Rhodium catalysis hydrosilylation

L. V. Dinh, J. Gladysz, Transition Metal Catalysis in Fluorous Media Extension of a New Immobilization Principle to Biphasic and Monophasic Rhodium-Catalyzed Hydrosilylations of Ketones and Enones , Tetrahedron Lett. 1999, 40,8995. 

A variety of transition metal complexes catalyze hydrosilylation of alkynes. Catalysis of hydrosilylation by rhodium gives T -alkenylsilanes from 1-alkynes.74... 

I 7 Well-Defined Surface Rhodium Siloxide Complexes and Their Application to Catalysis Table 7.5 Hydrosilylation of 1-hexadecene and allyl ethers by polyhydrosiloxane (7.1) . 

Catalysis of hydrosilylation by dimeric or by monomeric rhodium (and iridium) siloxide complexes occurs via preliminary oxidative addition of siUcon hydride followed by elimination of disiloxane (detected by GC/MS) to generate the square planar 16e hydride complex with an already coordinated molecule of alkene (Scheme 7.7). 

Scheme 7.8 Mechanism of heterogeneous catalysis of hydrosilylation by a surface rhodium (diene) siloxide complex.

Although the hydridorhodacarborane is formally a rhodium (III) derivative, it functions as a facile catalyst in alkenc isomerization, hydrogenation, hydroformylation, and hydrosilylation reactions 80). This catalyst system is extremely stable and may be recovered quantitatively from alkene isomerization and hydrogenation reactions. In addition to these reactions, the hydridorhodacarborane is very effective in the catalysis of deuterium exchange at terminal BH positions 59). These discoveries may soon lead to industrially useful metallocarborane catalysts. 

While platinum and rhodium are predominantly used as efficient catalysts in the hydrosilylation and cobalt group complexes are used in the reactions of silicon compounds with carbon monooxide, in the last couple of years the chemistry of ruthenium complexes has progressed significantly and plays a crucial role in catalysis of these types of processes (e.g., dehydrogenative silylation, hydrosilylation and silylformylation of alkynes, carbonylation and carbocyclisation of silicon substrates). 

Labande et al. tested the rhodium(I) complexes of diphenylphosphinoferrocenyl functionalised NHC ligands (Cp,Cp and Cp,Cp substitution) in the hydrosilylation of ketones finding these complexes of only moderate activity [186]. As no attempt was made for the chiral resolution of the catalysts prior to use in catalysis, the prochiral acetophenone could not be tested in asymmetric catalysis. 

All complexes have shown high catalytic activity, even at room temperature (in contrast to platinum catalysts). Hydrosilylation in the presence of phosphine-rhodium complexes occurred in air, because real catalyst (active intermediate) was formed after oxygenation and/or dissociation of phosphine, as reported previously [14]. The non-phosphine complexes 1 and 4 are also very efficient catalysts for the hydrosilylation of allyl glycidyl ether. Irrespective of the starting precursor, a tetracoordinated Rh-H species, responsible for catalysis, is generated under reaction conditions, as illustrated in Scheme 3. 

Nickel exhibits lower catalytic activity than platinum and rhodium catalysts , and in many cases, phosphine-nickel complexes cause the disproportionation of chlorohy-drosilanes giving complex results. However, in some cases, the regioselectivity in the hydrosilylation of styrene with trichlorosilane catalyzed by nickel complexes is quite different from that achieved by platinum or rhodium catalysts. For instance, a-adduct is exclusively formed by the catalysis of [Ni(CO)(ir-C5H5)]2 ... 

Our results demonstrate that during the course of a heterogeneous hydrosilylation, rhodium can move on the support (if free phosphino groups are still available on the support). It is very probable that part of the reaction could occur with soluble species. In the case of asymmetric catalysis, they are achiral and will lower the optical yield. It also is not clear at present if in homogeneous hydrosilylation similar phenomena do occur, which would increase the number of catalytically actives species. 

One of the most confusing problems in the hydrosilylation catalysis by rhodium complexes is the influence of molecular oxygen as a cocatalyst. This is a general phenomenon and also occurs in the presence of other transition metal complexes, such as platinum and ruthenium, particularly those with CO and phosphine ligands. The concerted mechanism of hydrosilylation processes catalyzed by Wilkinson catalyst involves predissociation of the phosphine from the complex. In this respect, molecular oxygen functions as a promoter since the dissociation of phosphine occurs more readily from [RhCl(02)(PPh3)3] than from the... 

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. 

Hydrosilanes undergo addition to carbon-carbon multiple bonds under catalysis by transition metal complexes. Nickel, rhodium, palladium, and platinum were used as catalytically active metals. By incorporating chiral ligands into the metal catalyst, the hydrosilylation can be performed analogously to other addition reactions with double bonds, for example, asymmetric hydrogenation to obtain optically active alkylsilanes. 

Chiral Phenanthrolines in Asymmetric Catalysis. Synthetic modifications on the Phenanthroline core can provide access to highly valuable chiral phenanthroline derivatives with important applications in asymmetric catalysis (Scheme 1). For example, LI has been utilized in copper-catalyzed allylic oxidations, L2 in palladium-catalyzed allylation reactions, and L3-type ligands in rhodium-catalyzed enantioselective hydrosilylation reactions of acetophenone. ... 

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