Silanes transition-metal hydrides

The well-established reducing properties of the Sn—H bond have led to a number of useful synthetic methods for forming a tin-metal bond. In particular, many early transition-metal hydrides, -amides and -silanes readily eliminate H2/amines/silanes upon treatment with the Sn—H bond. Some examples are illustrated in equations 79-83 for... 

The state of the art of reductions with metal hydrides a decade ago was the subject of comprehensive reviews. A detailed survey of reductions of carbonyl compounds with alkali and alkaline earth metal hydrides, borane and derivatives, alane and derivatives, metal borohydrides, metal aluminohydrides, silanes, stannanes and transition metal hydrides was compiled. The properties, preparation and applications of each reagent were discussed together with methods for their determination, handling techniques... 

The use of (i7-C5Hs)2TiCl2 as catalyst provides a more powerful reduction method similar to the use of LiAlH4 (230). In this case, reduction of optically active functional silanes occurs with a high degree of retention of configuration (eq. [65]). The intervention of a transition-metal hydride catalyst is probably involved in these reductions. 

Using transition metal hydrides, rather than the H-Si bond of silanes, as hydride donors offers several significant advantages. The steric and electronic properties... 

Chromium.—Photochemical reactions of, for example, Cr(C He)(CO)3 with silanes in hydrocarbon solvents give the silyl-transition-metal hydrides (18). The mechanism is thought to be dissociative, with loss of one carbon... 

The vinylsilane C-Si bond can also be formed from a silane by reductive cyclization/hydrosilylation of a 1,6- or 1,7-diyne. Reductive cyclization of diynes is an important ring-forming method catalyzed by transition metals, and silanes are common reductants in this process. However, in many cases the silane serves only as a hydride source, and the silyl group is not retained in the isolated product.95 Here, the focus is on the more rare methods which allow simultaneous C-C bond formation and vinylsilane installation. 

The reaction is catalysed by many transition-metal complexes, and a mechanism for the hydrosilylation of an alkene under transition-metal catalysis is depicted in Figure Si5.7. Initial coordination of the alkene to the metal is followed by cis addition of the silicon-hydrogen bond. A hydride migratory insertion and elimination of the product silane complete the cycle. 

Although a transition metal was not involved, the dehydrocoupling of the secondary hydrosilane, 1,1-dihydrotetraphenylsilole, to polymers with Mw values ranging from 4000 to 6000 (polydispersities ranged between 1.1 and 1.2) in the presence of catalytic quantities of Red-Al, L(N or K)-Selectride or Super-Hydride has been reported.1353 This is the only currently published example of polymeric material produced from a secondary silane. It is interesting that this secondary silane is a heterocyclic system where steric interactions (from substituents at silicon) have been reduced, although the fact that there are phenyl groups on the a-carbon, makes this a rather extraordinary result. 

The order of reactivity of this Ru/silane combination to various functional groups differs greatly from that of its Pd/silane/ZnCh analog. While the latter is very useful for allylic reductions and essentially useless for unsaturated esters, the Ru-based system exhibits opposite reactivity. This complementary che-moselectivity is illustrated by the reduction of cinnamyl cinnamate (Scheme 59), a substrate containing both an allylic carboxylate and an a, -unsaturated ester.Each of these can be reduced separately by silicon hydride and the appropriate transition metal catalyst. 

Silicon compounds with coordination number larger than four are the object of many studies first with respect to their application as catalysts in organic and inorganic syntheses and second as starting materials for the preparation of a broad variety of organosilicon compounds [1]. Additionally, hypervalent silicon hydride compounds can successfully be used as model compounds to study, for instance, the mechanism of nucleophilic substitution reactions, which is of great interest since the silicon atom is able to easily extend its coordination number [1]. Moreover, hypervalent silanes are suitable as starting materials for the synthesis and stabilization of low-valent silanediyl transition metal complexes [2-5]. 

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... 

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