Cyclobutanation theoretical considerations

The photochemistry of alkenes, dienes, and conjugated polyenes in relation to orbital symmetry relationships has been the subject of extensive experimental and theoretical studyThe analysis of concerted pericyclic reactions by the principles of orbital symmetry leads to a complementary relationship between photochemical and thermal reactions. A process that is forbidden thermally is allowed photochemically and vice versa. The complementary relationship between thermal and photochemical reactions can be illustrated by considering some of the reaction types discussed in Chapter 10 and applying orbital symmetry considerations to the photochemical mode of reaction. The case of [2Tr- -2Tr] cycloaddition of two alkenes, which was classified as a forbidden thermal reaction (see Section 10.1), can serve as an example. The correlation diagram (Figure 12.17) shows that the ground state molecules would lead to a doubly excited state of cyclobutane, and would therefore involve a prohibitive thermal activation energy. 

Bauld has carried out a large number of mechanistic studies of radical cation mediated Diels—Alder and cyclobutanation reactions, as discussed in detail in two recent reviews. Much of the above discussion concerning the concerted vs. stepwise nature of the dimerization reactions also applies to the addition of an alkene radical cation to a different alkene. Although the addition of alkene radical cations to dienes can lead to both cyclobutane and Diels-Alder products, the latter usually predominate for dienes with at least modest s-cis conformer populations. It is clear that in some cases the Diels-Alder adducts arise via rearrangement of initial divinylcyclobutane products. However, cyclobutanation and Diels-Alder adduct formation have been demonstrated to occur by independent pathways in other systems. There is also considerable experimental and theoretical data in support of a concerted but nonsynchronous mechanism for these reactions. 

This observation indicates that our earlier treatments of the decompositions of cyclobutane and of ethyl chloride [80.P1] are probably much too naive both may be considered to be dissociation reactions, and so the overall rotation could be important in determining the rate both reactions also involve curve crossings between different electronic states and it will take considerable theoretical effort to determine with confidence the relative magnitudes of these two opposing effects. 

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