Rotation around carbon-nitrogen bond

If it is assumed that 2,2 -bipyridine is bonded to the catalyst by both nitrogen atoms, then the position of the chemisorbed molecule on the metal is rigidly fixed. Unless two molecules of this base can be adsorbed at the required distance from each other and in an arrangement which is close to linear, overlap of the uncoupled electrons at the a-position cannot occur. The failure to detect any quaterpyridine would then indicate that nickel atoms of the required orientation are rarely, if ever, available. Clearly the probability of carbon-carbon bond formation is greater between one chemisorbed molecule of 2,2 -bipyridine and one of pyridine, as the latter can correct its orientation relative to the fixed 2,2 -bipyridine by rotation around the nitrogen-nickel bond, at least within certain limits. 

More illustrative is the structure of the fluorosilane which exhibits the same tricapped tetrahedron geometry. The three N-Si interactions take place in the frontal position towards the Si-F bond instead of the rear coordination observed in penta- and hexacoordinated structures. Furthermore, the benzylamino groups have the possibility not to be coordinated at silicon since these groups have the capability of free rotation around the carbon-nitrogen bond. 

FIGURE 3. Rotational energy profile around the methine carbon-nitrogen bond of isopropyldimethylamine (IDMA) as calculated by MM2-87. Reprinted with permission from Reference 72. Copyright (1992) American Chemical Society... 

The planarity of the amide linkers in the protein backbone restricts rotation around the carbon-nitrogen bond. This provides some restrictions on the number of conformers that can be adopted. The linkage joining amino acids in a polymer is quite stable, but not infinitely so, and it can be relatively easily hydrolyzed by enzymes to allow turnover of proteins within cells. This propitious combination of properties is conferred by the amide bonds linking the amino acids in the polymer polymers linked by ester, thioester, ether, or carbon-carbon bonds lack one or more these properties. 

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

Under the assumption that Baker s mechanism is valid, Farkas and Flynn draw the following conclusions from their results. The rate of the catalyzed reaction depends on the concentration of the amine complex and on the specific velocity constant for the reaction which involves this complex and the alcohol. Since the diazabicyclooctane is free from steric hindrance and the nitrogen atoms are readily accessible to the reactants, the formation of the complex between this molecule and the isocyanate molecules takes place more readily than with an amine having a carbon-nitrogen bond capable of free rotation around the nitrogen. Related to the postulate of higher... 

The bond between nitrogen and carbonyl carbon in amides is configurationally stable at room temperature, but being only a partial double bond, rotation around the a bond occurs at elevated temperatures, and the rotation barrier can be measured by means of H NMR spectroscopy. 

The peptide bond consists of an amide group. There is no free rotation around the peptide bond because the lone pair of electrons of the nitrogen atom interacts with the carbon and oxygen of the carbonyl group. This results in a resonance structure with a partially double bonded character. 

This structural feature has important implications for the three-dimensional conformations of peptides and proteins. There is free rotation around the bonds between the a-carbon of a given amino acid residue and the amino nitrogen and carbonyl carbon of that residue, but there is no significant rotation around the peptide bond. This stereochemical constraint plays an important role in determining how the protein backbone can fold. 

The second very important conclusion is related to the nature of the covalent bond between the nitrogen and the carbon atom. This bond has a single bond character in one of the resonance structures and a double bond character in the other. Hence, the amide bond has a bond order of 1.5, i.e. it has an intermediate character between a double and a single bond. In the chapter on alkanes we have mentioned that while the rotation around a single bond is free, the rotation around a double bond is forbidden. Because the amides bond has a partial double bond character the rotation around it is hindered. This evidence is important in the study of the stereochemistry of complex molecules with amide groups, such as polyamides and polypeptides. 

Inspection of these structures makes it clear that the chemical bonds between carbon and nitrogen have a double bond character. Consequently, the rotation around these bonds is restricted as is the rotation around the double bonds in alkenes. This restricted rotation makes polypeptide chains rigid. Because of structural rigidity polypeptide chains appear in two forms the a-helix and the P-sheet. 

Amides have a planar geometry. Even though the carbon-nitrogen bond is normally written as a single bond, rotation around that bond is restricted because of resonance. 

The molecule has two chiral elements. There is a chiral axis along the bond from the carbon at the 3-position of the thiophene to the nitrogen. This is because there is not free rotation around this single bond. There is also an asymmetric carbon at the methine carbon attached to nitrogen. Because of two chiral elements, there can be 1 or four possible stereoisomers. Dimethenamid with S-configuration at the chiral carbon can be prepared from S-methoxyisopropylamine. This single enantiomer amine is isolated from a racemic mixture of amines by enantioselective enzymatic acylation [58]. The enzyme selectively acylates only one enantiomer and the resultant amide can be readily separated from the unreacted free amine. The undesired enantiomer can then be racemized to provide a source of (after further separation) more of the desired enantiomer. 

The decreased double bond character of the carbon-nitrogen bond in both the S- and N-oxides and the consequent lower barrier to rotation around the carbon-nitrogen bond are also evident and supported by the PMR spectrum of TO, reported in Fig. 6. 

Spatial arrangement of heteroatom groups covered in this volume (N, P, O, S, Se, Te) gives rise to different isomers of transition metal complexes. For nitrogen and phosphorus (III) two isomers A and B may exist due to rotation around metal-heteroatom bond (Scheme 7). In the A-TS the lone pair of the heteroatom interacts with the multiple carbon-carbon bond, whereas in B-TS direct interaction is unlikely. Such different interactions may become a reason for changing relative stability of the transition states. 

An E-Z discrimination between isomeric oxaziridines (27) was made by NMR data (69JCS(C)2650). The methyl groups of the isopropyl side chains in the compounds (27) are nonequivalent due to the neighboring carbon and nitrogen centres of asymmetry and possibly due to restricted rotation around the exocyclic C—N bond in the case of the Z isomer. The chemical shift of a methyl group in (Z)-(27) appears at extraordinarily high field, an effect probably due to the anisotropic effect of the p-nitrophenyl group in the isomer believed to be Z. 

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. 

In this structure, the peptide group is indicated by the dashed lines. Because the peptide group itself is rigid and planar, there is no rotation around the bond between the carbonyl carbon atom and the nitrogen atom (the C —N bond). However, free rotation is possible around the bond between the a carbon and the carbonyl carbon atom (the C —C bond) and about the bond between the nitrogen atom and the alanyl o-carbon atom the (N—Ca bond). Thus, for every peptide group in a protein, there are two rotatable bonds, the relative angles of which define a particular backbone conformation. 

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