Diels-Alder reaction molecular orbital considerations

Although mixtures are often obtained, usually one predominates, the one indicated above. This regioselectivity, in which the ortho or para product is favored over the meta, has been explained by molecular-orbital considerations.864 When X = N02, regioselectivity to give the ortho or "para product was very high at room temperature, and this method, combined with subsequent removal of the N02 (see 0-82) has been used to perform regio-selective Diels-Alder reactions.865... 

Of the 18 systems, some of which are unstable and must be generated in the reaction has been accomplished for at least 15, but not in all cases with a carbon-carbon double bond (the reaction also can be carried out with other double bonds ). Not all aUcenes undergo 1,3-dipolar addition equally well. The reaction is most successful for those that are good dienophUes in the Diels-Alder reaction (15-60). The addition is stereospecific and syn, and the mechanism is probably a one-step concerted process, as illustrated above, " largely controlled by Frontier Molecular Orbital considerations. " In-plane aromaticity has been invoked for these dipolar cycloadditions. " As expected for this type of mechanism, the rates do not vary much with changes in solvent, " although rate acceleration has been observed in ionic liquids. " Nitrile oxide cycloadditions have also been done in supercritical carbon dioxide. There are no simple rules... 

Theoretical calculations have also permitted one to understand the simultaneous increase of reactivity and selectivity in Lewis acid catalyzed Diels-Alder reactions . This has been traditionally interpreted by frontier orbital considerations through the destabilization of the dienophile s LUMO and the increase in the asymmetry of molecular orbital coefficients produced by the catalyst. Birney and Houk have correctly reproduced, at the RHF/3-21G level, the lowering of the energy barrier and the increase in the endo selectivity for the reaction between acrolein and butadiene catalyzed by BH3. They have shown that the catalytic effect leads to a more asynchronous mechanism, in which the transition state stmcture presents a large zwitterionic character. Similar results have been recently obtained, at several ab initio levels, for the reaction between sulfur dioxide and isoprene. ... 

The Diels—Alder reaction may also be analyzed by a similar consideration of the molecular orbitals of butadiene and ethene. In this case, there are two possible HOMO—LUMO interactions. Since the phases of the 1,4-lobes of the HOMO of butadiene match with those in the LUMO of ethene, the [7r" s + TT s] cycloaddition is thermally allowed. We reach a similar conclusion by considering the symmetry of LUMO of butadiene and the HOMO of ethene (Figure 4.10). However, on energetic grounds the latter interaction will make a smaller contribution than the former. 

The frontier molecular orbital analysis led to the conclusion that butadiene will react with ethylene to give cyclohexene if both butadiene and ethylene collide in a suprafacial manner. However, ethylene will not react with ethylene in an analogous manner. We can extend these conclusions to the reaction of any conjugated diene with any alkene and to any alkene with another alkene. Presented below are several examples and practical considerations of the allowed 4 + 2 reaction (the Diels-Alder reaction). The suprafacial approach of both reactants has important consequences on the stereochemistry of the Diels-Alder reaction. 

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