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

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.