Rhodium complexes silyls

Enantiomerically pure 2-alkylidenetetrahydrofurans were prepared by TiCl4 mediated reactions of 1,3-bis-silyl enol ethers with enantiomerically pure epichlorohydrin <06TA892>. Rhodium complexes as the one shown below reacted in solution in the presence of triethylphosphine to afford 2,2-disubstituted-5-methylenetetrahydrofurans in good yield <06JA9642>. 

Rhodium complexes catalyze hydrosilylation-cyclization of 1,6-allenynes in the presence of (MeO SiH.77 To avoid complex product distributions, the use of substrates possessing fully substituted alkyne and allene termini is imperative. As shown in the cyclization of 1,6-allenyne 62a, the regiochemistry of silane incorporation differs from that observed in the rhodium-catalyzed hydrosilylation-cyclization of 1,6-enynes (see Section 10.10.2.3.2). For allenyne substrates, allene silylation occurs in preference to alkyne silylation (Scheme 40). 

When substituted silanes are used instead of hydrogen, the process is referred to as silylformylation or silylcarbonylation. Only rhodium complexes catalyze the transformation of unsaturated compounds to silylaldehydes via the silylformylation reaction. Iridium complexes also are able to catalyze the simultaneous incorporation of substituted silanes and CO into unsaturated compounds, although during the reaction other types of product are formed. In the presence of [ IrCl(C03) ] and [Ir4(CO)i2]) the alkenes react with trisubstituted silanes and CO to give enol silyl ethers of acyl silanes [58] according to Scheme 14.10. 

Hydroxy-directed hydrogenation of (phenyldimethylsilyl)allyl alcohols with a cationic rhodium complex provides a highly diastereoselective route to /3-silyl alcohols201,202. 

The hydrosilylation of 1-alkynes catalyzed by rhodium complexes proceeds predominantly in an awft -fashion, giving thermodynamically unfavorable (Z)-alkenylsilanes as the major product (up to 99%)3,106-108. For example, the hydrosilylation of 1-hexyne with HSiEt3 catalyzed by RhCl(PPh3), Rh4(CO)i2, Rh2Co2(CO)i2 and RhCo3(CO)i2 in toluene gives (Z)-l-silylhexene 85 as the major product (79-98%), (E)-l-silyl-l-hexene 86 (1-10%) and 2-silyl-l-hexene 87 (1-14%), in which the product ratio depends on the reaction conditions (vide infra) (equation 38) (see also Section III.B). 

The hydrosilylation of 3-butyn-l-ol, i.e. homopropargyl alcohol, with HSiMe2Ph catalyzed by (Cy3P)Pt(CH2=CH2)2 gives a 4.2 1 mixture of (E)-4-silyl- and (E)-3-silyl-3-buten-l-ols135, whereas the cationic rhodium complex-catalyzed reaction with HSiEt3 affords ( )-4-triethylsilyl-3-buten-l-ol (136) exclusively in 97% yield (equation 55)136. 

Using the metalloradical reactivity of the Rh(II)OEP (OEP = 2,3,7,8,12,13,17,18-octaethylphorphynato) dimer, the preparation of silyl rhodium complexes was achieved by the hydrogen elimination reaction with silanes I R SiH (R = R = Et, Ph R = Me, R = Ph, OEt). The Rh—Si bond length of 2.32(1) A, found when R = Et, is comparable to those in other Rh(III) complexes (Table 11). The crystal packing indicates that all the ethyl groups on the porphyrin periphery are directed toward the silyl group. Consequently, the aromatic part of one complex molecule is in contact with the aromatic part of the next molecule and the aliphatic part is in contact with the aliphatic part of the next molecule204. 

A chiral cationic rhodium complex has been shown to catalyse the enantioselective conjugate addition of silyl anion equivalents to cyclic a,fl-unsaturated ketones and esters, thus providing a facile access to chiral organosilicon compounds.247... 

Hydrosilanes, RsSiH, can be viewed as silylated hydrogen and used as a reducing reagent. Hydrosilation (equation 20) resembles hydrogenation in this sense. Reduction of prochiral ketones through hydrosilation has been investigated extensively with platinum and rhodium complexes having... 

Aldol Condensations. The rhodium complex has been utilized as a catalyst in aldol condensation of silyl enol ethers and... 

In place of dtphenylsilane, 1,2-bis(dimethylsilyl)ethane was applied to the reduction of several ketones by the combination of the Py box-rhodium complex and Silver(I) Trifluoromethanesulfonate to give the corresponding silyl enol ether exclusively (eq 6). ... 

Addition of the elements of Si—H to a carbonyl group produces silyl ethers which are synthetically equivalent to chiral secondary alcohols since the silyl groups are easily hydrolyzed. Hydrosilylation can be catalyzed by acids or transition metal complexes. Enantioselective hydrosilylation of prochiral ketones has been extensively studied using platinum or rhodium complexes possessing chiral ligands such as BMPP (86), DIOP (87), NORPHOS (88), PYTHIA (89) and PYBOX (90)." ... 

Similar reactions have been carried out on acetylene.In an interesting variation, thiocarbonates add to aUcynes in the presence of a palladium catalyst to give a p-phenylthio a,p-unsaturated ester.Aldehydes add to alkynes in the presence of a rhodium catalyst to give conjugated ketones.In a cyclic version of the addition of aldehydes, 4-pentenal was converted to cyclopentanone with a rhodium-complex catalyst. An intramolecular acyl addition to an alkyne was reported using silyl ketones, acetic aid and a rhodium catalyst. In the presence of a palladium catalyst, a tosylamide group added to an alkene unit to generate A-tosylpyrrolidine derivatives. 

On the other hand, divinyl-substituted organosilicon compounds in the presence of ruthenium and rhodium complexes containing or generating M-H and M-Si (M = Ru, Rh) bonds undergo competitive silylative coupling cyclization and polycondensation to give a mixture of oligomers and... 

The number of examples of highly selective dehydrogenative silylation is still limited. The most convincing examples are Ru3(CO)i2- and Fe3(CO)i2-cata-lyzed reactions of styrene [106, 114] and vinylsilane [115] with HSiEts, RuH2(H2)2PCy3)2-catalyzed reaction of ethylene with HSiEt3 [116], and cationic rhodium complex-catalyzed dehydrogenative silylation, e.g., [117], as well as the nickel equivalent of the Karstedt catalyst [105]. 

In 1986, Reetz et al. reported that chiral Lewis acids (B, Al, and ll) promoted the aldol reaction of KSA with low to good enantioselectivity [115]. The following year they also introduced asymmetric aldol reaction under catalysis by a chiral rhodium complex [116]. Since these pioneering works asymmetric aldol reactions of silyl enolates using chiral Lewis acids and transition metal complexes have been recognized as one of the most important subjects in modern organic synthesis and intensively studied by many synthetic organic chemists. 

Addition of certain dihydrosilanes, HjSiR R (R R ), to ketones in the presence of chiral rhodium complexes gives silyl ethers in an optically active form associated with the silicon atom, which are converted into optically active monohydrosilanes by the action of Grignard reagents, R MgX (R R, R ) ... 

A proposed mechanism for the rhodium-catalyzed alcoholysis is represented in Scheme 49 (77). In the first step, activation of the hydrosilane occurs through oxidative addition. Formation of the alkoxysilane then takes place by nucleophilic attack of a noncoordinated alcohol molecule. The dihydro-rhodium complex 143 thus obtained liberates a hydrogen molecule upon reductive elimination. Nucleophilic cleavage of the silicon-rhodium bond, without prior coordination of the alcohol at the rhodium is supported by results obtained in asymmetric alcoholysis (cf. Sect. II-D). Optical yields were shown to be little dependent on the catalyst ligands (in marked contrast with the asymmetric hydro-silylation), indicating but weak interaction between alcohol and catalyst during the reaction. Moreover, inversion of configuration at silicon, which occurs in the particular case of methanol as solvent, is not likely to occur in a reaction between coordinated silane and alcohol. 

If rhodium enolates are used in a catalytic cycle they can promote aldol reactions under reasonably mild conditions. For example, the aldol reactions of trimethylsilyl enol ethers and ketene silyl acetals (37) with aldehydes can be catalyzed by various rhodium(I) complexes, under essentially neutral conditions, to give p-trimethylsiloxy ketones and esters (38 equation 14 and Table 6). The study of Matsuda and coworkers suggests that use of the rhodium complex Rlu(CO)i2 (39 at 2 mol %) in benzene at 100 C gives best results for the formation of adduct (38 Table 6, entries 1-7). There is negligible diastereoselectivity in most cases. Various cationic ihodium complexes such as (40) also catalyze the reaction. Reetz and Vougioukas have found that this aldol reaction proceeds well with the more reactive ketene silyl acetals, (37) for R = OMe or OEt, in CH2CI2 at room temperature (Table 6, entries 8-13). The intermediacy of an ti -O-bound rhodium enolate, such as (41), in the catalytic cycle is like-... 

Preliminary examples of catalytic enantioselective aldol additions using rhodium complexes with chiral phosphine ligands attached, e.g. (42), have been disclosed by Reetz and Vougioukas (equation 15) although the low level of asymmetric induction so far obtain in the silylated aldol adduct (43) requires substantial enhancement before this reaction has any synthetic value. However, there is ample scope for future improvement by the use of more effective chiral diphosphine- odium complexes. 

Crossed aldol reactions of enol silyl ethers with aldehydes have been successfuly performed with the aid of catalytic amounts of the rhodium complex ((COD)Rh(DPPB)]+X (X = PFs or CIO4) or Rh4(CO)i2. Although the intermediacy of rhodium enolate has been suggested for these reactions, the fact that the same rhodium catalysts can promote the condensations of acetals as well (Scheme 38) tends to indicate that the reactive species may not be a metal enolate. 

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