Silicones ring-opening

The initial step in the reaction mechanism is formulated as an oxidative addition of the silacyclobutane to the transition-metal complex attaching Si to M (ring expansion). It is followed by a transfer of L2 from the metal to the silicon (ring opening) and polymer growth by insertion of further coordinated ring into the metal-carbon bond, similar to the mechanism proposed for olefin polymerization by Ziegler-type catalysts. 

Epoxides are regio- and stereoselectively transformed into fluorohydrins by silicon tetrafluoride m the presence of a Lewis base, such as diisopropyleth-ylamme and, m certain instances, water or tetrabutylammonium fluoride The reactions proceed under very mild conditions (0 to 20 C in 1,2-diohloroethane or diethyl ether) and are highly chemoselective alkenes, ethers, long-chain internal oxiranes, and carbon-silicon bonds remain intact The stereochemical outcome of the epoxide ring opening with silicon tetrafluoride depends on an additive used, without addition of water or a quaternary ammonium fluoride, as fluorohydrins are formed, whereas m the presence of these additives, only anti opening leading to trans isomers is observed [17, 18] (Table 2)... 

The regioselective ring opening of the silyl epoxides is facilitated by the stabilizing effect that silicon has on a positive charge in the -position. This facile transformation permits vinylsilanes to serve as the equivalent of carbonyl groups in multistep synthesis.149... 

To introduce intrinsic hydrophilicity into silicone like structures (necessary to provide wetting within high oxygen diffusion materials for contact lenses), the ring opening reaction by H20 addition was proved to be an adequate method (21) (Equation 6). 

A carbon-substituted five-membered ring, 24, was also successfully ring-opened via a silyl carbanion intermediate after initiator attack on a silicon adjacent to carbon to give a sequence-ordered C/Si... 

Sakurai, H. Yoshida, M. Synthesis of Polysilanes by New Procedures Part 1 Ring-opening Polymerizations and the Polymerization of Masked Disilenes. In Silicon-based Polymers The Science and Technology of their Synthesis and Application-, Jones, R. G., Ando, W., Chojnowski, J., Eds. Kluwer Dordrecht, 2000 pp 375-399. 

Chojnowski, J. Cypryk, M. Fortuniak, W. Kazmierski, K. Rozga-Wijas, K. Scibiorek, M. Controlled Synthesis of All Siloxane Architectures by Ring-Opening Polymerization. In Synthesis and Properties of Silicones and Silicone-Modified Materials Clarson, S. J., Fitzgerald, J. J., Owen, M. J., Smith, S. D., Van Dyke, M. E., Eds. AGS Symposium Series 838 American Chemical Society Washington, DC, 2003 pp 12-25. 

Extensive mechanistic investigation of the ring expansion 33 —> 34 was performed in order to differentiate between a ring-opening reaction to give a silyl radical 39 (path a), followed by the 6-endo cyclization, or a pentavalent silicon transition state 40 (path b). It was clearly demonstrated that the ring expansion proceeds via a pentavalent silicon transition state (Scheme 6.10) [16]. 

Denmark, S. E. Barsanti, P. A. Wong, K.-T. Stavenger, R. A. (1998) Enantioselective ring opening of epoxides with silicon tetrachloride in the presence of a chiral lewis base., J. Org. Chem., 63 2428-2429. 

Reymond, S. Brunei, J. M. Buono, G. (2000) New development in the enantioselective ring opening of meso-epoxides by various ion silicon sources catalyzed by an o-methoxyaryldiazaphosphonamide Lewis base., Tetrahedron Asymmetry, 11 4441-4445. 

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