Hydrosilylation, asymmetric

BINAM-derived NHCs (BINAM = l,l -bi(2-naphthylamine)) were also present in complex 120, in the presence of which various aryl alkyl 

---

Similar asymmetric hydrosilylation reactions were also performed using Rh-(R,R)-f-Bu-MiniPHOS, and the enantioselectivities obtained (80-97% ee) [29] are comparable with those obtained by use of the most effective ligands [ 125]. 

Scheme 29. Example of Rh-catalyzed asymmetric hydrosilylation reactions...

Yamano T, Taya N, Kawada M, Huang T, Imamoto T (1999) Tetrahedron Lett 40 2577 Brunner H, Nishiyama H, Itoh K (1993) Asymmetric hydrosilylation. In Ojima I (ed) Catalytic asymmetric synthesis. Wiley-VCH, New York, chap 6 Sawamura M, Kuwano R, Ito Y (1994) Angew Chem, Int Ed Engl 33 111 Kuwano R, Uemura T, Saitoh M, Ito Y (1999) Tetrahedron Lett 40 1327 Hayashi T (1993) Asymmetric allylic substitution and grignard cross-coupling. In Ojima I (ed) Catalytic asymmetric synthesis. WUey-VCH, New York, chap 7-1 Trost BM, Vranken DLV (1996) Chem Rev 96 395 Consiglio G,Waymouth RM (1989) Chem Rev 89 257... 

Chiral diamino carbene complexes of rhodium have been merely used in asymmetric hydrosilylations of prochiral ketones but also in asymmetric addition of aryl boron reagents to enones. 

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. 

An iron complex-catalyzed asymmetric hydrosilylation of ketones was achieved by using chiral phosphoms ligands [68]. Among various ligands, the best enantios-electivities (up to 99% ee) were obtained using a combination of Fe(OAc)2/(5,5)-Me-Duphos in THF. This hydrosilylation works smoothly in other solvents (diethylether, n-hexane, dichloromethane, and toluene), but other iron sources are not effective. Surprisingly, this Fe catalyst (45% ee) was more efficient in the asymmetric hydrosilylation of cyclohexylmethylketone, a substrate that proved to be problematic in hydrosilylations using Ru [69] or Ti [70] catalysts (43 and 23% ee, respectively). 

In 2008, Gade and coworkers reported that the asymmetric hydrosilylation of ketones was catalyzed by the Fe complex with a highly modular class of pincer-type ligand (Scheme 22) [71]. This Fe catalyst system showed both moderate to good... 

Scheme 22 Asymmetric hydrosilylation catalyzed by the Fe complex with the pincer-type ligand...

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). 

For the asymmetric hydrosilylation of 1,3-cyclohexadiene (42) (Scheme 3-17), the enantioselectivity is higher in the reaction with phenyldifluorosilane than that with trichlorosilane or methyidichlorosilane. The reaction of 42 with phenyldifluorosilane in the presence of a palladium catalyst coordinated with ferrocenylphosphine... 

Asymmetric HydrosilYtation of Dienes 85 Tab. 3-2 Asymmetric Hydrosilylation of 1,3-Cyclohexadiene (42)... 

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]. 

Intramolecular asymmetric hydrosilylation-oxidation of (alkenyloxy) hydrosilanes provides an efficient method for the preparation of optically active polyols from al-lylic alcohols. Cyclization of silyl ethers 54 of a meso-type allyUc alcohol in the pres-... 

Axially chiral spirosilane 61 was efficiently prepared by double intramolecular hydrosilylation of bis (alkenyl) dihydrosilane 60. By use of SILOP ligand, a C2 symmetric spirosilane which is almost enantiomerically pure was obtained with high di-astereoselectivity (Scheme 3-24) [65]. SILOP ligand is much more stereoselective for this asymmetric hydrosilylation than DlOP (5) though they have similar structure. 

---