Pd-Catalyzed Highly Enantioselective Synthesis of Planar Chiral Ferrocenylpyridine Derivatives

Access to chiral dihydrophenanthridines via a palladium(0)-catalyzed Suzuki coupling and C-H arylation cascade reaction using new chiral-bridged biphenyl bifunctional ligands

A class of chiral-bridged biphenyl phosphine-carboxylate bifunctional ligands CB-Phos has been developed and successfully applied to Pd(0)-catalyzed single enantioselective C-H arylation and a one pot cascade reaction involving Suzuki cross-coupling and C-H arylation. The catalytic system provides a new and convenient way for the synthesis of versatile chiral dihydrophenanthridines with rich structures and broad functional group tolerance. Good to excellent yields with high enantioselectivities were generally achieved. The reaction mechanism of the cascade reaction was also preliminarily discussed.

Design and Application of New Pyridine-Derived Chiral Ligands in Asymmetric Catalysis

ConspectusThe innovation of chiral ligands has been crucial for the asymmetric synthesis of functional molecules, as demonstrated by several types of widely applied "privileged" ligands. In this context, chiral pyridine-derived ligands, by far some of the oldest and most widely utilized ligands in catalysis, have attracted considerable research interest in the past half-century. However, the development of broadly applicable chiral pyridine units (CPUs) has been plagued by several intertwining challenges, thus delaying advancements in many asymmetric reactions.This Account aims to summarize the recent progress in new CPU-containing ligands, focusing on a rationally designed, modular, and tunable CPU developed in our laboratory. A significant problem thwarting conventional designs is the paradox between broad reactivity and stereoselectivity; that is, while enhanced stereoselectivity may be achieved by introducing chiral elements close to the N atom, the concomitant increase in local steric hindrance often limits catalytic activity and scope. Our newly developed CPU features a rigid [6-5-3] fused-ring framework and a tunable spirocyclic ketal side wall. The well-defined three-dimensional structure minimizes local (inner layer) steric hindrance and tunes the peripheral environment (outer layer) by remote substituents, thus securing reactivity and stereoselectivity. Different chelating ligands were readily assembled using this chiral structural module, with applications in mechanistically diverse transition-metal-catalyzed reactions. Thus, a series of chiral 2,2'-bipyridine ligands were successfully employed in the development of a general, efficient, and highly enantioselective nickel-catalyzed intermolecular reductive addition, Ullmann coupling of ortho-chlorinated aryl aldehydes, and carboxylation of benzylic (pseudo)halides with CO2. Notably, these chiral 2,2'-bipyridine ligands exhibited superior catalytic activity in the reactions compared to common N-based ligands. In addition, highly enantioselective iridium-catalyzed C-H borylation was developed using a CPU-containing N,B-bidentate ligand. Furthermore, mechanistically challenging, additive-free, and broad-scope transfer hydrogenative direct asymmetric reductive amination was achieved using a half-sandwich iridium catalyst supported by a chiral N,C-bidentate ligand. The new ligands demonstrated excellent performance in securing high catalytic activity and stereoselectivity, which, when combined with experimental and computational mechanistic investigations, supported the "double-layer control" design concept.Considering the broad applications of pyridine-derived ligands, the research progress described herein should inspire the creation of novel chiral catalysts and drive the development of many catalytic asymmetric reactions.

Design and synthesis of a new family of planar and central chiral ferrocenyl phosphine ligands

A family of planar chiral indene-fused ferrocenes were prepared through an intramolecular asymmetric C-H arylation in excellent yields (up to 99%) with excellent enantioselectivities (up to 99% ee). They were thereafter successfully transformed to chiral ferrocenyl phosphines, featuring both planar and central chiralities, in good yields (up to 83%) and excellent diastereoselectivities (up to 99% de) through highly diastereoselective phosphination. This protocol offers a general method for planar and central chiral ferrocenyl phosphines. The potential applications of the newly developed ligands were demonstrated by a Pd-catalyzed enantioselective allylic alkylation reaction, in which high enantioselectivity (92% ee) and good yield (89%) were obtained.

Enantioselective Assembly of Ferrocenes with Axial and Planar Chiralities via Palladium/Chiral Norbornene Cooperative Catalysis

The preparation of ferrocenes with both axial and planar chiralities poses a considerable challenge. Herein, we report a strategy for the construction of both axial and planar chiralities in a ferrocene system via palladium/chiral norbornene (Pd/NBE*) cooperative catalysis. In this domino reaction, the first established axial chirality is dictated by Pd/NBE* cooperative catalysis, while the latter planar chirality is controlled by the preinstalled axial chirality through a unique axial-to-planar diastereoinduction process. This method exploits readily available ortho-ferrocene-tethered aryl iodides (16 examples) and the bulky 2,6-disubstituted aryl bromides (14 examples) as the starting materials. Five- to seven-membered benzo-fused ferrocenes with both axial and planar chiralities (32 examples) are obtained in one step with constantly high enantioselectivities (>99% e.e.) and diastereoselectivities (>19:1 d.r.).

Ferrocene derivatives with planar chirality and their enantioseparation by liquid-phase techniques

In the last decade, planar chiral ferrocenes have attracted a growing interest in several fields, particularly in asymmetric catalysis, medicinal chemistry, chiroptical spectroscopy and electrochemistry. In this frame, the access to pure or enriched enantiomers of planar chiral ferrocenes has become essential, relying on the availability of efficient asymmetric synthesis procedures and enantioseparation methods. Despite this, in enantioseparation science, these metallocenes were not comprehensively explored, and very few systematic analytical studies were reported in this field so far. On the other hand, enantioselective high-performance liquid chromatography has been frequently used by organic and organometallic chemists in order to measure the enantiomeric purity of planar chiral ferrocenes prepared by asymmetric synthesis. On these bases, this review aims to provide the reader with a comprehensive overview on the enantioseparation of planar chiral ferrocenes by discussing liquid-phase enantioseparation methods developed over time, integrating this main topic with the most relevant aspects of ferrocene chemistry. Thus, the main structural features of ferrocenes and the methods to model this class of metallocenes will be briefly summarized. In addition, planar chiral ferrocenes of applicative interest as well as the limits of asymmetric synthesis for the preparation of some classes of planar chiral ferrocenes will also be discussed with the aim to orient analytical scientists towards 'hot topics' and issues which are still open for accessing enantiomers of ferrocenes featured by planar chirality.

Enantioseparation of planar chiral ferrocenes on cellulose-based chiral stationary phases: Benzoate versus carbamate pendant groups

In this study, the enantioseparation of 14 planar chiral ferrocenes containing halogen atoms, and methyl, iodoethynyl, phenyl, and 2-naphthyl groups, as substituents, was explored with a cellulose tris(4-methylbenzoate) (CMB)-based chiral column under multimodal elution conditions. n-Hexane/2-propanol (2-PrOH) 95:5 v/v, pure methanol (MeOH), and MeOH/water 90:10 v/v were used as mobile phases (MPs). With CMB, baseline enantioseparations were achieved for nine analytes with separation factors (α) ranging from 1.24 to 1.77, whereas only three analytes could be enantioseparated with 1.14 ≤ α ≤ 1.51 on a cellulose tris(3,5-dimethylphenylcarbamate) (CDMPC)-based column, used as a reference for comparison, under the same elution conditions. Pendant group-dependent reversal of the enantiomer elution order was observed in several cases by changing CMB to CDMPC. The impact of analyte and chiral stationary phase (CSP) structure, and MP polarity on the enantioseparation, was evaluated. The two cellulose-based CSPs featured by different pendant groups were also compared in terms of thermodynamics. For this purpose, enthalpy (ΔΔH°), entropy (ΔΔS°) and free energy (ΔΔG°) differences, isoenantioselective temperatures (T_iso ), and enthalpy/entropy ratios (Q), associated with the enantioseparations, were derived from van 't Hoff plots by using n-hexane/2-PrOH 95:5 v/v and methanol/water 90:10 v/v as MPs. With the aim to disclose the functions of the different substituents in mechanisms and noncovalent interactions underlying analyte-selector complex formation at molecular level, electrostatic potential (V) analysis and molecular dynamics simulations were used as computational techniques. On this basis, enantioseparations and related mechanisms were investigated by integrating theoretical and experimental data.

Enantioselective Synthesis of Both Axially and Planar Chiral Ferrocenes via Axial-to-Planar Diastereoinduction

Ferrocenes with planar chirality have emerged as an important class of scaffolds for ligands in asymmetric catalysis; however, ferrocene molecules with polychiral structures have not been well explored. Herein, both axially and planar chiral ferrocenes were synthesized via palladium/chiral norbornene cooperative catalysis and axial-to-planar diastereoinduction. In this work, chiral norbornene was used to stereoselectively control the aromatic axial chirality, and further selectivity induced C(sp²)-H activation for ferrocene planar chirality. Based on density functional theory calculations, the catalytic model of chiral norbornene with the substrate and the axial-to-planar diastereoinduction process were confirmed.

Synthesis of Planar Chiral 2-Aryl Aroylferrocenes via Palladium-Catalyzed C-C Bond-Cleavage/Ring-Opening Reaction

A palladium-catalyzed ring-opening reaction of optically active ferrocenyl tertiary alcohols for the construction of planar chiral ketones is reported. The stereochemistry of the hydroxyl group in ferrocenyl alcohols markedly affects reaction: ferrocenyl alcohols with a β-hydroxyl group show better reactivity and chemoselectivity than the corresponding α-hydroxyl analogues. The treatment of α-hydroxyl substrates with trifluoroacetic acid successfully realizes the inversion of the orientation of hydroxyl group to the corresponding β-analogues.

Chiral Catalysts for Pd0 -Catalyzed Enantioselective C-H Activation

In the past few decades, processes that involve transition-metal catalysis have represented a major part of the synthetic chemist's toolbox. Recently, the interest has shifted from the well-established cross-coupling reactions to C-H bond functionalization, thus making it a current frontier of transition-metal-catalyzed reactions. Constant progress in this field has led to the discovery of enantioselective methods to generate and control various types of stereogenic elements, thereby demonstrating its high value to generate scalemic chiral molecules. The present review is dedicated to enantioselective Pd⁰ -catalyzed C-H activation, which may be considered as an evolution of Pd⁰ -catalyzed cross-couplings, with a focus on the different chiral ligands and catalysts that enable these transformations.

Enantioselective Twofold C-H Annulation of Formamides and Alkynes without Built-in Chelating Groups

Twofold C-H annulation of readily available formamides and alkynes without built-in chelating groups was achieved. Ni-Al bimetallic catalysis enabled by a bulky BINOL-derived chiral secondary phosphine oxide (SPO) ligand proved to be critical for high reactivity and high selectivity. This reaction uses readily available formamides as starting materials and provides a concise synthetic pathway to a broad range of chiral ferrocenes in 40-98 % yield and 93-99 % ee.

Enantioselective Synthesis of Tertiary α,α-Diaryl Carbonyl Compounds Using Chiral N,N'-Dioxides under Umpolung Conditions

Brønsted acid-catalyzed addition of the chiral N,N'-dioxide into ynamides generated enolonium ions in situ which underwent enantioselective alkylation by indoles, pyrroles, and phenols, without racemization of the formed tertiary center. This external oxidant approach allows for the use of unmodified nucleophiles and does not leave trace groups from the oxidant, which significantly increases the synthetic efficiency and the product diversity. Furthermore, the byproduct of the N,N'-dioxide could be efficiently recycled into an optically pure form.

Synthesis of planar chiral ferrocenes via a Pd(0)-catalyzed syn-carbopalladation/asymmetric C–H alkenylation process

The Pd(0)-catalyzed tandem intermolecular syn-carbopalladation/asymmetric C–H alkenylation reaction of N-ferrocenyl propiolamides with aryl iodides has been realized, generating planar chiral ferrocene[1,2-d] pyrrolinones in good yields.

Catalytic Asymmetric C-H Arylation of (η6-Arene)Chromium Complexes: Facile Access to Planar-Chiral Phosphines

A catalytic asymmetric direct C-H arylation of (η6-arene)chromium complexes to obtain planar-chiral compounds is reported. The use of the hemilabile ligand H8-BINAP(O) is key to providing high enantioselectivity in this transformation. We show that this methodology opens the door to the synthesis of a variety of planar-chiral chromium derivatives which can be easily transformed into planar chiral mono- or diphosphines. Mechanistic studies, including synthesis and characterization of Pd and Ag complexes and their detection in the reaction mixture, suggest a Pd-catalyzed/Ag-promoted catalytic system where Ag carries out the C-H activation step.

Enantioselective Synthesis of Planar Chiral Pyridoferrocenes via Palladium-Catalyzed Imidoylative Cyclization Reactions

A highly efficient synthesis of planar chiral pyrido[3,4- b] ferrocenes by a palladium-catalyzed enantioselective isocyanide insertion/desymmetric C(sp²)-H bond activation reaction was developed. Various planar chiral pyridoferrocenes were obtained in high yields with good to excellent enantioselectivity under mild conditions (up to 99% yield, 99% ee), enabled by a unique SPINOL-derived phosphoramidite ligand.

Intramolecular Direct C-H Arylation via a Metallocenic Radical Pathway: Stereospecific Approach to Planar-Chiral Ferrocenes

Transition metal-free synthesis of planar-chiral 1,2-fused ferrocenes via intramolecular direct C-H bond arylation was achieved. The C-H arylation reactions promoted by a catalytic amount of 1,10-phenanthroline highlighted a unique planar-chiral metallocenic radical intermediate, generated from iodoferrocenes via a single-electron transfer process.

Enantioselective C-H Functionalization-Addition Sequence Delivers Densely Substituted 3-Azabicyclo[3.1.0]hexanes

An enantioselective C-H functionalization route to perfluoroalkyl-containing 3-azabicyclo[3.1.0]hexanes is disclosed. A modular and bench-stable diazaphospholane ligand enables highly enantioselective Pd(0)-catalyzed cyclopropane C-H functionalization using trifluoroacetimidoyl chlorides as electrophilic partners. In turn, the resulting cyclic ketimine products react smoothly with a broad variety of nucleophiles in one-pot processes enabling the rapid and modular construction of heavily substituted pyrrolidines.

P(O)R2-Directed Enantioselective C-H Olefination toward Chiral Atropoisomeric Phosphine-Olefin Compounds

An effective synthesis of chiral atropoisomeric biaryl phosphine-olefin compounds via palladium-catalyzed enantioselective C-H olefination has been developed for the first time. The reactions are operationally simple, tolerate wide functional groups, and have a good ee value. Notably, P(O)R₂ not only acts as the directing group to direct C-H activation in order to make a useful ligand but also serves to facilitate composition of the product in a useful manner in this transformation.

Synthesis of Planar Chiral Ferrocenes via Transition-Metal-Catalyzed Direct C-H Bond Functionalization

Ferrocenes are of great interest in the fields of materials science, organic synthesis, and biomedical research. Of particular significance is the fact that ferrocenes bearing planar chirality have been demonstrated to be highly efficient ligands or catalysts in asymmetric catalysis, some of which have been employed in the industrial synthesis of pharmaceuticals and agrochemicals. So far, the main methods for the synthesis of planar chiral ferrocenes involve diastereoselective directed ortho-metalation (DoM), enantioselective DoM, and chiral resolution. Despite the fact that these approaches are well developed and widely applied, the use of chiral auxiliaries or external stoichiometric chiral bases is required in most cases. Additionally, the practicality of these processes is hampered by the requirement of sensitive organometallic reagents, the poor compatibility with functional groups, and the low atom economy in some cases. Therefore, the development of highly efficient strategies to introduce planar chirality on the backbone of ferrocene that do not possess these limitations is highly desirable. Meanwhile, transition-metal-catalyzed asymmetric C-H bond functionalization reactions have attracted much attention over the past few years owing to their emerging potential for providing a straightforward approach for the preparation of chiral molecules. In addition to the majority of the work focusing on the installation of central chirality, methods for the catalytic asymmetric synthesis of planar chiral compounds via C-H bond functionalization have also been explored. In this Account, we summarize our recent efforts aimed at the development of novel methods to synthesize planar chiral compounds via asymmetric C-H bond functionalization and also highlight related achievements by other groups. First, we briefly introduce the precedent examples of diastereoselective and enantioselective synthesis of planar chiral ferrocenes. Subsequently, asymmetric syntheses of structurally diverse planar chiral ferrocenes via Pd [Pd(II), Pd(0)]-, Ir-, Rh-, Au-, and Pt-catalyzed C-H bond functionalization are described. These methods have impressive advantages over traditional approaches for the synthesis of functionalized planar chiral ferrocenes in terms of both step- and atom-economies. Notably, the products of these processes are easily transformed into a variety of new catalysts or ligands, which have been demonstrated to promote efficient asymmetric reactions. Moreover, DFT calculations have been conducted to explore the origin of the excellent enantioselectivity of Pd-catalyzed enantioselective C-H bond functionalization reactions. Progress made in the area of asymmetric C-H bond functionalization provides an effective platform for the design and synthesis of planar chiral ferrocenes.