Suprafacial geometry

Fora [4 + 2 -7r-electron cycloaddition (Diels-Aldei reaction), let s arbitrarily select the diene LUMO and the alkene HOMO. The symmetries of the two ground-slate orbitals are such that bonding of the terminal lobes can occur with suprafacial geometry (Figure 30.9), so the Diels-Alder reaction takes place readily under thermal conditions. Note that, as with electrocyclic reactions, we need be concerned only with the terminal lobes. For purposes of prediction, interactions among the interior lobes need not be considered. 

Figure 30.10 (a) Interaction of a ground-state HOMO and a ground-state LUMO in a potential [2 - 2] cycloaddition does not occur thermally because the antarafacial geometry is too strained, (b) Interaction of an excited-state HOMO and a ground-state LUMO in a photochemical [2 r 2] cycloaddition reaction is less strained, however, and occurs with suprafacial geometry. 

Contraceptive, steroid, 1083 Cope rearrangement, 1193-1194 suprafacial geometry of, 1194 Copolymer, 1210-1212 block. 1212 graft. 1212 table of. 1211... 

The symmetries of the terminal lobes of the HOMO and LUMO of the reactants in a [4 + 2] thermal cycloaddition allow the reaction to proceed with suprafacial geometry. 

The Cope rearrangement and the Claisen rearrangement involve reorganization of an odd number of electron pairs and proceed by suprafacial geometry. 

The Diels-Alder reaction is a thermal [4 + 2] cycloaddition, which occurs with suprafacial geometry. The stereochemistry of the diene is maintained in the product. 

The reaction of cyclopentadiene and cycloheptatrienone is a [6 + 4] cycloaddition. This thermal cycloaddition proceeds with suprafacial geometry since five electron pairs are involved in the concerted process. The n electrons of the carbonyl group do not take part in the reaction. 

The observed products A and B result from a [1,5] sigmatropic hydrogen shift with suprafacial geometry, and they confirm the predictions of orbital symmetry. C and D are not formed. 

This [2,3] sigmatropic rearrangement involves three electron pairs and should occur with suprafacial geometry. 

Restricting consideration to sipafacial-suprafacial geometries, two limiting confbimations are possible for the six-membered cyclic transition state, a four-center or chair conformation with staggered allyl units which resembles chair cyclohexane, and a six-center or boat confomnation with... 

Interaction of excited-state HOMO and LUMO in photochemical [2 -L 2] cycloaddition reactions. The reaction occurs with suprafacial geometry. 

Cycloadditions are controlled by orbital symmetry (Woodward-Hoffman rules) and can take place only if the symmetry of all reactant molecular orbitals is the same as the symmetry of the product molecular orbitals. Thus, an analysis of all reactant and product orbitals is required. A useful simplification is to consider only the frontier molecular orbitals. These orbitals are the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO). The orbital symmetry must be such that bonding overlap of the terminal lobes can occur with suprafacial geometry that is, both new bonds are formed using the same face of the diene. 

For the Diels-Alder reaction, we will select the HOMO of the diene and the LUMO of the alkene. Therefore, in the diene supplies the electrons. To accommodate the electron pair, the alkene uses ti. Both orbitals are antisymmetric, and the bonding of the terminal lobes occurs if the two molecules are arranged with suprafacial geometry (Figure 25.9). Note that we have to consider the signs of the wave functions only at the terminal lobes. 

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