Schenck mechanism

Not all sensitized photochemical reactions occur by electronic energy transfer. Schenck has proposed that many sensitized photoreactions involve a sensitizer-substrate complex. The nature of tWs interaction could vary from case to case. At one extreme this interaction could involve o-bond formation and at the other extreme involve loose charge transfer or exciton interaction (exciplex formation). The o Schenck mechanism for a photosensitized reaction is illustrated by the following hypothetical reaction  

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The Schenck Mechanism. Two mechanisms were originally proposed for photosensitized olefin reactions. One suggested that sensitization occurred by exchange energy transfer from an excited donor (S) to yield ground state donor and excited state acceptor.24,136... 

Additional mechanisms that have been established may be termed pho-tocatalytic. One of these is the Schenck mechanism, which involves an addition of the sensitizer (Sens) to the double bond and formation of a biradi-caloid intermediate that is free to rotate about the double bond and subsequently collapses to the sensitizer and olefin ... 

In a few less common procedures, the photocatalyst serves as a carrier of the hydrogen atom, subsequently forming a radical intermediate that must be regenerated via another hydrogen transfer step. For instance, a lower than stoichiometric amount of benzophenone is sufficient to trigger a photocatalysed addition of alkynes substituted with electron-withdrawing groups to cycloalkanes (575 is produced in less than 70% chemical yield Scheme 6.274).1491 In addition, photosensitized isomerization via the Schenck mechanism (Scheme 6.15 in Section 6.1.1) may also represent a photocatalytic radical process. 

Comparison of Terenin-Schenck Mechanism and the Singlet Oxygen Mechanism. Terenin (49, 50), and later Schenck (47), proposed the following mechanism for dye sensitized photo-oxygenation reactions ... 

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