Crystal structure of a cyclization photoproduct of 1-(4-chlorophenyl)-2-cyclooctylethanone, and reaction pathway in Norrish type II cyclizations

Abstract

The title compound is a minor solid-state photoproduct in the Norrish type II reaction of 1-(4-chlorophenyl)2-cyclooctylethanone. C16H21C10 , Mr= 264.79, triclinic, PI, a=14.335(3), b=15.122(3), c= 15.611 (2),/k, t~ = 76.90 (2), fl= 72.27 (1), 7= 68.00 (1) °, V = 2964 (I) A 3, Z = 8 (four molecules per asymmetric unit), D x = 1. 187 g cm -3, ,;[.(Cu Ka l) = 1.54053/~,/~ = 22.1 cm -~, F(000) = 1136, T= 295 K, R =0.069 for 2958 observed reflections. The four crystallographically independent molecules form a tetrameric hydrogen-bonding pattern. In each of the four molecules the cyclobutanol ring is trans-fused to the cyclooctane ring, and the hydroxyl group is cis to the nearest bridgehead H atom. All four cyclooctane rings exhibit disorder of the outer atoms. On the basis of the above structure, the major photoproduct has been established as the trans-fused-trans-OH isomer. Furthermore, from the known conformation of the reactant, the detailed reaction pathway of the cyclization reaction has been constructed. The pathway involves y-H abstraction by oxygen, followed by rotations about single bonds and angle bending, to allow overlap of radical p-orbitals and formation of a C-C bond. The motions required are minimum for the observed major trans-fused-trans-OH photoproduct, which can be formed in a topochemically controlled reaction.