Alcohols as Substrates in Transition-Metal-Catalyzed Arylation, Alkylation, and Related Reactions

Alcohols are abundant and attractive feedstock molecules for organic synthesis. Many methods for their functionalization require them to first be converted into a more activated derivative, while recent years have seen a vast increase in the number of complexity-building transformations that directly harness unprotected alcohols. This Review discusses how transition metal catalysis can be used toward this goal. These transformations are broadly classified into three categories. Deoxygenative functionalizations, representing derivatization of the C-O bond, enable the alcohol to act as a leaving group toward the formation of new C-C bonds. Etherifications, characterized by derivatization of the O-H bond, represent classical reactivity that has been modernized to include mild reaction conditions, diverse reaction partners, and high selectivities. Lastly, chain functionalization reactions are described, wherein the alcohol group acts as a mediator in formal C-H functionalization reactions of the alkyl backbone. Each of these three classes of transformation will be discussed in context of intermolecular arylation, alkylation, and related reactions, illustrating how catalysis can enable alcohols to be directly harnessed for organic synthesis.

Editing Tetrasubstituted Carbon: Dual C-O Bond Functionalization of Tertiary Alcohols Enabled by Palladium-Based Dyotropic Rearrangement

Many elegant asymmetric syntheses of enantioenriched tertiary alcohols have been developed, and both the transition metal-catalyzed and the radical-based peripheral functionalization of tertiary alcohols have attracted intensive research interest in recent years. However, directly editing tetrasubstituted carbons remains challenging. Herein, we report a Pd-catalyzed migratory fluoroarylation reaction that converts tertiary alcohols to α-fluorinated tertiary alkyl ethers in good to excellent yields. An unprecedented 1,2-aryl/PdIV dyotropic rearrangement along the C-O bond, integrated in a PdII-catalyzed domino process, is key to the dual functionalization of both the hydroxyl group and the tetrasubstituted carbon. This reaction, which is compatible with a broad range of functional groups, generates a tertiary alkyl fluoride and an alkyl-aryl ether functional group with inversion of the absolute configuration at the tetrasubstituted stereocenter.

Regioselective Olefination and Arylation of Arene-Tethered Diols Using the Easily Foldable Directing Groups

Arene-tethered diols constitute a valuable class of structural motifs of drug and bioactive natural product molecules. In this study, a regioselective protocol for olefination and arylation of arene-tethered 1,2-diols and 1,3-diols has been developed using easily foldable acetal structures for attaching pyridine and nitrile directing groups. The method overcomes the steric hindrance effect of the short-chain diols and affords products in high yield and regioselectivity. This efficient cascaded catalysis has been successfully utilized in the syntheses of natural products such as peucedanol, decursinol, and marmesin.

Strategic application of C-H oxidation in natural product total synthesis

The oxidation of unactivated C-H bonds has emerged as an effective tactic in natural product synthesis and has altered how chemists approach the synthesis of complex molecules. The use of C-H oxidation methods has simplified the process of synthesis planning by expanding the choice of starting materials, limiting functional group interconversion and protecting group manipulations, and enabling late-stage diversification. In this Review, we propose classifications for C-H oxidations on the basis of their strategic purpose: type 1, which installs functionality that is used to establish the carbon skeleton of the target; type 2, which is used to construct a heterocyclic ring; and type 3, which installs peripheral functional groups. The reactions are further divided based on whether they are directed or undirected. For each classification, examples from recent literature are analysed. Finally, we provide two case studies of syntheses from our laboratory that were streamlined by the judicious use of C-H oxidation reactions.

Hydrogen-bond-acceptor ligands enable distal C(sp3)-H arylation of free alcohols

The functionalization of C-H bonds in organic molecules is one of the most direct approaches for chemical synthesis. Recent advances in catalysis have allowed native chemical groups such as carboxylic acids, ketones and amines to control and direct C(sp3)-H activation1-4. However, alcohols, among the most common functionalities in organic chemistry5, have remained intractable because of their low affinity for late transition-metal catalysts6,7. Here we describe ligands that enable alcohol-directed arylation of δ-C(sp3)-H bonds. We use charge balance and a secondary-coordination-sphere hydrogen-bonding interaction-evidenced by structure-activity relationship studies, computational modelling and crystallographic data-to stabilize L-type hydroxyl coordination to palladium, thereby facilitating the assembly of the key C-H cleavage transition state. In contrast to previous studies in C-H activation, in which secondary interactions were used to control selectivity in the context of established reactivity8-13, this report demonstrates the feasibility of using secondary interactions to enable challenging, previously unknown reactivity by enhancing substrate-catalyst affinity.

Synthesis of highly substituted isoquinolines/isoquinolones by ruthenium (II)-catalyzed reaction of benzyl/α-methyl benzyl/benzoyl isocyanates with diaryl alkynes

An efficient and novel method has been developed for the synthesis of highly substituted isoquinolines/isoquinolones by Ru(II)-catalyzed intermolecular oxidative annulation of benzyl/benzoyl isocyanates with diaryl alkynes in the presence of Cs₂CO₃ as base and Cu(OTf)₂ as an oxidant at 120 °C for 1 h.

Redox-Neutral Ruthenium(II)-Catalyzed Enol-Directed Arene C-H Alkylation with Maleimides

An enol-assisted regioselective arene C-H alkylation with maleimides is developed under redox-neutral ruthenium(II) catalysis, offering a wide variety of valuable 3-aryl succinimides including amino acid embedded frameworks in good to excellent yields. The products were also aromatized to produce synthetically useful resorcinol-based biaryls. Mechanistic studies support an organometallic pathway with a reversible C-H metalation step for this reaction.

Palladium-catalyzed C(sp3)-H nitrooxylation of masked alcohols

A palladium-catalyzed β-C(sp³)-H nitrooxylation of aliphatic alcohols with AgNO₂ is reported. An 8-formylquinoline-derived oxime is installed as an exo-type directing group for sp³ C-H activation and selectfluor acts as the oxidant. The reaction tolerates a variety of functional groups and shows good selectivity for β-C-H nitrooxylation of alcohols.

Aryl Halides as Halogenation Reagents in the Bromination and Iodination of Arene-Tethered Diols

Aromatic halides constitute a valuable class of building blocks that are commonly used in organic synthesis. In this study, we demonstrate usage of aryl bromides and aryl iodides in C-Br or C-I bond formation. Methyl 2-bromobenzoate and 2-nitrophenyl iodides were developed as mild and effective bromination and iodination reagents for functionalization of arene-tethered diols. This efficient cascaded catalysis can be applied to the total syntheses of natural product Mafaicheenamine A and Claulamine A.

Iron-Catalyzed Photoinduced Remote C(sp3)-H Amination of Free Alcohols

We report a general photocatalytic protocol for the remote C(sp³)-H bond amination of free aliphatic alcohols. The electron transfer between the abundant and inexpensive catalyst FeCl₃ and simple alkanols under blue LED irradiation enables the alkoxy radical formation under mild redox-neutral conditions, with no need for additional oxidant and prefunctionalization. The subsequent selective 1,5-hydrogen atom transfer (HAT) and amination provide a simple and efficient way to access molecular complexity from readily available and bulk alcohols.

Weakly Coordinating, Hydroxyl Directed Ruthenium Catalyzed C-H Alkylation of Ubiquitous Benzyl Alcohols with Maleimides

Benzyl alcohols have been employed as effective coupling partners in Ru-catalyzed C-H functionalization reactions, and their annulation with maleimides then offers efficient synthesis of useful ortho substituted succinimide aromatic aldehydes and ketones. Detailed mechanistic studies have been demonstrated by performing preliminary reactions, deuterium studies, and competitive experiments.

Enantioselective α-Arylation of Primary Alcohols under Sequential One-Pot Catalysis

Secondary benzylic alcohols and diarylmethanols are common structural motifs of biologically active and medicinally relevant compounds. Here we report their enantioselective synthesis by α-arylation of primary aliphatic and benzylic alcohols under sequential catalysis integrating a Ru-catalyzed hydrogen transfer oxidation and a Ru-catalyzed nucleophilic addition. The method can be applied to various alcohols and aryl nucleophiles tolerating a range of functional groups, including secondary alcohols, ketones, alkenes, esters, NH amides, tertiary amines, aryl halides, and heterocycles.

Ligand-accelerated site-selective Csp2–H and Csp3–H alkynylations of alcohols via Pd(ii) catalysis

A ligand accelerated site-selective C–H alkynylation, including secondary and tertiary Csp3–H alkynylation of weakly coordinated yet synthetically promising alcohols, via putative 6, 7 and 8-membered palladacycle intermediates, was developed. 

C(sp3)-H Monoarylation of Methanol Enabled by a Bidentate Auxiliary

With the assistance of a practical directing group (COAQ), the first catalytic protocol for the palladium-catalyzed C(sp³)-H monoarylation of methanol has been developed, offering an invaluable synthesis means to establish extensive derivatives of crucial arylmethanol functional fragments. Furthermore, the gram-scale reaction, broad substrate scope, excellent functional group compatibility, and even the practical synthesis of medicines further demonstrate the usefulness of this strategy.

Ligand-Enabled β-Methylene C(sp3 )-H Arylation of Masked Aliphatic Alcohols

Despite recent advances, reactivity and site-selectivity remain significant obstacles for the practical application of C(sp3 )-H bond functionalization methods. Here, we describe a system that combines a salicylic-aldehyde-derived L,X-type directing group with an electron-deficient 2-pyridone ligand to enable the β-methylene C(sp3 )-H arylation of aliphatic alcohols, which has not been possible previously. Notably, this protocol is compatible with heterocycles embedded in both alcohol substrates and aryl coupling partners. A site- and stereo-specific annulation of dihydrocholesterol and the synthesis of a key intermediate of englitazone illustrate the practicality of this method.

Selective Cross-Dehydrogenative C(sp3)-H Arylation with Arenes

Selective C(sp³)-C(sp²) bond construction is of central interest in chemical synthesis. Despite the success of classic cross-coupling reactions, the cross-dehydrogenative coupling between inert C(sp³)-H and C(sp²)-H bonds represents an attractive alternative toward new C(sp³)-C(sp²) bonds. Herein, we establish a selective inter- and intramolecular C(sp³)-H arylation of alcohols with nondirected arenes that thereby provides a general pathway to access a wide range of β-arylated alcohols, including tetrahydronaphthalen-2-ols and benzopyran-3-ols, with high to excellent chemo- and regioselectivity.

Development of an Imine Chaperone for Selective C-H Functionalization of Alcohols via Radical Relay

The design of a radical relay chaperone to promote selective C-H functionalizations is described. A saccharin-based imine was found to be uniquely suited to effect C-H amination of alcohols via an in situ generated hemiaminal. This radical chaperone facilitates the mild generation of an N-centered radical while also directing its regioselective H atom transfer (HAT) to the β carbon of an alcohol. Upon β C-H halogenation, aminocyclization, and reductive cleavage, an NH2 is formally added vicinal to an alcohol. The development, synthetic utility, and chemo-, regio-, and stereoselectivity of this imine chaperone-mediated C-H amination is presented herein.

Sulfamyl Radicals Direct Photoredox-Mediated Giese Reactions at Unactivated C(3)-H Bonds

Alcohol-anchored sulfamate esters guide the alkylation of tertiary and secondary aliphatic C(3)-H bonds. The transformation proceeds directly from N-H bonds with a catalytic oxidant, a contrast to prior methods which have required preoxidation of the reactive nitrogen center, or employed stoichiometric amounts of strong oxidants to obtain the sulfamyl radical. These sulfamyl radicals template otherwise rare 1,6-hydrogen-atom transfer (HAT) processes via seven-membered ring transition states to enable C(3)-H functionalization during Giese reactions.

Sulfamate Esters Guide C(3)-Selective Xanthylation of Alkanes

Owing to the pervasiveness of hydroxyl groups in natural isolates, alcohol derivatives are alluring directing groups. Herein, an alcohol-derived sulfamate ester guides the light-initiated xanthylation of primary, secondary, or tertiary centers. This process enables formal directed deuteration, azidation, thiolation, and vinylation reactions.

Pd-catalysed selective C(sp3)-H arylation and acetoxylation of alcohols

A palladium catalyzed selective C(sp3)-H arylation and acetoxylation of alcohols using a practical bidentate auxiliary were developed. Masked alcohols were selectively arylated at the β-position with diverse aryl iodides for the first time. Moreover, an efficient and site-selective acetoxylation of various primary methyl, methylene, and benzylic C(sp3)-H bonds was performed by using cheap K2S2O8 as the external oxidant.

A computational study of cobalt-catalyzed C–H iodination reactions using a bidentate directing group with molecular iodine

A computational methodology was used to collect detailed mechanistic information on the cobalt-catalyzed C–H iodination of aromatic amides with molecular iodine using an N,N′-bidentate directing group.

Selective C(sp3 )-H and C(sp2 )-H Fluorination of Alcohols Using Practical Auxiliaries

Selective introduction of fluorine into molecules by the cleavage of inert C-H bonds is of central academic and synthetic interest, yet remains challenging. Given the central role of alcohols in organic chemistry as the most ubiquitous building blocks, a versatile and selective C(sp³ )-H and C(sp² )-H fluorination of simple alcohols, enabled by novel designed exo-directing groups, is described. C(sp² )-H bond fluorination was achieved by using a simple acetone oxime as auxiliary, whereas a new, modular and easily accessible bidentate auxiliary was developed for the efficient and site-selective fluorination of various primary methyl, methylene, and benzylic C(sp³ )-H bonds. Fluorinated alcohols can readily be accessed by the removal of auxiliaries, and significantly expands the synthetic prospect of the present procedure.

Directing activator-assisted regio- and oxidation state-selective aerobic oxidation of secondary C(sp(3))-H bonds in aliphatic alcohols

The regioselective conversion of an unactivated C(sp(3))-H bond of a methylene carbon (CH2) into a C-O single bond is an attractive reaction in organic synthesis. Herein, we present a strategy for a regio- and oxidation state-selective aerobic C-H oxidation based on an N-hydroxyamide-derived directing activator (DA), which is attached to a hydroxy group in alcohol substrates. The DA reacts with NOx species generated in situ from NaNO2, a Brønsted acid, and aerobic oxygen, and effectively generates an amidoxyl radical from the N-hydroxy moiety of the DA. Then, the amidoxyl radical promotes site-selective intramolecular C-H abstraction from methylenes with γ- (or δ-) selectivity. The thus-generated methylene radicals are trapped by molecular oxygen and NO. This process results in the predominant formation of nitrate esters as products, which suppresses undesired overoxidation. The products can be easily converted into alcohols after hydrogenolysis.

Sulfamate Esters Guide Selective Radical-Mediated Chlorination of Aliphatic C-H Bonds

Masked alcohols are particularly appealing as directing groups because of the ubiquity of hydroxy groups in organic small molecules. Herein, we disclose a general strategy for aliphatic γ-C(sp3 )-H functionalization guided by a masked alcohol. Specifically, we determine that sulfamate ester derived nitrogen-centered radicals mediate 1,6-hydrogen-atom transfer (HAT) processes to guide γ-C(sp3 )-H chlorination. This reaction proceeds through a light-initiated radical chain-propagation process and is capable of installing chlorine atoms at primary, secondary, and tertiary centers.

Cobalt(ii)-catalyzed chelation-assisted C–H iodination of aromatic amides with I2

An air stable and inexpensive cobalt-metal, mild and efficient catalytic system.

Radical-mediated intramolecular β-C(sp3)–H amidation of alkylimidates: facile synthesis of 1,2-amino alcohols

A new radical-mediated intramolecular β-C(sp³)–H amidation reaction of O-alkyl trichloro- or arylimidates is reported.

Nickel-catalyzed C-H/N-H annulation of aromatic amides with alkynes in the absence of a specific chelation system

The Ni-catalyzed reaction of aromatic amides with alkynes in the presence of KOBu t involves C-H/N-H oxidative annulation to give 1(2H)-isoquinolinones. A key to the success of the reaction is the use of a catalytic amount of strong base, such as KOBu t . The reaction shows a high functional group compatibility. The reaction with unsymmetrical alkynes, such as 1-arylalkynes, gives the corresponding 1(2H)-isoquinolinones with a high level of regioselectivity. This discovery would lead to the development of Ni-catalyzed chelation-assisted C-H functionalization reactions without the need for a specific chelation system.

Meta-Selective C-H Arylation of Aromatic Alcohols with a Readily Attachable and Cleavable Molecular Scaffold

The first example of meta-selective C-H arylations of arene alcohol-based substrates is described. The strategy involves the combination of the transient norbornene strategy with the quinoline-based acetal scaffold to achieve the formation of biaryl compounds. Both a two-step meta-arylation/scaffold cleavage process and a total telescoping procedure are described, highlighting the convenient attributes of attachment, removal, and recovery of the acetal scaffold. Moreover, the meta-arylated compounds can be further derivatized via ortho-selective functionalizations. These processes establish a foundation for catalytic polyfunctionalization of alcohol-based compounds.

Ligand-assisted palladium-catalyzed C-H alkenylation of aliphatic amines for the synthesis of functionalized pyrrolidines

The development of a ligand-assisted Pd-catalyzed C-H alkenylation of aliphatic amines is reported. Our studies indicated that an amino-acid-derived ligand renders the C-H bond activation step reversible and promotes the traditionally difficult alkenylation process. The C(sp³)-H alkenylation proceeds through a 5-membered-ring cyclopalladation pathway that allows access to complex aliphatic heterocycles that could be useful to practioners of synthetic and medicinal chemistry.

Recent advances in positional-selective alkenylations: removable guidance for twofold C–H activation

Recent advances in transition-metal catalyzed positional-selective alkenylations via twofold C–H activation directed by removable or traceless directing groups are reviewed.

Palladium(II)-Catalyzed ortho-Arylation of Aromatic Alcohols with a Readily Attachable and Cleavable Molecular Scaffold

A palladium(II)-catalyzed C-H arylation process of alcohols has been developed. The strategy utilizes a novel quinoline-based hemiacetal scaffold that can direct the selective C-H bond functionalization. This reaction provides a useful method to construct biaryl compounds of benzyl alcohols in good to excellent yields. The new molecular scaffold can be readily attached, removed, and recovered.