Si-H bond insertion

Skell and Owen observed the formation of methyl-substituted trisilanes from the reactions of thermally evaporated silicon atoms with methylsilanes. The most logical mechanism involves a di-insertion process Si atom reacts with- a substrate molecule by insertion into its Si-H bonds to form a methyl-substituted silylene, wich subsequently inserts into the Si-H bond of another substrate molecule to give the final product. 

This work also reveals that thermally evaporated Si atoms do not insert into C-H or C-Si bonds. 

As discussed below, the formation of Si2H from the reactions of recoil Si atoms with SiH4 may also involve an initial step of Si insertion into Si-H bonds. 

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Jacobsen, Panek and co-workers (86) investigated the intermolecular Si-H bond insertion of diazoesters. Bis(oxazolines) and diimines were found to be effective in this reaction, with diimine enf-88a providing optimal selectivities. As expected, enantioselectivity is a function of silane structure, with bulkier silanes providing higher selectivities but lower reactivity. Both CuOTf and Cu(OTf)2 catalyze this reaction but the Cu(II) precursors leads to much lower enantioselectivity (44% vs 83% at -40°C). 

Silanes can react with acceptor-substituted carbene complexes to yield products resulting from Si-H bond insertion [695,1168-1171]. This reaction has not, however, been extensively used in organic synthesis. Transition metal-catalyzed decomposition of the 2-diazo-2-phenylacetic ester of pantolactone (3-hydroxy-4,4-dimethyltetrahydro-2-furanone) in the presence of dimethyl(phenyl)silane leads to the a-silylester with 80% de (67% yield [991]). Similarly, vinyldiazoacetic esters of pantolactone react with silanes in the presence of rhodium(II) acetate to yield a-silylesters with up to 70% de [956]. 

As mentioned in Section HD, an important publication bearing on the question of dichlorosilane-metal reactions appeared in 198856. Boudjouk and coworkers treated t-Bu2SiX2, X = Cl, Br and I, with lithium and ultrasound activation in the presence of Et3SiH. Good yields (ca 60%) of EtjSi—SiH(Bu-t)2 were obtained in all three cases. This is rather persuasive evidence in favor of silylene formation, since Si—H bond insertion is characteristic for silylenes and it is difficult to see how the disilane could arise from the reaction of Et3SiH with a silylenoid95. 

Ground-state Si( P) in the gas phase with SiH, yields Si H, along with lesser amounts of SijHg through a combination of H abstraction and Si—H bond insertion ... 

The Si-H bond insertion reactions have been employed very effectively as a means of trapping various kinds of silylenes. (Only SiFj and SiClj do not insert into Si-H bonds.) The most popular trapping agents are silane, methylsilane, ethylsilane, and trimethylsilane. 

As in the case of C-H insertion, Si-H bond functionalization in aryl- and vinyl-diazoacetates takes place in the presence of dirhodium catalysts, and chiral ones result in asymmetric induction [60a]. In pioneering studies by Doyle et al, enantioselective Si-H bond insertion with chiral dirhodium catalysts was achieved [90]. Recently, Ball etal reported innovative studies with bis-acetate dirhodium complexes, bearing chelating nona-peptides (see Section 9.2.3.2), which catalyzed the enantioselective carbenoid insertion into Si-H bonds (Scheme 9.18) [46]. The optimization of the peptide bound to the dirhodium unit or the presence of a phosphite additive significandy improved the enantiose-lectivity of the silane products [46]. 

The insertion reaction of 16e -complexes into the Si-H bond has been investigated by Graham... 

A novel potentially useful route for the synthesis of metallo-silanols is based on the reaction of metallohydridosilanes with 1,1-dimethyldioxirane [7,8] resulting in the insertion of oxygen into the Si-H bond. This procedure is supplementary to the hydrolysis route, since, in addition to the transformation of the "normal" ferrio-hydridosilanes 9a,b to the silanols 10a,b, it guarantees the formation of chiral 10c, not available by hydrolysis, from "electron rich" phosphine iron fragment substituted 9c (Eq. (2)). 

Another hindered silane, di-t-butyldichlorodisilane, gives high yields of the reactive divalent species, di-t -butylsilylene, which was characterized by its insertion reactions into Si-H bonds(35) ... 

Fig. 25. Schematic diagram indicating a possible hydrogen diffusion mechanism, by which (a) the hydrogen leaves a Si—H bond for an interstitial position, (b) inserts into a strained Si—Si bond, (c) returns to an interstitial position, and (d) passivates a pre-existing dangling bond (Kakalios and Jackson, 1988). 

Fig. 26. A schematic diagram illustrating a possible hydrogen diffusion path, as in Fig. 25. The diagram indicates the Si—H bonds, the distribution of weak Si—Si bonds that can be broken by the insertion of a hydrogen atom, and mobile hydrogen interstitial states (Street etal., 1988). 

The product 33 (Amax = 345 nm) was formed as a film (estimated by the optical density of the UV spectrum to be several tens of nanometres thick) on a target quartz plate, the surface of which had been treated with 10 wt.% NaOH solution, followed by dichlorodimethylsilane and finally LiAlH4 to afford a surface terminated with -OSiMe2H groups. On the basis of trapping experiments (to confirm the formation of silylenes) and photo-CVD in the presence of styrene (to confirm the absence of radical species), a mechanism based on silylene formation and insertion into the surface Si-H bond was shown to be likely, as outlined in Scheme 19. 

The stereochemistry is assumed to involve retention of configuration. This assumption is very plausible, since retention has already been observed for Pt insertion in a Si-H bond, as well as for homogeneous and heterogeneous hydro-silylation and hydrogermylation . Moreover, the Si-H bond is known to react generally with retention of configuration . Finally, in a parent reaction of SiH with C02(C0)g, X-ray structural analysis reveals that the reaction proceeds with retention of configuration (see Sect. 2.2.2). 

As already briefly mentioned, the oxygen-atom insertion into Si—H bonds of silanes constitutes a selective method for the chemoselective preparation of silanols, which has been much less studied compared to the CH oxidation. This unique oxyfunctionalization of silanes is also highly stereoselective (equation 35) since, like the CH insertions, it proceeds with complete retention of configuration. A novel application of the SiH insertion process is the synthesis of the unusual iron complex with a silanediol functionality, in which selectively both Si—H bonds of the silicon atom proximate to the iron ligand are oxidized in the silane substrate (equation 36). ... 

Photochemical irradiation of (i-Pr3Si)3SiH (14) with light of 254 nm in either 2,2,4-trimethylpentane or pentane leads to the elimination of f-Pr3SiH and the generation of bis(triisopropylsilyl)silylene (/-Pr3Si)2Si (15). Silylene 15 can also be generated by the thermolysis of the same precursor 14 at 225 °C in 2,2,4-trimethyl-pentane (Scheme 14.11). Reactions of 15 include the precedented insertion into an Si H bond, and additions to the ti bonds of olefins, alkynes, and dienes. 

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