Expanded‐Ring N‐Heterocyclic Carbenes Efficiently Stabilize Gold(I) Cations,Leading to High Activity in π‐Acid‐Catalyzed Cyclizations

A series of six‐ and seven‐membered expanded‐ring N‐heterocyclic carbene (er‐NHC) gold(I) complexes has been synthesized using different synthetic approaches. Complexes with weakly coordinating anions [(er‐NHC)AuX] (X^?=BF₄^?, NTf₂^?, OTf^?) were generated in solution. According to their ¹³C NMR spectra, the ionic character of the complexes increases in the order X^?=Cl^?<NTf₂^?<OTf^?<BF₄^?. Additional factors for stabilization of the cationic complexes are expansion of the NHC ring and the attachment of bulky substituents at the nitrogen atoms. These er‐NHCs are bulkier ligands and stronger electron donors than conventional NHCs as well as phosphines and sulfides and provide more stabilization of [(L)Au⁺] cations. A comparative study has been carried out of the catalytic activities of five‐, six‐, and seven‐membered carbene complexes [(NHC)AuX], [(Ph₃P)AuX], [(Me₂S)AuX], and inorganic compounds of gold in model reactions of indole and benzofuran synthesis. It was found that increased ionic character of the complexes was correlated with increased catalytic activity in the cyclization reactions. As a result, we developed an unprecedentedly active monoligand cationic [(THD‐Dipp)Au]BF₄ (1,3‐bis(2,6‐diisopropylphenyl)‐3,4,5,6‐tetrahydrodiazepin‐2‐ylidene gold(I) tetrafluoroborate) catalyst bearing seven‐membered‐ring carbene and bulky Dipp substituents. Quantitative yields of cyclized products were attained in several minutes at room temperature at 1 mol % catalyst loadings. The experimental observations were rationalized and fully supported by DFT calculations.     

The heteronuclear bonding between heavier Group 14 elements and transition metals: a novel trioxystannate-iron complex with an unusual stannate fragment

A new trioxystannate-transition metal complex, {[Li][(Me(2)NCH(2)CH(2)O)(3)Sn-Fe(CO)(4)]}(2) (3), exhibiting an unusual type of the heteronuclear bonding between heavier Group 14 elements and transition metals, has been synthesised and completely characterised by NMR, Raman and IR spectroscopy in solution as well as by Raman spectroscopy and X-ray diffraction analysis in the solid state.     

New bifunctional organocatalysts based on (R,R)-cyclohexane-1,2-diamine for the asymmetric addition of nucleophiles to aldehydes

The amide and sulfamide derivatives of (1R,2R)-N,N-diethylcyclohexane-1,2-diamine can serve as organocatalysts for addition of Me₃SiCN and Et₂Zn to aldehydes.     

Chemoselectivity of [4 + 2] cycloaddition in <Emphasis Type="Italic">N</Emphasis>-maleyl- and <Emphasis Type="Italic">N</Emphasis>-allyl-2,6-difurylpiperidin-4-ones

On the basis of a transition state conformation analysis, an attempt was made to explain the high chemoselectivity of intramolecular [4 + 2] cycloaddition in 3-alkyl-2,6-difuryl-N-maleylpiperidin-4-ones. It was shown that the thermal Diels–Alder reaction in these piperidine derivatives takes place through the “boat” conformation and leads to the formation of hydrogenated 1-alkyl-4-(2-furyl)-2H-8,10a-epoxy-pyrido[2,1-a]isoindol-2-ones. The alternative regioisomers, 3-alkyl-4-(2-furyl)-2H-8,10a-epoxy-pyrido[2,1-a]isoindol-2-ones, are hardly formed at all. At the same time, the intramolecular Diels–Alder reaction in the isostructural 3-alkyl-N-allyl-2,6-difurylpiperidin-4-ones, takes place non-regioselectively from the “chair” conformation.     

Sulfur-containing alkenes—A new class of chelating ligands: Synthesis,coordination to palladium,and structure of the resulting complexes

Stable 1,2-disulfanylalkene palladium complexes [(RS-CH=CR′-SR)PdCl₂] were synthesized in 85–94% yield by reaction of palladium(II) chloride with sulfur-containing ligands RS-CH=C(R′)-SR (analogs of dithiolate ligands). The structure of the complexes was studied by NMR spectroscopy and quantum-chemical methods. The binding energy in palladium complexes with bis(arylsulfanyl)- and bis(alkylsulfanyl)alkenes was estimated (DFT) at 50 and 56 kcal/mol, respectively. Variation of substituents on the sulfur atoms is a convenient tool for fine tuning of the ligand properties and controlling the strength of the complex. The bite angle of the ligands does not depend on the substituent nature and is 88–89°, which is typical of square-planar complexes. According to the bite angle, the examined ligands are analogs of well known bidentate phosphine ligands, but the former are more labile since the corresponding binding energy is lower by 36 kcal/mol.     

Facile Access to 2-Selenoxo-1,2,3,4-tetrahydro-4-quinazolinone Scaffolds and Corresponding Diselenides via Cyclization between Methyl Anthranilate and Isoselenocyanates: Synthesis and Structural Features

A practical method for the synthesis of 2-selenoxo-1,2,3,4-tetrahydro-4-quinazolinone was reported. The latter compounds were found to undergo facile oxidation with H₂O₂ into corresponding diselenides. Novel organoselenium derivatives were characterized by the ¹H, ⁷⁷Se, and ¹³C NMR spectroscopies, high-resolution electrospray ionization mass spectrometry, IR, elemental analyses (C, H, N), and X-ray diffraction analysis for several of them. Novel heterocycles exhibited multiple remarkable chalcogen bonding (ChB) interactions in the solid state, which were studied theoretically.     

A Novel Family of Cage-like (CuLi,CuNa, CuK)-phenylsilsesquioxane Complexes with 8-Hydroxyquinoline Ligands: Synthesis,Structure, and Catalytic Activity

The first examples of metallasilsesquioxane complexes, including ligands of the 8-hydroxyquinoline family 1–9, were synthesized, and their structures were established by single crystal X-ray diffraction using synchrotron radiation. Compounds 1–9 tend to form a type of sandwich-like cage of Cu₄M₂ nuclearity (M = Li, Na, K). Each complex includes two cisoid pentameric silsesquioxane ligands and two 8-hydroxyquinoline ligands. The latter coordinates the copper ions and corresponding alkaline metal ions (via the deprotonated oxygen site). A characteristic (size) of the alkaline metal ion and a variation of characteristics of nitrogen ligands (8-hydroxyquinoline vs. 5-chloro-8-hydroxyquinoline vs. 5,7-dibromo-8-hydroxyquinoline vs. 5,7-diiodo-8-hydroxyquinoline) are highly influential for the formation of the supramolecular structure of the complexes 3a, 5, and 7–9. The Cu₆Na₂-based compound 2 exhibits high catalytic activity towards the oxidation of (i) hydrocarbons by H₂O₂ activated with HNO₃, and (ii) alcohols by tert-butyl hydroperoxide. Studies of kinetics and their selectivity has led us to conclude that it is the hydroxyl radicals that play a crucial role in this process.