Metal-Free Selective Synthesis of α,β-Unsaturated Aldehydes from Alkenes and Formaldehyde Catalyzed by Dimethylamine

α,β-Unsaturated aldehydes are important building blocks for the synthesis of a wide range of chemicals, including polymers. The synthesis of these molecules from cheap feedstocks such as alkenes remains a scientific challenge, mainly due to the low reactivity of alkenes. Here we report a selective and metal-free access to α,β-unsaturated aldehydes from alkenes with formaldehyde. This reaction is catalyzed by dimethylamine and affords α,β-unsaturated aldehydes in yields of up to 80 %. By combining Density Functional Theory (DFT) calculations and experiments, we elucidate the reaction mechanism which is based on a cascade of hydride transfer, hydrolysis and aldolization reactions. The reaction can be performed under very mild conditions (30-50 °C), in a theoretically 100 % carbon-economical fashion, with water as the only by-product. The reaction was successfully applied to non-activated linear 1-alkenes, thus opening an access to industrially relevant α,β-unsaturated aldehydes from cheap and widely abundant chemicals at large scale.

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.

Highly Enantioselective Construction of Multifunctional Silicon-Stereogenic Silacycles by Asymmetric Enamine Catalysis

We report the first highly enantioselective construction of silicon-stereocenters by asymmetric enamine catalysis. An unprecedented desymmetric intramolecular aldolization of prochiral siladials was thus developed for the facile access of multifunctional silicon-stereogenic silacycles in high to excellent enantioselectivity. With an enal moiety, these adducts could be readily elaborated for the diverse synthesis of silicon-stereogenic compounds, and for late-stage modification.

An Organocatalytic Oxy-Cope/Michael Cascade Reaction

Ethyl diazepane carboxylate catalyzes the oxy-Cope rearrangement of 4-hydroxy- and 4-alkoxy-1,5-hexadiene-2-carboxaldehydes via iminium ion activation. The resulting intermediate undergoes an intramolecular Michael reaction to furnish cyclopentane-containing products. The reaction proceeds with a range of substrates, including both cyclic and acyclic substrates, and tolerates substitution on the vinyl substituent. Substrates fused on a cycloalkane framework undergo net ring expansion/cyclopentannulation with a high degree of stereocontrol via chairlike transition states. The reaction extends iminium organocatalysis to the oxy-Cope rearrangement, embedded within a complexity-generating cascade transformation.

Aluminium-Catalyzed C(sp)-H Borylation of Alkynes

Historically used in stoichiometric hydroalumination chemistry, recent advances have transformed aluminium hydrides into versatile catalysts for the hydroboration of unsaturated multiple bonds. This catalytic ability is founded on the defining reactivity of aluminium hydrides with alkynes and alkenes: 1,2‐hydroalumination of the unsaturated π‐system. This manuscript reports the aluminium hydride catalyzed dehydroborylation of terminal alkynes. A tethered intramolecular amine ligand controls reactivity at the aluminium hydride centre, switching off hydroalumination and instead enabling selective reactions at the alkyne C−H σ‐bond. Chemoselective C−H borylation was observed across a series of aryl‐ and alkyl‐substituted alkynes (21 examples). On the basis of kinetic and density functional theory studies, a mechanism in which C−H borylation proceeds by σ‐bond metathesis between pinacolborane (HBpin) and alkynyl aluminium intermediates is proposed.

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.

Applications of Discrete Synthetic Macromolecules in Life and Materials Science: Recent and Future Trends

In the last decade, the field of sequence-defined polymers and related ultraprecise, monodisperse synthetic macromolecules has grown exponentially. In the early stage, mainly articles or reviews dedicated to the development of synthetic routes toward their preparation have been published. Nowadays, those synthetic methodologies, combined with the elucidation of the structure-property relationships, allow envisioning many promising applications. Consequently, in the past 3 years, application-oriented papers based on discrete synthetic macromolecules emerged. Hence, material science applications such as macromolecular data storage and encryption, self-assembly of discrete structures and foldamers have been the object of many fascinating studies. Moreover, in the area of life sciences, such structures have also been the focus of numerous research studies. Here, it is aimed to highlight these recent applications and to give the reader a critical overview of the future trends in this area of research.

Catalytic Foldamers: When the Structure Guides the Function

Enzymes are predominantly proteins able to effectively and selectively catalyze highly complex biochemical reactions in mild reaction conditions. Nevertheless, they are limited to the arsenal of reactions that have emerged during natural evolution in compliance with their intrinsic nature, three-dimensional structures and dynamics. They optimally work in physiological conditions for a limited range of reactions, and thus exhibit a low tolerance for solvent and temperature conditions. The de novo design of synthetic highly stable enzymes able to catalyze a broad range of chemical reactions in variable conditions is a great challenge, which requires the development of programmable and finely tunable artificial tools. Interestingly, over the last two decades, chemists developed protein secondary structure mimics to achieve some desirable features of proteins, which are able to interfere with the biological processes. Such non-natural oligomers, so called foldamers, can adopt highly stable and predictable architectures and have extensively demonstrated their attractiveness for widespread applications in fields from biomedical to material science. Foldamer science was more recently considered to provide original solutions to the de novo design of artificial enzymes. This review covers recent developments related to peptidomimetic foldamers with catalytic properties and the principles that have guided their design.

Three Ways Aliphatic Aldehydes React with Nonstabilized Azomethine Ylides

Aliphatic aldehydes readily react with nonstabilized azomethine ylides in one of the three ways to give oxazolidines, pyrrolidines, or Mannich bases, depending on the structure of the starting compound and the reaction conditions. The use of N-(methoxymethyl)-N-[(trimethylsilyl)methyl]benzylamine in DMF provided 5-alkyloxazolidines in 40–97% yields. On the other hand, three-component reactions of aliphatic aldehydes bearing one α-hydrogen with N-methyl(benzyl)glycine and formaldehyde gives Mannich bases in yields of 47–98%. A similar reaction of aldehydes bearing branched alkyl groups and two hydrogen atoms at the α-position proceeds as a domino process that gives 3-alkyl-3-formylpyrrolidines in yields of 34–93%.

Exploration of Diverse Reactive Diad Geometries for Bifunctional Catalysis via Foldamer Backbone Variation

What is the best spatial arrangement of a pair of reactive groups for bifunctional catalysis of a chemical transformation? The conformational versatility of proteins allows reactive group geometry to be explored and optimized via evolutionary selection, but it has been difficult for chemists to identify synthetic scaffolds that allow broad comparative evaluation among alternative reactive group geometries. Here we show that a family of helices, adopted predictably by oligomers composed partially or exclusively of β-amino acid residues, enables us to explore a range of orientations for a pair of pyrrolidine units that must work in tandem to catalyze a crossed aldol reaction. Thus, the crossed aldol reaction serves as an assay of reactive diad efficacy. We have chosen a test reaction free of stereochemical complexity in order to streamline our study of reactivity. The best geometry enhances the initial rate of product formation by two orders of magnitude. Our findings raise the possibility that rudimentary catalysts involving an isolated secondary structure might have facilitated the development of prebiotic reaction networks.

Expedient Organocatalytic Syntheses of 4-Substituted Pyrazolidines and Isoxazolidines

The efficient organocatalytic synthesis of heterocyclic systems of biological relevance is a subject of growing interest. We have found that the pyrrolidine/benzoic acid-catalyzed reaction of α-substituted propenals such as methacrolein, 2-benzylpropenal and 2-(n-hexyl)propenal with activated hydrazines takes place in very good yields (83%–99.6%) under very mild conditions to afford 4-substituted pyrazolidin-3-ols (as diastereomer mixtures); subsequent oxidation with PCC affords the corresponding-4-substituted-3-pyrazolidinones in essentially quantitative yields. In a similar way, 4-substituted isoxazolidinones are obtained with N-Cbz-hydroxylamine as a reagent. The use of chiral diarylprolinol trimethylsilyl ethers as catalysts allows the synthesis of several of these compounds in optically active form, in some cases with excellent enantioselectivity (up to 96:4 er). A preliminary evaluation of the biological activity shows that some of these compounds exhibit interesting antibacterial and antifungal activities.

Antimony(v) cations for the selective catalytic transformation of aldehydes into symmetric ethers, α,β-unsaturated aldehydes, and 1,3,5-trioxanes

1-Diphenylphosphinonaphthyl-8-triphenylstibonium triflate ([2][OTf]) was prepared in excellent yield. This antimony(v) cation was found to selectively catalyze the transformation of aldehydes into symmetric ethers, α,β-unsaturated aldehydes, and 1,3,5-trioxanes.

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

An efficient one-pot ruthenium-catalyzed hydrogen-transfer strategy for a direct access to α,β-unsaturated aldehydes has been developed. The employment of enolates prepared in situ from alcohols avoided handling unstable aldehydes and provided a very appealing route to different cinnamaldehydes substituted in position 2. A silica-grafted amine was used as phase-switch tag leading to a selective one-pot process in favor of cross-dehydrogenative coupling products.

Asymmetric total synthesis of (-)-amphidinolide V through effective combinations of catalytic transformations

An asymmetric total synthesis of (-)-amphidinolide V was accomplished. The synthesis features a base-catalyzed alkynyl silane alcoholysis/ring-closing enyne metathesis sequence for facile construction of a 1,3-diene motif. A diene RCM followed by a ring-contractive allylic transposition of cyclic silyl ethers was incorporated for the stereoselective installation of a functionalized 1,5-diene subunit. An efficient proline-mediated direct cross-aldol condensation of two advanced aldehyde intermediates was utilized for the construction of a key α,β-unsaturated epoxyaldehyde. This total synthesis demonstrates the prowess of metal-catalyzed transformations in complex molecule synthesis.

Non-asymmetric organocatalysis

Asymmetric organocatalysis is now an established methodology for the preparation of chiral compounds. However, these are not the only valuable molecules which can be conveniently obtained. Organocatalytic reactions affording achiral compounds are gaining momentum, opening unexplored pathways in the synthesis of densely functionalized aromatic moieties, olefins and useful molecules such as natural substances.

Diastereoselective multicomponent synthesis and anti-HSV-1 evaluation of dihydrofuran-fused derivatives

Enolizable 6-membered cyclic 1,3-dicarbonyls undergo an efficient and diastereoselective domino condensation/addition/heterocyclization reaction with arylaldehydes and phenacyl chloride, producing highly substituted dihydrofuran-fused derivatives. Ring size of the cyclic 1,3-dicarbonyls and the presence of at least one keto group are crucial to the reaction's success. The new compounds were evaluated in vitro for antiviral activity against herpes simplex virus type-1 (HSV-1). Interestingly, some of them appeared able to interfere with HSV-1 replication, without detection of cytotoxic effects.

Enamine catalysis

The reversible reaction of primary or secondary amines with enolizable aldehydes or ketones affords nucleophilic intermediates, enamines. With chiral amines, catalytic enantioselective reactions via enamine intermediates become possible. In this review, structure-activity relationships and the scope as well as current limitations of enamine catalysis are discussed.