Organomelemental fragmentations : I. Direct and retro fragmentations in organometallic benzyl compounds. Aromatisational metallation

Organometallic benzyl compounds are known to react with eletrophilic agents via two paths, viz., by direct substitution of the metal by the agent, and by attack of the ortho- and para-positions of the ring. The latter reaction leads to expulsion of the metal and formation of unstable triene intermediates (1-methylene-2,4-cyclohexadienes and 1-methylene-2,5-cyclohexadienes). By attributing these processes to electrophilic fragmentations (referred to as F_E) a new process may be predicted to occur, electrpohilic addition of metal salts to the exo-methylene site of trienes of this type. This process can be regarded as the reverse of the fragmentation reaction. The F_E representation proposed here forms the basis for a new organometallic synthesis, aromatisational metallation. It is shown that interaction of 1-methylene-6-ethyl-2,4-cyclohexadiene and 1-methylene-4-ethyl-2,5-cyclohexadiene with mercury dichloride in ether results in the formation of o- and p-ethylbenzylmercury chlorides, respectively.