Bidentate hydroxyalkyl NHC ligands for the copper-catalyzed asymmetric allylic substitution of allyl phosphates with Grignard reagents

Chiral NHC Ligands for Enantioselective Gold(I) Catalysis Under Aerobic Conditions: the Importance of Conformational Flexibility and Steric Hindrance of NHC Ligand on Reactivity

Gold(I) catalysis has been recognized as a valuable tool for the unique transformation of multiple carbon-carbon bonds. Enantioselective π-catalysis based on gold(I) complexes is, however, still underdeveloped due to lack of privileged ligands. Herein, we present an accessible method to a new family of stable yet catalytically active chiral NHC-Au(I)-Cl complexes. The key to preserving a simultaneous fine balance between reactivity and stability in this newly developed family appears to be sterically hindered, but conformationally flexible NHC ligands. These could be easily accessed on a multigram scale by merging sterically hindered anilines with commercially available amino alcohols and amines via a four-steps synthetic sequence without the need for chromatographic purification. Further investigations of the catalytic activity of NHC-Au-Cl complexes identified the OH functionality incorporated into the NHC core as crucial for the level of enantioselectivity as well as the TsO- anion responsible for the activation of NHC-Au(I)-Cl. Finally, NMR studies and X-ray investigations revealed for the first time that the widely accepted ion metathesis (NHC-Au-Cl to NHC-Au-OSO2 R) responsible for the activation of NHC-Au-Cl complexes does not take place (or it is very slow) in commonly used MeNO2 in contrast to DCM.

Chiral oxazolidines acting as transient hydroxyalkyl-functionalized N-heterocyclic carbenes: an efficient route to air stable copper and gold complexes for asymmetric catalysis

Optically pure oxazolidines were synthesized in nearly quantitative yields from chiral hydroxyalkyl-functionalized imidazolinium salts. Acting as transient chiral diamino N-heterocyclic carbenes (NHCs), these oxazolidines allowed the efficient formation of well-defined copper(i) and gold(i) hydroxyalkyl-NHC complexes, which could be isolated, for the first time, as air stable complexes after silica gel chromatography. Interestingly, X-ray analysis of gold complexes revealed that the hydroxyl-function is not chelated to the metal. Computational studies suggested that both cyclisation to produce oxazolidine and O–H bond elimination to form the transient carbene (prior to coordination) occur through a concerted mechanism. The novel chiral copper-catalysts, as well as oxazolidines alone (copper free), demonstrated excellent performances in asymmetric conjugate addition and allylic alkylation with high regio- and enantio-selectivities (up to 99% ee).

Well-defined chiral Cu(i) and Au(i) hydroxyalkyl NHC complexes were synthesized from oxazolidines. The copper-catalysts and oxazolidines alone (copper free), demonstrated excellent performances in asymmetric conjugate addition and allylic alkylation.

Recent Advances in Catalysis Involving Bidentate N-Heterocyclic Carbene Ligands

Since the discovery of persistent carbenes by the isolation of 1,3-di-l-adamantylimidazol-2-ylidene by Arduengo and coworkers, we witnessed a fast growth in the design and applications of this class of ligands and their metal complexes. Modular synthesis and ease of electronic and steric adjustability made this class of sigma donors highly popular among chemists. While the nature of the metal-carbon bond in transition metal complexes bearing N-heterocyclic carbenes (NHCs) is predominantly considered to be neutral sigma or dative bonds, the strength of the bond is highly dependent on the energy match between the highest occupied molecular orbital (HOMO) of the NHC ligand and that of the metal ion. Because of their versatility, the coordination chemistry of NHC ligands with was explored with almost all transition metal ions. Other than the transition metals, NHCs are also capable of establishing a chemical bond with the main group elements. The advances in the catalytic applications of the NHC ligands linked with a second tether are discussed. For clarity, more frequently targeted catalytic reactions are considered first. Carbon-carbon coupling reactions, transfer hydrogenation of alkenes and carbonyl compounds, ketone hydrosilylation, and chiral catalysis are among highly popular reactions. Areas where the efficacy of the NHC based catalytic systems were explored to a lesser extent include CO2 reduction, C-H borylation, alkyl amination, and hydroamination reactions. Furthermore, the synthesis and applications of transition metal complexes are covered.

ProPhenol Derived Ligands to Simultaneously Coordinate a Main-Group Metal and a Transition Metal: Application to a Zn-Cu Catalyzed Reaction

A new bifunctional ligand bearing chiral N-heterocyclic carbene (NHC) and prolinol moieties is presented. Utilizing the designed ligand, an in situ formed Cu/Zn hetero-bimetallic complex unlocks the asymmetric allylic alkylation reactions of allyl phosphates with zinc keto-homoenolates, leading to the formation of various γ-vinyl ketones with good regio- and enantio-selectivity. DF sT calculation supports that the chelation of allyl phosphates with catalyst promotes the S_N 2' addition and the ligand-substrate steric interactions account for the stereoselective outcome.

Catalytic Asymmetric Allylic Substitution with Copper(I) Homoenolates Generated from Cyclopropanols

By using copper(I) homoenolates as nucleophiles, which are generated through the ring-opening of 1-substituted cyclopropane-1-ols, a catalytic asymmetric allylic substitution with allyl phosphates is achieved in high to excellent yields with high enantioselectivity. Both 1-substituted cyclopropane-1-ols and allylic phosphates enjoy broad substrate scopes. Remarkably, various functional groups, such as ether, ester, tosylate, imide, alcohol, nitro, and carbamate are well tolerated. Moreover, the present method is nicely extended to the asymmetric construction of quaternary carbon centers. Some control experiments argue against a radical-based reaction mechanism and a catalytic cycle based on a two-electron process is proposed. Finally, the synthetic utilities of the product are showcased by means of the transformations of the terminal olefin group and the ketone group.

Enantioselective Tertiary Electrophile (Hetero)Benzylation: Pd-Catalyzed Substitution of Isoprene Monoxide with Arylacetates*

The enantioselective generation of quaternary carbon centers remains challenging but is of growing importance for the preparation of functional molecules. Metal catalyzed allylic alkylations of tertiary electrophiles can provide access to these substructures but remain generally incompatible with organometallic benzyl nucleophiles. Here we demonstrate that electron-deficient arylacetates can serve as benzyl nucleophile surrogates to generate enantioenriched acyclic molecules containing a quaternary carbon center via a two-step substitution-decarboxylation process using isoprene monoxide. Products are often obtained in >90 % ee using a commercially available catalyst. An array of electron-withdrawing functional groups on the arylacetate moiety are tolerated. The lactone generated by the initial substitution reaction can be used in further stereoselective transformations to prepare molecules with acyclic vicinal quaternary stereocenters.

Ligand Promoted Olefination of Anilides for Indirectly Introducing Fluorinated Functional Groups via Palladium Catalyst

We report a palladium-catalyzed, ligand promoted, C-H fluorine-containing olefination of anilides with 4-bromo-3,3,4,4-tetrafluorobutene as the fluorinated reagent, which has a potential transformation into other compounds due to its -CF₂CF₂Br functional group. -CF₂CF₂H was obtained by using the mild reducing agent sodium borohydride. Bioactive compounds such as aminoglutethimide derivative and propham were well-tolerated in this reaction, both of which highlight the synthetic importance of this method.

Influences of Phenyl Rings on NHC Ligands with Bicyclic Architectures

In addition to phosphanes, olefins, amines, and amides, over the past two decades N-heterocyclic carbene (NHC) has emerged as a useful alternative ligand. Based on a number of derivatization studies on NHC ligands, imidazol-2-ylidene and imidazolin-2-ylidene became the standard heterocyclic form, and bulky substituents have commonly been introduced on the nitrogen(s) adjacent to carbenic carbons. Our group previously developed NHCs equipped with noncarbenic carbons with a bicyclic architecture that gives them unique steric properties that make them bulky but accessible. In this study, we synthesized a novel type of NHC ligand that possesses a bicyclo[2.2.1]heptane architecture, and we compared five derivatives using copper-catalyzed allylic arylations with aryl Grignard reagents. The regioselectivity of the substitution obviously indicates that a phenyl ring over an active site has a characteristic effect on the resultant copper catalysts when γ-substitution is the major pathway.

Asymmetric synthesis via stereospecific C-N and C-O bond activation of alkyl amine and alcohol derivatives

This perspective showcases our development of benzylic and allylic amine and alcohol derivatives as electrophiles for stereospecific, nickel-catalyzed cross-coupling reactions, as well as the prior art that inspired our efforts. The success of our effort has relied on the use of benzyl ammonium triflates as electrophiles for cross-couplings via C-N bond activation and benzylic and allylic carboxylates for cross-couplings via C-O bond activation. Our work, along with others' exciting discoveries, has demonstrated the potential of stereospecific, nickel-catalyzed cross-couplings of alkyl electrophiles in asymmetric synthesis, and enables efficient generation of both tertiary and quaternary stereocenters.

Acyclic Quaternary Carbon Stereocenters via Enantioselective Transition Metal Catalysis

Whereas numerous asymmetric methods for formation of quaternary carbon stereocenters in cyclic systems have been documented, the construction of acyclic quaternary carbon stereocenters with control of absolute stereochemistry remains a formidable challenge. This Review summarizes enantioselective methods for the construction of acyclic quaternary carbon stereocenters from achiral or chiral racemic reactants via transition metal catalysis.

Ligand-Controlled Regiodivergent and Enantioselective Copper-Catalyzed Hydroallylation of Alkynes

A ligand-controlled regiodivergent and enantioselective copper-catalyzed intermolecular hydroallylation of alkynes with allylic phosphates and hydrosilanes has been achieved for the first time. The chiral bidentate sulfonate-containing N-heterocyclic carbene ligated CuCl complex leads to enantioenriched S_N 2'-type products, whereas the use of the IMesCuCl catalyst affords S_N 2-type products. Thus a range of chiral branched and achiral linear 1,4-dienes could be facilely synthesized from readily available alkynes in a regiodivergent manner.

Stereospecific, Nickel-Catalyzed Suzuki-Miyaura Cross-Coupling of Allylic Pivalates To Deliver Quaternary Stereocenters

Recognizing the importance of all-carbon, quaternary stereocenters in complex molecule synthesis, a stereospecific, nickel-catalyzed cross-coupling of allylic pivalates with arylboroxines to deliver products equipped with quaternary stereocenters and internal alkenes was developed. The enantioenriched allylic pivalate starting materials are readily prepared, and a variety of functional groups can be incorporated on both the allylic pivalate and the arylboroxine. Additional advantages include the use of a commercially available and air-stable Ni(II) salt and BISBI ligand, mild reaction conditions, and high yields and ee's. The observed stereoinversion of this reaction is consistent with an open transition state in the oxidative addition step.

PdII-Catalyzed Oxidative Aldehyde-sp2C-H Functionalization and Cyclization Using NHC with Mild Oxidant DMSO for the Selective Synthesis of Esters, Sugar-Based Analogues, and β-Hydroxy Chromanones: An 18O-Labeling Study

We assume formation of acyl-Pd^II-N-heterocyclic-carbene (NHC) organometalics for diverse C-O/O-C and C-C/C-O coupling catalysis of direct functionalization and cyclization reactions. We report the first use of dimethyl sulfoxide (DMSO) as an oxidant under an inert atmosphere to O₂-sensitive NHC for oxidative transformations. In situ generated imidazolium halides are utilized as a precursor of NHC and as a source of alkyl group for the sp²C-H functionalization of aldehydes to esters under mild conditions. In contrast to the reported NHC-catalyzed esterification strategies, the outstanding substrate scope of this mild catalysis approach is established through synthesis of thermally labile sugar-based chiral esters. Our competition experiments using various unsymmetrical imidazolium halides revealed an ascending rate of migratory aptitude among methyl ≪ allyl < crotyl < cinnamyl < benzyl moiety. DMSO is used as an oxidant for both esterification and cyclization reactions, and the transfer of the DMSO-oxygen to ester is confirmed using an ¹⁸O-labeling experiment. The diverse activity using DMSO-oxygen to acyl-Pd^II-NHC is verified by developing a unique C-C-coupled cyclization with side-chain hydroxylation of olefin to achieve valuable β-hydroxy chromanones.

Chiral Bidentate NHC Ligands Based on the 1,1'-Binaphthyl Scaffold: Synthesis and Application in Transition-Metal-Catalyzed Asymmetric Reactions

The use of the chiral 1,1'-binaphthyl scaffold to construct chiral ligands can be traced back for a long time. However, the development of bidentate NHC ligands based on the same backbone has only appeared recently. In this account, we describe the design and synthesis of a new family of chiral NHC ligands based on the 1,1'-binaphthyl scaffold and demonstrate the applications of these chiral NHC-metal complexes in the catalyzed oxidative kinetic resolution of secondary alcohols, asymmetric carbon-carbon bond formations, hydrosilylations, and cyclizations of 1,6-enynes. The chiral NHC ligands containing the 1,1'-binaphthyl backbone can be synthesized in good yields from enantiomerically pure 1,1'-binaphthyl-2,2'-diamine. These transition metals coordinated with chiral bidentate NHC ligands exhibit high catalytic activities and good enantioselectivities for a wide range of metal-catalyzed asymmetric reactions.

Iterative catalyst controlled diastereodivergent synthesis of polypropionates

Polypropionates are synthesized using a combination of a copper-catalyzed asymmetric allylic alkylation, ruthenium-catalyzed cross-metathesis and iridium-catalyzed asymmetric allylic etherification.

2,2,2-Trifluoroethyl Chlorooxoacetate--Universal Reagent for One-Pot Parallel Synthesis of N(1)-Aryl-N(2)-alkyl-Substituted Oxamides

A one-pot parallel synthesis of N(1)-aryl-N(2)-alkyl-substituted oxamides with 2,2,2-trifluoroethyl chlorooxoacetate was developed. The synthesis of a library of 45 oxamides revealed higher efficiency of this reagent over the known ethyl chlorooxoacetate. The reagent was successfully used to prepare the known oxamide-containing HIV entry inhibitors.

Regio- and enantioselective copper-catalyzed allylic alkylation of ortho-substituted cinnamyl bromides with Grignard reagents

A highly efficient method for the copper-catalyzed asymmetric allylic alkylation of ortho-substituted cinnamyl bromides with Grignard reagents is reported. The use of a catalytic system comprising CuBr·SMe2 and TaniaPhos as chiral ligands gives rise to a range of branched products with excellent regio- and enantioselectivity.

A computational study of the effects of ancillary ligands on copper(i)–ethylene interaction

Structural and electronic calculations on a ligand database are used to quantify the influence of the ancillary ligands and coordination mode on the electronic structure of Cu(i) ethylene complexes.

Catalyst-controlled reverse selectivity in C–C bond formation: NHC-Cu-catalyzed α-selective allylic alkylation with organolithium reagents

An efficient and highly α-selective copper-catalyzed allylic alkylation of allylic halides with organolithium reagents is presented.

Chiral N-heterocyclic carbene ligands bearing a pyridine moiety for the copper-catalyzed alkylation of N-sulfonylimines with dialkylzinc reagents

Amino acid-derived chiral imidazolium salts, each bearing a pyridine ring, were developed as N-heterocyclic carbene ligands. The copper-catalyzed asymmetric alkylation of various N-sulfonylimines with dialkylzinc reagents in the presence of these chiral imidazolium salts afforded the corresponding alkylated products with high enantioselectivity (up to 99 % ee). The addition of HMPA to the reaction mixture as a co-solvent is critical in terms of chemical yield and enantioselectivity. A wide range of N-sulfonylimines and dialkylzinc reagents were found to be applicable to this reaction.

Synthesis of quaternary carbon stereocenters by copper-catalyzed asymmetric allylic substitution of allyl phosphates with arylboronates

A copper-catalyzed asymmetric allylic substitution of γ,γ-disubstituted allyl phosphates with arylboronates has been developed for the construction of quaternary stereocenters. High regio- and enantioselectivities have been achieved by employing a hydroxy-bearing chiral N-heterocyclic carbene ligand, and both E and Z substrates provide the same enantiomer as the major product. The mechanistic aspect of this catalysis has also been investigated to find that a 1:1 copper/ligand complex is most likely responsible for the present asymmetric catalysis, and the reaction proceeds with almost perfect 1,3-anti stereochemistry with respect to the allylic electrophile.

A broadly applicable NHC-Cu-catalyzed approach for efficient, site-, and enantioselective coupling of readily accessible (pinacolato)alkenylboron compounds to allylic phosphates and applications to natural product synthesis

A set of protocols for catalytic enantioselective allylic substitution (EAS) reactions that allow for additions of alkenyl units to readily accessible allylic electrophiles is disclosed. Transformations afford 1,4-dienes that contain a tertiary carbon stereogenic site and are promoted by 1.0–5.0 mol % of a copper complex of an N-heterocyclic carbene (NHC). Aryl- as well as alkyl-substituted electrophiles bearing a di- or trisubstituted alkene may be employed. Reactions can involve a variety of robust alkenyl–(pinacolatoboron) [alkenyl–B(pin)] compounds that can be either purchased or prepared by various efficient, site-, and/or stereoselective catalytic reactions, such as cross-metathesis or proto-boryl additions to terminal alkynes. Vinyl-, E-, or Z-disubstituted alkenyl-, 1,1-disubstituted alkenyl-, acyclic, or heterocyclic trisubstituted alkenyl groups may be added in up to >98% yield, >98:2 S_N2′:S_N2, and 99:1 enantiomeric ratio (er). NHC–Cu-catalyzed EAS with alkenyl–B(pin) reagents containing a conjugated carboxylic ester or aldehyde group proceed to provide the desired 1,4-diene products in good yield and with high enantioselectivity despite the presence of a sensitive stereogenic tertiary carbon center that could be considered prone to epimerization. In most instances, the alternative approach of utilizing an alkenylmetal reagent (e.g., an Al-based species) represents an incompatible option. The utility of the approach is illustrated through applications to enantioselective synthesis of natural products such as santolina alcohol, semburin, nyasol, heliespirone A, and heliannuol E.