Eclipsing bonds, and

FIGURE 3 14 (a) A ball and spoke model and (b) a space filling model of the boat confor mation of cyclohexane Torsional strain from eclipsed bonds and van der Waals strain involving the flagpole hydrogens (red) make the boat less stable than the chair... 

Fig. 4. Solid-state structure of the Me3PB(C6F5)3 adduct, illustrating the nearly eclipsed bonds and close...

Out of the two conformations, the staggered conformation is the more stable as the C-H bonds and hydrogen atoms are as far apart from each other as possible. In the eclipsed conformation, both the bonds and the atoms are closer together and it can cause strain due to electron repulsion between the eclipsed bonds and between the eclipsed atoms. Therefore, majority of the ethane molecules are in the staggered conformation at any point of time. Since the energy difference between the staggered and eclipsed conformations is small and so it allows each ethane molecule to pass through an eclipsed conformation ... 

Fig. 2.34. Energy of tetrahedrally bound atom clusters of different structures (s/e - ratio between staggered and eclipsed bonds) and sizes (N - number of correlation layers) (after Grigorovici and M iilJl (1969)).

This flattening is due to the presence of an odd niimber of carbons in the ring and it means that there will be some torsion strain due to eclipsing bonds and atoms in this form of cycloheptane. Some twisting of the ring can occur to relieve this strain, but such pseudorotation may increase strain elsewhere in the molecule. This increase in strain makes conformations 49A and 49B for cycloheptane higher in energy than the chair conformations of cyclohexane. 

Total syntheses of tazettine and 6a-epi-pretazettine have been carried out by application of the Pd-catalyzed cydization of 169, in which a single pentacyclic product 170 was obtained, establishing a preference for an eclipsed orientation of the Pd—C (7-bond and alkene 7r-bond in the key intramolecular insertion step[l40]. 

Eclipsed bonds are characterized by a torsion angle of 0° When the torsion angle is approximately 60° we say that the spatial relationship is gauche, and when it is 180° we say that it is anti Staggered conformations have only gauche or anti relationships between bonds on adjacent atoms... 

At Its most basic level separating the total strain of a structure into its components is a qualita tive exercise For example a computer drawn model of the eclipsed conformation of butane using ideal bond angles and bond distances (Figure 3 8) reveals that two pairs of hydrogens are separated by a distance of only 175 pm a value considerably smaller than the sum of their van der Waals radii (2 X 120 pm = 240 pm) Thus this conformation is destabilized not only by the torsional strain associ ated with its eclipsed bonds but also by van der Waals strain... 

A second but much less stable nonplanar conformation called the boat is shown in Eigure 3 14 Like the chair the boat conformation has bond angles that are approximately tetrahedral and is relatively free of angle strain It is however destabi hzed by the torsional strain associated with eclipsed bonds on four of its carbons The... 

Figure 4.3. Energy versus bond rotation in methylsuccinic acid (schematic). The diagram shows the greater stability of staggered as compared with eclipsed forms, and the effect of size and dipole moment of substituents on the barriers. The slope of the curve at any point represents the force opposing rotation there. ( = energy of activation of rotation.) (After Gordon )...

The origin of the preference for the eclipsed conformation of propene can be explained in MO terms by focusing attention on the interaction between the double bond and the n component of the orbitals associated with the methyl group. The dominant interaction is a repulsive one between the filled methyl group orbitals and the filled n orbital of the double bond. This repulsive interaction is greater in the bisected conformation than in the eclipsed conformation. ... 

The staggered and eclipsed conformations interconvert by rotation around the C—C bond, and do so very rapidly. We ll see just how rapidly later in this section. 

After 0°, the rotation becomes a mirror image of what we ve already seen another gauche conformation is reached, another eclipsed conformation, and finally a return to the anti conformation. A plot of potential energy7 versus rotation about the C2-C3 bond is shown in Figure 3.9. 

The successful implementation of this strategy is shown in Scheme 4. In the central double cyclization step, the combined action of palladium(n) acetate (10 mol %), triphenylphosphine (20 mol %), and silver carbonate (2 equiv.) on trienyl iodide 16 in refluxing THF results in the formation of tricycle 20 (ca. 83 % yield). Compound 20 is the only product formed in this spectacular transformation. It is noteworthy that the stereochemical course of the initial insertion (see 17—>18) is guided by an equatorially disposed /-butyldimethylsilyl ether at C-6 in a transition state having a preferred eclipsed orientation of the C-Pd a bond and the exocyclic double bond (see 17). Insertion of the trisubstituted cycloheptene double bond into the C-Pd bond in 18 then gives a new organopal-... 

In contrast to the open-chain and dipolar models, which are based on conformations of the carbonyl compound not representing energy minima, Karabatsos proposed a different model assuming an early, reactant-like transition state in which the most stable conformation of the free carbonyl compound is preserved1314. Thus, the C-M bond eclipses the carbonyl double bond and, in order to minimize the energy of the transition state, the nucleophile approaches close to the small substituent on the stereogenic center (Figure 5). 

As a consequence of the molecular orbital interactions, ferrocene adopts an axially symmetrical sandwich structure with two parallel Cp ligands with a distance of 3.32 A (eclipsed conformation) and ten identical Fe-C distances of 2.06 A as well as ten identical C-C distances of 1.43 A [12]. Deviation of the parallel Cp arrangement results in a loss of binding energy owing to a less efficient orbital overlap [8]. All ten C-H bonds are slightly tilted toward the Fe center, as judged from neutron-diffraction studies [13]. 

Molecules of the simpUcity of ethane or the complexity of proteins and DNA adopt different conformations. In the case of ethane this gives rise to the notion of a staggered and eclipsed bond, whereas proteins form an array of complex structural elements and DNA - the famous double hehx. The understanding of the conformational properties of small molecules is an important factor in computational approaches contributing to drug discovery. 

Therefore, in the final analysis, the quadruple bond consists of one a bond, two it bonds, and one S bond. The configuration in which the Cl atoms are in eclipsed positions provides maximum overlap. From the rather intuitive analysis provided here, it is possible to arrive at a qualitative molecular orbital diagram like that shown in Figure 21.23 to describe the Re-Re bond. 

Fig. 2 Differences between wurtzite (WZ) and zincblende (ZB) forms, (a, b) Handedness of the fourth interatomic bond along the right (R) for WZ and left (L) for ZB. (c, d) Eclipsed (WZ) and staggered (ZB) conformation of atoms. Reprinted with permission from [23], Copyright 1992 by the American Physical Society...

Here also the rotation can occur about the two carbon bonds and is almost free. The energy difference between the eclipsed and staggered forms is 3.3 K gals/mole. This is also too low and permits ready interconversion. 

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