Insertion into Si-H bonds

The series of reactions above gives an insight into the reaction mechanism of reduction of CO2 to methanol, with three consecutive 2e -transfer steps (see also Chap. 8). 

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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). ... 

Insertion into Si H and Si Si Bonds. Silylenes, generated by thermolysis of cyclotrisilanes, inserted into the Si—Cl or Si—H bonds of monosilane to yield a variety of disilanes, which could be further functionalized. In contrast to carbenes, the insertion of silylenes into C—H bonds has not been observed. However, the insertion into Si—H bonds has been studied extensively. The occurrence of direct insertion has been indicated by formation of nongeminate homocoupling products. ... 

To date, chemical properties have been reported mainly for 59 and 61. Occupancy of the silicon 3p orbital by electrons from nitrogen greatly reduces the electrophilicity of these silylenes. This, together with probable aromatic stabilization, significantly mutes the behavior of 59 and 61 as silylenes. For example, 59 does not insert into Si—H bonds, or react with alkynes such as PhC=CPh367, Moreover, 59 shows no Lewis acidic behavior, even toward bases as strong as pyridine. 

It is assumed, in common with other reactions involving metal insertion into Si-H bonds (see below), that this reaction proceeds with retention of configuration at silicon. 

Because C-H bonds are usually less reactive towards dioxirane oxidation than heteroatoms and C-C multiple bonds, it is instructive to give a few general guidelines on the compatibility of functional groups within the substrate to be submitted to oxidative C-H insertion Substances with low-valent heteroatoms (N, P, S, Se, I, etc.), C-C multiple bonds, and C=X groups (where X is a N or S heteroatom) are normally not suitable for C-H insertions, because these functionalities react preferably. Even heteroarenes are more susceptible to dioxirane oxidation than C-H bonds, whereas electron-rich and polycyclic arenes are only moderately tolerant, but electron-poor arenes usually resist oxidation by dioxiranes. N-oxides and N-oxyl radicals are not compatible because they catalyze the decomposition of the dioxirane. Oxygen insertion into Si-H bonds by dioxirane is more facile than into C-H bonds and, therefore, silanes are not compatible. Substance classes normally resistant towards dioxirane oxidation include the carboxylic acids and their derivatives (anhydrides, esters, amides, and nitriles), sulfonic acids and their de-... 

The simplest silylene H2Si is formed by the pyrolysis of SilLt and the higher perhydrosilanes and it undergoes two typical types of reaction insertion and addition. Silylene readily inserts into Si-H bonds (Scheme 4) both in perhydro-and aUcylsilanes (organosilanes such as Et3 SiH are often used as chemical traps for R2Si species). ... 

The catalyst system MTO/H2O2 also catalyzes oxygen atom insertion into Si-H bonds. Silanols and disiloxanes are formed as products, with the latter being the major ones [3, 21 a]. When UHP is used as an oxygen source instead of aqueous H2O2, 1 catalyzes the oxidation of silanes to silanols in high conversions and excellent selectivities in favor of the silanol (eq. (10)). 

An analogy with carbene insertion into Si-H bonds has been proposed by Eaborn and coworkers61. Carbene insertions are supposed to involve a three-centre mechanism and proceed with retention of configuration at silicon71 (equation 17). 

See 5.8.2.2.1 for insertion into sp C—H bonds, 5.8.2.10.1 for insertipn into aldehyde C—H bonds, and 5.8.3.2.5 for insertion into Si—H bonds.) Steric effects play a predominant role in determining the various isomers observed in these reactions, but electronic effects are also important. 

Insertion into Si-H bonds. a-Silyl esters are similarly obtained. From R2Si(Cl)H the products are readily converted into o-(alkoxysilyl)alkanoic esters. Heterocyclization. Formation of a-alkylidene lactams by carbonylation of alkynyl amines and cyclization of 2-allyloxybenzylamines with allylic rearrangement are reported. 

SiMcj, may be conveniently generated by photolysis of o c/o-Si6Me,2, and this method has been used in the study of its insertion into Si—H bonds (e.g. in Me3SiH), Si—O bonds e.g. MejSiOEt)/ and into HCl. In low-temperature matrices it has been shown that SiMej may be converted by visible light (A = 450nm) into MeHSi = CH2. Annealing the photolysed matrix yields (4b). 

Rhodium carboxylates. 13,266 15, 278-286 16, 289-292 17,298-302 18, 306-307 a-Heterosubstituted esters. Rhodium carbenoids derived from a-diazo esters undergo X-H bond insertion. These processes are the basis for the synthesis of a-amino esters. - The insertion into Si-H bonds can be repeated, as shown in a preparation of 3-silaglutarate esters. ... 

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. 

Cetini and coworkers have studied the reactions of Si atoms with SiH4, SijH, and SijHg as a function of moderator concentrations. They used the nuclear transformation, Si(n,y) Si, as the hot atom source, and Ne as a moderator. In the SiH4 system, the yields of all the Si-labeled products decreased with moderation toward zero, indicating the total absence of thermal Si reactions with SiH4. This means that thermal Si atoms do not insert into Si-H bonds of silane. In the SijHj system, the SiHjSiHj yield does not change with moderation. Since the insertion of thermal Si into the Si-H bond is not operational, it was deduced that thermal Si atoms do undergo insertion into Si-Si bonds. 

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. 

SiHMe insertion into Si-H bonds during the vacuum uv photolysis of CH3SiH3. From the study of disilane pyrolysis, Tebben and Ring established the insertion of SiHj into SijHg to give Si3H which was immediately confirmed by Bowrey and Purnell in 1970 ... 

Both the ability of silylenes to insert into Si-H bonds and their inability to insert into C-H bonds have been described in the previous sections. Several... 

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]. 

RHODIUM-CATALYZED VINYLDIAZOESTERS INSERTION INTO Si-H BONDS. SYNTHESIS OF ALLYLSILANES... 

Two previously mentioned reagents, cyclic a-diazocarbonyl compounds (59) and enynones (61), proved to be effective promoters of Si-H insertion reactions also chiral catalyst (92) has been demonstrated to promote insertion into Si-H bonds in an enantioselective manner. ... 

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