Carbonyl complexes, hydrosilylation oxidation

Alkenyloxacarbenes, with tungsten carbonyls, 5, 675 Alkenyl oxiranes, for C-N bonds via amination, 10, 704 (Alkenyloxy)hydrosilanes, hydrosilylation-oxidation, 10, 832 Alkenylphosphonium iridium(III) complexes, preparation,... 

These complexes anchored to a solid via a ligand have been tested for a number of reactions including the hydrogenation, hydroformylation, hydrosilylation, isomerization, dimerization, oligomerization, and polymerization of olefins carbonylation of methanol the water gas shift reaction and various oxidation and hydrolysis reactions (see later for some examples). In most cases, the characterization of the supported entities is very limited the surface reactions are often described on the basis of well-known chemistry, confirmed in some cases by spectroscopic data and elemental analysis. 

A number of reactions, principally of olefinic substrates, that can be catalyzed by supported complexes have been studied. These include hydrogenation, hydrosilylation, hydroformylation, polymerization, oxidative hydrolysis, acetoxylation, and carbonylation. Each of these will be considered in turn together with the possibility of carrying out several reactions consecutively using a catalyst containing more than one kind of metal complex. 

Heterogcnized complexes have been used to catalyze a great number of reactions, such as hydrogenation [18], hydroformylation [19], ethylene oligomerization [20], hydrosilylation [21, 22], polymerization [23], telo-merization [24], oxidation [25], oligomerization of monoalkenc [26], methanol carbonylation [27], butadiene oligomerization [28], synthesis gas chemistry [29], and isomerization [30],... 

Molecular oxygen has become a commonly used co-catalyst for inactive or weakly active transition metal complexes [1-5]. In addition, other oxidizing agents, mainly peroxides, have recently been used in active rhodium complexes in particular, but also in metal carbonyls, as catalysts for hydrosilylation. The catalytic activity of bis(triphenylphosphine)carbonylrhodium(I) in the hydrosilylation of C=C and C=0 bonds can be much increased by the addition of about a 50 % molar excess of tert-butyl hydroperoxide [100]. Chromium triad carbonyls M(CO)e, where M = Cr, Mo, W, have been tested to examine the effect of various organic peroxides on the hydrosilylation of 2,3-dimethyl-1,3-butadiene by triethyl-, triethoxy- and methyldiethoxysilanes [100]. The evidence for organic oxidant promotion of RhCl(cod)phosphine-catalyzed hydrosilylation of 1-hexene was demonstrated previously [101]. 

A number of metal complexes catalyze the hydrosilylation of various carbonyl compounds by triethylsilane. Stereoselectivity is observed in the hydrosilylation of ketones as in the reactions of 4-t-butylcyclohexanone and triethylsilane catalyzed by ruthenium, chromium, and rhodium metal complexes (eq 4). Triethylsilane and Chlorotris(triphenylphosphine)rho-dium(I) catalyst effect the regioselective 1,4-hydrosilylation of Q ,/3-unsaturated ketones and aldehydes. Reduction of mesityl oxide in this manner results in a 95% yield of product that consists of 1,4- and 1,2-hydrosilylation isomers in a 99 1 ratio (eq 5). This is an exact complement to the use of phenylsilane, where the ratio of respective isomers is reversed to 1 99. ... 

The aim of this Chapter is to examine the application of well-defined N-hetero-cyclic carbene (NHC) complexes as well as the systems prepared in situ which involve free NHCs or the precursor salt for the reduction of imsaturated organic molecules such as alkynes, alkenes and carbonyl compounds. The most active complexes for such reductions contain electron-rich, late transition metals in low oxidation states. Herein, reductions useful for organic synthesis will be classified into four types aeeording to reductants used (i) hydrogenations, (ii) transfer hydrogenation, (iii) hydrosilylation and (iv) hydroboration. For examples of reduction reactions with systems containing non-classical NHC ligands, the reader is referred to Chapter 5. 

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