Double bonds rotation

Although essentially free rotation is possible around single bonds (Section 3.6), the same is not true of double bonds. For rotation to occur around a double bond, the -rrbond must break and re-form (Figure 6.2). Thus, the barrier to double-bond rotation must be at least as great as the strength of the 7r bond itself, an estimated 350 kj/mol (84 kcal/mol). Recall that the barrier to bond rotation in ethane is only 12 kj/mol. 

Iwata K, Ozawa R, Hamaguchi H (2002) Analysis of the solvent- and temperature-dependent Raman spectral changes of S1 trans-stilbene and the mechanism of the trans to cis isomerization dynamic polarization model of vibrational dephasing and the C=C double-bond rotation. J Phys Chem A 106 3614—3620... 

True double-bond rotations with low barriers have been proposed for 7 and 8(19). However, the possibility fora C —C2 rotation is not rigorously excluded,... 

With a double bond, rotation would destroy the tt bond that arises from overlap of p orbitals consequently, there is a very large barrier to rotation. It is of the order of 263 kJmol , which is very much higher than any of the barriers to rotation about single bonds that we have seen for conformational isomerism. Accordingly, cis and trans isomers do not interconvert under normal conditions. Ring systems can also lead to geometric isomerism, and cis and trans isomers... 

To explain these results, Lee and colleagues " showed that in the absence of Lewis acids the rotational barrier of the C=N double bond is fairly high, but in the presence of catalysts the rotational barrier is lowered. The complex formation of tosylate and AICI3 makes the double bond rotation possible and the product distribution is determined by the relative stability of the oxime E-Z isomers (equation 89). A cychc transition state affords the corresponding quinolinone 281 and the isoquinolinone 282. 

An interesting use is made of addition to a double bond by glutathione-dependent cis-tmns isomerases.76 One of them converts maleate to fumarate with a turnover number of 300 s"1. Similar enzymes, which participate in bacterial breakdown of aromatic compounds (Fig. 25-7), isomerize maleylacetoacetate and maleylpyruvate to the corresponding fumaryl derivatives (Eq. 13-20). The - SH group of bound glutathione is thought to add to the double bond. Rotation can then occur in the enolic intermediate. Thiocyanate ion catalyzes the isomerization of maleic acid nonenzy-matically, presumably by a similar mechanism. 

Maeda, K., Okamoto, Y., Morlender, N., Haddad, N., Eventova, I., Biali, S. E., and Rappoport, Z. (1995) Does the threshold enantiomerization route of crowded tetraarylethenes involve double bond rotation , J. Am. Chem. Soc. 117, 9686-9689. 

Two important cases in rotational isomerism are distinguished by considering the nature of the central bond. When it is a double bond, rotation of one form into another is hindered by a high potential... 

True double-bond rotation with low barriers108 has been proposed for compounds 61 (AG 47 kJ mol -1) and 62 (AG 58 kJ mol - ... 

Several compounds of the type 74 and 75 with aliphatic and aromatic N-substituents show double-bond rotation with barriers in the range of 80 to 38 kJ mol" while NMe2 torsional barriers are found13,121 in the range of 73.7 to 38 kJ mol-1. When R = Me, all barriers are lower than when R = H. When R1 (R2) is aromatic, a decrease in the C—N and an increase in the C=C barrier is observed. 

Betaines, with 1,2-vinylene linking group double bond rotation barriers, 60, 226 formation, 60, 205, 2( -9. 213-4. 216 nmr spectra. 60, 224-6, 227 theoretical calculations, 60, 240-1 Bicyclo(2.2.0]hexa-2,5-dienes, see Dewar benzenes... 

Figure 5.5. Hindered rotation about carbon-carbon double bond. Rotation would prevent overlap of p orbitals and would break n bond.

The operation required—rotation—is the same for interconversion of geometric and conformational isomers, and it has been suggested that they be called collectively ro/a/Zo/itf/(or torsional) isomers. Geometric isomers are thus double-bond rotational isomers, and conformational isomers are single-bond rotational isomers. 

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 E Z isomerization in several such compounds has been studied in different solvents . The rate of NH proton exchange was followed simultaneously using the N—Me doublet. The two processes were found to have very similar free-energy barriers ca 84kJmol ) which indicates that the E Z isomerization may not be a true double-bond rotation, but may proceed via one of the possible tautomeric forms with a C2—C3 single bond, a mechanism proposed earlier by Huisgen and coworkers for jS-aminoacrylates. For these compounds a lower limit to the barrier to uncatalysed rotation was found to be llOkJ mol , but significantly lower barriers were observed in the presence of traces of acid. 

The expected length of a single C-N bond is 1.45 A, as in the C -N bond, and that of a C = N double bond is 1.25 A. The actual length of the C -N peptide bond is 1.33 A, showing that it has partial double bond characteristics (40% double bond). Rotation can occur, in principle, around all three bonds [j/, q>, and w, where j/ = (p = w= %Q°. This means that for a protein of 100 residues there are 2 x 10 possible conformations, far more possible conformations than there would be protein molecules, even in a large sample. However, we know that a folded protein has a relatively stable conformation. This is due to many factors, one being the partial double bond characteristics of the C -N peptide bond that limits it to a trans conformation (with the exception of proline), the atoms of the side chains restrict bond rotation due to excluded volume effects that dictate... 

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