The NMR spectra of porphyrins: 22—Ring current effects in chlorins versus porphyrins

The proton NMR spectra of tetraphenylporphyrin, octaethylporphyrin and the analogous chlorins (7,8-dihydroporphyrins) are presented, and the chemical shift changes on chlorin formation are interpreted using a ring current model. In these compounds a general 10% reduction in the ring current occurs upon chlorin formation. Similar comparison of the chemical shifts of the corresponding dications and also of the protonated form of 2-vinylphylloerythrin methyl ester with the corresponding chlorin, methyl pyropheophorbide-a, shows that chlorin formation now has a much larger effect on the ring current, this reflecting the increased steric effects within the macrocycle which occur upon protonation. Variable temperature studies on the porphyrins and chlorins examined show clearly the effect of NH exchange processes and, in particular, novel intermolecular exchange processes with trifluoroacetic acid in the protonated species are recorded.     

Carbon-13 NMR spectra of some epoxides derived from dicyclopentadiene

The ¹³C NMR spectra of a series of epoxides derived from endo- and exo-dicyclopentadiene and their partially hydrogenated compounds were determined to examine the substituent effects arising from the introduction of the oxirane ring in comparison with those found in other ring systems. The ¹³C signals of some epoxides were assigned by using lanthanide shift reagents. Characteristic substituent effects exerted by an oxirane ring were demonstrated. Marked steric γ-effects of ?8—13 ppm were observed at the bridge carbon signal in the bicyclo[2.2.1]heptane skeleton. Differences were found in the substituent effects between endo- and exo-dicyclopentadiene epoxides, and have been discussed in relation to the molecular geometry.     

The NMR spectra of porphyrins 20—proton and 13C characterization of porphyrin atropisomers

The preparation, isolation and characterization by proton and ¹³C NMR of the four possible atropisomers of meso-tetra(2-methoxy-1-naphthyl)porphyrin is described. Chemical shift differences due to atropisomerization effects are observed in the porphyrin and naphthyl rings. Comparison of the naphthyl chemical shifts with those of the model compound 1-isopropenyl-2-methoxynaphthalene allows the shifts due to the porphyrin ring current to be isolated. The observed Δδ values of the naphthyl protons agree well with those calculated from the previously described porphyrin ring current model, and allow both the angle of tilt of the naphthyl ring and the dihedral angle of the 2-methoxy substituent to be estimated. In contrast, the Δδ values for the naphthyl carbons bear no relationship to the calculated ring current shifts. Calculations of the total ring current contribution (porphyrin plus naphthyl rings) at the different naphthyl rings of the unsymmetric type III isomer show that at least part of the observed atropisomerism effects are due to the long-range current shifts of the naphthyl rings. The results also provide a clear demonstration of the identity of the porphyrin ring current in the free base and porphyrin dication.     

The NMR spectra of the porphyrins 16—zinc(II) meso-tetraphenylporphyrin (Zn TPP) as a diamagnetic shift reagent. A quantitative ring current model

The facile preparation of zinc(II) meso-tetraphenylporphyrin (Zn TPP) and derivatives from substituted benzaldehydes and pyrrole, combined with the calculation of ring current shifts in these molecules, provides a useful series of selective diamagnetic shift reagents. The porphyrin-ligand equilibrium is examined for some nitrogenous bases and the complexation shifts (δP values) are obtained in precisely the same manner as LIS. The parameterization of the double-dipole model of the porphyrin ring current is given, with the inclusion of the phenyl ring currents and free rotation about the substrate-Zn bond. Precise agreement with the porphyrin proton chemical shifts and the complexation shifts of the geometrically rigid substrates of pyridine, 4-picoline and quinuclidine is obtained. In contrast, the ¹³C complexation shifts in these molecules are subject to additional effects other than the ring current shift.     

NMR spectra of chiral smectic liquid crystals differing in helical parameters

Chiral liquid crystals with three-ring rigid core were examined. The method based on the selectively reflected light was used to determine the helical pitch. The helical twist sense was worked out using the polarimetry method. ¹H and ¹³C NMR measurements were performed. The influence of three molecular structure parameters: the type of chiral centre, the length of non-chiral chain and substitution of benzene ring by fluorine atoms, on helical pitch, handedness of helical structure and values of chemical shift in proton and carbon spectra was determined. The change of the length of non-chiral terminal chain has the most significant influence on the temperature dependence of helical pitch. All tested parameters have the biggest influence on the values of chemical shift of atoms in the chiral centre.     

13C NMR studies of reduced porphyrin compounds. Aromatic delocalization pathways

¹³C NMR spectra of ms-tetraphenylchlorins, new aminoalkyl- and hydroxy-pyrroline substituted ms-tetra-phenylchlorins, and ms-tetraphenylisobacteriochlorins are presented and discussed. Significant changes in the chemical shifts of the α-pyrrole, α-pyrroline and meso skeletal carbons are found in chlorins and isobacteriochlorins in comparison to porphyrins. In contrast, the chemical shifts of the β-pyrrole carbons are almost unaffected by the structural modifications in chlorin and isobacteriochlorin. The chemical shifts of the α-pyrrole carbons in the various chlorins and isobacteriochlorins are strongly affected by the substitutents of the pyrroline ring, or by the introduction of an additional pyrroline ring in isobacteriochlorins. The results show that most of the electron density is concentrated in the unreduced part of the molecule, i.e. in the pyrrole rings and, especially, on the α-pyrrole carbons and is transferred or removed through the aromatic pathway by substituents on the pyrroline ring. These observations are supporting evidence that the α-pyrrole, α-pyrroline and meso-carbons are in the aromatic pathway and favour the proposal of a 16 atom dianion as the preferred delocalization pathway in chlorins and isobacteriochlorins.     

Proton NMR studies of reduced porphyrin compounds

Detailed ¹H NMR studies of ms tetraphenylchlorins (H₂TPC), new amino- and hydroxypyrroline substituted ms tetraphenylchlorins and ms tetraphenylisobacteriochlorins (H₂TPisoB) are presented and discussed. The results obtained are consistent with the general aspects of the ring current models as applied to the parent porphyrins. According to proton chemical shifts a gradual reduction in the magnitude of the ring current is observed in the order ms tetraphenylporphyrin (H₂TPP) > H₂TPC> ms tetraphenylbacteriochlorin (H₂TPB) > H₂TPisoB. The ¹H NMR spectra show chemical non-equivalence of the pyrroline ring protons due to adjacent substituents, and effects of steric constraints on the aminoalkyl substituents due to the close vicinity to the meso phenyl rings. The non-equivalence of the methylene protons of the pyrroline ring produces geminal coupling between the two methylene protons and vicinal coupling with the adjacent pyrroline proton, more pronounced in H₂TPC? OH and in . Restricted configuration of the methylene groups in the ethyl groups of H₂TPC? C(H)(CH₃)N(CH₂CH₃)₂ produces observable geminal coupling between the methylene protons. ¹H NMR reveals the difference between two types of meso phenyls in the chlorins, and three types of meso phenyls in isobacteriochlorins, as reflected in the chemical shifts of the o-phenyl protons.     

13C NMR spectra of benzo[b]thiophene and 1-(X-benzo[b]thienyl)ethyl acetate derivatives

Carbon-13 chemical shift assignments are reported for benzo[b]thiophene and 1-(X-benzo[b]thienyl)ethyl acetate derivatives, where X=? CH(OAc)CH₃ substituted at positions 2-7. Substituent chemical shift (SCS) effects for the ethyl acetate group are additive at all positions. A substantial upfield shift was observed at C-3, arising from the peri interaction of H-3 and the 4-ethyl acetate substituent. Carbon-13 relaxation times (T₁) and nuclear Overhauser enhancements (η) have been measured for benzo[b]thiophene and its derivatives, and the contributions of dipolar, T^DD₁, and spin rotation, T^SR₁, relaxation have been determined. Intramolecular dipole–dipole interactions are found to provide by far the most important spin-lattice relaxation mechanism whenever protons are bound directly to the carbons under investigation. Nonprotonated ring carbons are relaxed by both DD and SR mechanisms. Anisotropic motion has an easily observable effect on the DD contribution to T₁, and can form the basis for spectral assignments, as in 1-phenylethyl acetate. Long-range ¹³C? ¹H coupling constants were observed both between ring carbons and between ring carbons with ring side-chain hydrogens. These results have been used for the structure determination of the title compounds.     

NMR spectra of seleninium and 1- and 2-benzoseleninium cations

The analysis of the NMR spectra of seleninium and 1-and 2-benzoseleninium cations, recorded in deuterotrifluoroacetic acid, is reported. The effects of condensation on the spectral parameters of the protons of the seleninium ring were examined with particular reference to the shift to higher values of δ: this was consistent with the q values calculated by the HMO method. The ³J(HH) and ⁴J(HH) variations, including an interesting variation regarding the through-selenium ⁴J coupling, are also discussed.     

Synthesis,Aromaticity and Photophysical Behaviour of Ferrocene‐ and Ruthenocene‐Appended Semisynthetic Chlorin Derivatives

Two novel synthetic strategies to covalently link a metallocene electron‐donor unit to a chlorin ring are presented. In one approach, pyropheophorbide a is readily converted into its 13¹‐ferrocenyl dehydro derivative by nucleophilic addition of the ferrocenyl anion to the 13¹‐carbonyl group. In another approach, the corresponding 13¹‐pentamethylruthenocenyl derivative is synthesised from 13¹‐fulvenylchlorin by a facile ligand exchange/deprotonation reaction with the [RuCp*(cod)Cl] (Cp*=pentamethylcyclopentadienyl; cod=1,5‐cyclooctadiene) complex. The resulting metallocene–chlorins exhibit reduced aromaticity, which was unequivocally supported by ring‐current calculations based on the gauge‐including magnetically induced current (GIMIC) method and by calculated nucleus‐independent chemical shift (NICS) values. The negative ring current in the isocyclic E ring suggests the antiaromatic character of this moiety and also clarifies the spontaneous reactivity of the complexes with oxygen. The oxidation products were isolated and their electrochemical and photophysical properties were studied. The ruthenocene derivatives turned out to be stable under light irradiation and showed photoinduced charge transfer with charge‐separation lifetimes of 152–1029 ps.     

13C and 15N NMR spectra of 2,3-substituted 2H-azirines

The ¹³C and ¹⁵N NMR spectra of a series of 2,3-substituted 2H-azirines have been studied. The ¹⁵N chemical shift for the nitrogen in the azirine ring is found at much higher field than in acyclic imines with a considerable electronic effect for the substituents on the double bond. Cooperative steric and electronic effects associated with substituents on the unsaturated carbon atom of the ring were found to influence the shielding of the ¹³C and ¹⁵N nuclei. Reaction constants have been calculated for 2-alkyl(aryl)-3-phenylazirines. It has been shown that the azirine ring has a powerful —I effect (when compared with the phenyl ring) that exceeds the analogous value for the cyano group.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1181–1183, September, 1986.     

Carbon-13 NMR spectra of some chlorins and other chlorophyll degradation products

The ¹³C NMR spectra of the methyl esters of phaeophorbides-a and -b, mesophaeophorbides-a and -b, pyrophaeophorbide-a, mesopyrophaeophorbide-a, chlorin-e₆, mesochlorin-e₆, chlorin-p₆, rhodin-g₇, mesorhodin-g₇, phaeoporphyrin-a₅, 2-vinylphaeoporphyrin-a₅, rhodoporphyrin-XV, and 2-vinylrhodoporphyrin-XV, and of trans-octaethylchlorin, in deuteriochloroform and/or trifluoroacetic acid solution are reported. On the basis of comparisons within this comprehensive series and proton off-resonance decoupled spectra, assignments of most resonances are made; complete assignment of the quaternary “pyrrole” ring carbons was difficult to accomplish. A downfield shift of the α- and β-meso-carbons of chlorins in trifluoroacetic acid relative to deuteriochloroform is used to confirm that the Chlorobium chlorophylls (660) from Chloropseudomonas ethylicum are meso-methylated at the δ-position.     

1H NMR spectra and electronic structure of 3-arylmethyl-idenephthalide and 3-arylthiomethylidenephthalide derivatives

The ¹H NMR spectra of trans-3-phenylmethylidenephthalide and trans-3-phenylthiomethylidenephthalide derivatives were investigated. After applying a correction for the anisotropy of substituents and/or for changes of ring current in the substituted aromatic ring, linear correlations were obtained between the chemical shifts of protons of the substituted phenyl group and the methine group and s? constants of substituents. The influence of the bridge heteroatom on the transfer of electronic effects of substituents through the molecules under study is discussed.     

The 1H NMR spectra of zinc complexes of pyridine-2-carbaldehyde 2′-pyridylhydrazone

Proton magnetic resonance spectra at 100 MHz are described for some zinc complexes of the E- and Z-isomers of pyridine-2-carbaldehyde 2′-pyridylhydrazone in d₆-dimethylsulphoxide solution. Chemical shift data are discussed qualitatively in relation to factors such as the charge on the metal ion, the anisotropy of ligand nitrogen atoms, electric field effects caused by the dipole moment of nitrogen lone pairs, metal-nonbonded-hydrogen interactions, ring current effects and the conformational changes undergone by each isomer on coordination.     

The effects of lanthanide shift reagents on the NMR spectra of diaza-and dithiacyclophanes

The effect of a lanthanide shift reagent Eu(dpm)₃ on the spectra of 3 diazacyclophanes (Ia-c) has been studied together with the effect of a fluorinated shift reagent En(fod)₃d₂₇ on the nmr spectra of three dithiacyclophanes (IIa-c). The shift reagents remove certain unexpected accidental degeneracies in the original spectra and permit the assignment of the peaks in the expanded spectra to the individual methylene groups. The comparison of the chemical shifts for the methylene groups above the aromatic rings with those of related compounds provides evidence of an upfield shift deriving from the aromatic ring current. The synthesis of two new dithiacyclophanes (IIb-c) is reported.     

Carbon-13 NMR spectra of monosubstituted pyrazines

Carbon-13 NMR chemical shifts and one-bond carbon–hydrogen coupling constants have been obtained at 15·09 MHz. The trends in the carbon chemical shifts obtained for the pyrazines parallel those of monosubstituted benzenes and 2-substituted pyridines, except for the direct effect of substitution where the pyrazines resemble pyridines not benzenes. The substituent effects on the ¹³C NMR spectra are generally quite similar to those in the ¹H NMR spectra. The ¹³C NMR spectrum of the tautomeric hydroxypyrazine has been compared with the ¹³C NMR spectra of 2-, 3- and 4-hydroxypyridines. Hydroxy compounds that can exist as a cyclic amide show a large meta substituent effect on the chemical carbon shift.     

29Si NMR spectra of trimethylsilylated cyclic acyloins and ketones. 29Si chemical shifts as a measure of ring size

²⁹Si chemical shifts are reported for nine 1,2-bis(trimethylsiloxy)cycloalkenes and four 1-trimethylsiloxycycloalkenes, (Me₃SiO)_xC_nH_2n–2–x (x=1, 2). For cycloalkene derivatives with n?8 the silicon shift exhibits a strong dependence on the ring size, although the silicon is exocyclic and is separated by two bonds from the olefinic carbon atom. The dependence can be exploited for ring size determination of cyclic ketones after trimethylsilylation.     

The NMR spectra and conformations of cyclic compounds: IX—conformational studies of bicyclo(3,1,0)hexane derivatives by 13C NMR

The ¹³C spectra of α-thujene ( 1 ), isothujone ( 2 ), (?)isothujol ( 3 ), (+)neoisothujol ( 4 ), sabinol ( 5 ), dihydroumbellulone ( 6 ) and umbellulone ( 7 ) and the alcohol acetates are recorded and assigned. The C-6 chemical shift may be used in conjunction with the steric shift mechanism as a conformational probe in these molecules. The results obtained indicate that isothujol, neoisothujol, isothujone and dihydroumbellulone adopt boat-like conformations whilst sabinol has a much flatter conformation. Conjugation of the isolated double bond of α-thujene and the carbonyl group of dihydroumbellulone with the cyclopropane ring has virtually no effect on the ¹³C shifts, but those of umbellulone itself are anomalous, indicating conjugation of the α,β unsaturated ketone system with the cyclopropyl ring.     

Synthesis and 13C NMR spectra of cis- and trans-{2-(haloaryl)-2-[(1H-imidazol)-1-yl]methyl]}-1,3-dioxolane-4-methanols

Syntheses and ¹³C nmr spectra of a number of cis and trans 2-(haloaryl)-2-[(1H-imidazol-1-yl)rnethyl]-4-(hydroxymethyl)-1,3-dioxolanes are described. The haloaryl groups are 2,4-dichloro, 2,4-difluoro-, 4-chloro-and 4-bromophenyl. In these series, some of the cis compounds become available through crystalline bromo benzoates 5 . Separations of some trans isomers are achieved through fractional crystallizations of imidazolyl benzoate nitrates 6 . Stereochemical assignments are based primarily on one major ¹³C chemical shift difference, namely that of C-4 of the 1,3-dioxolane ring, the chemical shift of the trans isomers being 1.0-2.5 ppm downfield from that of the cis isomers.     

Amino alcohol based chiral solvating agents: synthesis and applications in the NMR enantiodiscrimination of carboxylic acids

Four optically active amino alcohols were synthesized via the ring opening of (R)-N-(2,3-epoxypropyl)phthalimide with (R)-2-phenyl glycinol, (1R,2S)-cis-1-amino-2-indanol, (R)-2-amino-1-butanol and (S)-phenyl ethylamine in 73-93% yields. The enantioselective recognition of these receptors towards the enantiomers of racemic carboxylic acids was studied by ¹H NMR spectroscopy. The molar ratio and the association constants of the chiral compounds with each of the enantiomers of the guests were determined by using Job plots and a non-linear least-squares fitting method, respectively. Large non-equivalent chemical shifts (up to 30.0 Hz) can be achieved in the presence of chiral amino alcohols 2 and 5. Amongst the chiral receptors used, compound 5 was found to be the best chiral shift reagent, and was effective in the determination of the enantiomeric excess of chiral carboxylic acids.