Dienes skewed

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. 

We can collect here two classes of molecules, characterized by very large skew angles, between 40° and 90°. Such a situation is found in the first instance in heteroannular dienes. 

Dienes in quasi-s-fraws conformation are found only in cyclic structures where perfect planarity is hindered. The DR also holds valid for this kind of conformation, as demonstrated by the considerations of Section II.D.l.a and also confirmed by all the reported calculations. Indeed, contrary to what is sometimes found for cisoid systems, the rotational strength evaluated by many types of calculation is invariably found to follow the diene rule for transoid systems. However, very small skew angles are usually found in real molecules and this implies that the main contribution to the observed optical activity cannot come from the weak intrinsic distortion, but is more likely to stem from the dissymmetric perturbations, notably of the allylic axial substituents. 

We consider it rather reasonable to suppose that when the distortion is small the n-electron system is delocalized, justifying the diene-picture whereas in the presence of large skew angles the contribution of localized double bonds can be much more important, supporting the olefin-picture. 

The skewed reagent ratios are based on an earlier observation (2) that higher yields of Diels-Adler adducts were obtained when the diene/ene reagent ratio consistently kept low during the course of the reaction. 

The reaction of chlorosulfonyl isocyanate with (7 )-l-isobutenylidene-2-phenylcyclopropane affords a mixture of optically active adducts 21-24, which possess the 1,2-dialkylidene structure and can be separated by column chromatography. The enantiomeric purity was determined by H-NMR techniques. In particular the adducts 21 and 22 contain a chiral, skewed diene system, and further addition of 4-phcnyl-3//-1,2,4-triazole-3,5(4W)-dionc produces a cycloadduct 25 or 26 with a stereogenic center, although with some loss of enantiomeric purity13. 

At low photochemic conversions the reversible transformation to the (2D hexatriene may dominate. However, the composition of the photostationary state depends upon the substitution pattern of the cyclo-hexadiene which controls the preferred conformation. It is believed that a planar 1,3-cyclohexadiene produces preferentially a bicyclo[2.2.0]hex-2-ene, e.g. molecules of the type (374), while dienes with skewed structures form hexatrienes. Another factor that changes dramatically the composition of the photolysis mixture is the wavelength of irradiation. At 254 nm the photostationary state mixture of cis-bicyclo[4.3.0]nona-2,4-diene (58) and ( ,Z,Z)-l,3,5-cyclononatriene (59) is 40% and 60%, respectively. At 300 nm, irreversible formatitm of tricyclo[4.i0.0 ]non-3-ene (374) becomes the preferred pathway. The ratio of extinction coefficients of (58) and (59) at the wavelength used would explain the shift in the photoequilibrium mixture. ... 

A dramatic example of prevalence of y-alkylation over a-alkylation is provided by cyanoethylation of 3)S-acetoxy-5a-pregn-17-en-21-al (186) with acrylonitrile in benzene (see Part II, Chapter 1, refs. 213—216). The signs of the Cotton effects exhibited by the isomeric homoannular dienes (187) produced in this reaction violate the helicity rule for skewed dienes. ... 

The rationalization for the formation of adduct 16 was somewhat more complex. Isomerization of (Z,Z)-hexadiene to ( ,Z)-hexadiene could be discounted since the cycloaddition of the ( ,Z)-diene and 9 led to the formation of sulfur epimer 15 exclusively. Assuming the reactive conformation of the (Z,Z)-diene is cisoid, only a nonplanar, helical skew arrangement can be achieved [Eq. (12)]. 

This is regarded as evidence that, of two different biradicals formed from cisoid and transold butadiene reacting with Cl2C=CF2, the latter gives only 1,2-adduct and the former collapses to 1,2- and 1,4-adducts in a ratio not dependent on temperature . However, more attention should probably be paid to the skewed conformations of dienes - . As (1,2-adduct/1,4-adduct) ratios for addition of Cl2C=CF2 to dienes fixed in non-lransoid conformation vary from less than 0.2 for cyclopentadiene to more than 99 for 1,2-dimethyl-enecyclobutane, Bartlett et have suggested that the distance between... 

The absolute configuration of aldehydes 10.2 and 10.10 was established by hydride reduction to diol 10.13 which showed a positive CD curve and thus a positive skewness of the 2(9),7(8)-diene. 

The case of a,/3-unsaturated carbonyl compounds is analogous to that of 1,3-dienes, in that electronic considerations favor coplanarity of the atoms of the C=C—C=0 system. The rotamers that appear important are the s-trans and s-cis conformations. Skew forms, unlike the case of 1,3-dienes, do not appear to represent potential energy minima. The torsional and van der Waals interactions that destabilize the s-cis conformation of 1,3-dienes seem, therefore, to be less important in a,j8-unsaturated carbonyl compounds. Microwave data indicate that the s-trans form is the only conformation present in acrolein in detectable amounts. ... 

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