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In light of the small ring strain of five/six-membered cyclic ethers, constructing complex molecules via ring-opening reactions has consistently been a highly challenging topic in organic synthesis. Induced by Lewis acids, the charge redistribution in cyclic ethers forms oxonium ylide intermediates, thereby activating the C–O bond and subsequently facilitating nucleophilic attack for ring opening. In recent years, a variety of novel Lewis acids, encompassing those with new metal centers and frustrated Lewis pairs (FLPs), have been effectively utilized to induce the formation of oxonium ylides, offering a diverse array of methods for the ring opening of five/six-membered cyclic ethers. This review conveys the extensive application advancements of diverse Lewis acid types for cyclic ether ring-opening reactions over the past two decades, originating from the perspective of the classification of Lewis acids. Furthermore, the substrate applicability and chemical transformation efficiency of these Lewis acids in the ring-opening reactions of cyclic ethers have also been discussed herein. [ABSTRACT FROM AUTHOR]

This review deals with the functionalization of double bonds carried out in the presence of a chiral catalyst exploiting the intramolecular attack to haliranium ions by nucleophilic nitrogen of amides or carbamates prepared from achiral aminoalkenes, and the C–N bonds formation leads to highly enantioenriched nonaromatic heterocycles. A range of protocols are reported, emphasizing the synthesis of many natural and biologically active products of pharmacological interest prepared according to this methodology. [ABSTRACT FROM AUTHOR]

The catalytic hydrodefluorination (HDF) with a bifunctional azairidacycle using HCOOK was examined for cyano- and chloro-substituted fluoroarenes, including penta- and tetrafluorobenzonitriles, tetrafluoroterephthalonitrile, tetrafluorophthalonitrile, 3-chloro-2,4,5,6-tetrafluoropyridine, and 4-cyano-2,3,5,6-tetrafluoropyridine. The reaction was performed in the presence of a controlled amount of HCOOK with a substrate/catalyst ratio (S/C) of 100 in a 1:1 mixture of 1,2-dimethoxyethane (DME) and H2O at an ambient temperature of 30 °C to obtain partially fluorinated compounds with satisfactory regioselectivities. The C–F bond cleavage proceeded favorably at the para position of substituents other than fluorine, which is in consonance with the nucleophilic aromatic substitution mechanism. In the HDF of tetrafluoroterephthalonitrile and 4-cyano-2,3,5,6-tetrafluoropyridine, which do not contain a fluorine atom at the para position of the cyano group, the double defluorination occurred solely at the 2- and 5-positions, as confirmed by X-ray crystallography. The HDF of 3-chloro-2,4,5,6-tetrafluoropyridine gave preference to the C–F bond cleavage over the C–Cl bond cleavage, unlike the dehalogenation pathway via electron-transfer radical anion fragmentation. In addition, new azairidacycles with an electron-donating methoxy substituent on the C–N chelating ligand were synthesized and served as a catalyst precursor (0.2 mol%) for the transfer hydrogenative defluorination of pentafluoropyridine, leading to 2,3,5,6-tetrafluoropyridine with up to a turnover number (TON) of 418. [ABSTRACT FROM AUTHOR]

In this study, we aimed to enhance the catalytic activity of perovskite catalysts and elucidate their catalytic behavior in the oxidative coupling of methane (OCM), using alkali-added LaAlO3 perovskite catalysts. We prepared LaAlO3-XY (X = Li, Na, K, Y = mol %) catalysts and applied them to the OCM reaction. The results showed that the alkali-added catalysts' activities were promoted compared to the LaAlO3 catalyst. In this reaction, ethane was first synthesized through the dimerization of methyl radicals, which were produced from the reaction of methane and oxygen vacancy in the perovskite catalysts. The high ethylene selectivity of the alkali-added catalysts originated from their abundance of electrophilic lattice oxygen species, facilitating the selective formation of C2 hydrocarbons from ethane. The high COx (carbon monoxide and carbon dioxide) selectivity of the LaAlO3 catalyst originated from its abundance of nucleophilic lattice oxygen species, favoring the selective production of COx from ethane. We concluded that electrophilic lattice oxygen species play a significant role in producing ethylene. We obtained that alkali-adding could be an effective method for improving the catalytic activity of perovskite catalysts in the OCM reaction. [ABSTRACT FROM AUTHOR]

Increasingly stringent regulations aimed at protection of the natural environment have stimulated the search for new synthetic methodologies in organic and medicinal chemistry having no or minimum harmful effect. An interesting approach is the use of alternative activation factors, microwaves (MW) or ultrasounds (US) and also their cross-combination, which has been tested in the fast and efficient creation of new structures. At present, an easy and green hybrid strategy ("Lego" chemistry) is generally recommended for the design of new substances from different chemistry building blocks. Often, selected biologically active components with specific chemical reactivities are integrated by a suitably designed homo- or heterodifunctional linker that modifies the functionality of the starting structure, allowing easy covalent linkage to another molecule. In this study, a fast introduction of heterodifunctional halogenoacidic linker to selected mono-, di- and triphenolic active substances, allowing their functionalization, was investigated. Nucleophilic substitution reaction was chosen to produce final ethers with the reactive carboxylic group from phenols. The functionalization was performed using various green factors initiating and supporting the chemical reactions (MW, US, MW-US). The benefits of the three green supporting methods and different conditions of reactions were analyzed and compared with the results of the reaction performed by conventional methods. [ABSTRACT FROM AUTHOR]

Geometries, equilibrium dissociation energies (De), and intermolecular stretching, quadratic force constants (kσ) are presented for the complexes B⋯CO2, B⋯N2O, and B⋯CS2, where B is one of the following Lewis bases: CO, HCCH, H2S, HCN, H2O, PH3, and NH3. The geometries and force constants were calculated at the CCSD(T)/aug-cc-pVTZ level of theory, while generation of De employed the CCSD(T)/CBS complete basis-set extrapolation. The non-covalent, intermolecular bond in the B⋯CO2 complexes involves the interaction of the electrophilic region around the C atom of CO2 (as revealed by the molecular electrostatic surface potential (MESP) of CO2) with non-bonding or π-bonding electron pairs of B. The conclusions for the B⋯N2O series are similar, but with small geometrical distortions that can be rationalized in terms of secondary interactions. The B⋯CS2 series exhibits a different type of geometry that can be interpreted in terms of the interaction of the electrophilic region near one of the S atoms and centered on the C∞ axis of CS2 (as revealed by the MESP) with the n-pairs or π-pairs of B. The tetrel, pnictogen, and chalcogen bonds so established in B⋯CO2, B⋯N2O, and B⋯CS2, respectively, are rationalized in terms of some simple, electrostatically based rules previously enunciated for hydrogen- and halogen-bonded complexes, B⋯HX and B⋯XY. It is also shown that the dissociation energy De is directly proportional to the force constant kσ, with a constant of proportionality identical within experimental error to that found previously for many B⋯HX and B⋯XY complexes. [ABSTRACT FROM AUTHOR]

Immobilization on Glyoxyl-agarose support (Gx) is one of the best strategies to stabilize enzymes. However, the strategy is difficult to apply at neutral pH when most enzymes are stable and, even when possible, produces labile derivatives. This work contributes to overcoming this hurdle through a strategy that combines solid-phase amination, presence of key additives, and derivative basification. To this end, aminated industrial lipases from Candida artarctica (CAL), Thermomyces lunuginosus (TLL), and the recombinant Geobacillus thermocatenulatus (BTL2) were immobilized on Gx for the first time at neutral pH using anthranilic acid (AA) or DTT as additives (immobilization yields >70%; recovered activities 37.5-76.7%). The spectroscopic evidence suggests nucleophilic catalysis and/or adsorption as the initial lipase immobilization events. Subsequent basification drastically increases the stability of BTL2-glyoxyl derivatives under harsh conditions (t1/2, from 2.1-54.5 h at 70 °C; from 10.2 h-140 h in 80% dioxane). The novel BTL2-derivatives were active and selective in fish oil hydrolysis (1.0-1.8 μmol of polyunsaturated fatty acids (PUFAs) min-1.g-1) whereas the selected TLL-derivative was as active and stable in biodiesel production (fatty ethyl esters, EE) as the commercial Novozyme®-435 after ten reaction cycles (~70% EE). Therefore, the potential of the proposed strategy in producing suitable biocatalysts for industrial processes was demonstrated. [ABSTRACT FROM AUTHOR]

The synthesis of optically active sulfinic acid esters has been accomplished by the acid catalyzed alcoholysis of optically active sulfinamides. Sulfinates are formed in this reaction with a full or predominant inversion of configuration at chiral sulfur or with predominant retention of configuration. The steric course of the reaction depends mainly on the size of the dialkylamido group in the sulfinamides and of the alcohols used as nucleophilic reagents. It has been found that bulky reaction components preferentially form sulfinates with retention of configuration. It has been demonstrated that the stereochemical outcome of the reaction can be changed from inversion to retention and vice versa by adding inorganic salts to the acidic reaction medium. The unusual stereochemistry of this typical bimolecular nucleophilic substitution reaction, as confirmed by kinetic measurements, has been rationalized in terms of the addition-elimination mechanism, A-E, involving sulfuranes as intermediates which undergo pseudorotations. [ABSTRACT FROM AUTHOR]

Organophosphates have been widely used in agrochemistry, as reagents for organic synthesis, and in biochemistry. Phosphate mimics possessing four unique substituents, and thereby a chirality center, are useful in transition metal catalysis and as nucleotide therapeutics. The catalytic, stereocontrolled synthesis of phosphorus-stereogenic centers is challenging and traditionally depends on a resolution or use of stochiometric auxiliaries. Herein, enantioenriched phosphorus centers have been synthesized using chiral nucleophilic catalysis. Racemic H-phosphinate species were coupled with nucleophilic alcohols under halogenating conditions. Chiral phosphonate products were synthesized in acceptable yields (33–95%) and modest enantioselectivity (up to 62% ee) was observed after identification of an appropriate chiral catalyst and optimization of the solvent, base, and temperature. Nucleophilic catalysis has a tremendous potential to produce enantioenriched phosphate mimics that could be used as prodrugs or chemical biology probes. [ABSTRACT FROM AUTHOR]

Protein cysteines often play crucial functional and structural roles, so they are emerging targets to design covalent thiol ligands that are able to modulate enzyme or protein functions. Some of these residues, especially those involved in enzyme mechanisms—including nucleophilic and reductive catalysis and thiol-disulfide exchange—display unusual hyper-reactivity; such a property is expected to result from a low pKa and from a great accessibility to a given reagent. New findings and previous evidence clearly indicate that pKa perturbations can only produce two–four-times increased reactivity at physiological pH values, far from the hundred and even thousand-times kinetic enhancements observed for some protein cysteines. The data from the molten globule-like structures of ribonuclease, lysozyme, bovine serum albumin and chymotrypsinogen identified new speeding agents, i.e., hydrophobic/electrostatic interactions and productive complex formations involving the protein and thiol reagent, which were able to confer exceptional reactivity to structural cysteines which were only intended to form disulfides. This study, for the first time, evaluates quantitatively the different contributions of pKa and other factors to the overall reactivity. These findings may help to clarify the mechanisms that allow a rapid disulfide formation during the oxidative folding of many proteins. [ABSTRACT FROM AUTHOR]

A straightforward and novel method for transformation of readily available 1,3-benzoxazines to secondary phosphonates and α-aminophosphonates using boron trifluoride etherate as catalyst is developed. The formation of phosphonates proceeds through ortho-quinone methide (o-QM) generated in situ, followed by a phospha-Michael addition reaction. On the other hand, the α-aminophosphonates were obtained by iminium ion formation and the subsequence nucleophilic substitution of alkylphosphites. This method can be also used for the preparation of o-hydroxybenzyl ethers through oxa-Michael addition. [ABSTRACT FROM AUTHOR]

MP2/aug-cc-pVTZ calculations were carried out for the ZFH3-B complexes (Z = C, Si, Ge, Sn and Pb; B = C2H2, C2H4, C6H6 and C5H5-; relativistic effects were taken into account for Ge, Sn and Pb elements). These calculations are supported by other approaches; the decomposition of the energy of interaction, Quantum Theory of Atoms in Molecules (QTAIM) and Natural Bond Orbital (NBO) method. The results show that tetrel bonds with π-electrons as Lewis bases are classified as Z···C links between single centers (C is an atom of the π-electron system) or as Z···π interactions where F‒Z bond is directed to the mid-point (or nearly so) of the CC bond of the Lewis base. The analogous systems with Z···C/π interactions were found in the Cambridge Structural Database (CSD). It was found that the strength of interaction increases with the increase of the atomic number of the tetrel element and that for heavier tetrel elements the ZFH3 tetrahedral structure is more deformed towards the structure with the planar ZH3 fragment. The results of calculations show that the tetrel bond is sometimes accompanied by the Z-H···C hydrogen bond or even sometimes the ZFH3-B complexes are linked only by the hydrogen bond interaction. [ABSTRACT FROM AUTHOR]