Hydrosilylation activation period

Herrmann et al. reported for the first time in 1996 the use of chiral NHC complexes in asymmetric hydrosilylation [12]. An achiral version of this reaction with diaminocarbene rhodium complexes was previously reported by Lappert et al. in 1984 [40]. The Rh(I) complexes 53a-b were obtained in 71-79% yield by reaction of the free chiral carbene with 0.5 equiv of [Rh(cod)Cl]2 in THF (Scheme 30). The carbene was not isolated but generated in solution by deprotonation of the corresponding imidazolium salt by sodium hydride in liquid ammonia and THF at - 33 °C. The rhodium complexes 53 are stable in air both as a solid and in solution, and their thermal stability is also remarkable. The hydrosilylation of acetophenone in the presence of 1% mol of catalyst 53b gave almost quantitative conversions and optical inductions up to 32%. These complexes are active in hydrosilylation without an induction period even at low temperatures (- 34 °C). The optical induction is clearly temperature-dependent it decreases at higher temperatures. No significant solvent dependence could be observed. In spite of moderate ee values, this first report on asymmetric hydrosilylation demonstrated the advantage of such rhodium carbene complexes in terms of stability. No dissociation of the ligand was observed in the course of the reaction. 

By far, the most W-Si bonds reported in the period that this review covers involve W(CO)n or (t]S-CsRs)W-containing compounds. A significant development has been that of a recyclable catalyst for the hydrosilylation of ketones the system begins with a polar liquid substrate (ketone or ester) and finishes with a non-polar liquid product (alkoxysilane). The rest state of the catalyst is a mixture of the [BlCgFsTH salts of 36 and 37 the tungsten complex is far more active than its molybdenum analog. 

During the coverage period of this chapter, reviews have appeared on the following topics reactions of electrophiles with polyfluorinated alkenes, the mechanisms of intramolecular hydroacylation and hydrosilylation, Prins reaction (reviewed and redefined), synthesis of esters of /3-amino acids by Michael addition of amines and metal amides to esters of a,/3-unsaturated carboxylic acids," the 1,4-addition of benzotriazole-stabilized carbanions to Michael acceptors, control of asymmetry in Michael additions via the use of nucleophiles bearing chiral centres, a-unsaturated systems with the chirality at the y-position, and the presence of chiral ligands or other chiral mediators, syntheses of carbo- and hetero-cyclic compounds via Michael addition of enolates and activated phenols, respectively, to o ,jS-unsaturated nitriles, and transition metal catalysis of the Michael addition of 1,3-dicarbonyl compounds. ... 

Synthesis. Synthesis of the copolymers was performed by a hydrosilylation reaction of poly(dimethylsiloxane-co-methylhydrosiloxane) (Petrarch System, Inc.) and a-olefins of various lengths (Aldrich). A round-bottomed flask equipped with a magnetic stirring bar, condenser, and calcium chloride tube was charged with a 50% solution of the reactants (up to 10% molar excess of a-olefin) in dry toluene. A solution of hydrogen hexachloroplatinate(IV) in diglyme-isopropyl alcohol (150 ppm Pt) was then added to the reaction mixture. The reaction mixture was stirred at 60 °C for 3 h. At the end of this period, the mixture was refluxed with activated charcoal for 1 h and filtered while hot. Finally the solvent and excess a-olefins were removed under reduced pressure (67 Pa at 100 °C). The reaction proceeded to completion as evidenced by the absence of the Si-H absorption at 2130 cm" in the IR spectra. Residual a-olefin in the purified polymers was determined by gas-liquid chromatography. In all polymers, residual a-olefin was less than 1.5 wt %. 

The mechanism of catalytic hydrosilylation is not well understood. Study of these reactions is hampered by their complexity induction periods are often involved, reaction conditions such as the nature of the catalyst and reacting groups are critical factors, and side-reactions, such as alkene rearrangements, are common. A widely accepted mechanism for homogeneous catalysis by platinum complexes is based on the work of Chalk and Harrod (Scheme 10)5,8,262. With Speier s catalyst, it appears that initially, and probably during the induction period, silane reduces the platinum to a Pt(0) or Pt(II) complex that is the active catalytic species265. 

Several mechanisms have been proposed for the platinum-catalyzed homogeneous hydrosilylation reaction. The most commonly invoked mechanism, proposed by Chalk and Harrod in 1965, consists of elementary steps similar to homogeneous hydrogenation, oxidative addition, migratory insertion, and reductive elimination (226). However, this mechanism fails to describe the indnction period or the presence of colloidal species at the end of the reaction. Lewis proposed an alternative mechanism based on the intermediacy of colloids that were detected by transmission electron microscopy after evaporation of catalsdically active solutions (227,228). 

In order to collect more information about the mechanism of the reaction, we devised three independent experiments in which the three reactants (alkyne, silane, and catalyst) were incubated two by two at 60 °C for 3h, before addition of the third component at 20 °C [35]. The results of these experiments are presented in Figure 5.20. As can be seen, the addition of complex 45 to a mixture of alkyne and silane displays a kinetic profile identical to what was observed previously (curve A). However, when the silane was added to the alkyne, previously incubated with the precatalyst, a considerable reduction of the catalytic activity occurred (curve B). It thus transpires that the alkyne triggers somehow the deactivation of the catalyst. In stark contrast, a dramatic acceleration of the reaction rate, concomitant with the disappearance of the induction period, was observed when the catalyst was heated with the silane prior to addition of the alkyne (curve C). This last effect is reminiscent of what was observed upon repeated addition of fresh reactants during the hydrosilylation of alkenes (see Figure 5.7). Therefore, treating the precatalyst with a silane before adding the alkyne leads to a particularly active and selective catalyst. 

In a very recent study, Oro et al. prepared MCM-41 supported NHC-Rh complexes 170 and tested them in the solvent-free hydrosilylation of acetophenone with HSiMe(OSiMe3)2 (Figure 13.20). Even if an induction period was observed, these catalysts eventually exhibited good catalytic activity. 

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