Cobalt catalysts, hydrosilylation using

For the hydrosilylation reaction various rhodium, platinum, and cobalt catalysts were employed. For the further chain extension the OH-functionalities were deprotected by KCN in methanol. The final step involved the enzymatic polymerization from the maltoheptaose-modified polystyrene using a-D-glucose-l-phosphalc dipotassium salt dihydrate in a citrate buffer (pH = 6.2) and potato phosphorylase (Scheme 59). The characterization of the block copolymers was problematic in the case of high amylose contents, due to the insolubility of the copolymers in THF. 

Deng and coworkers recently developed the first silyl-donor-functionalized Af-heterocyclic carbene (NHC) cobalt complexes for use as catalysts in hydrosilylation reactions. Of the catalysts screened, 144 proved superior in rapidly producing the desired silylated products 145 and 146 in reactions between 1-octene 143 and PhSiHj while minimizing isomerization side products 147 and 148. The... 

The syntheses of axially chiral biarylphosphine oxides based on alkynylphosphine oxide 2.157 and acetylene was carried out using chiral cobalt catalyst 2.159 [100, 105, 106]. The [24-24-2] cycloaddition, however, resulted in axially chiral biarylphosphine oxides 2.160 with moderate yield and enantioselectivity (Scheme 2.55). It was accompanied by a recovery of some of the corresponding phosphines. The newly obtained phosphines were used in the reaction of palladium-catalyzed hydrosilylation of unsymmetrical alkenes [100]. 

The first rhodium-catalyzed reductive cyclization of enynes was reported in I992.61,61a As demonstrated by the cyclization of 1,6-enyne 37a to vinylsilane 37b, the rhodium-catalyzed reaction is a hydrosilylative transformation and, hence, complements its palladium-catalyzed counterpart, which is a formal hydrogenative process mediated by silane. Following this seminal report, improved catalyst systems were developed enabling cyclization at progressively lower temperatures and shorter reaction times. For example, it was found that A-heterocyclic carbene complexes of rhodium catalyze the reaction at 40°C,62 and through the use of immobilized cobalt-rhodium bimetallic nanoparticle catalysts, the hydrosilylative cyclization proceeds at ambient temperature.6... 

In contrast to rhodium and iridium, monoatomic cobalt has not been investigated extensively as a hydrosilylation catalyst, though one report discusses the use of cobalt phosphine complexes.54... 

Whereas, plahnum complexes are used predominantly as efficient catalysts in the hydrosilylation of carbon-carbon mulhple bonds, cobalt and iron triad complexes play a cmcial role in the catalysis of other processes, such as the hydrosi-lylahon of C=0 and C=N, dehydrogenative silylation, sUylcarbonylahon, and silylation with vinylsilanes and disilanes. 

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). 

Reductive TVansformations. The utility of 1 was first demonstrated in the enantioselective hydrosilylation of ketones. Uniformly high enantioselectivity, yield, and turnover were observed for aromatic (and some aliphatic) ketones when using the complex derived from RhCls (eq 1). Lower enantioselection is observed with t-Bu-pybox or i-Pr-pybox cobalt(I). The derived l Sn(OTf)2 complex gives alcohol products with up to 58% ee using methano-lic polymethylhydrosiloxane. A cationic ruthenium(III) catalyst diverts the usual reduction pathway to enolsilane formation, particularly when the nature of the silane is modified (eq 2). ... 

Hydroformylation reactions are important from the industrial point of view and the two commonly used hydroformylation catalysts are either Rh or Co based. We thought it would be interesting to anchor a SiOs unit on a cobalt cluster via hydrosilylation. This would be a close model to a silica-supported cobalt cluster. Secondly, since the reactions of silanetriols have been demonstrated to afford three-dimensional metallasiloxanes, we anticipated that this silanetriol would react with substrates such as trialkylaluminums, affording cobalt carbonyl cluster anchored aluminosiloxanes. Such compounds would resemble a modified zeolite having on its surface catalytically active cobalt carbonyl moieties and might inspire the preparation of actual zeolite systems with these modifications. 

Transition metal carbonyls such as Co2(CO)8 and CoH(CO)4, formed in the reaction of R3SiH with dimer (but also Fe(CO)5 and M3(CO)i2 (M = Fe, Ru, Os)) have been found to be active catalysts for the hydrosilylation of olefins, dienes, unsaturated nitriles, and esters as well as for hydrosilylation C=0 and C=N bonds [56]. Hydrosilylation of phenylthioacetylenes in the presence of this catalyst is extremely regioselective [57]. Cobalt(I) complexes, e. g., CoH(X)2L3 (X = H, N), could be prospective candidates for investigation of the effectiveness of alkene hydrosilylation by trialkoxysilanes as well as dehydro-genative silylation [58]. Direct evidence for the silyl migration mechanism operative in a catalytic hydrosilylation pathway was presented by Brookhart and Grant [59] using the electrophilic Co cationic complex. 

Many other catalysts based on cobalt, ruthenium, rhodium, and platinum are now known to catalyze the hydrosilylation of alkenes, and the types of products can be controlled by the choice of catalyst and silane. Rhodium complexes, such as Wilkinson s catalyst, have been used frequently. A comprehensive treatment of selectivities is beyond the scope of this chapter, but several reviews provide information on the products formed from different catalysts and silanes.As one example, crotononitrile undergoes hydrosilylation in the presence of Wilkinson s catalyst to form the a-silyl nitrile product (Equation 16.23), and this regioselectivity contrasts with that for the reaction of the related acrylic acid ester in Equation 16.19 conducted with Speier s catalyst. 

As hydrosilylation catalysts, palladium, chromium, cobalt, rhodium and ruthenium, besides platinum, are also used. The additions of silanes to dienes with these catalysts are called 1,4-addition, for example, the reaction is shown in eq. (8.23) [14-17]. 

Hydrosilylation in the presence of a carbon electrophile is often accompanied by C-C bond formation. For example, three-component coupling of hydrosilane, alkyne, and y unsaturated aldehyde is suggested to proceed via oxanickelacycle intermediate to give (Z)-enol silyl ether (Scheme 3-28). Hydrosilylation of alkenes under a carbon monoxide atmosphere allows carbonyl incorporation, giving silyl enol ethers by using a cobalt or iridium catalyst (Schemes 3-29 and 3-30). Under similar reaction conditions in the presence of a rhodium catalyst, alkynes are converted to y silyl-substituted acroleins (Scheme 3-31). ... 

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