Amide Oxygen-Assisted Palladium-Catalyzed Hydration of Alkynes

Revealing the Role of Solvent in anti-Oxypalladation-Triggered Regiocontrolled Domino Reactions for the Synthesis of Benzazepine- and Tetrahydroquinoline-Containing Scaffolds: A Combined Computational and Experimental Study

Palladium-catalyzed regiocontrolled intramolecular oxypalladation-initiated cascades of multifunctional internal alkyne bearing an N-tosyl tether deliver functionalized benzazepine as an exclusive product via 6-endo-dig pathway in 1,4-dioxane solvent and tetrahydroquinoline scaffold as a major product via the 5-exo-dig pathway in the DMSO solvent. The role of the solvent in controlling the regioselectivity is still unknown which can be a major hurdle for further reaction development. Moreover, the reaction in DMSO suffered from having a mixture of products, and no exclusive formation of tetrahydroquinoline was achieved. Herein, we report a concerted computational and experimental study, revealing the role of the solvent in controlling the reaction outcome. DFT study revealed that the formation of the σ-vinylpalladium intermediate is reversible for the 5-exo-dig pathway while it is irreversible for the 6-endo-dig mechanism in 1,4-dioxane and consequently, the 5-exo-dig pathway is difficult to proceed. In contrast, both the 5-exo-dig and 6-endo-dig pathways are irreversible in DMSO. We predicted an exclusive formation of isobenzofuranone-fused chromane via the 5-exo-dig pathway when the N-tosyl tether is replaced by the O-tether in the internal alkyne in DMSO. The experimental study confirms the theoretical hypothesis and provides a highly chemo-divergent approach for the synthesis of biologically significant chromane with a large substrate scope and up to 95% yield at room temperature.

1-Butyl-3-methylimidazolium tetrafluoroborate as suitable solvent for BF3: the case of alkyne hydration. Chemistry vs electrochemistry

In order to replace the expensive metal/ligand catalysts and classic toxic and volatile solvents, commonly used for the hydration of alkynes, the hydration reaction of alkynes was studied in the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (BMIm-BF4) adding boron trifluoride diethyl etherate (BF3·Et2O) as catalyst. Different ionic liquids were used, varying the cation or the anion, in order to identify the best one, in terms of both efficiency and reduced costs. The developed method was efficaciously applied to different alkynes, achieving the desired hydration products with good yields. The results obtained using a conventional approach (i.e., adding BF3·Et2O) were compared with those achieved using BF3 electrogenerated in BMIm-BF4, demonstrating the possibility of obtaining the products of alkyne hydration with analogous or improved yields, using less hazardous precursors to generate the reactive species in situ. In particular, for terminal arylalkynes, the electrochemical route proved to be advantageous, yielding preferentially the hydration products vs the aldol condensation products. Importantly, the ability to recycle the ionic liquid in subsequent reactions was successfully demonstrated.

Autotandem Catalysis: Inexpensive and Green Access to Functionalized Ketones by Intermolecular Iron-Catalyzed Amidoalkynylation/Hydration Cascade Reaction via N-Acyliminium Ion Chemistry

Iron-based catalysts were applied in cascade-type reactions for the synthesis of different carbonyl compounds. The reactions proceeded by a new iron-catalyzed cascade of alkynylation/hydration by using both the σ- and π-Lewis acid properties of iron salts. The alkynylation reactions of several endo and exocyclic acetoxylactams were achieved with three different catalysts including FeCl 3 .6H 2 O, FeCl 3 , and Fe(OTf) 3 showing the efficiency of σ-Lewis acidity of iron (III) salts in catalyzing the alkynylation reaction. We also demonstrated that the reaction sequence could be shortened by the direct use of hydroxylactams, leading to an environmentally friendly protocol, avoiding the need to perform unnecessary lengthy steps. A combination of the hard/soft iron Lewis acid properties was then used to implement an unprecedented tandem intermolecular alkynylation/intramolecular hydration sequence allowing expedient access to a new carbonyl structures from trivial materials.

Aminium cation-radical catalysed selective hydration of (E)-aryl enynes

The hydration of carbon-carbon triple bonds is an important and atom economic synthetic transformation. Herein, we report a mild and selective method for the catalytic Markovnikov hydration of (E)-aryl enynes to the corresponding enones, mediated through the bench-stable aminium salt, tris(4-bromophenyl)ammoniumyl hexachloroantimonate (TBPA). The chemoselective and diastereoselective method proceeds under neutral metal-free conditions, delivering excellent product yields from terminal and internal alkyne units. The synthesis of biologically important (E)-3-styrylisocoumarins, including a formal synthesis of the natural product achlisocoumarin III, demonstrates the utility of this novel transformation.

Synthesis of 1,4,6-Tricarbonyl Compounds via Regioselective Gold(I)-Catalyzed Alkyne Hydration and Their Application in the Synthesis of α-Arylidene-butyrolactones

We report a direct, straightforward, and regioselective hydration of 1,4-enynes designed from Morita-Baylis-Hillman adducts. Under smooth conditions and short reaction times, gold-catalyzed hydration of internal alkynes provides synthetically useful ketones as single regioisomers in yields higher than 90%. The synthetic usefulness of this protocol was demonstrated by the conversion of selected ketones into biologically valuable α-alkylidene-γ-lactones upon reduction with sodium borohydride. In the course of the scope evaluation, we discovered that this methodology could also furnish α-arylidene-β,γ-butenolides.

Graphene oxide-iridium nanocatalyst for the transformation of benzylic alcohols into carbonyl compounds

A catalyst constructed from graphene oxide and iridium chloride exhibited high activity and reliability for the selective transformation of benzylic alcohols into aromatic aldehydes or ketones. Instead of thermal reaction, the transformation was performed under ultrasonication, a green process with low byproduct, high atomic yield and high selectivity. Experimental data obtained from spherical-aberration corrected field emission TEM (ULTRA-HRTEM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy and Raman spectra confirm the nanostructure of the title complex. Noticeably, the activity and selectivity for the transformation of benzylic alcohols remained unchanged within 25 catalytic cycles. The average turn over frequency is higher than 5000 h⁻¹, while the total turnover number (TON) is more than one hundred thousand, making it a high greenness and eco-friendly process for alcohol oxidation.

Graphene oxide–iridium nanostructure act as a robust catalyst exhibiting high activity and reliability for the selective transformation of benzylic alcohols into aromatic aldehydes or ketones.

Metal-Free Carbonyl-Assisted Regioselective Hydration of Alkynes: An Access to Dicarbonyls

Metal-free regioselective hydration of o-alkynylaldehydes with the assistance of neighboring carbonyl oxygen is disclosed. The developed protocol provides a facile route to synthesize a series of multisubstituted carbonyl containing scaffolds that enable the potential application toward the synthesis of highly diversified 5-azaindoles. γ-Carbolines and 2,8-diazacarbazoles can also be accessed directly without isolating the dicarbonyl compounds. The developed methodology is operationally simple and environment-friendly, tolerates a wide variety of functional groups, and is applicable toward large scale synthesis.

A Reusable MOF-Supported Single-Site Zinc(II) Catalyst for Efficient Intramolecular Hydroamination of o-Alkynylanilines

The exploitation of new and active earth-abundant metal catalysts is critical for sustainable chemical production. Herein, we demonstrate the design of highly efficient, robust, and reusable Zn^II -bipyridine-based metal-organic framework (MOF) catalysts for the intramolecular hydroamination of o-alkynylanilines to indoles. Under similar conditions homogeneous catalytic systems mainly provide hydrolysate. Our results prove that MOFs support unique internal environments that can affect the direction of chemical reactions. The Zn^II -catalyzed hydroamination reaction can be conducted without additional ligands, base, or acid, and is thus a very clean reaction system with regard to its environmental impact.

Tandem cyclization of o-alkynylanilines with isocyanides triggered by intramolecular nucleopalladation: access to heterocyclic fused 2-aminoquinolines

Herein, a novel strategy for the synthesis of various heterocyclic fused 2-aminoquinolines via palladium-catalyzed tandem cyclization of o-alkynylanilines with isocyanides has been developed. This process includes trans-oxy/aminopalladation, isocyanide insertion, elimination and 1,3-hydrogen migration. Besides high atom and step economy, this method shows good functional group compatibility with excellent chemo- and regioselectivities under mild reaction conditions.

Palladium-catalyzed hydroformylation of terminal arylacetylenes with glyoxylic acid

A simple, practical and governable palladium-catalyzed hydroformylation of terminal arylacetylenes has been disclosed. The reaction proceeds under syngas-free conditions, using readily available glyoxylic acid as the formyl source, under mild conditions, giving rise to a broad range of α,β-unsaturated aldehydes.

PtO2/PTSA system catalyzed regioselective hydration of internal arylalkynes bearing electron withdrawing groups

A highly efficient PtO₂/PTSA catalyst system for the hydration of a wide array of alkynes was developed. This method proved to be compatible with a large range of functional groups and the ketone products were obtained in high yields. The scope of this methodology was also extended to the synthesis of 3-aryl-isochromenones, -indoles and -benzofurans.

A highly efficient PtO₂/PTSA catalyst system for the hydration of a wide array of alkynes was developed.

Gold(I)-Catalyzed Cyclization/Carbonylation Cascade Reaction of 1,6-Diynes: An Access to β,γ-Unsaturated Ketones

A gold-catalyzed cyclization/carbonylation cascade reaction of 1,6-diynes is reported. The reaction goes through 6-exo-dig and 6-endo-dig cyclizations in sequence, followed by hydration to provide the β,γ-unsaturated ketones with moderate to high yields under mild reaction conditions. This is the first example of intercepting the postulated 1,3-oxazine vinylgold intermediate with another pendant alkyne, which not only verifies the proposed mechanism but also provides the ketone products with cyclized 1,2,3,6-tetrahydropyridine or 3,6-dihydro-2H-pyran frameworks from corresponding diynes.

TBHP-mediated highly efficient dehydrogenative cross-oxidative coupling of methylarenes with acetanilides

A TBHP-mediated dehydrogenative cross-oxidative-coupling approach has been developed for the synthesis of N-arylbenzamides from methylarenes and acetanilides. This cross-coupling method is free of transition metal catalysts and ligands, and no extra organic solvents are required, which make it an useful and attractive strategy for the straightforward construction of C–N bonds. Besides, this conversion is an important complement to the conventional C–N forming strategies.

Synthesis of 2,3-dihydro-1H-pyrroles by intramolecular cyclization of N-(3-butynyl)-sulfonamides

Hydroamination of 3-butynamine derivatives to give non-aromatic 2,3-dihydropyrroles was achieved by using PdCl₂ or AuCl as the catalyst.

KBr/K2S2O8-mediated dibromohydration of N-(2-alkynylaryl)acetamide

A neighbouring acetamino group-participated regioselective dibromohydration of N-(2-alkynylaryl)acetamide is described here for the synthesis of N-(2-(2,2-dibromo-2-arylacetyl)aryl)acetamide.