Nitrogen-carbon bonds, barriers rotation

H-NMR studies of oligocarbene Ru(II) complexes indicate a substantial barrier to rotation about the metal-carbene carbon and nitrogen-R bonds. This restricted rotation is thought to arise as a consequence of intramolecular non-bonding cis interactions of the carbene nitrogen-R substituents, and not because of any significant double bond character in ruthenium-carbene carbon (76). 

In the case of the carbon-nitrogen double bond, the reaction of isomerization can occur via rotation about the double bond, and by nitrogen inversion (XLVII). The latter process as a rule is strongly favored over rotation, resulting in a lowered barrier of overall isomerization as compared to ethylenes. For many imines (XLVII, c = H), this barrier is in the range 20-30 kcal/mol. Electronegative substituents on the nitrogen atom increase stability toward inversion, as evidenced by the relative stability of oximes (XLVII, c = OH) and hydrazones (XLVII, c = NRR ) [56]. 

These two polymers are characterized by a stable trans-configuration of the amide bond and by a high barrier of the rotation around the aryl carbon bond. The directions of rotational axes in all the chain units almost coincide. As a result, according to Tsvetkov the entire macromolecule acquires the shape of a crankshaft (Fig. 1). In the case of PBA, the retardation of rotation of the units may be also caused by a transfer of conjugation in the carbonyl-nitrogen system via a phenylene ring. 

Group IV.—Carbon. The effect of nearby inorganic moieties on kinetic characteristics of hindered rotation about a carbon to 5/>-element bond is illustrated by variable temperature n.m.r. studies of hindered rotation about carbon-oxygen in (36) (37), and about carbon-nitrogen in W-dimethylf Hglacetamide in various environments. For the latter compound the barrier is 210 kcalmol" in aqueous solution, but only 19 0 kcalmol when the compound is complexed by silver(i). Barriers to rotation about the phenanthroline-carbon to methyl-carbon bonds in methyl-substituted 1,10-phenanthroline complexes of chromium(iii) can be estimated from n.m.r. spectra. The interest here is that these barriers are solvent sensitive their variation with solvent may prove a useful probe in examining solvation and its effect on reactivity of this type of complex. ... 

The rotational barrier around the nitrogen-carbonyl carbon bond of W,W-dimethylformamide is approximately 87 kj mole". ... 

This IS an unusually high rotational energy barrier for a single bond and indicates that the carbon-nitrogen bond has significant double bond character as the reso nance picture suggests... 

Rotation about single bonds and conformational changes can be studied. Amides constitute a classic example. Because of the partial double bond character of the carbon-nitrogen bond as a consequence of the contribution of 2 to the electronic structure, there is an energy barrier to rotation about this bond. 

SCHEME 1. Conformational map of the RcRn and RcSn diastereomers of /V-ethyl-iV-methyl-2-aminobutane (EMAB). Interconversions among conformers within dashed boxes are fast on the NMR time scale at 104 K. Those between dashed boxes occur via rotations about the methine carbon-nitrogen bond with barriers which are DNMR-visible. The interconversion between the solid boxes occurs via nitrogen inversion (disstereomeric interconversion). The values in parentheses are MM2-80 results. Reprinted with permission from Reference 71. Copyright (1988) American Chemical Society... 

The barrier to rotation about the carbon-nitrogen bond in amides is 75 to 85 kJ/mol (18-20 kcal/mol). 

Contribution from resonance structure 3, which contains a formal double bond between carbon and nitrogen, is considered to be primarily responsible for the coplanarity and the high rotational barrier about the amide bond [58], The introduction of resonance structure 3 also implies that there is significant charge-delocalization from the nitrogen lone pair to the carbonyl oxygen. 

The barriers for methyl groups bonded to N-sp2 atoms are smaller than the respective barriers for methyl bonded to C-sp2 atoms in the corresponding carbon systems, suggesting facile bending of the N-sp2-CH3 groups in the rotational transition state. The authors also compare the steric size of the lone pair of the nitrogen in 84c and 84i and, curiously, different values are obtained in terms of van der Waals radii. 

This resonance representation correctly predicts a planar amide nitrogen atom that is sp2 hybridized to allow pi bonding with the carbonyl carbon atom. For example, formamide has a planar structure like an alkene. The C—N bond has partial double-bond character, with a rotational barrier of 75 kJ/mol (18 kcal/mol). 

Molecular orbital calculations of rotation barriers around the carbon-nitrogen bond in thioamides, amidinium salts, amidines and enamines have been described by Sandstrom61. 

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