Reagents, unstable, reactions Rearrangement

Most of the ortho photocycloadditions of alkenes and alkynes to the benzene ring that have been reported during the past 40 years have been tabulated in the 4 major sections of this chapter. Many ortho photocycloadducts are unstable under the conditions of irradiation. In some cases, they undergo spontaneous photochemical or thermal rearrangements in other cases, the investigators add a reagent that turns the unstable ortho adduct into a more stable product that can be isolated and identified. Tables 1-7, for the sake of comparison, list the primary ortho adducts, even if they have never been isolated or detected. In Section VI, the many types of secondary reactions that ortho photocycloadducts can undergo are discussed. 

No other derivatives of the tertiary cation, such as the aUylic alcohol in the last part, can r formed this way because the rearrangement is too fast. The reaction with PhSe-SePh and NaBH a trick to get this allylic alcohol. The true reagent is PhSe , formed by reduction of the Se-Se bcr. It is very nucleophilic and will attack even the tertiary epoxide to give a selenide. Oxidation to rh selenoxide leads to a fast concerted cis elimination. The intermediate selenide is unstable as wel m very smelly and must be oxidized immediately before it decomposes. 

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