Hydrosilylation carbonyl compounds

Until 1968, not a single nonenzymic catalytic asymmetric synthesis had been achieved with a yield above 50%. Now, barely 15 years later, no fewer than six types of reactions can be carried out with yields of 75-100% using amino acid catalysts, i.e., catalytic hydrogenation, intramolecular aldol cyclizations, cyanhydrin synthesis, alkylation of carbonyl compounds, hydrosilylation, and epoxidations. 

Highly enantioselective hydrosilylation of aliphatic and aromatic carbonyl compounds such as acetophenone, methyl phenethyl ketone 1813, or deuterobenz-aldehyde 1815 can be readily achieved with stericaUy hindered silanes such as o-tolyl2SiH2 or phenyl mesityl silane 1810 in the presence of the rhodium-ferrocene catalyst 1811 to give alcohols such as 1812, 1814, and 1816 in high chemical and optical yield [47] (Scheme 12.14). More recently, hydrosilylations of aldehydes... 

Beller and coworkers reported hydrosilylation reactions of organic carbonyl compounds such as ketones and aldehydes catalyzed by Fe(OAc)2 with phosphorus ligands (Scheme 21). In case of aldehydes as starting materials, the Fe(OAc)2/PCy3 with polymethylhydrosiloxane (PMHS) as an H-Si compound produced the corresponding primary alcohols in good to excellent yields under mild conditions [67]. Use of other phosphorus ligands, for instance, PPhs, bis(diphenylphosphino) methane (dppm), and bis(diphenylphosphino)ethane (dppe) decreased the catalytic activity. It should be noted that frans-cinnamaldehyde was converted into the desired alcohol exclusively and 1,4-reduction products were not observed. 

Scheme 2.9 Hydrosilylation of carbonyl compounds to silyl ethers...

The hydrosilylation of carbonyl compounds by EtjSiH catalysed by the copper NHC complexes 65 and 66-67 constitutes a convenient method for the direct synthesis of silyl-protected alcohols (silyl ethers). The catalysts can be generated in situ from the corresponding imidazolium salts, base and CuCl or [Cu(MeCN) ]X", respectively. The catalytic reactions usually occur at room tanperature in THE with very good conversions and exhibit good functional group tolerance. Complex 66, which is more active than 65, allows the reactions to be run under lower silane loadings and is preferred for the hydrosilylation of hindered ketones. The wide scope of application of the copper catalyst [dialkyl-, arylalkyl-ketones, aldehydes (even enoUsable) and esters] is evident from some examples compiled in Table 2.3 [51-53],... 

Fig. 2.10 Hydrosilylation catalysts of carbonyl compounds based on Cu-NHC complexes...

Scheme 2.10 Intermediates in the Cu-catalysed hydrosilylation of carbonyl compounds...

The reaction of carbonyl compounds, R(R )C=0, with PhSeSiMe3 and Bu3SnH in the presence of a catalytic amount of AIBN as a radical initiator results in the hydrosilylation of carbonyl compounds giving the corresponding silyl ethers, R(R )CH-OSiMe3.132 133... 

Although there are now several catalysts useful for hydrogenation of saturated carbonyl compounds to alcohols (see Section XII), an alternative approach has involved initial hydrosilylation (Chapter 9 in this volume) followed by acid hydrolysis [Eq. (41)]. The area first developed using principally the RhCl(PPh3)3 catalyst (207-210), and has since proved particularly useful in asymmetric syntheses (see Section III,A,4). Besides simple aliphatic and aromatic aldehydes and ketones, the ter-pene-ketones camphor and menthone were stereoselectively reduced to mainly the less stable alcohols e.g., camphor gave 9 (209). 

A single example of the reductive cyclization of allenic carbonyl compounds is reported, which employs a rhodium-based catalyst in conjunction with Et3SiH as terminal reductant.113 This protocol promotes hydrosilylation-cyclization to form both five- and six-membered rings with exceptional levels of yy -diastereocontrol. As revealed... 

Although detailed mechanistic studies are not reported, the postulated mechanism for the reductive cyclization of allenic carbonyl compounds involves entry into the catalytic cycle via silane oxidative addition. Allene silylrhodation then provides the cr-allylrhodium hydride A-18, which upon carbometallation of the appendant aldehyde gives rise to rhodium alkoxide B-14. Oxygen-hydrogen reductive elimination furnishes the hydrosilylation-cyclization product... 

For a review encompassing titanium-catalyzed hydrosilylation of alkenes and carbonyl compounds, see Harrod, J. F. Coord. Chem. Rev. 2000, 206-207, 493-531. 

A variety of carbonyl compounds react with PhSeSiMes in the presence Bu3SnH/AIBN to afford the corresponding hydrosilylation derivatives [59,60]. Generally good yields are obtained only for aromatic substituted aldehydes or ketones. Reactions (5.28) and (5.29) show this case for a few aldehydes... 

Scheme 5.9 Propagation steps for hydrosilylation of carbonyl compounds using...

Yu and co-workers have expanded upon Ojima s work through development of an effective Rh-catalyzed protocol for the cyclization/hydrosilylation of allenyl carbonyl compounds to form silylated vinylcycloalkanols and heterocyclic alcohols.For example, reaction of tosylamide 44 (X = NTs, R = H, n= ) and triethylsilane catalyzed by Rh(acac)(GO)2 (1 mol%) under GO (10 atm) at 70 °G for 8h gave the silylated vinyl pyrrolidinol 45 (X = NTs, R = H, n= ) in 74% yield with exclusive formation of the m-diastereomer (Equation (29)). The rhodium-catalyzed reaction was also effective for the cyclization of alleneones and for the formation of carbocycles, oxygen heterocycles, and six-membered cyclic alcohols (Equation (29)). However, Rh-catalyzed cyclization/hydrosilylation of allenyl carbonyl compounds that possessed substitution on an allenyl carbon atom was not established (Equation (29)). The efficiency of the Rh-catalyzed reaction of allenyl carbonyl compounds depended strongly on GO pressure. Reactions run under 10 atm GO were more efficient than were... 

Brunner H (1998) Hydrosilylation of carbonyl compounds. In Beller M, Bolm C (eds) Transition metals for organic synthesis, vol 2. Wiley-VCH, Weinheim, p 131... 

Hydrosilylation of various carbonyl compounds, enones and related functional groups catalyzed by Group VIII transition metal complexes, especially phosphine-rhodium complexes, have been extensively studied1,3, and the reactions continue to serve as useful methods in organic syntheses. 

Hydrosilylation of carbonyl compounds catalyzed by fluoride salts such as CsF and KF, originally developed by Corriu and coworkers161-165, has been modified to homogeneous systems and found applications in organic syntheses as an efficient and selective reduction method. 

Asymmetric hydrosilylation of prochiral carbonyl compounds, imines, alkenes and 1,3-dienes has been extensively studied and continues to be one of the most important subjects in the hydrosilylation reactions. This topic has been reviewed at each stage of its development as a useful synthetic method based on asymmetric catalytic processes1,3,187-189. In the last decade, however, substantial progress has been made in the efficiency of this reaction. Accordingly, this section summarizes the recent advances in this reaction. 

Hydrosilylation of carbonyl compounds. The definitive report on reduction of carbonyl compounds by hydrosilylation catalyzed by Wilkinson"scatalyst is available.1 Hydrosilylation can be used to effect regioselective 1,2- or 1,4-reduction of a,/ -enals or -enones by proper choice of the hydrosilane. In general, monohydrosilanes favor 1,4-adducts, whereas dihydrosilanes favor 1,2-adducts. The regioselectivity is also influenced by the substituents on silicon and on the substrates. The presence of a phenyl group on the enone system can effect dramatic changes in the selectivity. Examples ... 

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]. 

Hydrosilylation reactions catalyzed by iron carbonyl compounds often occur under drastic thermal conditions. Schroeder and Wrighton reported a photocatalyzed reaction oftrialkylsilanes with alkenes in the presence of Fe(CO)5 at low temperatures (0-50 °C) [48]. It is well known that irradiation of mononuclear metal carbonyls leads... 

The hydrosilylation of carbonyl compounds with polymethylhydrosiloxane (PMHS) or other alkoxysilanes can be catalyzed by TBAF with high efficiency [20]. The asymmetric version of this process has been developed by Lawrence and... 

High-valent rhenium dioxo complexes, ReC>2l(PR3)2, catalyse the hydrosilylation of carbonyl compounds.264 DFT calculations suggest rate-determining dissociative addition of the Si-H bond (of H-Si-alkyl3) across the Re=0 bond, with subsequent carbonyl coordination and reduction. 

Addition of hydrosilane to alkenes, dienes and alkynes is called hydrosilylation, or hydrosilation, and is a commercially important process for the production of many organosilicon compounds. As related reactions, silylformylation of alkynes is treated in Section 7.1.2, and the reduction of carbonyl compounds to alcohols by hydrosilylation is treated in Section 10.2. Compared with other hydrometallations discussed so far, hydrosilylation is sluggish and proceeds satisfactorily only in the presence of catalysts [214], Chloroplatinic acid is the most active catalyst and the hydrosilylation of alkenes catalysed by E PtCU is operated commercially [215]. Colloidal Pt is said to be an active catalytic species. Even the internal alkenes 558 can be hydrosilylated in the presence of a Pt catalyst with concomitant isomerization of the double bond from an internal to a terminal position to give terminal silylalkanes 559. The oxidative addition of hydrosilane to form R Si—Pt—H 560 is the first step of the hydrosilylation, and insertion of alkenes to the Pt—H bond gives 561, and the alkylsilane 562 is obtained by reductive elimination. 

The hydrosilylation of carbonyl compounds with polymethylhydrosiloxane (PMHS) or other alkoxysilanes can be catalyzed by TBAF, at high efficiency [9]. The asymmetric version of this process has been developed by Lawrence and coworkers using chiral ammonium fluoride 7c prepared via the method of Shioiri [10]. The reduction of acetophenone was performed with trimethoxysilane (1.5 equiv.) and 7c (10 mol%) in THF at room temperature, yielding phenethyl alcohol quantitatively with 51% ee (R) (Scheme 4.6). A slightly higher enantioselectivity was observed in the reduction of propiophenone. When tris(trimethylsiloxy)silane was used as a hydride source, the enantioselectivity was increased, though a pro-... 

Tris(pentafluorophenyl)borane [B(CgFj)3], is a powerful and selective Lewis acid catalyst used in many reactions in organic chemistry [1 ]. Parks and Piers [5] found that B(CgFj)3 catalyzes the hydrosilylation of carbonyl compounds. The silylation of alcohols with the formation of Ft as the only by-product [6] and the cleavage of silyl ether and ether bonds catalyzed by B(C F5)3 [7, 8] provide an... 

The term hydrosilation (or hydrosilylation) refers to the addition of a molecule containing a Si-H bond across the multiple bond of a substrate, usually an alkene, alkyne, or carbonyl compound (equation 1). The reaction can be promoted by UV-light, radiation (y- and X rays), radical initiators, Lewis acids, nucleophiles, or, most importantly, transition metal catalysts. Hydrosilation is related to the important processes of hydrogenation (see Hydrogenation) and hydroboration (see Hydroboration), all of which belong to the general reaction class of hydroelementation. 

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