Metallocenes, olefin insertion

Neutral dimethyl metallocenes of group 4, Cp2MMe2, are effective catalysts for the dehydropolymerization of primary silanes, via a a bond metathesis pathway.234 Given the topological similarities between the four centered transition states for olefin insertion into M-C bonds and a bond metathesis reactions, it was postulated that activation of these metallocenes by B(C6F5)3 might enhance dehydropolymerization of silanes in... 

Although 187-189 were not active catalysts for polymerization process, 187 and 189 proved to be active olefin hydrosilylation catalysts, presumably 187 first reacted with a silane to form a reactive metal hydride species. They are the first examples of d° metal complexes with non-Cp ligands in the catalytic hydrosilylation of olefins. The mechanism was believed to be consistent with that of other d° metallocene-based catalysts and included two steps 1) fast olefin insertion into the metal hydride bond and 2) a slow metathesis reaction with the silane. The catalysts exhibited a high regioselective preference for terminal addition in the case of aliphatic olefins... 

Czv-Symmetric Catalysts. Syndiotactic polymers have been formed using metallocene catalysts where the polymer chain end controls the syndiospecificity of olefin insertion. Resconi has shown that Cp 2MCl2 (M = Zr. Hf) derived catalysts produce predominantly syndiotactic poly(l-butene) with an approximate 2 kcal/mol preference for syndiotactic versus isotactic dyad formation." At —20 °C. Cp 2HfCl2/MAO produces poly(l-butene) with 77% rr triads. Pellecchia had reported that the diimine-ligated nickel complex 30 forms moderately syndiotactic polypropylene at —78 °C when activated with MAO ([rr] = 0.80)." " Olefin insertion was shown to proceed by a 1.2-addition mechanism." in contrast to the related iron-based systems which insert propylene with 2.1-regiochemistry. ... 

Olefin insertion into metallocene Mt—C bonds is largely predominantly primary. However, one of the features of most isospecific metallocene catalysts is their generally lower regioselectivity compared to heterogeneous Ziegler—Natta catalysts indeed, despite the fact that primary propene insertion is clearly favored by electronic factors (see section III.E), isolated secondary propene units are often detectable in i-PP samples and their presence is the signature of a metallocene catalyst. Tail-to-head propene insertions, currently referred to as secondary or 2,1 insertions, occur in i-PP from isospecific metallocene catalysts with high but opposite (with respect to primary insertions) enantioface selectivity (Scheme 14). 

Finally, systematic studies on olefins Insertion with transition metal based systems (not being part of a metallocene, though) have been reported by Sieg-bahn ° ° and Ziegler and their co-workers. 

Catalysts based on metallocenes belonging to the Cz, prochiral Cg, and Ci classes (see Chart 2) are, in principle, able to direct 1-olefin insertion. 

Isotactic Control. Olefin insertion in Q-sym-metric metallocenes occurs preferentially with the same face at both sites leading to an isotactic polymer. The isotacticity of the polymer chain depends on the metallocene structure. The chain-end control can be active, but for highly isotactic polymers it is difficult to check its presence as the pentads representing two consecutive wrong insertions have too low an intensity for a correct evaluation. 

Enantiomorphic Site with Chain-End Control. In the case of less stereoselective Cz-symmetric metallocene catalysts, the magnitude of chain-end control can be comparable to that of site control. In this case, obviously, the former has to be added to the model using Markovian statistics. The probability parameters are the same found for pure chain-end control p si re), i.e., the probability of insertion of a si monomer enantioface after a monomer inserted with the re face, p re si), p si si), and p re re). In this case, the metallocene chirality prevents the equiprob-ability of the si olefin insertion after a re inserted monomer (see structure on the left in Scheme 36) and re olefin insertion after a si inserted monomer (see structure on the right in Scheme 36). 

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