Rhodium-catalyzed hydrosilylation

Subsequently, cationic rhodium catalysts are also found to be effective for the regio- and stereoselective hydrosilation of alkynes in aqueous media. Recently, Oshima et al. reported a rhodium-catalyzed hydrosilylation of alkynes in an aqueous micellar system. A combination of [RhCl(nbd)]2 and bis-(diphenylphosphi no)propanc (dppp) were shown to be effective for the ( >selective hydrosilation in the presence of sodium dodecylsulfate (SDS), an anionic surfactant, in water.86 An anionic surfactant is essential for this ( )-selective hydrosilation, possibly because anionic micelles are helpful for the formation of a cationic rhodium species via dissociation of the Rh-Cl bond. For example, Triton X-100, a neutral surfactant, gave nonstereoselective hydrosilation whereas methyltrioctylammonium chloride, a cationic surfactant, resulted in none of the hydrosilation products. It was also found that the selectivity can be switched from E to Z in the presence of sodium iodide (Eq. 4.47). 

The rhodium-catalyzed intramolecular hydrosilylation of allylic alcohol derived silyl ethers has been described. Oxidative cleavage of the resulting cyclized hydrosilylation products affords a route to optically active diols (Scheme 28).129,130... 

Table 10 Impact of the chelating phosphine on levels of enantioselectivity in rhodium-catalyzed intramolecular hydrosilylation with [Rh(P-P)(acetone)2]+.

The rhodium-catalyzed cyclization/hydrosilylation of internal diyne proceeds efficiently with high stereoselectivity (Scheme 106). However, terminal diynes show low reactivity to rhodium cationic complexes. Tolerance of functionalities seems to be equivalent between the rhodium and platinum catalysts. The bulkiness of the hydrosilane used is very important for the regioselectivity of the rhodium-catalyzed cyclization/hydrosilylation. For example, less-hindered dimethylethylsilane gives disilylated diene without cyclization (resulting in the double hydrosilylation of the two alkynes), and /-butyldimethylsilane leads to the formation of cyclotrimerization compound. 

The first rhodium-catalyzed reductive cyclization of enynes was reported in I992.61,61a As demonstrated by the cyclization of 1,6-enyne 37a to vinylsilane 37b, the rhodium-catalyzed reaction is a hydrosilylative transformation and, hence, complements its palladium-catalyzed counterpart, which is a formal hydrogenative process mediated by silane. Following this seminal report, improved catalyst systems were developed enabling cyclization at progressively lower temperatures and shorter reaction times. For example, it was found that A-heterocyclic carbene complexes of rhodium catalyze the reaction at 40°C,62 and through the use of immobilized cobalt-rhodium bimetallic nanoparticle catalysts, the hydrosilylative cyclization proceeds at ambient temperature.6... 

The stoichiometric reaction of low-valent rhodium salts with l, -diynes to afford rhodacyclopentadiene complexes is well established and has been reviewed.73 733 The first rhodium-catalyzed reductive cyclization of a non-conjugated diyne has been reported only recently.74 743 The stereochemical outcome of the rhodium-catalyzed hydrosilylation-cyclization is dependent upon the choice of catalyst. Whereas reductive cyclization of 1,6-diyne 54a catalyzed by Rh4(CO)i2 provides modest yields of the Z-vinylsilane 54c, exposure of 54a to Wilkinson s catalyst... 

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). 

Asymmetric cyclization was also successful in the rhodium-catalyzed hydrosilylation of silyl ethers 81 derived from allyl alcohols. High enantioselectivity (up to 97% ee) was observed in the reaction of silyl ethers containing a bulky group on the silicon atom in the presence of a rhodium-BINAP catalyst (Scheme 23).78 The cyclization products 82 were readily converted into 1,3-diols 83 by the oxidation. During studies on this asymmetric hydrosilylation, silylrhodation pathway in the catalytic cycle was demonstrated by a deuterium-labeling experiment.79... 

L. V. Dinh, J. Gladysz, Transition Metal Catalysis in Fluorous Media Extension of a New Immobilization Principle to Biphasic and Monophasic Rhodium-Catalyzed Hydrosilylations of Ketones and Enones , Tetrahedron Lett. 1999, 40,8995. 

Tillack and co-workers developed a rhodium-catalyzed asymmetric hydrosilylation of butadiyne 258 to afford allenylsilane 260 (Scheme 4.67) [106]. Among more than 30 chiral phosphine ligands investigated, the highest enantioselectivity was observed when the catalyst was prepared from [Rh(COD)Cl]2 (1 mol%) and (S,S)-PPM 259 (2 mol%) to afford the optically active allene 260 with 27% ee. Other metals such as Ir, Pd, Pt or Ni were less effective for example, a nickel catalyst prepared from NiCl2 and (R,R)-DIOP 251 or (S,S)-PPM 259 gave the allene 260 with 7-11% ee. 

Scheme 4.67 Asymmetric synthesis of allene 260 by rhodium-catalyzed hydrosilylation of diyne 258.

A moderate pressure (>5 atm.) of CO in the reaction system leads to the selective formation of 29, while alkynes undergo rhodium-catalyzed hydrosilylation with a hydrosilane to afford vinylsilanes in the absence of CO. The presence of the rhodium complex is crucial for the smooth progression of siiyiformyiation, regardless of the presence of mononuclear or polynuclear complexes. This generalization is supported by the studies of many others [15]. The most important feature of this reaction is the excellent regioselectivity, which favors the formylation of the internal sp-carbon of the acetylenic bond of terminal... 

Table 2 Rhodium-catalyzed diyne cyclization/hydrosilylation...

Gyclization/hydrosilylation of enynes catalyzed by rhodium carbonyl complexes tolerated a number of functional groups, including acetate esters, benzyl ethers, acetals, tosylamides, and allyl- and benzylamines (Table 3, entries 6-14). The reaction of diallyl-2-propynylamine is noteworthy as this transformation displayed high selectivity for cyclization of the enyne moiety rather than the diene moiety (Table 3, entry 9). Rhodium-catalyzed enyne cyclization/hydrosilylation tolerated substitution at the alkyne carbon (Table 3, entry 5) and, in some cases, at both the allylic and terminal alkenyl carbon atoms (Equation (7)). 

Ojima has proposed a mechanism for the rhodium-catalyzed cyclization/hydrosilylation of enynes initiated by oxidative addition of the H-Si bond of the hydrosilane to form the Rh(iii) silyl hydride complex If (Scheme 7). Silylmetallation of the G=G bond of the enyne coupled with coordination of the pendant G=G bond could form... 

Widenhoefer and co-workers have developed an effective protocol for the asymmetric cyclization/hydrosilylation of functionalized 1,6-enynes catalyzed by enantiomerically enriched cationic rhodium bis(phosphine) complexes. For example, treatment of dimethyl allyl(2-butynyl)malonate with triethylsilane (5 equiv.) and a catalytic 1 1 mixture of [Rh(GOD)2] SbF6 and (i )-BIPHEMP (5 mol%) at 70 °G for 90 min gave the silylated alkylidene cyclopentane 12 in 81% yield with 98% de and 92% ee (Table 4, entry 1). A number of tertiary silanes were effective for the rhodium-catalyzed asymmetric cyclization/hydrosilylation of dimethyl allyl(2-butynyl)malonate with yields ranging from 71% to 81% and with 77-92% ee (Table 4, entries 1-5). Although the scope of the protocol was limited, a small number of functionalized 1,6-enynes including A-allyl-A-(2-butynyl)-4-methylbenzenesulfonamide underwent reaction in moderate yield with >80% ee (Table 4, entries 6-8). 

Shibata and co-workers have reported an effective protocol for the cyclization/hydrosilylation of functionalized eneallenes catalyzed by mononuclear rhodium carbonyl complexes.For example, reaction of tosylamide 13 (X = NTs, R = Me) with triethoxysilane catalyzed by Rh(acac)(GO)2 in toluene at 60 °G gave protected pyrrolidine 14 in 82% yield with >20 1 diastereoselectivity and with exclusive delivery of the silane to the G=G bond of the eneallene (Equation (10)). Whereas trimethoxysilane gave results comparable to those obtained with triethoxysilane, employment of dimethylphenylsilane or a trialkylsilane led to significantly diminished yields of 14. Although effective rhodium-catalyzed cyclization/hydrosilylation was restricted to eneallenes that possessed terminal disubstitution of the allene moiety, the protocol tolerated both alkyl and aryl substitution on the terminal alkyne carbon atom and was applicable to the synthesis of cyclopentanes, pyrrolidines, and tetrahydrofurans (Equation (10)). 

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... 

Rhodium carbonyl complexes also catalyze the cascade cyclization/hydrosilylation of 6-dodecene-l,l 1-diynes to form silylated tethered 2,2 -dimethylenebicyclopentanes. For example, reaction of ( )-85 with dimethylphenylsilane catalyzed by Rh(acac)(CO)2 in toluene at 50 °G under GO (1 atm) gave 86a in 55% yield as a single diastereomer (Equation (56)). Rhodium-catalyzed caseade cyclization/hydrosilylation of enediynes was stereospecific, and reaction of (Z)-85 under the conditions noted above gave 86b in 50% yield as a single diastereomer (Equation (57)). Rhodium(i)-catalyzed cascade cyclization/hydrosilylation of 6-dodecene-1,11-diynes was proposed to occur via silyl-metallation of one of the terminal G=G bonds of the enediyne with a silyl-Rh(iii) hydride complex, followed by two sequential intramolecular carbometallations and G-H reductive elimination. ... 

In contrast to the reactivity of 6-dodecene-1,11-diynes, rhodium-catalyzed reaction of l-dodecene-6,11-diynes with silane led not to cascade cyclization/hydrosilylation but rather to carbonylative tricyclization. For example, reaction of 87 [X = G(G02Me)2] and dimethylphenylsilane catalyzed by Rh(acac)(GO)2 in THE at room temperature under GO gave the cyclopenta[e]azulene 88 in 92% yield as the exclusive product (Scheme 22). Although the protocol was... 

Ojima has reported a rhodium-catalyzed protocol for the disilylative cyclization of diynes with hydrosilanes to form alkylidene cyclopentanes and/or cyclopentenes. As an example, reaction of dipropargylhexylamine with triethyl-silane catalyzed by Rh(acac)(GO)2 under an atmosphere of CO at 65 °G for 10 h gave an 83 17 mixture of the disilylated alkylidene pyrrolidine derivative 92b (X = N-//-hexyl) and the disilylated dihydro-1/ -pyrrole 92c (X = N-//-hexyl) in 76% combined yield (Equation (60)). Compounds 92b and 92c were presumably formed via hydrosilyla-tion and hydrosilylation/isomerization, respectively, of the initially formed silylated dialkylidene cyclopentane 92a (Equation (60)). The 92b 92c ratio was substrate dependent. Rhodium-catalyzed disilylative cyclization of dipro-pargyl ether formed the disilylated alkylidene tetrahydrofuran 92b (X = O) as the exclusive product in low yield, whereas the reaction of dimethyl dipropargylmalonate formed cyclopentene 92c [X = C(C02Et)2] as the exclusive product in 74% isolated yield (Equation (60)). 

Encouraged by these successful results, Saigo and co-workers tested ligand 78 in the rhodium-catalyzed hydrosilylation of ketones.56 Indeed, asymmetric hydrosilylation of acetophenone and tetralone using 78 as a chiral source led to considerably improved enantioselectivities (94% and 89% ee, respectively) compared to reactions performed with valinol-derived phosphorous-containing oxazoline 66 (82% and 59%, respectively).59,60 The equal accessibility of the two enantiomers of the m-2-amino-3,3-dimethyl-l-indanol backbone in 78 represented an additional advantage over oxazoline 66, which is derived from an amino alcohol of the chiral pool because (5)-tetralol could easily be obtained using (-)-78 in 97% yield and 92% ee (Scheme 17.30).56... 

Compared to the rhodium-catalyzed stereoselective reactions, studies on the iridium-catalyzed reactions have been limited until recently. Usually lower selectivities have been observed in the Ir(i)-catalyzed reactions.459,460 The asymmetric hydrosilylation of imines affords optically active secondary amines. These are very valuable compounds, but the studies on that reaction are quite limited.461 Close examinations of these reactions revealed that they proceed via a transfer hydrogenation. Other conditions such as the 2-propanol/base system in the presence of an appropriate metal complex have been employed as well, but only low selectivities were obtained.462... 

Alternatively, an insoluble fluorous support, such as fluorous silica [43], can be used to adsorb the fluorous catalyst. Recently, an eminently simple and effective method has been reported in which common commercial Teflon tape is used for this purpose [44]. This procedure was demonstrated with a rhodium-catalyzed hydrosilylation of a ketone (Fig. 9.27). A strip of Teflon tape was introduced into the reaction vessel and when the temperature was raised the rhodium complex, containing fluorous ponytails, dissolved. When the reaction was complete the temperature was reduced and the catalyst precipitated onto the Teflon tape which could be removed and recycled to the next batch. 

Compared to the rhodium-catalyzed asymmetric reactions, studies on the iridium-catalyzed ones have been limited until recently [8]. Hydrosilylation of acetophenone in the presence of a catalytic amount of lr(I) complex and the ligand 2 proceeded smoothly to give 1-phenylethanol in good yield, but with a much lower enantioselectivity (up to 23% ee) than the case of Rh(I) complex [6]. It is noteworthy that the alcohol of the opposite configuration was obtained when the metal is changed from Rh(I) to lr(I) [6] as has been also observed in similar reactions using chiral oxazolinylferrocenylphosphines as ligands [8]. 

Hydrosilylation. Rhodium(I) complexes catalyze the asymmetric hydrosilylation of prochiral ketones (eq 5), in the presence of (—)-sparteine. Secondary alcohols are obtained in up to 30% optical yield by this method. 

Rhodium-Catalyzed Asymmetric Hydrosilylation of Ketones. Complex 2 is a good catalyst for catalytic asymmetric hydrosilylation of ketones (eq 7). The reactions are carried out by using 1-naphthylphenylsilane at -40 °C in THF in the presence of 2 (1 mol%) for 3-4 days. Several types of ketones are hydrosilylated to afford optically active alcohols after acidic work-up. 

Asymmetric Hydrosilylation with Dihydrosilanes. Chiral alkoxysilanes have also been obtained in the rhodium catalyzed hydrosilylation of carbonyl compounds (68, 78, 79) (eq. [29]). 

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