Testing the 1,1,3,3-tetramethyldisiloxane linker in olefin metathesis

Compounds 12-15, possessing two styrenes connected by a silicon linker [1,1,3,3 tetramethyl-di-siloxane], were synthesized, characterized and used as model compounds for the ring-closing metathesis (RCM) catalyzed by commercially available ruthenium catalysts 1, 2 and 3. The RCM reactions of 12 and 15 in the presence of catalysts 1 or 2 resulted exclusively in the formation of (E)-stilbenes. The RCM reactions of 13 and 14, compounds possessing alkoxide substituents in the ortho position to styrene functionality, were not observed in the presence of 2, presumably due to the formation of inactive Hoveyda type ruthenium complexes. The RCM of mixture of 12 and 15, with 2, was used for the detailed examination of the mechanism of metathesis reactions investigated in this work. They revealed that both inter- and intramolecular metathesis is possible, in this case, despite the use of siloxane linker.