Hydrosilylation mechanism

Fig. 6. Hydrosilylation mechanism for chloroplatinic acid. L = unspecified ligand, presumably chlorine, hydrogen, or coordinated solvent.

C. Other Transition Metal Systems as Models for the Hydrosilylation Mechanism... 

The hydrogenation and hydrosilylation mechanisms using OsHCl(CO)(P Pr3)2 as catalyst show significant differences (Schemes 51 and 53). Thus, although the very stable Os (E)-CH=CHPh Cl(CO)(P Pr3)2 is the only complex observed under catalytic conditions in both cases, the hydrosilylation catalysis proceeds by initial reaction of HSiEt3 with OsHCl(CO)(P Pr3)2. The formation of cis-PhHC=CH(SiEt3) seems to occur by isomerization of a vinyl intermediate formed... 

The final cyclization manifold has been realized with a different ruthenium catalyst (Scheme 22). The cationic [Cp Ru(MeCN)3]PF6 induces exclusive endo-dig cyclization of both homopropargylic and bis-homopropargylic alcohols.29 73 The clean reaction to form a seven-membered ring is noteworthy for several reasons intramolecular exo-dig cyclization with bis-homopropargylic alcohols is not well established, the platinum-catalyzed case has been reported to be problematic,80 and the selectivity for seven-membered ring formation over the exo-dig cyclization to form a six-membered ring is likely not thermodynamic. The endo-dig cyclization manifold was thus significant evidence that a re-examination of alkyne hydrosilylation mechanisms is necessary (see Section 10.17.2). 

HYDROSILYLATION MECHANISM CHALK-HARROD VS. MODIFIED-CHALK-HARROD... 

Figure 13. Calculated reaction profile for the silylmetallation process required for the modified-Chalk-Harrod hydrosilylation mechanism.

Metal chemical shifts have not found extensive use in relation to structural problems in catalysis. This is partially due to the relatively poor sensitivity of many (but not all) spin 1=1/2 metals. The most interesting exception concerns Pt, which is 33.7% abundant and possesses a relatively large magnetic moment. Platinum chemistry often serves as a model for the catalytically more useful palladium. Additionally, Pt NMR, has been used in connection with the hydrosilyla-tion and hydroformylation reactions. In the former area, Roy and Taylor [82] have prepared the catalysts Pt(SiCl2Me)2(l,5-COD) and [Pt()i-Cl)(SiCl2Me)(q -l,5-COD)]2 and used Pt methods (plus Si and NMR) to characterize these and related compounds. These represent the first stable alkene platinum silyl complexes and their reactions are thought to support the often-cited Chalk-Harrod hydrosilylation mechanism. 

Duckett and Perutz have shown the stoichiometric reaction of the CpRh(C2H4)(SiR3)H (R = Et, i-Pr) complexes (Scheme 33)201. These complexes have been found to act as precursors to the catalytically active species for the hydrosilylation of ethene with Et3SiH but are not within the catalytic cycle. The mechanism proposed in Scheme 34 for the hydrosilylation of ethene was found to be equivalent to the Seitz-Wrighton hydrosilylation mechanism catalyzed by cobalt carbonyls complexes202. 

Figure 7.16 Proposed hydrosilylation mechanism with soluble catalysts of platinum or rhodium.

In order to better characterize the system, a further kinetic study was carried out on these two catalysts. Hydrosilylation mechanism has been thoroughly studied in the literature. This is a complex system, since the mechanism depends altogether on the catalyst, the reactants and the experimental conditions. This also explains why for each new reactant, a whole new experimental set-up has to be developed. In most cases already described, the limiting step is the insertion of platinum in the Si-H bond, leading to an apparent rate of reaction independent of double bond concentration ... 

Two remaining polymers have been encountered in the literature which fall under the category of silarylene polymers, but these polymers have no siloxane linkages. The first polymer which will be discussed has been used by Zelei and co-workers 22 as well as Ikeda and co-workers 23 in thermal degradation studies of 1 (poly(tetramethyl-p-silphenylenesilox-ane)). The polymer used was poly(p-dimethylsilphenylene) (101 and has been prepared by a method shown below. The second polymer is poly[l-(dimethylsilyl)-4-(dimethylethyl-ene)silylbenzene]) (H) and was prepared using a well known hydrosilylation mechanism in... 

Scheme 21 Proposed mechanism for the initiation of the light-induced hydrosilylation of an alkene.

Substantial advances have been made in the field of hydrosilylation [Eq. (108)] since 1965, when a comprehensive mechanism (Fig. 6) for catalysis by... 

The proposed mechanism for the hydrosilylation of olefins catalyzed by chloroplatinic acid is outlined in Fig. 6. Catalysis by square-planar or trigonal bipyramidal rf complexes can be similarly described (54, 55, 105). 

The mechanism for hydrosilylation in Figs. 6 and 7 clearly has much in common with suggestions regarding homogeneous transition metal catalysis for other processes involving olefins, such as hydrogenation, isomerization, the oxo reaction, and oligo- and polymerization. 

Consistent with this mechanism, HCo(CO)4 is an effective hydrosilylation catalyst 13, 14, 57). In the absence of olefin, RjSiCo(CO)4 is formed (Section II,B,4). 

A catalytic mechanism, which is supported by deuterium-labeling experiments in the corresponding Ru-catalyzed procedure [146], is shown in Scheme 47. Accordingly, the reactive Fe-hydride species is formed in situ by the reaction of the iron precatalyst with hydrosilane. Hydrosilylation of the carboxyl group affords the 0-silyl-A,0-acetal a, which is converted into the iminium intermediate b. Reduction of b by a second Fe-hydride species finally generates the corresponding amine and disiloxane. 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

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