Crowded conformation

Once we have established the factors responsible for conformational isomerism in 1,3-butadiene, we can attempt to put the insight that theory affords into use by designing dienic systems which will have a maximum chance of existing in a preferred crowded conformation, i. e. a gauche or cis conformation. 

The assignment of the configurations of the E- and Z-InFl geometrical isomers based on the spectral analysis data has been supported by X-ray diffraction analysis. The InFl molecule (Figure 20) is characterized by a sterically crowded conformation with a 1-methylindole substituent in the position tram to the carbonyl C(1)=0(1) group with respect to the C(2)=C(5) bond. The methyl group C(15) and the benzene ring of the indole moiety have an anti position with... 

FIGURE 17.7 Cyclopropyl constraints can be replaced by allylic strains. The less crowded conformers B and B were observed in the X-ray structures of 3 and 4. 

Process (a) proceeds by the more crowded conformation (see below), and so the tran5-alkene produced by process (b), would be expected to predominate. Use ISIS Draw and WebLab ViewerLite to look at the conformations if the Newman projections aren t helpful. 

A concerted syn-elimination requires the salt to adopt an eclipsed conformation that should be very sensitive to the steric interactions between the R groups. Then we should expect that a more crowded conformation (as in 7) would lead to a slower elimination process. This is consistent with the experimental observation that eryt/zro-dibromo derivatives react much faster than their threo-aaaXogs. In addition, the increasing ring strain for syn-eliminations in cyclic molecules could justify the slow debrominations observed for trans-l,2-dibromocycloalkanes. 

The three staggered conformations are different for valine because the three groups attached to Cp are different. The first staggered conformation (b) is less crowded and energetically most favored because the two methyl groups bound to Cp are both close to the small H atom bound to Ca-... 

FIGURE 6.3 Many of the possible conformations about an -carbon between two peptide planes are forbidden because of steric crowding. Several noteworthy examples are shown here. 

This is also the case in the decamethylmetal-locenes of Ru and Os but not in the iron analogue which has a staggered conformation, presumably due to steric crowding around the smaller metal. 

Further evidence of steric crowding owing to bulky phosphines is found in RhCl(CO)(PBu2 alkyl)2. Study of the 3IP NMR spectra at low temperature, freezing in the rotational conformers shows separate signals for each (Figure 2.25) [62],... 

A number of effects of conformation have been reported Crowding of the hydroxyl group results in faster oxidation and alcohols with axial hydroxyl groups are oxidised more readily than the equatorial isomers, e.g. cis-4-tert-butylcyclohexanol is oxidised about three times faster than the trans isomer at 25 °C. 

Steric crowding of intermediates, preventing groups from attaining correct conformations for maximum stabilization of departing anions, may be responsible for the observation that, in the alkaline hydrolysis of sterically crowded phosphonium salts, the group lost need not be that which is most stable as the anion. 

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