Propene conformations

The importance of steric interactions with cis hgands can be more clearly seen in alkene substituent effects. The barrier increases significantly in the PtCl(acac)(ene) series where ene = substituted ethylene compared to ene = ethylene. The m-Cl2(DMSO)Pt(alkene) series (6)-(9), studied by Boucher aud Bosuich, is particularly instructive. The rotatiou iu the ethylene begins to produce broadening about -47 °C, whereas the propene conformers broaden at —20 °C and... 

INDO methods. Dipole moments of states as a function of twist around bonds are examined and predictions made of photoisomerization effects. Energies, dipoles, and non-adiabatic couplings have also been computed for diradical and zwitterion states of ethylene and propene. Conformational analysis of flexible trans-dienes can be made by the technique of polarization spectroscopy in stretched polymer films as has already been mentioned. Vitamin D and its derivative have been selected as examples of the use of the technique. 

The C,H-eclipsed conformation is again the most stable, but because the overlap populations are much less, the translation into attractive terms is small and the barrier is dominated by the repulsive terms. In qualitative terms, the acetaldehyde barrier can be considered to be dominated by an attractive interaction between the 77-methyl group orbital and the C=0 antibonding orbital. The propene conformation is dominated by the repulsive interaction between the tt methyl group orbital and the filled tt orbital of the double bond. 

FIGURE 7 16 Poly mers of propene The mam chain IS shown in a zigzag conformation Every other carbon bears a methyl sub stituent and is a chirality center (a) All the methyl groups are on the same side of the carbon chain in isotactic polypropylene (b) Methyl groups alternate from one side to the other in syndiotactic polypropy lene (c) The spatial orienta tion of the methyl groups IS random in atactic polypropylene... 

There are two families of conformations available to terminal alkenes. These are the eclipsed and bisected conformations shown below for propene. The eclipsed conformation is more stable by about 2kcal/mol. ... 

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 case of a, -unsaturated caAonyl compounds is analogous to that of 1,3-dienes, in that stereoelectronic factors favor coplanaiity of the C=C—C=0 system. The rotamers that are important are the s-trans and s-cis conformations. Microwave data indicate that the s-trans form is the only conformation present in detectable amounts in acrolein (2-propenal). The equilibrium distribution of s-trans and s-cis conformations of a,fi-unsatuiated ketones depends on the extent of van der Waals interaction between substituents. Methyl vinyl ketone has minimal unfavorable van der Waals repulsions between substituents and exists predominantly as the s-trans conformer ... 

The lowest energy form is acetaldehyde, about 27 millihartrees below the 0° form of vinyl alcohol. As was true for propene, the vinyl alcohol conformer where the C-C-O-H dihedral angle is 0° is the lower energy conformer. ... 

Apply the bent-bond model to the preferred conformations of acetaldehyde and propene. Do bent-bonds maintain or remove eclipsing interactions in the equilibrium structures of the two molecules Formulate a simple rule based on the bent-bond model for predicting conformational preferences in systems containing trigonal atoms. 

A similar study was done with methyl acrylate as the dienophile.28 The uncatalyzed and catalyzed TSs are shown in Figure 6.7. As with propenal, the catalyzed reaction is quite asynchronous with C(2)-C(3) bonding running ahead of C(l)-C(6) bonding. In this system, there is a shift from favoring the exo-s-cis TS in the thermal reaction to the endo-s-trans TS in the catalyzed reaction. A large component in this difference is the relative stability of the free and complexed dienophile. The free dienophile favors the s-cis conformation, whereas the BF3 complex favors the s-trans conformation. 

A similar mechanism could operate also for bis(imino)pyridyl Fe(II)-based catalysts which have been reported to afford isospecific chain-end controlled propene polymerization occurring through secondary monomer insertion.138139 In fact, preliminary calculations indicate that the chirality of the growing chain would determine a chiral conformation of the bis-pyridyl ligand, which in turn would discriminate between the two monomer enantiofaces. 

So far we have discussed conformations of a molecule obtained by rotation along sp -sp3 bond i.e., between two tetrahedral carbon atoms. But there are many compounds in which one carbon is in a state of sp2 hybridisation. Examples are substituted alkenes where one carbon atom is tetrahedral and the other trigonal, for example propene ... 

As a consequence of the energy decrease in both u and 7r, only the —rr interaction need be considered when dealing with the rotational barrier in CH3CH=0. As in the propene case, the eclipsed conformer will be favored. However, as can be seen from the diagrams below, the eclipsed form will be favored to a lesser extent in acetaldehyde relative to propene. 

Experimentally, acetaldehyde is known to exist in the eclipsed conformation. It has a methyl rotational barrier of 1.16 kcal/mol94-96 as contrasted with a barrier of 2.00 kcal/mol in the case of propene. 

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