Conformational analysis eclipsed

Conformation and Conformational Analysis Conformation of Ethane Conformation of Propane Conformation of Butane Eclipsed and Staggered Eorms Ring Strains in Cycloalkanes Principles of Conformation 165... 

A rationalization - of observations (iv) and (v) is based on conformational analysis of sj —sjp single bond systems, for which an eclipsed conformation seems to be jxefened Of the three eclipsed conformations (9a), (9b) and (9c) of an allylic alcdiol system R (R )CHCHa->CR R conformation (9a) appears to be least sterically hindered and, therefore, most preferred. If this conformational preference is reflected in the transition state for hydroxylation, the major product can be seen to arise from the preferential approach of osmium tetroxide from the -ir-face opposite to that of the fxvexisting hydroxy or alkoxy group. An alternative rationalization, basetP on observations on the hydroxylation of... 

A conformational analysis of various substituted A-homo-steroids is reported. The preferred conformation of the amide group in some acetamido-substituted steroids is of the type (5), with anti-periplanar orientation of C—H and N—H bonds the eclipsing of the C—H and C=0 bonds is similar to that accepted for acetates of secondary alcohols. 

Conformation analysis is particularly important in aliphatic systems. Taking butane as an example, there is obviously an infinite number of possible dihedral angles that the two methyl groups can adopt with respect to each other. However, two of the conformers are of particular interest in that they represent the states of lowest and highest interaction between the hydrogens on the adjacent methyl groups these are the staggered and eclipsed conformers respectively. 

Nearly all side-chain prediction methods depend on the concept of side-chain rotamers. From conformational analysis of organic molecules, it was predicted long ago [157, 158] that protein side chains should attain a limited number of conformations because of steric and dihedral strain within each side chain and between the side chain and the backbone (dihedral strain occurs because of Pauli exclusion between bonding molecular orbitals in eclipsed positions) [159]. For sp3-sp3 hybridized bonds, the energy minima for the dihedral are at the staggered positions that minimize dihedral strain at approximately 60°, 180°, and —60°. For sp3-sp2 bonds, the minima are usually narrowly distributed around +90° or —90° for aromatics and widely distributed around 0° or 180° for carboxylates and amides (e.g., Asn/Asp y2 and Glu/Gln /3). 

The electrons in a C—H bond will repel the electrons in another C—H bond if the bonds get too close to each other. The staggered conformation, therefore, is the most stable conformation of ethane because the C—H bonds are as far away from each other as possible. The eclipsed conformation is the least stable conformation because in no other conformation are the C—H bonds as close to one another. The extra energy of the eclipsed conformation is called torsional strain. Torsional strain is the name given to the repulsion felt by the bonding electrons of one substituent as they pass close to the bonding electrons of another substituent. The investigation of the various conformations of a compound and their relative stabilities is called conformational analysis. 

Vibrational overtone spectroscopy has been applied to methyl-group conformational analysis in aromatic molecules (Henry, 1987). In addition to conformational data, very accurate information on the C—H bond lengths are obtained, showing that the methyl C—H bond eclipsing the ring plane is slightly shorter than the other methyl C—H bonds, in excellent agreement with ah initio calculations. 

The pseudorotation concept has been introduced by K. S. Pitzer [49] to describe the continuous interconversions between an infinite number of indefinite puckered conformations of the cyclopentane ring. PseudorotatiOTi [50] allows cyclopentane to relieve the ring strain, which would be induced by a 120° bond angle and the torsional strain by an eclipsed methylene group, if it were to adopt a planar conformation. A barrier to the planarity of cyclopentane of 22 kJ/mol has been reported [51]. The concept of pseudorotation has been applied for the first time to sugar furanoses by Hall et al. [52] studying the conformational analysis of pentofuranosyl fluorides. 

The selection of the initial values for stretching and bending variables can be made in consonance with the values recommended in ref. 22. However, it is frequent (and convenient) in conformational analysis to adopt a previously optimized conformation as an initial estimate for the geometries of other conformations. In the case of methanol, using the optimized geometry of the staggered conformer as initial geometry for the eclipsed conformation, the number of optimization cycles reduces to 5, with the consequent decrease in the computational time. 

In Chapter 1 we noted that radicals prefer planar structures, but the barrier to pyramidalization is very low. In fact, there is a conformational effect that leads to pyramidalization. Computational studies by Padden-Row and Houk have shown that even the simplest alkyl radicals are pyramidalized, and methyl radical is really the only totally planar radical. At right are shown three examples. The interpretation of these structures invokes simple conformational analysis. In each case, pyramidalization occurs in the direction that minimizes eclipsing interactions. 

Draw a relative energy diagram showing the conformational analysis of 1,2-dichloroethane. Clearly label all staggered conformations and all eclipsed conformations with the corresponding Newman projections. 

Sketch an energy diagram showing the conformational analysis of 2,2,3,3-tetramethylbutane. Use Table 4.6 to determine the energy difference between staggered and eclipsed conformations of this compound. 

Now let us consider a conformational analysis of ethane. Clearly, infinitesimally small changes in the dihedral angle between C—H bonds at each end of ethane could lead to an infinite number of conformations, including, of course, the staggered and eclipsed conformations. These different conformations are not aU of equal stability, however, and it is known that the staggered conformation of ethane is the most stable conformation (i.e., it is the conformation of lowest potential energy). The fundamental reason for this has recently come to light. 

A molecule s conformation changes from staggered to eclipsed millions of times per second at room temperature. As a result, the conformers carmot be separated from each other. At any one time, approximately 99% of the ethane molecules will be in a staggered conformation because of the staggered conformer s greater stability, leaving only 1% in less stable conformations. The investigation of the various conformers of a compound and their relative stabilities is called conformational analysis. 

There can be more than one staggered and one eclipsed conformation conformational analysis of butane... 

---