Hypervalent silicon hydrides

The hypervalent silicon hydride anion, SiHj (cf. SiFs below), has been synthesized as a reactive species in a low-pressure flow reactor ... 

Asymmetric reduction of alkyl aryl ketones with trialkoxysilanes is promoted by a catalytic amount of chiral nucleophiles [39]. The reactive species is a transiently prepared hypervalent silicon hydride. 2, 4, 6 -Trimethylacetophenone was reduced with equimolecular amounts of trimethoxysilane in the presence of the monolithio salt of (R)-BINAPHTHOL (substrate Li=20 l) in a 30 1 ether-TMEDA mixed solvent at 0 °C to afford the R product in 90% ee (Scheme 21) [40]. The presence of TMEDA was crucial to achieve high yield and enantiose-lectivity. Reduction of less hindered ketonic substrates preferentially gave the... 

Returning to the introductory sentence of this section, it may be noted that hypervalent silicon hydrides have been prepared as anions in the gas phase and their ion chemistry has been investigated by Squires and co-workers [238]. Thus, the synthetic methods for the precursor anions are known and it is perhaps only a question of time for pentavalent silicon radicals to be generated by NRMS as transient species in the gas phase. 

A completely different approach has been taken in the use of hypervalent silicon hydrides. It has been shown that pentacoordinated silanes possess enhanced reducing power. Thus, in the presence of a nucleophile, which may be external (reaction (a) in Scheme 2) or internal (reaction (b) in Scheme 2), hy-... 

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]. 

Open-chain alkanes, alkyl halide reduction, 29-31 Organosilicon hydrides bond strengths, 5-6 hypervalent silicon species, 9-11 ionic hydrogenation, 5 trivalent silicon species, 7-9 Orthoesters, reduction of, 97-99 Oxime reduction, 102... 

Summary BusSnH is an effective reagent for partial conversion of Si-Cl into Si-H groups. The hydrogenation mechanism postulates the coordination of the catalyst or the solvent to silicon giving a hypervalent intermediate in the first step, followed by the attack of tributyltin hydride by a single electron transfer or a synchronous hydride transfer. This mechanism implies that the intermediate containing a hypervalent silicon atom reacts faster than the starting tetracoordinate silane. 

In conclusion, one can look at the silyl hydride complexes in a number of ways, either as coordinated silane with a very elongated Si- -H bond or as complexes of hypervalent silicon moieties with an agostic Si-H bond [109]. It should be pointed out that the ability of Si to become hypervalent in the vicinity of a transition metal fragment has some important consequence on the reactivity of silane derivative in metathesis reaction with lanthanide complexes [115]. 

Keywords nonclassical interactions, agostic bond, hypervalent bond, silicon, hydride... 

While the last elass of eomplexes considered in this section, the compounds 145, closely resemble the usual silane a-complexes, other multicenter H Si interactions discussed above have spectroscopic and structural features common to both the IHI and a-complexes. This enigmatic situation can be explained well by the structure 132 in terms of a a-coordination of the Si-H bonds of the hypervalent ligand (H +iSiX3)" 1) to metal, which thus includes both the hypervalent interaction of the silicon with the hydride atoms and the a-complexation of the Si-H bonds to metals. The key features of complexes with multicenter H Si interactions are summarized in Table VIII, where a comparison with the IHI and the residual H-Si interactions in silane a-complexes is given. 

The Si- H interaction occurs in a wide variety of systems with one or two silyl groups such as 22 and 23 [79-85, 102, 103]. In the most general case, only one hydride is involved and the silicon centre is surrounded by five groups, a classical hypervalent situation for silicon [100, 101]. The presence of electron withdrawing atoms such as Cl on Si shortens the Si -H distance as shown by MP2 calculations on 22 [103] and DFT calculations on 23 [80], which agrees with the greater tendency of Si to become hypervalent in the presence of electronegative atoms. 

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