Triplet Acetylenes as Synthetic Equivalents of 1,2-Bicarbenes: Phantom n,* State Controls Reactivity in Triplet Photocycloaddition
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- 作者：Tarek A. Zeidan ; Serguei V. Kovalenko ; Mariappan Manoharan ; Ronald J. Clark ; Ion Ghiviriga ; and Igor V. Alabugin
- 刊名：Journal of the American Chemical Society
- 出版年：2005
- 出版时间：March 30, 2005
- 年：2005
- 卷：127
- 期：12
- 页码：4270 - 4285
- 全文大小：706K
- 年卷期：v.127,no.12(March 30, 2005)
- ISSN：1520-5126
文摘
Diaryl acetylenes, in which one of the aryl groups is either a pyridine or a pyrazine, undergoefficient triplet state photocycloaddition to 1,4-cyclohexadiene with formation of 1,5-diaryl substitutedtetracyclo[3.3.0.02,8.04,6]octanes (homoquadricyclanes). In the case of pyrazinyl acetylenes, the primaryhomoquadricyclane products undergo a secondary photochemical rearangement leading to diaryl substitutedtricyclo[3.2.1.04,6]oct-2-enes. Mechanistic and photophysical studies suggest that photocycloadditionproceeds through an electrophilic triplet excited state whereas the subsequent rearrangement to thetricyclooctenes 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 byphotophysical factors, such as the competition between the conversion of acetylene singlet excited stateinto the reactive triplet excited states (intersystem crossing: ISC) and/or to the radical-anion (photoelectrontransfer from the diene to the excited acetylene: PET). Dramatically enhanced ISC between 
-* S1 stateand "phantom" n,* 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 withthe polycyclic framework orients the terminal aryl (4-pyridyl, 4-tetrafluoropyridyl, phenyl, etc.) groups in analmost perfect 60
angle and renders such molecules promising supramolecular building blocks, especiallyin the design of metal coordination polymers.
-* S1 stateand "phantom" n,* 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 withthe polycyclic framework orients the terminal aryl (4-pyridyl, 4-tetrafluoropyridyl, phenyl, etc.) groups in analmost perfect 60 angle and renders such molecules promising supramolecular building blocks, especiallyin the design of metal coordination polymers.