Reversal of enantioselection in an aldol reaction catalyzed by sterically congested bis(oxazoline)–Cu(II) complexes

The use of sterically congested C₂-symmetrical bis(oxazoline) ligands with methylene and ethylene spacers between the oxazoline rings results in the reversal of the enantioselection for aldol reactions catalyzed by bis(oxazoline)–Cu(II) complexes.     

Reaction of oxide radical ion (O.–) with substituted pyrimidines

Pulse radiolysis technique has been used to investigate the reaction of oxide radical ion (O^.–) with 4,6-dihydroxy-2-methyl pyrimidine (DHMP), 2,4-dimethyl-6-hydroxy pyrimidine (DMHP), 5,6-dimethyl uracil (DMU) and 6-methyl uracil (MU) in strongly alkaline medium. The second-order rate constants for the reaction of O^.– with these compounds are in the range 2-5 × 10⁸ dm³ mol^–1 s^–1. The transient absorption spectra obtained with DHMP have two maxima at 290 and 370 nm and with DMHP have maxima at 310 and 470 nm. The transient spectrum from DMU is characterized by its absorption maxima at 310 and 520 nm and that of MU by its single maximum at 425 nm. The intermediate species were found to react with N,N,N,N-tetramethyl-p-phenylenediamine (TMPD) with high G(TMPD^.+) values ranged between 3.9 × 10^–7 molJ^–1 and 4.8 × 10^–7 molJ^–1. These radicals undergo decay by second-order kinetics (2k/ = 1.0-1.7 × 10⁶ s^–1). The reaction of O^.– with the selected pyrimidines is proposed to proceed through a hydrogen abstraction from the methyl group forming allyl type radicals. These are mainly oxidizing radicals and hence readily undergo electron transfer reactions with TMPD.     

Intramolecular Rearrangements of >Si(O–C·=O)(CH2–CH3) and >Si(CH2–CH·–CH3)(CH2–CH3) Radicals

Using ESR and IR spectroscopy, the structures of >Si(O–C^·=O)(CH₂–CH₃) (1) and >Si(CH₂–CH^·–CH₃)(CH₂–CH₃) (2) radicals were deciphered. The directions and kinetic parameters of reactions of intramolecular rearrangements in these radicals were determined. The reactions of hydrogen atom abstraction in radical (1) from the CH₂ and CH₃ groups were studied. It was found that the endothermic reaction of hydrogen atom abstraction from the methyl group occurs at a higher rate than the exothermic reaction with the methylene group. The differences are determined by changes in the size of a cyclic transition state. Based on the experimental data, the strengths of separate C–H bonds in surface fragments are compared. The rearrangement >Si(CH₂–CH^·–CH₃)(CH₂–CH₃) >Si(C^·(CH₃)₂)(CH₂–CH₃) was discovered and its mechanism was determined. One of its steps is the skeletal isomerization Si- (2)- _. (1)Si- (1)- _. (2). Experimental data are analyzed using the results of quantum-chemical calculations of model systems.     

Reaction of γ-(aminopropyl)siloxanes with phenolphthalein as a route to new siloxane-containing phenols

The reaction of γ-aminopropylorganosiloxanes with phenolphthalein yields monomeric and oligomeric N-alkylsiloxy-3,3-bis(4-hydroxyphenyl)phthalimidines. Their structures and compositions were confirmed by IR and NMR spectroscopy and by HPLC-MS. Studies by GLC and MALDI mass spectrometry showed that the action of the amine and of the water released in the reaction leads to the rearrangement of the siloxane bond in the course of the synthesis with the formation of linear and cyclic carbofunctional oligomeric siloxanephenols and mixed oligophenolaminosiloxanes. The possibility of modification of epoxy resins with the synthesized oligomers was revealed.     

Sol–gel synthesis of Zn-thiourea-SiO2 thin films from (EtO)3Si(CH2)3NHC(S)NHPh as molecular precursor

In order to study new and convenient sol–gel syntheses to homogeneous nanocomposites systems based on zinc sulphide clusters embedded in films of silica glass, a thiourea-functionalized silane ((EtO)₃Si(CH₂)₃NHC(S)NHPh, SilTu) was investigated as starting precursor. Electrospray ionization mass spectrometry (ESI-MS), FT-IR and NMR spectroscopies clearly evidence that SilTu coordinates Zn²⁺ ions in solution through the thiourea function. Both zinc acetate (Zn(CH₃COO)₂) and zinc chloride (ZnCl₂) were used as zinc source. The SilTuZn²⁺ complex is also stable under sol–gel conditions. Alcoholic solutions of SilTu, zinc acetate and tetraethoxysilane Si(OEt)₄ (TEOS) were used to prepare the thin films by dip-coating. Transparent, homogeneous and crack-free layers were obtained under annealing up to 600 °C. The samples were investigated by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). Beside ZnS formation, the presence of oxygenated zinc species was observed inside the silica matrix.     

Synthesis and crystal structures of sterically tuned ether functionalized NHC–silver(I) complexes: antibacterial and nucleic acid interaction studies

A series of new imidazolium salts (1–4) as N-heterocyclic carbene (NHC) precursors have been synthesized by successive N-alkylation method. Reactions of these salts with Ag₂O by varying the metal to salt ratio forms a series of new Ag(I)–NHC complexes (5–8). All compounds were characterized by physico-chemical and spectroscopic techniques. The molecular structures of 1 and 5 were characterized by single-crystal X-ray diffraction analysis. A comparative investigation of the bacterial growth inhibition potential of the salts and respective complexes indicates that 5–8 displayed good antibacterial activities on Staphylococcus aureus (ATCC 12600) and Escherichia coli (ATCC 11303) compared with the salts. Furthermore, it was observed that with increase in chain length at N-positions, the antibacterial activities also increased. Nuclease activity of the reported salts and Ag(I)–NHC complexes with nucleic acids (DNA and RNA) were also studied using agarose gel electrophoresis; the results show that the compounds do not have any apparent interaction with nucleic acids in the absence of hydrogen peroxide (H₂O₂). However, 5 and 8 were efficient in promoting the cleavage of nucleic acids in the presence of H₂O₂.     

Novel sterically congested chiral tripodal phosphite ligands: catalytic asymmetric hydrosilylation of ketones with a Rh(I)–TRISPHOS catalyst

A new chiral trisphosphite ligand, (S,S,S)-2,2′,2″-tris(2,4,8,10-tetra-tert-butyl-dibenzo[d,f][1,3,2]dioxaphosphepin-6-yl-6-oxy)tri-2-propylamine, (S,S,S)-TRISPHOS, was synthesized and coordination chemistry investigated. The Rh(I)–(S,S,S)-TRISPHOS complexes were found to be effective catalysts for the enantioselective hydrosilylation of ketones.     

Synthesis of (NHC)Rh(cod)Cl and (NHC)RhCl(CO)2 complexes – Translation of the Rh- into the Ir-scale for the electronic properties of NHC ligands

Twenty-three different Rh complexes of the (NHC)RhCl(cod) and (NHC)RhCl(CO)₂ type were synthesized from [RhCl(cod)]₂. The electron donating nature of the NHC ligands was changed in a systematic manner. The redox potentials of the various (NHC)RhCl(cod) and the ν(CO) of the various (NHC)RhCl(CO)₂ were determined. A correlation of the Rh redox potentials and the Rh ν(CO), respectively, with the related data from analogous (NHC)IrCl(cod) and (NHC)IrCl(CO)₂ complexes established two linear relationships. The linear regression (R² = 0.993) of the Rh and the Ir redox potentials results in an equation for the redox potential transformation: E_1/2(Ir) = 1.016 · E_1/2(Rh) ? 0.076 V. The linear regression (R² = 0.97) of the Rh and Ir ν_av(CO) results in an equation for the ν_av(CO) transformation: ν_av(CO)Ir = 0.8695 · ν_av(CO)Rh + 250.7 cm^?1. In this manner the Rh and the Ir-scale for the determination of the electron donating properties of NHC ligands are unified.     

Reaction kinetics and thermochemistry of the chemically activated and stabilized primary ethyl radical of methyl ethyl sulfide,CH3SCH2CH2•, with O2 to CH3SCH2CH2OO•

Oxidation of methyl ethyl sulfide (CH₃SCH₂CH₃, methylthioethane, MES) under atmospheric and combustion conditions is initiated by hydroxyl radicals, MES radicals, generated after loss of a H atom via OH abstraction, will further react with O₂ to form chemically activated and stabilized peroxyl radical adducts. The kinetics of the chemically activated reaction between the CH₃SCH₂CH₂• radical and molecular oxygen are analyzed using quantum Rice-Ramsperger-Kassel theory for k(E) with master equation analysis and a modified strong-collision approach to account for further reactions and collisional deactivation. Thermodynamic properties of reactants, products, and transition states are determined by the B3LYP/6-31+G(2d,p), M062X/6-311+G(2d,p), ωB97XD/6-311+G(2d,p) density functional theory, and CBS-QB3, G3MP2B3, and G4 composite methods. The reaction of CH₃SCH₂CH₂• with O₂ forms an energized peroxy adduct CH₃SCH₂CH₂OO• with a calculated well depth of 34.1 kcal mol⁻¹ at the CBS-QB3 level of theory. Thermochemical properties of reactants, transition states, and products obtained under CBS-QB3 level are used for calculation of kinetic parameters. Reaction enthalpies are compared between the methods. The temperature and pressure-dependent rate coefficients for both the chemically activated reactions of the energized adduct and the thermally activated reactions of the stabilized adducts are presented. Stabilization and isomerization of the CH₃SCH₂CH₂OO• adduct are important under high pressure and low temperature. At higher temperatures and atmospheric pressure, the chemically activated peroxy adduct reacts to new products before stabilization. Addition of the peroxyl oxygen radical to the sulfur atom followed by sulfur-oxygen double bond formation and elimination of the methyl radical to form S(= O)CCO• + CH₃ (branching) is a potentially important new pathway for other alkyl-sulfide peroxy radical systems under thermal or combustion conditions.     

Theoretical Studies on Reaction Mechanism of CH3SO with NO

The potential energy surface （PES） of CH3SO radical with NO reaction has been studied at MP2/6-311G（2df, p） and QCISD/6-311G（2df, p） levels. Geometries of the reactants, transition states （TS） and products were optimized at B3LYP/6-311G （d,p） level. The geometries of the transition states were found for the first time. The calculated results show that the reaction can proceed via singlet-state or triplet-state PES. Because of the high energy barrier of triplet surface, the singlet surface reactions are dominant. The topological analysis of electron density shows that there are two kinds of structaral transition states （the bifurcation-type ring structure transition state and the T-shaped conflict structure transition state） in the titled reaction. The total electronic density of the reactants, TS and products and the spin electronic density on the triplet surface were also discussed in this paper.     

Ab initio study of Eu3+–L (L=H2O,H2S,NH2CH3, S(CH3)2, imidazole) complexes

The ground states and binding energies of Eu³⁺–L (L=H₂O,H₂S,NH₂CH₃,S(CH₃)₂, imidazole) complexes has been determined using ab initio techniques. The binding is mostly electrostatic as expected. The empty f orbital is different for the S compounds, being a π-like orbital, while for the O and N containing ligands it is a σ-like orbital. However, the range in the binding energies for the different f holes is small.     

SbCl3-catalyzed solvent-free Friedel–Crafts reaction of phenols with mandelic acids to 3-aryl benzofuran-2(3H)-ones: Synthesis of spirocyclic 2,3-dihydrobenzofuran-2-ones

A facile, SbCl₃-catalyzed, one-pot, tandem Friedel–Crafts/lactonization reaction of phenols and mandelic acids has been developed under solvent-free conditions to afford 3-aryl benzofuran-2(3H)-ones in good to high yields (52–90%). Additionally, the utility of 3-aryl benzofuran-2(3H)-ones is demonstrated by using them as precursors in the synthesis of a new class of spirocyclic benzofuran-2-ones using classical synthetic methodologies.     

The reaction of Sn[CH(SiMe3)2]2 with ethyne: formation of (HCC)Sn[CH(SiMe3)2]2(μ-trans-CHCH)Sn[CH-(SiMe3)2]2(CHCH2)

The reaction of Sn[CH(SiMe₃)₂]₂ and ethyne at ambient temperature affords a mixture of products, from which the title compound has been separated and identified by IR, ¹H, and ¹³C NMR spectroscopy.     

Tetramethyl(2-methoxyethyl)cyclopentadienyl complexes of zirconium(iv). Crystal structure of [η5:η1−C5(CH3)4(CH2CH2OCH3)]ZrCl3

Several novel zirconium(iv) complexes with the chelating oxygen-containing cyclopentadienyl ligand, tetramethyl(2-methoxyethyl)cyclopentadiene, have been synthesized. [⁵:¹-Tetra-methyl(2-methyl)cyclopentadienyl]trichlorozirconium (2), bis[⁵-tetramethyl(2-methoxyethyl)cyclopentadienyl]dichlorozirconium (3), [⁵-pentamethylcyclopentadienyl][⁵-tetra-methyl(2-methoxyethyl)cyclopentadienyl]dichlorozirconium (4), and [⁵-tetra-methyl(2-methylthioethyl)cyclopentadienyl][⁵-tetramethyl(2-methoxyethyl)-cyclopentadienyl]dichlorozirconium (5) have been prepared from the corresponding lithium cyclopentadienide (l). The crystal structure of cyclopentadienyl complex2 has been established by X-ray analysis. The coordination OZr bond in compound2 exists both in the crystalline state and in solutions. No coordination of this type was observed in complexes3–5. Synthesized complexes2–5 are discussed in comparison with their sulfur-containing analogs.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1828–1832, July, 1996.     

Reaction of [HO(CH2)3]3P with α,β-Unsaturated Ketones Containing a Phenylpropanoid Backbone

Abstract

Interaction of 3,4-(MeO)₂-benzylideneacetone with [HO(CH₂)₃]₃P (THPP) was studied in CD₃OD by NMR to compare reactivity of a phenylpropanoid α,β-unsaturated ketone with a corresponding α,β-unsaturated aldehyde. In the presence of HCl, both the ketone and a related cinnamaldehyde first establish an equilibrium with the product formed by nucleophilic attack of the THPP at the C?O bond, [ArCH?CHCX(OD)PR₃]⁺Cl^?(X?H or CH₃, Ar?Ph or 3,4-(MeO)₂C₆H₃). The ketone salt then slowly transforms into [R₃PCH(Ar)CH(D)C(O)CD₃]⁺Cl^?, the phosphonium product of nucleophilic attack of THPP at the C?C bond, whereas the final product from the aldehyde is the (α-ether)phosphonium chloride [ArCH?CHCH(OCD₃)PR₃]⁺Cl^?. In aqueous media, in the absence of HCl, 4-HO-benzylideneacetone, which is similar to a lignin-type, α,β-unsaturated aldehyde model compound, interacts with THPP to afford a stable phosphonium zwitterion, in contrast to the previously studied aldehyde model, which forms dimeric, bisphosphonium products.     

Enantioselective addition of terminal alkynes to N-(diphenylphosphinoyl)imines catalyzed by Zn–BINOL complexes

Chiral nonracemic N-(diphenylphosphinoyl)-protected propargylic amines have been prepared by addition of terminal alkynes to N-(diphenylphosphinoyl)aldimines in the presence of dimethylzinc and 3,3′-dibromo-BINOL as catalyst. The reaction works with a variety of aromatic and heteroaromatic aldimines and with different alkynes, providing the expected products in generally good yields and enantiomeric excesses (up to 96%).     

Reaction between benzoyl isothiocyanate and 2,6-dimethylphenyl isocyanide: synthesis of a sterically congested ten-membered ring containing the S–S–S moiety

Abstract  

2,6-Dimethylphenyl isocyanide reacts with benzoyl isothiocyanate in a clean one-pot reaction to afford a sterically congested ten-membered ring containing the S–S–S moiety in nearly quantitative yield. Dynamic effects are observed in the ¹H nuclear magnetic resonance (NMR) spectrum of this compound for the restricted rotation around two of the Ar–N single bonds.     

(Me3Si)2CH]2(i)PrSi(NHC)Si═Si(Me)Si(i)Pr[CH(SiMe3)2]2]+: a molecule with disilenyl cation character

Reaction of the disilyne-NHC complex 1 [RLSi═SiR: (R = Si(i)Pr[CH(SiMe(3))(2)](2), L = NHC)] with MeOTf gave the cation 2 [RLSi═SiRMe](+), which is the first example of a base-stabilized heavy group 14 element analogue with vinyl cation character. Cation 2 has been fully characterized by multinuclear NMR spectroscopy and X-ray diffraction analysis. The molecular structure indicates that there are significant contributions from the NHC-stabilized cationic resonance structure 2A, the disilene-like structure 2B, and even some contribution from the silylene-like structure 2C.     

Gas-Phase Synthesis of 3-Vinylcyclopropene via the Crossed Beam Reaction of the Methylidyne Radical (CH; X2Π) with 1,3-Butadiene (CH2CHCHCH2; X1Ag)

The crossed molecular beam reactions of the methylidyne radical (CH; X²Π) with 1,3-butadiene (CH₂CHCHCH₂; X¹A_g) along with their (partially) deuterated counterparts were performed at collision energies of 20.8 kJ mol⁻¹ under single collision conditions. Combining our laboratory data with ab initio calculations, we reveal that the methylidyne radical may add barrierlessly to the terminal carbon atom and/or carbon−carbon double bond of 1,3-butadiene, leading to doublet C₅H₇ intermediates with life times longer than the rotation periods. These collision complexes undergo non-statistical unimolecular decomposition through hydrogen atom emission yielding the cyclic cis- and trans-3-vinyl-cyclopropene products with reaction exoergicities of 119±42 kJ mol⁻¹. Since this reaction is barrierless, exoergic, and all transition states are located below the energy of the separated reactants, these cyclic C₅H₆ products are predicted to be accessed even in low-temperature environments, such as in hydrocarbon-rich atmospheres of planets and cold molecular clouds such as TMC-1.     

Reaction of chlorine with platinum(IV) triamine containing N,N-dimethylethylenediamine. The crystal structure of [Pt{(CH3)2N(CH2)2NH2}PyCl3]Cl·H2O[Pt{(CH3)2N(CH2)2NCl}PyCl3], and [Pt{(CH3)2N(CH2)2NH2}PyCl4]

The Platinum(II) diamine with N,N-dimethylethylenediamine (N,N-dimeEn) [Pt{(CH₃)₂N(CH₂)₂NH₂}Cl₂] (I) was synthesized. The reaction of the diamine with pyridine gave Pt(II) tetramine [Pt{(CH₃)₂N(CH₂)₂NH₂}Py₂]Cl₂ (II), which was oxidized with chlorine to give Pt(IV) triamine Pt{[(CH₃)₂N(CH₂)₂}PyCl₃]Cl · H₂O (III). The reaction of III with chlorine (chloroamidation) yielded chloroimide [Pt{(CH₃)₂N(CH₂)₂NCl}PyCl₃] (IV). The IR spectra of complexes I–IV and UV/Vis spectra of III and IV were studied. X-Ray diffraction analysis was performed for III (monoclinic crystals, space group P2₁/c, a = 7.7437(6), b = 8.1100(7), c = 28.52992(2) Å, β = 93.7280(10)°, Z = 4, R _hkl = 0.0420) and IV (orthorhombic crystals, space group Pna2₁, a = 15.7825(12), b = 7.4447(6), c = 12.3099(6) Å, Z = 4, R _hkl = 0.0539). During oxidation of Pt(II) tetramine with chlorine, the pyridine molecule is removed from the cis position relative to the (CH₃)₂N group (trans position relative to the NH₂ group) of N,N-dimethylethylenediamine. The reaction of chloroimide complex IV with concentrated HCl (dechloroamidation) at 20°C afforded the initial complex III; that at 100°C, gave triamine III together with Pt(IV) diamine [Pt(N,N-dimeEn)Cl₄] (V) (monoclinic crystals, space group P2₁/n, a = 7.1278(5), b = 11.5384(8), c = 12.7501(9) Å, β = 93.23(10)°, Z = 4, R _hkl = 0.0239).