Atom-efficient gold(I)-chloride-catalyzed synthesis of α-sulfenylated carbonyl compounds from propargylic alcohols and aryl thiols: substrate scope and experimental and theoretical mechanistic investigation

Synthesis of α-sulfenylated carbonyl compounds under metal-free conditions

An efficient synthesis of α-sulfenylated carbonyl compounds from propargylic alcohols and aryl thiols under heating conditions is described. The method is characterized by mild conditions, simple operation, metal-free catalysis and good functional group tolerance. Mechanistic studies suggest that the reaction involves a radical pathway and an isomerization process.

Synthesis of dithioacetals via gold-catalysed hydrothiolation of vinyl sulfides

The synthesis of unsymmetrical dithioacetals based on gold catalysis is described. Although many approaches to the preparation of symmetrical dithioacetals have been developed, the methods to access unsymmetrical ones remain limited. In this regard, we report a mild synthetic method with a broad substrate scope. Screening of various gold catalysts identified a catalyst, which allows the hydrothiolation of both activated and unactivated vinyl sulfides with high efficiency. Moreover, the reaction displays broad compatibility for both aryl and aliphatic thiols.

Mild and Expeditious Synthesis of Sulfenyl Enaminones of l-α-Amino Esters and Aryl/Alkyl Amines through NCS-Mediated Sulfenylation

Sulfenylation or selenylation of enaminones of l-α-amino esters requires mild reaction conditions due to the presence of a racemization-prone chiral center and reactive side chains. An N-chlorosuccinimide (NCS)-mediated methodology has been developed for rapid sulfenylation of enaminones of l-α-amino esters and aryl/alkyl amines at room temperature in open air under metal-free conditions. Enaminones of l-α-amino esters bearing aliphatic, aromatic, and heterocyclic side chains react efficiently with diverse aryl/alkyl/heteroaryl thiols (R1SH) in the presence of NCS to afford a library of biologically important sulfenyl enaminones in good-to-excellent yields (71-90%). Under similar reaction conditions, the enaminones also react with benzeneselenol to produce selenyl enaminones in good yield (73-83%). The NCS-mediated pathway generates sulfenyl chloride (R1SCl) as an intermediate which leads to rapid sulfenylation of enaminones through cross-dehydrogenative coupling (CDC) under mild reaction conditions.

From Propargylic Alcohols to Substituted Thiochromenes: gem-Disubstituent Effect in Intramolecular Alkyne Iodo/hydroarylation

This work describes the 6-endo-dig cyclization of S-aryl propargyl sulfides to afford 2H-thiochromenes. The substitution at the propargylic position plays a crucial role in allowing intramolecular silver-catalyzed alkyne hydroarylation and N-iodosuccinimide-promoted iodoarylation. Additionally, a PTSA-catalyzed thiolation reaction of propargylic alcohols was developed to synthesize the required tertiary S-aryl propargyl ethers. The applicability of merging these two methods is demonstrated by synthesizing the retinoic acid receptor antagonist AGN194310.

Assessing Methodologies to Synthesize α-Sulfenylated Carbonyl Compounds by Green Chemistry Metrics

α-Sulfenylated carbonyl compounds are important both as active pharmaceutical ingredients and as intermediates in organic synthesis. Owing to their relevance in synthetic organic chemistry, this Minireview focuses on assessing the most relevant synthetic procedures based on green chemistry metrics. The Minireview starts with the traditional routes and then focuses on more recently developed methodologies. These routes include sulfenylating reagents using organocatalysis, cross-dehydrogenative couplings using in situ halogenations to prevent reactive intermediates in high concentrations, oxidative couplings using terminal oxidants such as DDQ or TEMPO, and redox-neutral couplings using transition metal catalysis. These methodologies have been evaluated on the basis of atom economy, E factor, and the safety and toxicity of the transformations and the solvents used. Besides using green metrics to evaluate these novel methodologies, the synthetic utility is also assessed with regard to the availability of starting materials and the generality of the reactions. This Minireview aims to inspire researchers to apply green assessments to other methodologies and also for them to take measures to increase the greenness of their developed transformations.

Difluoroacetaldehyde N-Triftosylhydrazone (DFHZ-Tfs) as a Bench-Stable Crystalline Diazo Surrogate for Diazoacetaldehyde and Difluorodiazoethane

Despite the growing importance of volatile functionalized diazoalkanes in organic synthesis, their safe generation and utilization remain a formidable challenge because of their difficult handling along with storage and security issues. In this study, we developed a bench-stable difluoroacetaldehyde N-triftosylhydrazone (DFHZ-Tfs) as an operationally safe diazo surrogate that can release in situ two low-molecular-weight diazoalkanes, diazoacetaldehyde (CHOCHN₂ ) or difluorodiazoethane (CF₂ HCHN₂ ), in a controlled fashion under specific conditions. DFHZ-Tfs has been successfully employed in the Fe-catalyzed cyclopropanation and Doyle-Kirmse reactions, thus highlighting the synthetic utility of DFHZ-Tfs in the efficient construction of molecule frameworks containing CHO or CF₂ H groups. Moreover, the reaction mechanism for the generation of CHOCHN₂ from CF₂ HCHN₂ was elucidated by density functional theory (DFT) calculations.

High-Atom Economic Approach To Prepare Chiral α-Sulfenylated Ketones

Chiral α-sulfenylated ketones are versatile building blocks, although there are still several limitations with their preparation. Here we report a new two-step procedure, consisting of Pd-catalyzed hydrothiolation of propargylic alcohols followed by an enantioselective Rh isomerization of allylic alcohols. The isomerization reaction is the key step for obtaining the ketones in their enantioenriched form. The new methodology has a high atom economy and induces good to high levels of enantioselectivity; no waste is produced. A mechanism involving a Rh-hydride-enone intermediate is proposed for the isomerization reaction.

Mechanisms of CO2 Incorporation into Propargylic Amine Catalyzed by Ag(I)/Amine Catalysts

Density functional theory calculations are carried out to explore the detail mechanisms of CO₂ incorporation into propargylic amine catalyzed by Ag(I)/amine catalysts. Our calculations reveal that the whole reaction involves Lewis acid catalysis and Lewis base catalysis stages, and the outcomes of this reaction critically depend on the basicity of amine. A weaker base (i.e., DABCO) makes the Ag center more acidic, thus favoring the Lewis acid catalysis, resulting in benzoxazin-2-one. However, the following rearrangement of benzoxazin-2-one requires a stronger base (i.e., DBU) to stabilize its deprotonated form. Thus, the product selectivity could be subtly tuned by the choice of amine and the condition control, consistent with the experimental observations.

How the Coordinated Structures of Ag(I) Catalysts Affect the Outcomes of Carbon Dioxide Incorporation into Propargylic Amine: A DFT Study

Density functional theory calculations have been carried out to explore the detailed mechanisms for carbon dioxide incorporation of N-unsubstituted propargylic amine catalyzed by Ag(I) catalysts. We show that the reaction undergoes substrate adsorption or displacement, isomerization from amine-coordinated species to the alkyne-coordinated species, CO₂ attack, and proton transfer, giving the carbamate intermediate. Subsequently, the reaction would bifurcate at the intermolecular ring-closing step, which produces five-membered ring (5MR) and six-membered ring (6MR) products at the same time, thus raising a regioselectivity issue. Our calculations reveal that the outcomes of the reaction critically depend on the coordination number and the basicity of the ligands. Higher coordinate number and stronger basicity of the ligands would stabilize the 5MR transition state over the 6MR counterpart. Such a preference can be rationalized by using transition state energy decomposition. All of these results could promote the rational design of noble metal/organic base combined catalysts with higher selectivity.

A zinc-catalyzed oxidative reaction of ynamides with phenols and thiophenols: highly site-selective synthesis of versatile α-aryloxy amides and α-arylthio amides

A zinc-catalyzed oxidative reaction of ynamides with phenols and thiophenols under mild reaction conditions has been developed, which provides various α-aryloxy amides and α-arylthio amides in moderate to good yields, respectively. Importantly, high chemoselectivity is achieved by such a non-noble metal-catalyzed alkyne oxidation.

Metal Catalysis in Thiolation and Selenation Reactions of Alkynes Leading to Chalcogen-Substituted Alkenes and Dienes

This review covers recent achievements in metal-catalyzed Z-H and Z-Z (Z=S, Se) bond addition to the triple bonds of alkynes-a convenient and atom-efficient way to carbon-element bond formation. Various catalytic systems (both homogeneous and heterogeneous) developed to date to obtain mono- and bis-chalcogen-substituted alkenes or dienes, as well as carbonyl compounds or heterocycles, starting from simple and available alkynes and chalcogenols or dichalcogenides are described. The right choice of metal and ligands allows us to perform these transformations with high selectivities under mild reaction conditions, thus tolerating unprotected functional groups in substrates and broadening ways of further modification of the products. The main aim of the review is to show the potential of the catalytic methods developed in synthetic organic chemistry. Thus, emphasis is made on the scope of reactions, types of products that can be selectively formed, convenience, and scalability of the catalytic procedures. A brief mechanistic description is also given to introduce new readers to the topic.