Photochemical reactions basic ideas

This chapter follows on from chapter 12 where we introduced some basic ideas on stereocontrol. Since then we have met many stereospecific reactions such as pericyclic reactions including Diels-Alder (chapter 17), 2 + 2 photochemical cycloadditions (chapter 32), thermal (chapter 33) cycloadditions, and electrocyclic reactions (chapter 35). Then we have seen rearrangements where migration occurs with retention at the migrating group such as the Baeyer-Villiger (chapters 27 and 33), the Amdt-Eistert (chapter 31) and the pinacol (chapter 31). 

There are variations on the diabatic process. One important feature is that some reactions will have a small barrier on S that separates the initial excited state geometry from the funnel geometry. This can adversely affect photochemical efficiency and produce temperature dependent quantum yields. Still, the basic idea of finding geometries in which the excited state and ground state are close in energy is central to photochemistry. 

The Nazarov cyclization is an example of a 47r-electrocyclic closure of a pentadienylic cation. The evidence in support of this idea is primarily stereochemical. The basic tenets of the theory of electrocyclic reactions make very clear predictions about the relative configuration of the substituents on the newly formed bond of the five-membered ring. Because the formation of a cyclopentenone often destroys one of the newly created centers, special substrates must be constructed to aUow this relationship to be preserved. Prior to the enunciation of the theory of conservation of orbital symmetry, Deno and Sorensen had observed the facile thermal cyclization of pentadienylic cations and subsequent rearrangements of the resulting cyclopentenyl cations. Unfortunately, these secondary rearrangements thwarted early attempts to verify the stereochemical predictions of orbital symmetry control. Subsequent studies with Ae pentamethyl derivative were successful. - The most convincing evidence for a pericyclic mechanism came from Woodward, Lehr and Kurland, who documented the complementary rotatory pathways for the thermal (conrotatory) and photochemical (disrotatoiy) cyclizations, precisely as predicted by the conservation of orbital symmetry (Scheme 5). 

With the idea of avoiding the potential bimolecular coupling reaction of the radical centers in the solution-phase chemical oxidation reactions, a photochemical approach was adopted. Diazo compounds 10 and 12 were treated with the Rh catalyst under basic conditions to give the poly(acetylene)s 43 [R and m- /7-(/i-C,9H39)C6H4]C(N2)] of 200000 [8]. While photolysis of the diazo groups proceeded smoothly on neat films at 2 K and broad EPR si-... 

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