Page 1113 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

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CHAPTER 12 π * 2 AA π * 1 AS σ * 2 AA σ * 1 SA
Photochemistry

π 2 SA π 1 SS σ 2 AS σ 1 SS
two ground state ethenes cyclobutane

Fig. 12.17. Orbital correlation for two ground state ethenes and cyclobutane. The
symmetry designations apply, respectively, to the horizontal and vertical planes for
two ethene molecules approaching one another in parallel planes.

symmetrical approach as in the thermal reaction, then the same array of orbitals is
involved, but the occupation of the orbitals is different: the (SS) orbital is doubly
1
occupied, but (SA) and 2 ∗ (AA) are singly occupied. The reaction is therefore
2
allowed in the sense that there is no high-energy barrier. Although the correlation
diagram illustrated in Figure 12.18 might suggest that the product would initially be
formed in an excited state, this is not necessarily the case. The concerted process can
involve a transformation of the reactant excited state to the ground state of product.
The nature of this transformation is discussed shortly.
Consideration of the HOMO-LUMO interactions also indicates that the [2 +2 ]
addition is allowed photochemically. The HOMO in this case is the excited alkene ∗
∗
orbital. The LUMO is the of the ground state alkene, and a bonding interaction is
present between both pairs of carbons where new bonds must be formed. Similarly,
the concept of aromatic transition states shows that the reaction has an antiaromatic 4
combination of basis set orbitals, which predicts an allowed photochemical reaction.
Thus, orbital symmetry considerations indicate that photochemical [2 +2 ] cycload-
dition of alkenes is feasible.

HOMO

LUMO

HOMO-LUMO combination for Aromatic Mobius
[2 + 2] photocycloaddition. basis set orbial
array for [2 + 2]
photocycloaddition

* π * AS σ * σ * SA
π 2 AA 1 2 AA 1

π 2 SA π 1 SS σ 2 AS σ 1 SS

Fig. 12.18. Orbital correlation diagram for one ground state ethene and one ethene
in an excited state. The symmetry designations apply, respectively, to the horizontal
and vertical planes for two ethene molecules approaching one another in parallel
planes.