Conjugated unsaturated systems 1,3-butadiene

In molecules, the conjugation of unsaturated systems, i.e., possessing molecular orbitals built up from Ti-atomic orbitals (2p-orbitals in hydrocarbons benzene), is, in general, a planarity factor (e.g., butadiene). Nevertheless, steric hindering can oppose this tendency to make a planar system (e.g., biphenyl, cyclooctatetraen ). The same phenomena are encountered in crystals. 

As an example of the utility of this model, consider the norbomadiene molecule (see Fig. 12-7a). According to our understanding of unsaturated systems, the double bonds in this molecule should behave like isolated ethylene double bonds and not like the conjugated bonds of butadiene. But these bonds are at an orientation and proximity allowing significant overlap between AO lobes beneath the molecule. If we treat this system with our two-level model, the unperturbed MOs are the bonding... 

LUMO of a simple carbonyl group. The nearest thing you have met so far (in Chapter 7) are the orbitals of butadiene (C=C conjugated with C=C), which we can compare with the a,(3-unsaturated aldehyde acrolein (C=C conjugated with C=0). The orbitals in the 7t systems of butadiene and acrolein are shown here. They are different because acrolein s orbitals are perturbed (distorted) by the oxygen atom (Chapter 4). You need not be concerned with exactly how the sizes of the orbitals are worked out, but for the moment just concentrate on the shape of the LUMO, the orbital that will accept electrons when a nucleophile attacks. 

A conjugated double bond system undergoes 1,4-addition (Thiele s rule) for example, butadiene and an equimolar quantity of bromine yield 1,4-dibromo-2-butene (90%). On the other hand, chlorination of butadiene in the liquid or vapor phase furnishes about equal amounts of 1,2-and 1,4-addition products. Other polyfunctional compounds resulting from this method of preparation include dihalogenated acids, esters, aldehydes, and ketones. < The addition of bromine to unsaturated ethers yields dibromo ethers which are used as intermediates in the synthesis of olefins (method 21) and olefinic alcohols (method 99) ... 

In the polymerization of ethylene by (Tr-CjHsljTiClj/AlMejCl [111] and of butadiene by Co(acac)3/AlEt2Cl/H2 0 [87] there is evidence for bimolecular termination. The conclusions on ethylene polymerization have been questioned, however, and it has been proposed that intramolecular decomposition of the catalyst complex occurs via ionic intermediates [91], Smith and Zelmer [275] have examined several catalyst systems for ethylene polymerization and with the assumption that the rate at any time is proportional to the active site concentration ([C ]), second order catalyst decay was deduced, since 1 — [Cf] /[Cf] was linear with time. This evidence, of course, does not distinguish between chemical deactivation and physical occlusion of sites. In conjugated diene polymerization by Group VIII metal catalysts -the unsaturated polymer chain stabilizes the active centre and the copolymerization of a monoolefin which converts the growing chain from a tt to a a bonded structure is followed by a catalyst decomposition, with a reduction in rate and polymer molecular weight [88]. 

Conjugated systems, including 1,3-butadienes, diene-allenes, enyne-allenes, enediynes, dienediynes, and enediallenes, were synthesized via organoboranes. A wide array of cascade cyclization reactions involving high energy intermediates, such as o-isotoluenes, biradicals, o-quinodimethanes, benzocyclobutadienes, and enyne-ketenes, were initiated from these unsaturated compounds. 

We now show how the conjugated carbonyl group of a,j8-unsaturated aldehydes and ketones can enter into reactions that involve the entire functional system. These reactions are 1,4-additions of the type encountered with conjugated dienes, such as 1,3-butadiene (Section 14-6). The reactions proceed by acid-catalyzed, radical, or nucleophilic addition mechanisms, depending on the reagents. 

The polymerization of some conjugated dienes (butadiene, isoprene, chloroprene) by an ether-free Grignard system ((C4H9)2Mg—C4H9MgI] yielded polymers that possessed slightly lower than theoretical unsaturation (44, 56). Cyclization was postulated as having occurred in order to explain this result. 

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