Iridium complexes hydrosilylation

New N-heterocyclic carbene rhodium and iridium complexes derived from 2,2 -diaminobiphenyl were successfully synthesized and their structures unambiguously characterized by X-ray diffraction (XRD) analysis. These are cata-lytically active for the hydrosilylation of ketones with diphenylsilane, although an NHC—rhodium complex was found to be the best among those investigated [45]. 

Platinum complexes have been mainly used in the hydrosilylation of carbon-carbon bonds, and ruthenium complexes in the metathesis and silylative coupling of olefins with vinylsilanes. Most of these processes (except for olefin metathesis) may also proceed efficiently in the presence of rhodium and iridium complexes. 

In contrast to the Pt(0) and Pt(II) complexes and the corresponding Rh(I) and Rh(III) complexes, the iridium complexes have rarely been employed as hydrosilylation catalysts [1-4]. Iridium-phosphine complexes with d metal configura-tion-forexample, [Ir(CO)Cl(PPh3)2] (Vaska s complex) and [Ir(CO)H(PPh3)3]-were first tested some 40 years ago in the hydrosilylation of olefins. Although they underwent oxidative addition with hydrosilanes (simultaneously to Rh(I) com-... 

The complexes of the composition [ Ir( t-X)(diene) 2], where X = halogen, OH, OMe (e.g. [IrCl(CO)(cod)]) appeared to be very effective catalysts for the hydrosilylation of allyl chloride by trialkoxy- and alkylalkoxy-silanes [22]. Other iridium complexes have been subsequently reported as catalysts for the synthesis of silane... 

Computational and catalytic studies of the hydrosilylation of terminal alkynes have been very recently reported, with the use of [ Ir( r-Cl)(Cl)(Cp ) 2] catalyst to afford highly stereoselectively P-Z-vinylsilanes with high yields (>90%) [35]. B-isomers can be also found among the products, due to subsequent Z —> E isomerization under the conditions employed. The catalytic cycle is based on an lr(lll)-lr(V) oxidahve addition and direct reductive elimination of the P-Z-vinylsilane. Other iridium complexes have been found to be active in the hydrosilylation of phenylacetylene and 1-alkynes for example, when phenylacetylene is used as a substrate, dehydrogenative silylation products are also formed (see Scheme 14.5 and Table 14.3). 

Iridium complexes are known to be generally less active in hydrosilylation reactions when compared to rhodium derivatives, although iridium-based catalysts with bonded chiral carbene ligands have been used successfully in the synthesis of chiral alcohols and amines via hydrosilylation/protodesilylation of ketones [46-52] and imines [53-55], The iridium-catalyzed reaction of acetophenone derivatives with organosubstituted silanes often gives two products (Equation 14.3) ... 

As mentioned above, iridium complexes are also active in the formation of amines via the hydrosilylation/protodesUylation of imines. In the presence of 2 equiv. of HSiEts, the cationic complex [lr bis(pyrazol-l-yl)methane (CO)2][BPh4] (C4) catalyzes the reduction of various imines, including N-alkyl and N-aryl imines and both aldimines and ketimmes. Excellent conversions directly to the amine products were achieved rapidly at room temperature in a methanol solution (Scheme 14.7) [53]. 

The above-mentioned iridium complex, C4, as well as [ lr[bis(l-methylimidazol-2-yl)methane (CO)2][BPh4] (C5), also appeared very efficient in a one-pot tandem hydroamination/hydrosilylation reaction of 4-pentyn-l-amine with HSiEts to form 2-methylpyrroline, and then subsequently l-(triethylsilyl)-2-methylpyrrohdine with an essentially quantitative yield (Equation 14.5) [54]. 

Among the latter group, iridium complexes (though less common than rhodium) and perhaps also ruthenium play crucial roles in many of the above-mentioned transformations of silicon compounds, leading to the creahon of sihcon-carbon bonds. Examples include the hydrosilylation or dehydrogenahve silylation of alkenes and alkynes, the hydroformylahon of vinylsilanes, and the silyhbrmylation of alkynes as well as activation of the sp C—H of arenes (by disilanes) and alkenes (by vinylsilanes). 

Iridium complexes with O-donor ligand environments, Ir(triso)(ol)2 [triso = tris(diphenyloxophosphoranyl)methanide ol = C2H4, cyclooc-tene), catalyze the hydrosilylation and dehydrogenative silylation of ethylene with triphenylsilane.35 Diphenylmethylsilane can also be used in the... 

Crabtree and Chianese have extended the scope of Hoveyda s ligand by making the imidazolium salt 39 in two steps from l,l/-diamino-2,2/-binaphthyl (Fig. 10) [80]. They prepared neutral rhodium and iridium complexes with that ligand precursor and applied these complexes in the asymmetric hydrosilylation of acetophenone. Moderate enantioselectivities were obtained with the iridium derivative (up to 60% ee) whilst the rhodium catalysts only gave low enantioselectivities. 

A successful study of non-phosphine iridium complexes Ir", Ir , and Ir e. g., IrX(cod)2 [60], IrH2(triso)(SiMePh2)2 [61, 62], Ir(triso)(coe)2 (coe = cyclooctene triso = tris(diphenyloxophosphoranyl)methanide), Ir(triso)(C2H4)2 [61], has demonstrated effective hydrosilylation of alkenes and alkynes. Iridium phosphine complexes, e. g., Ir(C=CPh)(CO)2PCy2 [63] and IrCl(CO)(PPh3)2 [64], are also found to be active for hydrosilylation of phenylacetylene and 1-hexyne. 

Interesting, optically acive complexes were obtained upon reaction of organosilicon hydrides. The reactions of R3SiH with transition metal complexes represent a key step in catalytic hydrosilylation reactions. Catalytic activation of silicon hydrides has been proposed to arise from an oxidative addition process to a transition metal centre. Such a process has been shown to be reversible via a reductive elimination step (equation 9). The stereochemistry of addition to an iridium complex was shown to occur in a cis fashion59 (equation 10). 

Malacca, R., Poli, R., Manoury, E. (2010). Asymmetric hydrosilylation, transfer hydrogenation and hydrogenation of ketones catalyzed by iridium complexes. Coordination Chemistry Reviews, 254, 729—752. http //dx.doi.Org/10.1016/j.ccr.2009.09.033. 

Ir(I)-oxazolinyl-phosphane complexes are also effective catalysts for hydrosilylation of imines (246). Chiral poisoning of iridium complexes containing racemic DIOP with PRONOP provides an enantioselective catalyst (247). Iridium complexes containing phosphinooxazoline or dithioether ligands (like DIOSPr or DIOSPh) catalyze the enantioselective hydrogenation of functionalized olefins (248). 

Chiral rhodium complexes have provided high yields but moderate ee values in the asymmetric hydrosilylation of acetophenone [56,125]. Chiral iridium complexes, in which the NHC bears a hydroxyamide, provided moderate to good yields (up to 85%) in the hydrosilylation of a wide range of aryl methyl ketones. More importantly, high enantioselective induction was observed (up to 92%) at room temperature [126]. 

Apart from hydrosilylation of alkenes/ " catalytic transformations involving organosilicon compounds have been extensively studied, mainly by Marciniec and co-workers.Iridium complexes have shown to be active catalysts for these processes incorporation of CO and silanes into organic substrates has been reported via silylcarbo-... 

As an alternative, iridium complexes show exciting catalytic activities in various organic transformations for C-C bond formation. Iridium complexes have been known to be effective catalysts for hydrogenation [1—5] and hydrogen transfers [6-27], including in enantioselective synthesis [28-47]. The catalytic activity of iridium complexes also covers a wide range for dehydrogenation [48-54], metathesis [55], hydroamination [56-61], hydrosilylation [62], and hydroalkoxylation reactions [63] and has been employed in alkyne-alkyne and alkyne - alkene cyclizations and allylic substitution reactions [64-114]. In addition, Ir-catalyzed asymmetric 1,3-dipolar cycloaddition of a,P-unsaturated nitriles with nitrone was reported [115]. 

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... 

Iridium(l) precursors [lr(cod)(L)] with bidentate N-heterocyclic carbene ligands L3 appeared slightly less active in the hydrosilylation of acetophenone with diphe-nylsilane than did the similar rhodium complexes, giving respectively yields of 85% of I and 15% of II for the Pr substituent, and 83% of I and 17% of II for the benzyl moiety, after 2 h reaction at room temperature [47]. However, when carbene ligands of type L3 were used a significant increase in the ee-value of the sec-phenethyl alcohol R isomer of up to 60% was observed. 

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