The 2-methylenenorborn-3-yl ↔ norborn-2-en-2-ylmethyl allylic radical system

Abstract

Hydrogen abstraction from both 2-methylenenorbornane and 2-methylnorborn-2-ene gives rise to the same resonance-stabilised allylic radical system, the major canonical species of which are 2-methylenenorborn-3-yl and norborn-2-en-2-ylmethyl radicals. Chain transfer of the allylic radical system with t-butyl hypobromite gives solely 3-exo-bromo-2-methylenenorbornane. No 2-bromomethylnorborn-2-ene is formed; if it is assumed that no allylic rearrangement occurs, this suggests the occurrence of chain transfer to give the thermodynamically more stable product. With N-bromosuccinimide, varying, but smaller (one tenth to one quarter) amounts of 3-endo-bromo-2-methylenenorbornane are also formed; this shows that the stereochemistry of chain transfer is dependent on reagent size. Attempts at allylic chlorination of 2-methylenenorbornane with t-butyl hypochlorite and sulphuryl chloride were unsuccessful; only products of addition to the double bond were formed.