Palladium-Catalyzed Hydrovinylation of Vinyl Triflates with Alkynes An Approach to the Synthesis of 3-Vinylfuran-2(5H)-ones

Alkynoates as Versatile and Powerful Chemical Tools for the Rapid Assembly of Diverse Heterocycles under Transition-Metal Catalysis: Recent Developments and Challenges

Heterocycles, heteroaromatics and spirocyclic entities are ubiquitous components of a wide plethora of synthetic drugs, biologically active natural products, marketed pharmaceuticals and agrochemical targets. Recognizing their high proportion in drugs and rich pharmacological potential, these invaluable structural motifs have garnered significant interest, thus enabling the development of efficient catalytic methodologies providing access to architecturally complex and diverse molecules with high atom-economy and low cost. These chemical processes not only allow the formation of diverse heterocycles but also utilize a range of flexible and easily accessible building units in a single operation to discover diversity-oriented synthetic approaches. Alkynoates are significantly important, diverse and powerful building blocks in organic chemistry due to their unique and inherent properties such as the electronic bias on carbon-carbon triple bonds posed by electron-withdrawing groups or the metallic coordination site provided by carbonyl groups. The present review highlights the comprehensive picture of the utility of alkynoates (2007-2019) for the synthesis of various heterocycles (> 50 types) using transition-metal catalysts (Ru, Rh, Pd, Ir, Ag, Au, Pt, Cu, Mn, Fe) in various forms. The valuable function of versatile alkynoates (bearing multifunctional groups) as simple and useful starting materials is explored, thus cyclizing with an array of coupling partners to deliver a broad range of oxygen-, nitrogen-, sulfur-containing heterocycles alongside fused-, and spiro-heterocyclic compounds. In addition, these examples will also focus the scope and reaction limitations, as well as mechanistic investigations into the synthesis of these heterocycles. The biological significance will also be discussed, citing relevant examples of drug molecules highlighting each class of heterocycles. This review summarizes the recent developments in the synthetic methods for the synthesis of various heterocycles using alkynoates as readily available starting materials under transition-metal catalysis.

The effects of amines on oxidative homo-coupling of terminal alkynes promoted by copper salts

The effects of all kinds of amines on homo-couplings (Glaser reactions) of terminal alkynes promoted by copper salts and the Sonogashira coupling reactions were studied systematically. Diethylamine (2° amine) can serve as an excellent solvent, base and coordination ligand in the oxidative homo-coupling of terminal alkynes and several modified Glaser coupling procedures have been developed which are based on a catalytic amount of cuprous salts (CuI, CuBr or CuCl) with diethylamine systems. Homo-coupling of terminal acetylenes in the Sonogashira reaction could be inhibited by using triethylamine (3° amine) as reaction medium, and the cross-coupling products were formed as the exclusive products.

A Novel Cu(II)-Catalysed Sonogashira Reaction

In the presence of triphenylphosphine, a novel Cu(II)-catalysed Sonogashira reaction of aryl iodides and bromides with terminal alkynes has been developed, which generates the corresponding cross-coupling products in good yields.

Bifunctional Ligand Enables Efficient Gold-Catalyzed Hydroalkenylation of Propargylic Alcohol

Using the previously designed biphenyl-2-ylphosphine ligand, featuring a remote tertiary amino group, the first gold-catalyzed intermolecular hydroalkenylation of alkynes has been developed. Synthetically valuable conjugated dienyl alcohols are formed in moderate to good yields. A range of alkenyltrifluoroborates are allowed as the alkenyl donor, and no erosion of alkene geometry and/or the propargylic configuration are detected. DFT calculations confirm the critical role of the remote basic group in the ligand as a general-base catalyst for promoting this novel gold catalysis with good efficiency.

Hydrotalcite-supported palladium nanoparticles as catalysts for the hydroarylation of carbon–carbon multiple bonds

Palladium nanoparticles supported on hydrotalcites catalyze the hydroarylation reaction of carbon–carbon multiple bonds.

Diphenylphosphinoethane-functionalized polystyrene resin-supported Pd(0) complex — A highly active and recyclable catalyst for the Stille reaction under aerobic conditions

The polymer-supported palladium(II) diphenylphosphinoethane complex is a highly active catalyst that could be used for the Stille coupling reaction between aryl iodides and (or) chlorides and organostannanes at room temperature under aerobic conditions. This heterogeneous palladium catalyst is efficient, air-stable, and recyclable.

Palladium-catalyzed synthesis of 2-(aminomethyl)indoles from ethyl 3-(o-trifluoroacetamidophenyl)-1-propargyl carbonate

[reaction: see text] The palladium-catalyzed reaction of ethyl 3-(o-trifluoroacetamidophenyl)-1-propargyl carbonate with piperazines in the presence of Pd(PPh(3))(4) in THF at 80 degrees C affords 2-(piperazin-1-ylmethyl)indoles in excellent yields. Good to excellent yields are also obtained with other secondary amines.

The Mechanism of the Phosphine-Free Palladium-Catalyzed Hydroarylation of Alkynes

The mechanism of the Pd-catalyzed hydroarylation and hydrovinylation reaction of alkynes has been studied by a combination of experimental and theoretical methods (B3LYP), with an emphasis on the phosphine-free version. The regioselectivity of the hydroarylation and hydrovinylation shows unexpected differences, which could be attributed mainly to the higher steric demand of the cyclohexenyl group as compared to the phenyl group. Hydroarylation of alpha,beta-acetylenic carbonyl substrates yields a very unusual anti-Michael selectivity, which is shown to result from reaction of the nonconjugated double bond, leaving the conjugation intact. In all cases were the regioselectivities reproduced by the calculations.

Palladium(0) alkyne complexes as active species: a DFT investigation

Alkynes have been found to be excellent ligands for Pd(0); the stability of a range of alkyne-Pd(0) complexes, and their reactivity in oxidative addition, have been investigated by DFT methods.

An efficient, regio- and stereoselective palladium-catalyzed route to tetrasubstituted olefins

An efficient, regio- and stereoselective palladium-catalyzed route to tetrasubstituted olefins has been developed, which involves the intermolecular coupling of an aryl iodide, an internal alkyne, and an arylboronic acid. The reaction involves cis-addition of the aryl group from the aryl halide to the less hindered or less electron-poor end of the alkyne, while the aryl group from the arylboronic acid adds to the other end. [reaction: see text]

Palladium-catalyzed arylation of alpha-methylene-gamma-butyrolactone: 3-benzylfuran-2(5H)-ones vs (Z)-benzylidene-gamma-butyrolactones and their reduction to 3-benzyl-gamma-butyrolactones

[reaction: see text] The palladium-catalyzed arylation of the alpha-methylene-gamma-butyrolactone proceeds in good yields and may be directed toward the synthesis of 3-benzylfuran-2(5H)-ones when the starting aryl iodides contain strongly electron-withdrawing groups. The combined palladium-catalyzed arylation/hydrogenation of the alpha-methylene-gamma-butyrolactone represents a new simple entry into functionalized alpha-benzyl-gamma-butyrolactones.