Hydrosilylation iron catalysts

Abstract Organic syntheses catalyzed by iron complexes have attracted considerable attention because iron is an abundant, inexpensive, and environmentally benign metal. It has been documented that various iron hydride complexes play important roles in catalytic cycles such as hydrogenation, hydrosilylation, hydro-boration, hydrogen generation, and element-element bond formation. This chapter summarizes the recent developments, mainly from 2000 to 2009, of iron catalysts involving hydride ligand(s) and the role of Fe-H species in catalytic cycles. 

The comparison of a bis(imino)pyridine iron complex and a pyridine bis (oxazoline) iron complex in hydrosilylation reactions is shown in Scheme 24 [73]. Both iron complexes showed efficient activity at 23°C and low to modest enantioselectivites. However, the steric hindered acetophenone derivatives such as 2, 4, 6 -trimethylacetophenone and 4 -ferf-butyl-2, 6 -dimethylacetophenone reacted sluggishly. The yields and enantioselectivities increased slightly when a combination of iron catalyst and B(CeF5)3 as an additive was used. 

Scheme 4.27 Asymmetric hydrosilylation of ketone 66 with iron catalysts according to Nishiyama and Furuta [59],...

As for hydrogenations, predous metal catalysts have dominated the study of catalytic hydrosilylations, with Rh and Pt catalysts being prevalent in catalytic hydrosilylations. Compared to hydrogenations, however, there are more examples of cheap metals that catalyze the hydrosilylation reaction. In addition to the tungsten catalysts discussed above, several examples of iron catalysts for hydrosilylation are discussed in Chapter 4. Examples of catalytic hydrosilylations with Mn, Ti and Cu catalysts are briefly mentioned below. 

Scheme 4-328. Chiral iron catalysts and ligands for the hydrosilylation of ketones.

Employment of chiral bis(oxazolinylphenyl)amines such as SJS)-BopsL-dpm (Scheme 4-328) as ligands for iron catalysts leads to almost quantitative yields and high enantioselectivities for the asymmetric hydrosilylation of ketones and asymmetric conjugate hydrosilylation of enones with (diethoxy)methylsilane as reductant (Scheme 4-329). Both enantiomers of the hydrosilylation product can be obtained from the same chiral ligand by a slight variation of the reaction conditions. The mixed catalyst system of (S -Bopa-dpm and iron(II) acetate provides the (/ )-enantiomer of the alcohol... 

Bis(imino)pyridine iron complex 5 acts as a catalyst not only for hydrogenation (see 2.1) but also for hydrosilylation of multiple bonds [27]. The results are summarized in Table 10. The reaction rate for hydrosilylations is slower than that for the corresponding hydrogenation however, the trend of reaction rates is similar in each reaction. In case of tra s-2-hexene, the terminal addition product hexyl (phenyl)silane was obtained predominantly. This result clearly shows that an isomerization reaction takes place and the subsequent hydrosilylation reaction dehvers the corresponding product. Reaction of 1-hexene with H2SiPh2 also produced the hydrosilylated product in this system (eq. 1 in Scheme 18). However, the reaction rate for H2SiPh2 was slower than that for H3SiPh. In addition, reaction of diphenylacetylene as an atkyne with phenylsilane afforded the monoaddition product due to steric repulsion (eq. 2 in Scheme 18). 

An iron complex-catalyzed asymmetric hydrosilylation of ketones was achieved by using chiral phosphoms ligands [68]. Among various ligands, the best enantios-electivities (up to 99% ee) were obtained using a combination of Fe(OAc)2/(5,5)-Me-Duphos in THF. This hydrosilylation works smoothly in other solvents (diethylether, n-hexane, dichloromethane, and toluene), but other iron sources are not effective. Surprisingly, this Fe catalyst (45% ee) was more efficient in the asymmetric hydrosilylation of cyclohexylmethylketone, a substrate that proved to be problematic in hydrosilylations using Ru [69] or Ti [70] catalysts (43 and 23% ee, respectively). 

Unlike Pt catalysts, the triad complexes of iron and cobalt catalyze competihvely both dehydrogenative silylation and hydrosilylation [11]. The reaction can proceed via a complex containing the o-alkyl and o-silylalkyl ligands (Scheme 14.1). 

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. 

Detailed mechanistic studies with respect to the application of Speier s catalyst on the hydrosilylation of ethylene showed that the process proceeds according to the Chalk-Harrod mechanism and the rate-determining step is the isomerization of Pt(silyl)(alkyl) complex formed by the ethylene insertion into the Pt—H bond.613 In contrast to the platinum-catalyzed hydrosilylation, the complexes of the iron and cobalt triads (iron, ruthenium, osmium and cobalt, rhodium, iridium, respectively) catalyze dehydrogenative silylation competitively with hydrosilylation. Dehydrogenative silylation occurs via the formation of a complex with cr-alkyl and a-silylalkyl ligands ... 

In a more recent report from Seki and Murai, Fe3(CO)i2 is shown to exhibit complete selectivity in the catalytic dehydrogenative silylation of styrenes.32 No products resulting from hydrosilylation are observed with the iron complex catalyst, in comparison to the minor amounts of hydrosilylated... 

Exclusive formation of silylstyrenes 76 is achieved when the reactions of styrene and 4-substituted styrenes with HSiEt3 are catalyzed by Fe3(CO)i2 or Fe2(CO)9100. Other iron-triad metal carbonyl clusters, Ru3(CO)i2 and Os3(CO)i2, are also highly active catalysts, but a trace amount of hydrosilylation product 77 is detected in the Ru-catalyzed reactions and the Os-catalyzed reactions are accompanied by 3-12% of 77 (equation 31)100. Mononuclear iron carbonyl, Fe(CO)5, is found to be inactive in this reaction100. 

Most examples in the literature on hydrosilylation with iron complexes as catalyst concern Fe(CO)5 or related iron carbonyl compounds [41]. The first use of iron pentacarbonyl was reported for the reaction of silicon hydrides with alkenes at 100-140 °C to form saturated and unsaturated silanes according to Scheme 4.20 [42, 43]. 

Figure 4 Paramagnetic iron complexes 13-15 are catalysts for hydrosilylation...

The carbon-nitrogen double bonds of nitrones N1-N3 (Fig. 14) were catalytical-ly reduced with diphenylsilane in the presence of Ru2Cl4(Tol-BINAP, L24)2(NEt3) to give hydroxylamines in high % ees [56]. The hydroxylamine HI was obtained in 63% yield with 86% ee (S) and the hydroxylamine H3 was formed in 91% ee. It was also proposed that this process opened a new access to optically active amines from racemic amines, via nitrones and hydroxylamines. The iron complex [(Cp)2Fe2(HPMen2> L25)(CO)2] was reported to be a catalyst in the asymmetric hydrosilylation of ketones under irradiation, where acetophenone was reduced in up to 33% ee [57]. 

More well-defined iron compounds have also been used as catalyst precursors for aldehyde and ketone hydrosilylation. Nikonov [110] reported that [(Tl -C5H5)Fe(PR3)(CH3CN)2] is an active hydrosilyation catalyst, although the substrate scope has yet to be explored. Using a ligand displacement reaction originally pioneered by Campora and coworkers [111], our laboratory reported that... 

Carbonyl compounds can be reduced efficiently in hydrosilylation reactions with an inorganic solid acid or base catalyst present [108, 109]. Iron montmorillonite catalyses hydrosilylation reactions most effectively (e.g. equation 4.21) [108], while sodium montmorillonite is completely inactive. 

The catalytic addition of organic and inorganic silicon hydrides to alkenes, ary-lalkenes, and cycloalkenes as well as their derivatives with functional groups leads to their respective alkyl derivatives of silicon and occurs according to the anti-Markovnikov rule. However, under some conditions (e.g., in the presence of Pd catalysts), this product is accompanied by a-adduct (i.e., the one containing an internal silyl group). Moreover, dehydrogenative silylation of alkenes with hydrosilanes, which proceeds particularly in the presence of iron- and cobalt-triad complexes as related to hydrosilylation (and very often its side reaction), is discussed. 

Iron pentacarbonyl was the first reported metal carbonyl catalyst for hydrosilylation, and although this reaction occurs under mild conditions (temperature below 100°C), it takes a somewhat unexpected course. In the presence of this catalyst, the hydrosilylation of ethylene and its derivatives is accompanied by the dehydrogenative silylation (3). In excess of alkene, vinyl trisubstituted silanes are produced almost exclusively. 

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