Homogeneous hydrosilylation, transition

Silanes have also found use in colloid preparation. An interesting example for the use of organosilanes in the formation of organosols of transition metals is the reduction of platinum(II) complexes, especially cyclooctadieneplatinum dichloride, to colloidal platinum by trialkoxysilanes and trialkylsilanes. Hus was reported by Lewis and coworkers [60] who demonstrated that the presumably homogeneous hydrosilylation catalysts formed from platinum compounds in the presence of silanes were in fact colloidal in nature, and raised the prospect for colloid activity in many homogeneous catalyst systems (see Section 6.5). The same reduction method with organosilanes has been used for the preparation of colloids of rhodium and palladium. [61]... 

The most common catalyst used to date is chloroplatinic acid (also known, after its discoverer, as Speier s catalyst) it is now clear that, contrary to earlier views (23), hydrosilylation is a homogeneous process (25, 208). A major problem is that of reproducibility, and efforts are being made to utilize soluble transition metal complexes. Information about such systems has been used in the interpretation of some related catalytic heterogeneous reactions (232). 

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. 

Two significant communications indicate the considerable potential of transition metal complexes as multifunctional homogeneous catalysts in the silane field (5, 53). Here the same catalyst activates silanes toward different substrates and it is probable that all proceed via a common metal hydrido intermediate. Both Co2(CO)8 and (Ph3P)3CoHX [X = H2, N2, or (H)Si(OEt)j] catalyze 0-silylation and hydrosilylation the hydrogen on Si may be replaced by R O, R COO, R CONH, or R3SiO [e.g., Eqs. (117)-(120)], and excellent yields of silylated product result. Phenolic groups do... 

The author hopes that this chapter has convinced the readers of the value of homogeneous catalysis for the synthesis of organophosphorus compounds and for organo-heteroatom compounds in a broader sense. Hydrosilylation and hydroboration are indispensable modern synthetic reactions in this category. The H-P addition reactions herein described joins them as a third member. Although this chapter does not cover, the addition reactions of the S-P and Se-P bonds in thiophosphates [39] and selenophosphates [40] to alkynes also proceed in the presence of transition metal catalysts. In view of the wide use of phosphorus compounds, the new procedures will find practical applications. 

The first successful achievements using asymmetric homogeneous transition metal catalysis were obtained in the asymmetric hydrogenation of alkenes24 25, This method has been successfully used in many synthetic applications (Section D.2.5.1.)26-29. In addition, chirally modified versions of the transition metal catalyzed hydrosilylation of olefins and carbonyl compounds (Sections D.2.3.1. and 2.5.1.) and olefin isomerization (Section D.2.6.2.) have been developed. Transition metal catalyzed asymmetric epoxidation constitutes one of the most powerful examples of this type (Section D.4.5.2.). 

The stereochemistry of addition to acetylenes has been studied for various types of homogeneous catalysts. In practice, cis addition seems to be a common feature of hydrosilylation catalyzed by transition metal complexes, although this mechanistic scheme does not require a cis addition. Benkeser (39) found that the addition of trichlorosilane to 1-alkenes in the presence of homogeneous (chloro-platinic acid) catalysts induces a cis approach and results in the trans product. Several series of experiments with optically active trisubstituted silanes have indicated that when catalyzed by platinum complexes, the hydrosilylation process occurs overall with retention of configuration at the asymmetric silicon center. 

A proposed mechanism [9] for the hydrosilylation of olefins catalyzed by platinum(II) complexes (chloroplatinic acid is thought to be reduced to a plati-num(II) species in the early stages of the catalytic reaction) is similar to that for the rhodium(I) complex-catalyzed hydrogenation of olefins, which was advanced mostly by Wilkinson and his co-workers [10]. Besides the Speier s catalyst, it has been shown that tertiary phosphine complexes of nickel [11], palladium [12], platinum [13], and rhodium [14] are also effective as catalysts, and homogeneous catalysis by these Group VIII transition metal complexes is our present concern. In addition, as we will see later, hydrosilanes with chlorine, alkyl or aryl substituents on silicon show their characteristic reactivities in the metal complex-catalyzed hydrosilylation. Therefore, it seems appropriate to summarize here briefly recent advances in elucidation of the catalysis by metal complexes, including activation of silicon-hydrogen bonds. 

N.L. Lewis - Hydrosilylation, a Homogeneous Reaction Really Catalalyzed by Colloids, in Homogeneous Transition-Metal Catalyzed Reactions, W.R. Moser,... 

A review on the industrial applications of homogeneous catalysts is particularly welcome. The proceedings of recent symposia and a general text have been published in addition to reviews on metal cluster catalysts, activation of saturated hydrocarbons by metal complexes in solution, catalysis by arene Group-VIB tricarbonyls, titanocene-catalysed reactions ofalkenes, transition-metal hydrides in catalysis, the mechanisms of the catalytic cyclization of aliphatic, hydrocarbons, asymmetric hydrosilylation and asymmetric synthesis. A n.m.r. study of the conformations of chelated Diop and a MO study of organo-metallic migration reactions are also of interest. Polymer supported catalysts have been reviewed and the relationship between cross-linking of the polymer and catalytic activity has been discussed. ... 

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