Photochemistry of 5H-dibenzo[a,c] cycloheptene
Date
1990
Authors
Budac, David Patrick
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Abstract
The photochemistry of 5H-dibenzo[a,c)cycloheptene (DAC) (6) has been studied in CH₃CN, aqueous CH₃CN and other organic solvents. The aim of the investigation was to determine whether this compound behaved as an ionizing carbon acid in S₁. It has already been reported, based on work done in this group, that a related compound, 5H-dibenzo[a,d)cycloheptene (DAD) (1), was a very strong carbon acid in S₁. Since 1 and a derivative were the only excited state carbon acids reported it was important to see whether 6 would exhibit similar behaviour.
Exchange of the methylene protons of DAC (6) with D2O was observed on photolysis in D₂O-CH₃CN solution, under conditions where there was no exchange without irradiation. Quantum yields for deuterium incorporation in 6 have been measured. These results indicate that 6 is indeed an ionizing carbon acid in S₁.
In addition to carbon acid behaviour, a competing formal di-π-methane rearrangement to give dibenzonorcaradiene (DBN) (50) was observed in all solvents. In 100% CH₃CN, this rearrangement was the exclusive photochemical reaction. Detailed investigations of this formal di-π-methane rearrangement showed that two mechanisms were operative. In S₁ the rearrangement was via a 1,7-hydrogen shift followed by electrocyclic ring closure. From T₁, a true di-π-methane mechanism operated. Quantum yields for the rearrangement have been measured in a variety of solvents.
The exchange process is believed to involve initial deprotonation of the methylene proton by the solvent as base (H₂O or D₂O), forming a carbanion, which is then protonated. The kinetics of the deprotonation step have been studied by fluoresence quenching and lifetime measurements.