Planar dienes

The two most stable conformations of conjugated dienes are the s cis and s trans The s trans conformation is normally more stable than the s cis Both conformations are planar which allows the p orbitals to overlap to give an extended tt system... 

Diene moieties, reactive in [2 + 4] additions, can be formed from benzazetines by ring opening to azaxylylenes (Section 5.09.4.2.3). 3,4-Bis(trifluoromethyl)-l,2-dithietene is in equilibrium with hexafluorobutane-2,3-dithione, which adds alkenes to form 2,3-bis-(trifluoromethyl)-l,4-dithiins (Scheme 17 Section 5.15.2.4.6). Systems with more than two conjugated double bonds can react by [6ir + 2ir] processes, which in azepines can compete with the [47t + 27t] reaction (Scheme 18 Section 5.16.3.8.1). Oxepins prefer to react as 47t components, through their oxanorcaradiene isomer, in which the 47r-system is nearly planar (Section 5.17.2.2.5). Thiepins behave similarly (Section 5.17.2.4.4). Nonaromatic heteronins also react in orbital symmetry-controlled [4 + 2] and [8 + 2] cycloadditions (Scheme 19 Section 5.20.3.2.2). 

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. 

The Hiickel rule predicts aromaticity for the six-7c-electron cation derived from cycloheptatriene by hydride abstraction and antiaromaticity for the planar eight-rc-electron anion that would be formed by deprotonation. The cation is indeed very stable, with a P Cr+ of -1-4.7. ° Salts containing the cation can be isolated as a product of a variety of preparative procedures. On the other hand, the pK of cycloheptatriene has been estimated at 36. ° This value is similar to those of normal 1,4-dienes and does not indicate strong destabilization. Thus, the seven-membered eight-rc-electron anion is probably nonplanar. This would be similar to the situation in the nonplanar eight-rc-electron hydrocarbon, cyclooctatetraene. 

Reductive coupling reaction of fluonnated vinyl iodides or bromides has been used as a route to fluorinated dienes [246, 247, 248, 249, 250. Generally, the vinyl iodide is heated with copper metal in DMSO or DMF no 1 ntermediate perfluorovmy I-copper reagent is detected. Typical examples are shown m equations 163-165 [246, 247, 249. The X-ray crystal structure of perfluorotetracyclobutacyclooctatetraene, prepared via coupling of tetrafluoro-l,2-diiodocyclobutene with copper, is planar... 

The absolute configuration of the cycloaddition product obtained by the reaction of ketones with activated dienes catalyzed by (S)-t-Bu-BOX-Cu(II) (S)-21b points also to an intermediate in which the geometry around the central copper atom is square-planar similar to 26 above, and that the diene approaches the carbonyl functionality in an endo fashion. 

Benzo[fl]- (a), benzo[fc]- (b) and benzo[c]flnorenes (c) bearing a diene group (93) in spiro geometry are three possible combinatorial isomers wherein the direction of fnsion of the naphthalene is different (Fig. 15). The n reaction centers of the diene gronps are snbject to spiro-conjngation [98, 99, 102] with the planar aromatic n system. The effect of perturbation arising from spiro-conjngation on... 

Further evidence for the importance of mesomeric ion, 166b, comes from a careful examination of the solvolytic behavior of the cyclic bromodienes, 167 (153). In these systems, the planarity of the diene double bonds precludes... 

X-ray structure analysis showed that macrocycle 57 was essentially planar, with the twist angle of the benzene rings from the plane of the macrocycle being less than 2°. Most of the strain was seemingly contained in the triple bonds, as these were bent from linearity by 10.1° to 12.3°. Despite its strained nature, the macrocycle showed remarkable stability. Decomposition occurred above 300°C on attempted melting. No reaction was observed between 57 and cyclopenta-diene at room temperature. 

Diels-Alder reactions are found to be little influenced by the introduction of radicals (cf. p. 300), or by changes in the polarity of the solvent they are thus unlikely to involve either radical or ion pair intermediates. They are found to proceed stereoselectively SYN with respect both to the diene and to the dienophile, and are believed to take place via a concerted pathway in which bond-formation and bond-breaking occur more or less simultaneously, though not necessarily to the same extent, in the transition state. This cyclic transition state is a planar, aromatic type, with consequent stabilisation because of the cyclic overlap that can occur between the six p orbitals of the constituent diene and dienophile. Such pericyclic reactions are considered further below (p. 341). 

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. 

While SN4 and STD exhibit essentially D2d symmetry, SN3 has Cs symmetry with a planar diene ring and an envelope-shaped cyclopentene ring. The maximum torsion in the folded ring of SN3 is 20.2°. The spiro-connection of two five-membered ring systems leads to some strain at the spiroatom (101.4° to 101.8° at A1 compared to 109.5° for tetrahedral... 

The possibility of delocalization of the ji electrons of the diene system across the formally a bond between carbon atoms 2 and 3 forces the molecule to be planar, thus we have two limiting conformations6 ... 

In molecules like 2, or 3, the 1,3-diene chromophore is planar. Formally, the presence of R differentiates the two vertical halves of the molecule, which becomes chiral10. The physical meaning of this differentiation is that it induces dissymmetric vibrations, which determine, in turn, a dynamic twist of the chromophore. 

In 4 and 5 the chromophore is planar and the optical activity arises from the lack of a vertical symmetry plane (i.e. that bisecting the diene moiety), owing to the presence of the Ci—CH3 bond, which has no counterpart in the other half of the molecule11. 

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. 

A few examples will illustrate the case. The parent trans-diene derivatives 31a and 3235 have nearly planar chromophores, but the Cotton effects are quite strong and opposite in sign (+15 and —27.9, respectively). This can be attributed mainly to the allylic axial C—CH3 bonds, which provide a positive contribution for compounds 31 and a negative for 32. Furthermore, the As values of P-chiral s-trans-31 are strongly dependent on the polarizability of the allylic C—X bond. 

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