S-trans Conformations

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... 

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 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... 

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 ... 

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. 

Examine the models 1,3-butadiene in Figure 10.6 on/.earning By Modeling and compare space-filling models of the s-cis and s-trans conformation. 

One interesting phenomenon was the effect of the boron substituent on enantioselectivity. The stereochemistry of the reaction of a-substituted a,/ -unsatu-rated aldehydes was completely independent of the steric features of the boron substituents, probably because of a preference for the s-trans conformation in the transition state in all cases. On the other hand, the stereochemistry of the reaction of cyclopentadiene with a-unsubstituted a,/ -unsaturated aldehydes was dramatically reversed on altering the structure of the boron substituents, because the stable conformation changed from s-cis to s-trans, resulting in production of the opposite enantiomer. It should be noted that selective cycloadditions of a-unsubsti-tuted a,/ -unsaturated aldehydes are rarer than those of a-substituted a,/ -unsatu-... 

The diene must adopt what is called an s-cis conformation, meaning "cis-like" about the single bond, to undergo a Diels-Alder reaction. Only in the s-cis conformation are carbons 1 and 4 of the diene close enough to react through a cyclic transition state. In the alternative s-trans conformation, the ends of the diene partner are too far apart to overlap with the dienophile p orbitals. 

Which of the following dienes have an s-cis conformation, and which have an s-trans conformation Of the s-trans dienes, which can readily rotate to s-cis ... 

We assume that the double bonds in 1,3-butadiene would be the same as in ethylene if they did not interact with one another. Introduction of the known geometry of 1,3-butadiene in the s-trans conformation and the monopole charge of 0.49 e on each carbon yields an interaction energy <5 — 0.48 ev between the two double bonds. Simpson found the empirical value <5 = 1.91 ev from his assumption that only a London interaction was present. Hence it appears that only a small part of the interaction between double bonds in 1,3-butadiene is a London type of second-order electrical effect and the larger part is a conjugation or resonance associated with the structure with a double bond in the central position. 

For reactions of A-acyliminium ions with alkenes and alkynes one has to distinguish between A-acyliminium ions locked in an s-trans conformation and those which (can) adopt an s-cis conformation. The former type reacts as a (nitrogen stabilized) carbocation with a C —C multiple bond. Although there are some exceptions, the intramolecular reaction of this type is regarded as an anti addition to the 7t-nucleophile, with (nearly) synchronous bond formation, the conformation of the transition state determining the product configuration. 

Simple 1,3-dienes also undergo a thermal monocyclopropanation reaction with methoxy(alkyl)- and methoxy(aryl)carbene complexes of molybdenum and chromium [27]. The most complete study was carried out by Harvey and Lund and they showed that this process occurs with high levels of both regio-and diastereoselectivity. The chemical yield is significantly higher with molybdenum complexes [27a] (Scheme 7). Tri- and tetrasubstituted 1,3-dienes and 3-methylenecyclohexene (diene locked in an s-trans conformation) fail to react [28]. The monocyclopropanation of electronically neutral 1,3-dienes with non-heteroatom-stabilised carbene complexes has also been described [29]. 

Vinylindoles have been studied extensively and used in the synthesis of carbazoles, alkaloids and other classes of pharmacologically active compounds. MMX force field calculations have shown that coplanar s-cis and. s-trans conformations of 3-vinylindole (84, Figure 2.11) are the most stable conformers they exhibit only slight differences in their thermodynamic stabilities [86]. 

The enantioselectivity was explained by the preference for a transition state 28, in which the diene approaches the face of the coordinated dienophile, which adopts an s-trans conformation (Fig. 14). 

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. 

The reactivity of diazo carbonyl compounds appears to be related to the conformational equilibria between s-cis and s-trans conformations. A concerted rearrangement is favored by the s-cis conformation.237 The /-butyl compound 19, which exists in the s-trans conformation, gives very little di-/-butylketene on photolysis.238 A similarly... 

Since energy transfer from high triplet energy donors to both s-tram and s-cis conformations should be diffusion controlled, and the piperylene is more than 95% in the s-trans conformation at room temperature, the role of the c-T and c-C isomers can be neglected to a first approximation. 

Gawronski et al.21 have used NMR spectroscopy, especially NOE experiments in order to assign conformation of the chiral calixsalen-type macrocycles, products from the [3 + 3] cyclocondensation of the trans-1,2-diaminocyclohexane with hydroxydialdehydes. It was shown that the macrocycles had C3-symmetrical structure and s-syn conformation of imine C—H/cyclohexane axial C—H bond systems and s-trans conformation of the bis-imine unit. 

There are two possible planar conformations of 1,3-butadiene the s-cis and the s-trans conformations. 

The s-trans conformation of 1,3-butadiene is the predominant one at room temperature. 

The observation that the overwhelming product from the cycloaddition of 3 to 1,3-butadiene (12) is a cyclobutane derivative 31 and the proportion of the [4 -I- 2] adduct increases in the order 12 < 26 6 is in accord with the increasing diene reactivity in this series. Whereas cyclopentadiene readily combines with most dienophiles at low temperatures, 1,3-butadiene, mainly owing to its predominant s-trans conformation, enters into [4 + 2] cycloadditions only at elevated temperatures. 

The ground state structure of butadiene has been extensively studied using different kinds of theoretical methods19,21,23,31,34,36. For this molecule, several conformations associated with rotation around the single C—C bond are possible. Experimental evidence shows that the most stable one is the planar s-trans conformation. All theoretical calculations agree with this fact. 

Furthermore, whenever the 1,3-diene group is not embedded in a rigid structure, e.g. in 612 and 713, it assumes the most stable s-trans conformation, just as in the case of 1,3-butadiene. Again dissymmetrically disposed substituents perturb the n —> it transition, which acquires some magnetic moment parallel to the electric one. 

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