Solvent-Dependent Chemoselectivity in Ruthenium-Catalyzed Cyclization of Iodoalkyne−Epoxide Functionalities

Rhodium and Isothiourea Dual Catalysis: Enantiodivergent Transformation of Terminal Alkynes

A dual rhodium/isothiourea catalytic system was developed for the enantiodivergent transformation of terminal alkynes. Under synergistic rhodium/isothiourea dual catalysis, terminal alkynes can be creatively utilized as precursors for C1-ammonium enolate species, which subsequently participate in [4 + 2] and [2 + 2] annulation reactions with α,β-unsaturated ketimines or ketones, respectively. A wide range of chiral lactams and lactones were obtained in excellent yields and stereoselectivities (up to >20:1 dr, 98% ee).

Oxidation of Alkynes via Catalytic Metal-Vinylidenes

Metal-vinylidenes, generated by treatment of terminal alkynes with transition metals, are very useful intermediates in modern synthetic chemistry as shown by the high number of transformations in which they are involved. When a metal-vinylidene is generated in the presence of an oxidant, its immediate oxidation to a ketene occurs. In this short review, recent synthetic applications of the oxidation of alkynes via ketene intermediates from initially formed metal-vinylidenes­ are highlighted.

1 Introduction

2 Oxidation of Metal-Vinylidenes with Internal Oxidants

3 Oxidation of Metal-Vinylidenes with External Oxidants

4 Conclusions

Ruthenium-Catalyzed Oxidative Amidation of Alkynes to Amides

Complex CpRuCl(PPh₃)₂ catalyzes reactions of terminal alkynes with 4-picoline N-oxide and primary and secondary amines to afford the corresponding amides. The reactions occur in chlorinated solvent and aqueous medium, showing applications in peptide chemistry. Stoichiometric studies reveal that the true catalysts of the processes are the vinylidene cations [CpRu(═C═CHR)(PPh₃)₂]⁺ which are oxidized to the Ru(η²-CO)-ketenes by the N-oxide. Finally, nucleophilic additions of primary and secondary amines to the free ketenes yield the corresponding amides.

Multicomponent Reactions, Solvent-Free Synthesis of 2-Amino-4-aryl-6-substituted Pyridine-3,5-dicarbonitrile Derivatives, and Corrosion Inhibitors Evaluation

A number of 2-amino-4-aryl-6-substituted pyridine-3,5-dicarbonitrile derivatives were synthesized via one-pot multicomponent condensation reactions of different aromatic aldehydes with malononitrile and different primary amines, using different molecular ratios and different reaction conditions to achieve considerable product yields. Moreover, we succeed, for the first time to develop a new method to synthesize the aforementioned under the fusion condition without using solvent and catalysts. With this method, a wide range of novel 2-amino-3,5-dicyano-4-aryl-6-substituted aminopyridine derivatives were synthesized with high yields and board substrate of functional groups. The synthesized pyridine derivatives were found to have a corrosion inhibition efficiency, the rate of which increased with the increasing concentration of the derivatives. The structures of the new compounds were elucidated by spectroscopic data and elemental analyses.

Ruthenium-Catalyzed Cycloisomerization of 2-Alkynylanilides: Synthesis of 3-Substituted Indoles by 1,2-Carbon Migration

We developed ruthenium-catalyzed cycloisomerization of alkynylanilides that gave 3-substituted indoles in high yields. The reaction proceeded via the disubstituted vinylidene ruthenium complex that was formed by the 1,2-carbon migration.

Unprecedented one-pot chemocontrolled entry to thioxoimidazolidinones and aminoimidazolones: synthesis of kinase inhibitor leucettamine B

A novel and highly chemoselective protocol for the construction of thioxoimidazolidinone and aminoimidazolone frameworks was explored, and the influence of the reaction conditions on product formation was studied to establish two distinct approaches for their selective formation. In this one-pot reaction, ambient temperature generally resulted in the formation of thioxoimidazolidinones, whereas microwave irradiation provided aminoimidazolones exclusively. An attempt to elucidate the observed chemoselectivity is described, and the products were confirmed by X-ray studies. One-pot synthesis toward Leucettamine B, a marine alkaloid, was achieved on the basis of this protocol.

Beyond conventional routes, an unprecedented metal-free chemoselective synthesis of anthranilate esters via a multicomponent reaction (MCR) strategy

A hitherto unreported route for the synthesis of anthranilate esters is demonstrated using 2-nitrobenzaldehyde, malonitrile and an alcohol or amine via a metal and oxidant free multicomponent reaction (MCR) strategy.

Rhodium-catalyzed oxygenative [2 + 2] cycloaddition of terminal alkynes and imines for the synthesis of β-lactams

A rhodium-catalyzed oxygenative [2 + 2] cycloaddition of terminal alkynes and imines has been developed, which gives β-lactams as products with high trans diastereoselectivity. In the presence of a Rh(I) catalyst and 4-picoline N-oxide, a terminal alkyne is converted to a rhodium ketene species via oxidation of a vinylidene complex and subsequently undergoes a [2 + 2] cycloaddition with an imine to give rise to the 2-azetidinone ring system. Mechanistic studies suggest that the reaction proceeds through a metalloketene rather than free ketene intermediate. The new method taking advantage of catalytic generation of a ketene species directly from a terminal alkyne provides a novel and efficient entry to the Staudinger synthesis of β-lactams under mild conditions.

Synthesis of fully-substituted pyridines and dihydropyridines in a highly chemoselective manner utilizing a multicomponent reaction (MCR) strategy

An efficient protocol has been developed for the synthesis of pyridines and 1,4-dihydropyridines based on chemoselective multicomponent reactions.

Reactions of aromatic S- and O-enynes with two methyl substituents on the olefinic unit assisted by a ruthenium complex: tandem cyclization and product selectivity

Ruthenium-assisted cyclizations of two enynes, HC≡CCH(OH)(C6H4)X-CH2CH=CMe2 (X=S (1a), O (1b)), each of which contains two terminal methyl substituents on the olefinic parts, are explored. The reaction of 1a in CH2Cl2 gives the vinylidene complex 2a from the first cyclization and two side products, 3a and the carbene complex 4a with a benzothiophene ligand. The same reaction in the presence of HBF4 affords 4a exclusively. Air oxidation of 4a in the presence of Et3N readily gives an aldehyde product. In MeOH, tandem cyclizations of 1a generate a mixture of the benzothiochromene compound 10a and the carbene complex 7a also with a benzothiochromene ligand. First, cyclization of 1b likewise proceeds in CH2Cl2 to give 2b. Tandem cyclization of 1b in MeOH yields comparable products 10b and 7b with benzochromene moieties, yet with no other side product. The reaction of [Ru]Cl with HC≡CCH(OH)(C6H4)S-CH2CH=CH2 (1c), which contains no methyl substituent in the olefinic part, in MeOH gives the carbene complex 15c with an unsubstituted thiochromene by means of a C-S bond formation. Structures of 3a and 15c are confirmed by X-ray diffraction analysis. The presence of methyl groups of enynes 1a and 1b promotes sequential cyclization reactions that involve C-C bond formation through carbocationic species.

Microwave-assisted solvent-dependent reaction: chemoselective synthesis of quinoxalin-2(1H)-ones, benzo[d]imidazoles and dipeptides

A microwave-assisted solvent-dependent chemoselective reaction dealing with 4-arylidene-2-phenyloxazol-5-ones and diverse ortho-diamines to achieve three types of molecular scaffolds, 3-benzylquinoxalin-2(1H)-ones, benzimidazole and β-amino dipeptides is reported. The procedures feature short reaction time, good to excellent yields, operational simplicity, as well as easily available starting materials. These compounds are of importance for organic chemistry and medicinal chemistry research.

On the validity of Au-vinylidenes in the gold-catalyzed 1,2-migratory cycloisomerization of skipped propargylpyridines

A mechanism of the Au-catalyzed cycloisomerization of propargylpyridines has been investigated. Both DFT computational and experimental results strongly support generation of a Au-carbene via a cyclization/proton transfer sequence over the previously proposed path involving a Au-vinylidene intermediate. For the β-Si-substituted Au-carbene (G = SiR(3)), a 1,2-Si migration was shown to be kinetically favored over a 1,2-H shift. This study highlights the importance of alternative pathways that could explain reactivities commonly attributed to an alkyne-vinylidene isomerization in Au catalysis.

Metal vinylidenes as catalytic species in organic reactions

Organic vinylidene species have found limited use in organic synthesis owing to their inaccessibility. In contrast, metal vinylidenes are much more stable and may be readily accessed through transition-metal activation of terminal alkynes. These electrophilic species may be trapped by a number of nucleophiles. Additionally, metal vinylidenes can participate in pericyclic reactions and processes that involve migration of a metal ligand to the vinylidene species. This Focus Review addresses the reactions and applications of metal vinylidenes in organic synthesis.

Catalytic carbophilic activation: catalysis by platinum and gold pi acids

The ability of platinum and gold catalysts to effect powerful atom-economic transformations has led to a marked increase in their utilization. The quite remarkable correlation of their catalytic behavior with the available structural data, coordination chemistry, and organometallic reactivity patterns, including relativistic effects, allows the underlying principles of catalytic carbophilic activation by pi acids to be formulated. The spectrum of reactivity extends beyond their utility as catalytic and benign alternatives to conventional stoichiometric pi acids. The resulting reactivity profile allows this entire field of catalysis to be rationalized, and brings together the apparently disparate electrophilic metal carbene and nonclassical carbocation explanations. The advances in coupling, cycloisomerization, and structural reorganization--from the design of new transformations to the improvement to known reactions--are highlighted in this Review. The application of platinum- and gold-catalyzed transformations in natural product synthesis is also discussed.

Metal Vinylidenes and Allenylidenes in Catalysis: Applications in Anti-Markovnikov Additions to Terminal Alkynes and Alkene Metathesis

The involvement of a catalytic metal vinylidene species was proposed for the first time in 1986 to explain the regioselective formation of vinyl carbamates directly from terminal alkynes, carbon dioxide, and amines. Since this initial report, various metal vinylidenes and allenylidenes, which are key activation intermediates, have proved extremely useful for many alkyne transformations. They have contributed to the rational design of new catalytic reactions. This 20th anniversary is a suitable occasion to present the advancement of organometallic vinylidenes and allenylidenes in catalysis.

Short and Efficient Synthesis of Coronene Derivatives via Ruthenium-Catalyzed Benzannulation Protocol

[reaction: see text] TpRuPPh3(CH3CN)2PF6 (3 mol %) was very active in catalytic benzannulation of 1-phenyl-2-ethynylbenzenes in dichloroethane (60 degrees C, 36 h) to afford phenanthrene in 95% yield. This method is applicable to the synthesis of various polycyclic aromatic hydrocarbons via two- and four-fold benzannulations, including various substituted coronene derivatives (53-86% yields) using this catalyst at a moderate loading (10 mol %).