Influence of substituents on carbonyl carbon chemical shifts in 4-substituted bornane-2,3-diones and the preparation of bornane-2,3-dione

¹³C Chemical shifts of the 4-substituted boniane-2,3-dione(1–6) have been assigned. The shielding of the CO carbons brought about by electron withdrawing substituents is attributed to a field effect of the substituent which serves to increase the CO bond order. For the substituent bearing carbons C(4), enhanced shielding is noted and these carbons exhibit small substituent chemical shifts. A preparative method leading to borane-2,3-dione is described and briefly discussed.     

Preparation and characterization of symmetrical bis[4-chloro-2-pyrimidyl] dichalcogenide (S, Se, Te) and unsymmetrical 4-chloro-2-(arylchalcogenyl) pyrimidine: X-ray crystal structure of 4-chloro-2-(phenylselanyl) pyrimidine and 2-(p-tolylselanyl)-4-chloropyrimidine

Synthesis of a novel class of multinucleate pyrimidine chalcogen (S/Se/Te) derivatives has been successfully attempted for the first time by the selective substitution of chlorine at the C-2 position of 2,4-dichloropyrimidine with nucleophilic dichalcogenide anion E₂²⁻ (E = S, Se, Te) to afford bis[4-chloro-2-pyrimidyl] dichalcogenide. The highly electrophilic nature of 2,4-dichloropyrimidine compared to aryl chlorides has been further exploited to prepare a variety of 4-chloro-2-(arylchalcogenyl) pyrimidine compounds by substituting the chlorine exclusively at the C-2 position of 2,4-dichloropyrimidine with a variety of chalcogen bearing aryl anions ArE⁻ (Ar = phenyl, 1-naphthyl, p-tolyl, 4,6-dimethyl-2-pyrimidyl, 2-pyridyl, 4-methyl-2-pyridyl). All the newly prepared symmetrical and unsymmetrical pyrimidyl chalcogen compounds have been thoroughly characterized with the help of various spectroscopic techniques viz., NMR (¹H, ¹³C, ⁷⁷Se), FT-IR and mass spectrometry (in representative cases). The crystal structures of 4-chloro-2-(phenylselanyl) pyrimidine and 2-(p-tolylselanyl)-4-chloropyrimidine have been determined by X-ray crystallography.     

Multinuclear NMR studies on CF3 substituted sulfur,selenium and tellurium compounds

A systematic investigation of thirty-four CF₃Se(II, IV) and eight CF₃Te(II, IV) compounds by ¹³C, ¹⁹F, ⁷⁷Se and ¹²⁵Te NMR spectroscopy resulted in some general features for chemical shifts and coupling constants which agree with the trends of reported ¹⁹F and new ¹³C NMR data of CF₃S(II, IV) compounds. Moreover, the NMR spectra of molecules of the type E=CXY (E = chalcogen, X, Y = halogen) and substances containing a CSe double bond have been studied. From the comparison of these NMR data with those of CF₃ substituted chalcogen compounds, a partial double bond character of the carbon-fluorine and carbon-chalcogen bond in CF₃ substituted chalcogen compounds can be derived:

     

Carbon-13 nuclear magnetic resonance spectra: III—2-heteroadamantanes

The ¹³C n.m.r. spectra of some 2-heteroadamantanes and 1-substituted 2-heteroadamantanes are reported. The influences of the heteroatoms in the adamantane framework, and those of the substituents attached to it, on the ¹³C chemical shifts of the adamantane carbons are investigated and compared with related compounds such as the corresponding heterocyclohexane derivatives and 2-mono- and 2,2-disubstituted adamantanes. The nonadditivity of the substituent effects for 1-substituted 2-heteroadamantanes, especially for the geminally substituted carbons, is substantially confirmed. In addition, the influences of a missing CH₂ group and of NCH₃ carbons upon the ¹³C chemical shifts of the carbons in the adamantane system are described.     

The additivity of NMR carbon–carbon spin–spin coupling constants

Fluorene-9-¹³C, fluorenone-9-¹³C, acenaphthenone-11-¹³C, acenaphthenone-12-¹³C, 1-methylcyclopentanol-1-¹³C and 1-methylcyclopentene-1-¹³C were synthesized to obtain J(CC) values between the natural carbons and the labeled carbons. Each of these compounds possessed at least one asymmetric dual-path coupling, i.e., coupling between the labeled carbon and another carbon via simultaneous two- and three-bonded coupling paths. Model ¹³C-labeled compounds were synthesized where necessary to give expected values of the constituent mono-path couplings. Values of these dual-path couplings (²⁺³)J suggested that the observed value is the (at least approximate) algebraic sum of the two constituent J values.     

13C NMR study of ortho-, meta- and para- substituted phenyldiphenylamines: Substituent effect correlations

The ¹³C chemical shifts of 11 substituted triphenylamines have been determined and the assignment of these resonances made using intensities, ¹H and ¹⁹F couplings and predictions from bond additivity relationships. ¹³C chemical shifts at carbons bearing the substituent and at carbons ortho to the substituent correlated reasonably well with the Q parameter. A multiple regression analysis of chemical shifts with the field and resonance parameters of Swain and Lupton and the Q parameter produced significantly better correlations than those obtained when Q was omitted for these positions. ¹³C chemical shift correlations for carbons meta and para to the substituent were not significantly better than when Q was omitted. Significant correlations were obtained between field and resonance parameters and ¹³C chemical shifts of C-o and C-p, and C-i, C-o, C-m and C-p of the non-substituent bearing phenyl rings in ortho- and para-substituted phenyldiphenylamines, respectively.     

Investigation of substituent effects of chalcones by 13C n.m.r. spectroscopy

¹³C chemical shifts for 23 para- and meta-substituted chalcones of the types 1 and 2 have been determined. The aromatic shieldings are compared with previous results for other aromatic derivatives. Correlations of the ¹³C chemical shifts of vinyl carbons and carbonyl carbons as well as ring carbons with Hammett σ parameters, π electron densities and the reactivity parameters of Swain and Lupton provide a consistent picture of electronic effects transmitted through the carbon framework of the compounds studied.     

CRAPT: an improved version of APT with compensation for variations in JCH

A modified version of the attached proton test (APT) sequence for ¹³C spectral editing, which we call CRisis‐APT (CRAPT), is developed and tested on representative organic compounds. CRAPT incorporates ¹³C compensation for refocusing inefficiency with synchronized inversion sweeps (CRISIS) pulses in combination with ¹H broadband inversion pulses to give improved compensation for variations in ¹J_CH along with improved refocusing efficiency. It is shown that CRAPT gives edited ¹³C spectra with only small losses in sensitivity (between 8% and 15% for strychnine, 1 , menthol, 2 , cholecalciferol, 3 , and isotachysterol, 4 ), compared with basic ¹³C spectra obtained on the same compounds. CRAPT also gives significantly better signal/noise than DEPTQ for nonprotonated carbons. Therefore, we conclude that CRAPT is an improvement over APT or DEPTQ or a combination of DEPT135 with a full ¹³C spectrum for routine ¹³C spectral editing of organic compounds. Copyright © 2014 John Wiley & Sons, Ltd.     

13C NMR spectroscopic studies of the behaviors of carbonyl compounds in various solutions

¹³C NMR spectroscopic studies were performed for carbonyl compounds having a hydroxyl group, a carboalkoxy group, an acetoxy group, or a carboxyl group in various solvents with different polarities for observation of their behaviors of ¹³C NMR chemical shifts of carbonyl carbons in solutions. It was found that the chemical shifts of the carbonyl carbons in ¹³C NMR have good correlation with the empirical parameter for solvent polarities, E_T^N, depending on the structures. Inter- or intramolecular hydrogen bonding and dipolar-dipolar interactions appear to play a key role in this observation.     

Insect pheromone components: Use of 13C NMR spectroscopy for assigning the configuration of CC double bonds of monoenic or dienic pheromone components and for quantitative determination of Z/E mixtures

Several insect sex pheromone components and structural analogues have been characterized by ¹³C NMR spectroscopy. The evaluation of the difference between the chemical shift values of the allytic carbons present in the diunsaturated pheromone components and those of the carbons having the same position on the n-alkane chain of the corresponding saturaated compounds was used to assign the configuration of the CC double bonds present into such dienic compounds. The technique avoids the need to use the nuclear Overhausser effect; it allows quantitative evaluation, with good accuracy, of the stereoisomeric composition of mixtures of synthetic monoenic or dienic pheromone components such as (Z)- and (E)-9-tetraceden-1-yl acetate and (Z)- and (E)-9,11-dodecadien-1-yl acetate.     

Determination of relative signs of 13C?13C coupling constants in doubly labelled compounds

Analysis of the noise decoupled ¹³C spectra of doubly ¹³C labelled compounds where the two labelled carbons are identical, makes the determination of reltive signs of ¹³C? ¹³C coupling constants possible in a very simple way. The involved carbon form AA'X or AA'B spin systems.     

Identification of halogen substituents in natural products by measurement of 13c spin-lattice relaxation times and integrated intensities

A method is proposed for differentiating brominated carbons from chlorinated carbons by means of natural-abundance ¹³C NMR spectroscopy. The basis of the method is that the spin-lattice relaxation behaviour of brominated carbons is influenced by carbon-bromine scalar interactions, which can lead to shortened ¹³C spin-lattice relaxation times and reduced values of the nuclear Overhauser enhancement. C-Cl scalar interactions make a negligible contribution to the spin-lattice relaxation of chlorinated carbons. These effects are illustrated by measurement of the ¹³C spin-lattice relaxation times and integrated intensities of chloro-, bromo and iodobenzene and chloro-, bromo- and iodocyclohexane. The method is then tested on four polyhalogenated marine natural products. The results indicate that ¹³C relaxation measurements can be used to distinguish brominated carbons from chlorinated carbons in the case of halogenated quaternary carbons, sp² hydridized methine carbons and some sp³ hydridized methine carbons, but not in the case of halogenated methylene carbons or gem-dihalo substituted methine carbons.     

A 13C NMR study of actinomycin D and related model peptides

¹³C n.m.r. pulsed Fourier transform spectra were measured and interpreted for actinomycin D and for two related peptide derivatives: BOC-Pro-Sar-OCH₃ and BOC-Sar-Meval-OCH₃. Actinomycin D, specifically enriched with ¹³C in the chromophoric C-methyl and in the peptide N-methyl carbons, was produced biosynthetically. Enrichment of specific N-methyl carbons in the model peptides was effected synthetically. Spectral assignments relied on the use of the enriched samples and particularly on off-resonance and selective low power ¹H decoupling experiments. Poor correlation was observed between some of the ¹³C chemical shifts in the model compounds and those of the analogous carbons in actinomycin D. Ten of our ¹³C assignments for actinomycin D differ from those published by Hollstein, Breitmaier and Jung.     

Dynamische NMR-Untersuchungen der behinderten Rotation am N?C(X)-Bindungsfragment. XVII. Einfluß des Zentralmetallatoms auf die Rotationsbarriere der behinderten C,N-Rotation und die 13C-NMR-Spektren von Chelatkomplexen des 1,1-Diethyl-3-benzoyl-thio(seleno)-harnstoffs

Dynamic N.M.R. Studies of Hindered Rotation on N? C(X) Bond Increment. XVI. Central Metal Atom Influence on Rotational Barriers of the Restricted C, N-Rotation and the ¹³C N.M.R. Spectra of the Metal Complexes of 1,1-Diethyl-3-benzoyl-thio(seleno) Urea The rotational barriers about the partial C, N-double bond Et₂N? C(X) in the metal complexes of 1,1-diethyl-3-benzoyl-thio(seleno) urea are submitted, corrected for the compounds of low Δν value. The barriers are dependent on the electric polarizibility of the metal ion and the chalcogen atom X (S, Se). The results are discussed. In the ¹³C n.m.r. spectra of the investigated compounds also the most electron attractive metal ions influence characteristic variations of ¹³C chemical shifts.     

13C n.m.r. chemical shifts of carbodiimides

The ¹³C n.m.r. chemical shifts of the sp-hybridized carbons in dialkylcarbodiimides have values of δc ? 140. These shifts are compared with those of similarly hybridized carbons occuring in other classes of compounds.     

Pulsed fourier transform NMR of substituted aryltrimethyltin derivatives : IV. Proton and carbon-13 NMR data for ortho-, 2,6-, and poly-substituted aryltrimethyltins

Proton and carbon-13 NMR data recorded in the Fourier transform mode are reported for ten ortho-substituted, six 2,6-disubstituted, and six miscellaneous polysubstituted aryltrimethyltin compounds. Although ¦¹J(¹³C¹H)¦ and ¦²J(¹¹⁹SnC¹H)¦ coupling constants are rather insensitive to substituent variation, tin methyl proton chemical shifts reflect the increasing inductive effects as methyl-, chloro-, fluoro-, and trifluoromethyl-groups are brought into juxtaposition with the trimethyltin moiety. Resonances in the natural-abundance carbon-13 NMR spectra for the tin derivatives are assigned on the basis of additivity relationships, proton undecoupled spectra, and relative magnitudes of ¦J(¹¹⁹Sn¹³C)¦ and ¦J(¹³C¹⁹F)¦ coupling constants. Mutually deshielding γ-, δ-, and ?-effects in the carbon-13 chemical shifts of substituent carbons are rationalized in terms of steric crowding between the trimethyltin group and neighboring substituents. Deshieldings in ring carbons formally para- to conjugating substituents are discussed in terms of the steric inhibition of resonance model. Previous conclusions concerning lack of significant higher coordination at tin in aryltin derivatives bearing substituents with lone pair electrons are corroborated in this work.     

Pairwise effects of chlorine substituents on the 13C NMR chemical shifts of dichlorobicyclo[2.2.1]heptanes (norbornanes)

The ¹³C NMR spectra of nine dichlorinated bicyclo[2.2.1]heptanes (norbornanes) have been measured and assigned. The pairwise effects of chlorine substituents which cause deviations from the additivity of single-substituent effects were investigated and are discussed. The largest effect found is the high-field shift of carbons bearing vicinal cis substituents. In the case of geminal substitution deviations from additivity were found to be to low field and large in the γ, smaller in the β and negligible in the α chemical shifts. The observed deviations for 1,3-disubstituted cases vary from ?3.2 to +1.1 ppm at different carbons, allowing no simple explanation. Replacement of α-hydrogen in a diaxial 1,3-arrangement by CH₃, OH or CI causes the single substituent effect, namely the γ_a effect, to change considerably.     

On the HALA effect in the NMR carbon shielding constants of the compounds containing heavy p‐elements

The heavy atom (HA) effect on the NMR isotropic carbon shielding constants is computationally investigated in the series of model ethanes, ethylenes, and acetylenes, C_βH₃? C_αH₂? XH_n, C_βH₂? C_αH? XH_n, C_βH?C_α? XH_n (n = 0, 1, 2, or 3 depending on X), where X covers p‐elements in the 13–17 groups of the 3–6 periods in as many as 60 compounds. Compounds under study provide diverse bonding situations for the α‐ and β‐carbons, which are characterized by the consecutive increase of the s‐character of the C_β? C_α and C_α? X bonds, being one of the factors influencing spin‐orbit part of the HA on light atom effect (SO‐HALA). The “chalcogen dependence,” “pnictogen dependence,” “tetrel dependence,” and “triel dependence” are established for the 16th, 15th, 14th, and 13th groups, respectively. A well‐known “normal halogen dependence” for the ¹³C NMR chemical shifts, established much earlier for the compounds containing 17th group elements, also revealed itself in all three series under investigation. The dependence of the spin‐orbit effects size depending on the number of the lone electron pairs (LEPs) on HA X has also been investigated. The comparison of theoretical ¹³C NMR chemical shifts with experiment is performed for three representative tellurides. The HALA effect in this series has been shown to be strongly dependent on the number of tellurium LEPs.     

13C-NMR-Spektren isomerer Diazaphenanthrene

The ¹³C chemical shifts of eleven isomeric diazaphenathrenes (1.5-? 1.10-, 2.7-, 4.5-? 4.7-, and 5.6-DAP) have been determined and iteratively assigned by means of comparison with suitable model compounds. The data obtained (132 points) were used to test the relationship between ¹³C chemical shifts and HMO charge densities. The best correlation with a standard deviation S(E) = 4.8 ppm was found for the chemical shifts, relative to phenanthrene, of tertiary carbons. The different slopes for correlations of tertiary and quarternary carbons (275–300 vs 540–550 ppm/electron) are most probably due to different ΔE values for both types of carbons.     

Unequivocal 13C NMR assignment of cyclohexadienyl rings in bromotyrosine‐derived metabolites from marine sponges

Bromotyrosine‐derived compounds are commonly isolated from Verongida sponges and are a major class of marine natural products. Here we report on the unequivocal ¹³C NMR assignment of the brominated carbons at positions C‐2 and C‐4 of the cyclohexadiene ring, two carbons whose resonances are often incorrectly assigned. Interpretation of HMBC data acquired for a series of known bromotyrosine analogues, which included ianthesine E (1), aerothionin (2), 11‐hydroxyaerothionin (3), and 11,19‐dideoxyfistularin‐3 (4), allowed us to unequivocally assign the carbons in question, C‐2 and C‐4, through the observance of unique HMBC correlations from the C‐1 hydroxyl proton. Here we present the complete 2D NMR data sets recorded in DMSO‐d₆ for 2–4 that were used to confirm the assignment and establish the working model. Using this model, a survey of the literature revealed that many members of this structure class had been wrongly assigned. This paper serves to reassign those compounds whose ¹³C NMR assignment at positions C‐2 and C‐4 of the cyclohexadiene ring should be reversed. Copyright © 2012 John Wiley & Sons, Ltd.