Chiral ligands for asymmetric hydrosilylation

Diselenides as Chiral Ligands for Asymmetric Hydrosilylation of Ketones and Related Reactions... 

Johannsen and co-workers found that the phosphoramidite of axially chiral BINOL (IH-9) is an effective chiral ligand, and they obtained 32 with 99 % ee in 87 % yield from styrene. Also they prepared 1-trichlorosilyl-l-phenylpropane (35) with 98 % ee in 91 % yield by the hydrosilylation of /S-methylstyrene (34) with this ligand, and chiral 1-phenyl-1-propanol was prepared from 35 [21]. Furthermore, they claimed that arylmonophosphinoferrocene was an efficient ligand. In particular, the p-MeO-Ph-MOPF (VIII-8) they synthesized was the most effective ligand for asymmetric hydrosilylation of styrene, and ultrafast asymmetric hydrosilylation occurred with TOF exceeding 180000 h [22]. 

Palladium complexes also catalyze hydrosilylation, and particular emphasis has been placed on the use palladium catalysts for asymmetric hydrosilylation. The most selective of these catalysts contains a binaphthyl monophosphine ligand. - Finally, lanthanides have also been used for hydrosilylation. Lanthanide-metallocene catalysts can be highly active for the hydrosilylation of olefins, and lanthanides bearing chiral ligands catalyze asymmetric hydrosilylation with measurable enantiomeric excess. ... 

The MOP range of ligands designed by Hayashi has proved remarkably useful for asymmetric hydrosilylation reactions.59 MOP ligands are a series of enantiomerically pure monophosphine ligands whose chirality is due to 1,1 -binaphthyl axial chirality. 

Kumada et al. have examined a number of chiral ferrocenylphosphines as ligands for asymmetric reactions catalyzed by transition metals. They are of interest because they contain a planar element of chirality as well as an asymmetric carbon atom. They were first used in combination with rhodium catalysts for asymmetric hydrosilylation of ketones with di- and trialkylsilanes in moderate optical yields (5-50%). High stereoselectivity was observed in the hydrogenation of a-acetamidoacrylic acids (equation 1) with rhodium catalysts and ferrocenylphosphines. ... 

It was described earlier that the diselenide 2 acted as chiral ligand for the Rh(I)-catalyzed asymmetric hydrosilylation of unfunctionalized alkyl aryl ketones with diphenylsilane in tetrahydrofuran [6]. When the reaction was carried out in methanol as solvent, it gave directly a chiral alcohol, not a hydro-silylated product [7] (Scheme 5). 

In Section 4, it is described that chlorotris(triphenylphosphine)rhodium(I) (7) is quite an effective catalyst for the hydrosilylation of carbonyl compounds. For this reason, extensive studies on asymmetric hydrosilylation of prochiral ketones to date have been based on employing rhodium(I) complexes with chiral phosphine ligands. The catalysts all prepared in situ are rhodium(I) complexes of the type, (BMPP>2Rh(S)a (8) [40] and (DIOP)Rh(S)Cl (6) [41], and a cationic rhodium(III) complex, [(BMPP)2lUiH2(S)2] Q04 (5) [42], where S represents a solvent molecule. An interesting polymer-supported rhodium complex (V) [41], and several chiral ferrocenylphosphines [43], recently developed as chiral ligands, have also been employed for asymmetric hydrosilylation of ketones. Included in this section also are selective asymmetric hydrosilylation of a,0-unsaturated carbonyl compounds and of certain keto esters. 

A curious reversal of configuration is observed when Ir-catalyst is used instead of Rh-catalyst for the same chiral ligand -. For example, the asymmetric hydrosilylation followed by desilylation of acetophenone catalyzed by 207/[Rh(COD)Cl]2 gives (R)-1-phenylethanol with 91% ee, while the same reaction catalyzed by 207/[Ir(COD)Cl]2 yields (S)-alcohol with 96% ee (Scheme 21). The rationale for this remarkable reversal in the direction of asymmetric induction has not been given clearly, but either a change... 

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. 

The Rh-catalysed asymmetric hydrosilylation of prochiral ketones has been studied with complexes bearing monodentate or heteroatom functionalised NHC ligands. For example, complexes of the type [RhCl(l,5-cod)(NHC)] and [RhL(l,5-cod)(NHC)][SbFg ], 70, where L = isoquinoline, 3,5-lutidine and NHC are the chiral monodentate ligands 71 (Fig. 2.11). 

---