Intermolecular Photocyclizations of N‐(ω‐Hydroxyalkyl)tetrachlorophthalimide with Alkenes Leading to Medium‐ and Large‐Ring Heterocycles—Reaction Modes and Regio‐ and Stereoselectivity of the 1,n‐Biradicals

Abstract

A new photocyclization strategy by using intermolecular tandem reactions between N-(omega-hydroxyalkyl)-4,5,6,7-tetrachlorophthalimides (1, 2, and 3) and a series of acyclic and cyclic alkenes is reported. Electron transfer of the triplet-excited phthalimide with the alkene and regioselective trapping of the alkene cation radical by the hydroxyl group at the phthalimide side chain gives a triplet 1,n-biradical, which after intersystem crossing (ISC) leads to regio- and diastereoselective synthesis of polycyclic heterocycles with an N,O-containing medium to large ring. Regio- and diastereoselectivity in the cyclizations are clarified by unambiguous steric structure assignments of the products by X-ray diffraction or extensive 2D NMR measurements. The diastereoselectivity is decided by the stereochemical course of the ISC process of the triplet 1,n-biradicals. These intermolecular photoreactions also furnish a new strategy to generate triplet 1,n-biradicals. Therefore, in photoreactions of 1 and 2 with phenylcyclohexene, the unprecedented stereoselective formation of products by intramolecular hydrogen-atom transfer in the 1,n-biradical intermediate was found (9 and 23). These facts provide direct verification to the reaction pathways of the 1,n-biradicals and give a new insight into the factors deciding reaction-pathway partitioning and stereoselectivity.

1,n‐Hydrogen‐Atom Transfer (HAT) Reactions in Which n≠5: An Updated Inventory

“…An unprecedented highly stereoselective internal disproportionation of biradical 60.2 a was shown to occur as side reaction in the photocyclization of N ‐(ω‐hydroxyalkyl)‐tetrachlorophthalimide 60.1 a with 1‐phenylcyclohexene, after intersystem crossing (ISC) to the singlet excited state (Scheme ) 113. In this reaction, the photoinduced electron transfer from the triplet excited state of the phthalimide to the alkene is followed by intermolecular coupling of the triplet ion–radical pair to give biradical 60.2 a .…”