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Photochemical Pinacol Rearrangement
Mary Hoang1, Timothy Gadosy, Hedieh Ghazi
1Department of Chemistry, York University, Toronto, Ontario M3J 1P3, Canada, and Steacie Institute for Molecular Science, National Research Council Canada, Ottawa, Ontario K1A 0R6, Canada.
The Journal of Organic Chemistry
|October 24, 2001
Summary
Photolysis of 9,9'-bifluorene-9,9'-diol yields fluorenone and a pinacol rearrangement product. The substituted 9-fluorenyl cation formed is relatively insensitive to solvent effects, indicating competing rearrangement and quenching pathways.
Area of Science:
- Organic Chemistry
- Photochemistry
- Carbocation Chemistry
Background:
- 9,9'-Bifluorene-9,9'-diol (1) undergoes photochemical reactions.
- Pinacol rearrangement is a known reaction pathway for diols.
- Solvent effects play a crucial role in carbocation stability and reactivity.
Purpose of the Study:
- To investigate the photochemical behavior of 9,9'-bifluorene-9,9'-diol (1).
- To elucidate the mechanism of product formation, including pinacol rearrangement.
- To characterize the transient species formed during photolysis and study their decay kinetics.
Main Methods:
- Irradiation of diol 1.
- Laser flash photolysis in various solvents (2,2,2-trifluoroethanol, hexafluoro-2-propanol).
- Spectroscopic analysis of transient species (UV-Vis absorption).
- Kinetic studies of cation decay.
Main Results:
- Irradiation produced 9-fluorenone and spiro[9H-fluorene-9,9'(10'-H)-phenanthren]-10'-one (4) via pinacol rearrangement.
- Product distribution was solvent-dependent, favoring ketone 4 in trifluoroethanol.
- Laser flash photolysis identified transients absorbing at 350, 505, and 700 nm, assigned to a substituted 9-fluorenyl cation (5).
- Decay kinetics of cation 5 showed relative insensitivity to solvent compared to other fluorenyl cations.
Conclusions:
- Photoheterolysis of diol 1 generates a substituted 9-fluorenyl cation (5).
- The pinacol rearrangement product (4) formation is influenced by solvent properties.
- Unimolecular rearrangement competes with nucleophilic quenching for cation 5, with less solvent dependence observed.