Electrooxidative palladium- and enantioselective rhodium-catalyzed [3 + 2] spiroannulations

Recent Advances in Asymmetric Organometallic Electrochemical Synthesis (AOES)

ConspectusIn recent years, our research group has dedicated significant effort to the field of asymmetric organometallic electrochemical synthesis (AOES), which integrates electrochemistry with asymmetric transition metal catalysis. On one hand, we have rationalized that organometallic compounds can serve as molecular electrocatalysts (mediators) to reduce overpotentials and enhance both the reactivity and selectivity of reactions. On the other hand, the conditions for asymmetric transition metal catalysis can be substantially improved through electrochemistry, enabling precise modulation of the transition metal's oxidation state by controlling electrochemical potentials and regulating the electron transfer rate via current adjustments. This synergistic approach addresses key challenges inherent in traditional asymmetric transition metal catalysis, particularly those related to the use of redox-active chemical reagents. Furthermore, the redox potentials of molecular electrocatalysts can be conveniently tuned by modifying their ligands, thereby governing the reaction regioselectivity and stereoselectivity. As a result, the AOES has emerged as a powerful and promising tool for the synthesis of chiral compounds.In this Account, we summarize and contextualize our recent efforts in the field of AOES. Our primary strategy involves leveraging the controllability of electrochemical potential and current to regulate the oxidation state of organometallics, thereby facilitating the desired reactions. An efficient asymmetric synthesis platform was established under mild conditions, significantly reducing the reliance on chemical redox reagents. Our research has been systematically categorized into three sections based on distinct electrolysis modes: asymmetric transition metal catalysis combined with anodic oxidation, cathodic reduction, and paired electrolysis. In each section, we highlight our innovative discoveries tailored to the unique characteristics of the respective electrolysis modes.In many transformations, transition metal-catalyzed reactions involving traditional chemical redox reagents and those utilizing electrochemistry exhibit similar reactivities. However, we also observed notable differences in certain cases. These findings include the following: (1) Enhanced efficiency in asymmetric electrochemical synthesis: for instance, the Rh-catalyzed enantioselective electrochemical functionalization of C-H bonds demonstrates superior efficiency. (2) Expanded scope of transformations: certain transformations, previously challenging in traditional transition metal catalysis, can be achieved through electrochemistry due to the tunability of redox potentials. A notable example is the enantioselective reductive coupling of aryl chlorides, which significantly expands the range of accessible transformations. Additionally, our mechanistic studies explore unique techniques intrinsic to electrochemistry, such as controlled potential electrolysis experiments, the impact of electrode materials on catalyst performance, and cyclic voltammetry studies. These investigations provide a more intuitive understanding of the behavior of metal catalysts through the study of electrochemical mechanisms, which can also guide the design of new catalytic systems.The advancements in this field offer a robust platform for environmentally friendly and sustainable selective asymmetric transformations. By integrating electrochemistry with transition metal catalysis, we have developed a versatile approach for organic synthesis that not only enhances the efficiency and selectivity of reactions but also reduces the environmental impact. We anticipate that this Account will stimulate further research and innovation in the realm of AOES, leading to the discovery of new catalytic systems and the development of more sustainable synthetic methodologies.

Rhodaelectro-Catalyzed Synthesis of Pyrano[3,4-b]indol-1(9H)-ones via the Double Dehydrogenative Heck Reaction between Indole-2-carboxylic Acids and Alkenes

A rhodaelectro-catalyzed double dehydrogenative Heck reaction of indole-2-carboxylic acids with alkenes has been developed for the synthesis of pyrano[3,4-b]indol-1(9H)-ones. The weakly coordinating carboxyl group is utilized twice as a directing group to activate the C-H bonds throughout the reaction. This reaction precedes an acceptorless dehydrogenation under exogenous oxidant-free conditions in an undivided cell with a constant current.

Metallaelectro-catalyzed alkyne annulations via C-H activations for sustainable heterocycle syntheses

Alkyne annulation represents a versatile and powerful strategy for the assembly of structurally complex compounds. Recent advances successfully enabled electrocatalytic alkyne annulations, significantly expanding the potential applications of this promising technique towards sustainable synthesis. The metallaelectro-catalyzed C-H activation/annulation stands out as a highly efficient approach that leverages electricity, combining the benefits of electrosynthesis with the power of transition-metal catalyzed C-H activation. Particularly attractive is the pairing of the electro-oxidative C-H activation with the valuable hydrogen evolution reaction (HER), thereby addressing the growing demand for green energy solutions. Herein, we provide an overview of the evolution of electrochemical C-H annulations with alkynes for the construction of heterocycles, with a topical focus on the underlying mechanism manifolds.

Electro- and Mechanochemical Strategy as a Dual Synthetic Approach for Biologically Relevant 3-Nitro-imidazo-[1,2-a]pyridines

We herein disclose a dual synthetic approach involving electrochemical and mechanochemical strategies for diversely functionalized 3-nitro-2-aryl-immidazo[1,2-a]pyridines. Both methods offer a practical and straightforward alternative route for accessing this important class of biologically promising nitrogen-containing heterocycles. Significant advantages of the newly developed methods include mild and energy-efficient reaction conditions, avoidance of transition metal catalysts, external heating and additional oxidants, shorter reaction times, good to excellent yields, broad substrate scope, gram-scale applicability, operational simplicity, and eco-friendliness. Furthermore, a synthetic application was extended by successfully reducing synthesized 3-nitro-2-aryl-immidazo[1,2-a]pyridines to their corresponding amino derivatives.

Electrorearranged Difunctionalization of 4-Hydroxy-α-benzopyrones

We herein report the exploration of an electrosynthetic strategy as a highly efficient and straightforward alternative protocol for accessing diversely substituted and biologically promising alkyl 2-hydroxy-3-oxo-2,3-dihydrobenzofuran-2-carboxylates through an electrorearranged difunctionalization of 4-hydroxycoumarins, involving the singlet oxygen insertion from molecular oxygen, at ambient temperature. The present method is notably more advantageous than the previously reported photochemical conversion regarding yields and reaction times, substrate scope and functional group tolerability, operational simplicity, and scalability.

Electrochemical Enantioselective C-H Annulation by Achiral Rhodium(III)/Chiral Brønsted Base Domino Catalysis

Rhodium(III)-catalyzed enantioselective C-H activation has emerged as a powerful tool for assembling enabling chiral molecules. However, this approach is significantly hampered by the cumbersome synthetic routes for preparing chiral rhodium catalysts. In sharp contrast, we herein report on an electrochemical domino catalysis system that exploits an achiral Cp*-rhodium catalyst along with an easily accessible chiral Brønsted base for an enantioselective C-H activation/annulation reaction of alkenes by benzoic acids. Our strategy offers an environmentally benign and most user-friendly approach for assembling synthetically useful chiral phthalides in good enantioselectivity, employing electricity as the sustainable oxidant.

Organocatalytic Asymmetric Dearomative Spirocyclization/Oxa-Michael Addition Sequence: Synthesis of Polycyclic Tetralones

Herein, an organocatalytic asymmetric dearomative spirocyclization/oxa-Michael addition sequence with a newly designed substrate having two naphthol motifs has been developed. The reaction proceeds through in situ chiral vinylidene ortho-quinone methide (VQM) intermediate formation, dearomative spirocyclization of naphthol, and an oxa-Michael addition reaction. The densely functionalized tetralone products were formed in high yields with high diastereo- and enantioselectivities.

Electrooxidative Rhodium(III)/Chiral Carboxylic Acid-Catalyzed Enantioselective C-H Annulation of Sulfoximines with Alkynes

The combination of achiral Cp*Rh(III) with chiral carboxylic acids (CCAs) represents an efficient catalytic system in transition metal-catalyzed enantioselective C-H activation. However, this hybrid catalysis is limited to redox-neutral C-H activation reactions and the adopt to oxidative enantioselective C-H activation remains elusive and pose a significant challenge. Herein, we describe the development of an electrochemical Cp*Rh(III)-catalyzed enantioselective C-H annulation of sulfoximines with alkynes enabled by chiral carboxylic acid (CCA) in an operationally friendly undivided cell at room temperature. A broad range of enantioenriched 1,2-benzothiazines are obtained in high yields with excellent enantioselectivities (up to 99 % yield and 98 : 2 er). The practicality of this method is demonstrated by scale-up reaction in a batch reactor with external circulation. A crucial chiral Cp*Rh(III) intermediate is isolated, characterized, and transformed, providing rational support for a Rh(III)/Rh(I) electrocatalytic cycle.

Ni-Catalyzed 1,5-Sigmatropic Ester Shift on Cyclopentadiene Rings: Regioselective Conversion of 5,5-Disubstituted Cyclopentadienes to CH2-Cyclopentadienes

Described herein is a nickel(II)-catalyzed regioselective rearrangement of 5,5-disubstituted cyclopentadienes to fully functionalized CH2-cyclopentadienes via successive 1,5-sigmatropic shifts of the ester group on the quaternary carbon and hydrogen under mild basic conditions. The obtained CH2-cyclopentadienes were also readily applied in the preparation of highly functionalized dibenzo[e,g]azulene derivatives in two steps.

Electrochemical cascade migratory versus ortho-cyclization of 2-alkynylbenzenesulfonamides

Efficient control over several possible reaction pathways of free radicals is the chemical basis of their highly selective transformations. Among various competing reaction pathways, sulfonimidyl radicals generated from the electrolysis of 2-alkynylbenzenesulfonamides undergo cascade migratory or ortho-cyclization cyclization selectively. It is found that the incorporation of an extra 2-methyl substituent biases the selective migration of the acyl- over vinyl-linker of the key spirocyclic cation intermediate and thus serves as an enabling handle to achieve the synthetically interesting yet under-investigated cascade migratory cyclization of spirocyclic cations.

Scalable Diastereoselective Electrosynthesis of Spiro[benzofuran-2,2'-furan]-3-ones

Spirobenzofuran scaffolds, because of their three-dimensional structure, are incorporated into several valuable natural products and drug candidate molecules. Herein, with the assistance of electrosynthesis, we introduce a novel electrochemical approach for achieving spirobenzofurans in a user-friendly and operationally simple undivided cell setup under constant current. This metal-catalyst-free electrochemical procedure afforded spiro[benzofuran-2,2'-furan]-3-ones with high diastereoselectivity. Compatibility with gram-scale synthesis along with the convenient accessibility of reaction instruments and starting materials collectively raised the importance of this protocol compared to previous challenging methods. Furthermore, mechanistic cognizance of this reaction is obtained by the investigation of the cyclic voltammetry spectra of reactants.

Palladium-Catalyzed Intermolecular Dearomatization Annulation Cascade Reaction of Furans for Stereoselective Access to 2,5-Dihydrofurans

A novel palladium-catalyzed intermolecular dearomatization of furans with alkynes via a three-component formal [3 + 2] spiroannulation/allylic substitution cascade reaction has been successfully developed for the stereoselective assembly of spiro 2,5-dihydrofuran frameworks. High step economy and efficacy as well as excellent stereoselectivity were achieved for a broad substrate scope. Two new C-C bonds and one new C-O bond were generated sequentially in a one-pot manipulation. The yielded spiro 2,5-dihydrofuran skeleton bearing a tetrasubstituted carbon center constitutes the core structure for plenty of useful natural products or corresponding analogues. This work represents a significant advancement in the dearomatization strategy for furan heterocycles and provides a practical methodology for expedited access to complex spiro dihydrofuran scaffolds.

The crucial role of silver(I)-salts as additives in C-H activation reactions: overall analysis of their versatility and applicability

Transition-metal catalyzed C-H activation reactions have been proven to be useful methodologies for the assembly of synthetically meaningful molecules. This approach bears intrinsic peculiarities that are important to be studied and comprehended in order to achieve its best performance. One example is the use of additives for the in situ generation of catalytically active species. This strategy varies according to the type of additive and the nature of the pre-catalyst that is being used. Thus, silver(I)-salts have proven to play an important role, due to the resulting high reactivity derived from the pre-catalysts of the main transition metals used so far. While being powerful and versatile, the use of silver-based additives can raise concerns, since superstoichiometric amounts of silver(I)-salts are typically required. Therefore, it is crucial to first understand the role of silver(I) salts as additives, in order to wisely overcome this barrier and shift towards silver-free systems.

Rhodium-Catalyzed Asymmetric C-H Functionalization Reactions

This review summarizes the advancements in rhodium-catalyzed asymmetric C-H functionalization reactions during the last two decades. Parallel to the rapidly developed palladium catalysis, rhodium catalysis has attracted extensive attention because of its unique reactivity and selectivity in asymmetric C-H functionalization reactions. In recent years, Rh-catalyzed asymmetric C-H functionalization reactions have been significantly developed in many respects, including catalyst design, reaction development, mechanistic investigation, and application in the synthesis of complex functional molecules. This review presents an explicit outline of catalysts and ligands, mechanism, the scope of coupling reagents, and applications.

Palladium-Catalyzed C-H Arylation/Arene Dearomatization Domino Reaction: Expeditious Access to Spiro[4,5]fluorenes

A palladium-catalyzed C-H arylation/arene dearomatization of α-aryl-β-naphthol with o-dihalobenzenes was realized in a redox-neutral manner. This bimolecular domino reaction was initiated by an in situ-formed Pd(II) species generated from the dihalobenzene, followed by phenolic-group-directed C-H activation, biaryl cross-coupling, and naphthol dearomatization, thus rendering the rapid assembly of a class of spiro[4,5]fluorenes in high yields with good chemoselectivity. Remarkably, malononitrile-derived spirofluorene 6 was found to exhibit mechanoresponsive luminescence, which can be applied to optical memory devices.

Cobaltaelectro-Catalyzed C-H Annulation with Allenes for Atropochiral and P-Stereogenic Compounds: Late-Stage Diversification and Continuous Flow Scale-Up

The 3d metallaelectro-catalyzed C-H activation has been identified as an increasingly viable strategy to access valuable organic molecules in a resource-economic fashion under exceedingly mild reaction conditions. However, the development of enantioselective 3d metallaelectro-catalyzed C-H activation is very challenging and in its infancy. Here, we disclose the merger of cobaltaelectro-catalyzed C-H activation with asymmetric catalysis for the highly enantioselective annulation of allenes. A broad range of C-N axially chiral and P-stereogenic compounds were thereby obtained in good yields of up to 98% with high enantioselectivities of up to >99% ee. The practicality of this approach was demonstrated by the diversification of complex bioactive compounds and drug molecules as well as decagram scale enantioselective electrocatalysis in continuous flow.

Pd-catalyzed regioselective rollover dual C-H annulation cascade: facile approach to phenanthrene derivatives

Annulations of unsaturated systems through C-H activation represent a powerful tool for producing multicyclic scaffolds. Having coordinating centers in both annulation partners (a dual coordination strategy) would afford remarkable selectivities in the outcomes. Along this concept, we report herein a Pd-catalyzed regioselective rollover cascade dual C-H annulation of o-arylphenols with alkynols for constructing phenanthrene scaffolds. Control, KIE and deuteration studies were conducted to determine the reaction mechanism, and downstream transformations and scaled-up reactions were carried out to assess the robustness of the transformation.

Electrochemical C-H Sulfonylation of Hydrazones

We have developed an electrochemical method for the direct C-H sulfonylation of aldehyde hydrazones using sodium sufinates as the sulfonylating agent under supporting electrolyte-free conditions. This straightforward sulfonylation strategy afforded a library of (E)-sufonylated hydrazones with high tolerance of various functional groups. The radical pathway of this reaction has been revealed by the mechanistic studies.

Rh(III)-catalyzed (5 + 2)-cycloaddition reactions of ortho-hydroxyethyl phenols with internal alkynes: efficient synthesis of benzoxepines

An unprecedented (5 + 2)-cycloaddition reaction of ortho-hydroxyethyl phenol and internal alkyne was developed. This Rh(III)-catalyzed reaction provided benzoxepine derivatives which have very high biological significance. A wide range of ortho-hydroxyethyl phenols and internal alkynes were studied to provide the benzoxepines in high yields.

Pd-Catalyzed intermolecular asymmetric allylic dearomatization of 1-nitro-2-naphthols with MBH adducts

An asymmetric allylic dearomatization reaction of 1-nitro-2-naphthol derivatives with Morita-Baylis-Hillman (MBH) adducts has been developed. By utilizing Pd catalyst derived from Pd(OAc)₂ and Trost ligand (R,R)-L1, the reaction proceeded smoothly in 1,4-dioxane at room temperature, affording substituted β-naphthalenones in good yields (up to 92%) and enantioselectivity (up to 90% ee). A range of substituted 1-nitro-2-naphthols and MBH adducts were found to be compatible under the optimized conditions. This reaction provides a convenient method for the synthesis of enantioenriched 1-nitro-β-naphthalenone derivatives.

Transition Metal-Catalyzed C-H Functionalization Through Electrocatalysis

Electrochemically promoted transition metal-catalyzed C-H functionalization has emerged as a promising area of research over the last few decades. However, development in this field is still at an early stage compared to traditional functionalization reactions using chemical-based oxidizing agents. Recent reports have shown increased attention on electrochemically promoted metal-catalyzed C-H functionalization. From the standpoint of sustainability, environmental friendliness, and cost effectiveness, electrochemically promoted oxidation of a metal catalyst offers a mild, efficient, and atom-economical alternative to traditional chemical oxidants. This Review discusses advances in the field of transition metal-electrocatalyzed C-H functionalization over the past decade and describes how the unique features of electricity enable metal-catalyzed C-H functionalization in an economic and sustainable way.

Enantio- and Regioselective Electrooxidative Cobalt-Catalyzed C-H/N-H Annulation with Alkenes

In recent years, the merging of electrosynthesis with 3d metal catalyzed C-H activation has emerged as a sustainable and powerful technique in organic synthesis. Despite the impressive advantages, the development of an enantioselective version remains elusive and poses a daunting challenge. Herein, we report the first electrooxidative cobalt-catalyzed enantio- and regioselective C-H/N-H annulation with olefins using an undivided cell at room temperature (up to 99 % ee). ^t Bu-Salox, a rationally designed Salox ligand bearing a bulky tert-butyl group at the ortho-position of phenol, was found to be crucial for this asymmetric annulation reaction. A strong cooperative effect between ^t Bu-Salox and 3,4,5-trichloropyridine enabled the highly enantio- and regioselective C-H annulation with the more challenging α-olefins without secondary bond interactions (up to 96 % ee and 97 : 3 rr). Cyclovoltametric studies, and the preparation, characterization, and transformation of cobaltacycle intermediates shed light on the mechanism of this reaction.

Electrochemical Regioselective C(sp2)-H Selenylation and Sulfenylation of Substituted 2-Amino-1,4-naphthoquinones

A straightforward and efficient electrochemical method for regioselective C(sp²)-H selenylation and sulfenylation of substituted 2-amino-1,4-naphthoquinones has been unearthed. This oxidative cross-coupling reaction avoids using transition metal catalysts, oxidants, and high temperatures. The other notable advantages of this protocol are the tolerance of diverse functional groups, mild reaction conditions at ambient temperature, energy efficiency, good to excellent yields, short reaction times (in minutes), gram-scale applicability, and eco-friendliness.

Rh-Catalyzed [3+2] Annulation of Cyclic Ketimines and Alkynyl Chloride: A Strategy for Accessing Unsymmetrically Substituted and Highly Functionalizable Indenes

Alkynyl chlorides were found to be extraordinarily novel electrophiles, which could afford a single regioisomer of the [3+2] annulation adducts with cyclic ketimines by rhodium catalysis. The alkenyl chloride moiety in the products provided a valuable functional handle for further diverse transformations. Therefore, this research provided not only a synthetic protocol for accessing unsymmetrically substituted indenyl amines but also a highly divergent solution for decorating the substituting group by postmanipulation of the chloride.

Selective Electrochemical Synthesis of 9-Aryl-10-sulfonyl Substituted Phenanthrene from Alkynes and Sulfonyl Hydrazides

An efficient electrochemical synthesis of sulfonated phenanthrenes via the reaction of internal alkynes with sulfonyl hydrazides has been established. The protocol does not require a metal catalyst or external oxidants, providing a green and mild route to functionalized phenanthrenes. Moreover, the compatibility of various functional groups and decagram-scale experimental conditions demonstrate the practicality of the electrochemical strategy.

Silver-Free C-H Activation: Strategic Approaches towards Realizing the Full Potential of C-H Activation in Sustainable Organic Synthesis

The activation of carbon-hydrogen bonds is considered as one of the most attractive techniques in synthetic organic chemistry because it bears the potential to shorten synthetic routes as well as to produce complementary product scopes compared to traditional synthetic strategies. However, many current methods employ silver salts as additives, leading to stoichiometric metal waste and thereby preventing the full potential of C-H activation to be exploited. Therefore, the development of silver-free protocols has recently received increasing attention. Mechanistically, silver can serve various roles in C-H activation and thus, avoiding the use of silver requires different approaches based on the role it serves in a given process. In this Review, we present the comparison of silver-based and silver-free methods. Focusing on the strategic approaches to develop silver-free C-H activation, we provide the reader with the means to develop sustainable methods for C-H activation.

Electrochemical Palladium‐Catalyzed Oxidative Carbonylation‐Cyclization of Enallenols

Herein, we report an electrochemical oxidative palladium-catalyzed carbonylation-carbocyclization of enallenols to afford γ-lactones and spirolactones, which proceeds with excellent chemoselectivity. Interestingly, electrocatalysis was found to have an accelerating effect on the rate of the tandem process, leading to a more efficient reaction than that under chemical redox conditions.

Palladium(0)-Catalyzed Intermolecular Asymmetric Allylic Dearomatization of Substituted β-Naphthols with Morita-Baylis-Hillman (MBH) Adducts

Pd-catalyzed intermolecular asymmetric allylic dearomatization of substituted β-naphthol derivatives with Boc-protected Morita-Baylis-Hillman (MBH) adducts was developed. The reaction occurs smoothly in 1,4-dioxane at room temperature in the presence of [Pd(C₃H₅)Cl]₂ (2.5 mol %), (S, S_p)-PHOX ligand (5.5 mol %), and Li₂CO₃ (1.0 equiv). A series of dearomatized products were afforded in moderate to excellent yields and enantioselectivity (up to 99% yield, 97% ee). Furthermore, the compatibility with gram-scale reaction and mild conditions make the current method synthetically useful.

Lewis Acid-Catalyzed [3 + 2]-Cyclization of Iodonium Ylides with Azadienes: Access to Spiro[benzofuran-2,2'-furan]-3-ones

A highly regioselective synthesis of spiro[benzofuran-2,2'-furan]-3-ones has been explored via Lewis acid-catalyzed [3 + 2] cyclization of iodonium ylides with azadienes. The acidity of the Lewis acid was significantly strengthened with strong hydrogen bond donors, thereby promoting the enolization isomerization of iodonium ylides for the subsequent cycloaddition. This reaction was compatible with a broad range of substrates under the mild reaction conditions, and efficiently delivered spiro-heterocycles with excellent stereoselectivity.

Rapid synthesis of spirodienones via electrochemical dearomative spirocyclization in flow

An electrochemical dearomative spirocyclization in flow has been developed, featuring the use of electrons as the clean oxidant in a minimum amount of electrolytes to afford diverse spirodienones in a short reaction time.