Synthesis of benzotri(benzonorbornadienes) (BTBNDs): Rigid, cup-shaped molecules with high electron density within the cavity

Cyclotrimerisation of benzo-polycyclic bromostannylalkenes 8a-d with Cu(NO3 )(2). 3H(2)O in THF affords benzotri(benzonorbornadienes) 3a-d as a mixture of the syn and anti isomers. The ratio of syn to anti is close to the 1:3 statistical value in most cases (i.e. in cyclotrimers Sa,b,d), but highly i n favour of the anti isomer in 3c, where steric hindrance by the methoxy gr oups plays an important role in the stereochemistry of the cyclotrimerisati on. The substrates for the cyclotrimerisation, i.e. the bromostannyl alkene s 8a-d, were prepared from bromoalkenes 7a-d by treatment with base (LDA) a nd quenching with trimethyltin chloride. Tn turn, bromoalkenes 7a-d were pr epared from alkenes 5a-d by radical bromination-elimination. The reaction c onditions used were designed to minimise Wagner-Meerwein rearrangements tha t would lead to unwanted bromo isomers. The cup-shaped syn cyclotrimers 3a- d exhibit high electron density within the cavity as determined by AM1 semi empirical calculations of their electrostatic potential surfaces and are va luable substrates for supramolecular chemistry. As an example, it is shown that fullerene C-60 is drawn into solution in acetonitrile by complexation with both the syn and anti trimer of benzonorbornadiene 3a.