Trans conformation stability

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... 

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. 

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]. 

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. 

Substituents may play a crucial role in the conformation of quinolizidine systems. Thus, compound 49 shows a trans-conformation 50 with all three hydroxyl groups in equatorial positions. For its diastereomer 51, a -conformation 52 was initially proposed, but the H NMR data point at the /raor-conformation 53, with axial orientation of the hydroxy substituents and presumably stabilized by an intramolecular hydrogen bond <2004T3009>. 

Chiral bis-(binaphthophosphole) (bis(BNP)) ligands have been used in the asymmetric hydroformylation of styrene. In solution, the free diphospholes display fluxional behavior. Consistent with their structure, the reaction of the bis(BNP) compounds with platinum(II) derivatives gives either cis chelate mononuclear complexes or trans phosphorus-bridged polynuclear derivatives. Coordination to platinum enhances the conformational stability of bis(BNP)s and diastereomeric complexes can be detected in solution. In the presence of SnCl2, the platinum complexes give rise to catalysts that exhibit remarkable activity in the hydroformylation of styrene. Under optimum conditions, reaction takes place with high branched selectivity (80-85%) and moderate enantio-selectivity (up to 45% ee). 

We can extend the above theoretical approach to isoconjugate molecules such as acrolein and glyoxal. Arguing as before, we conclude that the relative order of conformational stability will be gauche > tram > cis if pi nonbonded interactions dictate the preferred conformation, cis > gauche > trans if sigma interactions are dominant, and trans > gauche > cis if steric effects are the most important factor. 

Consequently, the nA-04 interaction stabilizes the trans conformation more than the cis. 

We note here that sigma conjugative interactions are also operative in CH3-CO-X systems and they stabilize the cis conformation relative to the trans conformation ... 

For example, by combining the previous statements, Bingham formulates the following rule. Electron delocalization will, therefore, stabilize trans conformations relative to the corresponding cis structures when only bonding or nonbonding molecular orbitals are occupied. Cis conformations will be stabilized (less destabilized) relative to trans when antibonding molecular orbitals are also filled. ... 

The enone system has to preferably adopt an s-cis or s-trans conformation in the transition state. Which one is favored may depend on the nature of the Lewis acid. It is generally accepted that Lewis acid complexation dramatically stabilizes the s-trans conformation204. The s-cis conformation, however, may be the more reactive conformation. The dienophile may react selectively in this conformation, if the s-trans and s-cis conformations are in equilibrium. 

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