Dienes distorted

Proper selection of plastic matrix fire retardants and property enhancers offers acceptable combinations of impact properties and heat-distortion temperature (HDT) values for fire retardant plastics. This can be demonstrated by fire retardant styrenics. Fire retardant enhancers have special interest as property enhancers for example the addition of a highly flammable material such as ethylene propylene diene terpolyer (EPDM), dramatically improves the bromine efficiency of octabromodiphenyl oxide in ABS by increasing char-forming without changing the Sb-Br reaction. 

A pyridine derivative related to dien with respect to the number and distribution of N-donor atoms, namely bis(2-pyridylmethyl)amine (bpma), also gives comparable complexes with Cd, e.g., [Cd(bpma)2](C104)2 with potentially three isomers (including a pair of enantiomers). As shown by the structure analysis (C2/c, Z = 4), a distorted octahedral fac-isomer with symmetry 2 —C2 has been isolated, necessarily with both enantiomers in the crystal lattice. No significant difference in the two kinds of Cd—N bonds (rav(Cd—N) 235.0 pm) is observed.189 Solid-state 13C NMR spectra of this complex and related Mn and Zn complexes have been discussed. 

The first correlation between the sign of the CD band allied to the lA —> 1B transition and the molecular chirality was formulated just taking into account the lowering of symmetry of distorted dienes only9 15. 

The diene chirality rule (hereafter referred to as DR) constitutes a simple tool for correlating the sign of the lowest energy tt —> n transition (] A —> 1B in C2 symmetry) of the distorted diene to the chirality (left or right-handed) of the chromophore. The validity of this rule is based on the assumption that the CD of the distorted chromophore is determined by its intrinsic helicity alone and that external dissymmetric perturbations have only minor effects on the optical activity. 

This point of view finds its justification in the following observations. Compounds 8 (pyrocalciferol) and 9 (isopyrocalciferol), having opposite absolute configurations of the stereogenic centres near the dienes, show lA —> 1B Cotton effects at about 275 nm of the same sign and intensity. The reason for this is that only the twist of the chromophore determines the optical activity in fact the diene moieties are distorted in the same sense in 8 and 9, as found by X-ray diffraction16. 

The diene rule has had much importance in the interpretation of the optical activity of distorted dienes, as in the case of occidentalol, epi-occidentalol and fraws-occidentalol, 10, 11 and 12, respectively17. 

The problem of interpreting the chiroptical properties of highly distorted dienes remains, however, a different question (see Section II.D.3 below). 

Another, very notable, case where the two definitions are in conflict is that of het-eroannular cisoid dienes. As we have mentioned, this was just the class of molecules that stimulated the introduction of the AAR. Here, in order to have the correct results one should refer the chirality of the axial substituent to the individual double bonds (olefin-picture), as depicted in Figure 6 and in the upper parts of Figure 7(b) and (c). The case of heteroannular dienes is anyway peculiar, because in these compounds the chromophore is unusually distorted. This case is treated in the following section. 

The authors pointed out that for M-distorted dienes a positive CD is found, while the P-distorted dienes show a negative CD, supporting the above correlation. 

Other interesting examples of highly distorted dienes are 29 and 30. In these cases there is a further difficulty. For 3039, for instance, two low-energy conformations in equilibrium are possible These two conformations are differentiated essentially by the interaction between the methylidene groups and the equatorial and axial hydrogens on the ring. 

A peculiar case of a strongly distorted diene is represented by cw,fraws-l,3-cycloocta-diene40 ... 

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. 

The two representations lead to the same result in the majority of the cases studied so far, where agreement with experiment is rather good and there are only few exceptions. The cases in which the two pictures are in conflict can be divided into two classes strongly versus normally distorted dienes (0 + 30°). The former is typified by the heteroannular dienes, which seem to obey the olefin-picture . The latter, much less well characterized, seems to be better interpreted in terms of the diene-picture . 

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. 

In both cyclic and acyclic dienes which can achieve the necessary geometry the [1, 5] shift is commonly observed because the activation energy is lowest for the transition states involving minimal distortion. This is particularly so in cyclopentadienes and indenes. The preference for [1, 5] over [1, 3] shifts is demonstrated by thermal rearrangement of 1-duterioindene. At 200°C, deuterium became scrambled over all three non-benzenoid carbons. 

Pyramidalization is thought not to be relevant to the different face selectivity of 4 + 2 cycloaddition reactions of diene systems exocyclic to the bicyclo[2,2,l]heptane structure, because selectivity persists for additions to dienes which are not significantly distorted (Gallucci et al., 1985). 

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