Anti-Markovnikov reaction hydrosilylation

Rhodium(I) and ruthenium(II) complexes containing NHCs have been applied in hydrosilylation reactions with alkenes, alkynes, and ketones. Rhodium(I) complexes with imidazolidin-2-ylidene ligands such as [RhCl( j -cod)(NHC)], [RhCl(PPh3)2(NHC)], and [RhCl(CO)(PPh3)(NHC)] have been reported to lead to highly selective anti-Markovnikov addition of silanes to terminal olefins [Eq. 

The ratio of the three products depends on the reacting silane and alkyne, the catalyst, and the reaction conditions. Platinum catalysts afford the anti-Markovnikov adduct as the main product formed via syn addition.442- 146 Rhodium usually is a nonselective catalyst404 and generally forms products of anti addition.447 151 Minor amounts of the Markovnikov adduct may be detected. Complete reversal of stereoselectivity has been observed.452 [Rh(COD)Cl]2-catalyzed hydrosilylation with Et3SiH of 1-hexyne is highly selective for the formation of the Z-vinylsilane in EtOH or DMF (94-97%). In contrast, the E-vinylsilane is formed with similar selectivity in the presence of [Rh(COD)Cl]2-PPh3 in nitrile solvents. 

Hydrosilylation is also a very useful chemical modification which leads to silane modified polymers with special properties [60-62]. Silane modified polymers have improved adhesion to fillers and better heat resistance. It also acts as a reactive substrate for grafting or moisture catalysed room temperature vulcanisation. Guo and co-workers [61] carried out catalytic hydrosilylation of BR using RhCl(PPh3)3 as the catalyst. Hydrosilylation reactions followed anti-Markovnikov rule as shown in the Scheme 4.4. 

The amount of catalyst in such cases is rather high 1000-5000 ppm and selectivity towards anti-Markovnikov addition is lower (80-90%), compared to hydrosilylation in the presence of platinum based catalyst. The synthesis of phenylethenyl substituted siloxanes is of commercial importance, driven by potential application in personal care products. Such materials should be in the form of fluids and thus in order to preserve this requirement two approaches have been exploited. One of them involved substitution of less than 100% phenylethenyl moieties, the other made use of 1-hexene as a co-reactant, leading to decreased crystallinity of the final materials. Depending on the structure of (methylhydrido)siloxanes and reaction conditions the resulting silicon fluids exhibited refraction indices ranging from 1.527 to 1.574 (Table 1). 

Catalytic hydrosilylation of alkenes performed in the presence of a chiral catalyst results in the formation of chiral silanes. Initially platinium catalysts of the type L PtCl2, L = (/ )-benzyl-(methyl)phenylphosphine (BMPP) or (/ )-methyl(phenyl)propylphosphine and 1,1-disubstituted prostereogenic alkenes, such as a-methylstyrene, 2,3-dimethyl-l-butene and 2-methyl-l-butene, were used however, the stereoselectivity was low4,5. A slightly higher stereoselectivity is obtained when dichlorobis[(/ )-benzyl(methyl)phenylphosphine]nickel [Ni(BMPP)2Cl2] is used as the catalyst. Note that two chiral silanes are formed in this reaction, both of which are products of anti-Markovnikov addition. The major product is the expected dichlorosilane 3, while the byproduct is an anomalous chlorosilane 4 both products were separated by fractional distillation and the major product methylated to give the trimethylsilanes 56 7. 

Complexes of the 4 type catalyze the hydroboration of various olefins with catecholborane at ambient temperature [173], The proposed mechanism of the hydroboration reaction - although not within the scope of this book - parallels that of the hydrogenation and hydrosilylation reactions. The architecture of both olefins (terminal > terminal disubstituted > internal disubstituted > trisubstituted) and organolanthanides (TOF(La) 10 TOF(Sm) TOF(5) = 4 TOF(4) affects the rate of hydroboration, which for 4(La CH(SiMc3)2) and 1-hexene is TOF = 200 h , for example. The observed high regioselectivities are exclusively anti-Markovnikov. For smaller metal centers (Y, Zr, Ti) and other ligand systems (bis(cyclopentadienyl), bis(benzamidinato)) inactivation of the catalyst by catecholborane or Lewis base-metal complex induced disproportionation of catecholborane appeared to compete effectively with the catalytic conversion [174]. 

For hydrosilylation of alkenes, the reaction rate increases with temperature and hence many of these reaetions have been performed at 100 °C. Higher reaction rates are obtained for silanes with very electronegative substituents and low steric requirements (e.g. HSi(OEt)3 > HSi(i-Pr)3). Terminal alkenes usually are hydrosilylated in an anti-Markovnikov sense to give terminal silanes. Internal alkenes tend not to react (e.g. cyclohexene), or iso-merize to the terminal alkene which is then hydrosilylated (eq 10). Conversely, terminal alkenes may be partially isomerized to un-reactive internal alkenes before the addition of silane can occur. 1,4-Additions to dienes are frequently observed, and the product distributions are extremely sensitive to the silane used (eq 11). 

The hydrosilylation of alkenes produces terminal alkylsilane products. Several examples of these reactions described in Speier s original paper are shown in Equations 16.18-16.22. These examples first show that the terminal anti-Markovnikov products are formed from a-olefins (Equation 16.18). These results also show that linear products are formed from the hydrosilylation of a,S-unsaturated esters with Speier s catalyst (Equation 16.19). Reactions of internal olefins are more complex. Reactions of imsubstituted cyclic alkenes form a single symmetrical product (Equation 16.20). However, as shown in Equations 16.21a and 16.21b, reactions of internal olefins form the same major product as reactions of terminal olefins. This result was corifusing at the time, but the now weU-known isomerization of secondary alkyl complexes to primary alkyl complexes accounts for this result. More details about this isomerization process are given in Section 16.3.5 that covers the mechanism of hydrosilylation. Finally, the silane can affect regioselectivity of the hydrosilylation of alkenes catalyzed by Speier s catalyst. Reaction of dichlorosilane with 2-hexene formed the 2- and 3-alkylsilanes without formation of the terminal alkylsilane (Equation 16.22). ... 

The catalytic addition of organic and inorganic silicon hydrides to alkenes, ary-lalkenes, and cycloalkenes as well as their derivatives with functional groups leads to their respective alkyl derivatives of silicon and occurs according to the anti-Markovnikov rule. However, under some conditions (e.g., in the presence of Pd catalysts), this product is accompanied by a-adduct (i.e., the one containing an internal silyl group). Moreover, dehydrogenative silylation of alkenes with hydrosilanes, which proceeds particularly in the presence of iron- and cobalt-triad complexes as related to hydrosilylation (and very often its side reaction), is discussed. 

Marko and co-workers reported the activity of monomeric and moisture and air-stable NHC-Pt such as 80 (Figure 13.12). Complex 80 was applied to the hydrosilylation of a variety of terminal alkenes and only the anti-Markovnikov adducts were obtained, isomerization by-products representing less than 2% of the reaction products. This catalyst tolerated functional groups such as ethers, carbonyl groups and esters, but internal alkenes were inert under similar conditions. 

Using an alternative to Speier s catalyst, Chauhan and coworkers reported that recyclable platinum nanoclusters can function as catalysts in PBD hydrosilylation (Fig. 14). These nanoclusters, which were prepared via reduction of Me2Pt(COD) and recovered after the reaction by centrifugation, showed consistent activity up to five cycles of consecutive hydro-silylations. Complete conversion of 1,2-PBD was achieved with a variety of silane structures and yielded the hydrosilylation product via anti-Markovnikov addition at the terminal positions of 1,2-butadiene units. The retention of a narrow molecular weight (M /M =. 4—. 5) in the GPC analysis confirmed that no chain scission or cross-finking occurred in the polymer chains during hydrosilylation. 

Ni" complexes 137, bearing hemilabile picolyl-NHC ligands, were active in hydrosilylation reactions of styrenes (Figure 13.15). The silylated products were mainly the Markovnikov addition products when styrene and 4-meth-ylstyrene were used, but anti-Markovnikov products when a-methylstyrene was the chosen substrate. 

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