Synthesis of an octamethyl-18-crown-6 derivative and the X-ray crystal structure of its 2:1 complex with borane-ammonia

The synthesis of 2,2,3,3,11,11,12,12-octamethyl-1,4,7,10,13,16-hexaoxacyclooctadecane (2) from pinacol (3) by a sequence of reactions (3→4→5→6+5→2) involving alkylation (3→ 4), ozonolysis and reduction (4→5), tosylation (5→6), and cyclisation (5+6→2) is reported. With borane-ammonia, the octamethyl-18-crown-6 derivative 2 forms a crystalline 2:1 complex, (BH₃NH₃)₂ · 2. X-Ray crystallography reveals the two guest BH₃NH₃ molecules are hydrogen bonded in a centrosymmetric manner to the opposite faces of the host 2, which adopts an all-gauche (ag⁺a ag^-a)₃ conformation.     

Stabilization of β-turn conformations in Pro-X sequences by disulphide bridging. Synthesis and solution conformations of five cyclic cystine peptides

Five cyclic peptide disulphides of the type

have been synthesized, where X = Gly (1), L-Ala (2), D-Ala (3), Aib (4) and L-Leu (5). ¹H NMR studies at 270 MHz in CDCl₃ and (CD₃)₂SO provide evidence of a Pro-X β-turn conformation, stabilized by a transannular 4→1 hydrogen bond involving the Cys(4) NH, in all the peptides. In addition peptides 2, 4 and 5 also possess a second intramolecular hydrogen bond involving the -NHMe group. The spectroscopic data are consistent with a consecutive type III β-turn conformation for peptides 2, 4 and 5, a type I(III) β-turn structure for 1 and a type II turn for 3.     

Preparation of benzo[4,5]thiazolo[3,2-a]chromeno[4,3-d]pyrimidin-6-one derivatives using MgO-MgAl2O4 composite nano-powder

MgO-MgAl₂O₄ nanocomposite was prepared from the co-precipitation of Mg(NO₃)₂ and Al(NO₃)₃ salts, characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and fourier transform infrared spectroscopy (FTIR) techniques and evaluated in the synthesis of thirty five derivatives of benzo[4,5]thiazolo[3,2-a]chromeno[4,3-d]pyrimidin-6-ones (d₁-d₃₄) via the multi-component reaction of 4-hydroxycoumarins, aldehydes, and 2-aminobenzothiazole derivatives under solvent free condition. The catalytic activity of MgO-MgAl₂O₄ nanocomposite and the synthesis of the above mentioned compounds were investigated under thermal solvent free (times: 1.4–3 h; yields: 75–95%), ultrasonic irradiation (US) conditions (times: 1–2.5 h; yields: 69–97%) and using high-speed ball milling (HSBM) technique (times: 0.7–2.5 h; yields: 67–97%). In all cases, the products were obtained in excellent yields. Nuclear Magnetic Resonance (NMR) and MASS spectroscopy were used to characterize the structure of the desired product. The mechanism for the preparation of compounds d₁-d₃₄ was proposed and confirmed by ¹H NMR investigations.     

Study of lithium complexes with benzo-15-crown-5 ether by X-ray diffraction and IR spectroscopy

The complex formation of lithium with benzo-15-crown-5 (B15C5) was investigated. The complexes LiB15C5H₂OX, where X = Cl^? (1), I^? (2), (3), (5), and LiBF₄B15C5 (4) were synthesized and studied by IR spectroscopy. Complexes 1–4 were examined by X-ray diffraction. According to IR spectroscopy data, the crown ether conformation changes upon dissolution. The interaction of the extracted complex with the solvent was identified.     

Conformational properties of simple monocyclic conjugated enediynes

An investigation employing the AM1 semi-empirical SCF MO method to calculate structure optimization and conformational interconversion pathways for Z-cyclonona-3-ene-1,5-diyne (1), Z-cyclodeca-3-ene-l,5-diyne (2), and Z-cycloundeca-3-ene-1, 5-diyne (3) has been undertaken. The plane-symmetrical half-chair conformation of 1 is calculated to be 23.8 kJ mol⁻¹ more stable than the planar C_2v transition-state geometry. Compound 2 has two energy-minimum conformations with the twistboat (C₂) form being 31.1 kJ mol⁻¹ more stable than the twist (C₁) geometry. The calculated energy barrier for interconversion of C₂ and C₁ conformations is 37.4 kJ mol⁻¹. The unsymmetrical twist-boat form of 3 is calculated to be the most stable conformation. The twist-boat geometry can undergo two degenerate processes to achieve a time-averaged symmetry of C_2v.     

NAD(P)H Models 20: Chemoselective metal ion catalyzed reduction of α-keto-β,γ-unsaturated esters by 1,4-dihydropyridine derivatives

Ethyl 2-oxo-4-aryl-3-butene-1-oates 1a–1c are reduced by NAD(P)H models (1-n-propyl-1,4-dihydronicotinamide (4) and Hantzsch ester(5)), in presence of magnesium perchlorate. One equivalent of the reductant reduces the substrates selectively to the corresponding 2-oxo-4-arylbutanoates (6,10a,b). An additional equivalent and higher temperature, converts ethyl 2-oxo-4-phenylbutanoate (6) to ethyl 2-hydroxy-4-phenylbutanoate (7). Reduction of ethyl 2-oxo-4-phenyl-3-buten-1-oate (1a) by Hantzsch ester in C₂H₅OD or by Hantzsch ester-4,4-d₂ in C₂H₅OH, leads to direct transfer of the hydrogen or deuterium, respectively, without isotopic scrambling. These results have been interpreted to support the hydride transfer mechanism.     

Redox thermochemistry of SrFe1-xCoxO3-d

The enthalpy of oxidation of SrFe_1-xCo_xO_3-d with x=0.33 and 0.67 has been determined by adiabatic calorimetry; average values for x=0.33 and 0.67 are -67±11 and -49.5±9 kJ (mol O₂)^-1. These data and the previously reported value for SrFeO_3-d suggest that the enthalpy of oxidation for pure (perovskite-type) SrCoO_3-d is close to zero. Earlier reported composition - partial pressure data for SrFe_0.67Co_0.33O_3-d are reproduced when preferential oxidation of iron is assumed for low partial pressures of oxygen. This revised version was published online in August 2006 with corrections to the Cover Date.     

Synthesis and structures of binuclear half-sandwich cobalt (III) ortho-carboranedithiolato complexes with silyl-bridged bis(cyclopentadienyl) ligands

Five binuclear half-sandwich cobalt complexes, [(η⁵-C₅H₄)Co(CO)I₂]₂SiMe₂ (3), [(η⁵-C₅H₄)Co(S₂C₂B₁₀H₁₀)]₂SiMe₂ (4), [(η⁵-C₅H₄)]₂Co₂(μ₂-S₂C₂B₁₀H₁₀)SiMe₂ (5), [(η⁵-C₅H₃)CoI₂](μ-I)[(η⁵-C₅H₃)Co(CO)I](SiMe₂)₂ (8), [(η⁵-C₅H₃)Co(S₂C₂B₁₀H₁₀)]₂(SiMe₂)₂ (9), were successfully synthesized in moderate yield by the reactions of corresponding ligands, (C₅H₅)₂SiMe₂ (1) and (C₅H₄)₂(SiMe₂)₂ (6), respectively. The molecular structures of 3, 5, 6, 8 and 9 was determined by X-ray crystallographic analysis, which distinctly depict various molecular structures containing the Cp rings and the metal centers with halide or 1,2-dicarba-closo-dodecaborane-1,2-dithiolato ligands. For the (η⁵-C₅H₄)₂SiMe₂ complexes, coordination of the fragments CpCo favors a exo conformation. With the rigid structure of the di-bridged ligand (C₅H₄)₂(SiMe₂)₂, only cis isomers of the corresponding (η⁵-C₅H₃)₂(Si₂Me₂)₂ complexes are formed. All the complexes have been well characterized by elemental analysis, NMR and IR spectra.     

Synthesis, crystal structure, hydrogen bonding and spectroscopy of transition-metal complexes with the ligand (N,N′-bis(2-pyridylmethyl)-1,3-propanediamine)

Six mononuclear complexes are reported with the tetradentate ligand N,N′-bis(2-pyridylmethyl)-1,3-propanediamine, (abbreviated as pypn) i.e. [Cu(pypn)(ClO₄)₂](H₂O)_1/2 (1), [Fe(pypn)Cl₂](NO₃) (2), [Zn(pypn)Cl](ClO₄) (3), [Co(pypn)(NCS)₂](ClO₄) (4), [Co(pypn)(N₃)₂](ClO₄) (5), [Zn(pypn)(NCS)₂] (6). The synthesis and X-ray crystal structures of all six compounds and their spectroscopic properties are presented.The geometry of the Cu²⁺, Co³⁺, Zn²⁺, Fe³⁺ ions is essentially octahedrally based, with the mm conformation (for Cu) and m_sf conformations for the other 3 metal ions; in compound 3 the geometry around the Zn²⁺ is distorted trigonal bipyramidal. The stabilisation of the crystal lattices is maintained by interesting, relative strong hydrogen bonds.     

Non-chair six-membered-ring conformations. Preference for a twist-boat (or skew) structure in α- -sorbopyranose derivatives

The conformational preferences for 2,3-O-isopropylidene-α- -sorbopyranose derivatives 3–6 were determined by using ¹H NMR data and empirical force field calculations. Proton NMR studies of 3–6 indicate that a twist-boat (or skew) conformation (³S₀) prevails over possible chair forms for each compound. Force-field calculations (MM2, MNDO, AM1) on a model 2,3-O-isopropylidene-α- -sorbopyranose system (18) indicate that the ³S₀ conformation is among the low-energy structures. X-Ray crystallographic analysis of α- -sorbopyranose sulfamate 3, a compound with potent anticonvulsant activity, demonstrates that the ³S₀ skew conformation is manifested in the solid state, as well.     

Role of ligand conformation in the structural diversity of copper(II) and silver(I) coordination polymers containing the angular dipyridyl ligand bearing amide spacer

The new dipyridyl ligands N,N′-(methylenedi-p-phenylene)bis(pyridine-4-carboxamide), L1, and N,N′-(methylenedi-p-phenylene)bis(pyridine-3-carboxamide), L2, incorporating amide spacers have been synthesized and reacted with metal salts to give complexes of the types [Cu(L1)₂X₂]_∞ (X = Cl, 1 and X = Br, 2), {[Cu(L1)₂(DMF)](NO₃)₂}_∞, 3, {[Ag₂(L1)₂](SO₄)}_∞, 4, and {[Cu(L2)(DMSO)₂(NO₃)](NO₃)}_∞, 5. All compounds have been characterized by spectroscopic methods and their structures determined by X-ray crystallography.Complexes 1, 2 and 3 form 1-D double-stranded polymeric chains showing rhombic molecular squares with approximate dimensions of 16.95 × 19.13 Å² for 1, 17.03 × 19.06 Å² for 2 and 16.66 × 19.94 Å² for 3. Complex 4 forms infinite 1-D zigzag polymeric chains, which are interlinked through a series of Ag–O interactions to form wavy 1-D ladder like chains, and complex 5 forms 1-D sinusoidal chains. While the L1 ligands in complexes 1, 2 and 3 adopt the cis conformation and that in complex 4 adopts trans conformation, the L2 ligand in complex 5 adopts the trans-anti conformation. The ligand conformations also differ in the dihedral angles between the pyridyl and phenyl rings. All complexes exhibit emissions which may be tentatively assigned as intraligand (IL) π → π* transition.     

Asymmetric synthesis of (R)-[2,2-2H2]-1-aminocyclopropane-1-phosphonic acid (ACPP derivative) conformationally constrained ACC analogue using a chiral sulfinyl auxiliary

The asymmetric cyclopropanation of vinylphosphonate using (S)-dimethylsulfonium-(p-tolylsulfinyl)methylide was applied to obtain a dideuterated cyclopropyl sulfoxide. A three-step synthesis of enantiopure (+)-(1R)-1-amino-2,2-dideuteriocyclopropanephosphonic acid (+)-17-d₂ was developed.     

(+)-Neomenthyl- and (−)-phenylmenthyl-substituted cyclopentadienyl and indenyl yttrocenes as catalysts in asymmetric hydroamination/cyclization of aminoalkenes (AHA)

The chiral, terpenoid-substituted yttrocene [(η⁵-neomenthylCp)₂Y{o-C₆H₄CH₂NMe₂}] (1) can be prepared via facile arene elimination starting from [Y(o-C₆H₄CH₂NMe₂)₃]. Compound 1 retains a C₁-symmetric structure in solution on the NMR time scale, due to tight binding of the amine donor. The (−)-phenylmenthyl-substituted complexes [(η⁵-(−)-phenylmenthylCp)₂Y(μ-Cl)₂Li(OEt₂)₂] (5) and [(η⁵-(−)-phenylmenthylCp)₂YN(SiMe₃)₂] (6) were prepared via salt metathesis. Reaction of YCl₃ with the planar chiral (1-neomenthylindenyl)lithium predominantly produced a single, C₂-symmetric, racemic-like diastereomer. The X-ray crystal structure analysis confirmed that [(η⁵-(+)-NMInd)₂Y(μ-Cl)₂Li(Et₂O)₂] (7) represents the same p-S, p-S metallocene diastereomer and adopts a very similar conformation as observed by Erker in his zirconocene complexes. Complex 7 reacts with LiN(SiMe₃)₂ to form [(η⁵-(+)-NMInd)₂YN(SiMe₃)₂] (8) with retention of configuration. Complexes 1, 6 and 8 showed moderate to good catalytic activity in asymmetric hydroamination/cyclizations of aminoalkenes, but enantioselectivities were limited to a maximum of 38% ee for the sterically most hindered catalyst 8. The indenyl complex 8 is prone to protolytic loss of an indenyl ligand at low (?0.5%) catalyst loading, if sterically undemanding aminoalkene substrates are applied.     

Mechanisms of elimination reactions—XXVII: The reactions of 1-arylethyldimethylsulfonium bromides with sodium ethoxide in ethanol

Treatment of 1-phenylethyldimethylsulfonium bromide 1 with sodium ethoxide in ethanol at 35° affords four products by five reaction paths: styrene 2 by E2 and α'-β mechanisms, ethyl 1-phenylethyl ether 3 and methyl 1-phenylethyl sulfide 4 by the S_N2 mechanism, and methyl o-ethylbenzyl sulfide 5 by a Sommelet-Hauser rearrangement. Isotope effects of 5.9 and 3.5 for E2 and α'-β reactions were estimated by comparing the deuterium content of methyl sulfide from β-d₃, and α-d₇ (all α-positions exchanged) sulfonium salt. Product analyses on products from the unsubstituted, p-methyl, p-fluoro, p-bromo and m-chloro sulfonium salts were combined with overall rates to calculate partial rates for the formation of each product. These partial rates were fitted to the Hammett equation to give values for each product as follows: 2, 0.95±0.19; 3, ?0.63±0.41; 4, 0.95±0.11; 5, 4.84±0.38. The mechanistic implications of the results are discussed.     

Trimethylsilyl-functionalised bis(indenyl)iron(II) complexes: solid-state structure of [η-1,3-(SiMe3)2C9H5]2Fe

The synthesis of the bis(η⁵-indenyl)iron sandwich complexes (η⁵-1-SiMe₃-C₉H₆)₂Fe (3a), (η⁵-2-SiMe₃-C₉H₆)₂Fe (3b), [η⁵-1,2-(SiMe₃)₂C₉H₅]₂Fe (4a) and [η⁵-1,3-(SiMe₃)₂C₉H₅]₂Fe (4b), by the reaction of the appropriate lithium indenide salts [prepared from 1-SiMe₃-C₉H₇ (2a), 2-SiMe₃-C₉H₇ (2b), 1,2-(SiMe₃)₂C₉H₆ (2c) or 1,3-(SiMe₃)₂C₉H₆ (2d)] with ferrous chloride (1) in a 2:1 molar ratio is discussed. The solid-state structure of 4b was determined by single-crystal X-ray diffractometry. Complex 4b exists in a gauche conformation, showing that the indenyl ligands are sterically imposed by the bulk of the Me₃Si substituents. The average Fe-C distance is 2.091(3) Å. Cyclovoltammetric studies indicate that 3 and 4 are redox-active with one-electron oxidations [E^1/2=−270 to −360 mV versus Fc/Fc⁺, Fc=(η⁵-C₅H₅)₂Fe].     

Übergangsmetall-methylen-komplexe: LVI. Organoruthenium-komplexe mit offenkettigen und carbocyclischen alkyliden-brücken: Synthese,konstitution, isomerie,protonierung und thermolytischer abbau

The eighteen new μ-alkylidene ruthenium complexes 5a–r and 5t are very easily and cleanly obtained along the diazoalkane or the hydrazone routes that involve treatment of the dinuclear, metal-metal doubly bonded precursor compound [(η⁵-C₅H₅)Ru(μ-NO)]₂ (3) either with the diazoalkanes oxidizing agent (e.g., MnO₂), with the respective hydrazones. Similarly, sulfur dioxide adds cleanly to the RuRu double bond of 3, thus giving the complex (μ-SO₂)[(η⁵-C₅H₅)Ru(NO)]₂ (5s). Regardless of the nature of the carbene bridge ligands, the dimetallacyclopropanes exhibit, in contrast to their iron analogues, exclusively terminal nitrosyl ligands. cis/trans-Isomerism with predominating amounts of the trans-isomers is observed for the derivatives that display unsymmetrically substituted carbene bridges.Treatment of the μ-methylene- and μ-ethylidene complexes (μ-CH₂)[(η⁵-C₅H₅)Ru(NO)]₂ (5a) and (μ-CHCH₃)[(η⁵-C tetrafluoroboric acid or trifluoromethanesulfonic acid in diethyl ether yields, at ambient temperature, quantitatively the ionic complexes 6a,b and 7a,b, respectively, which were shown by ¹H NMR spectroscopy to contain metal-metal bridging hydrogen functionalities. The reaction of hydrogen bromide with 5a under the same conditions gives the neutral bromo(methyl) complex 6d. This latter compound results from the isolable ionic intermediate of composition [(μ-CH₂)(μ-H){(η⁵C₅H₅)Ru(NO)}₂]⁺Br^? (6c), which reaction stems from the nucleophilicity of the halide ion present in 6c.     

The structure and racemization of 1,2-bis(pentaphenylphenyl)benzene

1,2-Bis(pentaphenylphenyl)benzene (2) was synthesized by the cycloaddition of 1,2-bis(phenylethynyl)benzene and tetracyclone. Its X-ray structure was determined, and the molecule adopts a C₂-symmetric conformation in the crystal. Monomethoxy and dimethoxy derivatives of compound 2 were also prepared, and dynamic NMR studies of these compounds yielded a free energy of activation for racemization (ΔG³_rac) of 20.3?kcal/mol at 423?K. The results are compared with estimates of ΔG³_rac for 2 by various DFT methods.     

Mehrfachbindungen zwischen hauptgruppenelementen und übergangsmetallen: XXXVI. Metallorganische oxide der vanadium-reihe: derivate der schlüsselverbindung (η5-C5H5)VOCl2

The synthesis of the organovanadium(V) oxide (η⁵-C₅H₅)VOCl₂ (2a) from the low-valent precursor compound (η⁵-C₅H₅)V(CO)₄ (1a) has been applied to the permethylated derivative of composition (η⁵-C₅Me₅VOCl₂ (2b). Exchange of bromine for chlorine in oxodichlorides 2a and 2b is effected by boron tribromide, yielding the oxodibromide derivatives of composition (η⁵-C₅R₅)VOBr₂ (R = H, 3a; R = CH₃, 3b). The methoxy derivatives 4a and 4b are synthesized directly from the dichloro precursors 2a and 2b, respectively, by treatment with a slight excess of sodium methoxide. Diarylvanadium(V) compounds (η⁵-C_5R)VOX₂ (e.g., R = CH₃, X = C₆H₅; 5b), are obtained from 2a,2b via the Grignard route. ⁵¹V NMR spectroscopy is an easy and powerful method of detecting novel organic vanadium oxides since the chemical shifts vary greatly even with small changes in the ligands attached to the metal.     

Synthesis,structures and full characterization of p-tert-butylcalix[5]arene mono-, di-, tri- and pentaanionic ligand precursors

The synthesis, complete characterization, and solid state conformation of a new series of p-tert-butylcalix[5]arene (Bu^tC5) mono-, di-, tri- and pentaanions are reported. X-ray structures of the alkali metal salts illustrate the strong influence of the alkali metal ion on the structure of the calixanion. The strength of the alkali metal base and its reaction stoichiometry play an important role in the conformation and level of deprotonation of the resulting anion. Reaction of Bu^tC5 in a 2:1 molar ratio with alkali metal bases M₂CO₃ (M = Rb, Cs), or in a 1:1 ratio with M₂CO₃ (M = Na, K), MOH (M = Na, K, Rb, Cs) or MH (M = Li, Na) produces Bu^tC5 monoanions, but Bu^tC5 reacts in a 1:1 molar ratio with M₂CO₃ (M = Rb, Cs) or a 1:2 molar ratio with MOC(CH₃)₃ (M = Na, K) to afford Bu^tC5 dianions. Due to the steric bulkiness of the Bu^t group no polymeric structures are observed. Alkali metal salts of trianionic Bu^tC5 were obtained in high yields from reactions of Bu^tC5 with MOC(CH₃)₃ (M = Li, Na, K), BuⁿLi, LiH and LiOH in a 1:3 molar ratio. Pentaanionic Bu^tC5 salts were obtained by the reaction with MOC(CH₃)₃ (M = Li, Na, K) or BuⁿLi in a 5:1 ratio. X-ray crystal structures of Bu^tC5 · Na and Bu^tC5 · Cs indicate that the size of the alkali metal influences the level of cation-π arene interactions and therefore the conformation of the Bu^tC5 unit; for example, Bu^tC5 · Na has a cone conformation while Bu^tC5 · Cs shows a flattened cone conformation. Cation-π arene interactions are observed in most of the calixarene salts.     

Synthesis and structures of the dinuclear tin(II) complexes [R = Ph, X(Z) = CPh; R = SiMe3, X(Z) = PPh2] and of related compounds

Tin(II) compounds containing the ligands [CH(C₆H₃Me₂-2,5)C(Bu^t)NSiMe₃]⁻ (≡ L¹), [CH(Ph)C(Ph)NSiMe₃]⁻ (≡L²), [CH(SiMe₃)P(Ph)₂NSiMe₃]⁻ (≡ L³),

Full-size image (3K)

(≡ L⁴), [C(Ph)C(Ph)NSiMe₃]²⁻ (≡ L⁵), and [C(SiMe₃)P(Ph)₂NSiMe₃]²⁻ (≡ L⁶) are reported: the transient SnBr(L¹) (1) and SnBr(L²) (2), Sn(L¹)₂ (3) [P.B. Hitchcock, J. Hu, M.F. Lappert, M. Layh, J.R. Severn, J. Chem. Soc., Chem. Commun. (1997) 1189], the labile Sn(L²)₂ (4), [Sn(L⁵)]₂ (5), SnCl(L³) (6), Sn(L³)₂ (7), [Sn(L⁶)]₂ (8), Sn(L⁴)₂ (9) and Pb(L⁴)₂ (10). They were prepared from (i) SnBr₂ and K(L¹) (1, 3) or K(L²) (2, 4, 5); (ii) SnCl₂ and Li(L³) (6–9); or (iii) PbCl₂ and Li(L⁴) (10). Each of 1, 3 and 5–10 has been characterised by multinuclear NMR spectra; 3, 5, 6, 8, 9 and 10 by EI-mass spectra, but only 3, 5, 8, 9 and 10 were isolated pure and furnished X-ray quality crystals. Of greatest novelty are the title binuclear fused tricyclic ladder-like compounds 5 and 8. Quantum chemical calculations, on alternative pathways to 5 from 2 and to 8 from 7, are reported.