Hydrosilylation rhodium complexes

Herrmann et al. reported for the first time in 1996 the use of chiral NHC complexes in asymmetric hydrosilylation [12]. An achiral version of this reaction with diaminocarbene rhodium complexes was previously reported by Lappert et al. in 1984 [40]. The Rh(I) complexes 53a-b were obtained in 71-79% yield by reaction of the free chiral carbene with 0.5 equiv of [Rh(cod)Cl]2 in THF (Scheme 30). The carbene was not isolated but generated in solution by deprotonation of the corresponding imidazolium salt by sodium hydride in liquid ammonia and THF at - 33 °C. The rhodium complexes 53 are stable in air both as a solid and in solution, and their thermal stability is also remarkable. The hydrosilylation of acetophenone in the presence of 1% mol of catalyst 53b gave almost quantitative conversions and optical inductions up to 32%. These complexes are active in hydrosilylation without an induction period even at low temperatures (- 34 °C). The optical induction is clearly temperature-dependent it decreases at higher temperatures. No significant solvent dependence could be observed. In spite of moderate ee values, this first report on asymmetric hydrosilylation demonstrated the advantage of such rhodium carbene complexes in terms of stability. No dissociation of the ligand was observed in the course of the reaction. 

Asymmetric hydrosilylation can be extended to 1,3-diynes for the synthesis of optically active allenes, which are of great importance in organic synthesis, and few synthetic methods are known for their asymmetric synthesis with chiral catalysts. Catalytic asymmetric hydrosilylation of butadiynes provides a possible way to optically allenes, though the selectivity and scope of this reaction are relatively low. A chiral rhodium complex coordinated with (2S,4S)-PPM turned out to be the best catalyst for the asymmetric hydrosilylation of butadiyne to give an allene of 22% ee (Scheme 3-20) [59]. 

DKR via hydrosilylation was also investigated in the presence of the methyl-DuPhos rhodium complex. When MOM protected 31 was tested, a low yield of desired diasteromer was observed with modest 70% ee. On the other hand, the unprotected thiol-ketone 33 gave 89% ee of the desired anti-diastereomer in 40%... 

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). 

The ketone hydrosilylation shown in Fig. 7 was used as a test reaction. This can be catalyzed by the fluorous rhodium complexes 16-Rf6 and 16-Rfs under fluorous/organic liquid/liquid biphase conditions [55,56]. These red-orange compounds have very httle or no solubihty in organic solvents at room temperature [57]. However, their solubilities increase markedly with temperature. Several features render this catalyst system a particularly challenging test for recovery via precipitation. First, a variety of rest states are possible (e.g., various Rh(H)(SiR3) or Rh(OR )(SiR3) species), each with unique solubility properties. Second, the first cycle exhibits an induction period, indicating some fundamental alteration of the catalyst precursor. 

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]. 

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. 

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... 

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). 

Rhodium complexes with l,3-bis(di-fert-butylphosphino)methane (dtbpm), [(dtbpm) RhCl]2/PPh3 (89), (dtbpm)RhS i(OEt)3 (PMe3) (90) and (dtbpm)RhMe(PMe3) (91) are found to be effective catalysts for the hydrosilylation of an internal alkyne, 2-butyne, with HSi(OEt)3 at ambient temperature without solvent to yield (E)-2-triethoxysilyl-2-butene with complete stereoselectivity in quantitative yield using a proper concentration of the catalysts, i.e. >0.05 mol% for 89, >0.4 mol% for 90 and 91114. When the reaction is carried out at lower catalyst concentrations, i.e. 0.1 mol% for 90 or 91, (Z)-product is formed via frans-addition in 7-13% yield. 

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. 

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. 

Inspired by the chiral phosphine/oxazoline ligands developed by Helmchen and Pfaltz [131], Crudden and coworkers, have prepared a chiral NHC-oxazoline possessing a rigid backbone (Fig. 14) [ 132 ]. The rhodium complex 74 has been used in the catalytic hydroboration of olefins and the hydrosilylation of prochiral ketones with enantiomeric excesses that did not exceed 10%. 

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. 

Hydrosilylation of a-methylstyrene with trimethyl- or dimethylphenylsilane in the presence of chiral rhodium complexes A or B results in the formation of chiral 2-phenylpropylsilanes 6 with low optical purity7. 

Cationic rhodium(I) complexes such as Rh[(5,5)-ethyl-DuPHOS](cod) +X- (X=OTf, PFe, BF4, SbFe) are usually employed as precatalysts for enantioselective hydrogenation " or hydrosilylation reactions. The precatalysts can be prepared from the chiral ligand and [Rh(cod)2] X -complexes by a standard method. The corresponding Rh[(5, 5)-ethyl-DuPHOS](nbd) complex can be accessed equally well by the method of Schrock and Osborne or by exchange of cod in the relevant rhodium complex by norbornadiene (nbd). ... 

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)." ... 

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