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Conformational Analysis f Organic Carbonyl Compounds. Part 3. A 1H AND 13C Nuclear Magnetic Resonance Study of Formyl and Acetyl Derivatives of Benzo[b]thiophene
No full textThe conformational analysis of formyl and acetyl derivatives of benzo[b]thiophen was carried out by employing nmr chemical shifts (1-H and 13-C) and coupling constants. By measuring the lanthanide-induced shifts (LIS) and simulating experimental chemical shifts the relative conformer stability was determined. The results show that all the molecules examined are present almost completely in the Z conformation and that the stabilization of one conformer in this heterocyclic system seems due almost exclusively to the mesomeric interaction originating in the trans-arrangement of the C=O bond and the C=C bond having the higher double-bond character
A 1H NMR Spectra of the 2-Trifluoroacetyl Derivatives of Benzo[b]furan and Benzo[b]thiophene
No full textProton magnetic resonance spectroscopy of title compounds were carefully examined
Synthesis of 3,3'- and 4,4'-bis(alkylsulfanyl)-2,2'-bithiophenes from the corresponding dibromo derivatives through lithiation
No full textBoth 3,3'- and 4,4'-dibromo-2,2'-bithiophenes, through bromine-lithium exchange with butyllithium followed by reaction with a dialkyl disulfide (Me(2)S(2) or Bu(2)S(2)), have been converted into the corresponding bis(alkylsulfanyl) derivatives
INTERNAL-ROTATION AROUND SINGLE BONDS AND CONFORMATIONAL PREFERENCES IN HETEROCYCLIC-ANALOGS OF BENZYL METHYL SULFOXIDE STUDIED WITH NMR TECHNIQUES
No full textThe conformational equilibrium related to the internal rotation processes occurring in sulphoxides of the type ArCH2SOCH3 (where Ar=2-thienyl, 2-furyl, 2-pyridinyl and (3-methyl)2-pyridinyl rings) were studied with H-1 and C-13 NMR spectroscopy. Proton chemical shifts and long-range coupling constants (n)J(H, H) were obtained from the iterative analysis of the multiplets and were employed, together with C-13 chemical shifts, long-range (n)J(C, H) and relaxation parameters (NOE and non-selective T-1 values) to obtain stereochemical relationships between the protons present in these molecules. Conformational predictions at a qualitative level were also derived from total molecular energies calculated with the semi-empirical AM1/MNDO method as a function of internal coordinates. The different approaches converged to indicate that the heterocyclic rings adopt an average orientation similar to the perpendicular orientation of the phenyl ring in benzyl methyl sulphoxide and, as regards rotation around the CH2-S bond, the prevalent conformer shows that the methyl group is symmetrically oriented with respect to the methylenic protons, The barriers for internal rotation are rather low and the equilibrium between conformers is dependent on the medium properties. Attempts to obtain conformational results were performed for the molecule of omeprazole, an antiulcer drug which contains the ArCH2SO-R moiety (Ar and R are substituted 2-pyridinyl and 2-benzimidazolyl groups, respectively). With respect to the other compounds examined, the orientation of the Ar ring does not significantly differ and the benzimidazole ring seems to prefer an orientation stereochemically equivalent to that of the methyl group
Long-range 13C-1H Spin-Spin Coupling Constants in the Conformational Analysis of Formil Derivatives of Furan and Thiophene, Magn. Reson. Chem., 25, 804-810
No full textThe long-range 13C-1H Constants were employed in the conformational analysis of formil derivatives of furan and thiophen
Conformational Analysis f Organic Carbonyl Compounds. Part 3. A 1H and 13C Nuclear Magnetic Resonance Study of Formyl and Acetyl Derivatives of Benzo[b]furan
No full textA conformational study of formyl and acetyl derivatives of benzo[b]furan provides evidences that in the case of 2- and 7-derivatives, the E,Z-conformational mixture is solvent dependent: the Z prevailing in solvent with higher polarity
Electron Transfer in the Reactions of Organic Trichloromethyl Derivatives with Iron(II) Chloride
No full textReduction of trichloromethyl derivatives RCCl3 [1, R = Ph;2, R = PhC(O); 3, R = EtOC(O)] with iron(II) chloride in acetonitrile, has been studied in order to examine the mechanism of the electron transfer (ET) process and the reactions of the radicals formed. Substrates 1-3 afforded different product compositions and the cause was identified as differences in the reactivity of radicals which is substantially of two types: reductive coupling and proton abstraction after further reduction to a carbanion coordinated to the metal ion. Compound 1 gave only coupling products, compound 2 only hydrogenated products and compound 3 a mixture of coupling and hydrogenated products depending on experimental conditions. Proton abstraction by the carbanion was found to occur from water molecules, which should be present in the coordination shell of the metal ion, and not from the solvent. The different behaviour of compounds 1-3 is attributed to the presence of substituents which are able to stabilize the radical and carbanionic intermediates. Rate constants at different temperatures were measured and the activation parameters calculated. The three substrates differ only slightly in reaction rates, in the order 1 > 2 > 3. Activation enthalpies are very close to each other and this agrees with the almost equal values of C-Cl bond dissociation energies of compounds 1-3, empirically determined. Large, negative entropies of activation were found, suggesting that an ordered activation complex should be formed in order that electron transfer from the metal ion to the organic halide can take place
INTERNAL-ROTATION AND CONFORMATIONAL PREFERENCES IN 1,2-DIARYL DERIVATIVES OF 1,1,2,2-TETRACHLOROETHANE - A H-1 DNMR AND X-RAY STRUCTURAL STUDY
No full textThe title compounds show in their H-1 NMR spectra recorded at room temperature signals of different line-width characteristic of molecules with slow internal rotation around sterically crowded single bonds. From the DNMR study in [H-2(6)]acetone solution of the 4,4'-(compound 1) and 3,3'-bis(benzoic acid) dimethyl ester (compound 2) and of the 3,3'-bispyridine (compound 3) derivative the dynamic parameters of two internal rotation processes, with different energies of activation. were extracted. The internal rotation process with higher energy barrier (Delta G* = 13-14 kcal mol(-1)) was assigned to the anti reversible arrow gauche conformer interconversion, and that with lower energy barrier (Delta G* = 8-9 kcal mol(-1)) to the rotation of aromatic rings around the C(sp(3))-C(sp(2)) bond in the gauche conformer. The conformer populations measured at low temperature showed that the gauche conformer is the more abundant. The structural features of the conformers of compounds 1-3 were also investigated theoretically at a semi-empirical level with the AM1/MNDO method. The results of these calculations show that the anti conformer is the most stable one, yet the gauche form has higher polarity and this result could explain the increase of the gauche population in polar solvents. For compound 3 calculations were extended to the whole energy hypersurface having as coordinates the angles of rotation around the central C(sp(3))-C(sp(3)) and C(sp(3))-C(sp(2)) bonds. The theoretical free-energies of activation, even if largely underestimated, show that the barriers for the different internal processes follow the order: barrier for rotation around the C(sp(3))-C(sp(3)) bond > barrier for rotation around the C(sp(3))-C(sp(2)) bond in the gauche conformer > barrier for rotation around the C(sp(3))-C(sp(2)) bond in the anti conformer. This order fits the trend of the first two barriers. experimentally determined. For compound 3 and for the 4,4'-bisbenzonitrile (compound 4) derivative, the crystal and molecular structure was obtained from X-ray analysis: the molecules have crystallographically dictated 1 symmetry with perfectly staggered anti conformation. Longer C-Cl bond lengths and smaller Cl-C-Cl bond angles than those present in structurally similar compounds indicate steric crowding around the C(sp(3))-C(sp(3)) bond. Solid state conformations are in excellent agreement with the calculated ground-state rotational structure of these molecules.The title compounds show in their 1H NMR spectra recorded at room temperature signals of different line-width characteristic of molecules with slow internal rotation around sterically crowded single bonds. From the DNMR study in [2H6]acetone solution of the 4,4′- (compound 1) and 3,3′-bis(benzoic acid) dimethyl ester (compound 2) and of the 3,3′-bispyridine (compound 3) derivative the dynamic parameters of two internal rotation processes, with different energies of activation, were extracted. The internal rotation process with higher energy barrier (ΔG* = 13-14 kcal mol-1) was assigned to the anti ⇌ gauche conformer interconversion, and that with lower energy barrier (ΔG* = 8-9 kcal mol-1) to the rotation of aromatic rings around the C(sp3)-C(sp2) bond in the gauche conformer. The conformer populations measured at low temperature showed that the gauche conformer is the more abundant. The structural features of the conformers of compounds 1-3 were also investigated theoretically at a semi-empirical level with the AM1/MNDO method. The results of these calculations show that the anti conformer is the most stable one, yet the gauche form has higher polarity and this result could explain the increase of the gauche population in polar solvents. For compound 3 calculations were extended to the whole energy hypersurface having as coordinates the angles of rotation around the central C(sp3)-C(sp3) and C(sp3)-C(sp2) bonds. The theoretical free-energies of activation, even if largely underestimated, show that the barriers for the different internal processes follow the order: barrier for rotation around the C(sp3)-C(sp 3) bond > barrier for rotation around the C(sp3)- C(sp2) bond in the gauche conformer > barrier for rotation around the C(sp3)-C(sp2) bond in the anti conformer. This order fits the trend of the first two barriers, experimentally determined. For compound 3 and for the 4,4′-bisbenzonitrile (compound 4) derivative, the crystal and molecular structure was obtained from X-ray analysis: the molecules have crystallographically dictated 1 symmetry with perfectly staggered anti conformation. Longer C-Cl bond lengths and smaller Cl-C-Cl bond angles than those present in structurally similar compounds indicate steric crowding around the C(sp3)-C(sp3) bond. Solid state conformations are in excellent agreement with the calculated ground-state rotational structure of these molecules
CONFORMATIONAL PREFERENCE OF THE METHYLSULFINYL GROUP BONDED TO THE FURAN AND THIOPHENE RINGS - A THEORETICAL APPROACH
No full textThe ground state conformations and transition states for conformer interconversion in the two isomers of the of furan (1 and 2) and thiophene (3 and 4) were identified through a theoretical approach based on ab-initio molecular orbital calculations. Two minima and two transition states were found in the potential energy profiles for all the molecules, but the behaviour of 2-methylsulphinyl furan (1) differs somewhat from that of the other compounds. The minima correspond to the two orientations of the S-O bond with respect to the heterocyclic heteroatom X (X = O or S), labelled X,O-cis an X,O-trans. The conformer having the S-O bond oriented s-cis with respect to the internal double bond with higher pi density (X,O-cis for the 3-methylsulphinyl derivatives and X,O-trans for the 2-methylsulphinyl derivatives) has a twist angle phi (between the S-O bond and the heterocyclic ring) of a few degrees. Larger distortions are present in the conformers with opposite orientation of the S-O bond, and the twist angle has the highest value (phi = 62.8-degrees) in compound 1. The transition states correspond to the methyl group being nearly coplanar with the heterocyclic ring and this situation seems to be due mostly to a loss of pi electron delocalization. More complex is the transition state situation for compound 1, since a barrier in the potential energy for internal rotation is imposed also by the repulsion between the heterocyclic and sulphinyl oxygen atoms. The conformer populations were estimated from the potential energ
Synthesis and Characterization of Isomeric alpha,alpha‘-Bithienyls with beta-Methylthio Substituents
No full textThe three isomeric bithienyl 1,2,3, formally related to the dimerization of 3-(methylsulfanyl)thiophene, have been synthsized by the metal-catalysed coupling between a metallated and a halogenated derivative of thiophene of suitable structure. The 1-H and 13-C NMR data are discussed
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