Monosubstituted hydrosilylation

Hydrosilylation of monosubstituted and. em-disubstituted olefins (Reactions 5.3 and 5.4) are efficient processes and have been shown to occur with high regioselectivity (awti-Markovnikov) in the case of both electron-rich and electron-poor olefins [25]. For cis or trans disubstituted double bonds, hydrosilylation is still an efficient process, although it requires slightly longer reaction times and an activating substituent (Reaction 5.5) [25]. Any hydrosilylation product has been observed with 1,2-dialkyl-and 1,2-diaryl-substituted olefins, due to the predominant reversible addition of (TMS)3Si radical to the double bond [19]. 

The addition of gem-disubstituted olefins, CH2=CXY, on polysilane 2 also worked well [23,24], For example, the addition of 2-methoxypropene and methylenecyclohexane afforded the expected adducts with 73% and 77% degrees of substitution, although a higher loss of molecular weight with respect to the hydrosilylation of monosubstituted olefins is observed. Copolymer 21, containing both mono- and disubstituted olefins, was made from 2 in a single reaction by adding 50 mol% vinyl acetic acid and an excess of 2-methoxypro-pene to the THF-polymer solution [24],... 

The C2-symmetric 2,6-bis(2-oxazolin-2-yl)pyridine (pybox) ligand was originally applied with Rh for enantioselective hydrosilylation of ketones [79], but Nishiyama, Itoh, and co-workers have used the chiral pybox ligands with Ru(II) as an effective cyclopropanation catalyst 31 [80]. The advantages in the use of this catalyst are the high enantiocontrol in product formation (>95 % ee) and the exceptional diastereocontrol for production of the trans-cyclopropane isomer (>92 8) in reactions of diazoacetates with monosubstituted olefins. Electronic influences from 4-substituents of pyridine in 31 affect relative reactivity (p = +1.53) and enantioselectivity, but not diastereoselectivity [81]. The disadvantage in the use of these catalysts, at least for synthetic purposes, is their sluggish reactivity. In fact, the stability of the intermediate metal carbene has allowed their isolation in two cases [82]. 

The hydrosilylation of monosubstituted and /jem-disubstiluted olefins (equation 28 and 29) are efficient processes and have been shown to occur with high regioselectivity... 

Early reports stated that the course of reaction is strongly dependent on the reaction conditions (L e. the employed catalyst)20-28. Benkeser20-22 and his co-wor-kers intensively investigated hydrosilylation of monosubstituted acetylenes 1 [R = i-prop- (la) and t-But- (lb)]. 

An alternative route to the hydrosilylation product of monosubstituted alkynes (29) is that which involves silylmetallation of the alkynes (equation 16). A cuprate reagent (34) prepared from CuCN and 2 mol of Me2PhSiLi adds in a syn manner to the bond with the Me2PhSi group at the terminal carbon. Quenching with water affords a product of type (31). 

Hydrosilylation of monosubstituted alkenes with palladium catalysts and trichlorosilane follows a course which favors branched products. By using a chiral phosphine ligand, asymmetric reaction is feasible. Initially, menthyldiphenylphosphine (MDPP, 93) and neomenthyldiphenylphosphine (NMDPP, 94) were employed with little success. Later, (/ )-/VA -dimethyl-l-[(S)-2-diphenylphosphinoferroce-nyl]ethylamine [(R)-(S)-PPFA] (95) and its enantiomer were prepared, and these have proved to be the... 

The regioselectivity of olefin insertion varies with the complex used in the reaction [27, 34]. In the hydrosilylation of a monosubstituted olefin, the use of complexes with larger metals and more open ligands provide increased yields of the product derived from reversed ( 2,1 ) insertion (Eq. 16). These results reveal that a variety of complexes give excellent selectivity for terminal insertion, but the conditions to elevate the amount of 2,1 insertion remain elusive. 

Hydrosilylation of alkenes and alkynes This reaction can be effected with 1 and an initiator at 90°. Reaction with monosubstituted and gem-disubstituted alkenes shows high an -Markovnikov regioselectivity. cis- or rrons-Disubstituted and trisubstituted alkenes are hydrosilylated in high yield but require longer reaction times. 

In general, radical hydrosilylation of alkenes cannot be conducted with tri-alkylsilanes, which is due to a rather strong Si—H bond in the latter. However, the hydrosilylation of carbon-carbon multiple bonds with modified silanes such as tris(trimethylsilyl)silane has been successfully used in radical hydrosilylation (16). The reversible addition of tris(trimethylsilyl)silyl [(TMSlsSi] radical to the C=C bonds is due to the ability of this radical to isomerize alkenes. The hydrosilylation of monosubstituted and gem-disubstituted olefins are efficient processes and have been shown to proceed with high regioselectivity for both electron-rich and electron-poor olefins (140). Walton and Studer presented the results of the radical hydrosilylation with silylated cyclohexadienes as radical initiators (141). The bisvinylic methylene group acts as the hydrogen donor in these reactions. H-transfer leads to a cyclohexadienyl radical (2) that subsequently rearranges to provide er -butyldimethylsilyl radical and arene (3) (see Scheme 20) (141). 

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