S-cis

The Phase Change Upon CycUzadou of Different s-cis Cyclobutadiene Isomers ... 

D-A rxns are sensitive to steric effects of the dienephiles, particularly at the I- and 2-postions. Steric bulk at the I-position may prevent approach of the dienophile while steric bulk at the 2-position may prevent the diene from adopting the s-cis conformation. 

Additional evidence for electron delocalization m 1 3 butadiene can be obtained by considering its conformations Overlap of the two rr electron systems is optimal when the four carbon atoms are coplanar Two conformations allow this coplananty they are called the s cis and s trans conformations... 

The letter sms cis and s trans refers to conformations around the C—C single bond m the diene The s trans conformation of 1 3 butadiene is 12 kJ/mol (2 8 kcal/mol) more stable than the s cis which is destabilized by van der Waals strain between the hydrogens at C 1 and C 4... 

Examine the models of 1 3 butadiene in Figure 10 6 on Learn mg By Modeling and com pare space filling models of the s CIS and s trans confor mation... 

FIGURE 10 6 Confor mations and electron delo calization in 1 3 butadiene The s CIS and the s trans con formations permit the 2p or bitalsto be aligned parallel to one another for maxi mum TT electron delocaliza tion The s trans conformation is more stable than the s CIS Stabilization resulting from tt electron de localization is least in the perpendicular conformation which IS a transition state for rotation about the C 2—C 3 single bond The green and yellow colors are meant to differentiate the orbitals and do not indicate their phases... 

The Diels-Alder cycloaddition is one example of a pencyclic reaction, which is a one step reaction that proceeds through a cyclic transition state Bond formation occurs at both ends of the diene system and the Diels-Alder transition state involves a cyclic array of six carbons and six tt electrons The diene must adopt the s cis conformation m the transition state... 

The two most stable conformations of conjugated dienes are the s cis and s trans The s trans conformation is normally more stable than the s cis Both conformations are planar which allows the p orbitals to overlap to give an extended tt system... 

Examples of prolate near-symmetric rotors are the s-trans and s-cis isomers of crotonic acid, shown in Figure 5.8, the a axis straddling a chain of the heavier atoms in both species. The rotational term values for both isomers are given approximately by Equation (5.37) but, because A and B are different for each of them, their rotational transitions are not quite coincident. Figure 5.9 shows a part of a low-resolution microwave spectmm of crotonic acid in which the weaker series of lines is due to the less abundant s-cis isomer and the stronger series is due to the more abundant s-trans isomer. 

Bufa-1,3-diene is one of many examples of molecules in which torsional motion may convert a sfable isomer info anofher, less sfable, isomer. The more sfable isomer in fhis case is fhe s-trans form, shown in Figure 6.44(e), and fhe less sfable one is fhe s-cis form, ... 

Figure 6.44 Torsional vibrations in (a) toluene, (b) phenol, (c) ethylene, (d) methyl alcohol, (e) s-lraws-buta-l,3-diene and (1) s-cis-buta-l,3-diene...

Conformation. The exact conformation of the isoprene molecule is stiU in doubt. It is generally accepted that rotation is restricted around the central C—C single bond. Isoprene may be considered as an equiHbrium of two conformations, namely a cisoid s-cis) conformation in which both vinyl groups are located on the same side of the C—C bond, and a transoid s-trans) one with the vinyl groups located on the opposite sides of the bond. The predominance of the trans-planar or nonplanar configuration has been supported by experimental data (10—14). 

Butadiene, the simplest conjugated diene, has been the subject of intensive theoretical and experimental studies to understand its physical and chemical properties. The conjugation of the double bonds makes it 15 kJ/mole (3.6 kcal/mol) (13) more thermodynamically stable than a molecule with two isolated single bonds. The r-trans isomer, often called the trans form, is more stable than the s-cis form at room temperature. Although there is a 20 kJ/mole (4.8 kcal/mol) rotational barrier (14,15), rapid equiUbrium allows reactions to take place with either the s-cis or r-trans form (16,17). 

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 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 ... 

When larger alicj i groups are substituted for methyl, the mole fraction of the s-cis form progressively increases as the size of the alkyl group increases. ... 

An unfavorable methyl-methyl interaction destabilizes the s-trans conformation of 4-methyl-3-penten-2-one relative to the s-cis conformation, and the equilibrium favors the s-cis form. 

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