1,3-Butadiene, 1,2-addition reactions molecular orbitals

Butadiene is known to be a substance in which the double bonds can react simultaneously as, for example, in the Diels-Alder reaction and in 1,4 additions of halogens. In the simple molecular orbital treatment, butadiene is treated as a system with localized cr bonds and delocalized u bonds. 

According to the frontier orbital theory, the orbitals that control these reactions are the aforementioned HOMO of one reactant and the LUMO (lowest unoccupied molecular orbital) of the other reactant. So, for this reaction, we have two possible scenarios interaction between HOMO ij/2 of butadiene and LUMO (p of ethylene or that between HOMO ti of ethylene and LUMO of butadiene. As the following illustration indicates, both possibilities lead to bonding overlap between the interacting orbitals of the reactants so the reaction is allowed, as we aU know. In addition, as fotmd by theory, between these two possible scenarios, we favor the interaction between the HOMO of the electron-rich reactant (butadiene in this case) with the LUMO of the electron-poor reactant (ethylene). 

A review (37 refs.) on the application of molecular orbital calculations to the stereoselectivity of addition reactions of nitro-enitol derivatives has been published in Japanese. Cycloaddition of 1-acetoxy- or 1-trimethylsilyloxy-butadiene to l,2-dideoxy-l-nitro-D-g/ co-hept-l-enitol tetraacetate gave mainly the cyclohexane derivatives 41. Additions of alcohols, carbon radicals and phosphoiyl... 

If butadiene and an appropriately substituted ethylene approach and begin to overlap as in Equation 5.2, there is a favorable phase relationship using the HOMO of the diene and the LUMO of ethylene (the frontier molecular orbitals) for a face-to-face joining. This is, of course, the familiar Diels-Alder reaction, and it is thermally allowed. With respect to both components of the reaction, the reaction occurs from the same face, termed suprafacial addition. Because there are four % electrons in butadiene and two % electrons in ethylene, the Diels-Alder reaction is named as a [ 4 -i- 2 ] reaction. The stereochemical consequences of this approach are further illustrated in Section 8.6. 

In case of unsymmetrical 1, 3-dienes for example 2-methyl-l, 3-butadiene, i.e., isoprene, reaction same type of interactions are involved. Additional methyl group does not affects much to the coefficients of molecular orbitals. 

To explain the pericychc reaction of 1,3,5-hexatriene by frontier molecular orbital theory, we need only consider the symmetry of 713 and 714. The HOMO is 713 it is symmetric. The LUMO is 714 it is antisymmetric. Note that this order of symmetry is opposite to that of 1,3-butadiene. The symmetry of the HOMO alternates with each additional double bond. The difference in the chemistry of polyenes noted earlier in this chapter for An 71 and 4 + 2 7t systems results from this difference in symmetry for the highest energy occupied molecular orbital involved in the reaction (Figure 25.3). 

Theoretical.—Molecular orbital calculations have been reported for the thiiranium ions derived from ethylene, propylene, isobutene, and butadiene. The most stable system involves a p-configuration of the valence electrons of sulphur. Differences in the extension of the lowest unoccupied MO s on the ethylene carbon atoms were said to be important in determining the regio-specificity of ring-opening addition reactions with chloride ion. Theoretical calculations on C2H4SH indicate that a bridged thiiranium structure would be more stable as an intermediate than an open structure in electrophilic additions of RSX to olefins. The barrier to pyramidal inversion at the sulphur atom was calculated to be 78.11 kcal mol . 

The principles of conservation of orbital synunfetry can be applied to inter-molecular cycloaddition reactions and to what can be thought of as their reverse, the fragmentation of one molecule into two or more smaller fragments. Let us consider first the Diels-Alder reaction, a reaction of great synthetic usefulness as well as the subject "oT many theoretical and mechanistic speculations and studies. In its simplest form, this reaction is the addition of an olefin to a conjugated diene to give a cyclohexene. It is termed a [4 -I- 2]-cycloaddition reaction, since it involves a four-7r-electron system and a two-7r-electron system. All available data indicate that the reaction is a concerted one that proceeds stereospecifically by cis addition to the double bond. This stereospecificity is exemplified by the formation of stereoisomeric products from the reaction of butadiene with ethyl maleate and ethyl fumarate ... 

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