Dienes molecular orbitals

Fig. 11.13. Coefficients and relative energies of dienophile and diene molecular orbitals. [From K.. N. Houk, J. Am. Chem. Soc. 95 4092 (1973).]...

Stability of Conjugated Dienes Molecular Orbital Theory 4133... 

Coefficients and relative energies of dienophile and diene molecular orbitals... 

A plot of the rate constants of the Diels-Alder reaction of dienes with maleic anhydride and tetracyanoethylene against the energy separation of the highest-occupied diene molecular orbital and lowest-unoccupied dienophile orbitals fits broadly the predicted curve, but cyclohepta-1,3-diene reacts much more slowly than predicted. ... 

The mechanism of the Diels-Alder reaction is best understood on the basis of a molecular orbital approach To understand this approach we need to take a more detailed look at the rr orbitals of alkenes and dienes... 

Let us now examine the Diels-Alder cycloaddition from a molecular orbital perspective Chemical experience such as the observation that the substituents that increase the reac tivity of a dienophile tend to be those that attract electrons suggests that electrons flow from the diene to the dienophile during the reaction Thus the orbitals to be considered are the HOMO of the diene and the LUMO of the dienophile As shown m Figure 10 11 for the case of ethylene and 1 3 butadiene the symmetry properties of the HOMO of the diene and the LUMO of the dienophile permit bond formation between the ends of the diene system and the two carbons of the dienophile double bond because the necessary orbitals overlap m phase with each other Cycloaddition of a diene and an alkene is said to be a symmetry allowed reaction... 

The Diels-Alder reaction is believed to proceed m a single step A deeper level of understanding of the bonding changes m the transition state can be obtained by examining the nodal properties of the highest occupied molecular orbital (HOMO) of the diene and the lowest unoccupied molecular orbital (LUMO) of the dienophile... 

The problems associated with predicting regioselectivity in quinone Diels-Alder chemistry have been studied, and a mechanistic model based on frontier molecular orbital theory proposed (85). In certain cases of poor regioselectivity, eg, 2-methoxy-5-methyl-l,4-ben2oquinone with alkyl-substituted dienes, the use of Lewis acid catalysts is effective (86). 

Perfluoroalkyl groups adjacent to multiple bond systems lower the frontier molecular orbitals (FMOs) Therefore, cycloaddition reactions preferentially occur with electron-rich multiple-bond systems The preference of bis(trifluoromethyl)-substituted hetero-l,3-dienes for polar reacuons makes them excellent model compounds for developing new types of diene reactions deviating from the well documented Diels-Alder scheme (pathway 1) A systematic study of the reactions of diene (1 =2-3=4)-dienophile (5=6) combinations reveals new synthetic possibilities that have not yet been fully exploited as tools for preparative organic cherrustry (equation 25)... 

Most reactions discussed can be classified into two types of [n s+iAs cycloadditions, the normal and inverse electron-demand cycloaddition reactions, based on the relative energies of the frontier molecular orbitals of the diene and the dieno-phile (Scheme 4.2) [4]. 

Conjugated enones are more stable than nonconjugated enones for the same reason that conjugated dienes are more stable than nonconjugated dienes (Section 14.1). Interaction between the tt electrons of the C=C bond and the tt electrons of the C=0 group leads to a molecular orbital description for a conjugated enone that shows an interaction of the tt electrons over all four atomic centers (Figure 23.3). 

Figure 23.3 The - bonding molecular orbitals of a conjugated erone (propenal) and a conjugated diene (1,3-butadiene) are similar in shape and are spread over the entire %< system.

Cycloaddition reactions are those in which two unsaturated molecules add together to yield a cyclic product. For example, Diels-AJder reaction between a diene (four tt electrons) and a dienophile (two tt electrons) yields a cyclohexene. Cycloadditions can take place either by suprafacial or antarafacial pathways. Suprafacial cycloaddition involves interaction between lobes on the same face of one component and on the same face of the second component. Antarafacial cycloaddition involves interaction between lobes on the same face of one component ancl on opposite faces of the other component. The reaction course in a specific case can be found by looking at the symmetry of the HOMO of one component and the lowest unoccupied molecular orbital (LUMO) of the other component. 

J 7i-Cycloadducts at the C4 — C5 azepine positions are also formed with 1,2,3,4-tetra-chloro-5,5-dimethoxycyclopentadiene260 261 and with hexachlorocyclopentadiene.261 The reactivity of ethyl l//-azepine-l-carboxylate towards cyclopentadienones has been studied in terms of frontier molecular orbital theory which predicts that dimethyl 2-oxo-4,5-diphenyl-cyclopenta-1 (5),3-diene-1,3-dicarboxylate (20) should be more reactive towards the 1/f-azepine than other more common cyclopcntadienone derivatives.262 In fact, in refluxing benzene, the cyclopentadienone and ethyl 1/f-azepine-l-carboxylate (1) form a mixture of the [4 + 2] n-endo,anti-adduct 22, produced by Cope rearrangement of the initially formed [2 + 4] 7T-adduct 21, and the c.w-adduct 23, a rare example of a [6 + 4] rc-cycloadduct. At room temperature, only the [6 + 4] adduct 23 and a small amount of the [2 + 4] adduct 21 are obtained, the latter rearranging to the [4 + 2] adduct 22 on warming.262 Other [4 + 2] 7r-adducts with cyclopentadienones have been prepared similarly.263... 

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