NMR-Spectroscopic, computational and mass-spectrometric investigations on the cis/trans analogues of 2,3,4-trihydronaphthalene-1-one

NMR-Spectroscopic, computational and mass-spectrometric studies of the cis/trans isomers of N-[8-(acetylamino)-4-(2,2-dimethyl-1,1-diphenyl-silapropoxy)-6-fluoro-5-methyl-1-one-2,3,4-trihydronaphthyl]acetamide (1a and 1b), obtained as intermediates in the synthesis of an important class of alkaloid molecules, are reported. ¹H and ¹³C NMR analyses show an unusual axial preference of the TBDPSi- (tert-butyldiphenylsilyl) group in position 4 in both the isomers. Mass spectrometric evidence demonstrates that trans isomer has a higher affinity for ammonium ions than the cis isomer and that only the ammonium adduct [1b+NH₄]⁺ and the protonated molecule [1b+H]⁺ show the fragmentation in which loss of benzene is observed. Moreover, molecular mechanics and semi-empirical calculations indicate that a group of trans conformers tend to place one of the phenyl rings of the TBDPSiO- group in a offset π-stacked geometry with the compound's aromatic ring. The combination and the detailed analyses of these experimental and theoretical results could support the π-π interaction obtained as a conformational preference in the trans isomer.     

An analysis of the vibrational structure of the UV absorption spectrum of acryloyl chloride vapor

The vibrational structure of the UV absorption spectrum of acryloyl chloride vapor was analyzed to obtain detailed information about the torsional vibration levels of the trans and cis isomers in the ground (S ₀) and excited (S ₁) electronic states. The spectrum contained 114 absorption bands of which ～90% were assigned. The 0-0 transition frequencies of the trans and cis acryloyl chloride isomers were determined. Several Deslandres tables for torsional vibrations from the 0–0 transition frequencies and local origins corresponding to the fundamental frequencies of two isomeric molecule forms in both electronic states were constructed. The corresponding systems of torsional vibration levels were determined, and the harmonic frequencies θ_ε and anharmonicity coefficients x ₁₁ of the trans and cis isomers were calculated. The torsional vibration levels in the ground electronic state obtained from the vibrational structure of the UV spectrum are compared with those determined using the Fourier spectrum. The 0–3 torsional transition of the trans isomer was found to be displaced because of Fermi resonance with a fundamental frequency.     

Synthetic polyisoprenes studied by Fourier transform Raman spectroscopy

Fourier transform Raman spectra are presented for the cis-1,4 and trans-1,4 isomers of polyisoprenes Vibrational intensities are used to determine quantitatively the amounts of each isomer in the microstructure. Improvements over previous work are suggested for the quantitative assessment of 1,4 microstructure. Also, changes in the Raman spectrum due to oxidative degradation show that preferential oxidative degradation for the vinyl-3,4 units occurs. The α and β forms of trans-1,4 polyisoprene were studied the ν(CC) bands resolved were identified 4 cm⁻¹ apart. A study of the copolymerization of methyl methacrylate with isoprene showed that the 1,4 form is the most favoured form produced on copolymerization. Accurate cis-1,4 and trans-1,4 microstructural information could not, however, be determined.     

Identification of cis/trans isomers of methyl ester and oxazoline derivatives of unsaturated fatty acids using GC–FTIR–MS

Identification of cis/trans isomers of unsaturated fatty acids cannot usually be achieved by GC-MS (gas chromatography-mass spectrometry) without reference substances. In this study a GC-FTIR-MS system (gas chromatography-Fourier transform-mass spectrometry) was used to identify fatty acid methyl esters (FAMEs) and differentiate between the cis/trans isomers. Besides methyl esters, 2-alkenyl-4,4-dimethyloxazoline derivatives (DMOX), which have been used to locate double bond positions of unsaturated fatty acids, were examined with respect to their suitability for cis/trans differentiation. A combined GC-FTIR-MS system with a wide band (4000–550 cm^?1) mercury cadmium telluride (MCT) detector was used in series and parallel to identify 31 reference unsaturated fatty acids, including 7 pairs of cis/trans isomers. Serum samples of healthy persons and commercially available fish oil were analyzed as examples of complex mixtures. Using splitless injection the detection limit for the less sensitive IR detector was 25 ng/μl in case of the weak cis and trans bands. In the FTIR spectra cis/trans isomers were identified by analysis of bands arising from C? H out-of-plane (oop) bending: for both the FAME and DMOX derivatives cis-1,2-disubstituted double bonds give a strong band near 720 cm^?1 and the corresponding trans isomers near 967 cm^?1. cis Isomers could be identified further by a band at 3012 cm^?1. With the combined data of the GC-FTIR-MS system it is now possible to identify polyunsaturated fatty acids with regard to the discrimination of cis/trans isomers.     

Conformational analysis,barriers to internal rotation,ab initio calculations and vibrational assignment of fluoroacetone

The infrared (3500-20 cm⁻¹) and Raman (3200-10 cm⁻¹) spectra have been recorded for gaseous and solid fluoroacetone (1-fluoro-2-propanone), CH₂FC(O)CH₃. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. These data have been interpreted on the basis that the molecule exists predominantly in the cis (fluorine atom oriented cis to the methyl group) conformation in the vapor but for the liquid a second conformer having a trans orientation (fluorine atom oriented trans to the methyl group) is present. From a study of the Raman spectrum of the liquid at variable temperatures the trans conformation has been determined to be more stable than the cis form by 416 ± 54 cm⁻¹ (1.19 ± 0.15 kcal mol⁻¹) and is the only conformation present in the spectrum of the annealed solid. The asymmetric torsional fundamental for the more stable cis conformer has been observed in the far infrared spectrum of the gas at 69.6 cm⁻¹ with six accompanying hot band transitions proceeding to lower frequency. The corresponding mode for the high energy trans conformer is extensively overlapped but is distinguishable at ∼65 cm⁻¹. From these data the asymmetric torsional potential function governing internal rotation about the CC bond has been determined and the potential coefficients are: V₁ = 675 ± 2, V₂ = 991 ± 5, V₃ = 74 ± 1 and V₄ = 54 ± 2 cm⁻¹. The cis to trans and trans to cis barriers are 1332 ± 5 and 731 ± 5 cm⁻¹, respectively, with an enthalpy difference of 601 ± 8 cm⁻¹ (1.72 ± 0.02 kcal mol⁻¹). From ab initio calculations at the 3-21G and 6-31G* basis set levels optimized geometries for both the cis and trans conformers have been obtained and the potential surface governing internal rotation of the asymmetric top determined. The observed vibrational frequencies with their assignments for both the cis and trans conformers are compared to those from the ab initio calculations. All of these results are compared to the corresponding quantities for some similar molecules.     

Structure of polypiperylene chain segments according to high-resolution 13C NMR spectra

The microstructure of stereoregular 1,4-trans-and 1,4-cis-polypiperylenes, as well as polymers prepared from the trans-and cis-piperylene isomers via cationic polymerization in the presence of TiCl₄, was studied by high-resolution ¹³C NMR spectroscopy. Polypiperylene synthesized through the cationic polymerization of the cis isomer had a more diversified morphology of the macromolecular chain, i.e., had higher relative amounts of 1,2-cis-units and combinations of irregular-addition 1,4-trans-units. It was shown that ¹³C NMR spectra give the most comprehensive and independent information on the details of structure of the piperylene macromolecular chain.     

Synthesis and structure characterization of ladder-like polymethylsilsesquioxane (PMSQ) by isolation of stereoisomer

The stereo cyclic siloxane compounds having tetra hydroxyl and tetra methyl group were synthesized by hydrolysis of tetrahydrido tetramethyl cyclosiloxane consisting of four stereoisomers. Among the stereo cyclic isomers, cis-trans-cis tetrahydroxyl tetramethyl cyclosiloxane (2) was separated by recrystallization and was used as a monomer species for preparation of ladder-like polymethylsilsesquioxane (PMSQ). The isolated isomer was directly polymerized in THF with potassium carbonate (K₂CO₃) by systematic ring condensation. The hydrolyzed stereoisomers were characterized by ¹H and ²⁹Si NMR, and HPLC. The structure of the synthesized PMSQs were characterized by size exclusion chromatography (SEC), MALDI-TOF mass, ²⁹Si NMR, and small angle X-ray scattering (SAXS).     

Multinuclear two-dimensional NMR: Assignments of natural abundance polypeptide 13C, 1H and 15N chemical shifts and demonstration of isomer interconversion

The polypeptide carbobenzoxy-glycyl-L -prolyl-L -leucyl-L -alanyl-L -proline (0.2 M in DMSO-d₆) was investigated using ¹³C, ¹H and ¹⁵N NMR in natural abundance at 4.7 tesla. The existence of cis–trans-Gly-Pro and -Ala-Pro bonds permits up to four isomers, and all four were observed (in a 60:30:7:3 ratio). ¹³C shifts of the proline β-CH₂ resonances are consistent only with the 60% form being trans–trans. The 30% form is either trans–cis or cis–trans (order as above) and was tentatively assigned as cis-trans on the basis of relaxation behavior. Refocused INEPT studies aided the ¹³C assignments. The ¹⁵N data were obtained using both NOE and INEPT excitation, with signals evident for the three major isomers. The spectra were analysed by starting from the ¹³C data, which were assigned based on known regularities in peptide spectra. A ¹³C? ¹H heteronuclear two-dimensional chemical shift correlation experiment allowed direct assignment of proton shifts for major and minor isomers. The NH proton shifts were assigned by running a homonuclear two-dimensional chemical shift correlation experiment and noting the correlation with the previously assigned α-CH protons. The ¹⁵N resonances were then assigned from a ¹⁵N? ¹H heteronuclear two-dimensional chemical shift correlation experiment, relating the ¹⁵N signals directly to the NH proton resonances. Isomer interconversion between the two major isomers was demonstrated by performing a magnetization transfer homonuclear 2D experiment. Off-diagonal intensity was noted relating the major and minor isomer alanine NH proton, as well as for the major and minor isomer leucine NH protons.     

Solid-state isomerisation reactions of (η-C5H4R)M(CO)2(PR3)I (M=W, Mo; R=Bu, Me; R=Ph, OPr3)

The cis and trans monosubstituted cyclopentadienyl tungsten and molybdenum complexes (η⁵-C₅H₄R)M(CO)₂(L)I (1) (M=W, R=Me, ^tBu, L=P(OⁱPr)₃, PPh₃; M=Mo, R=Me, L=PPh₃) have been synthesised and fully characterised by elemental analysis and IR and NMR spectroscopy. It was found that 1 underwent a thermal solid-state ligand isomerisation reaction and that the favoured direction of the isomerisation reaction is related to the melting points of the cis and trans isomers, i.e., with intermolecular forces in the solid state. No obvious relationship between the melting point and the metal, the ring-substituent or the ligand was observed. Crystal structure determinations of the cis and trans isomers of (η⁵-C₅H₄Me)W(CO)₂(PPh₃)I reveal that a limited amount of isomer conversion can be accommodated in the unit cell of the trans isomer, prior to crystal fragmentation. The rearrangement of the molecules within the unit cell, during isomerisation, also leads to disorder in the crystal.     

Metalloporphyrins as chemical shift reagents: the unambiguous NMR characterization of the cis- and trans-isomers of meso-(bis)-4′-pyridyl-(bis)-4′-carboxymethylphenylporphyrins

The condensation of pyrrole with 4-pyridylcarboxyaldehyde and methyl 4-formyl benzoate under Adler-Longo conditions yielded the series of meso-(4′-pyridyl)/(4′-carboxymethylphenyl)porphyrins as a mixture. Careful column chromatography afforded each isomer in pure form. In this paper we focus on the two bis-substituted isomeric meso-porphyrins, 5,10-bis(4′-pyridyl)-15,20-bis(4′-carboxymethylphenyl)porphyrin and 5,15-bis(4′-pyridyl)-10,20-bis(4′-carboxymethylphenyl)porphyrin, respectively, 4′-cis and 4′-transDPyDMeP. The assignment of the geometry of the two isomers was performed by ¹H NMR spectroscopy on the trinuclear adducts [(4′-cisDPyDMeP){Ru(TPP)(CO)}₂] and [(4′-transDPyDMeP){Ru(TPP)(CO)}₂], obtained by selective coordination of [Ru(TPP)(CO)(EtOH)] (TPP=tetraphenylporphyrin) to the peripheral nitrogen atoms. The axially bound ruthenium porphyrins act as chemical shift reagents on the central porphyrin, allowing a clear distinction of the pyrrole proton resonances and consequent unambiguous assignment of the geometry of each isomer based upon symmetry considerations.     

First separation and characterization of cis and trans 1,2-bisaryloxy perfluorocyclobutanes

4-Bromotrifluorovinyloxybenzene (4-Br-TFVE) undergoes cyclodimerization upon heating at 150 °C affording cis and trans isomers of 1,2-bis(4-bromophenoxy)hexafluorocylcobutane. Stereoisomers were separated by selective crystallization, confirmed by single crystal X-ray, and further characterized by NMR. Remarkable difference in the solid state structures include aryl to aryl dihedral angles of 12° for the trans isomer and 88° for the cis isomer. Polymers containing roughly equal amounts of cis and trans fused perfluorocyclobutyl (PFCB) rings should be expected to have low crystallinity due to the marked difference in the two unit cells as is observed.     

Microwave spectrum,structure, dipole moment and internal rotation of ethyl fluorosilane

Microwave spectra of the trans and gauche isomers of ethyl fluorosilane and their eleven isotopically substituted species have been measured. The r_s structures of the two isomers were determined from the observed moments of inertia. The molecular structures found for the two isomers in the present study are compared with those of analogous molecules. Dipole moments of the two isomers were determined by Stark-effect measurements and are also compared with those of analogous molecules. The energy difference between the trans and gauche isomers was obtained from the relative intensity measurements of the spectra and the barrier to internal rotation of the methyl group for the gauche isomer was obtained from the A—E splittings of the spectra in the first excited methyl torsional state. The V₃ value was 2775 ± 25 cal mol^?1.     

Meso-tetracinnamylporphyrin: Synthesis,characterization and the catalytic activity of its Mn(III) complex in olefin epoxidation with tetra-n-butylammonium hydrogen monopersulfate

Meso-tetracinnamylporphyrin (H₂tcp) has been synthesized and characterized by UV–Vis, ¹H NMR and IR spectroscopies. Protonation of H₂tcp with HCl is accompanied by a blue shift (77 cm⁻¹) of the Q(0,0) band, probably indicating a decrease in the efficiency of the π-electron donation from the meso-substituents to the a_2u orbital in comparison with the corresponding dication of meso-tetraphenylporphyrin (H₂tpp). The unusual NH signal multiplicity observed in ¹H NMR spectrum of H₂tcp shows evidence of a long distance coupling between the vinyl protons and NH ones. This long range coupling can occur between the remote centers using the a_2u orbital as a connector. Mntcp(OAc) shows higher catalytic activity (ca. 1.2-fold higher) than Mntpp(OAc) in olefin epoxidation with tetra-n-butylammonium hydrogen monopersulfate (TBAO). The low ratio (∼1.5) of cis- to trans-stilbene oxide, in competitive oxidation of cis- and trans-stilbene with TBAO, strongly suggests the involvement a common high-valent manganese oxo complex as the reactive intermediate responsible for oxygen atom transfer.     

Syntheses and Characterization of a Pair of Isomers of Heteroleptic Bis(Bidentate) Ruthenium(II) Complexes with Two Different Monodentate Ligands

Two isomers of heteroleptic bis(bidentate) ruthenium(II) complexes with dimethyl sulfoxide (dmso) and chloride ligands, trans(Cl,N_bpy)- and trans(Cl,N_Hdpa)-[Ru(bpy)Cl(dmso-S)(Hdpa)]⁺ (bpy: 2,2′-bipyridine; Hdpa: di-2-pyridylamine), are synthesized. This is the first report on the selective synthesis of a pair of isomers of cis-[Ru(L)(L′)XY]ⁿ⁺ (L≠L′: bidentate ligands; X≠Y: monodentate ligands). The structures of the ruthenium(II) complexes are clarified by means of X-ray crystallography, and the signals in the ¹H NMR spectra are assigned based on ¹H–¹H COSY spectra. The colors of the two isomers are clearly different in both the solid state and solution: the trans(Cl,N_bpy) isomer has a deep red color, whereas the trans(Cl,N_Hdpa) isomer is yellow. Although both complexes have intense absorption bands at λ≈440–450 nm, only the trans(Cl,N_bpy) isomer has a shoulder band at λ≈550 nm. DFT calculations indicate that the LUMOs of both isomers are the π* orbitals in the bpy ligand, and that the LUMO level of the trans(Cl,N_bpy) isomer is lower than that of the trans(Cl,N_Hdpa) isomer due to the trans effect of the Cl ligand; thus resulting in the appearance of the shoulder band. The HOMO levels are almost the same in both isomers. The energy levels are experimentally supported by cyclic voltammograms, in which these isomers have different reduction potentials and similar oxidation potentials.     

Selective formation of thecis isomer of the nitridotechnetium(V) complex with 2,2′:6′,2″-terpyridine

The reaction of [TcNCl₂(PPh₃)₂] with 2,2′:6′,2″-terpyridine producedcis-[TcNCl₂(terpy)] selectively. The resulting complexes were characterized by¹H NMR and IR spectroscopy. The geometries of thecis andtrans isomers were estimated by theoretical calculations following a density functional method. Thecis isomer is likely more stable than thetrans one with respect to thetrans influence of the nitrido ligand. Furthermore, the behavior of nitridotechnetium complexes in polar solvents was compared to Os-analogues.     

Regioselective and enantioselective synthesis of seven-membered ring cyclic arylguanidine and urea derivatives

Cycloaddition reaction of 2-vinylpyrrolidines with carbodiimides in the presence of palladium acetate and dpppentane affords seven-membered ring cyclic arylguanidines in good yields and conversions. When 1-butyl-4-methyl-2-vinylpyrrolidine 1c was used as a mixture of trans and cis isomers in a 4:1 ratio, and reacted with bis(2-chlorophenyl)carbodiimide 2a, high stereoselectivity was achieved and only the trans seven-membered cyclic guanidine was obtained. A methyl group in the 4-position of the pyrrolidine ring and the chloro substituent in the ortho position of the carbodiimide may be responsible for the enhanced product ratio in favor of the trans isomer.     

The investigation of stereogenic properties of cyclotriphosphazene derivatives with two different chiral centres

Hexachlorocyclotriphosphazene N₃P₃Cl₆ and gem-disubstituted cyclotriphosphazene derivatives N₃P₃Cl₄X₂ (X = Ph, PhS, PhNH) were reacted with N-methyl-1,3-propanediamine and 3-amino-1-propanol to give compounds (9a-12a, 9b-12b) which exist as cis and trans geometric isomers and are two different racemic isomers, respectively to describe the stereogenic properties of a series of chiral cyclotriphosphazene compounds with two different centres of chirality. The geometric isomers were separated by column chromatography on silica gel and analysed by elemental analysis, mass spectrometry, and ³¹P and ¹H NMR spectroscopies, and also the geometric forms (cis or trans) of 9b, 10a, 11a, 11b and 12a have been determined by the X-ray crystallography. The enantiomers of all racemic compounds have been analysed by the changes in ³¹P NMR spectra on addition of a Chiral Solvating Agent (CSA), (R)-(+)-2,2,2-trifluoro-1-(9′-anthryl)ethanol. On the other hand, the racemic forms of chiral cyclotriphosphazene derivatives have been confirmed by contribution of chiral HPLC methods which have been developed for this study.     

Occurrence of oleic and 18:1 methyl-branched acyl chains in lipids of Rhodobacter sphaeroides 2.4.1

The fatty acids (FAs) composition of lipids extracted from Rhodobacter sphaeroides 2.4.1 was investigated by gas chromatography–mass spectrometry (GC–MS) analysis of the corresponding FA methyl esters (FAMEs), obtained through trans-esterification of the original lipid species. A GC stationary phase based on a highly polar ionic liquid (IL) was selected, aimed to enhance the separation of isomeric FAMEs with particular emphasis on positional and geometrical isomers of monounsaturated 16:1 and 18:1 fatty acyl chains. The occurrence of 18:1 cis-Δ⁹ (oleic) acid, a positional isomer of the well-known and most predominant 18:1 cis-Δ¹¹ (cis-vaccenic) acid, has been demonstrated here for the first time. Furthermore a methyl branched 18:1 FA was also identified and its structure tentatively assigned as 11-methyl-Δ¹²-octadecenoic acid (most likely as trans isomer). The unprecedented observation about 18:1 cis-Δ⁹ FA occurrence in R. sphaeroides 2.4.1 is, even indirectly, supported by a biosynthetic pathway postulated with the aid of the Kyoto Encyclopedia of Genes and Genomes (KEGG) database. The concurrent presence of 16:1 cis-Δ⁷ and 18:1 cis-Δ⁹ FAs suggested the existence of parallel and/or complementary processes to those invoked for the formation of most common 16:1 cis-Δ⁹ and 18:1 cis-Δ¹¹ FAs. A further route was hypothesized for the trans FAs biosynthesis in wild-type cells of R. sphaeroides.     

Cationic copolymerization of anethole: Reactivity of geometric isomers of α,β-disubstituted ethylenes

Cationic copolymerizations of anethole were carried out under various conditions in order to confirm the relative reactivities of its geometric isomers. trans-Anethole was more reactive than cis-anethole in copolymerizations with p-methoxystyrene or styrene, but less reactive in the mutual copolymerization of cis- and trans-anethole; i.e., the trans isomer was more reactive to a growing chain end with little steric hindrance. Thus the intrinsic reactivity of an olefinic double bond to carbonium ion is greater for the trans isomer than for the cis isomer. This idea is supported by ¹³C NMR spectra, since the signal of the olefinic β-carbon of the trans isomer is at higher field than that of the cis isomer. The behavior of anethole was compared with the results observed in vinyl ethers, where the cis isomer was always more reactive irrrspective of the structure of the growing chain end. In addition, the dependence of monomer reactivity ratios on polymerization conditions is discussed.     

Direct separation and quantitative determination of glimepiride isomers by high performance liquid chromatography

A simple and sensitive high‐performance liquid chromatographic procedure for the determination of the trans isomer of glimepiride is reported. Chromatography accomplished direct separation of the cis and trans isomers of glimepiride on a Dikmonsil C18 (250×4.6 mm, 5 μm) column with a mobile phase consisting of methanol‐acetonitrile‐NH₄Ac buffer solution (1.5 mol L^–1, pH = 4.5) (1.1 : 1.3 : 1.0, v/v) at a flow rate 0.5 mL min^–1. The resolution (R_S) was 1.73 with a retention time of 24.885 and 23.018 min for the cis and the trans isomer, respectively. A standard linear calibration curve was established for the trans isomer of glimepiride over the range of 4.95–198.00 μg mL^–1 with a correlation coefficient of 0.99997. This method has been successfully used to analyze four different kinds of glimepiride product.