal and equivalent at first sight, they should be recognized as different classes of reactions, because they lead to different reaction optimization and control strategies. The closed-shell mechanism involves heterolytic bond cleavages, which give rise to highly localized charges to form at the transition state. In the open-shell pathway, bonds are broken homolytically avoiding localized charges to accumulate on molecular fragments at the transition states. As a result, functional groups with inductive effects may exert a substantial influence on the energies of the intermediate and transition states, whereas no such effect is expected if the mechanism proceeds through the open-shell mechanism. If these functional groups are placed in a way that opens an electronic communication pathway to the molecular sites where charges accumulate, for example, using hyperconjugation, electron donating groups may stabilize a positive charge at that site. An instructive example is discussed, where this stereoelectronic effect allowed for rendering the oxidative addition diastereoselective. No such control is possible, however, when the open-shell reaction pathway is followed, because the inductive effects of functional groups have little to no effect on the stabilities of radical-like substrate states that are encountered when the bonds are broken in a homolytic fashion. Whether the closed-shell or open-shell mechanism for oxidative addition is followed is determined by the ordering of the d-orbital dominated frontier orbitals. If the highest occupied molecular orbital (HOMO) is oriented in space in such a way that will give the organic substrate easy access to the valence electron pair, the closed-shell mechanism can be followed. If the shape and orientation of the HOMO is not appropriate, however, an alternative pathway involving singlet excited states of the metal that will invoke the matching radicaloid cleavage of the organic substrate will dominate the oxidative addition. This novel paradigm for formally analyzing and understanding oxidative additions provides a new way of systematically understanding and planning catalytic reactions, as demonstrated by the in silico design of room-temperature Pauson-Khand reactions.

The origin of the ligand-controlled regioselectivity in Rh-catalyzed [(2 + 2) + 2] carbocyclizations: steric vs. stereoelectronic effects

Density functional theory calculations demonstrate that the reversal of regiochemical outcome of the addition for substituted methyl propiolates in the rhodium-catalyzed [(2 + 2) + 2] carbocyclization with PPh₃ and (S)-xyl-binap as ligands is both electronically and sterically controlled.

Density functional theory calculations demonstrate that the reversal of regiochemical outcome of the addition for substituted methyl propiolates in the rhodium-catalyzed [(2 + 2) + 2] carbocyclization with PPh₃ and (S)-xyl-binap as ligands is both electronically and sterically controlled. For example, the ester functionality polarizes the alkyne π* orbital to favor overlap of the methyl-substituted terminus of the alkyne with the p_π-orbital of the alkenyl fragment of the rhodacycle during alkyne insertion with PPh₃ as the ligand. In contrast, the sterically demanding xyl-binap ligand cannot accommodate the analogous alkyne orientation, thereby forcing insertion to occur at the sterically preferred ester terminus, overriding the electronically preferred orientation for alkyne insertion.

Mechanisms and stereoselectivities of the Rh(i)-catalyzed carbenoid carbon insertion reaction of benzocyclobutenol with diazoester

Mechanisms and stereoselectivities of a Rh(i)-catalyzed carbenoid carbon insertion reaction of benzocyclobutenol with diazoester have been investigated using the DFT method.

Reactivity and chemoselectivity of allenes in Rh(I)-catalyzed intermolecular (5 + 2) cycloadditions with vinylcyclopropanes: allene-mediated rhodacycle formation can poison Rh(I)-catalyzed cycloadditions

Allenes are important 2π building blocks in organic synthesis and engage as 2-carbon components in many metal-catalyzed reactions. Wender and co-workers discovered that methyl substituents on the terminal allene double bond counterintuitively change the reactivities of allenes in [Rh(CO)2Cl]2-catalyzed intermolecular (5 + 2) cycloadditions with vinylcyclopropanes (VCPs). More sterically encumbered allenes afford higher cycloadduct yields, and such effects are also observed in other Rh(I)-catalyzed intermolecular cycloadditions. Through density functional theory calculations (B3LYP and M06) and experiment, we explored this enigmatic reactivity and selectivity of allenes in [Rh(CO)2Cl]2-catalyzed intermolecular (5 + 2) cycloadditions with VCPs. The apparent low reactivity of terminally unsubstituted allenes is associated with a competing allene dimerization that irreversibly sequesters rhodium. With terminally substituted allenes, steric repulsion between the terminal substituents significantly increases the barrier of allene dimerization while the barrier of the (5 + 2) cycloaddition is not affected, and thus the cycloaddition prevails. Computation has also revealed the origin of chemoselectivity in (5 + 2) cycloadditions with allene-ynes. Although simple allene and acetylene have similar reaction barriers, intermolecular (5 + 2) cycloadditions of allene-ynes occur exclusively at the terminal allene double bond. The terminal double bond is more reactive due to the enhanced d-π* backdonation. At the same time, insertion of the internal double bond of an allene-yne has a higher barrier as it would break π conjugation. Substituted alkynes are more difficult to insert compared with acetylene, because of the steric repulsion from the additional substituents. This leads to the greater reactivity of the allene double bond relative to the alkynyl group in allene-ynes.

Computationally designed and experimentally confirmed diastereoselective rhodium-catalyzed Pauson-Khand reaction at room temperature

The computational analysis of the rhodium-catalyzed Pauson-Khand reaction indicates that the key transition state is highly charge-polarized, wherein different diastereoisomers have distinctively different charge polarization patterns. Experimental studies demonstrate that chloro-enynes provide the optimal σ-electron-withdrawing group to promote polarization and thereby reduce the activation barrier to provide a highly diastereoselective reaction at room temperature.

Density functional theory study of the mechanisms and stereochemistry of the Rh(I)-catalyzed intramolecular [3+2] cycloadditions of 1-ene- and 1-yne-vinylcyclopropanes

The mechanisms, structures of all stationary points involved, and kinetic and thermodynamic parameters of the Rh(I)-catalyzed intramolecular [3+2] cycloaddition reactions of 1-ene- and 1-yne-vinylcyclopropanes (1-ene-VCPs and 1-yne-VCPs) have been investigated using density functional theory (DFT) calculations. The computational results showed that the [3+2] reactions of 1-ene/yne-VCPs studied here occur through a catalytic cycle of substrate-catalyst complex formation, cyclopropane cleavage, alkene/alkyne insertion, and reductive elimination. Alkene/alkyne insertion is the rate- and stereoselectivity-determining step of these multistep [3+2] cycloadditions. The experimentally observed high reactivity of 1-yne-VCPs compared to 1-ene-VCPs is well rationalized by the differences of steric effects in the alkyne/alkene insertion transition states. DFT calculations unveiled that the relative orientation of the tethers in the 1-ene/yne-VCPs plays a key role in controlling the stereochemistry of the [3+2] cycloadducts. In addition, DFT calculation results are used to explain why, in some cases, the formation of the β-hydride elimination byproduct can compete with the [3+2] pathway.

The mechanism of the rhodium(I)-catalyzed [2 + 2 + 1] carbocyclization reaction of dienes and CO: a computational study

The rhodium(I) catalyzed [2 + 2 + 1] carbocyclization of tethered diene-enes to afford substituted hexahydropentalenones with high levels of diastereoselectivity was modeled using density functional theory. Previously, this transformation was observed to be facile, whereas the analogous bis-ene substrate could not be cyclized under any reasonable conditions. To establish a conceptual understanding of the carbocyclization mechanism and to identify the functional role of the diene fragment we analyzed the simulated reaction mechanisms using the two parent systems. We discovered a thus far unrecognized, but intuitively plausible, role of the CO ligand for controlling the electron density at the metal center, which affects the feasibility of oxidative addition and reductive elimination steps that are key components of the mechanism. Our calculations suggest that the diene moiety is unique and required because of its ability to undergo a eta(2)-->eta(4) reorganization allowing for the thermoneutral expulsion of one CO ligand, which in turn generates an electron-rich, coordinatively saturated Rh(I) center that can efficiently promote the oxidative addition with a low barrier. A number of functionalization strategies were considered explicitly to derive a rational plan for optimizing the catalysis and to expose the roles of the different components of the reactant-catalyst complex.