Synthesis of α,β-unsaturated aldehydes based on a one-pot phase-switch dehydrogenative cross-coupling of primary alcohols

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 in cross-coupling of alcohols via borrowing hydrogen catalysis

Application of the borrowing hydrogen strategy facilitates utilization of abundantly available alcohols for linear or branched long-chain alcohols. Selective synthesis of such alcohols is highly challenging and involves the utilization of transition metal catalysts towards the desired cross-coupled product. Herein, we have highlighted recent advances (from 2015 to 2023) towards the synthesis of higher alcohols. Major focus has been given to the development of ligands, including transition metal catalysts. Judicious catalyst design plays a key role in the alkylation process and is summarised in this review.

Proline-catalyzed synthesis of α-substituted (E)-α,β-unsaturated aldehydes from epoxides

A novel, simple and metal-free tandem approach for synthesizing α-substituted (E)-α,β-unsaturated aldehyde derivatives through acid-catalyzed epoxide rearrangement and organocatalyzed aldol condensation processes has been described. This transformation has a broad substrate scope under mild conditions, including epoxides and aldehydes containing diverse functional groups, resulting in moderate to high yields of the desired products. Eventually, large-scale reactions and the synthesis of some bioactive molecules are used to demonstrate the potential applicability of the developed method.

Intertwining Olefin Thianthrenation with Kornblum/Ganem Oxidations: Ene-type Oxidation to Furnish α,β-Unsaturated Carbonyls

A widely applicable, practical, and scalable synthetic method for efficient ene-type double oxidation of alkenes is reported via a two-step alkenyl thianthrenium umpolung/Kornblum-Ganem oxidation strategy. This chemo- and stereoselective procedure allows easy access to various α,β-unsaturated carbonyls that may be otherwise difficult or cumbersome to synthesize by conventional methods. For α-olefins, this metal-free transformation can be tuned according to synthetic needs to produce either the elusive (Z)-unsaturated aldehydes or their (E) counterparts. Moreover, this strategy has enabled streamlined synthesis of distinct butadienyl pheromones and kairomones.

α-Functionally Substituted α,β-Unsaturated Aldehydes as Fine Chemicals Reagents: Synthesis and Application

α-Functionalized α,β-unsaturated aldehydes is an important class of compounds, which are widely used in fine organic synthesis, biology, medicine and pharmacology, chemical industry, and agriculture. Some of the 2-substituted 2-alkenals are found to be the key metabolites in plant and animal cells. Therefore, the development of efficient methods for their synthesis attracts the attention of organic chemists. This review focusses on the recent advances in the synthesis of 2-functionally substituted 2-alkenals. The approaches to the preparation of α-alkyl α,β-unsaturated aldehydes are not included in this review.

Saegusa Oxidation of Enol Ethers at Extremely Low Pd-Catalyst Loadings under Ligand-free and Aqueous Conditions: Insight into the Pd(II)/Cu(II)-Catalyst System

A highly efficient and practical Pd(II)/Cu(OAc)₂-catalyst system of Saegusa oxidation, which converts enol ethers to the corresponding enals with a number of diverse substrates at extremely low catalyst loadings (500 mol ppm) under ligand-free and aqueous conditions, is described. Its synthetic utility was demonstrated by large-scale applications of the catalyst system to important nature molecules. This work allows Saegusa oxidation to become a highly practical approach to preparing enals and also suggests new insight into the Pd(II)/Cu(II)-catalyst system for dehydrogenation of carbonyl compounds and decreasing Pd-catalyst loadings.

Synthesis of α,β-unsaturated carbonyl compounds by carbonylation reactions

Carbonylation reactions represent one of the most important tool box for the synthesis of α,β-unsaturated carbonyl compounds which are key building blocks in organic chemistry. This paper summarizes the most important advances in this field.

Base-Mediated Transition-Metal-Free Dehydrative C-C and C-N Bond-Forming Reactions from Alcohols

In recent years, there has been an increasing interest in using alcohols as alkylating agents for C-C and C-N bond-forming processes employing mainly TM-catalysts. Although BH-catalysis looks like a green atom economy process since water is the only by-product, it often suffers from one or more drawbacks, such as the use of expensive noble metal complexes, capricious ligands, and toxic organic solvents. Therefore, straightforward, efficient, atom economy and environmentally benign alternative protocols are desirable. This review aims to summarize the current knowledge within the published literature about dehydrative processes developed without TM-catalysts. The most recent contributions to this topic have been reviewed keeping into account the new findings reported in this area. The features, strengths, and limitations of these alcohol-based C-C and C-N bond-forming processes has also been taken into account.

NMR and experimental reinvestigation of the condensation reaction between γ-methylene-α,β-unsaturated aldehydes and propargyl aldehydes

An experimental and computational study of the reaction between γ-methylene enals and propargyl aldehydes uncovered a deconjugative aldol condensation.

C-Alkylation by Hydrogen Autotransfer Reactions

The development of practical, efficient, and atom-economical methods for the formation of carbon-carbon bonds remains a topic of considerable interest in current synthetic organic chemistry. In this review, we have summarized selected topics from the recent literature with particular emphasis on C-alkylation processes involving hydrogen transfer using alcohols as alkylation reagents. This review includes selected highlights concerning recent progress towards the modification of catalytic systems for the α-alkylation of ketones, nitriles, and esters. Furthermore, we have devoted a significant portion of this review to the methylation of ketones, alcohols, and indoles using methanol. Lastly, we have also documented recent advances in β-alkylation methods involving the dimerization of alcohols (Guerbet reaction), as well as new developments in C-alkylation methods based on sp (3) C-H activation.

Synthesis of pyrazoles from 1,3-diols via hydrogen transfer catalysis

1,3-Diols engage in ruthenium-catalyzed hydrogen transfer in the presence of alkyl hydrazines to provide 1,4-disubstituted pyrazoles. Regioselective synthesis of unsymmetrical pyrazoles from β-hydroxy ketones is also described.

Stereoselective α-indolylation of enals via an organocatalytic formal cross-coupling with indoles

A novel organocatalytic one-pot cascade bromination–Michael addition dearomatization–nucleophilic rearrangement aromatization cascade process for the direct α-indolylation of unfunctionalized enals from readily available indoles with good yields and high E selectivity has been developed with great simplicity and practicality.

Substitution of alcohols by N-nucleophiles via transition metal-catalyzed dehydrogenation

This review summarizes the recent advances in transition metal-catalyzed dehydrogenative substitution of alcohols by N-nucleophiles since 2009.