Chemistry of metal encapsulated polysiloxane gel as reusable catalysts

Treatment of polymethylhydrosiloxane (PMHS) with α, ω-diol in the presence of several transition metal catalysts results in dehydrogenative silylation to form insoluble polysiloxane gel, to which metallic species used as the catalyst for cross-linking are effectively entrapped. The formed gel behaves as a reusable catalyst in organic synthesis. In a typical example, treatment of α, ω-diols with a triruthenium cluster, (μ ₃, η ², η ³, η ⁵- acenaphthylene)Ru ₃ (CO) ₇ gives polysiloxange gel containing ruthenium carbonyl species, which catalyzes alkene isomerization of allyl ethers to the corresponding vinyl ethers. It is well known that platinum salts and complexes behave as efficient catalysts for hydrosilylation of alkenes. Reaction of PMHS with α, ω-dienes gives rise to cross-linking through Si-C bond formation to afford polysiloxane gel containing platinum species useful for catalytic hydrogenation of nitroarenes. Platinum-catalyzed hydrosilylation is also useful for crosslinking of PMHS by α, ω-dienes containing a bipyridine or bisoxazoline unit. Ligation of copper species to the formed gel leads to reusable gel catalysts for atom transfer radical cyclization and asymmetric cyclopropanation.