Photochemical C2-C6 cyclization of enyne-allenes: detection of a fulvene triplet diradical in the laser flash photolysis.
A series of enyne-allenes, with and without benzannulation at the ene moiety and equipped with aromatic and carbonyl groups as internal triplet sensitizer units at the allene terminus, was synthesized. Both sets, the cyclohexenyne-allenes and benzenyne-allenes, underwent thermal C(2)-C(6) cyclization exclusively to formal ene products. In contrast, the photochemical C(2)-C(6) cyclization of enyne-allenes provided formal Diels-Alder and/or ene products, with higher yields for the benzannulated systems. A raise of the temperature in the photochemical cyclization of enyne-allene 1b' led to increasing amounts of the ene product in relation to that of the formal Diels-Alder product. Laser flash photolysis at 266 and 355 nm as well as triplet quenching studies for 1b,b' indicated that the C(2)-C(6) cyclization proceeds via the triplet manifold. On the basis of a density functional theory (DFT) study, a short-lived transient (tau = 30 ns) was assigned as a triplet allene, while a long-lived transient (tau = 33 micros) insensitive to oxygen was assigned as fulvene triplet diradical. An elucidation of the reaction mechanism using extensive DFT computations allowed rationalization of the experimental product ratio and its temperature dependence.
Lehrstuhl für Organische Chemie II, Ruhr-Universität Bochum, Universitätsstrasse 150, D-44801 Bochum, Germany. goebu@chem.gla.ac.uk
Götz
G
Bucher
Atul A
AA
Mahajan
Michael
M
Schmittel
D000466:Alkadienes / Q000737:chemistry*; D002244:Carbon / Q000737:chemistry*; D003500:Cyclization; D003517:Cyclopentanes / Q000737:chemistry*; D007834:Lasers; D010782:Photolysis; D013056:Spectrophotometry, Ultraviolet; D013237:Stereoisomerism
D000466:Alkadienes; D003517:Cyclopentanes; C017332:fulvene; C025947:propadiene; D002244:Carbon