Skew conformations

Dienes would be expected to adopt conformations in which the double bonds are coplanar, so as to permit effective orbital overlap and electron delocalization. The two alternative planar eonformations for 1,3-butadiene are referred to as s-trans and s-cis. In addition to the two planar conformations, there is a third conformation, referred to as the skew conformation, which is cisoid but not planar. Various types of studies have shown that the s-trans conformation is the most stable one for 1,3-butadiene. A small amount of one of the skew conformations is also present in equilibrium with the major conformer. The planar s-cis conformation incorporates a van der Waals repulsion between the hydrogens on C—1 and C—4. This is relieved in the skew conformation. 

The barrier for conversion of the skew conformation to the s-trans conformation is 3.9kcal/mol. This energy maximum presiunably refers to the conformation (transition state) in which the two n bonds are mutually perpendicular. Various MO calculations find the s-trans conformation to be 2-5 kcal/mol lower in energy than either the planar or skew cisoid conformations. Most high-level calculations favor the skew conformation over the planar s-cis, but the energy differences found are quite small. ... 

The two -OH groups in l,2 5,6-di-0-cyclohexylidene-wii/o-inositol and its di-O-isopropylidene analog are trans. The X-ray crystal structure of the latter compound suggests that the ring is in a skew conformation with the 0-3 and 0-4 -OH groups both in ax positions, but NMR studies and ab initio calculations indicate that a mixture of the skew and chair conformations, with 0-3 and 0-4 both in eq positions, is present. Formation of a dibutylstannylene acetal presumably locks these two compounds in the latter conformation. 

Butadiene can exist in two planar conformations, s-trans and s-cis. They belong to the C2h and C2v symmetry point group, respectively. Obviously, both forms have symmetry planes. A skewed conformation, instead, has only a C2 axis. We can choose the reference frames depicted in Figure 12, characterized by z as the symmetry axis and x directed along the single bond. 

Certain physical properties show that rotation about the single bond is not quite free. For ethane there is an energy barrier of about 3 kcal mol-1 (12 kJ mol-1). The potential energy of the molecule is at a minimum for the staggered conformation, increases with rotation, and reaches a maximum at the eclipsed conformation. The energy required to rotate the atoms or groups about the carbon-carbon bond is called torsional energy. Torsional strain is the cause of the relative instability of the eclipsed conformation or any intermediate skew conformations. 

Rather more theoretical work has been done with respect to tris(ethylenediamine) complexes. By considering the two stable chiral (skew) conformations of the five-membered chelate rings (designated k and S as above), four unique conformers of M(en)"+ can be constructed for each absolute configuration (A or A) (12) kkk, kkS, kSS, and 886. In this representation, A(1U) is energetically equivalent to A(<5<5<5). 

Figure 9 shows some of the conformations of S-chairphos(S-l,3-bis(diphenylphosphino)butane). A chair and a skew conformation exist that have equatorially disposed methyl groups. The chair conformation that is intrinsically more stable is expected to be preferred (33, 34, 37). The other chair conformation with an axially disposed methyl group is expected to be less stable. 

A tentative inference may be drawn from these results the skewphos complex is predominantly in the skew conformation in solution, but minor stereochemical encumbrances may tip the ring into the chair conformation. 

The basic skeleton of these compounds is 2,9-dioxa-bicyclo[3.3.1]nonane (57), which has limited steric flexibility. 1,7-Anhydroheptoses adopt predominantly the twin-chair conformation 57a, provided that no endo substituents are present at positions C-3 and C-7 to cause steric interactions. Otherwise, flattened boat-like and skew conformations may also play an important role. 

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