Diene relative energies

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

Examine conformational energy profiles for Z-penta-1,3-diene and E,E-hexa-2,4-diene together with transition-state geometries for cycloadditions with TCNE (Z-penta-1,3-diene+TCNE and E,E-hexa-2,4-diene+TCNE, respectively). Predict the rates of Diels-Alder reactions involving these two dienes, relative to that for cycloaddition of E-penta-1,3-diene with TCNE. 

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]. 

Table 21. Computed relative energies of cis and trans diene systems...

Thus, the observations that (a) dienes quench the photoreduction reaction (b) the isomerization and dimerization of dienes is sensitized by the it,n carbonyl triplet and (c) there is a lack of photocycloaddition products with dienes, taken in conjunction with the relative energy levels of carbonyl compounds and dienes, form a consistent picture. 

Sensitizers whose triplet energies are less than 60 kcal mole 1 are of lower energy than the butadiene r-trans triplet and they must therefore seek out an r-cis diene (triplet energy of r-cis butadiene is 53 kcal mole-1) before they can transfer their energy. The biradical produced by attack of an. f-cis triplet on an r-trans ground-state molecule can form a cyclohexene without bond rotation (see Scheme 2), and thus the relative amount of cyclohexene increases. When the... 

The most important stabilizing interaction of the transition states of Diels-Alder reactions with inverse electron demand is due to the second term of Equation 15.3. In this case, the denominator of the second term is substantially smaller than that of the first term. This is because the HOMO of an electron-rich dienophile is closer to the LUMO of an electron-poor diene than is the HOMO of the same diene relative to the LUMO of the same dienophile (Figure 15.24, column 4). We saw the reason for this previously acceptors lower the energies of all 7F-type MOs donors increase these energies. 

Relative energies of conjugated, isolated, and cumulated dienes compared with alkynes, based on molar heats of hydrogenation. 

The molar heats of hydrogenation and the relative energies of cyclohexene, cyclohexa-1,4-diene, cyclohexa-1,3-diene, and benzene. The dashed lines represent the energies that would be predicted if every double bond had the same energy as the double bond in cyclohexene. 

Earlier in the chapter you saw the epoxidation of a diene to give a monoepoxide only one of the double bonds reacted. This is quite a usual observation dienes are more nucleophilic than isolated alkenes. This is easy to explain by looking at the relative energy of the HOMO of an alkene and a diene—this discussion is on p. 168 of Chapter 7. Dienes are therefore very susceptible to protonation... 

Dipolar cycloaddition reactions are generally classified into three types, dipole HO controlled, dipole LU controlled or HO,LU controlled, depending upon the relative energies of the dipole and dipolarophile frontier molecular orbitals. If the energy gap separating the dipole HOMO from the dipolarophile LUMO is smaller than that between the dipole LUMO and the dipolarophile HOMO, then the reaction is said to be dipole HO controlled. If the dipole LUMO-dipolarophile HOMO energy gap is smaller, then dipole LU control prevails. If the energy difference between the dipole HOMO and the dipolarophile LUMO is about the same as that between the dipole LUMO and the dipolarophile HOMO, dien neither interaction dominates and HO,LU control is operable. 

Both the geometrical characteristics of the transition states (attack directionality or pyramidalization of the reacting rites) and the structure of the reactive conformers can be altered by changing reaction conditions so that the relative energy level of the transition states, and therefore the stereoselection, can be changed. A classic example of the effect of reaction conditions is the difference in stereoselectivity of the Diels-Alder reaction of acrylates derivatives with dienes when performed thermally or in the presence of Lewis acids [71-74] (Figure 1.17) ( 9.3). In the thermal reaction, several conformers of the dienophile can intervene, while in the presence of Lewis acids, only the s-trans conformer of the ester takes part to the reaction. 

Fig. 10.5. Coefficients and relative energies of dienophile and diene frontier MOs. Orbital energies are given in eV. The sizes of the circles give a relative indication of die orbital coefficient. Z stands for a conjugated EWG, e.g., C=0, C=N. NO2 C is a conjugated substituent widiout strong electronic effect, e.g., phenyl, vinyl X is a conjugated ERG, e.g., OCH3, NH2. From J. Am. Chem. Soc., 95, 4092 (1973).

As you saw with allyl systems, the presence of conjugation leads to slabilizniion. The result is lower energy for conjugated dienes relative to the others. Again, both resonance and molecular-orbital explanations are applicable. 

IS. Conformational Isomerism of Diene Systems Table 19. Computed relative energies of the conformers of 1,3-butadiene... 

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