Torsional isomers

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, ... 

Step through the sequence of structures depicting rotation about the carbon-carbon bond in the two dibromoethane isomers l,2-dibromo-l,2-diphenylethane A andfi). For each, plot energy (vertical axis) vs. BrCCBr torsion angle (horizontal axis), and identify all minimum-energy structures. Which of these are reactive conformers , that is, conformers which are set up for either syn or anti elimination of HBr Which are non-reactive conformers , that is, which do not meet the requirements for elimination Do the reactive conformers correspond only to syn elimination, only to anti elimination, or are both pathways represented Which alkene would these reactive conformers lead to Are your results consistent with the observation that each isomer of the starting material gives only one alkene Explain. 

Useful diastereoselectivity is observed if one torsional isomer predominates for steric reasons. 

Intramolecular allylzinc aldehyde additions lead, in one synthetic step, to bicyelic a-methylene-y-lactones41. Interestingly, the (Z)-isomer gave only the r/5-fused bicyclic products, which arises from a. vyw-selectivc addition, while the ( >isomeric bromide furnished a cisjtrans mixture (yield 56%, cisjtrans 72 28)41 indicating that the double-bond torsion and the intramolecular carbonyl addition have similar rates. A similar sequence was the key step in a confertin synthesis28. 

The trisulfane molecule exists as two conformers which have been termed as cis- and trans-HzSi. While the trans-form is a helical molecule of C2 symmetry with the motif ++ (or — for the enantiomer), the cfs-form is of Q symmetry with the motif +- (identical to -+). Both forms have been detected by rotational spectroscopy [17, 45, 46]. The motif gives the order of the signs of the torsion angles at the SS bonds. The geometrical parameters [17] are presented in Table 4. The trans-isomer is by only 1 kj mol more stable than the cfs-form but the barrier to internal rotation from tmns to cis is 35 kJ mor [46]. The dipole moments were calculated by ab initio MO theory at the QCISD/TZ+P level as 0.68 D (trans) and 2.02 D (cis) [46]. For geometrical parameters of cis- and trans-trisulfane calculated at the MP2/6-311++G> > level, see [34]. 

Fig. 2 Three conformational isomers of the tetrasulfane molecule H2S4 differing by the signs of the torsional angles at the three SS bonds (after [47])...

Isomers are, of course, possible when the equatorial ligand lacks a plane of symmetry, as in the corrinoids [see (HI)]. All the acetamide side chains project to one side of the corrin ring, which we shall call the upper side, and all the propionamide side chains and the nucleotide side chain to the lower side. Isomers are then theoretically possible whenever the two axial ligands are different, and their existence has been shown experimentally for corrinoids where one axial ligand is CN , Me, or Et and the other is H2O or is absent [for further details see Section 8.2 of ref. (136)]. Salen and BAE also show minor deviations from planarity due to the bending of the two halves (mentioned above in Section II,B,2) and to torsion about the C—C bonds in the ethylene bridge (see references in Table I), but these are not expected to give rise to separable isomers. 

With less hindered hydride donors, particularly NaBH4 and LiAlH4, confor-mationally biased cyclohexanones give predominantly the equatorial alcohol, which is normally the more stable of the two isomers. However, hydride reductions are exothermic reactions with low activation energies. The TS should resemble starting ketone, so product stability should not control the stereoselectivity. A major factor in the preference for the equatorial isomer is the torsional strain that develops in the formation of the axial alcohol.117... 

Figure 7 Plot of energy vs torsion angle from an energy profiling study resulting from rotating the oxazoline ring of the 5 isomer of WIN52084 about the phenyl ring.

The product ratio of exo-halide 28 and endo-isomer 29 was largest for large halogen transfer agents XY. XY apparently approaches 2 7 preferentially from the less shielded exo-side. The torsional effect5discussed before is probably also of importance. Similar results were obtained more recently for the transfer of hydroxy groups from peracids to 2763). 

In Table 7 the six-membered monocyclic dienes are represented by the conjugated 1,3-cyclohexadiene and its isomer 1,4-cyclohexadiene. 1,3-Cyclohexadiene has a nonplanar equilibrium conformation that is primarily influenced by three factors -electron interaction (optimal for a planar conformation) angle strain and torsion strain (both optimal for a planar conformation). The reduced overlap between the two --orbital systems is, for the observed C=C—C=C angle of 18°, estimated at ca 10% and should therefore not influence the conjugation stabilization drastically, compared to a conformation with coplanar C=C bonds. 

Figure 4.83 Optimized conformers of W2H10, showing (a) staggered (

An alternative approach for determining the relative pi electronic stabilization of two torsional isomers utilizes a molecular dissection into two open shell radical fragments. This approach is illustrated by examining torsional isomerism in butadiene and 1,3,5-hexatriene. The ir MO s of the conformational isomers of 1,3,5-hexatriene can be constructed from the union of the n MO s of two formal allyl radicals. The two regiochemical modes of union of interest will be designated cis and tram ... 

Table 10. Relative stability of the torsional isomers of YCH2CH2X molecules...

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