Rotation around double bonds

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. 

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. 

For ds-trans isomerization in highly condensed media at very low temperature <77 K, liu and Hammond postulated mechanisms called one-bond flip and hula-twist mecdianism [35-38]. According to the H-T mechanism, isomerization takes place not by the one-bond rotation around the double bond but by the concomitant twist of the double bond and the adjacent single bond to accomplish the double-bond isomerization (Figure 4.10). These mechanisms were assumed to reduce free volume... 

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... 

As is inversely proportional to solvent viscosity, in sufficiently viscous solvents the rate constant k becomes equal to k y. This concerns, for example, reactions such as isomerizations involving significant rotation around single or double bonds, or dissociations requiring separation of fragments, altiiough it may be difficult to experimentally distinguish between effects due to local solvent structure and solvent friction. 

Figure B2.4.1. Proton NMR spectra of the -dimethyl groups in 3-dimethylamino-7-methyl-l,2,4-benzotriazine, as a fiinction of temperature. Because of partial double-bond character, there is restricted rotation about the bond between the dunethylammo group and the ring. As the temperature is raised, the rate of rotation around the bond increases and the NMR signals of the two methyl groups broaden and coalesce.

However, the descriptors cannot be considered independently as there is no free rotation around the double bond, In order to take account of this rigidity, the descriptors of the two units have to be multiplied to fix a descriptor of the complete stereoisomer. 

One part of the molecule (dark blue and red) rotates 180° around a double bond between two carbon atoms (green). The geometry of the molecule is changed by this rotation from a trans form to a cis form. Carbon atoms are blue, hydrogen atoms gray and the oxygen atom red. 

A pure double bond between C and O would permit free rotation around die C —N bond. 

Rotational barriers for bonds which have partly double bond character are significantly too low. This is especially a problem for the rotation around the C-N bond in amides, where values of 5-10 kcal/mol are obtained. A purely ad hoc fix has been made for amides by adding a force field rotational term to the C-N bond which raises the value to 20-25 kcal/mol, and brings it in line with experimental data. Similarly, the barrier for rotation around the central bond in butadiene is calculated to be only 0.5-2.0 kcal/mol, in contrast to the experimental value of 5.9 kcal/mol. 

Figure 6.2 The n bond must break for rotation to take place around a carbon-carbon double bond.

The lack of rotation around carbon-carbon double bonds is of more than just theoretical interest it also has chemical consequences. Imagine the situation for a disubstifitted alkene such as 2-butene. Disubstitilted means that two substituents other than hydrogen are bonded to the double-bond carbons.) The two methyl groups in 2-bulene can be either on the same side of the double bond or on opposite sides, a situation similar to that in disubstitutecl cycloalkanes (Section 4.2). 

The general catalytic cycle for the coupling of aryl-alkenyl halides with alkenes is shown in Fig. 9.6. The first step in this catalytic cycle is the oxidative addition of aryl-alkenyl halides to Pd(0). The activity of the aryl-alkenyl halides still follows the order RI > ROTf > RBr > RC1. The olefin coordinates to the Pd(II) species. The coordinated olefin inserts into Pd—R bond in a syn fashion, p-Hydrogen elimination can occur only after an internal rotation around the former double bond, as it requires at least one /I-hydrogen to be oriented syn perpendicular with respect to the halopalladium residue. The subsequent syn elimination yields an alkene and a hydridopalladium halide. This process is, however, reversible, and therefore, the thermodynamically more stable (E)-alkene is generally obtained. Reductive elimination of HX from the hydridopalladium halide in the presence of a base regenerates the catalytically active Pd(0), which can reenter the catalytic cycle. The oxidative addition has frequently assumed to be the rate-determining step. 

It has been mentioned (p. 9) that the two carbon atoms of a C—C double bond and the four atoms directly attached to them are all in the same plane and that rotation around the double bond is prevented. This means that in the case of a molecule WXC=CYZ, stereoisomerism exists when W X and Y Z. There are two and only two isomers (E and F), each superimposable on its mirror image unless... 

Conversely, there are compounds in which nearly free rotation is possible around what are formally C=C double bonds. These compounds, called push-pull or captodative ethylenes, have two electron-withdrawing groups on one carbon and two electron-donating groups on the other (66). The contribution of di-ionic... 

Although at first glance addition to the central carbon and formation of what seems like an allylic carbonium ion would clearly be preferred over terminal addition and a vinyl cation, a closer examination shows this not to be the case. Since the two double bonds in allenes are perpendicular to each other, addition of an electrophile to the central carbon results in an empty p orbital, which is perpendicular to the remaining rr system and hence not resonance stabilized (and probably inductively destabilized) until a 90° rotation occurs around the newly formed single bond. Hence, allylic stabilization may not be significant in the transition state. In fact, electrophilic additions to allene itself occur without exception at the terminal carbon (54). 

Duong and Gaudemer studied the alkylation of (presumably) [Co -(DMG)2X], where X is pyridine, aniline, or water, by the cis and trans isomers of )S-bromostyrene (PhCH=CHBr) and the methyl ester of )3-chloroacrylic acid (CHCl=CHCOOMe) in 50% aqueous methanol, and found that the configuration of the double bond remained unchanged, i.e., the halogen had simply been replaced by cobalt. They suggested that the reaction involved the addition of cobalt, followed by the elimination of the halide ion (apparently without rotation around the C—C bond), i.e.. 

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