The preparation of cis- and trans-[PtH(C6Cl5)(PEt3)2 and a study of the “hydride-donor capacity” of the complexes trans- [PtH(C6X5(PEt3)2] (X = F and Cl)

The preparations of cis- and trans-[PtH(C₆Cl₅)(PEt₃)₂] by thermal decomposition of cis- and trans-[Pt(OCHO)(C₆Cl₅)(PEt₃)₂], respectively, are reported. Also described are cis- and trans-[Pt(SnCl₃)(C₆Cl₅)(PEt₃)₂], obtained by treating SnCl₂ with cis- and trans-[PtCl(C₆,Cl₅)(PEt₃)₂], respectively. It is shown that while trans- [PtH(C₆Cl₅)(PEt₃)₂] does not form hydride-bridged complexes in the presence of trans-(PtH(MeOH)(PEt₃)₂]⁺, the corresponding complex trans-[PtH(C₆)(PEt₃)₂] reacts with the same solvento complex, in methanol, giving labile [(PEt₃)₂HPt(-μH)Pt(C₆F₅)(PEt₃)₂]⁺.     

Analyse conformationnelle de derives bicyclo (4.1.0) heptaniques cis—I

¹H NMR conformational studies of cis-3(4) epoxy bicyclo (4.1.0) heptanes endo and exo, and of related alcohols (3-norcaranols) has been carried out using Eu(dpm)₃ as shift reagent. Comparatively the conformational analysis of cis and trans-1(2), -4(5) diepoxy cyclohexanes has been performed. The experimental data agree with a predominant planar conformation of the two -3(4) epoxy bicyclo (4.1.0) heptanes and of the cis diépoxy cyclohexane, but do not distinguish between the planar conformation and the $\text{1}\text{1}$ equilibrium of the two equivalent boat conformations of the trans diepoxyde. For the two alcohols, they adopt the same semi chair form in which the hydroxyl group is axial for the exo isomer and equatorial for the endo.     

PhP-PPh group bound to 1,8-positions of naphthalene: Preparation of cis isomer and synthesis of binuclear complex

A thermodynamically less stable cis isomer of 1,2-diphosphacycle was prepared from the corresponding trans isomer. Diphosphine, in which a PhP-PPh bond bridges the 1,8-positions of naphthalene, 1,2-diphenyl-1,2-dihydronaphtho[1,8-cd][1,2]diphosphole (1), was first prepared according to a previously reported method, and the trans isomer of 1 was irradiated in tetrahydrofuran with UV-vis light to reach equilibrium with cis-1 in a trans:cis ratio of 1:2. When a similar photochemical conversion was carried out using a saturated hexane solution of trans-1, cis-1 was precipitated in a good yield of 94%. The configuration of cis-1 was confirmed by X-ray analysis. Both cis- and trans-1 diphosphine ligands were used for the preparation of binuclear gold complexes. The crystal structure of (μ-cis-1)-[AuCl]₂ demonstrated that the two lone pairs of cis-1 are suitably directed for arrangement of the two gold centers in close proximity to each other. The two independent (μ-cis-1)-[AuCl]₂ molecules in the crystal were found to form a dimer through the multiple intermolecular interaction among the gold centers.     

An AB initio study of the cis- and trans -conformers of 1,3-butadiene,acrolein and glyoxal: evidence for a stabilizing interaction in cis-acrolein

The cis- and trans-conformers of 1,3-butadiene, acrolein and glyoxal have been studied using the 4-31G and (7, 3) basis sets with full geometry optimization. The changes in geometry in going from the cis- to the trans-structures indicate that the terminal carbon and oxygen atoms in cis-acrolein are closer together than would otherwise be expected from the butadiene and glyoxal data, and that the hydrogens of the CH₂ group are also affected. These results, together with a comparison of the changes in force constants for key structural elements, suggest that some sort of attractive interaction is present in cis-acrolein. ΔE_T, ΔE_K, ΔV_ee, ΔV_nn and ΔV_en are reported for the cis-trans isomerizations, the special geometrical features of the cis-conformer being reflected in ΔV_nn for the acrolein reaction. The positive ΔE_T values for the disproportionation reactions of both conformers2 acrolein → 1,3-butadiene + glyoxal show that the acrolein framework provides the most stable combination of the H₂C, CH- and O structural elements, while the bigger value for the cis-conformers affords additional evidence that the interaction in cis-acrolein is stabilizing in nature. However, in going from the trans- to the cis-structure neither the change in f_C-H,C-H for the C-H that would be involved in an H- bond, nor the change in charge on the atoms in the C-H ? O unit. based on Mulliken population analyses, are in accord with what would be expected for a hydrogen-bonding type of interaction. Instead the stabilization may be a manifestation of the different pattern of charge distribution along the polyene chain of acrolein, i.e. with the ends oppositely charged, compared to butadiene and glyoxal, in which the ends carry the same charge.     

cis–trans Geometrical isomerization of bis(2-pyridinecarboxylato)nitrosylruthenium complex accompanying electrochemical one-electron reduction

cis-[Ru(NO)(CH₃CN)(pyca)₂] and trans-[Ru(NO)(OH)(pyca)₂] (pyca = 2-pyridinecarboxylato) were synthesized and characterized by X-ray crystallography. Electrochemical behaviors of cis-[Ru(NO)(CH₃CN)(pyca)₂] and cis-[Ru(NO)(CH₃O)(pyca)₂] in acetonitrile were studied. These complexes showed two reduction processes in CH₃CN. The controlled potential electrolyses of cis-[Ru(NO)(CH₃O)(pyca)₂] in a methanol–acetonitrile mixed solution were performed at the potential of the first reduction process. trans-[Ru(NO)(CH₃O)(pyca)₂] was isolated from the electrolyzed solution and characterized by IR and CV. The cis–trans geometrical change reaction occurred in the electrochemical one-electron reduction of cis-[Ru(NO)(CH₃O)(pyca)₂].     

Mechanochemical-induced cis-to-trans isomerization of poly(2-ethynyl-3-n-octylthiophene) prepared with a Rh complex catalyst

Highly stereoregular polymerization of 2-ethynyl-3-n-octylthiophene was successfully performed with a [Rh(norbornadiene)Cl]₂ catalyst to produce the corresponding polymers in fairly high yields by using triethylamine or a mixture of it with other solvents as the polymerization solvent. We found that the obtained polymer using CHCl₃ was a mixture of cis-transoid form, ca. 68% and trans-transoid form, ca. 32% unlike our previous conjecture. Further, we found that the cis-to-trans isomerization can be also induced when the pristine predominant cis polymer was subjected to mechanochemical grinding (MCG) treatment at 77 K using a mortar filled with liquid nitrogen to decrease the cis content from ca. 68% to ca. 7%. The polymers obtained before and after the MCG treatment were characterized in detail using ¹H NMR, laser Raman, solution UV-vis, diffuse reflective UV-vis, and ESR methods in order to determine the geometry of the main-chain CC bonds in the polymer. The data showed that the polymer obtained by the treatment has a fairly distorted trans conjugation length, i.e., bent trans structure in which less mobile unpaired electrons generated by the rotational scission of the original cis CC bonds are stabilized.     

Hydrogen bonding in cis and trans 1-t butylcyclohexan-4-ol

The homogeneous autoassociation of cis and trans 1-t-butylcyclohexan-4-ol has been investigated by means of IR spectroscopy. The temperature and concentration dependences of this interaction have been examined in CCl₄ as solvent. The results demonstrate a lower degree of association in the case of the cis compound, especially because of the steric hindrance of the acceptor function of this substance. The enthalpy of associatio-n-ΔH is found at about 13 [kJ/mole] for the trans compound and may be somewhat lower for the cis alcohol. The absorbance of the associated OH band of the trans compound is constant with varying concentration and/or temperature, but for the cis alcohol it increases with increasing concentration and decreasing temperature.     

Triazolopeptides: chirospecific synthesis and cis/trans prolyl ratios of structural isomers

As cis/trans prolyl isomerization plays a crucial role in various biological processes, peptide mimics capable of modifying the cis/trans Xaa-Pro ratio are of particular interest. A practical approach toward proline derived triazolopeptides employing [3+2] azide-alkyne cycloadditions as the key reaction step and the analysis of their cis/trans prolyl ratios are reported. Structural investigations indicated the adjustability of both the cis-percentage and the conformational stability toward intramolecular H-bonding effects.     

Vibrational spectra and quantum mechanical assignments for 1-nitropropene

The i.r. spectra of cis and trans 1-nitropropene have been recorded in the vapor phase and in various solvents. Theoretical calculations at the Hartree—Fock level with a 3-21G(d) basis set were carried out yielding vibrational frequencies for nitromethane and cis and trans 1-nitropropene. Assignment of fundamentals was assisted by solvent effects on 1-nitropropene. Evidence is obtained for inter-molecular hydrogen bonding.     

Microwave spectra of cis and trans 2-furylisocyanate conformers

The microwave spectrum of 2-furylisocyanate has been obtained in the frequency region from 8 to 40 GHz. This spectrum is attributed to the ground state of the cis and trans configurations. The rotational constants for both ground vibrational states have been determined. Two sets of vibrational satellites are observed and assigned to the modes of C-N torsion and CNC bending. The microwave results show and MINDO/3 calculations confirm that the barrier between the cis and trans conformers is high and that the cis conformer is more stable than the trans by 3 kcal mol^?1.     

Tris(acetonitrile)tricarbonylchromium(0) catalyzed 1,4-addition of hydrogen to 1,3-dienes

The Cr(CO)₃(CH₃CN)₃ complex is found to catalyze the 1,4-addition of hydrogen to 1,3-dienes such as 2-methyl-1,3-butadiene, trans-1,3-pentadiene, and trans, trans-2,4-hexadiene at low temperature (40°) and low H₂ pressure (20 psi). For trans, trans-2,4-hexadiene the only product obtained when D₂ is used is 2,5-dideuterio-cis-3-hexene. The catalytic 1,4-hydrogenation can be carried out in neat dienes, and turnover numbers for the catalyst of greater than 3000 have been observed.     

Addition of lithium ethyl fluoroacetate to cis and trans α,β-epoxyaldehydes. Access to C2 fluorinated butyrolactones

The aldolisation reaction of lithium ethyl fluoroacetate with cis and trans α,β-epoxyaldehydes in their racemic forms proceeds with good C₃-OH diastereoselectivity and much less at the C₂-F carbon atom. A two-step reaction on the major aldol compounds (iodination, lactonisation) led to racemic functionalised C₂ fluorinated lactones, possessing a C₂/C₃cis relationship between the fluorine and hydroxyl groups.     

Infrared studies of the less stable cis form of N-methylformmaide and N-methylacetamide in low-temperature nitrogen matrices and vibrational analyses of the trans and cis forms of these molecules

Recently studies of less stable rotational isomers became possible using a combination of low-temperature matrix-isolation infrared spectroscopy and either the high-temperature nozzle technique or irradiation with ultraviolet or near-infrared light. In this paper the infrared spectra of the less stable cis form of N-methylformamide and N-methylacetamide and their variously deuterated species were obtained in low-temperature nitrogen matrices using the high-temperature nozzle technique. The assignments of the bands due to the cis form as well as the trans form were made on the basis of normal-coordinate calculations.     

A molecular orbital study of the cis-trans energy difference in glyoxal

The barrier to internal rotation around the C-C bond in glyoxal, and the energy difference between the cis and the trans form of the molecule have been estimated using (7,$\text{3}\text{4}$) and (9,$\text{5}\text{4}$) Gaussian basis sets including polarization functions. The predicted energy of the cis form relative the ground state trans form was found to be substantially higher (4.9–6.3 kcal mol^?1) than the experimental value of 3.2 kcal mol^?1.     

Deamination process in the formation of a copper(II) complex with glutamic acid and a new ligand derived from guanidinoacetic acid: Synthesis,characterization, and molecular modeling studies

A deamination process was observed after copper(II) complexation reaction with guanidinoacetic (Gaa) and glutamic acids (Glu), forming the binuclear copper(II) complex K₂Cu₂C₁₆H₂₃N₇O₁₂ · 1/2H₂O (1), which was characterized by elemental analysis (CHN), spectroscopy methods (IR and EPR), powder X-ray diffraction, thermogravimetric analysis (TGA), and mass spectrometry. A new ligand, namely biguanide-1,5-diethanoate (Bge) (C₆H₉N₅O₄), was formed during complexation, probably due to the reaction between two Gaa species and the consequent release of a significant amount of ammonia, thus, characterizing the deamination process. In complex 1, Bge behaved as a tetradentated ligand, using its oxygen and nitrogen atoms as coordinating sites to both Cu(II) ions. In addition, Glu has coordinated to Cu(II) through its α-N and O atoms. Theoretical calculations of the cis–cis, cis–trans, and trans–trans isomers of 1, considering three prototropic forms of the Bge ligand, were carried out using semi-empirical quantum mechanics (PM3/d). DFT (B3LYP and B3P86) calculations of complex 1, in which a hydrogen atom replaced the side chain of Glu, were also carried out using the 6-31G(d) basis set and the LanL2DZ effective core potential for the transition metal. Based on experimental and theoretical data, we concluded that the trans–trans isomer of the binuclear copper(II) complex 1 should be the most stable, although the occurrence of other isomers, even if in minor quantities, should not be disregarded.     

Electrostatic forces that determine O,O-trans versus O,S-cis conformers in the aminated porphyrin complexes of Cd(N-NHCO-2-C4H3O-tpp)(OAc) and Cd(N-NHCO-2-C4H3S-tpp)(OAc)

Two new diamagnetic, mononuclear and aminated porphyrin complexes of O,O-trans-Cd (3-trans) and O,S-cis-Cd (4-cis) have been synthesized and characterized by ¹H, ¹³C NMR spectroscopy. The crystal structures of (acetato)(N-2-furancarboxamido-meso-tetraphenylporphyrinato)cadmium(II) [Cd(N-NHCO-2-C₄H₃O-tpp)(OAc); 3-trans] and (acetato)(N-2-thiophenecarboxamido-meso-tetraphenylporphyrinato)cadmium(II) [Cd(N-NHCO-2-C₄H₃S-tpp)(OAc); 4-cis] were determined. The coordination sphere around Cd²⁺ is a distorted square-based pyramid in which the apical site is occupied by a bidentate chelating OAc⁻ group for 3-trans and 4-cis. The plane of three pyrrole nitrogen atoms [i.e., N(1), N(2), N(4) for 3-trans and N(1), N(2), N(3) for 4-cis] strongly bonded to Cd²⁺ is adopted as a reference plane 3N. The N(3) and N(4) pyrrole rings bearing the 2-furancarboxamido (Fr) and 2-thiophenecarboxamido groups in 3-trans and 4-cis, respectively, deviate mostly from the 3N plane, thus orienting separately with a dihedral angle of 33.4° and of 31.0°. In 3-trans, Cd²⁺ and N(5) are located on different sides at 1.06 and −1.49 Å from its 3N plane, while in 4-cis, Cd²⁺ and N(5) are also located on different sides at 1.04 and −1.53 Å from its 3N plane. An attractive electrostatic interaction between the Cd²⁺ and O(4)⁻ atoms in furan stabilizes the O,O-trans conformer of 3. A repulsive electrostatic interaction between Cd²⁺ and S(1)⁺ destabilizes the O,S-trans conformer of 4. Both of these repulsive and the mutually attractive interactions between S(1)⁺ and O(3)⁻ atoms favor the O,S-cis rotamer of 4 both in the vapor phase and in low polarity solvents. NOE difference spectroscopy, HMQC and HMBC were employed for the unambiguous assignment of the ¹H and ¹³C NMR resonances of 3-trans and 4-cis in CDCl₃ at 20 and −50 °C.     

Simultaneous determination of the trans and cis forms of zearalenone in cereal products by high-performance liquid chromatography with voltammetric detection

Zearalenone is shown by n.m.r. to exist in the trans and cis forms both in methanolic solution and in cereal products. Conversion of the trans to the cis isomer is enhanced by the effects of ultraviolet irradiation. A method is described for the simultaneous determination of the two isomers by using high-performance liquid chromatography with voltammetric detection. A linear calibration is operative in the range 3–300 ng of zearalenone. Concentrations of trans- and cis-zearalenone in wheat and corn down to 5 ng g^-1 can be determined in 10-g cereal samples.     

The rhodophytin and chondriol natural products; structures of several new acetylenes from laurencia, and a reassignment of structure for cis-rhodophytin

The structures of five new halogenated vinyl acetylenes are described which are natural products from various species of the red seaweed Laurencia. The structure of epoxyrhodophytin (1) was determined by spectral, chemical, and X-ray diffraction analyses. The structures of trans-rhodophytin (5) and trans-chondriol (8) are based on chemical and spectral comparisons with the previously reported compounds cis-rhodophytin (4) and cis-chondriol (7). The structures of cis-chondrin (14) and trans-chondrin (15) were secured by synthesis from cis- and trans-chondriol, respectively. The reactivity of these enol-ethers to various conditions of catalytic hydrogenation has been examined in detail. Hydrogenation yields an unexpected and facile incorporation of oxygen into the reaction products. Based upon these studies, and upon combustion analysis, the structure of rhodophytin has been revised as the vinyl ether rather than the vinyl peroxide originally proposed.     

Theoretical study of the force field and vibrational assignments of acetamide and deuterated analogues

The complete harmonic force constants of acetamide have been evaluated by ab initio calculations at the Hartree-Fock level with the 4–31G(d) basis set. The force field was scaled to compensate for the systematic overestimations of the Hartree-Fock-level force constants by empirical factors using the matrix isolation IR spectra of CH₃CONH₂ and CD₃CONH₂. A normal coordinate treatment has been carried out with the scaled force field to analyze the vibrational spectra of CH₃CONH₂. A normal coordinate treatment has been carried out with the scaled force field to analyze the vibrational spectra of CH₃COND₂, cis-CH₃CONHD and trans-CH₃CONHD. The effect of cis/trans isomerism of CH₃CONHD on the fundamental bands was well reproduced by the calculations. The fundamental vibrations were also predicted for CD₃COND₂, cis-CD₃CONHD and trans-CD₃CONHD.     

RuO4-catalyzed oxidative polycyclization of the Cs-symmetric isoprenoid polyene digeranyl. An unexpected stereochemical outcome

The RuO₄-catalyzed oxidative polycyclization of digeranyl, a C_s-symmetric tetraene possessing a repetitive 1,5-diene structural motif, has been studied. The required substrate has been synthesized by Ti(III)-mediated tail-to-tail homocoupling of geranyl bromide. The process afforded two hitherto unknown isomeric tris-tetrahydrofuran products possessing unexpected all-threo cis-trans-cis and cis-trans-trans relative configuration. The new stereochemical outcome is explained based on previously formulated chelation or steric control models on the basis of structural differences between digeranyl and previously studied isoprenoid polyenes farnesyl acetate, geranylgeranyl acetate and squalene.