1.4- Hydrosily lation

This reaction typifies the two possibilities of reaction routes for M-catalyzed addition of an S-X (or Se-X) bond to alkyne (a) oxidative addition of the S-X bond to M(0) to form 94, (b) insertion of alkyne into either the M-S or M-X bond to provide 95 or 96 (c) C-X or C-S bond-forming reductive elimination to give 97 (Scheme 7-21). Comparable reaction sequences are also discussed when the Chalk-Harrod mechanism is compared with the modified Chalk-Harrod mechanism in hydrosily-lations [1,3]. The palladium-catalyzed thioboratiori, that is, addition of an S-B bond to an alkyne was reported by Miyaura and Suzuki et al. to furnish the cis-adducts 98 with the sulfur bound to the internal carbon and the boron center to the terminal carbon (Eq. 7.61) [62]. 

For rhodium-catalyzed enantioselective enyne cycloisomerization and hydrosily-lation-cyclization, see (a) C. Ping,... 

Intramolecular hydrosilation. Tamao et al. have extended their hydrosily-lation-oxidation sequence (12,243-245) to allyl and homoallyl alcohols as an approach to 1,3-diols. When applied to cyclic homoallylic alcohols, only a cis-1,3-diol is obtained, presumably by way of a cyclic intermediate (equation I). Acyclic homoallylic alcohols can be converted by this approach to either syn- or anti-1,3-... 

A1C13 can interact electrophilically with allenes to form zwitterionic intermediate 37, which would react with HSiR3 to produce the (E)-vinylic silane 39 by a hydrosily-lation [22], This hydrosilylation shows trans-diastereoselectivity. 

In the next example, the strategy for the formation of fused cyclic ethers utilized the formation of an intermediate a-heterosubstituted carbon radical, generated by the reaction of (TMS)3Si radical with A-(ethoxycarbonyl)-l, 3-thiazolidine derivative [38]. This intermediate gives intramolecular C — C bond formation in the presence of proximate 1,2-disubstituted double bonds (Reaction 7.27). However, when terminal double bonds are used, the hydrosily-lation of the double bond by (TMS)3SiH can compete with the reduction and prevent forming the desired C—C bond formation. 

In this chapter, recent advances in asymmetric hydrosilylations promoted by chiral transition-metal catalysts will be reviewed, which attained spectacular increase in enantioselectivity in the 1990s [1], After our previous review in the original Catalytic Asymmetric Synthesis, which covered literature through the end of 1992 [2], various chiral Pn, Nn, and P-N type ligands have been developed extensively with great successes. In addition to common rhodium and palladium catalysts, other new chiral transition-metal catalysts, including Ti and Ru complexes, have emerged. This chapter also discusses catalytic hydrometallation reactions other than hydrosily-lation such as hydroboration and hydroalumination. 

In the 1990s, no improvement has been reported for the asymmetric hydrosily lation of imines (Scheme 2.3) by using chiral Rh-catalysts. Imines 59 and 60 were reduced with 2 by using DIOP-Rh catalysts to the corresponding secondary amines 61 and 62 with 65% ee and 66% ee, respectively [43], Chiral Ru-catalyst, however, achieved higher enantioselectivity in the asymmetric reduction of a particular imine (see below). 

Cationic rhodium complexes, e. g., [Rh(cod)2] BF [48] and [Rh(coe)] C104 [49], have recently appeared as regio- and stereoselective catalysts for hydrosily-lation of alkynes and exclusive formation of vinylsilanes instead alkylsilanes in the hydrosilylation of alkenes. 

Mechanism of asymmetric induction in catalytic hydrogenation, hydrosily-lation, and cross-combination by metal complexes (A,(9-heterocycles as ligands) 02UK39. 

Fig. 7 shows the FTIR spectra of the Si nanoparticles. Curve (a) is the spectrum of dried Si nanoparticles after hydrosily lation, while curve (b) is an experimental spectrum from pure acrylic acid for comparison. Upon comparing these spectra, peaks attributed to C=C bonds are clearly present in spectrum (b), but are almost completely absent in spectrum (a). Simultaneously, the Si-CH2 peak is observed in spectrum (a). These observations confirm that the hydrosilylation proceeds as expected and Si-C bonds replace the C=C and Si-H bonds. Although the nanoparticles were dried in air, the Si-0 peak at 1080 cm 1 is quite small. 

CH, Figure 7. Chemical stracture and cross-linking by hydrosili-lation of a chiral copolymer... 

By using a difunctional dihydrosiloxane and/or divinylsiloxane as hydrosily-lation substrates, polycarbosiloxanes can be obtained that differ from their poly-carbosilane analogues in the thermal and mechanical properties (359-362). The organosilicon polymers of this type, containing both Si—O and Si—C bonds in their backbones, are also called hybrid silicones. Because polycarbosiloxanes have properties intermediate between those of linear polydimethysiloxanes and saturated hydrocarbons, they have been synthesized recently to improve thermal and fuel resistance properties of classical silicones (Scheme 39). 

Vinylpolymers + Rh-catalyst (XXXII) hydrosily- lation ketones opt. act. alcohols (see Scheme 3) v2.5 [92,93]... 

A dihydrodisilene nickel complex with metallacycle character is obtained from the reaction of the silylene hydride with Ni(COD)2 (5). Additionally, a silole is isolated upon the treatment of the silylene hydride with diphenylacetylene via [2 + 2 + 1] cycloaddition pathway (6). Further prospects of reactivity involve the hydrosily-lation by activation of terminal Si—H bond. 

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