Silicon—carbon bonds metal hydrides

Since silene is an unstable species, various transition metal-silene complexes coordinated by the silicon-carbon double bond have been reported. In 1970, Pannel reported the formation of silene by irradiation of an iron complex (Eq. 6) [8]. He obtained an iron-TMS complex that was apparently formed from silene and an iron-hydride complex generated from the starting iron complex by /3-hydrogen elimination [8]. Wrighton confirmed the existence of tungsten-and iron-silene complexes by examination of NMR spectra obtained at low temperature (Eqs. 7 and 8) [9]. 

Silatungstacyclopropane (35) is subject to addition reactions as well. Methanol, H2, and H-TMS all add to the metal-carbon bond. The first two reactions lead to metal hydrides while the last reagent gives a metallasilane. Trimethylphosphine addition to (35) causes attack of cyclopentadienide at silicon and cleaves the silicon-metal bond <90JA6405>. 

Hydrosilylation.—This reaction is catalysed by the usual homogeneous catalysts. In some cases the mechanism involves insertion of the alkene into a metal-hydrogen bond, as in hydrosilylation of butadiene in the presence of PdL(PPh3)2, with L = p-benzoquinone or maleic anhydride. In other cases concerted addition of the silicon hydride to the carbon-carbon double bond is indicated, as in hydrosilylations catalysed by rhodium(i) catalysts such as RhCl(PPh3)3. In the reaction of silanes with hex-l-ene in the presence of this catalyst, rates depend on the stability of the intermediate adduct RhClH(SiR3)(PPh3)2 such an adduct was isolated in one case. Hydrosilylation of ethylene by trimethylsilicon hydride... 

P-Hydrogen eliminations and p-aryl eliminations from alkoxo and amido complexes are also known. Such eliminations have been shown to occur by migratory de-insertion pathways, as well as alternative p-hydride abstraction mechanisms. P-Hydrogen eliminations from metal-silyl complexes are rare because the silicon-carbon double bond in the product is weak. For similar reasons, p-hydrogen eliminations from metal-thiolate complexes are rare. 

The mechanism of hydrosilylation involves a sequence of elementary reactions described in the earlier chapters of the book. The most commonly cited mechanism for hydrosilylation was first described by Chalk and Harrod and involves oxidative addition of the silane, insertion of an olefin into the metal-hydride bond, and reductive elimination to form the silicon-carbon bond in the organosilane product. More recently, a related but distinct mechanism involving insertion of the olefin into the silyl group has been recognized, and this mechanism is often called the modified Chalk-Harrod mechanism. Before these steps are described, some of the mechanistic issues regarding the specific systems of Speier s catalyst and Karstedt s catalyst are described briefly. 

Reaction with Further Electrophiles of Group IVA (Sl,Ge,Sn). IV-Silylated aziridines can be prepared from ethyleneimine by amination of chlorosilanes in the presence of an HC1 acceptor, by dehydrocondensation with an organosilicon hydride or by cleavage of a silicon—carbon bond in 2-furyl-, 2-thienyl-, benzyl-, or allylsilanes in the presence of an alkali metal catalyst (262—266). N-Silylated aziridines can react with carboxylic anhydrides to give acylated aziridines, eg, A/-acetylaziridine [460-07-1] in high yields (267). At high temperatures, A/-silylaziridines can be dimerized to piperazines (268). Aldehydes can be inserted... 

Germanium forms a wide range of compounds with hydrogen, silicon, the heavier group 14 elements, some main group metals, and transition metals. Catenated hydrides, Ge H2 +2, have already been discussed. The majority have carbon bonded to Ge and are included in the next article (see Germanium Organometallic Chemistry). 

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