Silicon hydrides alkenes

Silicon hydrides can also oxidatively add to low-valent transition metal complexes forming a metal hydride silyl complex which can undergo subsequent insertion reactions. This elementary step forms the basis for the hydrosilylation process for alkenes and ketones. 

As an example, the propagation steps for the reductive alkylation of alkenes are shown in Scheme 7.1. For an efficient chain process, it is important (i) that the RjSi radical reacts faster with RZ (the precursor of radical R ) than with the alkene, and (ii) that the alkyl radical reacts faster with the alkene (to form the adduct radical) than with the silicon hydride. In other words, the intermediates must be disciplined, a term introduced by D. H. R. Barton to indicate the control of radical reactivity [5]. Therefore, a synthetic plan must include the task of considering kinetic data or substituent influence on the selectivity of radicals. The reader should note that the hydrogen donation step controls the radical sequence and that the concentration of silicon hydride often serves as the variable by which the product distribution can be influenced. 

Figure 5.7. Proposed mechanism for surface hydrosilylation. The initial loss of silicon hydride generates a silicon dangling bond. Reaction between the silicon and an alkene molecule leads to an attached alkyl radical, which may abstract a hydrogen atom from a neighboring silicon.

Effenberger, F., Gotz, G., Bidlingmaier, B. and Wezstein, M. Photoactivated preparation and patterning of self-assembled monolayers with 1-alkenes and aldehydes on silicon hydride surfaces. Angewandte Chemie-International Edition 37, 2462 (1998). 

Most examples in the literature on hydrosilylation with iron complexes as catalyst concern Fe(CO)5 or related iron carbonyl compounds [41]. The first use of iron pentacarbonyl was reported for the reaction of silicon hydrides with alkenes at 100-140 °C to form saturated and unsaturated silanes according to Scheme 4.20 [42, 43]. 

The combination of silicon hydrides and a Pd° catalyst is essentially useless for reduction of electron-deficient alkenes. However, addition of catalytic amounts of zinc chloride creates a new three-component mixture that enables rapid conjugate reduction of a,p-unsaturated ketones and aldehydes. In fact, soluble palladium complexes of various oxidation states were equally efficient catalysts, an obvious practical advantage of this approach. The generality of the method with respect to the substrate, its experimental simplicity, and its easy applicability to large-scale work make it a method of choice for conjugate reduction of unsaturated ketones and aldehydes. 

The silicon hydrides do not spontaneously add to alkenes either. However, the hydrosilation, or hydrosilylation reaction, of olefins is of significant utility in the preparation of alkyl-subtituted silanes with the use of either radical or transition metal catalysis. The preferred metal catalysts for hydrosilation are platinum complexes. Chloro-platinic acid will catalyze hydrosilations with halosilanes, alkylarylhalosilanes, alkoxy-silanes, and siloxanes that in many cases are quantitative under ambient conditions. Yields and conversions are generally poorer for alkyl,- and arylsilanes. Many other coordination complexes have been found to catalyze the hydrosilation reaction, and these can provide certain advantages, particularly in regiochemistry. Some typical hydrosilation reactions are shown in Table... 

CH2CH2C=CH2 for a hydrogen in Cl3SiH. This may be achieved through addition of the silicon hydride to the appropriate alkene ... 

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... 

The Reaction of Alkenes and Alkynes with Silicon Hydrides. 

The catalytic addition of organic and inorganic silicon hydrides to alkenes, ary-lalkenes, and cycloalkenes as well as their derivatives with functional groups leads to their respective alkyl derivatives of silicon and occurs according to the anti-Markovnikov rule. However, under some conditions (e.g., in the presence of Pd catalysts), this product is accompanied by a-adduct (i.e., the one containing an internal silyl group). Moreover, dehydrogenative silylation of alkenes with hydrosilanes, which proceeds particularly in the presence of iron- and cobalt-triad complexes as related to hydrosilylation (and very often its side reaction), is discussed. 

The hydrosilylation of alkenes and alkynes catalyzed by transition metal complexes is often accompanied by side reactions such as isomerization, polymerization, and hydrogenation of unsaturated compounds and/or redistribution and dehydrogenation of silicon hydrides as well as reactions in which both substrates take part, such as dehydrogenative silylation (4,13). The latter reaction, which in certain conditions permits direct production of unsaturated silyl compound, has been a subject of intense study over the last two decades. 

Treatment of a y-iodoallylic alcohol with NaH and -Bu2SiHCl afforded the corresponding di-f-butyloxysilanes which were exposed to UV irradiation in the presence of 10% hexabutylditin in the so-caUed unimolecular chain transfer (UMCT) reaction of silicon hydrides to afford the reduced alkene (eq 1). ... 

Why do you think that a homologous series of silicon hydrides, Si H2 , analogous to the alkenes, does not exist ... 

In addition to the volatile silanes, silicon also forms non-volatile hydrides with formulae (SiHj) , but little is known about their structure. Silicon, however, does not form unsaturated hydrides corresponding to the simple alkenes. 

Other Lewis acid-catalyzed hydrosilylations of alkenes and alkynes on hydride-terminated silicon surfaces result in a surface modified with alkyl and alkenyl functionalities. ... 

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