Thermally-reversible photoisomerization of trans-dinitrobis(tri-N-propylphosphine)palladium(II)

UV irradiation of trans-dinitrobis(tri-n-propylphosphine)palladium(II) in MeOH yields a photostationary trans-cis mixture which reverts quantitatively in the dark to the trans-form. The cis-isomer, which can be isolated in crystalline form by irradiation of trans-species in n-hexane, has been characterized by UV, IR ¹H and ³¹P NMR spectroscopy. Irradiation of a 1/1 mixture of trans-[(PBu₃ⁿ)₂Pd(NO₂)₂] and trans-[(PPr₃ⁿ)₂Pd(NO₂)₂] gives almost entirely a 1/1 mixture of the corresponding cis-isomer, indicating an intramolecular process.     

Interactions intramoléculaires: XXIII—Constantes de couplage et interactions gauches avec un groupement t-butyle (cyclohexanones disubstituées 3, 4)

The ¹H NMR spectra of a series of cis and trans-3R,4 X-cyclohexanones (-2,2,6,6-d₄) are analysed. By comparison of their ³J coupling constants with those of cyclohexane homologues we obtain information about the chair–chair equilibrium constants for R = CH₃, X = CN, the chair structure of cis isomers with an equatorial t-butyl group, and a conformational heterogeneity with trans (CH₃)₃C and CN groups. This latter situation is analysed by means of a simplified but controlled Karplus relationship, on the basis of a mixture of two conformers; this involves a diequatorial chair and a boat form with a dihedral angle Φ₃₄ of about ?6°.     

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.     

The stereochemistry of 2,4-disubstituted γ-butyrolactones and 2,4,6-trisubstituted-5,6-dihydro-4H-1, 3-oxazines

2,4-Disubstituted butyrolactones and 2,4,6-trisubstituted-5,6 dihydro-4H-1,3-oxazines show similar features in their ¹H and ¹³C- NMR spectra. Two geminal ring hydrogens of cis isomers give rise to a complex ABXY spectra when the substituent is alkyl or aryl. In spectra of trans isomers these patterns are degenerated. When R is OMe(in 4) or OCOMe (in 6) the difference in chemical shifts of geminal protons and vicinal coupling constants cannot be used for diagnosis. In ¹³C spectra ring carbons C-2 and C-3 in lactones and C-4 and C-5 in oxazine of trans isomers show a small but consistent shift to higher fields.     

Carbocationic Cyclizations: IX. Rearrangement of Long-Lived 4-(2-Biphenylyl)-1,2,3,4-tetramethylcyclobutenyl Cation into trans- and cis-4,5,6,6-Tetramethyl-4,5,6-trihydrocyclopenta[j,k]phenanthren-5-yl Cations

According to the ¹H and ^{1 3}C NMR data, long-lived 4-(2-biphenylyl)-1,2,3,4-tetramethylcyclobutenyl cation generated by protonation of 3-(2-biphenylyl)-1,2,3-trimethyl-4-methylenecyclobutene in super-acids undergoes cyclization which launches further rearrangements finally leading to formation of a mixture of trans- and cis-4,5,6,6-tetramethyl-4,5,6-trihydrocyclopenta[j,k]phenanthren-5-yl cations.     

Conformational analysis: XIV—A 1H n.m.r. conformational study of methyl substituted 2-oxo-1,3,2-dioxathians to confirm the predominance of chair forms in the trimethylene sulphite series

Seven isomeric 4,5,6-trimethyl-2-oxo-1,3,2-dioxathians, cis-4-trans-6-dimethyl-r-2-oxo-1,3,2-dioxathian and two isomeric 4,5,5,6-tetramethyl-2-oxo-1,3,2-dioxathians were prepared and their ¹H n.m.r. spectra analysed. The values of the vicinal coupling constants reported earlier for the cis-4-trans-6 compound were shown to be erroneous. In all cases the values of the vicinal coupling constants (and those of the chemical shifts) are indicative of a single chair conformation or a chair-chair equilibrium, in contrast to earlier reports on the significant contribution of twist forms but in agreement with an electron diffraction study. The chair form is ? 31 kJ mol^?1 thermochemically more stable than the twist form.     

Thermally Induced trans-to-cis Isomerization and Its Photoinduced Reversal Monitored using Absorption and Luminescence: Cooperative Effect of Metal Coordination and Steric Substituent

For ethene derivatives with large groups the cis-isomer is often quite unstable and unavailable. Herein, we report an exception of two stable coordination complexes, (cis- L )ZnCl₂, starting from trans-1,2-bis(1-R-benzo[d]imidazol-2-yl)ethene (R=H, L1 ; R=CH₃, L2 ) ligands under solvothermal condition (T ≥140 °C). Using the intensity of the absorption and luminescence spectra as probes we proposed its progressive cis-to-trans reversal upon irradiation with UV light, which was confirmed by powder X-ray diffraction (PXRD). Similar results observed in the series of (cis- L2 )M^IICl₂ [M=Fe ( 4 ), Co ( 5 ), Ni ( 6 )] demonstrate the universal strategy. The results of PXRD, NMR spectroscopy, ESI-MS and DFT calculations support the above conclusion. NMR spectroscopy indicates that irradiation of 1 converts an optimized 71 % of the cis-isomer to trans, whereas the free trans- L1 ligand transforms to only 15 % cis-isomer under similar conditions.     

N″-sulphonylformamidrazones; preparation and characterisation

N′-Sulphonylformamidrazones have been prepared in good yields from the reaction between N-sulphonylformimidates and di- or tri-substituted hydrazines.The actual tautomeric form present has been shown to be a hydrazide imide by ¹³C NMR spectroscopy. ¹H and ¹³C NMR spectra showed the existence of a cis and a trans isomer in solution, arising from hindered rotation around the CN¹ bond.     

Synthesis and conformational analysis of some cyanomethylene derivatives of piperidines

Five 4-dicyanomethylene derivatives 6–10, N-cyanoacetyl-cis-2,6-diphenylpiperidin-4-one 11 and 4-cyano(ethoxycarbonyl)-methylene-cis-2,6-bis(o-chlorophenyl)piperidine 12 were synthesised by condensing the appropriate piperidin-4-ones 13–17 with malononitrile/ethylcyanoacetate and their ¹H and ¹³C NMR spectra were recorded. The ¹H-¹H COSY spectrum for 6 and NOESY spectra for 8, 10, and 11 were also recorded. Based on coupling constants and the results obtained from NOESY spectra boat conformation for 10 and epimerised chair conformations for 8 and 9 have been proposed. Other derivatives adopt normal chair conformations. Theoretical calculations and the ¹H and ¹³C chemical shifts also support the above conformations. Mass spectra were also recorded for 6–12.     

On the stereochemistry of 1,4-diheterocyclanes. IV—carbon-13 NMR spectra and structural properties of 1,4-dioxane-2,3-diols and their methyl-substituted derivatives

The 1,4-dioxane-2,3-diols and a number of their methyl-substituted derivatives were synthesized and their ¹³C NMR spectra measured. The configurational properties were studied on the basis of the spectral data and chemical equilibration. For the parent compound the trans configuration is slightly more stable. The hydroxyl groups are predominantly axial. Introduction of a methyl group at the 2 position causes the cis configuration of the hydroxyl substituents to become more stable. The substituent effects on the chemical shifts were calculated and used to test the conformational homogeneity of the compounds. The results indicate a biased (chair) conformation for all the methyl-substituted derivatives which were studied. Methyl groups have strongly predominating equatorial orientation in each case studied.     

Synthesis and 1H and 13C NMR structural analysis of cis‐ and trans‐2‐imino‐1,3‐ and ‐3,1‐perhydrobenzoxazines and their 3‐ and 1‐N‐methyl derivatives

cis‐ and trans‐2‐imino‐1,3‐ and ‐3,1‐perhydrobenzoxazines and the N‐methyl derivatives of the latter were synthesized from the corresponding cyclic 1,3‐amino alcohol with cyanogen bromide. The configurations of the studied compounds were confirmed by ¹H and ¹³C NMR spectra. All trans‐fused compounds exist in biased chair–chair conformations as expected, whereas the cis‐fused 1,3‐benzoxazines attain exclusively the O‐in conformations. The cis‐fused 3,1‐benzoxazines, especially the 1‐methyl‐substituted derivatives, tend to favor the N‐out form, obviously owing to the favorable axial orientation of this N‐methyl. Copyright © 2003 John Wiley & Sons, Ltd.     

Ring distortion effect on δ13C values of bicyclo[4.n.0] alkanes

The ¹³C NMR spectra of trans- and cis-fused bicyclo[4.n.0]alkanes, where n=4, 3 and 2, were determined. The distortion of the six membered ring, which is due to the strain arising from the 1,2 fusion of cyclohexane with a smaller ring, is apparent mainly for the chemical shifts of the ring junction carbons. Both for the trans-and for the cis-fused hydrocarbons the results support a cyclohexane in a chair conformation, distorted to a different degree according to the size of the other ring. The geometries of the different compounds in the series were calculated.     

Conformational composition of the 2,4,6-trimethyl-1,3,2-dioxaborinane stereoisomers

A study of conformational transformations of 2,4,6-trimethyl-1,3,2-dioxaborinane cis- and trans-isomers in non-empirical quantum-chemical HF/6-31G (d) and PBE/3z approximations showed that the transformation routes include the sofa conformers with different orientations of the substituents at the ring atoms C⁴ and C⁶ through the transition states corresponding to diequatorial, axial-equatorial, and dixial conformations of the 2,5-twist form. The calculation and ¹H NMR data show that the conformational equilibrium of cis-isomer is almost entirely shifted to diequatorial sofa, and the trans-form is characterized by the interconversion between the two axial-equatorial sofa conformers.     

NMR studies of the configuration of the isomeric 2,3-dimethyloxetanes

The ¹H and ¹³C NMR spectra of cis- and trans-2,3-dimethyloxetane have been re-examined and the published NMR assignments shown to be in error.     

Stereoselective synthesis and characterization of novel trans-4-(thiophenyl)pyrazolyl-β-lactams and their C–3 functionalization

A stereoselective synthesis and C–3 functionalization of a long series of novel hybrid 4-(thiophenyl)pyrazolyl-β-lactams have been carried out. The divergent substrate scope and mechanistic insights were examined to delineate the generality of reaction that favored trans-β-lactams 4a-q almost exclusively in all cases. The C–3 functionalization was achieved by Lewis acid assisted nucleophilic substitution reaction of cis-3-chloro-β-lactams 6a-e to afford cis-3-monosubstituted-β-lactams 7a-e. The cis stereochemistry of β-lactams 7a-e was further established by stereospecific desulfurization with Raney-nickel, in representative cases (7a,b), leading to the formation of cis-β-lactams 8a,b. The structures and stereochemical assignments for synthesized β-lactams have been unambiguously confirmed using FT-IR, 1D NMR (¹H and ¹³C), 2D NMR (¹H–¹H COSY, ¹H–¹³C HSQC and ¹³C DEPT–135), elemental analysis (CHN), mass spectrometry (ESI-MS) and single crystal X-ray crystallography, in representative cases (4b,e). The cis and trans configuration of the hydrogen/chloro/nucleophile substituent at C–3 was assigned with respect C4–H of the β-lactam ring.     

Selective detection of multiplets via double quantum coherence: A 19F NMR study of perfluorodecalin

Commercially obtained perfluorodecalin (octadecafluorodecahydronaphthalene) contains a mixture of cis- and trans-isomers, the ¹⁹F NMR signals of which overlap. The spectrum can be simplified and assignments can be made by using techniques based on double quantum coherence. The INADEQUATE method eliminates peaks due to the cis-isomer, and the DOUBTFUL method selectively detects two quartets of the trans-isomer.     

Efficient synthesis of cis-thiazolidinethiones derived from ephedrines

The reaction of chlorodeoxypseudoephedrine or chlorodeoxynorpseudoephedrine hydrochlorides with sodium dithiocarbonate in stirring ethanol at 0 °C to stereoselectively afford the corresponding cis-thiazolidinethiones in good yields (81% and 95%) is reported. The in situ formation of a cis-aziridine to explain the presence of trans-thiazolidinethione as a side product is proposed when the same reaction was carried out at room temperature. In addition, a 70:30 mixture of trans-isomers of a thiazolidinethione/isothiazolidinethione was formed when a cis-aziridine NH was reacted with carbon disulfide in refluxing ethanol. The analogous reaction with cis-aziridine N-Me stereoselectively affords the corresponding cis-thiazolidinethione. The ¹H and ¹³C NMR data of the thiazolidinethiones were assigned. cis-3,4-Dimethyl-5-phenylthiazolidine-2-thione was crystallized from ethanol and its X-ray diffraction structure was analyzed.     

1H and 13C NMR studies of some germacrones and isogermacrones

The ¹H and ¹³C NMR spectra of the trans,trans-, cis,cis- and cis-C-3–C-4, trans-C-7–C-8-germacrones and of the cis-C-2–C-3, trans-C-7–C–8, trans-C-2–C-3, cis-C-7–C-8- and cis,cis-isogermacrones are analysed. The last two isogermacrones are new compounds. The C-2–C-3 double bond in the previously described isogermacrone is found to be of cis configuration, contrary to the hitherto accepted trans arrangement.     

Conformational analysis of <Emphasis Type="Italic">cis-</Emphasis> and <Emphasis Type="Italic">trans</Emphasis>-isomers of 2,5-dimethyl-1,3-dioxane

The conformational isomerization of cis- and trans-isomers of 2,5-dimethyl-1,3-dioxane was investigated by means of ab initio RHF//6-31G(d) and PBE//ccpVDZ quantum-chemical methods. It has been shown that in comparison with cis-form the potential energy surface of trans-isomer has a less number of stationary points and includes the main minimum belonging to the diequatorial chair form. In the case of cisisomer the main minimum corresponds to the C 2e5a chair form. The values of potentional barriers of conformational isomerization were also determined. On the basis of experimental (NMR ¹H) and theoretical vicinal coupling constants we determine the value ΔG ⁰ of chair-chair inversion for the trans-isomer molecule.     

1H and 13C NMR studies on 1,3,2-dioxarsolanes

The ¹H and ¹³C NMR spectra of a series of 1,3,2-dioxarsolanes have been obtained at 2.1 T and some at 9.4 T. The chemical shifts and spin-spin coupling constants have been obtained from complete spectral analyses of the ¹H and proton-coupled ¹³C spectra. The spectral data are interpreted on the basis of two rapidly interconverting half-chair conformers with a pseudoaxial substituent at arsenic. Unique assignment of syn/anti or cis/trans geometries have been made from ¹H or ¹³C NMR spectroscopy alone. The syn and trans isomers of the 4-methyl- and 4,5-dimethyl-1,3,2-dioxarsolanes, respectively, appear to be conformationally biased towards the forms with pseudoequatorial methyl groups. The general trends in the geminal and vicinal ¹H? ¹H and ¹³C? ¹H coupling constants are interpreted in terms of stereospecific, electronegativity and lone-pair effects of the oxygen heteroatoms and conformational factors. The NMR data on the 1,3,2-dioxarsolanes are discussed with reference to related 1,3-dithoxa- and 1,3-dithia- five-membered rings with As, P, S or C at the 2-position.