Synthesis and Organocatalytic Applications of Imidazol(in)ium-2-thiocarboxylates

COS-triggered oxygen/sulfur exchange of isatins: chemoselective synthesis of functionalized isoindigos and spirothiopyrans via self-condensation and the thio-Diels-Alder reaction

Herein, we present the first organocatalytic oxygen/sulfur atom exchange reaction (O/S ER) of isatins by employing carbonyl sulfide (COS) as a novel sulfuring reagent under mild reaction conditions. 8-Diazabicyclo[5.4.0]undec-7-ene (DBU) exhibited excellent activity in this approach. Remarkably, the chemical transformations of in situ generated 3-thioisatins can be tuned via the judicious choice of reaction solvents in a one pot process, enabling the selective formation of either functionalized isoindigos in CH₃CN via a self-condensation process or spirothiopyrans in DMSO in the presence of conjugated dienes via the thio-Diels-Alder reaction. Mechanistic studies with experimental and density functional theory approaches revealed that the O/S ER between isatins and COS results in the formation of 3-thioisatins as the key intermediates, which further undergo solvent-controlled transformations to generate isoindigos or spirothiopyrans, respectively. The easily-accessible substrates and operational simplicity make the process suitable for further exploration. The practicality of this transformation was demonstrated by the gram-scale synthesis of isoindigo-based drug molecules and donor-acceptor conjugated polymers.

Progress toward colorimetric and fluorescent detection of carbonyl sulfide

We report here that a fluorescent benzobisimidazolium salt (TBBI) can be used for the fluorescent and colorimetric detection of carbonyl sulfide (COS) over related heterocumulenes including CO2 and CS2 in wet MeCN. The reaction between TBBI and COS in the presence of fluoride yields a highly fluorescent (λem = 354 nm) and colored product (λmax = 321, 621 nm), that is readily observed by the naked eye. We view these results as a first step toward developing activity-based probes for COS detection.

Access to 1,3-oxazine-2,4-diones/1,3-thiazine-2,4-diones via organocatalytic CO2/COS incorporation into allenamides

Organocatalyzed [4 + 2] annulation of CO₂/COS with allenamides is firstly reported to synthesize 1,3-oxazine-2,4-diones and 1,3-thiazine-2,4-diones in moderate to excellent yields under mild reaction conditions. The catalytic potential of a series of Lewis base CO₂ and COS adducts are particularly noted for this process, which features high regio- and chemo-selectivity, step-economy, facile scalability, and easy product derivatization. This study offers the potential for the application of organocatalytic systems for CO₂ and COS chemical transformation.

Organocatalytic cycloaddition of carbonyl sulfide with propargylic alcohols to 1,3-oxathiolan-2-ones

Lewis base-COS adducts were firstly studied as organocatalysts for the cyclization of propargylic alcohols with carbonyl sulfide.

Synthesis, characterization, and gas-phase fragmentation of rhenium-carbonyl complexes bearing imidazol(in)ium-2-dithiocarboxylate ligands

Five complexes with the generic formula [ReBr(CO)₃(κ²-S,S'-S₂C·NHC)] were obtained by reacting [ReBr(CO)₅] with a set of representative imidazol(in)ium-2-dithiocarboxylate zwitterions. These ligands are the adducts of N-heterocyclic carbenes (NHCs) and carbon disulfide. The monometallic Re(i) compounds were further coupled with Na[Re(CO)₅] to afford bimetallic Re(0) species. Depending on the experimental conditions, either octacarbonyl dimers [Re₂(CO)₈(μ₂-κ¹-S,κ¹-S'-S₂C·NHC)] or hexacarbonyl clusters [Re₂(CO)₆(κ²-S,S'-κ³-S,C,S'-S₂C·NHC)] were isolated. All the products were fully characterized using various analytical techniques. Single crystal XRD analysis helped establish with certainty the various binding modes exhibited by the NHC·CS₂ ligands. With bite angles ranging from ca. 104 to 130°, these zwitterions displayed a remarkable flexibility, which also permitted significant twists of the thiometallated rings to preserve a staggered arrangement of the carbonyl groups in the bimetallic systems. Monitoring the chemical shift of the CS₂^- moiety by ¹³C NMR spectroscopy was most useful to detect its change of hapticity upon decarbonylation of the octacarbonyl compounds into hexacarbonyl derivatives. IR spectroscopy was another very convenient tool to identify the type of complex formed in a reaction, based on the pattern of its carbonyl vibration bands. Advanced mass spectrometry techniques showed that all the compounds underwent partial or total decarbonylation in the gas phase with no concomitant fragmentation of the bimetallic assemblies into monometallic ions.

Monothiocarbamates Strongly Inhibit Carbonic Anhydrases in Vitro and Possess Intraocular Pressure Lowering Activity in an Animal Model of Glaucoma

A series of monothiocarbamates (MTCs) were prepared from primary/secondary amines and COS as potential carbonic anhydrase (CA, EC 4.2.1.1) inhibitors, using the dithiocarbamates, the xanthates, and the trithiocarbonates as lead compounds. The MTCs effectively inhibited the pharmacologically relevant human (h) hCAs isoforms I, II, IX, and XII in vitro and showed KIs spanning between the low and medium nanomolar range. By means of a computational study, the MTC moiety binding mode on the CAs was explained. Furthermore, a selection of MTCs were evaluated in a normotensive glaucoma rabbit model for their intraocular pressure (IOP) lowering effects and showed interesting activity.

A cyclic (alkyl)(amido)carbene: synthesis, study and utility as a desulfurization reagent

The synthesis and study of a cyclic (alkyl)(amido)carbene is described.

Probing the Diastereoselectivity of Staudinger Reactions Catalyzed by N-Heterocyclic Carbenes

The reaction of ethylphenylketene with 1,3-dimesitylimidazol-2-ylidene (IMes) or 1,3-dimesitylimidazolin-2-ylidene (SIMes) afforded the corresponding azolium enolates in high yields. The two zwitterions were fully characterized by various analytical techniques. Their thermal stabilities were monitored by thermogravimetric analysis and the molecular structure of SIMes⋅EtPhCCO was determined by means of X-ray crystallography. A mechanism was proposed to account for the trans-diastereoselectivity observed in the [2+2] cycloaddition of ketenes and N-protected imines catalyzed by N-heterocyclic carbenes and an extensive catalytic screening was performed to test its validity. The steric bulk of the NHC catalyst markedly affected the cis/trans ratio of the model β-lactam product. The nature of the solvent used to carry out the Staudinger reaction also significantly influenced its diastereoselectivity. Conversely, the nature of the substituent on the N-sulfonated imine reagent and the reaction temperature were less critical parameters.

Liberation of N-heterocyclic carbenes (NHCs) from thermally labile progenitors: protected NHCs as versatile tools in organo- and polymerization catalysis

The thermally triggered release of catalytically active, free NHCs from various heat-sensitive progenitors is discussed.

N-Heterocyclic carbenes (NHCs) as organocatalysts and structural components in metal-free polymer synthesis

The chemistry of N-heterocyclic carbenes (NHCs) has witnessed tremendous development in the past two decades: NHCs have not only become versatile ligands for transition metals, but have also emerged as powerful organic catalysts in molecular chemistry and, more recently, in metal-free polymer synthesis. To understand the success of NHCs, this review first presents the electronic properties of NHCs, their main synthetic methods, their handling, and their reactivity. Their ability to activate key functional groups (e.g. aldehydes, esters, heterocycles, silyl ketene acetals, alcohols) is then discussed in the context of molecular chemistry. Focus has been placed on the activation of substrates finding analogies with monomers (e.g. bis-aldehydes, multi-isocyanates, cyclic esters, epoxides, N-carboxyanhydrides, etc.) and/or initiators (e.g. hydroxy- or trimethylsilyl-containing reagents) employed in such "organopolymerisation" reactions utilizing NHCs. A variety of metal-free polymers, including aliphatic polyesters and polyethers, poly(α-peptoid)s, poly(meth)acrylates, polyurethanes, or polysiloxanes can be obtained in this way. The last section covers the use of NHCs as structural components of the polymer chain. Indeed, NHC-based photoinitiators, chain transfer agents or functionalizing agents, as well as bifunctional NHC monomer substrates, can also serve for metal-free polymer synthesis.