Porphyrin-based enediynes

The photochemistry of the BC has been largely expanded by metal-mediated strategies aiming to achieve control over enediyne reactivity. The effects of direct incorporation of metal centers in metalloenediynes, porphyrin-based enediynes, as well as inhibitory complexation of simple acyclic and cyclic enediynes are discussed in the following. 

SCHEME 30.16 Suggested reaction energy profiles for ground- and excited-state Bergman cyclization of porphyrin-based enediynes. Reprinted with permission from Nath et al. [29a]. American Chemical Society. 

Zaleski and coworkers also prepared an interesting family of enediynes based on the porphyrin core [28]. These porphyrinic enediynes (Scheme 30.14) show systematic red shifts in their electronic spectra as a function of the number of conjugated alkyne units ( 13nm/alkyne), which indicate involvement of the enediyne units in the electronic excited states. Photochemical activation of the free base of tetraphenyl octakis(phenylethynyl) porphyrin derivative only led to the reduction of the porphyrin backbone via H-atom addition at the opposing me o-positions, suggesting significant activation barriers for the photo-BC. 

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