Stereochemistry of the cyclotrimerisation of enantiopure polycyclic bromostannylalkenes: Mechanistic considerations on the coupling of alkenyl stannanes by copper(II) nitrate

The cyclotrimerisation of enantiopure 1-bromo-2-trimethylstannylbenzonorbornadiene 2, contrary to the expectations, affords predominantly the trimer anti-4. This observation suggests that the reaction proceeds mainly via a Sn-Sn coupling to produce the dimer anti-5 and a tin-copper product that triggers halogen-metal exchange on the dimer thus allowing a second coupling with the starling reagent eventually leading to the anti-trimer. The little hut consistent formation of the isomer syn-4 and meso dimer 5 can arise from a racemisation of the bromine-copper-tin intermediate possibly via an alkyne structure.