Photochemical C-2-C-6 Cyclization of Enyne-Allenes: Detection of a Fulvene Triplet Diradical in the Laser Flash Photolysis

Bucher, G. , Mahajan, A. A. and Schmittel, M. (2008) Photochemical C-2-C-6 Cyclization of Enyne-Allenes: Detection of a Fulvene Triplet Diradical in the Laser Flash Photolysis. Journal of Organic Chemistry, 73(22), pp. 8815-8828. (doi: 10.1021/jo801689w)

Abstract

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 mu s) 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.

• Item Type: Articles
• Keywords: BASIS-SETS BERGMAN CYCLIZATION BIRADICAL CYCLIZATION COMPUTATIONS CYCLIZATION CYCLOAROMATIZATION DENSITY Density functional theory DENSITY-FUNCTIONAL THEORY DEPENDENCE DIELS-ALDER ENEDIYNES FORMAL DIELS-ALDER laser flash photolysis MECHANISM MYERS-SAITO PHOTODYNAMIC THERAPY SYSTEM SYSTEMS TEMPERATURE TEMPERATURE-DEPENDENCE THERMAL CYCLIZATION UNIT UNITS
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Bucher, Dr Goetz
• Authors: Bucher, G., Mahajan, A. A., and Schmittel, M.
• College/School: College of Science and Engineering > School of Chemistry
• Journal Name: Journal of Organic Chemistry
• ISSN: 0022-3263