Lewis-acid-catalyzed photodimerization of coumarins and N-methyl-2-quinolone.
The BF(3)-catalyzed photodimerization of parent coumarin (1), three 6-alkylcoumarins (2-4) and N-methyl-2-quinolone (5) in dichloromethane was studied by time-resolved UV-vis spectroscopy. The lowest triplet state properties in the absence and presence of BF(3) were outlined, in particular the effect of self-quenching which initiates dimerization. The quantum yield of intersystem crossing (Phi(isc)) of 1-4 increases with BF(3) concentration, approaching Phi(isc) = 0.3. Phi(isc) and the relative quantum yield of dimerization go along, thereby favoring an overall triplet mechanism in both the direct and BF(3)-catalyzed photodimerization. The product ratio of 5 changes strongly with the BF(3) concentration from 100%anti-hh for the free quinolone to 100%syn-ht for the 1:1 complex.
Görner
Helmut
H
Max-Planck-Institut für Bioanorganische Chemie, Mülheim an der Ruhr, Germany. goerner@mpi-muelheim.mpg.de
Wolff
Thomas
T
D000143:Acids / Q000737:chemistry; D001880:Boranes / Q000737:chemistry* / Q000528:radiation effects; D002384:Catalysis; D003374:Coumarins / Q000737:chemistry* / Q000528:radiation effects*; D019281:Dimerization; D015394:Molecular Structure; D010777:Photochemistry; D011789:Quantum Theory; D015363:Quinolones / Q000737:chemistry* / Q000528:radiation effects*; D014466:Ultraviolet Rays
D000143:Acids; D001880:Boranes; D003374:Coumarins; D015363:Quinolones; C021274:boron trifluoride; C538772:N-methyl-2-quinolone