S-trans diene

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 ... 

In the third sequence, the diastereomer with a /i-epoxide at the C2-C3 site was targeted (compound 1, Scheme 6). As we have seen, intermediate 11 is not a viable starting substrate to achieve this objective because it rests comfortably in a conformation that enforces a peripheral attack by an oxidant to give the undesired C2-C3 epoxide (Scheme 4). If, on the other hand, the exocyclic methylene at C-5 was to be introduced before the oxidation reaction, then given the known preference for an s-trans diene conformation, conformer 18a (Scheme 6) would be more populated at equilibrium. The A2 3 olefin diastereoface that is interior and hindered in the context of 18b is exterior and accessible in 18a. Subjection of intermediate 11 to the established three-step olefination sequence gives intermediate 18 in 54% overall yield. On the basis of the rationale put forth above, 18 should exist mainly in conformation 18a. Selective epoxidation of the C2-C3 enone double bond with potassium tm-butylperoxide furnishes a 4 1 mixture of diastereomeric epoxides favoring the desired isomer 19 19 arises from a peripheral attack on the enone double bond by er/-butylper-oxide, and it is easily purified by crystallization. A second peripheral attack on the ketone function of 19 by dimethylsulfonium methylide gives intermediate 20 exclusively, in a yield of 69%. 

FIGURE 8. Sector rules for exocyclic s-trans dienes. Top bond-centred , bottom atom-centred rule. The ellipsoid represents the rest of the molecule and also defines the direction opposite to the observer. In both cases two planes are defined by the transition dipole and the plane containing the diene only the third plane is located differently. The bond-centred rule holds for cyclohexylidene compounds, while the atom-centred rule should apply to adamantylidene derivatives... 

The crystal structures of a number of s-trans (diene)metal complexes (3) have been determined10-14. The diene ligand in all s-trans complexes is distinctly non-planar the torsional angle between the two olefin groups is between 114° and 127°. In general, the terminal carbon to metal distance is greater than for the internal carbon to metal distance, and the Cl—C2/C3—C4 bonds are shorter than the C2—C3 bond. 

The reaction < —40 °C of svw-vinyl substituted acyclic (/p-allyljlVKCOjjCp complexes 57 (M = Mo, W Cp = Cp, Cp ) with CF3CO2H or a mixture of BF3 and an aldehyde generates the s-trans (diene)M(CO)2Cp/ cations 58a or 58b respectively, which may be isolated by precipitation from ether (Scheme 15)30,88. At higher temperature... 

The preparation and characterization of several octahedral Ru(II) complexes containing s-trans coordinated dienes have been reported. The Zn mediated reduction of Ru(acac)3 in the presence of a 1,3-diene affords (diene)Ru(acac)2 complexes as a mixture of diastereo-mers (eg. 129)13a b. Reaction of [(trispyrazolylborate)RuCl]jt or [(NH3)4Ru(acetone)2]2+ [CIO4 ]2 with acyclic dienes yields complex 130 or cation 131 respectively130,14. Coordination of the ligand as an s-trans diene was indicated either by crystal structure or by determining C2v symmetry on the basis of NMR spectroscopy. 

The application of exciton coupling between the benzoate and the s-trans-diene or -enoate chromophore in axially dissymmetric molecules derived from hydroxy-substituted adaman-tanone allowed determination of not only the absolute but also the relative configuration. All benzoates 2-5 of 4R configuration show negative exciton Cotton effects, with amplitudes lower for (2 )-adamantylidene compounds 2, 4 compared with 2Z-isomers 3, 5, in which the two interacting chromophores are at a closer distance143. 

Butadiene, the parent conjugated diene, can in principle attain two planar conformations, namely s-frans-butadiene and. v-m-butadiene. In reality, the majority of the acyclic 1,3-butadiene derivatives exhibit global conformational minima that are at least close to the s-trans-diene situation.1,2 For butadiene itself the s-trans-C4H6 conformer is more stable than the i-cw-isomer by ca. 3 4 kcal mol-1, although the s-trans- - s-cis-butadiene interconversion is kinetically rapid (AG 7 kcal mol-1). Consequently, reactions via the less favorable conformations are not uncommon (e.g., the Diels-Alder reaction) (Scheme 1). 

The [(s-trans-diene)ZrCp2] complex (s-trans-1) equilibrates with the [(s-cA-diene)ZrCp2] isomer (x-cA-l) via a reactive high lying (r 2-butadiene) metallocene intermediate (2) [A(s-trans-1 s-cis-l, 283 K) = 22.7 0.3 kcal mol-1]. Syntheses of the (butadiene)zirconocene system carried out under kinetic control invariably led to pure s-trans-1, whereas a ca. 1 1 equilibrium of s-trans-1 and. v-ci.v-l was obtained under conditions of thermodynamic control.5,6 The cr,7i-structured s-cis-l isomer undergoes a dynamic ring-flip automerization process (see Scheme 2) that is rapid on the NMR time scale [AG futom = 12.6 0.5 kcal mol ].5... 

Green has also used the method of H+ addition to a -CHO-substituted (allyl)metal complex for the preparation of (s-trans-diene) complex derivatives of a Group 8 metal. The Ru complex 35 was synthesized in that way.33 Stable [(s-tra ,s-r 4-conjugated diene) Ru(acac)2] isomers have been described34 and related [(s-tra s-r 4-diene)Ru(trispyrazolylbo-rate)Cl] systems are stable and isolable.35 Eventually, Mashima et al. have described the reaction of all-tra ,s-l,8-diphenyloctatetraene with [Ru(acac)3] under reducing conditions. A dimetallic complex (36) was isolated and characterized by X-ray diffraction, that contained a linear array of two (s-tra ,s-r 4-diene)Ru subunits36 (see Scheme 10). 

In order to undergo Diels-Alder reaction, this s-trans diene would have to rotate to an s-cis arrangement. In an s-cis conformation, however, the two circled methyl groups experience steric strain by being too close to each other, preventing the molecule from adopting this conformation. Thus, Diels-Alder reaction doesn t occur. 

Schleyer proposed one last alternative method for ganging ASE. Noting that many of these better methods (especially those analogons to Reaction 3.24) require computation of many compounds, he developed the isomerization stabilization energy (ISE) method, particularly useful for strained aromatic systems. ISE measures the energy realized when an isomeric compound converts into its aromatic analog. Benzene itself cannot be analyzed by the ISE method, however, toluene can, and the ASE values of toluene and benzene are expected to be quite similar. The conversions of two different isomers into toluene provide the ISE for toluene (Reactions 3.27a and 3.27b). Both of these reactions do not conserve s-cis/s-trans diene conformations. Reaction 3.28 can be added once to Reaction 3.27a and twice to Reaction 3.27b to give the corrected ISE values of -32.0 and -28.9 kcal mol , respectively. 

In addition, the experimental observation of (.s-tra/u-i7 -diene)metal-locene complexes is kinetically facilitated by the special appearance of the energy prohle of the dienemetallocene system. For the parent system there is evidence (27-31) that the stable (butadiene)ZrCp2 isomers [about equal amounts of (s-trans- (3a) and (s-cu-T -butadiene)zirconocene (Sa) are obtained under equilibrium conditions (22, 23)] are connected through a reactive (T7 -butadiene)zirconocene intermediate (4a) which rapidly equilibrates with s-trans diene complex 3a, but is separated from the (s-cis-diene)metalIocene isomer Sa by a rather substantial activation barrier (Fig. 1). 

Conformation and chiroptical properties of dienes and polyenes 2. Planar s-trans-dienes... 

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