Radical cascade transformations of tris(o-aryleneethynylenes) into substituted benzo[a]indeno[2,1-c]fluorenes

Oligomeric o-aryleneethynylenes with three triple bonds undergo cascade radical transformations in reaction with a Bu3SnH/AlBN system. These cascades involve three consecutive cycle closures with the formation of substituted benzo[a]indeno[2,1-c]fluorene or benzo[1,2]fluoreno[4,3-b]silole derivatives. The success of this sequence depends on regioselectivity of the initial attack of the Bu3Sn radical at the central triple bond of the o-aryleneethynylene moiety. The cascade is propagated through the sequence of 5-exo-dig and 6-exo-dig cyclizations which is followed by either a radical attack at the terminal Ar substituent or radical transposition which involves H-abstraction from the terminal TMS group and 5-endo-trig cyclization. Overall, the transformation has potential to be developed into an approach to a new type of graphite ribbons.