Induction period silanes

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. 

A number of features of the silane and germane polymerization reactions show unequivocally that there are at least two independant mechanisms operating. This dichotomy is most evident in situations where the products of reaction can be compared for the induction period that precedes the formation of compound J, and the products that are produced after the formation of 1 (12,15,20). 

The remaining catalyst system, a combination of the commercially available metallocenes, Cp2MCl2 (M = Ti, Zr, Hf) with BuLi was introduced by Corey and co-workers.3m 48,49 With this system there is no induction period when used for the condensation of PhSiH3, and secondary silanes can be coupled at reasonable rates at higher temperatures. 

The addition of methyldichlorosilane to styrene has been carried out homogeneously in the presence of [PtCl2(CgH8)]2 246). This reaction is first-order with respect to the styrene and the silane, and first-order with respect to the platinum complex. In aromatic solvents, an induction period was observed which was attributed to coordination of the aromatic or a second molecule of styrene to the platinum. This necessity of a vacant site was further emphasized by the finding that addition did not occur in donor solvents such as tetrahydrofuran, ether, or pyridine. 

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. 

Zeolite powder has the lowest flowability of all detergent powder components. Bridging in the silos and screwers is a common event. An improvement can be obtained by modifying the surface by sorption of different additives (e.g.. Silanes [57]), or modifying the surface as early as the induction period, as was discussed above. 

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