Amide restricted rotation

A nitrogen atom at X results in a variable downfield shift of the a carbons, depending in its extent on what else is attached to the nitrogen. In piperidine (45 X = NH) the a carbon signal is shifted by about 20 p.p.m., to ca. S 47.7, while in A-methylpiperidine (45 X = Me) it appears at S 56.7. Quaternization at nitrogen produces further effects similar to replacement of NH by A-alkyl, but simple protonation has only a small effect. A-Acylpiperidines show two distinct a carbon atoms, because of restricted rotation about the amide bond. The chemical shift separation is about 6 p.p.m., and the mean shift is close to that of the unsubstituted amine (45 X=NH). The nitroso compound (45 X = N—NO) is similar, but the shift separation of the two a carbons is somewhat greater (ca. 12 p.p.m.). The (3 and y carbon atoms of piperidines. A- acylpiperidines and piperidinium salts are all upfield of the cyclohexane resonance, by 0-7 p.p.m. 

C—N bond and restricts rotation around it. The amide bond is therefore planar, and the N—H is oriented 180° to the C=0. 

The extent of restricted rotation about the amide band of (38) was used to compare the electron-withdrawing process of phosphonium salts (38, Y = alkyl) and chalcogenides (38, Y = O or S) with the more conventional electron-withdrawing groups. These phosphorus groups were found to exert a — A7 effect comparable with that of a nitro-group. 

In the case of the molecule in Structure 6.14, only the protons of the piperidone ring would be affected by restricted rotation about the amide bond. As far as the aromatic protons are concerned, there is no anisotropic difference in the environment they experience, because the piperidone has a plane of symmetry through it. 

Restricted rotation about the C—N bond of amides was studied by Gutowsky and Holm in the earliest days of NMR spectroscopy (31). A number of papers have been published since then. However, due to various difficulties, the barriers... 

Table 4.18 Constants for Extraction of Actinide Nitrate Adducts with Phosphoric Acid Trialkylesters (Free Rotation of the P Substituents) and with A,A-Dialkylamides (Restricted Rotation Around the Amide Moiety)...

Exchange broadening is frequently observed in amides due to restricted rotation about the N-C bond of the amide group. 

The restricted rotation about amide bonds often occurs at a rate that gives rise to observable broadening in NMR spectra. 

The restricted rotation in amide bonds results from the partial double bond character of the C-N bond. 

These substrates show restricted rotation about the amide bond With no ortho-substituent present, the svn-form predominates with adjacent phenyl rings. Ortho-substitution increases the proportion at equlibrium of the rotamer with anft -phenyl rings. Reduction of the anti-mUmm can lead only to replacement of halogen by hydrogen. For the chlorobcnzanilides, the formation of a product with the chlorine... 

A 3360 cm N-H amide stretch Two bands indicating restricted rotation about the N-CO bond resulting in stereoisomers... 

The two values represent two rotamers that are present in a ratio of 2.2-2.8 to 1. Compound is not symmetrical due to restricted rotation around the amide bond. 

Furancarboxamides and furanthiocarboxamides somewhat resemble the oximes in that there are two sources of isomerism. The rotational isomerism about the C(2)—C(O) bond has not been observed, however — only restricted rotation in the amide link. This is easier than in the benzamides since the furan ring is electron-releasing thus reducing the N—C(O) interaction and giving an energy barrier of about 58kJmol 1. There is little difference... 

In contrast, resonance stabilization is less in an amide because the resonance forms Ai and A2 given below are very different in energy. Nevertheless, because an amide is a resonance hybrid of Ai and A2, it is predicted diat tliere should be some double-bond character in die bond between carbon and nitrogen. This is in fact die case since many amides show restricted rotation around die C-N bond (typical of a 7r bond). Moreover, die nitrogen atom in amides is nearly planar and not very basic, also indicating that the lone pail" is delocalized. 

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