In hydrosilation catalysis

Unlike the iron triad complexes, cobalt triad compounds, particularly those of Rh and Ir are much more active in hydrosilation catalysis, although the cobalt carbonyl Co2(CO)g has played a very important role in furthering understanding of the catalytic cycle with transition metals. Recently, a simple cobalt salt, CoBr2, in conjunction with added Bu"P, Znl2 and BU4NBH4 was shown to afford 1,4-hydrosilated isoprene (silicon at C-4) in 90% isolated yield, where most metal catalysts derived from Ru, Rh, Pd and Pt usually lead to a mixture of regioisomers. ... 

These metals, belonging to groups on either side of the main block of transition metals (Group VIIIB or 8, 9 and 10) commonly examined in hydrosilation catalysis, have only been employed very rarely to facilitate hydrosilation. Very few reports have emerged during the last 15 years on catalysts based on these metals and their use has generally been connected with special cases of hydrosilation. 

Platinum(O) phosphine complexes undergo a variety of oxidative addition reactions with compounds containing Group 14 elements. These reactions are of widespread interest because similar processes are probably involved in the catalysis by platinum complexes of reactions such as the hydrosilation of alkenes and the disilylation of dienes and alkenes. 

An important potential target of asymmetric hydrosilation catalysis is to the reduction of the cyclopenten-l,4-diones, 1, producing the 4-silyloxyeyclopentenones, 2, which are key intermediates in the synthesis of singleisomer prostaglandins (18-22). 

Based in part on the article Hydrosilation Catalysis by John F. Harrod which appeared in the Encyclopedia of Inorganic Chemistry, First Edition. 

Other substituent groups are usually introduced via Hydrosilation reactions. In this important reaction, Si-H bonds add across C=C or C=C bonds in 1,2 fashion to give products. Many transition metals and their complexes will catalyze hydrosilation, but the usual catalyst is platinum see Hydrosilation Catalysis). Thus CF3CH2CH2Si(Me)Cl2 is made by hydrosilation of 3,3,3-trifluoropropene with methyldichlorosilane (equation 12). n-Alkylmethyldichlorosilanes can be made similarly, by hydrosilation of l-aUcenes. Vinylmethyldichlorosilane may be obtained in analogous fashion by hydrosilation of acetylene, as shown in equation (13). An alternate route to the same compound is hydrosilation of vinyl chloride, followed by dehydrochlorination (equations 14 and 15). 

Photolysis of dinuclear complexes such as Co2(CO)g and Mn2(CO)io can in principle lead to 17 electron metal radicals which can participate in further catalysis with silanes. Such species might be anticipated to act in one of two ways. Abstraction of a hydrogen from a H-Si bond in a manner which might favor radical chain reaction catalysis of hydrosilation and might be attractive given the lower homolytic dissociation energy of Si-H bonds (relative to H-H). Alternatively, the highly labile 17 e species provide routes for active substitution of alkene substrates and or silanes into the coordination sphere. 

This fundamentally sound model of the catalytic cycle continues to be widely accepted, but its simplistic picture cannot explain some common observations and the formation of certain products in hydrosilations catalyzed by several transition metals other than platinum. The most difficult to explain and accommodate via the Chalk-Harrod mechanism are formation of vinylsilanes, highly-colored solutions often associated with gray-black metal precipitation, and frequently an induction period before the catalytic cycle kicks off in earnest. The deeper question of whether the catalysis occurs homogeneously or on the surface of colloidal metal has been an additional mystery that has shrouded the catalysis, especially in light of the fact that... 

The first example of acid catalysis appeared in a 1934 patent in which it is claimed that surface catalysts, particularly hydrosilicates of large surface area , known at that time under the trade name Tonsil, Franconit, Granisol, etc. lead to a smooth addition of the olefine to the molecule of the primary aromatic amine . Aniline and cyclohexene were reacted over Tonsil at 230-240°C to give, inter alia, the hydroamination product, N-cyclohexylaniline [47]. 

The range of reactions which have been examined is wide (248) and includes hydrogenations (256), ammonia synthesis (257), polymerizations (257), and oxidations (258). Little activity has occurred in this area during the past few years. Recent reports of the effects of sonication on heterogeneous catalysis include the liquefaction of coal by hydrogenation with Cu/Zn (259), the hydrogenation of olefins by formic acid with Pd on carbon (260), and the hydrosilation of 1-alkenes by Pt on carbon (261). 

A consistently useful theory for a mechanism for homogeneous catalysis of hydrosilation has been expounded by Chalk and Harrod (//-/Id). This theory is accepted, adapted, and relied on to give a qualitatively rational explanation for numerous data in the following discussions. 

Asymmetric catalysis by chiral rhodium complexes in both hydrogenation and hydrosilation have been found. Kagan (42) recently reported that the chiral phosphine rhodium complex [(—)-DIOPRhCl]2 gave the most efficient asymmetric syntheses observed to date. This complex is ... 

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