Stille coupling metal catalysts

The third-generation dendrimer palladium complex (31, containing 24 PdC groups) was applied in the Stille coupling of methyl-2-iodobenzoate with 2-(tributyl-stannyl)thiophene in DMF (Scheme 10 lmol% catalyst). In contrast to what was observed with the monomer (PPh3)2PdCl2, no palladium metal formation was... 

Various aromatic dibromides have been cross-coupled with difunctional tributyltin aromatic compounds in the presence of palladium-based catalysts to yield poly(arylene)s [scheme (12)] [149-151]. The mechanistic pathway of this coupling, known as Stille coupling [152], follows an oxidative addition-trans-metallation-reductive elimination sequence. 

Historically, high-pressure free radical copolymerization has been used to produce highly branched, ill-defined copolymers of ethylene and various polar monomers. Although these materials are in production and extensively used throughout the world, the controlled incorporation of polar functionality coupled with linear polymer structure is still desired to improve material properties. Recent focus in this area has led to the development of new transition metal catalysts for ethylene copolymerization however, due to the electro-philicity of the metal centers in these catalysts, polar functional groups often coordinate with the metal center, effectively poisoning the catalyst. There has b een some success, but comonomer incorporation is hard to control, leading to end-functionalized, branched polyethylenes [44, 46]. These results are undesirable due to low incorporation of polar monomer into the polymer as well... 

The detailed discussion of transmetallation reactions with organolithium compounds (1, XM = RLi), and coupling reactions (5) in the presence of transition metal catalysts (particularly the Stille reactions) is deferred until Sections 22.1 and 22.2. 

Reviews (a) Farina, V., Krishnamurthy, V., Scott, W.J. The Stille Reaction Wiley, New York, 1999. (b) Mitchell, T. N. in Metal-Catalyzed Cross-Coupling Reactions Diederich, F., Stang, P.J., Eds. Wiley-VCH, Weinheim, Germany, 1998, Chapter 4. (c) Tsuji, J. Palladium Reagents and catalysts John Wiley Sons, Chichester, 1995. (d) Stan-forth, S.P. Tetrahedron 1993, 54, 263. (e) Farina, V., Krishnamurthy, V., Scott,... 

Palladium diphosphine complexes located at the core have also been found to be efficient, recoverable catalysts in a Stille coupling involving iodobenzene ant tributylvinyltin. This palladium complex, composed of a dendrimer of generation 3 bearing only one metallic center located at the core, was prepared in situ by reacting the dendrimer (Figure 38) with Pd(OAc)2. The obtained complex (1 or 5 mol.%) appeared to be a very efficient catalyst under mild conditions. Steric hindrance around the metallic center due to the dendrimer shell allowed the system to be recycled several times without loss of activity. 

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