Chiral 4-MeO-Pyridine (MOPY) Catalyst for Enantioselective Cyclopropanation: Attenuation of Lewis Basicity Leads to Improved Catalytic Efficiency

The design of pyridine-derived organocatalysts aims at the increase of their Lewis basicity, however such an approach is not always efficient. For example, strongly Lewis basic DMAP is completely inefficient as catalyst in the cyclopropanation reaction. Herein we disclose an alternative approach that relies on attenuation of DMAP Lewis basicity. Specifically, the replacement of 4-dimethylamino substituent in DMAP for 4-MeO group delivered a highly efficient catalyst for cyclopropanation of electron-deficient olefins with α-bromoketones. Kinetic studies provide compelling evidence that the superior catalytic efficiency of 4-MeO pyridine (MOPY) is to be attributed to the favorable balance between Lewis basicity and leaving group ability. The use of chiral, enantiomerically pure MOPY catalyst has helped to achieve high enantioselectivities (up to 91 : 9 er) in the previously unreported pyridine-catalyzed cyclopropanation reaction.

The emission efficiency of cationic solid state luminophores is directly proportional to the intermolecular charge transfer intensity

Cationic luminophores have recently emerged as a class of efficient emitters in both the solid state and solutions. However, the underlying processes that secure the emission in these luminophores are poorly understood. Here, we employ charge transfer integral (CTI) analysis in combination with X-ray single crystal data to uncover the emission mechanism in a series of pyridinium luminophores. We demonstrate that the solid state photoluminescence quantum yield (Φ) of cationic luminophores is directly proportional to the charge transfer (CT) intensity within a network of molecules in the crystal lattice. Electrostatic intermolecular interactions between π⁺-systems in the crystal lattice provide a disproportionately high contribution to the CT intensity and therefore are instrumental in achieving high Φ. In addition, the strength of electrostatic interactions can be increased by a through-space (TS) electron-donation strategy. Hence, electrostatic interactions can be utilized as a tool to achieve radiative CT, which is useful in the development of efficient luminophores, sensors and nonlinear optical materials.

Synthetic Access to Fluorocyclopropylidenes

Herein we report an approach for the straightforward preparation of fluorocyclopropylidene group from aldehydes and ketones via Julia-Kocienski olefination using the newly developed reagent 5-((2-fluorocyclopropyl)sulfonyl)-1-phenyl-1H-tetrazole. Derivatization of monofluorocyclopropylidene compounds includes hydrogenation to deliver fluorocyclopropylmethyl compounds and fluorinated cyclobutanones. The utility of the described method is demonstrated by the synthesis of a fluorocyclopropyl-containing analogue of ibuprofen. Bioisosteric replacement of isobutyl with the fluorocyclopropyl group may be used for tuning biological properties of drug molecules.

Acylative Dynamic Kinetic Resolution of Secondary Alcohols: Tandem Catalysis by HyperBTM and Bäckvall's Ruthenium Complex

Non-enzymatic dynamic kinetic resolution (DKR) of secondary alcohols by enantioselective acylation using an isothiourea-derived HyperBTM catalyst and racemization of slowly reacting alcohol by Bäckvall's ruthenium complex is reported. The DKR approach features high enantioselectivities (up to 99:1), employs easy-to-handle crystalline 4-nitrophenyl isobutyrate as the acylating reagent, and proceeds at room temperature and under an ambient atmosphere. The stereoinduction model featuring cation-π system interactions between the acylated HyperBTM catalyst and π electrons of an alcohol aryl subunit has been elaborated by DFT calculations.

Metal-free glycosylation with glycosyl fluorides in liquid SO2

Liquid SO₂ is a polar solvent that dissolves both covalent and ionic compounds. Sulfur dioxide possesses also Lewis acid properties, including the ability to covalently bind Lewis basic fluoride ions in a relatively stable fluorosulfite anion (FSO₂ ^-). Herein we report the application of liquid SO₂ as a promoting solvent for glycosylation with glycosyl fluorides without any external additive. By using various temperature regimes, the method is applied for both armed and disarmed glucose and mannose-derived glycosyl fluorides in moderate to excellent yields. A series of pivaloyl-protected O- and S-mannosides, as well as one example of a C-mannoside, are synthesized to demonstrate the scope of the glycosyl acceptors. The formation of the fluorosulfite species during the glycosylation with glycosyl fluorides in liquid SO₂ is proved by ¹⁹F NMR spectroscopy. A sulfur dioxide-assisted glycosylation mechanism that proceeds via solvent separated ion pairs is proposed, whereas the observed α,β-selectivity is substrate-controlled and depends on the thermodynamic equilibrium.

trans-Fluorine Effect in Cyclopropane: Diastereoselective Synthesis of Fluorocyclopropyl Cabozantinib Analogs

Investigation of the trans-fluorine effect on the hydrolysis rate of diethyl 2-fluorocyclopropane-1,1-dicarboxylate provides synthetic access to both diastereomers of the fluorocyclopropyl analog of cabozantinib, a c-Met and VEGFR-2 inhibitor used as a first-line treatment for thyroid cancer and as a second-line treatment for renal cell carcinoma. Despite some known potent examples, there are only a few drug molecules that contain fluorocyclopropane moieties. Herein, we present a case study in which the monofluoro analog of a known cyclopropane-containing drug molecule displays an improved in vitro profile compared to the parent nonfluorinated structure. The fluorocyclopropane moiety may offer valuable fine-tuning options for lead optimization in drug discovery.

Cation–π interactions secure aggregation induced emission of planar organic luminophores

The use of non-covalent intermolecular π⁺–π interactions between quaternary pyridinium or imidazolium cations and aromatic π systems is an efficient approach to achieve AIE in planar purely organic luminophores.

Preparative-Scale Synthesis of Vedejs Chiral DMAP Catalysts

A scalable synthesis of chiral Vedejs-type DMAP catalysts is reported. The key step of the synthesis is amination of the enantiomerically pure 4-chloropyridine derivative using well-defined ZnCl₂(amine)₂ complexes. A series of Zn(II)-amine complexes have been synthesized to explore the scope of the ZnCl₂-mediated amination of 4-halopyridines. Mechanistic studies support a Zn(II)-facilitated nucleophilic aromatic substitution as a plausible mechanism for the chlorine-to-amine exchange.