Formamide rotation barrier

A similar rotational analysis has been done with formamide derivatives. It is known that thioformamide has a larger rotational barrier than formamide, which can... 

The effects of the substituents on nitrogen on rotational barriers were discussed by Yoder and Gardner (34) for formamides and acetamides. The pertinent data, given in Table 5, suggest that the barriers to rotation of formamides are not affected by the bulkiness of the alkyl group on nitrogen, but such a conclusion... 

A semiempirical study of rotational barriers of various thioamides and thioureas was published by Feigel and Strassncr111" and a careful, high level ab-initio study of formamide and thioformamide has been carried out by Chu and coworkers1116. 

The same methodology was applied to the study of the role of conjugation in the stability and rotational barriers of formamide and thioformamide [100]. Here it was found that resonance accounts for roughly one-half of the rotational barrier of formamide and for two-thirds in the case of thioformamide. 

An understanding of the internal rotation about the amide bond is important because of its relevance to protein structure. Formamide is the simplest amide. The coplanarity and the remarkable rotational barrier about the C-N bond in formamide can be rationalized by resonance between the n electrons of the carbonyl group and the lone pair of the nitrogen atom [1, 50]. According to VB theory, the Jt electronic structure of formamide may be described by six resonance structures. 

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). 

A similar conformational analysis has been done with formamide derivatives, with secondary amides, and for hydroxamide acids. It is known that thioformamide has a larger rotational barrier than formamide, which can be explained by a traditional picture of amide resonance that is more appropriate for the thioformamide than formamide itself. Torsional barriers in a-keto amides have been reported, and the C—N bond of acetamides, thioa-mides, enamides carbamates (R2N—C02R), and enolate anions derived... 

Formamide, 252 interaction diagram, 257 rotation barrier, 260 structure, 256... 

The existence of A-formylaminoazoles (pyrazoles and indazoles) in the Z configuration <93JCS(P2)377> has been proved (x-ray structures) for A-(pyrazol-l-yl)formamide (61) and A-(inda-zol-l-yl)formamide (62) the rotation barriers about the amide bond are similar although a little lower than those of 7V-phenylformamide. 

Compared to the corresponding formamides and acetamides, the non-bonding interaction between the sulfur atom and the hydrogen atom of the methyl group attached to the nitrogen atom is important. Two rotamers of the a,p-un-saturated thioamides 6 and 7 are observed in NMR spectra, and their rotational barriers can be elucidated. The isomers 6, where the thiocarbonyl and benzyl groups are located in a cis position with respect to the C-N single bond. 

The C-N rotational barrier in thioformamide is larger than in formamide. Although the C=S bond is not strongly polarized and the C=S carbon is considerably less electron deficient than the amide C=0 carbon, the Jt 5 energy in thioamides is much lower than the in amides (see Chapter 5 for a more detailed discussion of acceptor ability of orbitals). Due to the lower donor/acceptor energy gap, the n reso-... 

S. Tsuzuki and K. Tanabe, /. Chem. Soc., Perkin Trans. 2, 1255 (1991). Basis Set and Electron Correlation Effects on die Internal Rotational Barrier Heights of Formamide and Acetamide. 

In the case of formamide the dependence upon the C-N II bond order is corroborated by the increase in the rotational barrier which accompa-... 

Effect of Substituents on Nitrogen on the Barrier to Rotation of Formamides and Acetamides (RCONR2) ... 

Siddall and his co-workers (46) have examined the barriers to rotation of a series of 2,6-disubstituted anilides. Af-Ethyl-A/-(2,6-xylyl)formamide (9) was recrystallized as a uranyl nitrate complex, and one isomer, which at equilibrium was favored by a factor of 3 1, was enriched up to a 30 1 ratio. The kinetics of rotation were examined at 0 to 29°C. The Arrhenius activation energy was 26 3 kcal/mol and log A was 18.5 2.4 hr-1. Siddall and Gamer (47) were able to obtain an almost pure isomer (which also predominated at equilibrium 1.3 1 for the ethyl compound and 1.1 1 for the methyl compound) of Ar-alkyl- V-(2-methyl-4,6-dibromophenyl)-l-naphthamide (10). The half-lives of... 

Siddall (48) also reported that the barriers to rotation in N-substituted N-(2-chloro-6-methylphenyl)formamides (11) were high, but not high enough for the isolation of atropisomers. The exact barriers were not reported but, if one compares them with those in compound 9, the barriers to rotation of these compounds are lowered by the substitution of the chloro group for the methyl on the aromatic ring. 

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