Carbon-13 NMR studies of substituted naphthalenes. II—rotational barriers and conformations in some naphthaldehydes and azulenaldehydes

Proton-coupled and noise-decoupled ¹³C NMR spectra of 1-azulenecarbaldehyde, 1-acetylazulene and 1,3-azulenedicarbaldehyde have been studied, and complete assignments have been made based on the ¹³C? ¹H coupling constants, additivity of substituent effects (SIS), and previous assignment for the parent hydrocarbon. The barriers to rotation of the aldehyde group in the above azulenecarbaldehydes and in some naphthaldehydes have been determined by ¹³C dynamic NMR, (DNMR), resulting in free energies of activation of 42.7, 26.8 and 34.4 kJ mol^?1 for 1-azulenecarbaldehyde, 1-naphthaldehyde and 2-naphthaldehyde respectively. The same order of barriers is obtained by CNDO/2 calculations. A 4-methoxy substituent in 1-naphthaldehyde and a 6-methoxy substituent in 2-naphthaldehyde increases the rotational barrier by 4.6 and 2.9 kJ mol^?1, respectively, whereas a 3-methoxy substituent in 2-naphthaldehyde reduces the barrier by 6.7 kJ mol^?1. The conformations of the dominant rotamers are deduced from ¹³C chemical shifts to be Z for 1-azulenecarbaldehyde and 1-naphthaldehyde, and E for 2-naphthaldehyde.     

NMR study of orgnasilicon compounds : X. The transmission of substituent effects in phenoxysilanes as studied by 29Si and 13C NMR

²⁹Si and ¹³C NMR chemical shifts for a series of meta and para substituted phenoxytrimethylsilanes are given and compared with those in phenyltrimethylsilanes using the formal single and dual substituent parameter analysis of substituent effects.The silicon chemical shift is found to be about twice as sensitive to substituent effects in phenoxytrimethylsilanes as in phenyltrimethylsilanes. The chemical shift sensitivity to substituent effects, ?, is considered to be a product of two factors, ?^el and ?^shield, which describe the sensitivity of the electron density to substituent effects and the sensitivity of the shielding to the electron density, respectively.Using ¹³C chemical shifts and CNDO/2 net atomic charges, it is shown that the substituent effects propagate within XC₆H₄ fragment of phenoxysilanes no better than in phenylsilanes. The ¹³C chemical shifts of the terminal methyl groups are affected by the substituents in the former series of compounds much less than in the latter. An increase in the relative basicity of oxygen is accompanied by an increase in silicon shielding in phenoxytrimethylsilanes.According to CNDO/2 calculations, the substituents cause larger changes in net atomic charges on the silicon atom if it is bonded directly to the benzen ring rather than via the oxygen bridge. In spite of the fact that the possibility of a dative O·→Si interaction, not reflected by the CNDO/2 calculations, cannot be completely excluded, the results that the increased silicon shift sensitivity to substituent effects in phenoxysilanes is due to higher sensitivity of silicon shielding (?^shield) to electron density in these compounds rather than to a bettr transmission of electronic effects (?^el). The existing theory of silicon shielding must be improved or refined if it has to accomodate the increased sensitivity in the phenoxysilanes.     

13C NMR spectra of thiophenes. III—Bromothiophenes

The ¹³C NMR spectra of all bromo substituted thiophenes have been obtained at 15·085 MHz. ¹³C signal assignments for monobromothiophenes have been confirmed by comparison with the spectra of their partially deuteriated derivatives; a revision is made for the assignment in 3-bromothiophene. The substituent effect of bromine is generally additive for the ¹³C chemical shifts. The substituent effect on various types of ¹³C, ¹H coupling constants is also obtained and discussed.     

13C n.m.r. of organosulphur compounds: II—13C chemical shifts and conformational analysis of methyl substituted thiacyclohexanes

¹³C n.m.r. spectra of methyl substituted thianes and thianium cations have been determined. The magnitude of the ¹³C substituent effects of an equatorial methyl group or of a gem-dimethyl grouping appear to depend in a systematic way on whether the carbon atom concerned is adjacent to, or removed from, the heteroatom. The shieldings are discussed in relation to the conformational properties of the thiane ring. Moreover, the average ¹³C substituent parameters obtained from conformationally biased systems are applied to potentially mobile systems to assess the position of the conformational equilibrium.     

Electrochemical reduction of organic and organometallic compounds. Polarization effect in radical anions

Half-wave potentials of electrochemical reduction and electron affinities of X-B-R_C compounds belonging to 23 reaction series depend not only on the inductive and resonance effects but also on the polarization effect of the X substituent, which was not taken into account previously. In some cases, the contribution of the polarization effect reaches 50% of the overall substituent effect. The main factors responsible for the magnitude of polarization effect in X-B-R _C ^? radical anions are the natures of reaction center R_C and bridging moiety B and the distance between the substituent X and reaction center R_C.     

Phosphol-3-ene 1-oxides and -sulfides. A systematic investigation via carbon-13 nuclear magnetic resonance spectroscopy

¹³C NMR chemical shifts and ¹³C? ³¹P couplings are reported for 18 phosphol-3-ene 1-oxides and 18 corresponding sulfides. The effects of methyl substitution at positions 3 and 4 on the carbon shifts have been systematically explored and substituent parameters derived. One bond couplings from phosphorus to C-2 and C-5 have been related to the sum of the exocyclic substituent group electronegativities (covalent boundary potential values).     

N.m.r. studies in the heterocyclic series: XVII—carbon-13 spectra of indazole derivatives

The ¹³C chemical shifts of 17 indazole derivatives are discussed as a function of substituent effects and the N-1—H or N-2—H structure of indazole. The presence in solution of the N-1—H tautomer is confirmed.     

77Se n.m.r. studies of organoselenium compounds: III—substituent effects in 4,4′-disubstituted diphenyl selenides and 4,4′-disubstituted diphenylmethanes studied by 1H, 13C and 77Se spectroscopy

⁷⁷Se N.m.r. spectra of 4,4′-disubstituted diphenyl selenides have been obtained. The chemical shifts could be correlated with substituent constants of the Hammett type or with Swain and Lupton two-parameter equations. No correlations were observed between these ⁷⁷Se shifts and the ¹³C shifts of the CH₂ group of 4,4′-disubstituted diphenylmethanes, the latter shifts showing no correlation with substituent constants. On the other hand linear correlation was observed between the ⁷⁷Se shifts of the 4,4′-disubstituted diphenyl selenides and the ¹H shifts of the CH₂ group of the diphenylmethanes, the ¹³C shifts of their 1-carbons and the ¹⁹F shifts of 4-substituted fluorobenzenes.     

Multinuclear NMR and vibrational spectroscopy studies of the substituent effects in benzensulphonamide inhibitors of the enzyme carbonic anhydrase

Substituent effects on the electronic structure of sixteen biologically active benzensulphonamide derivatives were investigated by means of ¹⁵N, ¹³C, ¹H NMR, and IR spectroscopy, as well as by quantum chemical calculations. Good correlations were found between the spectroscopic data and both substituent constant and computed electronic charges. On this basis the substituent effects are interpreted in terms of electronic charge perturbation, which is linearly transmitted from the substituent to the biofunctional −SO₂NH₂ group. The resonance nature of the substituent seems most important in determining the ¹⁵N chemical shifts, which follow a “reverse” trend; i.e., electron-donor substituents induce downfield ¹⁵N shifts.     

13C and 1H chemical shifts in 2-benzylidene[3]ferrocenophane-1,3-diones. Elucidation of the substituent dependence of carbonyl chemical shifts in different carbonyl derivatives

The ¹³C and ¹H chemical shifts of the ferrocene moiety, as well as the carbonyl carbons and styrene moiety, of substituted 2-benzylidene[3]ferrocenophane-1,3-diones have been assigned. Correlations of ¹³C substituent chemical shifts of both carbonyl carbons with the Hammett constants have been found, and the effect of the transmission of substituent effects on these chemical shifts through the styrene moiety is discussed. An explanation is given for the different sensitivities of the carbonyl carbon chemical shifts to the electronic effect of substituents in mono- and dicarbonyl derivatives.     

An NMR study of the conformational equilibrium of 5-fluoro-1,3-dioxan in solution. Dependence on solvent

The ¹H and ¹⁹F NMR parameters of 5-fluoro-1,3-dioxan ( 1 ) dissolved in a number of solvent systems are interpreted on the basis of fast inversion between two chair conformations. In cyclohexane solution the two chair conformations are almost equally populated, whereas in more polar solvents, such as chloroform, the conformation having the fluorine substituent in an axial position is strongly preferred. Addition of acetic acid to a solution of 1 in cyclohexane increases the preference of the fluorine substituent for the axial orientation. Possible reasons for these observations are discussed.     

THE SULFENAMIDE CHIRAL UNIT. TORSIONAL BARRIERS IN TRINITROBENZENESULFENAMIDES AND PERHALOBENZENSULFENAMIDES

Abstract

The free energies of activation for degenerate racemization of sulfenamides, measured by DNMR spectroscopy provide information about the N-S formal single bond since stereomutation is accomplished by torsion about this bond. The S-N torsional barrier in benzenesulfenamides (arSNR¹R²) is singnificantly increased by the presence of substituents which are electron withdrawin (by resonance). However, while the barrier is increased by para and^a/single ortho substituent, the introduction of a second ortho substituent lowers the barrier.     

The electron impact induced fragmentation of aromatic aldoximes—III. On the loss of HCNO,OH⋅ and substituents and the role of substituent effects in the decomposition of cyclohexadiene type intermediate ions

The mechanisms for loss of HCNO, OH^˙ and the substituent X^˙ from aromatic aldosimes were elucidated with the aid of deuterium labelling, metastable ion characteristics and substituent effects. It is proposed that the loss of HCNO occurs through a cyclohexadiene type intermediate ion generated via a 6-membered ring hydroxyl hydrogen transer to the ortho position of the phenyl ring. This is followd by a second step which involves the trnsfer of a hydrogen atom from the ortho position to C-1. It is inferred from the corelation with the mesomeric effect (σ_R⁺) of substituents that this step is rate determining. Loss of OH^˙ and X^˙ proceed via the same cyclohexadiene type intermediate ion but, depending upon the substituent, other pathways are also followed.     

Substituent effect of the nitro group on alkanes in carbon-13 n.m.r. spectroscopy

Linear correlation coefficients were obtained for the substituent effect of the nitro group in primary nitroalkanes. The solvent effect of chloroform is also discussed, as well as the influence of the scalar relaxation on the half-width of α-carbons in ¹³C n.m.r. spectroscopy.     

The Derivation of Inductive Substituent Constants from pKa Values of 4-Substituted Quinuclidines. Polar effects. Part I

Thermodynamic pK_a-values have been determined for 38 4-substituted quinuclidinium perchlorates. They are remarkably sensitive to the polar effect of the substituent and cover a range of 3.63 pK_a units. Furthermore, they vary linearly and almost equally with temperature since the contribution of the TΔS° term to the free energy of ionization is relatively small and constant. The magnitude of the polar effect of the 4-cyano group varies with the solvent and appears to depend on its ability to form hydrogen bonds to the substituent rather than its dielectric constant. New inductive substituent constants σ_I^q are derived from the pK_a values. Their correlation with known inductive constants is only fair or unsatisfactory, especially as regards the relative order of hydrogen and the alkyl groups. The discrepancies can be ascribed mainly to the different models used to derive the substituent constants.     

13C-NMR.-Substituenteninkremente der N3-Gruppe in Azido-1,4-benzochinonen

¹³C-NMR. Substituent Increments of the Azido-group in 1,4-Benzoquinones The empirical substituent increments of the azido-group in 1,4-benzoquinones have been derived from the ¹³C-NMR. chemical shifts of 2-azido-5-chloro-1,4-benzoquinone. The validity of the obtained values was tested by comparison of the empirical and computed ¹³C-NMR. chemical shifts of other azido-chloro-1,4-benzoquinones.     

Polar effects. Part 12. Inductivity and carbon participation in the solvolysis of 4-substituted 2-adamantyl arenesulfonates

The rate constants (log k) for the solvolysis of 4^e-substituted 2^e- and 2^a-adamantyl p-nitrobenzenesulfonates 14 and 15 , respectively, in 80% EtOH correlate linearly with the respective inductive substituent constants σ. Therefore, relative rates are controlled by the I effect of the substituents at C(4). The derived reaction constants, or inductivities, ρ_I of −0.80 and −0.64 for the series 14 and 15 , respectively, are far smaller than those previously determined for 6-substituted 2-norbornyl and 2-bicyclo[2.2.2]octyl sulfonates, in which the partial structure containing the substituent and the leaving group is the same. The ratio of the retained and inverted adamantanols obtained upon hydrolysis of the series 14 falls from 2.85 for R = CH₃ to ca. 1 for R = CN, i.e. as the substituent at C(4) becomes more electron-attracting. In the 2^a-series 15 this ratio is uniformly higher. These findings confirm that the 2-adamantyl cation is weakly bridged and that through-space induction in carbocations involves graded bridging of the cationic center by neighboring C-atoms.     

Direct and indirect substituent effects on 1J(CH) in vinyl derivatives and related compounds

The variation in the one–bond couplings ¹J(CH) in vinyl derivatives with substituent has been examined. For the geminal proton ¹J correlates very badly with substituent electronegativity but extremely well with σ_I, if conjugating substituents are excluded. In the case of halogen substituents the marked stereospecificity of ¹J(CH) for the cis and trans protons can be rationalised in terms of an intrinsic dependence of π_CH on the dihedral angle between the coupling atoms and the perturbing substituent, with an additional positive increment to the cis coupling due to direct interaction of the substituent non-bonding electrons or to orbital circulation of the substituent electrons. The intrinsic specificity of β-substituent effects on ¹J(CH) is also found in analogous compounds containing C?N and C?O bonds.     

Porphyrins. 29. Mass spectra of functionally substituted tetraarylporphyrin derivatives

The mass spectral behavior of functionally substituted meso(p-adamantylphenyl)triphenylporphyrins and their p-methoxy-substituted analogs was studied. Breakdown of the M⁺ ions of these compounds proceeds chiefly-with a loss of the functional substituent in the adamantane ring with the formation of ions, which make it possible to determine the nature of this substituent.For Communication 28, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1642–1644, December, 1993.     

Azoles and azines. 62. Structure of 2-aryl-1,3-oxazine-4,6-diones

¹H,¹³C NMR, IR, and UV spectra have been studied for solutions of a number of potentially tautomeric 2-aryl-1,3-oxazine-4,6-diones and their 5-methyl substituted analogs with variation of substituent at the para position of the benzene ring, as well as compounds with a fixed structure that simulates possible tautomeric forms. The data have been compared with the results of quantum chemical calculations carried out in SSO MO LCAO approximation by the CNO, CNDO/2, and MPNDO/3 methods. In DMSO and THF solution the test compounds exist predominantly as 2-aryl-4-hydroxy-6H-1,3-oxazin-6-ones. The para substituent in the benzene ring does not affect the composition of the tautomer mixture significantly.For Communication 61, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 386–394, March, 1987.