Catalytic enantioselective C-H functionalization of alcohols by redox-triggered carbonyl addition: borrowing hydrogen, returning carbon

Chemical Syntheses and Chemical Biology of Carboxyl Polyether Ionophores: Recent Highlights

A central goal of chemical biology is to develop molecular probes that enable fundamental studies of cellular systems. In the hierarchy of bioactive molecules, the so-called ionophore class occupies an unflattering position in the lower branches, with typical labels being "non-specific" and "toxic". In fact, the mere possibility that a candidate molecule possesses "ionophore activity" typically prompts its removal from further studies; ionophores-from a chemical genetics perspective-are molecular outlaws. In stark contrast to this overall poor reputation of ionophores, synthetic chemistry owes some of its most amazing achievements to studies of ionophore natural products, in particular the carboxyl polyethers renowned for their intricate molecular structures. These compounds have for decades been academic battlegrounds where new synthetic methodology is tested and retrosynthetic tactics perfected. Herein, we review the most exciting recent advances in carboxyl polyether ionophore (CPI) synthesis and in addition discuss the burgeoning field of CPI chemical biology.

Regio- and Enantioselective Iridium-Catalyzed N-Allylation of Indoles and Related Azoles with Racemic Branched Alkyl-Substituted Allylic Acetates

Cyclometallated π-allyliridium C,O-benzoates modified with (S)-tol-BINAP, which are stable to air, water, and SiO₂ , catalyze highly enantioselective N-allylations of indoles and related azoles. This reaction complements previously reported metal-catalyzed indole allylations in that complete levels of N versus C3 and branched versus linear regioselectivity are observed.

Diastereo- and Enantioselective Access to Stereotriads through a Flexible Coupling of Substituted Aldehydes and Alkenes

A flexible redox-neutral coupling of aldehydes and alkenes enables rapid access to stereotriads starting from a single stereocenter with perfect levels of enantio- and diastereoselectivity under mild conditions. The versatility of the method is highlighted by the installation of heteroatoms along the tether, which enables a route to structurally diverse building blocks. The formal synthesis of (+)-neopeltolide further demonstrates the synthetic utility of this approach.

Conversion of Aldehydes to Branched or Linear Ketones via Regiodivergent Rhodium-Catalyzed Vinyl Bromide Reductive Coupling-Redox Isomerization Mediated by Formate

A regiodivergent catalytic method for direct conversion of aldehydes to branched or linear alkyl ketones is described. Rhodium complexes modified by P ^tBu₂Me catalyze formate-mediated aldehyde-vinyl bromide reductive coupling-redox isomerization to form branched ketones. Use of the less strongly coordinating ligand, PPh₃, promotes vinyl- to allylrhodium isomerization en route to linear ketones. This method bypasses the 3-step sequence often used to convert aldehydes to ketones involving the addition of pre-metalated reagents to Weinreb or morpholine amides.

Catalytic Enantioselective Synthesis of Chiral Organofluorine Compounds: Alcohol-Mediated Hydrogen Transfer for Catalytic Carbonyl Reductive Coupling

Alcohol-mediated carbonyl addition has enabled catalytic enantioselective syntheses of diverse fluorine-containing compounds without the need for stoichiometric metals or discrete redox manipulations. Reactions of this type may be separated into two broad categories: redox-neutral hydrogen auto-transfer reactions wherein lower alcohols and n-unsaturated pronucleophiles are converted to higher alcohols and corresponding 2-propanol mediated carbonyl reductive couplings.

Theoretical Study of Ruthenium(0)-Catalyzed Transfer Hydrogenative Cycloaddition of Cyclohexadiene and Norbornadiene with 1,2-Diols to Form Bridged Carbocycles

The recent success of Krische et al. ( Angew. Chem., Int. Ed. 2017 , 56 , 14667 -14671 ) in achieving a ruthenium(0)-catalyzed transfer hydrogenative cycloaddition of 1,2-diols with cyclohexadiene and norbornadiene in excellent yield with exo- and diastereoselectivity represents an exciting development in the synthesis of bridged carbocycles. In the present work, the possible catalytic mechanisms and origin of the exo- and diastereoselectivity for cyclohexadiene and norbornadiene were studied in detail by density functional theory calculations. The theoretical results indicate that the exoselective pathway for the cyclohexadiene substrate proceeds by a novel two-step successive C-C coupling, while the endoselective pathway undergoes the C-C coupling reaction in a conventional concerted manner. The origin of the preferential chemoselectivity of dione-cyclohexadiene C-C coupling over aromatization to benzene was investigated. Aromatization to benzene is unfavorable because of the large distortion energy of the three-membered ring in the transition state of hydrogen migration. From distortion/interaction analysis, for norbornadiene, the distortion energy plays the main role in determining the exoselectivity.

Successive Nucleophilic and Electrophilic Allylation for the Catalytic Enantioselective Synthesis of 2,4-Disubstituted Pyrrolidines

Successive nucleophilic and electrophilic allylation mediated by the bis-Boc-carbonate derived from 2-methylene-1,3-propane diol enables formation of enantiomerically enriched 2,4-disubstituted pyrrolidines. An initial enantioselective iridium-catalyzed transfer hydrogenative carbonyl C-allylation is followed by Tsuji-Trost N-allylation using 2-nitrobenzenesulfonamide. Subsequent Mitsunobu cyclization provides the N-protected 2,4-disubstituted pyrrolidines.

Enantioselective and site-specific copper-catalyzed reductive allyl-allyl cross-coupling of allenes

A copper-catalyzed asymmetric reductive allyl-allyl cross-coupling reaction of allenes with allylic phosphates wherein allenes were used as allylmetal surrogates has been achieved for the first time. This protocol provides an efficient and straightforward route to optically active 1,5-dienes in a highly enantioselective and site-specific fashion. Furthermore, all-carbon quaternary stereogenic centers could also be constructed with this protocol. The versatility of the products is demonstrated through a diverse array of further transformations of the enantioenriched 1,5-dienes.

Rhodium-Catalyzed Aldehyde Arylation via Formate-Mediated Transfer Hydrogenation: Beyond Metallic Reductants in Grignard/Nozaki-Hiyami-Kishi-Type Addition

The first intermolecular carbonyl arylations via transfer hydrogenative reductive coupling are described. Using rhodium catalysts modified by tBu2PMe, sodium formate-mediated reductive coupling of aryl iodides with aldehydes occurs in a chemoselective fashion in the presence of protic functional groups and lower halides. This work expands the emerging paradigm of transfer hydrogenative coupling as an alternative to pre-formed carbanions or metallic reductants in C═X addition.

Cyclometalated Iridium-PhanePhos Complexes Are Active Catalysts in Enantioselective Allene-Fluoral Reductive Coupling and Related Alcohol-Mediated Carbonyl Additions That Form Acyclic Quaternary Carbon Stereocenters

Iridium complexes modified by the chiral phosphine ligand PhanePhos catalyze the 2-propanol-mediated reductive coupling of diverse 1,1-disubstituted allenes 1a-1u with fluoral hydrate 2a to form CF3-substituted secondary alcohols 3a-3u that incorporate acyclic quaternary carbon-containing stereodiads. By exploiting concentration-dependent stereoselectivity effects related to the interconversion of kinetic ( Z)- and thermodynamic ( E)-σ-allyliridium isomers, adducts 3a-3u are formed with complete levels of branched regioselectivity and high levels of anti-diastereo- and enantioselectivity. The utility of this method for construction of CF3-oxetanes and CF3-azetidines is illustrated by the formation of 4a and 6a, respectively. Studies of the reaction mechanism aimed at illuminating the singular effectiveness of PhanePhos as a supporting ligand in this and related transformations have led to the identification of a chromatographically stable cyclometalated iridium-( R)-PhanePhos complex, Ir-PP-I, that is catalytically competent for allene-fluoral reductive coupling and previously reported transfer hydrogenative C-C couplings of dienes or CF3-allenes with methanol. Deuterium labeling studies, reaction progress kinetic analysis (RPKA) and computational studies corroborate a catalytic mechanism involving rapid allene hydrometalation followed by turnover-limiting carbonyl addition. A computationally determined stereochemical model shows that the ortho-CH2 group of the cyclometalated iridium-PhanePhos complex plays a key role in directing diastereo- and enantioselectivity. The collective data provide key insights into the structural-interactional features of allyliridium complexes required to enforce nucleophilic character, which should inform the design of related cyclometalated catalysts for umpoled allylation.

Enantioselective iridium-catalyzed carbonyl isoprenylation via alcohol-mediated hydrogen transfer

Highly enantioselective iridium catalyzed carbonyl (2-vinyl)allylation or "isoprenylation" is achieved via hydrogen auto-transfer or 2-propanol-mediated reductive coupling from primary alcohol or aldehyde reactants, respectively. Using this method, asymmetric total syntheses of the terpenoid natural products (+)-ipsenol and (+)-ipsdienol were achieved.

Enantioselective Ir-Catalyzed Bidirectional Reductive Coupling

In the presence of a chiral iridium complex, commercially available 3-chloro-2-chloromethyl-1-propene (1) was selectively activated for various reductive couplings. Depending on the reaction conditions it allows a selective mono- or bidirectional condensation with one or two external aldehydes with excellent enantiocontrol (>90% ee). This approach occurring simply under mild conditions and avoiding premetalated reagents constructs rapidly chiral homoallylic alcohols, key precursors of important molecular fragments such as furans, pyrans, ketodiols, or 1,3,5-polyols.

Enantioselective Redox-Neutral Coupling of Aldehydes and Alkenes by an Iron-Catalyzed "Catch-Release" Tethering Approach

The reductive coupling of aldehydes and alkenes is an emerging technology that holds the potential to reinvent carbonyl addition chemistry. However, existing enantioselective methods are limited to form "branched" products. Herein, we present a directed enantio- and diastereoselective alkylation of aldehydes with simple olefins to selectively yield linear coupling products. This is achieved by redox-neutral remote functionalization, whereby a tethering "catch-release" strategy decisively solves the key problems of reactivity and selectivity.

Catalytic Enantioselective Carbonyl Propargylation Beyond Preformed Carbanions: Reductive Coupling and Hydrogen Auto-Transfer

Chiral metal complexes catalyze enantioselective carbonyl propargylation via reductive coupling or as hydrogen auto-transfer processes, in which reactant alcohols serve dually as reductant and carbonyl proelectrophile. Unlike classical propargylation protocols, which rely on allenylmetal reagents or metallic reductants (e.g. NHK reactions), reductive protocols for carbonyl propargylation can occur in the absence of stoichiometric metals, precluding generation of metallic byproducts. Propargylations of this type exploit both enyne and propargyl halide pronucleophiles.

First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands

The use of 3d metals in de/hydrogenation catalysis has emerged as a competitive field with respect to "traditional" precious metal catalyzed transformations. The introduction of functional pincer ligands that can store protons and/or electrons as expressed by metal-ligand cooperativity and ligand redox-activity strongly stimulated this development as a conceptual starting point for rational catalyst design. This review aims at providing a comprehensive picture of the utilization of functional pincer ligands in first-row transition metal hydrogenation and dehydrogenation catalysis and related synthetic concepts relying on these such as the hydrogen borrowing methodology. Particular emphasis is put on the implementation and relevance of cooperating and redox-active pincer ligands within the mechanistic scenarios.

Iridium-Catalyzed Asymmetric Allylic Substitution Reactions

In this review, we summarize the origin and advancements of iridium-catalyzed asymmetric allylic substitution reactions during the past two decades. Since the first report in 1997, Ir-catalyzed asymmetric allylic substitution reactions have attracted intense attention due to their exceptionally high regio- and enantioselectivities. Ir-catalyzed asymmetric allylic substitution reactions have been significantly developed in recent years in many respects, including ligand development, mechanistic understanding, substrate scope, and application in the synthesis of complex functional molecules. In this review, an explicit outline of ligands, mechanism, scope of nucleophiles, and applications is presented.

Regio- and Enantioselective Iridium-Catalyzed Amination of Racemic Branched Alkyl-Substituted Allylic Acetates with Primary and Secondary Aromatic and Heteroaromatic Amines

The air- and water-stable π-allyliridium C,O-benzoate modified by ( S)-tol-BINAP, ( S)-Ir-II, catalyzes highly regio- and enantioselective Tsuji-Trost-type aminations of racemic branched alkyl-substituted allylic acetates using primary or secondary (hetero)aromatic amines. Specifically, in the presence of ( S)-Ir-II (5 mol%) in DME solvent at 60-70 °C, α-methyl allyl acetate 1a (100 mol%) reacts with primary (hetero)aromatic amines 2a-2l (200 mol%) or secondary (hetero)aromatic amines 3a-3l (200 mol%) to form the branched products of allylic amination 4a-4l and 5a-5l, respectively, as single regioisomers in good to excellent yield with uniformly high levels of enantioselectivity. As illustrated by the conversion of heteroaromatic amine 3m to adducts 6a-6g, excellent levels of regio- and enantioselectivity are retained across diverse branched allylic acetates bearing normal alkyl or secondary alkyl substituents. For reactants 3n-3p, which incorporate both primary and secondary aryl amine moieties, regio- and enantioselective amination occurs with complete site-selectivity to furnish adducts 7a-7c. Mechanistic studies involving amination of the enantiomerically enriched, deuterium-labeled acetate 1h corroborate C-N bond formation via outer-sphere addition.

Alternating oligo(o,p-phenylenes) via ruthenium catalyzed diol-diene benzannulation: orthogonality to cross-coupling enables de novo nanographene and PAH construction

Ruthenium(0) catalyzed diol-diene benzannulation is applied to the conversion of oligo(p-phenylene vinylenes) 2a-c, 5 and 6 to alternating oligo(o,p-phenylenes) 10a-c, 11-13. Orthogonality with respect to conventional palladium catalyzed biaryl cross-coupling permits construction of p-bromo-terminated alternating oligo(o,p-phenylenes) 10b, 11-13, which can be engaged in Suzuki cross-coupling and Scholl oxidation. In this way, structurally homogeneous nanographenes 16a-f are prepared. Nanographene 16a, which incorporates 14 fused benzene rings, was characterized by single crystal X-ray diffraction. In a similar fashion, p-bromo-terminated oligo(p-phenylene ethane diol) 9, which contains a 1,3,5-trisubstituted benzene core, is converted to the soluble, structurally homogeneous hexa-peri-hexabenzocoronene 18. A benzothiophene-terminated pentamer 10c was prepared and subjected to Scholl oxidation to furnish the helical bis(benzothiophene)-fused picene derivative 14. The steady-state absorption and emission properties of nanographenes 14, 16a,b,d,e,h and 18 were characterized. These studies illustrate how orthogonality of ruthenium(0) catalyzed diol-diene benzannulation with respect to classical biaryl cross-coupling streamlines oligophenylene and nanographene construction.

Silver/NBS-Catalyzed Synthesis of α-Alkylated Aryl Ketones from Internal Alkynes and Benzyl Alcohols via Ether Intermediates

The silver hexafluoroantimonate/ N-bromosuccinimide (NBS)-catalyzed synthesis of α-alkylated aryl ketones with a tertiary carbon center from internal alkynes and benzyl alcohols is reported. This reaction proceeds via the etherification of benzyl alcohols with an in situ generated benzyl bromide, formed by the reaction of benzyl alcohol with a catalytic amount of NBS and AgSbF₆. Ag-catalyzed C-O cleavage of the ether leads to a tolyl radical, which undergoes addition to the alkyne, ultimately leading to the α-alkylated aryl ketone products.

Enantioconvergent Biocatalytic Redox Isomerization

Alcohol dehydrogenases can act as powerful catalysts in the preparation of optically pure γ-hydroxy-δ-lactones by means of an enantioconvergent dynamic redox isomerization of readily available Achmatowicz-type pyranones. Imitating the traditionally metal-mediated "borrowing hydrogen" approach to shuffle hydrides across molecular architectures and interconvert functional groups, this chemoinspired and purely biocatalytic interpretation effectively expands the enzymatic toolbox and provides new opportunities in the assembly of multienzyme cascades and tailor-made cellular factories.

Enantioselective Ruthenium-Catalyzed Benzocyclobutenone-Ketol Cycloaddition: Merging C-C Bond Activation and Transfer Hydrogenative Coupling for Type II Polyketide Construction

The first enantioselective intermolecular metal-catalyzed cycloadditions of benzocyclobutenones via C-C bond oxidative addition are described. In the presence of a ruthenium(0) complex modified by ( R)-DM-SEGPHOS, tetralone-derived ketols and benzocyclobutenones combine to form cycloadducts with complete regio- and diastereoselectivity and high enantioselectivity. Using this method, the "bay region" substructure of the angucycline natural product arenimycin was prepared.

Redox-Annulations of Cyclic Amines with 2-(2-Oxoethyl)malonates

Amines such as 1,2,3,4-tetrahydroisoquinoline undergo redox-neutral annulations with 2-(2-oxoethyl)malonates in the presence of catalytic amounts of benzoic acid. These reactions install a fully saturated five-membered ring and provide access to structures closely related to the natural products crispine A and harmicine.

Catalytic Enantioselective Allylations of Acetylenic Aldehydes via 2-Propanol-Mediated Reductive Coupling

Cyclometalated π-allyliridium C,O-benzoates modified by ( S)-SEGPHOS or ( S)-Cl,OMe-BIPHEP catalyze enantioselective 2-propanol-mediated reductive couplings of diverse nonmetallic allyl pronucleophiles with the acetylenic aldehyde TIPSC≡CCHO. Absolute stereochemistries of the resulting secondary homoallylic-propargylic alcohols were assigned using Rychnovsky's competing enantioselective conversion method.

Intermolecular Metal-Catalyzed Reductive Coupling of Dienes, Allenes, and Enynes with Carbonyl Compounds and Imines

Metal-catalyzed reductive coupling has emerged as an alternative to the use of stoichiometric organometallic reagents in an increasingly diverse range of carbonyl and imine additions. In this review, the use of diene, allene, and enyne pronucleophiles in intermolecular carbonyl and imine reductive couplings are surveyed, along with related hydrogen autotransfer processes.

Divergent Coupling of Anilines and Enones by Integration of C-H Activation and Transfer Hydrogenation

Cp*Rh^III /Ir^III complexes are known to play important roles in both C-H activation and transfer hydrogenation (TH). However, these two areas evolved separately. They have been integrated in redox- and chemodivergent coupling reactions of N-pyridylanilines with enones. The iridium-catalyzed coupling with enones leads to the efficient synthesis of tetrahydroquinolines through TH from ⁱ PrOH. Counterintuitively, ⁱ PrOH does not serve as the sole hydride source, and the major reaction pathway involves disproportionation of a dihydroquinoline intermediate, followed by the convergent and iterative reduction of quinolinium species.

Ruthenium(0)-Catalyzed Cycloaddition of 1,2-Diols, Ketols, or Diones via Alcohol-Mediated Hydrogen Transfer

Merging the characteristics of transfer hydrogenation and carbonyl addition, a broad new class of ruthenium(0)-catalyzed cycloadditions has been developed. As discussed in this Minireview, fused or bridged bicyclic ring systems are accessible in a redox-independent manner in C-C bond-forming hydrogen transfer reactions of diols, α-ketols, or 1,2-diones with diverse unsaturated reactants.

Organocatalytic reductive coupling of aldehydes with 1,1-diarylethylenes using an in situ generated pyridine-boryl radical

A pyridine-boryl radical promoted reductive coupling reaction of aldehydes with 1,1-diarylethylenes has been established.

A pyridine-boryl radical promoted reductive coupling reaction of aldehydes with 1,1-diarylethylenes has been established via a combination of computational and experimental studies. Density functional theory calculations and control experiments suggest that the ketyl radical from the addition of the pyridine-boryl radical to aldehydes is the key intermediate for this C–C bond formation reaction. This metal-free reductive coupling reaction features a broad substrate scope and good functional compatibility.

Traceless Silylation of β-C(sp3)-H Bonds of Alcohols via Perfluorinated Acetals

We report the silylation of primary C-H bonds located β to secondary and tertiary alcohols by exploiting perfluorinated esters as traceless directing groups. The conversion of a secondary or tertiary alcohol to a perfluoroalkyl ester and conversion of the ester to the corresponding silyl acetals by hydrosilylation allows for selective β-C(sp3)-H silylation catalyzed by the combination of [Ir(cod)OMe]2 and Me4Phen (3,4,7,8-tetramethyl-1,10-phenanthroline) to form 6-membered dioxasilinane. Tamao-Fleming oxidation of these dioxasilinane leads to 1,2 diols. The developed sequence was applied to a series of natural products containing hydroxyl groups.

Catalytic enantioselective OFF ↔ ON activation processes initiated by hydrogen transfer: concepts and challenges

Hydrogen transfer initiated processes are eco-compatible transformations allowing the reversible OFF ↔ ON activation of otherwise unreactive substrates. The minimization of stoichiometric waste as well as the unique activation modes provided by these transformations make them key players for a greener future for organic synthesis. Long limited to catalytic reactions that form racemic products, considerable progress on the development of strategies for controlling diastereo- and enantioselectivity has been made in the last decade. The aim of this review is to present the different strategies that enable enantioselective transformations of this type and to highlight how they can be used to construct key synthetic building blocks in fewer operations with less waste generation.