Green synthesis of various saturated S-heterocyclic scaffolds: an update

Development of reliable and eco-friendly novel schemes for the synthesis of organic compounds is an important step in the field of organic and medicinal chemistry. Green chemistry-based strategies involving use of catalysts, green solvents, atom economic reactions etc. play a key role because of their exceptional ability to minimize the toxicity or hazards of the side products and processes. With the use of these green techniques, a number of researchers were able to synthesis a wide range of heterocyclic compounds. This chapter highlights the potential and diverse biological activities of saturated sulphur containing heterocyclic compounds including thiirane, thiane, thiolane and many more. The aim of this chapter is to provide fresh perspective on the various techniques employed for the formation of C–S bond by summarizing all green synthesis from 2016 to 2021.

Synthesis and characterization of trigonal bipyramidal FeIII complexes and their solution behavior

A series of air-stable trigonal bipyramidal Fe^III complexes supported by a redox non-innocent NNN pincer ligand, Cz ^tBu(Pyr^R)₂ ^- (R = iPr, Me, or H), were synthesized, fully characterized, and utilized for the investigation of the interaction between acetone and the Fe^III center. The magnetic moments determined from the paramagnetic ¹H NMR spectra in conjunction with EPR and Mössbauer spectroscopy indicate the presence of a high-spin ferric center. Cyclic voltammetry studies feature two quasi-reversible events corresponding to a metal-centered Fe^III/II reduction around -0.40 V (vs. Fc) and a ligand-centered Cz ^tBu(Pyr^R)₂/Cz ^tBu(Pyr^R)₂ ^•+ oxidation at potentials near +0.70 V (vs. Fc). UV-Visible spectroscopy in CH₂Cl₂ showcases ligand-metal charge transfer (LMCT) bands, as well as coordination of acetone to Cz ^tBu(Pyr^H)₂FeCl₂. In situ IR spectroscopy and solution conductivity (κ) measurements of Cz ^tBu(Pyr^R)₂FeCl₂ with varied equivalents of acetone reveal that acetone is weakly associated with the iron center.

Mg2+ ion-catalyzed polymerization of 1,3-dioxolane in battery electrolytes

Mg²⁺ ions in electrolytes can catalyze the polymerization of 1,3-dioxolane to form poly-DOL while anion pairs affect their capability.

NMR Quantification of the Effects of Ligands and Counterions on Lewis Acid Catalysis

The relative Lewis acidity of a variety of metal-ligand catalyst complexes is quantified using ³¹P NMR spectroscopy. Three ³¹P NMR probes, including two new bidentate binding probes, are compared on the basis of different binding modes (i.e., monodentate vs bidentate) and the relative scale of their downfield shift upon binding to Lewis acid complexes. Bidentate coordination of catalyst complexes including metal catalysts, ligands, and counterions were assessed due to their importance to asymmetric catalysis. The effect of ligands, counterions, and additives on Lewis acidity is quantified and correlated to reaction yield at an early time point as an approximation for catalytic activity/efficiency and chelation mode in two organic transformations. Binding studies were performed under catalytically relevant conditions, giving further applicability to synthesis. Insight into activation modes are revealed through this analysis.

Direct Amination of Alcohols Catalyzed by Aluminum Triflate: An Experimental and Computational Study

Among the best-performing homogeneous catalysts for the direct amination of activated secondary alcohols with electron-poor amine derivatives, metal triflates, such as aluminum triflate, Al(OTf)₃ , stand out. Herein we report the extension of this reaction to electron-rich amines and activated primary alcohols. We provide detailed insight into the structure and reactivity of the catalyst under working conditions in both nitromethane and toluene solvent, through experiment (cyclic voltammetry, conductimetry, NMR spectroscopy), and density functional theory (DFT) simulations. Competition between aniline and benzyl alcohol for Al in the two solvents explains the different reactivities. The catalyst structures predicted from the DFT calculations were validated by the experiments. Whereas a S_N 1-type mechanism was found to be active in nitromethane, we propose a S_N 2 mechanism in toluene to rationalize the much higher selectivity observed when using this solvent. Also, unlike what is commonly assumed in homogeneous catalysis, we show that different active species may be active instead of only one.

Reaction pathways at the initial steps of trioxane polymerisation

Cleavage and (re)formation of oxymethylene moieties are essential steps during the build-up of polyoxymethylene (POM), a technically relevant high-performance polymer.

Studies in Cyclization of Aromatic Epoxy-acyclicpolyprenes: Lewis Superacids and Titanocene Chloride

Two catalytic cascade cyclization methods (radical and cationic) to obtain aromatic polycyclic diterpenes hydroxylated at C3 starting from aromatic epoxypolyprenes were developed. The catalytic use of the Lewis superacid bismuth triflate produces a good yield of cyclized 2 from epoxypolyprene 3. This research may well direct future efforts to the synthesis of bioactive natural products.

Synthesis of pyrrolo[1,2-a]naphthyridines by Lewis acid mediated cycloisomerization

Functionalized pyrrolo[1,2-a]naphthyridines were synthesized by application of PtCl₂ and Bi(OTf)₃ as simple Lewis acids in a cycloisomerization reaction.

Design of stereoelectronically promoted super lewis acids and unprecedented chemistry of their complexes

A new family of stereoelectronically promoted aluminum and scandium super Lewis acids is introduced on the basis of state-of-the-art computations. Structures of these molecules are designed to minimize resonance electron donation to central metal atoms in the Lewis acids. Acidity of these species is evaluated on the basis of their fluoride-ion affinities relative to the antimony pentafluoride reference system. It is demonstrated that introduced changes in the stereochemistry of the designed ligands increase acidity considerably relative to Al and Sc complexes with analogous monodentate ligands. The high stability of fluoride complexes of these species makes them ideal candidates to be used as weakly coordinating anions in combination with highly reactive cations instead of conventional Lewis acid-fluoride complexes. Further, the interaction of all designed molecules with methane is investigated. All studied acids form stable pentavalent-carbon complexes with methane. In addition, interactions of the strongest acid of this family with very weak bases, namely, H2, N2, carbon oxides, and noble gases were investigated; it is demonstrated that this compound can form considerably stable complexes with the aforementioned molecules. To the best of our knowledge, carbonyl and nitrogen complexes of this species are the first hypothetical four-coordinated carbonyl and nitrogen complexes of aluminum. The nature of bonding in these systems is studied in detail by various bonding analysis approaches.