Tin and silicon hydrides

A number of other thiocarbonyl derivatives have been used [222] (sec below), as well as various hydrogen atom sources (tin and silicon hydrides,... 

Moreover, the reactivity differences between the tin and silicon hydrides can be exploited for functional-group differentiation. For example, in the presence of Pd(0), tri-butyltin hydride reduces a,/3-unsaturated aldehydes and ketones rapidly but silicon hydrides are unable to do so. Thus, the treatment of a mixture of an allylic acetate and an unsaturated ketone with tin hydride and Pd(0) catalyst results in total conjugate reduction of the latter and unreacted allylic acetate. In contrast, employment of silicon hydride provides complementary chemoselectivity allylic reduction was completed before any reduction of the Michael acceptor could be detected (Scheme 15). 

Owing to space limitations, only a review of the most important concepts of the radical reactions mediated by tin and silicon hydrides will be discussed. However, these reactions have been excellently reviewed elsewhere [34]. 

Sequential reactions can be divided into four main types, depending on the type of reactions involved in the sequence. These sequences are (a) radical/radical, (b) radical/anionic, (c) anionic/radical, and (d) anionic/anionic. Though methodologies utilizing tin and silicon hydrides have dominated these reactions, Sml2 is proving to be an excellent alternative and in some cases, the reagent of choice. 

Since its introduction by Barton and McCombie, the deoxygenation of thionocarbonyl derivatives of alcohols has become an important synthetic reaction and a valuable method for the generation of carbon-centered radicals.3-4 Xanthates, thionobenzotes, thionocarbonyl imidazolides, aryloxy thionocarbonate, N-phenylthionocarbamates and oxalate esters are conveniently deoxygenated with tin or silicon hydrides in boiling benzene or toluene.4-5... 

Lewis acids based on boronic acid derivatives or main group elements such as mercury, tin and silicon form strong bonds to anions with considerable covalency, exemplified by hydride sponge and the anticrowns. 

Nowadays, there is a wide variety of hydride reagents but this section will concern itself mostly with those derived from boron, aluminum, tin and silicon. Even within this restricted group there is a wide... 

The method seems to offer promise of alkylating certain boron hydrides (compare 126). In addition, hydrides of tin and silicon seem to undergo an analogous reaction. Thus it has been observed that triphenyltin hydride reacts with methyllithium to yield mainly lithium hydride and triphenyl-methyltin (50). 

Alternatively tin and silicon atoms of stannylsilanes can be halogenated via hydride-substituted derivatives. Hydridosilylstannanes 10 are easily accessible by the reaction of diorganofluorosilanes with diorganodichlorostannanes and magnesium (Eq. 1). 

Raasch has extended his studies of the reactivity of bis(trifluoro-methyl)thioketen (141). Scheme 8 gives a survey of the reactions that have been carried out. Thus common nucleophilic agents such as alcohols, thiols, primary, and secondary amines, etc., react with (141) with conservation of the C=S double bond, whereas the C==C double bond remains in the products of the reactions of (141) with certain ethers, dimethylaniline, and hydrides of tin and silicon. Compound (141) reacted with a variety of olehnic substances in accordance with the general ene-reaction formalism, yielding allyl vinyl sulphides (148). The formation... 

The UV-vis spectral analysis confirms the appearance of a new charge-transfer absorption band of the complexes of colorless a-donors (R3MH) and the n-acceptor (TCNE). In accord with Mulliken theory, the absorption maxima (Act) of the [R3MH, TCNE] complexes shift toward blue with increasing ionization potential of the metal hydrides (i.e., tin > germanium > silicon) as listed in Table 8. 

Limited studies of the germanium and tin hydride analogs of the silicon hydrides show that they share this ability to function as hydride sources in ionic hydrogenations however, their relatively greater reactivity toward acids appears to restrict their practical applications in organic synthesis.24,25... 

The reactions of atoms or radicals with silicon hydrides, germanium hydrides, and tin hydrides are the key steps in formation of the metal-centered radicals [Eq. (1)]. Silyl radicals play a strategic role in diverse areas of science, from the production of silicon-containing ceramics to applications in polymers and organic synthesis.1 Tin hydrides have been widely applied in synthesis in radical chain reactions that were well established decades ago.2,3 Germanium hydrides have been less commonly employed but provide some attractive features for organic synthesis. 

The review is divided into sections according to the type of metal hydride for convenience in discussing the information systematically. At one extreme, kinetic studies have been performed with many types of silicon hydrides, and much of the data can be interpreted in terms of the electronic properties of the silanes imparted by substituents. At the other extreme, kinetic studies of tin hydrides are limited to a few stannanes, but the rate constants of reactions of a wide range of radical types with the archetypal tin hydride, tributylstannane, are available. Kinetic isotope effects for the various hydrides are collected in a short section, and this is followed by a section that compares the kinetics of reactions of silicon, germanium, and tin hydrides. 

Trialkyltin hydrides represent an important class of reagents in organic chemistry because of their utility in radical reactions. However, problems of toxicity and the difficulty of product purification made trialkyltin hydrides less than ideal reagents. Several workup procedures and structurally modified trialkyltin hydrides have been developed to facilitate the separation of tin residues from the reaction mixture. Tris(trimethylsilyl)silicon hydride has also been synthesized and is often used successfully in radical reactions. However, its reactivity is different from that of trialkyltin hydrides in a number of important respects. Other tin hydride surrogates are also available. ... 

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