Fast Enantio‐ and Chemoselective Arylation of Ketones with Organoboronic Esters Enabled by Nickel/N‐Heterocyclic Carbene Catalysis

Nickel-Catalyzed Enantioselective Carbonyl Addition of Aryl Chlorides and Bromides to Aldehydes

The Ni-catalyzed enantioselective addition reaction of aryl halides to aldehydes was studied with cyanobis(oxazoline) as chiral ligands and Mn as reductant. Aryl and heteroaryl bromides reacted with phenyl aldehyde at room temperature to produce dibenzyl alcohols in 16-99 % yields with 53-92 % ees. Moreover, the coupling of phenyl chloride with a variety of aryl, heteroaryl and alkyl aldehydes was demonstrated in the presence of cyanobis(oxazoline)/Ni(II) at 60 °C in generally high yields with moderate enantioselectivities.

Nickel-Catalyzed Enantioselective Reductive Arylation of Common Ketones

A nickel complex of chiral bisoxazolines catalyzed the stereoselective reductive arylation of ketones in high enantioselectivity. A range of common acyclic and cyclic ketones reacted without the aid of directing groups. Mechanistic studies using isolated complex of a chiral bis(oxazoline) (L)Ni(Ar)Br revealed that Mn reduction was not needed, while Lewis acidic titanium alkoxides were critical to ketone insertion.

Nickel-Catalyzed Asymmetric Propargyl-Aryl Cross-Electrophile Coupling

Performing asymmetric cross-coupling reactions between propargylic electrophiles and aryl nucleophiles is a well-established method to build enantioenriched benzylic alkynes. Here, a catalytic enantioselective propargyl-aryl cross-coupling between two electrophiles was achieved for the first time in a stereoconvergent manner. Propargylic chlorides were treated with aryl iodides as well as heteroaryl iodides in the presence of a chiral nickel complex, and manganese metal was used as a stoichiometric reductant, allowing for the construction of a propargyl C-aryl bond under mild conditions. An alternative dual nickel/photoredox catalytic protocol was also developed for this cross-electrophile coupling in the absence of a metal reductant. The potential utility of this conversion is demonstrated in the facile construction of stereoenriched bioactive molecule derivatives and medicinal compounds based on the diversity of acetylenic chemistry. Detailed experimental studies have revealed the key mechanistic features of this transformation.

Highly efficient α-arylation of aryl ketones with aryl chlorides by using bulky imidazolylidene-ligated oxazoline palladacycles

α-Aryl derivatives of carbonyl compounds are important building blocks. Herein, we presented an efficient catalytic system for the α-arylation of aryl ketones with inactive aryl chlorides by firstly using N,N'-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr)-ligated chiral oxazoline palladacycles, and tolerated a wide range of substrates at low catalyst loadings, leading to the desired products in good to excellent yields.

Dynamic kinetic asymmetric arylation and alkenylation of ketones

Despite the importance of enantioenriched alcohols in medicinal chemistry, total synthesis, and materials science, the efficient and selective construction of enantioenriched tertiary alcohols bearing two contiguous stereocenters has remained a substantial challenge. We report a platform for their preparation through the enantioconvergent, nickel-catalyzed addition of organoboronates to racemic, nonactivated ketones. We prepared several important classes of α,β-chiral tertiary alcohols in a single step with high levels of diastereo- and enantioselectivity through a dynamic kinetic asymmetric addition of aryl and alkenyl nucleophiles. We applied this protocol to modify several profen drugs and to rapidly synthesize biologically relevant molecules. We expect this nickel-catalyzed, base-free ketone racemization process to be a widely applicable strategy for the development of dynamic kinetic processes.

Enantioselective rhodium-catalyzed addition of arylboronic acids to N-heteroaryl ketones: synthesis of α-hydroxy acids

The enantioselective addition of arylboronic acids to N-heteroaryl ketones provides a convenient access to chiral α-heteroaryl tertiary alcohols, yet addition reactions of this type have been challenging due to catalyst deactivation. In this report, an efficient rhodium-catalyzed addition of arylboronic acids to N-heteroaryl ketones is established, affording a variety of valuable α-heteroaryl alcohols with excellent functional group compatibility. The employment of the WingPhos ligand containing two anthryl groups is crucial for this transformation. In particular, a range of chiral benzoxazolyl-substituted tertiary alcohols were formed with excellent ee values and yields by employing a Rh loading as low as 0.3 mol%, which can serve as a practical protocol to furnish a series of chiral α-hydroxy acids after hydrolysis.

C2-Symmetric N-Heterocyclic Carbenes in Asymmetric Transition-Metal Catalysis

The last decades have witnessed a rapid growth of applications of N-heterocyclic carbenes (NHCs) in different chemistry fields. Due to their unique steric and electronic properties, NHCs have become a powerful tool in coordination chemistry, allowing the preparation of stable metal-ligand frameworks with both main group metals and transition metals. An overview on the use of five membered monodentate C2-symmetric N-heterocyclic carbenes (NHCs) as ligands for transition-metal complexes and their most relevant applications in asymmetric catalysis is offered.

Enantio- and Regioselective Ni-Catalyzed para-C-H Alkylation of Pyridines with Styrenes via Intermolecular Hydroarylation

Direct asymmetric functionalization of the pyridyl C-H bond represents a longstanding challenge in organic chemistry. We herein describe the first enantioselective para-C-H activation of pyridines through the use of a Ni-Al bimetallic catalyst system and N-heterocyclic carbene (NHC) ligand for intermolecular hydroarylation of styrenes. The reaction procceds in high to excellent enantioselectivities (up to 98.5:1.5 er) and high site-selectivities for both styrene and pyridine components (up to >98:2). Consequently, a broad range of enantioenriched 1,1-diarylalkanes containing pyridine moieties could be prepared in a single step with 100% atom economy. Computational studies supported a mechanism involving a ligand-to-ligand H-transfer (LLHT) and reductive elimination sequence, with LLHT being the rate- and enantioselectivity-determining step. DFT studies indicate that the π-π stacking interaction between the NHC aryl fragment and trans-styrenes is critical for high reactivity and enantiocontrol.

Synthesis of W-Phos Ligand and Its Application in the Copper-Catalyzed Enantioselective Addition of Linear Grignard Reagents to Ketones

The asymmetric catalytic addition of linear Grignard reagents to ketones has been a long-standing challenge in organic synthesis. Herein, a novel family of PNP ligands (W-Phos) was designed and applied in copper-catalyzed asymmetric addition of linear Grignard reagents to aryl alkyl ketones, allowing facile access to versatile chiral tertiary alcohols in good to high yields with excellent enantioselectivities (up to 94 % yield, 96 % ee). The process can also be used to synthesize chiral allylic tertiary alcohols from more challenging α,β-unsaturated ketones. Notably, the potential utility of this method is demonstrated in the gram-scale synthesis and modification of various densely functionalized medicinally relevant molecules.

Photoassisted Cobalt-Catalyzed Asymmetric Reductive Grignard-Type Addition of Aryl Iodides

Grignard addition is one of the most important methods used for syntheses of alcohol compounds and has been known for over a hundred years. However, research on asymmetric catalysis relies on the use of organometallic nucleophiles. Here, we report the first visible-light-induced cobalt-catalyzed asymmetric reductive Grignard-type addition for synthesizing chiral benzyl alcohols (>50 examples, up to 99% yield, and 99% ee). This methodology has the advantages of mild reaction conditions, good functionality tolerance, excellent enantiocontrol, the avoidance of mass metal wastes, and the use of precious metal catalysts. Kinetic realization studies suggested that migratory insertion of an aryl cobalt species into the aldehyde was the rate-determining step of the reductive addition reaction.

Nickel-Catalyzed Intermolecular Asymmetric Addition of Aryl Iodides across Aldehydes

Enantioenriched alcohols comprise much of the framework of organic molecules. Here, we first report that chiral nickel complexes can catalyze the intermolecular enantioselective addition of aryl iodides across aldehydes to provide diverse optically active secondary alcohols using zinc metal as the reducing agent. This method shows a broad substrate scope under mild reaction conditions and precludes the traditional strategy through the pre-generation of organometallic reagents. Mechanistic studies indicate that an in situ formed arylnickel, instead of an arylzinc, adds efficiently to aldehydes, forming a new C-C bond and a chiral nickel alkoxide that may be turned over by zinc powder.

Rh-Catalyzed Coupling of Aldehydes with Allylboronates Enables Facile Access to Ketones

We present herein a novel strategy for the preparation of ketones from aldehydes and allylic boronicesters. This reaction involves the allylation of aldehydes with allylic boronicesters and the Rh-catalyzed chain-walking of homoallylic alcohols. The key to this successful development is the protodeboronation of alkenyl borylether intermediate via a tetravalent borate anion species in the presence of KHF 2 and MeOH. This approach features mild reaction conditions, broad substrate scope, and excellent functional group tolerance. Mechanistic studies also supported that the tandem allylation and chain-walking process was involved.

Enantioselective Ni/N-Heterocyclic Carbene-Catalyzed Redox-Economical Coupling of Aldehydes, Alkynes, and Enones for Rapid Construction of Acyclic All-Carbon Quaternary Stereocenters

Acyclic quaternary carbon stereocenters exist widely in natural products and bioactive molecules, but their enantioselective construction remains a prominent challenge. In particular, multicomponent enantioselective couplings of simple precursors to acyclic all-carbon quaternary stereocenters are very rare. We describe herein an N-heterocyclic carbene (NHC)-Ni catalyzed redox-economical three-component reaction of aldehydes, alkynes, and enones that proceeds in a highly chemo-, regio-, and enantioselective manner. A wide variety of valuable acyclic α-quaternary chiral ketones were synthesized in a single step with 100% atom economy. This reaction proceeds through the formation of a transient cyclic enolate followed by an aldol reaction/ring-opening sequence. The strategy is expected to inspire new and efficient approaches to generate other acyclic quaternary stereocenters.

Designing successful monodentate N-heterocyclic carbene ligands for asymmetric metal catalysis

Chiral ligands are of particular importance in asymmetric transition-metal catalysis. Although the development of effective chiral monodentate N-heterocyclic carbenes (NHCs) has been slow, a growing amount of papers have been published in recent years showing their unique efficiency as chiral ancillary ligands. Herein we provide an overview of NHC structures that accomplish high levels of enantioselectivity (≥90% ee) and give guidelines to their use and thoughts on the future of this field.

Enantioconvergent Arylation of Racemic Secondary Alcohols to Chiral Tertiary Alcohols Enabled by Nickel/N-Heterocyclic Carbene Catalysis

The direct upgrading reaction of simple and readily available achiral alcohols via C-H functionalization is an ideal strategy to prepare value-added chiral higher alcohols. Herein, we disclose the first enantioconvergent upgrading reaction of simple racemic secondary alcohols to enantioenriched tertiary alcohols. An N-heterocyclic carbene (NHC)-nickel catalyst was leveraged to enable this highly efficient formal asymmetric alcohol α-C-H arylation via a dehydrogenation using phenyl triflate as a mild oxidant followed by asymmetric addition of arylboronic esters to the transient ketones. Mechanistic studies and control experiments were conducted to reveal the possible reasons for the exceptional control over chemo- and enantioselectivity.

Low-Temperature Nickel-Catalyzed C-N Cross-Coupling via Kinetic Resolution Enabled by a Bulky and Flexible Chiral N-Heterocyclic Carbene Ligand

The transition-metal-catalyzed C-N cross-coupling has revolutionized the construction of amines. Despite the innovations of multiple generations of ligands to modulate the reactivity of the metal center, ligands for the low-temperature enantioselective amination of aryl halides remain a coveted target of catalyst engineering. Designs that promote one elementary reaction often create bottlenecks at other steps. We here report an unprecedented low-temperature (as low as -50 °C), enantioselective Ni-catalyzed C-N cross-coupling of aryl chlorides with sterically hindered secondary amines via a kinetic resolution process (s factor up to >300). A bulky yet flexible chiral N-heterocyclic carbene (NHC) ligand is leveraged to drive both oxidative addition and reductive elimination with low barriers and control the enantioselectivity. Computational studies indicate that the rotations of multiple σ-bonds on the C₂ -symmetric chiral ligand adapt to the changing needs of catalytic processes. We expect this design would be widely applicable to diverse transition states to achieve other challenging metal-catalyzed asymmetric cross-coupling reactions.

Regio- and Trans-Selective Ni-Catalyzed Coupling of Butadiene, Carbonyls, and Arylboronic Acids to Homoallylic Alcohols under Base-Free Conditions

We herein report a Ni-catalyzed three-component coupling of 1,3-butadiene, carbonyl compounds, and arylboronic acids as a general synthetic approach to 1,4-disubstituted homoallylic alcohols, an important class of compounds, which have previously not been straightforward to access. The reaction occurs efficiently using a Ni(cod)₂ catalyst without any external base and ligand at ambient temperature and allows a highly regioselective and trans-selective 1,4-dicarbofunctionalization of feedstock butadiene in a single operation. This simple and practical protocol could apply to a comprehensive scope of substrates. The neutral conditions show extraordinary tolerance for even highly base-sensitive functional groups.