Gerasimos J. Karabatsos

Structural studies by nuclear magnetic resonance—XV : Conformations and configurations of oximes

Abstract Conformations and configurations were assigned to oximes of several aldehydes and ketones from analysis of their 60-Mc. NMR spectra. Interpretation of the spin-spin coupling data from the cis isomers (hydroxy cis to hydrogen) of the aldoximes in terms of rotamers I and II, whereby a single bond eclipses the double bond, led to the following: For-α-monosubstituted acetaldoximes, when R was changed from methyl to t-butyl, ΔH° for I ⇄ II varied from + 500 to +4,500 cal/mol. The α,α-disubstituted acetaldoximes behaved similarly. Interpretation of the analogous data from the trans isomers led to the conclusion that, whereas the eclipsing conformation XII is the most stable conformation of the α,ga-disubstituted acetaldoximes, both eclipsing XIII and bisecting XIV conformations, or other conformations in-between the two, are important. From the chemical shift data, the s-cis conformation about the NO bond is excluded as the equilibrium conformation of the oximes.

Structural studies by nuclear magnetic resonance—XI: Conformations and configurations of oxime o-methyl ethers

Conformations and configurations were assigned to several aldehyde and ketone oxime O methyl ethers from analysis of their 60-Mc. NMR spectra. Interpretation of the data from the cis-isomers (methoxy cis to hydrogen) of the aldehyde derivatives in terms of rotamers I and II, whereby a single bond eclipses the double bond, led to the following conclusions: For α-mono-substituted derivatives, when R was varied from methyl to t-butyl, ΔH° for I → II varied from +390 to +4,5000 cal/mole. A similar range was obtained for α,α,-disubstituted derivatives. Interpretation of the data from the trans-isomers led to the conclusion that, whereas I is the only significant rotamer for α,α-disubstituted derivatives, for α-mono-substituted I and III are equally important.

Structural studies by nuclear magnetic resonance—XIX : N,N-dimethylhydrazones and general comments on configurational and conformational isomerism☆

Configurations and conformations were assigned to several aldehyde and ketone N,N-dimethyldrazones from analyses of their 60-Mc NMR spectra. Aldehyde, but not ketone, N,N-dimethylhydrazones exist exclusively in the syn configuration. Interpretation of the vicinal spin-spin coupling constants, JH1Hα, led to the conclusion that II and III are the minimum energy conformations of these compounds. The ΔH0 values for II ⇋ III were found to be more positive than those of N-Me imines (Z = Me) but less positive than those of oximes (Z = OH) and their O-methyl ethers (Z = Me). General comments are made on several aspects of configurational and conformational isomerism about single, double and partial double bonds, and on the usefulness and limitations of NMR in studying various problems associated with such isomerism.

Structural studies by nuclear magnetic resonance—XVIII : Conformational and configurational isomerism of N-alkyl imines☆

Abstract Conformations and configurations were assigned to several N-Me imines and N-alkyl acetaldimines from analyses of their 60-Mc NMR spectra. All aldimines exist exclusively in the syn configuration. Interpretation of the vicinal and long-range (across 4 and 5 and bonds) spin-spin coupling constants of N-Me aldimines led to the conclusion that I and II are the minimum energy conformations of these compounds. The ΔH° values for I ⇌ II fall between those of aldehydes, which are usually negative, and those of oximes and their O-methyl ethers, which are positive. Interpretation of the long-range (across 4 and 5 bonds) spin-spin coupling constants of N-alkyl acetaldimines led to the conclusion that III is far more stable than IV. The greater thermodynamic stabilities of the syn over the anti isomers of aldimines and III over IV have been explained in terms of nonbonded interactions across the CN double bond. The results of some of these calculations are given.

Structural studies by nuclear magnetic resonance—XVI : Conformations and configurations of hydrazones

Abstract Conformations and configurations were assigned to several hydrazones of aldehydes and ketones from analysis of their 60-Mc NMR spectra. Azines and hydrazone dimers were found to be frequent contaminants of the hydrazones, especially after the hydrazones, or their solutions, were let stand at room temperature. From the vicinal spin-spin coupling constants, J H 1 H α , the following conclusions were reached regarding the relative stabilities of rotamers I and II: (a) ΔG° 40 for V a ⇄ VI was +400, +500 and +700 cal/mole for R equal to methyl, isopropyl and phenyl, respectively. (b) ΔG° 40 for VII ⇄ VIIIa was +200, +600 and +700 cal/mol for R equal to methyl, ethyl and isopropyl, respectively. Interpretation of the spin-spin coupling constants of the trans isomers (amino group trans to H 1 ) led to the conclusion that, whereas IX is the only significant rotamer of α,α-disubstituted acetaldehyde hydrazones, both X and XI—with a low barrier of interconversion between them—are significant rotamers of the α-monosubstituted compounds.

Structural studies by nuclear magnetic resonance—XII: Conformations and configurations of n-methylphenylhydrazones

Abstract Conformations and configurations were assigned to several aliphatic N-methylhydrazones from analysis of their 60-Mc NMR spectra. Interpretation of the vicinal spin-spin coupling constants, J H 1 Hα , of the syn isomers (N-methylamino cis to hydrogen) of the aldehyde N-methylhydrazones in terms of rotamers I and II gave ΔH° values that are similar to those found in hydrazones, oximes, and oxime O-methyl ethers. The anisotropic effects of the CN double bond, the association between solute and solvent, several aspects of the chemical shifts, and other problems related to configurational isomerism about the CN double bond are discussed.Abstract Conformations and configurations were assigned to several aliphatic N-methylhydrazones from analysis of their 60-Mc NMR spectra. Interpretation of the vicinal spin-spin coupling constants, J H 1 Hα , of the syn isomers (N-methylamino cis to hydrogen) of the aldehyde N-methylhydrazones in terms of rotamers I and II gave ΔH° values that are similar to those found in hydrazones, oximes, and oxime O-methyl ethers. The anisotropic effects of the CN double bond, the association between solute and solvent, several aspects of the chemical shifts, and other problems related to configurational isomerism about the CN double bond are discussed.

Asymmetric induction—V : Effect of R on asymmetric induction in additions to abcC-COR compounds

Abstract The diastereomeric product ratios from reactions involving the addition of various reagents to the carbonyl of φCH 3 HC-COR compounds varies over a wide range. This variation is interpreted as due partly to changes in the structure of the diastereomeric transition states with changes in the size of R. For example, when R is smaller than the φCH 3 HC group the diastereomeric product ratios are close to those predicted on the assumption that 1 and 2 are the minimum energy diastereomeric transition states; and when R is larger than the φCH 3 HC group, the diastereomeric product ratios are large and closer to those that one might expect if 5 and 6 were the minimum energy transition states.

Structures of the transition states of some intermolecular hydride transfer reactions

The large kH/kD isotope effects, 7–9, obtained in the hydride transfer reactions of triarylmethanes with triarylmethyl cations support linear rather than nonlinear transition states.

Structural studies by nuclear magnetic resonance—XXIII : Conformational analysis of substituted phenylacetaldehydes

Abstract The vicinal spin-spin coupling constants between the aldehydic and α-protons of phenylacetaldehyde, p-methylphenylacetaldehyde, p-methoxyphenylacetaldehyde, p-chlorophenylacetaldehyde, and 2,6-dichlorophenylacetaldehyde were studied at 60 MHz as a function of temperature and solvent. The following conclusions were drawn. (1) A threefold barrier to rotation about the sp2-sp3 carbon-carbon bond best fits the data for the substituted phenylacetaldehydes. (2) The stability of the rotamer, where the phenyl group eclipses the carbonyl group, increases with increasing solvent dielectric constant. (3) The free energy and enthalpy values for 1⇌2 are solvent dependent, being more negative in solvents of high dielectric constant. (4) Local dipole-dipole interactions are more important than overall dipole-dipole interactions in determining rotamer stability.