Styrene, hydrosilation

With cyclohexene, polymerization occurs more rapidly than hydrosilation. After polymerization has proceeded to completion, there is a slow hydrosilation to introduce cyclohexyl groups onto the polymer chain, to a maximum extent of about 50 per cent of the Si-H groups. With more reactive olefins, such as styrene, hydrosilation occurs more rapidly than polymerization and the polymerization reaction is suppressed. As in the polymerization reaction, the reactivity of primary silanes is much greater than... 

SCN L = Me2SO, Et2SO, Et2S, NH3), based on the strong-base anionite AB-17-8 and the strong-acid cationite KY-2-8. Many of these metal complexes exhibit catalytic activity during styrene hydrosilation by methyldichlorosilane and are stable enough to be used repeatedly. 

Catalyst Metal content m Degree of exchange W Styrene hydrosilation by methyl-dichlorosilane ... 

Titanocene catalysts do not catalyze the hydrosilation of most internal olefins, although they can attach active olefins such as styrene, or norbornene to the growing polymer chain ends. The zirconocene-based catalysts, on the other hand, can be powerful hydrosilation catalysts and the remarkable copolymer synthesis shown in Equation 3 can be easily achieved under mild conditions (V7). 

Our experiences with chlorosilanes (12) indicate some of the processes that take place during an induction period. The experiences are synop-sized as though they were obtained in one experiment. The results are in general accord with those described by Benkeseref al. (13), who studied the hydrosilation of styrene in ethylbenzene. 

In the presence of electron-donor solvents or organic bases, hydrosilation of styrene did not occur. An amount of pyridine, quinoline, or triphenylphosphine equivalent to [C6H5CH=CHPtCl2]2 in solutions at 20°C gave a long induction period followed by hydrosilation at rates much slower than those that had been observed in absence of the bases. So-... 

Solvents, PhCH2CH2EtSiCl2, PhMeSiCl2, benzene, toluene, hexane, excess styrene, or excess MeCl2SiH, had little or no influence on the rate of hydrosilation of styrene with MeCl2SiH. In these solvents, kinetics of hydrosilation were first-order in catalyst and zero-order for reagents, even when one of the reagents was in great excess over the other (18). 

To obtain reproducible kinetic data, Reiksfel d had to add styrene to a solution of [PhCH=CH2PtCl2]2 before MeCl2SiH was added. When the silane was added first and styrene was withheld for about 10 minutes, the rate of hydrosilation was noticeably retarded. 

Asymmetric hydrosilation of prochiral olefins other than 1,1-disubsti-tuted ones was unsuccessful because they were unreactive under the conditions employed. Styrene afforded a 20% yield of 1-phenylethyl- and 50% of 2-phenylethylmethyldichlorosilane, both inactive. The 1-phenylethyl isomer had a possibility of activity with its asymmetric center, PhMeC HSi=. 

The square-planar complex (34) NiCI2-(P-/i-Bu3)2 was a better catalyst than the tetrahedral complex NiBr2 (PPh3)2 for hydrosilation of styrene with trichlorosilane at temperatures of 150°-170°C. A nickel(0) complex, Ni[P(OPh)3]4, was as good as NiCl2(NC5H5)4, which was best among known nickel catalysts for this reaction. Addition of copper(I) chloride... 

Regiospecificity of the reaction is normally high but with some alkenes, such as styrene, considerable amount of the /3-isomer (38.5%) is produced exceptionally in addition to the a-isomer (58.7%). Interestingly, alkenes having an inner double bond undergo hydrosilation with isomerization to give exclusively terminal products, as shown in equation (24). 

Another recent report described the asymmetric hydrosilation of styrene using chiral phosphine ligands such as methyldiphenylphosphine 160). Enantiomeric excesses of 5% of S isomer of 2-phenylethyl-trichlorosilane were obtained. The mechanism almost certainly involved insertion of Pd(II) into a silicon-hydride bond, followed by addition of Pd-hydride across the olefinic bond and elimination of Pd(II) to give the silicon-carbon bond. 

The planar clusters [W2Pd2Cp2(CO)6(PR3)2] (3e, 3f) catalyze the hydrogenation and isomerization of 1,5-cyclooctadiene and the hydrogenation of phenylacetylene to a mixture of styrene (major) and ethylbenzene. These clusters also catalyze the photoinitiated hydrosilation of 1-pentene and butadiene oligomerization. ... 

The cluster [Co2Ni(//3-CMe)Cp(CO)6] has been used to catalyze the homogeneous hydroformylation of 1-pentene to hexanal and 2-methylpentanal. Hydro-formylation of styrene was achieved under mild conditions with moderate to high branched-to-normal selectivity. This cluster could be recovered in high yield (>90%) after catalysis. It also catalyzes the hydrosilation of acetophenone. ... 

B.i.a. Palladium-Catalyzed Hydrosilation ofAlkenes. Aside from the above-mentioned study by Takahashi and co-workers and an isolated example of Pd-catalyzed hydrosilation of styrene with HSiCls by Kiso, Yamamoto, Tamao, and Kumada, the first systematic study of Pd-catalyzed hydrosilation of alkenes most probably was that by Tsuji and co-workers reported in 1974. In this study, the scope of Pd-catalyzed hydrosilation with respect to (i) substrate structural type, (ii) silanes, and (iii) catalysts including phosphines was delineated for the first time. Some representative results obtained by using HSiCl3 and Pd(PPh3)4 are shown in Scheme 4, which indicate that not... 

On the other hand, the hydrosilation of styrenes with HSiClj in the presence of a Pd-phosphine complex has been shown to proceed via hydropaUadation rather than silyl-paUadation.t Specifically, the following set of results were obtained. 

Under the same reaction conditions, styrene was shown to undergo rapidly the expected hydrosilation to give a-trichlorosilylethylbenzene in 93% yield, whereas allyl-benzene did not produce any hydrosilation product, the only product being ( )-/3-methyl-styrene (49%) (Eqs. 2 and 3). 

Graft Copolymers. The hydrosilation technique described above for difunctional PDMS macroinitiators is also suitable for pendant functional polymers. For example, commercially available poly(dimethylsiloxane-5rar-vinylmethylsiloxane) was reacted with 2-(4-chloromethylphenyl)ethyldimethylsilane in a hydrosilation analogous to the top reaction shown in Scheme 3 to yield PDMS containing pendant benzyl chloride moieties. ATRP of styrene proceeded with first order kinetic consumption of monomer. An increase in molecular weight was observed from M = 6600 to 14,800. Unlike the triblock copolymers, here the polydispersity increased from 1.8 to 2.1 due to an inconsistent number of initiating sites per PDMS chain. Again, the product was isolated as a soluble white powder after purification 34). 

Complexes of nickel and nickel are suitable catalysts for the hydrosilation of styrene [165]. The choice of catalyst influences the product formed. An example [166] is given in Scheme 3.4. 

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