Organosilicon compounds silanes, activation

Stereoselective enzymatic hydrolyses of esters represent a further type of biotransformation that has been used for the synthesis of optically active organosilicon compounds. The first example of this particular type of bioconversion is illustrated in Scheme 15. Starting from the racemic (l-acetoxyethyl)silane rac-11, the optically active (l-hydroxyethyl)silane (5)-41 was obtained by a kinetic racemate resolution using porcine liver esterase (PLE E.C. 3.1.1.1) as the biocatalyst7. The silane (5)-41 (isolated with an enantiomeric purity of 60% ee bioconversion not optimized) is the antipode of compound (R)-41 which was obtained by an enantioselective microbial reduction of the acetylsilane 40 (see Scheme 8). 

Enantioselective enzymatic amide hydrolyses can also be applied for the preparation of optically active organosilicon compounds. The first example of this is the kinetic resolution of the racemic [l-(phenylacetamido)ethyl] silane rac-84 using immobilized penicillin G acylase (PGA E.C. 3.5.1.11) from Escherichia coli as the biocatalyst (Scheme 18)69. (R)-selective hydrolysis of rac-84 yielded the corresponding (l-aminoethyl)silane (R)-85 which was obtained on a preparative scale in 40% yield (relative to rac-84). The enantiomeric purity of the biotransformation product was 92% ee. This method has not yet been used for the synthesis of optically active silicon compounds with the silicon atom as the center of chirality. 

Recent studies on the direct reaction of elemental silicon with alkyl chlorides such as methyl chloride, activated alkyl chlorides, polychloro-methanes, (chloromethyl)silanes, (dichloromethyl)silanes, etc. are summarized in this review. In the direct reaction of elemental silicon with activated alkyl chlorides and polychloromethanes, the decomposition of the reactants can be suppressed and the production of polymeric carbosilanes reduced by adding hydrogen chloride to the reactants. These reactions provide a variety of new organosilicon compounds containing Si-H and Si- Cl functionalities, which should find considerable application in the silicone industry. 

Hydrosilylation of unsaturated organosilicon compounds has also found several applications in molecular and polymer organosilicon chemistry. In particular, the addition of polyfunctional silicon hydrides to poly(vinyl)organosiloxane, catalyzed exclusively by Pt compounds and providing an activated cure for silicon rubber [10], has been of great practical importance. Hydrosilylation of the vinyl group at silicon seems to be effective synthetic method for preparation of oligomers and polymers with a linear or cyclolinear stmcture (polyhydrosilylation), and can occur either via the addition of dihydro-carbosilanes and -siloxanes to divinyl-silanes and -siloxanes [25, 26] or by intermolecular hydrosilylation [4] (eq. (1)). 

It is obvious from this table that the change of structure of the organosilicon compounds can have a more pronounced effect on the activation energy of chemical stage than on the heat of H complex formation. Moreover, steric effects of substituents at Si atom in these compounds have a more dominant role in the chemical interaction than in the complex stability. This is apparent from comparison of the Qajs change at adsorption of alkoxy-silanes with the observed chemisorption energies. The E., increases at enhancement (the absolute value) of a steric constant of the alkoxy substituent at Si atom. 

The major products containing Si are siliconesf which are polymeric compounds having Si—O bonds. Silicones are used in various industrial fields such as the electric and electronic industries, business machinery and tools, constructions, foods, medical treatments, fibers, plastics, papers, pulps, paints, rubbers, etc. The kind of goods manufactured are three thousand or more, and increasing all the time [2]. Therefore, the number of patent applications for silicones and silanes are many and there are 4000-5000 per year in Japan. The field of organosilicon compounds is the most active of the organometallic compounds [2]. 

Apart from hydrosilylation of alkenes/ " catalytic transformations involving organosilicon compounds have been extensively studied, mainly by Marciniec and co-workers.Iridium complexes have shown to be active catalysts for these processes incorporation of CO and silanes into organic substrates has been reported via silylcarbo-... 

Chapter III basically collects contributions on the role of transition metals in organosilicon-based chemistry. The review of the Wacker-Silicone Awardee T. Don Tilley on Transition Metals in Organosilicon Chemistry ftindamentally describes two different strategies toward the exploitation of reactive metal-silicon bonds. One type involves the early transition metals, since d° metal-silicon single bonds are unsupported by metal-to-silicon 7t-backbonding and are therefore weaker than other transition metal-silicon bonds these M-Si bonds activate various unsaturated compounds via insertion and participate in the activation of single bonds (e.g., C-H and Si-C), possibly being responsible for the action as catalysts for the dehydropolymerization of hydrosilanes to poly silanes. Most recently, even the catalytic silylation of methane has been observed. 

---