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Triplet Acetylenes as Synthetic Equivalents of
1,2-Bicarbenes: Phantom n,π* 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
photophysical factors, such as the competition between the conversion of acetylene singlet excited state
into the reactive triplet excited states (intersystem crossing: ISC) and/or to the radical-anion (photoelectron
transfer from the diene to the excited acetylene: PET). Dramatically enhanced ISC between π-π* S1 state
and “phantom” n,π* triplet excited state is likely to be important in directing reactivity to the triplet pathway.
The role of PET can be minimized by the judicious choice of reaction conditions (solvent, concentration,
etc.). From a practical perspective, such reactions are interesting because “capping” of the triple bond with
the polycyclic framework orients the terminal aryl (4-pyridyl, 4-tetrafluoropyridyl, phenyl, etc.) groups in an
almost perfect 60° angle and renders such molecules promising supramolecular building blocks, especially
in the design of metal coordination polymers.
Introduction
stituents on the acetylenic termini (e.g., tetrafluoropyridyl )
TFP) into respective indene products (Scheme 1).4,5 In contrast
The triple bond is among the most important functional
groups in organic chemistry. As a result, further understanding
of the chemical and photochemical reactions of the acetylenic
moiety is a desirable goal. Recently, photochemistry of acety-
lenes received a lot of attention from the point of view of their
biological activity.1,2 However, despite continuing effort,3 the
photochemistry of alkynes is still considerably less studied than
the photochemistry of alkenes.
to the photochemical Bergman cyclization,6-9 the C1C5 cy-
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(5) A topologically similar but mechanistically different C1C5 cyclization of
enediyne radical-cations was also reported: Ramkumar, D.; Kalpana, M.;
Varghese, B.; Sankararaman, S.; Jagadeesh, M. N.; Chandrasekhar, J. J.
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† Florida State University.
‡ University of Florida.
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J. AM. CHEM. SOC. 2005, 127, 4270-4285
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