Carbon-13 NMR spectra of bromine-containing esters

¹³CHBrCOOR fragments of bromine-containing mono- and dicarboxylates show ¹³C NMR signals in the range 38–45 ppm, at 12–15 ppm to high field of their chloro analogues. The introduction of one bromine atom into the α-CH₂ group causes a 6–13 ppm shift of the carbon signal, depending on the type of ester. The replacement of H with Br in ¹³CH₂(COOR)₂ has only a negligible influence on the ¹³CH₂ signal position. The ¹³CBr₂COOR fragment signals appear in the range 50–54 ppm. ¹³C NMR data, including the chemical shift values, signal multiplicities and spin-spin couplings, make possible the identification of isomers present in isomer mixtures of bromine-containing esters. The ¹³COOR chemical shifts of diastereomers of α,β-dibromoesters differ noticeably from each other, and the ¹³C NMR spectra can thus be used to supply preliminary information about the stereochemistry of bromine addition to α,β-unsaturated acids.     

NMR study of the spatial structure of synthetic pyrethroids

The spatial isomers of the new synthetic analogs of ethyl permithrinic ether and permethrin were investigated by NMR (¹H, ¹³C, DEPT (distortionless enhancement by polarization transfer), COSY (correlation spectroscopy), CHCORR (heteronuclear (C, H) shift correlation spectroscopy), ROESY (rotating-frame Overhauser effect spectroscopy)). Several tendencies were revealed in the ¹H and ¹³C chemical shifts of the α atoms of the substituents in the 2nd and 3rd positions of the cyclopropane ring. For substituents cis-orientated relative to the ester group, the spectra show a paramagnetic shift of the ¹H signals and the diamagnetic shift of the ¹³C signals relative to the trans-orientated substituents. The ¹H and ¹³C chemical shifts of the α atoms of the substituents in the 2nd and 3rd positions of the cyclopropane ring permit an unambiguous determination of the stereochemistry of ethyl permethrinic ether and permethrin analogs.     

13C NMR spectroscopic evidence for the existence of the monocation of pyropheophytin a

Pyropheophytin a, which is an unsymmetric porphyrin, has been titrated with trifluoroacetic acid (TFA) in tetrahydrofuran, the protonation reaction being followed by ¹³C NMR spectroscopy. TFA was added in small increments to a 0.28 M solution of pyropheophytin a in tetrahydrofuran, and the chemical shift changes of the macrocyclic carbons were determined as a function of the TFA increments. On the addition of TFA the signals of the α-carbons of ring II experienced a large upfield change, whereas the signals of all other macrocyclic carbons moved only slightly downfield or remained constant. These observations were interpreted as indicating the formation of a monocation in which the proton is attached to the nitrogen of ring II. The ¹³C protonation shifts of pyropheophytin a were compared with those previously reported for symmetric porphyrins. On the basis of this comparison, the basicity of the macrocyclic nitrogen atoms, the N–H tautomerism and the electron delocalization in structurally different porphyrin macrocycles are discussed.     

Synthesis of new crown ethers and their metal complexes: 1H and 13C NMR study

A new type of crown ethers containing a diphenyl ether unit has been prepared, the ring size ranging from 12 to 36. ¹H and ¹³C NMR spectra of both free ligands and their metal-ion complexes have been recorded. For 18- and 21-membered compounds a general downfield shift was observed for both methylene and aromatic proton resonances on metal-ion complexation. The stoichiometry of K⁺ and Na⁺ complexes was deduced from chemical shift dependence on metal-ion concentration. The K⁺ and Na⁺ complexes of 18- and 21-membered rings have a guest to host ratio of 1:1, whereas the K⁺ salt of the 15-membered ring exists as a 1:2 complex in solution. The ¹H shift observed on salt formation was attributed to electric-field and conformational effects. The ¹³C resonances for the aryl carbons, C-1, C-2 and C-3, and the α-methylene carbon in 15- and 18-membered rings were shifted upfield when an equivalent amount of KSCN was added in CDCI_3?DMSO-d₆. The shift changes were independent of the anion, and similar results were obtained for SCN^?, Br^?, and I^? salts. The upfield shift is explained by conformational factors. The spectral changes were slight for 12- and 36-membered rings. In 15- and 18-membered rings, complexation induces conformational changes which force the C-α carbon into the plane of the benzene ring. The solution conformation of these molecules is discussed.     

19F and 1H NMR of aldimines of fluoroamino acids with pyridoxal-5′-phosphate

¹H and ¹⁹F NMR spectra were obtained for six Schiff bases (aldimines) formed by pyridoxal-5′-phosphate (PLP) with four fluorinated or their two parent non-fluorinated α-amino acids (phenylalanine and α-aminobutyric acid). pK_A Values were derived from ¹H and ¹⁹F titration curves. The imine nitrogen of the aldimines is very basic (～13) and sensitive to the electron withdrawing effect of fluorine. The pyridine nitrogen is less basic in the aldimines (～7) than in PLP (8.12) and is less sensitive to the electron withdrawing effect of the fluorine than is the imine nitrogen. The phosphate group has a pK in the same range (～6) and the chemical shifts of some nuclei are sensitive to both pK values. Protonation of the aldimine causes the ¹H signal to shift downfield at the methyl protons of the pyridine ring and at the aldehydic proton of the aldimine for the high pK value (except for the aldimines prepared from the β-fluorophenylalanine), but upfield at the aromatic proton and at the aldehydic proton of the aldimine for the low pK. Protonation of the aldimine causes the ¹⁹F signal of an aryl fluorine to shift downfield but gives an upfield shift at a β-fluorine. These data are related to the highly conjugated electronic structure of the Schiff bases.     

Comparative conformational studies of cyclic derivatives by 13C NMR spectroscopy

The ¹³C chemical shift of the substituted or functionalized carbon of various medium and large rings is plotted against ring size (6–15 carbons). The curves thus obtained allow a conformational analysis of the corresponding derivatives.     

The diphenyliodonium-2-carboxylates: An NMR structure investigation

Diphenyliodonium-2-carboxylate, phenyl 4-methylphenyliodonium-2-carboxylate and phenyl 5-methyl-phenyliodonium-2-carboxylate have been examined by ¹H and ¹³C NMR spectroscopy. Each compound showed one proton considerably upfield from the others. From the two substituted compounds this was deduced to be H-6 on the ring containing the carboxylate group. Shift reagents and relaxation measurements were used to make the ¹³C chemical shift assignments. These data are most consistent with a cyclic, neutral structure for these iodonium carboxylates. The iodine is in the center of a trigonal bipyramid with the unsubstituted phenyl and carboxylate group apical, which places the C-6 proton in the shielding region of the adjacent phenyl group.     

DFT‐GIAO 1H and 13C NMR prediction of chemical shifts for the configurational assignment of 6β‐hydroxyhyoscyamine diastereoisomers

¹H and ¹³C NMR chemical shift calculations using the density functional theory–gauge including/invariant atomic orbitals (DFT–GIAO) approximation at the B3LYP/6‐311G++(d,p) level of theory have been used to assign both natural diastereoisomers of 6β‐hydroxyhyoscyamine. The theoretical chemical shifts of the ¹H and ¹³C atoms in both isomers were calculated using a previously determined conformational distribution, and the theoretical and experimental values were cross‐compared. For protons, the obtained average absolute differences and root mean square (rms) errors for each comparison showed that the experimental chemical shifts of dextrorotatory and levorotatory 6β‐hydroxyhyoscyamines correlated well with the theoretical values calculated for the (3R,6R,2′S) and (3S,6S,2′S) configurations, respectively, whereas for ¹³C atoms the calculations were unable to differentiate between isomers. The nature of the relatively large chemical shift differences observed in nuclei that share similar chemical environments between isomers was asserted from the same calculations. It is shown that the anisotropic effect of the phenyl group in the tropic ester moiety, positioned under the tropane ring, has a larger shielding effect over one ring side than over the other one. Copyright © 2009 John Wiley & Sons, Ltd.     

The study of longifolene by two-dimensional NMR spectroscopy

The complete assignment of the ¹³C NMR spectrum of longifolene was achieved from double quantum coherence measurements, while combined evaluation of a ¹H? ¹³C heteronuclear chemical shift correlation diagram and a homonuclear ¹H J-resolved diagram provided all proton chemical shifts. Conformational information on the seven-membered ring of the tricyclic sesquiterpene was obtained from proton chemical shift considerations.     

13C NMR spectra of a number of penta- and hexacyclic triterpenoids derived from glycyrrhetic acid

The¹³C NMR spectra of 22 derivatives of 18α- and 18β-glycyrrhetic acids that have been investigated and an assignment of the signals has been made. It has been shown that a modification of the carboxy group of glycyrrhetic acid leads mainly to a change in the chemical shifts of the α-, β-, and γ-carbon atoms of ring E. The assignment of a number of signals has been confirmed by the use of the shift reagent Eu(fod)_s. It has been established that the C₂₈ and C₁₆ signals are the most sensitive to a change in the C₁₈ configuration in the spectra of glycyrrhetic acid derivatives.

     

Investigation of the exo and endo isomers of dioxolane-type benzylidene derivatives of carbohydrates by 13C NMR spectroscopy

The absolute configurations of nine 2,3-O-benzylidene-α-L-rhamno- and α-D-mannopyranoside diasteteomeric pairs were determined and the ¹³C NMR spectra of further thirteen α-L-rhamno- and α-D-mannopyranosides, having various substituents, were completely assigned.Four ¹³C shifts were found suitable for the determination of the absolute configuration of the dioxolane skeleton. (1) The chemical shift of the acetal carbon in the endo isomers is between 103.9 and 104.7 ppm whereas for the exo isomers this region extends from 102.8 to 103.4 ppm; (2) The formation of the dioxolane ring causes a deshielding effect for the bridgehead carbons, in the exo isomers this effect is more pronounced for C-3 whereas in the endo isomers for C-2. For C-4, shielding effect was found in the exo isomers and deshielding effect in the endo ones; (3) The chemical shift of the quaternary carbon of the phenyl group is greater in the exo isomers than in the endo ones; (4) The difference between the shift of the acetal carbon and that of the quaternary carbon of the phenyl group in the exo isomers is greater than 35.4 ppm, in the endo isomers is less than 33.7 ppm.     

13C NMR spectral characterization of epimeric rotenone and some related tetrahydrobenzopyranofurobenzopyranones

The ¹³C nuclear magnetic resonance (nmr) spectra of epimers of rotenone and four 12a-hydroxy-analogues were examined to determine the stereochemical effect of the B/C ring fusion involving the 6a- and 12a-carbon centers. Chemical shift differences between the epimeric carbon resonances of cis- and trans-6a,12a-compounds were notably larger than those of diastereoisomers derived from the same B/C ring junction stereochemistry. Results of the spectral analysis have been useful for the quantification of mixtures of epimers and for the measurement of rates of epimerization and oxygenation.     

Effective consideration of ring structures in CAST/CNMR for highly accurate C NMR chemical shift prediction

A new function that effectively takes into account ring structural environments achieves extensive highly accurate prediction of ¹³C NMR chemical shift in the CAST/CNMR system. The approach adapts a fast and flexible ring perception algorithm and a new CAST coding method for the ring information. ¹³C NMR chemical shift prediction is performed for complicated polycyclic natural products and their synthetic intermediates as the demonstration, which shows the reliability of the function in extending the scope of the practically accurate ¹³C NMR prediction for wide range of organic compounds.     

13C-{1H} NMR spectra of episulphide copolymers

Ethylene sulphide-isobutylene sulphide and propylene sulphide-isobutylene sulphide copolymers have been prepared using anionic catalysts and investigated by ¹³C-{¹H} NMR spectroscopy. The carbon-13 NMR spectra are assigned in terms of diad and triad sequences. There is discussion of the effects of mono- or dimethyl substitution in the α, β, γ or δ positions on the chemical shift of the main chain carbon atoms. It has also been shown that for isobutylene sulphide, as for propylene sulphide, under the influence of an anionic catalyst, there is a normal ring opening only at the primary carbon atom.     

The Tricyclooctanone Approach to the Total Synthesis of Steroids. The Cyclization of the A-CD Unit

The first steps of a novel approach to the total synthesis of 9, 11-dehydroestrone via tricyclo[3.3.0.0^2,8]octan-3-one (2) are described. One route involves a tandem-type transformation of the key intermediate 3 (A-CD unit) consisting of cyclopropane cleavage and ring B closure to afford C, 18-bisnor-13 α, 17 α-estradiol derivatives. E.g. the 3-methoxy-9 ζ-hydroxy-17 α-methanesulfonyloxy derivative (-)- 6 has been synthesized in 8 steps and 10% overall yield from 1,3-cyclohexadiene. As an alternative, the A-CD type intermediate 4b has been prepared and could be used for a ring C enlargement prior to cyclization.     

NMR Studies of the Structures of Cyclic Phosphorus Compounds

Abstract

[RNPOCl]₃ (R = Ph, p-Tol, p-MeOC₆H₄) has been shown by ³¹P n.m.r. to consist mainly of the 2α, 4α, 6β(C₂) isomer (AB₂ spin system), along with small amounts of the symmetric (C₃) isomer (2α, 4α, 6α). The ³¹P shifts are almost independent of the nature of the group R. Confirmation of the ring structure requires the determination of J_BB which is achieved by second-order analysis of ¹³C or ¹H spectra. Similar techniques have been applied to the phosphorus-oxygen ring system [PhPO₂]₃. We have discovered a relationship between the orientation of N-phenyl substituents on cyclophosphazane ring systems and the ¹³C n.m.r. shifts of the ring carbons. In cyclodiphosphazanes the phenyl rings are coplanar with the P-N ring, whereas in cyclotriphosphazanes the phenyl rings are perpendicular to the P-N-P plane; the ¹³C shifts of the ortho and para carbons are quite different in these two situations, and we have used these shifts to obtain structural information on a number of cyclophosphazanes. Rotation of the ring about the C-N bond has also been studied by low-temperature n.m.r.     

57Fe NMR Chemical shifts and 57fe, 13C coupling constants in α-ferrocenyl carbocations. Direct metal participation in the stabilization of metallocenyl carbocations

The ¹³C{⁵⁷Fe} double resonance method has been used to investigate ⁵⁷Fe-enriched samples of ferrocene derivatives, α-ferrocenyl carbocations and carbonyl complexes with various σ- and π-hydrocarbon ligands. In the α-ferrocenyl carbocations the ⁵⁷Fe resonances span a 1200 ppm range, being a sensitive tool of direct iron participation in the stabilization. The ⁵⁷Fe resonances in the carbocations [FcCH₂][HSO₄] (I), [FcCHMe][HSO₄] (II), [FcCHPh][HSO₄] (III), [FcCHC₅H₄ Mn(CO)₃][CF₃CO₂] (IV), [(Fc)₂CH][BF₄] (V), [FcCHC₅H₄RuC₅Hs][BF₄] (VI) and [FcCMe₂][HSO₄] (VII), ?523.6, ?219.3, 221.0, 368.7, 699.0, 405.0 and 288.5 ppm, respectively, relative to ferrocene, are interpreted in terms of rehybridization of the iron non-bonding d orbitals (shielding effect and the electron withdrawing effect of the substituent in the cyclic ligand (deshielding effect). The role of rehybridization of non-bonding iron orbitals in the low-frequency shift of the ⁵⁷Fe resonances, in addition to that in the previously investigated complex [(C₅H₅)₂FeH][BF₃OH] (?1109.3 ppm), has been demonstrated for bridge-substituted [3]ferrocenophanes, whose ring tilting induces a low-frequency shift of up to 340 ppm relative to their unbridged analogues.The ¹³C NMR spectra of carbocations V and VI reveal a temperature dependence due to the rotation around the C(1)C_α exocyclic bonds. In carbocation VI the ruthenium atom effectively competes with the iron atom to bond with C_α whereas in carbocation V two equivalent metal atoms possess the same ability for such bonding; as a result, complex V has a more pronounced “carbenium ion” nature than IV and VI, as indicated by the relative positions of the ¹³C_α resonances in carbocations IV, V and VI: δ 122.4, 147.2 and 116.9 ppm, respectively.The values of ⁵⁷Fe, ¹³C coupling constants for α-ferrocenyl carbocations exclude Fe-C_α σ-bonding and support a structure in which the iron atom is π-bonded with six carbon atoms of a fulvenoid ligand. According to the data on ⁵⁷Fe resonances and ⁵⁷Fe, ¹³C coupling constants in α-ferrocenyl carbocations the strength of FeC_α bonding is markedly influenced by the electronic effect of the substituent at C_α, being even lower in carbocation I than that of Fe-cyclopentadienyl carbon atoms.     

Discrimination by NMR spectroscopy of isomeric ar,ar′-disubstituted [2.2] paracyclophanes carrying identical substituents

 Four diastereomers are formed when precursors of the type 2-R-1,4-(CH₂X)₂C₆H₃ are coupled in the synthesis of symmetrical [2.2]paracyclophanes with one substituent R per aromatic ring. The stereochemistry of the diastereomers is derived from the symmetry of their ¹H NMR spectra and from NOE difference and 2D C,H-COSY and -COLOC spectra for the case of R=CO₂CH₃. This substituent causes only small ¹H chemical shift effects in the distant aromatic ring; so the results of chemical shift increment calculations should be regarded with caution. For the present compounds, the use of ¹³C shift increments is safer, but the disadvantage of increment calculations is the need for the mono-substituted derivative and the analysis of its spectra. Received: 24 May 1996/Accepted: 1 July 1996     

The determination of sulfoxide configuration in six-membered rings using NMR spectroscopy and DFT calculations

Cyclic six-membered ring sulfoxides and sulfones were prepared by a stepwise in situ oxidation of the corresponding sulfides with meta-chloroperbenzoic acid in an NMR tube. The oxidation was followed by NMR spectra and the ¹H and ¹³C NMR data were collected. The geometries of all of the compounds were optimized using the DFT B3LYP/6-31G^∗∗ method and the ¹³C and ¹H NMR chemical shifts were calculated for geometry-optimized structures with the DFT B3LYP/6-31++G^∗∗ method. The calculated ¹³C NMR chemical shifts induced by oxidation (Δδ values) are in very good agreement with the experimental data and can be used to determine the oxidation state of the sulfur atom (-S-, -SO-, -SO₂-). The characteristic differences of the induced oxidation chemical shifts of carbon atoms at the α- and β-position to sulfur were successfully used for distinguishing between the diastereoisomeric sulfoxides.     

Carbon-13 NMR spectra of some 4-substituted phenacyl bromides

The ¹³C NMR signals for some 4- substituted phenacyl bromides were assigned. The experimental chemical shifts of the aromatic ring carbons are in close agreement with those calculated using substituent chemical shifts. Both the carbonyl and the α-methylene carbons exhibit upfield shifts compared with those of the corresponding 4-substituted acetophenones.