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Triplet Acetylenes as Synthetic Equivalents of 1,2-Bicarbenes: Phantom n,* State Controls Reactivity in
 

Summary: Triplet Acetylenes as Synthetic Equivalents of
1,2-Bicarbenes: Phantom n,* State Controls Reactivity in
Triplet Photocycloaddition
Tarek A. Zeidan, Serguei V. Kovalenko, Mariappan Manoharan, Ronald J. Clark,
Ion Ghiviriga, and Igor V. Alabugin*
Contribution from the Department of Chemistry and Biochemistry, Florida State UniVersity,
Tallahassee, Florida 32303-4390, and Chemistry Department, UniVersity of Florida,
GainesVille, Florida, 32611-720
Received October 11, 2004; E-mail: alabugin@chem.fsu.edu
Abstract: Diaryl acetylenes, in which one of the aryl groups is either a pyridine or a pyrazine, undergo
efficient triplet state photocycloaddition to 1,4-cyclohexadiene with formation of 1,5-diaryl substituted
tetracyclo[3.3.0.02,8
.04,6
]octanes (homoquadricyclanes). In the case of pyrazinyl acetylenes, the primary
homoquadricyclane products undergo a secondary photochemical rearangement leading to diaryl substituted
tricyclo[3.2.1.04,6
]oct-2-enes. Mechanistic and photophysical studies suggest that photocycloaddition
proceeds through an electrophilic triplet excited state whereas the subsequent rearrangement to the
tricyclooctenes proceeds through a singlet excited state. Chemical and quantum yields for the cycloaddition,
in general, correlate with the electron acceptor character of aryl substituents but are attenuated by

  

Source: Alabugin, Igor - Department of Chemistry and Biochemistry, Florida State University

 

Collections: Chemistry; Biology and Medicine