Recent Advances On Direct Formylation Reactions

Aldehydes serve as the key functional group in organic synthesis and are valuable intermediates. The various advanced methods of direct formylation reactions have been reviewed in this article. Overcoming the drawbacks of the traditional methods of formylation, newer methods involving homo and heterogenous catalysts, one pot reactions, solvent free techniques are elaborated, which can be performed under mild conditions and using inexpensive resources.

Formyl Radical Generation from α-Chloro N-Methoxyphthalimides Enables Selective Aldehyde Synthesis

The aldehydes installation by radical formylation constitutes an attractive synthetic strategy. However, the generation of formyl radicals for organic synthesis applications remains unknown. Herein we report the first formyl radical generation from α-chloro N-methoxyphthalimides, which selectively synthesize aldehydes by alkene hydroformylation under mild photoredox conditions. The aldehydes can be installed on acrylates, acrylamides, vinyl sulfones, vinyl ketones, and complex steroids by radical hydroformylation in excellent chemoselectivity and regioselectivity. The concerted hydrochloride elimination for the formyl radical generation from α-chloro methoxy radicals is established by experimental and computational approaches.

(NH4)2S2O8-Mediated Metal-Free Decarboxylative Formylation/Acylation of α-Oxo/Ketoacids and Its Application to the Synthesis of Indole Alkaloids

A metal-free method for the formylation/acylation of indoles and β-carbolines with (NH₄)₂S₂O₈ via direct decarboxylative cross-coupling of α-oxo/ketoacids in moderate to good yields is described. The reaction occurs between ambient temperature and 40 °C under mild reaction conditions with commercially available starting materials. This methodology can be expanded to some biologically active indole alkaloids like pityriacitrins, eudistomins Y₁ and Y₃, and marinacarbolines A-D.

Recent Advances in Photoredox-Mediated Radical Conjugate Addition Reactions: An Expanding Toolkit for the Giese Reaction

Photomediated Giese reactions are at the forefront of radical chemistry, much like the classical tin-mediated Giese reactions were nearly forty years ago. With the global recognition of organometallic photocatalysts for the mild and tunable generation of carbon-centered radicals, chemists have developed a torrent of strategies to form previously inaccessible radical intermediates that are capable of engaging in intermolecular conjugate addition reactions. This Review summarizes advances in photoredox-mediated Giese reactions since 2013, with a focus on the breadth of methods that provide access to crucial carbon-centered radical intermediates that can engage in radical conjugate addition processes.

Cascade Cross-Coupling of Dienes: Photoredox and Nickel Dual Catalysis

Chemical transformations based on cascade reactions have the potential to simplify the preparation of diverse and architecturally complex molecules dramatically. Herein, we disclose an unprecedented and efficient method for the cross-coupling of radical precursors, dienes, and electrophilic coupling partners via a photoredox- and nickel-enabled cascade cross-coupling process. The cascade reaction furnishes a diverse array of saturated carbo- and heterocyclic scaffolds, thus providing access to a quick gain in C-C bond saturation.

Continuous-flow Synthesis of Aryl Aldehydes by Pd-catalyzed Formylation of Aryl Bromides Using Carbon Monoxide and Hydrogen

A continuous-flow protocol utilizing syngas (CO and H2 ) was developed for the palladium-catalyzed reductive carbonylation of (hetero)aryl bromides to their corresponding (hetero)aryl aldehydes. The optimization of temperature, pressure, catalyst and ligand loading, and residence time resulted in process-intensified flow conditions for the transformation. In addition, a key benefit of investigating the reaction in flow is the ability to precisely control the CO-to-H2 stoichiometric ratio, which was identified as having a critical influence on yield. The protocol proceeds with low catalyst and ligand loadings: palladium acetate (1 mol % or below) and cataCXium A (3 mol % or below). A variety of (hetero)aryl bromides at a 3 mmol scale were converted to their corresponding (hetero)aryl aldehydes at 12 bar pressure (CO/H2 =1:3) and 120 °C reaction temperature within 45 min residence time to afford products mostly in good-to-excellent yields (17 examples). In particular, a successful scale-up was achieved over 415 min operation time for the reductive carbonylation of 2-bromo-6-methoxynaphthalene to synthesize 3.8 g of 6-methoxy-2-naphthaldehyde in 85 % isolated yield. Studies were conducted to understand catalyst decomposition within the reactor by using inductively coupled plasma-mass spectrometry (ICP-MS) analysis. The palladium could easily be recovered using an aqueous nitric acid wash post reaction. Mechanistic aspects and the scope of the transformation are discussed.

Palladium/Rhodium Cooperative Catalysis for the Production of Aryl Aldehydes and Their Deuterated Analogues Using the Water-Gas Shift Reaction

A novel Pd/Rh dual-metallic cooperative catalytic process has been developed to effect the reductive carbonylation of aryl halides in moderate to good yield. In this reaction, water is the hydride source, and CO serves both as the carbonyl source and the terminal reductant through the water-gas shift reaction. The catalytic generation of the Rh hydride allows for the selective formation of highly hindered aryl aldehydes that are inaccessible through previously reported reductive carbonylation protocols. Moreover, aldehydes with deuterated formyl groups can be efficiently and selectively synthesized using D2 O as a cost-effective deuterium source without the need for presynthesizing the aldehyde.

Selective nickel-catalyzed dehydrogenative–decarboxylative formylation of indoles with glyoxylic acid

Herein we present a new strategy for the dehydrogenative–decarboxylative coupling of indoles with glyoxylic acid.

Mild, Redox-Neutral Formylation of Aryl Chlorides through the Photocatalytic Generation of Chlorine Radicals

We report a redox-neutral formylation of aryl chlorides that proceeds through selective 2-functionalization of 1,3-dioxolane through nickel and photoredox catalysis. This scalable benchtop approach provides a distinct advantage over traditional reductive carbonylation in that no carbon monoxide, pressurized gas, or stoichiometric reductant is employed. The mild conditions give unprecedented scope from abundant and complex aryl chloride starting materials.