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.

Recent Advances on Direct Functionalization of Indoles in Aqueous Media

Indoles and their derivatives have dominated a significant proportion of nitrogen-containing heterocyclic compounds and play an essential role in synthetic and medicinal chemistry, pesticides, and advanced materials. Compared with conventional synthetic strategies, direct functionalization of indoles provides straightforward access to construct diverse indole scaffolds. As we enter an era emphasizing green and sustainable chemistry, utilizing environment-friendly solvents represented by water demonstrates great potential in synthesizing valuable indole derivatives. This review aims to depict the critical aspects of aqueous-mediated indoles functionalization over the past decade and discusses the future challenges and prospects in this fast-growing field. For the convenience of readers, this review is classified into three parts according to the bonding modes (C-C, C-N, and C-S bonds), which focus on the diversity of indole derivatives, the prominent role of water in the chemical process, and the types of catalyst systems and mechanisms. We hope this review can promote the sustainable development of the direct functionalization of indoles and their derivatives and the discovery of novel and practical organic methods in aqueous phase.

Well-Defined NNS-Mn Complex Catalyzed Selective Synthesis of C-3 Alkylated Indoles and Bisindolylmethanes Using Alcohols

Herein, we demonstrated Mn-catalyzed selective C-3 functionalization of indoles with alcohols. The developed catalyst can also furnish bis(indolyl)methanes from the same set of substrates under slightly modified reaction conditions. Mechanistic studies reveal that the C-3 functionalization of indoles is going via a borrowing hydrogen pathway. To highlight the practical utility, a diverse range of substrates including nine structurally important drug molecules are synthesized. Furthermore, we also introduced a one-pot cascade strategy for synthesizing C-3 functionalized indoles directly from 2-aminophenyl ethanol and alcohol.

Molecular Iodine-Catalyzed Selective C-3 Benzylation of Indoles with Benzylic Alcohols: A Greener Approach toward Benzylated Indoles

Iodine-catalyzed selective C-3 benzylation of indoles with benzylic alcohols is developed. The reaction proceeds with molecular iodine as the catalyst under ligand-, metal-, and base-free conditions and tolerates wide functionalities. The experimental observations account for the halogen-bond activation mechanistic pathway for the molecular iodine catalysis.

Synthesis of 3-Aryl-3-Sulfanyl Azetidines by Iron-Catalyzed Thiol Alkylation with N-Cbz Azetidinols

New small-ring derivatives can provide valuable motifs in new chemical space for drug design. 3-Aryl-3-sulfanyl azetidines are synthesized directly from azetidine-3-ols in excellent yield by a mild Fe-catalyzed thiol alkylation. A broad range of thiols and azetidinols bearing electron-donating aromatics are successful, proceeding via an azetidine carbocation. The N-carboxybenzyl group is a requirement for good reactivity and enables the NH-azetidine to be revealed. Further reactions of the azetidine sulfides demonstrate their potential for incorporation in drug discovery programs.

Catalytic Organic Reactions in Water toward Sustainable Society

Traditional organic synthesis relies heavily on organic solvents for a multitude of tasks, including dissolving the components and facilitating chemical reactions, because many reagents and reactive species are incompatible or immiscible with water. Given that they are used in vast quantities as compared to reactants, solvents have been the focus of environmental concerns. Along with reducing the environmental impact of organic synthesis, the use of water as a reaction medium also benefits chemical processes by simplifying operations, allowing mild reaction conditions, and sometimes delivering unforeseen reactivities and selectivities. After the "watershed" in organic synthesis revealed the importance of water, the development of water-compatible catalysts has flourished, triggering a quantum leap in water-centered organic synthesis. Given that organic compounds are typically practically insoluble in water, simple extractive workup can readily separate a water-soluble homogeneous catalyst as an aqueous solution from a product that is soluble in organic solvents. In contrast, the use of heterogeneous catalysts facilitates catalyst recycling by allowing simple centrifugation and filtration methods to be used. This Review addresses advances over the past decade in catalytic reactions using water as a reaction medium.

Direct Substitution of Alcohols in Pure Water by Brønsted Acid Catalysis

With the increasing concern for sustainability, the use of environmentally friendly media to perform chemical processes has attracted the attention of many research groups. Among them, the use of water, as the unique solvent for reactions, is currently an active area of research. One process of particular interest is the direct nucleophilic substitution of an alcohol avoiding its preliminary transformation into a good leaving group, since one of the by-products in this approach would be water. The direct substitution of allylic, benzylic, and tertiary alcohols has been achieved through S_N1-type reactions with catalytic amounts of Brønsted or Lewis acids; however, organic solvents are often required. In this review, the pioneering S_N1 approaches performed in pure water and in the absence of a metal based Lewis acid are compiled and discussed.

Cationic palladium(ii)-catalyzed dehydrative nucleophilic substitutions of benzhydryl alcohols with electron-deficient benzenethiols in water

An efficient direct nucleophilic substitution of benzhydryl alcohols with electron-deficient benzenethiols using cationic Pd(ii) catalysts as Lewis acids in water is reported.