Copper-Hydride-Catalyzed Enantioselective Processes with Allenyl Boronates. Mechanistic Nuances, Scope, and Utility in Target-Oriented Synthesis

Enantioselective Allylboration of Acetylene: A Versatile Tool for the Stereodivergent Synthesis of Natural Products

Efficient catalytic methods that allow the use of simple and abundant chemical feedstocks for the preparation of synthetically versatile compounds are central to modern synthetic chemistry. Acetylene is a basic feedstock with a remarkable production over one million tons per year, although it is underutilized in the stereoselective synthesis of fine chemicals. Here we report a facile catalytic multicomponent reaction that allows for the enantio- and diastereoselective allylboration of acetylene gas. This process is catalyzed by a chiral copper catalyst, operates without specialized equipment or pressurization, and provides chiral skipped dienes bearing stereodefined and orthogonally functionalized olefins with excellent levels of chemo-, regio-, enantio- and diastereoselectivity. The combined stereochemical features and orthogonal functionalization make the products privileged structural scaffolds to access the complete set of stereoisomers of the chiral skipped diene core through simple enantio- and diastereodivergent pathways. The utility of the method is demonstrated with the enantioselective synthesis of three bioactive natural skipped diene products, namely (+)-Nyasol, (+)-Hinokiresinol and Phorbasin C, and other related synthetically relevant chiral molecules.

Regio- and enantioselective synthesis of acyclic quaternary carbons via organocatalytic addition of organoborates to (Z)-Enediketones

The chemical synthesis of molecules with closely packed atoms having their bond coordination saturated is a challenge to synthetic chemists, especially when three-dimensional control is required. The organocatalyzed asymmetric synthesis of acyclic alkenylated, alkynylated and heteroarylated quaternary carbon stereocenters via 1,4-conjugate addition is here catalyzed by 3,3´-bisperfluorotoluyl-BINOL. The highly useful products (31 examples) are produced in up to 99% yield and 97:3 er using enediketone substrates and potassium trifluoroorganoborate nucleophiles. In addition, mechanistic experiments show that the (Z)-isomer is the reactive form, ketone rotation at the site of bond formation is needed for enantioselectivity, and quaternary carbon formation is favored over tertiary. Density functional theory-based calculations show that reactivity and selectivity depend on a key n→π* donation by the unbound ketone's oxygen lone pair to the boronate-coordinated ketone in a 5-exo-trig cyclic ouroboros transition state. Transformations of the conjugate addition products to key quaternary carbon-bearing synthetic building blocks proceed in good yield.

Copper-Catalyzed Enantioselective Borylative Allyl-Allyl Coupling of Allenes and Allylic gem-Dichlorides

A catalytic asymmetric reaction between allenes, bis(pinacolato)diboron, and allylic gem-dichlorides is reported. The method involves the coupling of a catalytically generated allyl copper species with the allylic gem-dichloride and provides chiral internal 1,5-dienes featuring (Z)-configured alkenyl boronate and alkenyl chloride units with high levels of chemo-, regio-, enantio-, and diastereoselectivity. The synthetic utility of the products is demonstrated with the synthesis of a range of optically active compounds. DFT calculations reveal key noncovalent substrate-ligand interactions that account for the enantioselectivity outcome and the diastereoselective formation of the (Z)-alkenyl chloride.

Fused Tricyclic Guanidine Alkaloids: Insights into Their Structure, Synthesis and Bioactivity

A marine natural product possesses a diverse and unique scaffold that contributes to a vast array of bioactivities. Tricyclic guanidine alkaloids are a type of scaffold found only in marine natural products. These rare skeletons exhibit a wide range of biological applications, but their synthetic approaches are still limited. Various stereochemical assignments of the compounds remain unresolved. Batzelladine and ptilocaulins are an area of high interest in research on tricyclic guanidine alkaloids. In addition, mirabilins and netamines are among the other tricyclic guanidine alkaloids that contain the ptilocaulin skeleton. Due to the different structural configurations of batzelladine and ptilocaulin, these two main skeletons are afforded attention in many reports. These two main skeletons exhibit different kinds of compounds by varying their ester chain and sidechain. The synthetic approaches to tricyclic guanidine alkaloids, especially the batzelladine and ptilocaulin skeletons, are discussed. Moreover, this review compiles the first and latest research on the synthesis of these compounds and their bioactivities, dating from the 1980s to 2022.

Copper-Catalyzed Diastereo- and Enantioselective Borylative Cyclization

Copper-catalyzed enantioselective borylative cyclization with various electrophiles via difunctionalization of unsaturated hydrocarbons is a powerful tool for the generation of interesting boron-containing carbocycles and heterocycles processes involving a chiral organocopper intermediate. Alkenes, allenes, and alkynes are versatile and easily accessible substrates that can be subjected to a wide range of reactions to produce densely functionalized, enantioenriched products. In this chapter, I discuss copper-catalyzed alkenes, allenes, and alkynes borofunctionalization and enantioselective cyclization via chiral organocopper intermediate. Copper-catalyzed enantioselective borylative cyclization and regiodivergent functionalization of alkenes, allenes, and alkynes, as well as the current mechanistic understanding of such processes, are given special attention in this review.

Enantio- and Diastereoselective Copper-Catalyzed Allylboration of Alkynes with Allylic gem-Dichlorides

Allylic gem-dichlorides are shown to be efficient substrates for catalytic asymmetric allylboration of alkynes. The method employs a chiral NHC-Cu catalyst capable of generating in a single step chiral skipped dienes bearing a Z-alkenyl chloride, a trisubstituted E-alkenyl boronate and a bis-allylic stereocenter with excellent levels of chemo-, regio- enantio- and diastereoselectivity. This high degree of functionalization makes these products versatile building blocks as illustrated with the synthesis of several optically active compounds. DFT calculations support the key presence of a metal cation bridge ligand-substrate interaction and account for the stereoselectivity outcome.

Acyclic Quaternary Carbon Stereocenters through Transition-Metal-Catalyzed Enantioselective Functionalization of Unsaturated Hydrocarbons

Acyclic quaternary carbon stereocenters occur frequently in natural products, bioactive molecules, and pharmaceutical compounds. Construction of a carbon stereogenic center attached to four different carbons with defined spatial arrangement is a daunting challenge in asymmetric catalysis. Significant efforts have been directed towards the stereoselective construction of such acyclic quaternary carbon stereocenters. In particular, catalytic generation of acyclic quaternary carbon stereocenters through functionalization of unsaturated hydrocarbons is an extremely attractive approach because unsaturated hydrocarbons are easily accessible both in industry and in organic synthesis. In this short review, we summarize the recent advances achieved in this research area, with the aim to inspire future development.

1 Introduction

2 Acyclic Quaternary Carbon Stereocenters through Functionalization of Allenes

3 Acyclic Quaternary Carbon Stereocenters through Functionalization of Dienes

4 Acyclic Quaternary Carbon Stereocenters through Functionalization of Mono-alkenes

5 Acyclic Quaternary Carbon Stereocenters through Functionalization of Alkynes

6 Summary and Outlook

Ligation of Boratabenzene and 9-Borataphenanthrene to Coinage Metals

The reactions of boratabenzene and borataphenanthrene anions with group 11 Ph₃PMCl reagents furnished η² coordination complexes, with the exception of the copper boratabenzene species that adopted an η⁶ mode. The binding of arene ligands to copper in an η⁶ manner is rare, and altering the ancillary ligand on copper to an N-heterocyclic carbene switched the binding of the boratabenzene to η², indicating that such ligands are capable of vacating coordination sites. The η² coordination complexes bind side-on, akin to olefins, via a borataalkene unit, although with the carbon atom much more proximal to the metal center than boron.

Cross-metathesis of Allenes. Mechanistic Analysis and Identification of a Ru-CAAC as the Most Effective Catalyst

The first examples of cross-metathesis between two different allenes is disclosed. First- and second-generation Ru complexes were found to be ineffective, at most affording only oligomeric products. The exception was a first-generation complex bearing a bidentate phenyl isopropoxy ligand (i.e., PCy₃ is not released upon initiation), reactions with which afforded a 1,3-disubstituted allenyl boronate in 22% yield. On the basis of mechanistic studies designed to gain deeper understanding of the reasons for the ineffectiveness of different Ru catalysts, it was discovered that phosphine-free Ru-CAAC complexes have the steric and electronic attributes to be highly effective. The results of these investigations pave the way for development of additional olefin metathesis reactions that generate allenes.

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.

Allenes and Dienes as Chiral Allylmetal Pronucleophiles in Catalytic Enantioselective C=X Addition: Historical Perspective and State-of-The-Art Survey

The use of allenes and 1,3-dienes as chiral allylmetal pronucleophiles in intermolecular catalytic enantioselective reductive additions to aldehydes, ketones, imines, carbon dioxide and other C=X electrophiles is exhaustively catalogued together with redox-neutral hydrogen auto-transfer processes. Coverage is limited to processes that result in both C-H and C-C bond formation. The use of alkynes as latent allylmetal pronucleophiles and multicomponent C=X allylations involving allenes and dienes is not covered. As illustrated in this review, the ability of allenes and 1,3-dienes to serve as tractable non-metallic pronucleophiles has evoked many useful transformations that have no counterpart in traditional allylmetal chemistry.

Diastereo-, Enantio-, and anti-Selective Formation of Secondary Alcohol and Quaternary Carbon Stereocenters by Cu-Catalyzed Additions of B-Substituted Allyl Nucleophiles to Carbonyls

A general method for the synthesis of secondary homoallylic alcohols containing α-quaternary carbon stereogenic centers in high diastereo- and enantioselectivity (up to >20:1 dr and >99:1 er) is disclosed. Transformations employ readily accessible aldehydes, allylic diboronates, and a chiral copper catalyst and proceed by γ-addition of in situ generated enantioenriched boron-stabilized allylic copper nucleophiles. The catalytic protocol is general for a wide variety of aldehydes as well as a variety of 1,1-allylic diboronic esters. Hammett studies disclose that diastereoselectivity of the reaction is correlated to the electronic nature of the aldehyde, with dr increasing as aldehydes become more electron poor.

Sulfonate N-Heterocyclic Carbene-Copper Complexes: Uniquely Effective Catalysts for Enantioselective Synthesis of C-C, C-B, C-H, and C-Si Bonds

A copper-based complex that contains a sulfonate N-heterocyclic carbene ligand was first reported 15 years ago. Since then, these organometallic entities have proven to be uniquely effective in catalyzing an assortment of enantioselective transformations, including allylic substitutions, conjugate additions, proto-boryl additions to alkenes, boryl and silyl substitutions, hydride-allyl additions to alkenyl boronates, and additions of boron-containing allyl moieties to N-H ketimines. In this review article, we detail the shortcomings in the state-of-the-art that fueled the development of this air stable ligand class, members of which can be prepared on multigram scale. For each reaction type, when relevant, the prior art at the time of the advance involving sulfonate NHC-Cu catalysts and/or subsequent key developments are briefly analyzed, and the relevance of the advance to efficient and enantioselective total or formal synthesis of biologically active molecules is underscored. Mechanistic analysis of the structural attributes of sulfonate NHC-Cu catalysts that are responsible for their ability to facilitate transformations with high efficiency as well as regio- and enantioselectivity are detailed. This review contains several formerly undisclosed methodological advances and mechanistic analyses, the latter of which constitute a revision of previously reported proposals.

Catalytic enantioselective allene-anhydride approach to β,γ-unsaturated enones bearing an α-all-carbon-quarternary center

A protocol of highly regio- and enantioselective copper-catalyzed hydroacylation of the non-terminal C[double bond, length as m-dash]C bond in 1,1-disubstituted terminal allenes with anhydrides has been developed. Both aromatic and aliphatic carboxylic anhydrides are applicable to the efficient construction of all carbon quarternary centers connected with a versatile C[double bond, length as m-dash]C bond and a useful ketone functionality. The synthetic potentials of the enantioenriched products have also been demonstrated. Density functional theory (DFT) calculations were performed to explain the steric outcome of the products: the hydroacylation proceeds through a six-membered transition state and the ligand-substrate steric interactions account for the observed enantioselectivity although the chiral ligand is far away from the to-be-genetated chiral center.

Cu-Catalyzed Diastereo- and Enantioselective Reactions of γ,γ-Disubstituted Allyldiboron Compounds with Ketones

A catalytic diastereo- and enantioselective method for the preparation of complex tertiary homoallylic alcohols containing a vicinal quaternary carbon stereogenic center and a versatile alkenylboronic ester is disclosed. Transformations are promoted by 5 mol % of a readily available copper catalyst bearing a bulky monodentate phosphoramidite ligand, which is essential for attaining both high dr and er. Reactions proceed with a wide variety of ketones and allylic 1,1-diboronate reagents, which enables the efficient preparation of diverse array of molecular scaffolds.

Enantioselective Synthesis of 4-Allyl Tetrahydroquinolines via Copper(I) Hydride-Catalyzed Hydroallylation of 1,2-Dihydroquinolines

CuCl/(R,R)-Ph-BPE-catalyzed asymmetric hydroallylation of 1,2-dihydroquinolines, prepared from readily available quinolines, was developed. The optically active tetrahydroquinolines (THQs) bearing an allylic functionality at position 4 were obtained in good yields and excellent enantioselectivity. The introduced allylic groups are amenable to diverse transformations, thus offering chances to rapidly expand the THQ libraries.

Nickel-catalyzed allyl–allyl coupling reactions between 1,3-dienes and allylboronates

A nickel-hydride catalysis has been developed to facilitate the allyl–allyl cross-coupling reactions between 1,3-dienes and allyl-B(pin) in excellent regioselectivity.