Dehydrocoupling mechanism

Insoluble solid polymers were isolated in 82% yield for Ti, 95% yield for Zr, and 80% yield for Hf. TGA ceramic residue yields were 72% for Ti, 73% for Zr, and 74% for Hf. The weight average molecular weights of the oily polymers were 4120 for Ti, 9020 for Zr, and 5010 for Hf. The TGA ceramic residue yields of the soluble oily polymers were ca. 14%. The dehydrocoupling mechanism of 4 should be similar to the sigma-bond metathesis for the dehydrocoupling of phenylsilane.11,12... 

Scheme 8 o--Bond metathesis polymerization mechanism proposed in Zr-catalyzed dehydrocoupling. 

Preliminary studies of the mechanism of dehydrocoupling suggested that the principal turnover controlling process is not transfer of two hydrogen... 

Woo s combinative catalyst system of CpjMCE/Hydride is different from the catalyst systems using Cp2MCl2/2 alkyllithiums of Corey, Tanaka, and Harrod. Real catalytic species in the dehydrocoupling of hydrosilanes could be a metallocene hydride based on a sigma-bond metathesis mechanism.12,1315 Inorganic hydrides effectively produce a metallocene hydride whereas alkyllithium can produce a metallocene hydride via... 

Fig. 1. Possible mechanism for the formation of polysilole 2 from the dehydrocoupling of 1.

A plausible mechanism for the redistribution-dehydrocoupling of 4 with Cp2ZrCl2/ Red-Al is shown in Fig. 3.57... 

The most widely accepted mechanism for Group 4 metallocene promoted dehydrocoupling reactions is the a-bond metathesis mechanism proposed by Tilley as shown in Fig. 3 3h,3°,30,4°,96 reac on of the metallocene... 

The mechanism for dehydrocoupling utilizing electron-rich complexes has not yet been established. The most commonly proposed route involves oxidative addition of the hydrosilane to the metal center followed by reductive elimination of two silyl groups to form the Si Si bond related to that proposed by Curtis and Epstein.140... 

Scheme 1. Proposed mechanism for the dehydrocoupling/redistribution of PhCH(SiH3)-SiH3 by Cp2MCl2/Red-Al catalysts.

Scheme 2. The Ojima mechanism for dehydrocoupling/redistribution of R2SiH2.

In summary, although there are some indications that the stereochemistry of silane dehydrocoupling may not be completely random, the effect is not dramatic and can be rationalized by any of the mechanisms discussed previously. In the case of radical coupling, steric interaction between the substituents on the end silicons would tend to favor syndiotactic enchainment. Coupling in the coordination sphere of the catalyst would involve interactions between catalyst substituents and substituents at the end of the silicon chain. 

It is clear that catalytic dehydrocoupling is a versatile technique for the synthesis of E-E and E-E bonds. The scope of the methodology, in terms of the types of reactions that can be carried out and the selectivities that are possible, has been broadly established. There is still much to be done in unequivocally establishing the mechanisms and the types of catalytic intermediates involved. 

Scheme 9. The mechanism proposed for CpjTIH-catalyzed dehydrocoupling of BujSnH and Ph,P = Te. (In the scheme, Ti = CpjTi.)...

SCHEME 18.2 Proposed mechanism for the formation of polysilanes by metallocene-catalyzed dehydrocoupling of alkylsilanes, RSiHj, via metallocene-silanediyl intermediates. ... 

SCHEME 18.4 Mechanism suggested by Don Tilley et al. for the dehydrocoupling of RSiHj using CpCp HfHCl (Cp = T15-C5H5 Cp = Ti -CjMej ) as a catalyst. The catalytic cycle involves two o-bond metathesis reactions that pass through four-center transition states. " ... 

SCHEME 18.30 Proposed mechanism of the n-BuLi catalyzed dehydrocoupling of tris(hydridosilyl-ethylene)boranes and ammonia. (1) deprotonation with formation of an amide, (2) substitution of silicon-bonded hydride with amide, (3) polymerization through condensation. 

Harrod s preliminary suggestions for the dehydrocoupling of hydrosilanes by early transition metal catalysts involved the formation of metal-silylene intermediates, Cp2M=SiRR [138a]. The most plausible mechanism for the condensation reaction, however, has been presented elegantly by Tilley and co-work-ers [144] and involves ff-bond metathesis from M-H species generated from the catalyst precursor. 

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