Cobalt-Catalyzed Domino Transformations via Enantioselective C-H Activation/Nucleophilic [3 + 2] Annulation toward Chiral Bridged Bicycles

Selective synthesis of chiral bridged (hetero)bicyclic scaffolds via asymmetric C-H activation constitutes substantial challenges due to the multiple reactivities of strained bicyclic structures. Herein, we develop the domino transformations through an unprecedented cobalt-catalyzed enantioselective C-H activation/nucleophilic [3 + 2] annulation with symmetrical bicyclic alkenes. The methods offer straightforward access to a wide range of chiral molecules bearing [2.2.1]-bridged bicyclic cores with four and five consecutive stereocenters in a single step. Two elaborate salicyloxazoline (Salox) ligands were synthesized based on the rational design and mechanistic understanding. The well-defined chiral pockets generated from asymmetric coordination around the trivalent cobalt catalyst direct the orientation of bicyclic alkenes, leading to excellent enantioselectivity.

Copper-catalyzed asymmetric allylic substitution of racemic/meso substrates

The synthesis of enantiomerically pure compounds is a pivotal subject in the field of chemistry, with enantioselective catalysis currently standing as the primary approach for delivering specific enantiomers. Among these strategies, Cu-catalyzed asymmetric allylic substitution (AAS) is significant and irreplaceable, especially when it comes to the use of non-stabilized nucleophiles (pK a > 25). Although Cu-catalyzed AAS of prochiral substrates has also been widely developed, methodologies involving racemic/meso substrates are highly desirable, as the substrates undergo dynamic processes to give single enantiomer products. Inspired by the pioneering work of the Alexakis, Feringa and Gennari groups, Cu-catalyzed AAS has been continuously employed in deracemization and desymmetrization processes for the synthesis of enantiomerically enriched products. In this review, we mainly focus on the developments of Cu-catalyzed AAS with racemic/meso substrates over the past two decades, providing an explicit outline of the ligands employed, the scope of nucleophiles, the underlying dynamic processes and their practical applications.

Enantioselective Palladium(II)-Catalyzed Desymmetrizative Coupling of 7-Azabenzonorbornadienes with Alkynylanilines

A PdII -catalyzed, domino enantioselective desymmetrizative coupling of 7-azabenzonorbornadienes with alkynylanilines is disclosed herein. This operationally simple transformation generates three covalent bonds and two contiguous stereocenters with excellent enantio- and diastereo-selectivity. The resulting functionalized indole-dihydronaphthalene-amine conjugates served as an appealing platform to streamline the diversity-oriented synthesis (DOS) of other valuable enantioenriched compounds. DFT calculations revealed that the two stabilizing non-covalent interactions contributed to the observed enantioselectivity.

Catalytic Asymmetric Ring-Opening Reactions of Unstrained Heterocycles Using Cobalt Vinylidenes

Cobalt catalysts promote highly enantioselective ring-opening reactions of 2,5-dihydrofurans using vinylidenes. The products are acyclic organozinc compounds that can be functionalized with an electrophile. The proposed mechanism involves the generation of a cobalt vinylidene species that adds to the alkene by a [2+2]-cycloaddition pathway. Ring-opening then occurs via outer-sphere β-O elimination assisted by coordination of a ZnX2 Lewis acid to the alkoxide leaving group. DFT models reveal that competing inner-sphere syn β-H and β-O elimination pathways are suppressed by the geometric constraints of the metallacycle intermediate. These models rationalize the observed stereochemical outcome of the reaction.

Asymmetric alkene-alkene reductive cross-coupling reaction via visible-light photoredox/cobalt dual catalysis

The first example of asymmetric alkene-alkene reductive coupling is demonstrated via visible-light-fueled photoredox/cobalt dual catalysis. The desymmetrization reaction provided products (>20 examples) with up to five chiral centers in single-step operation in up to 95% yields with very high relative (>99 : 1 dr) and absolute stereochemistry (up to 98 : 2 er) control. The preliminary mechanistic investigations suggested that the critical mechanistic steps involved light-mediated controlled low-valent cobalt complex generation, oxidative ene-ene cyclization, and protonation.

Palladium-catalyzed stereocontrolled ring-opening of 7-oxabenzonorbornadienes with organic carboxylic acids

A palladium-catalyzed tandem reaction of 7-oxabenzonorbornadienes with organic carboxylic acids that provides cis-1,2-dihydro-l-naphthyl derivatives in moderate to good yields in a highly diastereoselective manner is described. A possible reaction mechanism involving syn acylation followed by insertion and diastereoselective ring-opening with 7-oxabenzonorbornadienes is proposed to account for the reaction.

Nickel-Catalyzed Diastereoselective Cross-Electrophile Ring Opening of 7-Oxabenzonorbornadienes with Aromatic Aldehydes

In this protocol, we developed a nickel-catalyzed diastereoselective cross-electrophile ring opening reaction of 7-oxabenzonorbornadienes with aromatic aldehydes as the electrophilic coupling partner utilizing Zn as the stoichiometric reductant. In this reaction, a challenging stereoselective bond formation between two disubstituted sp³-hybridized carbon centers has been achieved, furnishing a variety of 1,2-dihydronaphthalenes with full diastereocontrol of three consecutive stereogenic centers.

A synergistic Rh(I)/organoboron-catalysed site-selective carbohydrate functionalization that involves multiple stereocontrol

Site-selective functionalization is a core synthetic strategy that has broad implications in organic synthesis. Particularly, exploiting chiral catalysis to control site selectivity in complex carbohydrate functionalizations has emerged as a leading method to unravel unprecedented routes into biologically relevant glycosides. However, robust catalytic systems available to overcome multiple facets of stereoselectivity challenges to this end still remain scarce. Here we report a synergistic chiral Rh(I)- and organoboron-catalysed protocol, which enables access into synthetically challenging but biologically relevant arylnaphthalene glycosides. Our method depicts the employment of chiral Rh(I) catalysis in site-selective carbohydrate functionalization and showcases the utility of boronic acid as a compatible co-catalyst. Crucial to the success of our method is the judicious choice of a suitable organoboron catalyst. We also determine that exquisite multiple aspects of stereocontrol, including enantio-, diastereo-, regio- and anomeric control and dynamic kinetic resolution, are concomitantly operative.

Cobalt(III)-catalyzed asymmetric ring-opening of 7-oxabenzonorbornadienes via indole C-H functionalization

Asymmetric ring-opening of 7-oxabenzonorbornadienes is achieved via Co-catalyzed indole C-H functionalization. The utilization of chiral Co-catalyst consisting of a binaphthyl-derived trisubstituted cyclopentadienyl ligand resulted in high yields (up to 99%) and excellent enantioselectivity (>99% ee) for the target products with tolerance for diverse functional groups. Opposite diastereoselectivities are obtained with chiral Co-catalyst or Cp*CoI₂CO. Combined experimental and computational studies suggest β-oxygen elimination being the selectivity-determining step of the reaction. Meanwhile, the reactions of 7-azabenzonorbornadiene could also be executed in a diastereodivergent manner.

Diastereodivergent Desymmetric Annulation to Access Spirooxindoles: Chemical Probes for Mitosis

Spirooxindoles have emerged as promising architectures for engineering biologically active compounds. The diastereodivergent construction of unique scaffolds of this type with full control of continuous chiral centers including an all-carbon quaternary stereogenic center is yet to be developed. Here, we report an unprecedented diastereodivergent desymmetric [3 + 3] annulation of oxabicyclic alkenes with enals enabled by N-heterocyclic carbene (NHC)/Rh cooperative catalysis, leading to a series of enantiomerically enriched spirooxindole lactones with excellent enantioselectivities (up to >99% ee) and diastereoselectivities (up to >95:5 dr). The combined catalyst system comprises a rhodium complex that controls the configuration at the electrophilic carbon and an NHC catalyst that controls the configuration at the nucleophilic oxindole-containing carbon; thus, four stereoisomers of the spirooxindole products can be readily obtained simply by switching the configurations of the two chiral catalysts. Transformations of the chiral spirooxindoles delivered synthetically useful compounds. Importantly, those chiral spirooxindoles arrested mammalian cells in mitosis and exhibited potent antiproliferative activities against HeLa cells. Significantly, both absolute and relative configurations exert prominent effects on the bioactivities, underscoring great importance of catalytic asymmetric diastereodivergent synthesis beyond creating useful tools for the exploration of structure-activity relationships.

Additive-free synthesis of fused tricyclic cyanoisoxazolidines using in situ formed cyanonitrones

Herein, we disclose the first report on the generation of cyanonitrone in situ from diazoacetonitrile and nitrosoarene, and its subsequent [3+2] cycloaddition with oxabicyclic alkenes to access fused tricyclic cyanoisoxazolidines. Further, this methodology could be extended to access fused tricyclic trifluoromethylated and phosphonylated isoxazolidines. Surprisingly, the reductive ring-opening of cyanoisoxazolidines was followed by a spontaneous lactonization to produce fused tricyclic amino lactones. Moreover, the N-O bond of the obtained tricyclic trifluoromethylated isoxazolidines could be cleaved to obtain 1,3-amino alcohols.

Transition-metal-catalyzed domino reactions of strained bicyclic alkenes

This review presents a comprehensive overview of transition-metal-catalyzed domino reactions of strained bicyclic alkenes, including both homo- and heterobicyclic alkenes. These compounds are important synthons in organic synthesis, providing an important platform for the construction of biologically/medicinally significant compounds which bear multiple stereocenters. The review has been divided according to the metal used in the reaction. An overview of the substrate scope, reaction conditions, and their potential applications in organic synthesis is discussed. A comprehensive outlook on the reactivity paradigms of homo- and heterobicyclic alkenes is discussed and should shed light on future directions for further development in this field.

C-H bond activation and sequential addition to two different coupling partners: a versatile approach to molecular complexity

Sequential multicomponent C-H bond addition is a powerful approach for the rapid, modular generation of molecular complexity in a single reaction. In this approach, C-H bonds are typically added across π-bonds or π-bond isosteres, followed by subsequent coupling to another type of functionality, thereby forming two σ-bonds in a single reaction sequence. Many sequential C-H bond addition reactions have been developed to date, including additions across both conjugated and isolated π-systems followed by coupling with reactants such as carbonyl compounds, cyanating reagents, aminating reagents, halogenating reagents, oxygenating reagents, and alkylating reagents. These atom-economical reactions transform ubiquitous C-H bonds under mild conditions to more complex structures with a high level of regiochemical and stereochemical control. Surprising connectivities and diverse mechanisms have been elucidated in the development of these reactions. Given the large number of possible combinations of coupling partners, there are enormous opportunities for the discovery of new sequential C-H bond addition reactions.

Scandium-Catalyzed Benzylic C(sp3)-H Alkenylation of Tertiary Anilines with Alkynes

This work describes the chemo- and stereoselective benzylic C(sp³)-H alkenylation of tertiary ortho-methyl anilines with internal alkynes using a simple β-diketiminato scandium catalyst. This protocol offers an efficient method for the synthesis of a new family of tertiary ortho-allylanilines in high yields. The resultant alkenylation products facilely underwent further chemical transformation to other valuable anilines. A cationic scandium benzyl species was isolated from a stoichiometric reaction and confirmed to be the catalytic intermediate.

Catalytic Asymmetric β‐Oxygen Elimination**

A catalytic enantioselective β‐O‐elimination reaction is reported in the form of a zirconium‐catalyzed asymmetric opening of meso‐ketene acetals. Furthermore, a regiodivergent β‐O‐elimination is demonstrated. The reaction proceeds under mild conditions, at low catalyst loadings, and produces chiral monoprotected cis‐1,2‐diols in good yield and enantiomeric excess. The combination with a Mitsunobu reaction or a one‐pot hydroboration/Suzuki reaction sequence then gives access to additional diol and aminoalcohol building blocks. A stereochemical analysis supported by DFT calculations reveals that a high selectivity in the hydrozirconation step is also important for achieving high enantioselectivity, although it does not constitute the asymmetric step. This insight is crucial for the future development of related asymmetric β‐elimination reactions.

Iridium-catalyzed hydroacylation reactions of C1-substituted oxabenzonorbornadienes with salicylaldehyde: an experimental and computational study

An experimental and theoretical investigation on the iridium-catalyzed hydroacylation of C₁-substituted oxabenzonorbornadienes with salicylaldehyde is reported. Utilizing commercially available [Ir(COD)Cl]₂ in the presence of 5 M KOH in dioxane at 65 °C, provided a variety of hydroacylated bicyclic adducts in up to a 95% yield with complete stereo- and regioselectivity. The mechanism and origins of selectivity in the iridium-catalyzed hydroacylation reaction has been examined at the M06/Def2TZVP level of theory. The catalytic cycle consists of three key steps including oxidative addition into the aldehyde C–H bond, insertion of the olefin into the iridium hydride, and C–C bond-forming reductive elimination. Computational results indicate the origin of regioselectivity is involved in the reductive elimination step.

Copper-Catalyzed Hydroamination of Oxa- and Azabenzonorbornadienes with Pyrazoles

An efficient and highly chemo- and stereoselective copper-catalyzed hydroamination of oxa- and azabenzonorbornadienes with various pyrazole derivatives is described. This catalytic process is promoted by the presence of N-heterocyclic carbene ligands and KOt-Bu under mild and simple reaction conditions, and allows for the direct synthesis of new and versatile functionalized oxa(aza)benzonorbornyl pyrazoles starting from readily available oxa(aza)bicyclic alkenes. The synthetic utility of this method was demonstrated by the transformation of the obtained products into pyrazolyl-substituted naphthalenes.

A Short Total Synthesis of Benzophenanthridine Alkaloids via a Rhodium(III)-Catalyzed C-H Ring-Opening Reaction

The biologically important naturally available benzophenanthridines were prepared efficiently in three steps with overall good yields. A new synthetic methodology involving a rhodium(III) catalyzed redox-neutral ring-opening of 7-azabenzonorbornadienes with aromatic aldoximes is developed to synthesize the target molecules. The developed C-H ring-opening reaction is highly diastereoselective and compatible with various sensitive functional group substituted aromatic aldoximes as well as substituted 7-azabenzonorbornadienes. The ring-opening products were transformed into highly sensitive 13,14-dehydrobenzo phenanthridine derivatives by HCl hydrolysis. Subsequently, 13,14-dehydrobenzophenanthridines were converted into biologically important benzophenanthridine alkaloids in the presence of DDQ. A possible reaction mechanism was proposed for the C-H ring-opening reaction and supported by the deuterium labeling studies.

Benzocarbazole Synthesis via Visible-Light-Accelerated Rh(III)-Catalyzed C-H Annulation of Aromatic Amines with Bicyclic Alkenes

A visible-light-accelerated Rh(III)-catalyzed C-H annulation of aromatic amines with bicyclic alkenes for the synthesis of benzocarbazole derivatives was developed. In this approach, with the cooperation of rhodium catalysis and visible-light irradiation, various aromatic amines reacted with oxabicyclic alkenes and azabicyclic alkenes smoothly at room temperature, delivering the corresponding bridged oxa or aza tetrahydro benzocarbazoles in good to excellent yields. Moreover, a series of benzo[b]carbazoles were synthesized conveniently through further aromatization in one pot. The potential of this method was demonstrated via directing-group removal, derivatization, a scale-up reaction, and fluorescence investigations.

Iron-catalysed enantioselective carbometalation of azabicycloalkenes

The first enantioselective carbometalation reaction of azabicycloalkenes has been achieved by iron catalysis to in situ form optically active organozinc intermediates, which are amenable to further synthetic elaborations. The observed chiral induction, along with the DFT and XAS analyses, reveals the direct coordination of the chiral phosphine ligand to the iron centre during the carbon-carbon and carbon-metal bond forming step. This new class of iron-catalysed asymmetric reaction will contribute to the synthesis and production of bioactive molecules.

Transition-Metal-Catalyzed Directing Group Assisted (Hetero)aryl C-H Functionalization: Construction of C-C/C-Heteroatom Bonds

Transition-metal-catalyzed C-H functionalization is one of the fascinating scientific fronts in organic synthesis for the formation of conjugated arenes and has emerged as a benchmark to revolutionize the synthetic enterprise since past decades. In this realm, chelation-guided functionalization of C-H bonds using an exogenous directing group has received considerable attention recently for the expedient regioselective construction of C-C and C-heteroatom bonds as an efficient and sustainable alternative. This article outlines our contribution towards a wide variety of transformations that have been achieved by the directed C-H functionalization through the fine tuning of catalytic systems.

Recent Advances in Transition-Metal-Catalyzed C–H Functionalization Reactions Involving Aza/Oxabicyclic Alkenes

Bicyclic alkenes, including oxa- and azabicyclic alkenes, readily undergo activation with facial selectivity in the presence of transition-metal complexes. This is due to the intrinsic angle strain on the carbon–carbon double bonds in such unsymmetrical bicyclic systems. During the past decades considerable progress has been made in the area of ring opening of bicyclic strained rings by employing the concept of C–H activation. This short review comprehensively compiles the various C–H bond activation assisted reactions of oxa- and azabicyclic alkenes, viz., ring-opening reactions, hydroarylation, and annulation reactions.

1 Introduction

2 Reactions of Heterobicyclic Ring Systems

2.1 Ring-Opening Reactions of Oxa- and Azabenzonorbornadienes

2.1.1 Reactions Using 7-Oxabenzonorbornadienes

2.1.2 Reactions Using 7-Azabenzonorbornadienes

2.2 Hydroarylation Reactions

2.3 Annulation Reactions

2.4 Other Reactions

3 Conclusion