Free-radical reactions of halogenated bridged polycyclic compounds. Part XV. The reaction of methyl-lithium with some chlorine-substituted norbornenes and norbornadienes
Abstract
The reaction of methyl-lithium with chlorine-substituted norbornenes and norbornadienes not containing C-5 or C-6 substituents results in the replacement of a bridge chlorine atom by a methyl group. The methyl group prefers to enter a position anti to a double bond, and product formation can be accommodated in terms of the formation of C-7 radical pairs. If a C-5 endo-halogen atom is also present, reaction products are different as typified by the formation, in the proportions 20 : 1, of 3,4,5,6-tetrachloro-7-methyltricyclo[4,1,0,03,7]hept-4-ene and 1,2,3,5-tetrachloro-4,6-dimethylnorborna-2,5-diene as products from 5-endo-bromo-1,2,3,4,7,7-hexachloronorborn-2-ene. The formation of the tricyclic product is explicable in terms of C-5 and C-7 radical pairs, but diene formation is more complex, and is best accounted for by initial formation and combination of a C-7 radical pair, followed by the formation ands ubsequent rearrangement of a methyl-substituted C-5 radical. Tricyclic and diene products result from other compounds having halogen substituents at both C-5 and C-7. A choice between reaction paths is finely balanced, and depends on both the position and the reactivity of the various halogen substituents.