Asymmetric hydrosilylations of imines

Several successful results have been obtained in the asymmetric hydrogenation and asymmetric hydrosilylation of imines.101 An efficient enantioselective hydrogenation of the ON double bond was developed by Burk and Feast-er,101a who used [ R h (CO D) (D u P h o s) ] C Fi SO3 in the hydrogenation of N-aroylhydrazone 98. 

Scheme 6-46. Titanocene-catalyzed asymmetric hydrosilylation of imines.

Chiral oxazolinylphosphines were used as effective ligands for the iridium-catalyzed asymmetric hydrosilylation of imines to afford the corresponding sec-amines with high enantioselectivities (up to 89% ee) after hydrolysis in almost quantitative yields (Equation 14.6) [55]. The following derivatives as efficient ligands were used (see also Table 14.4) ... 

Scheme 11.7 Asymmetric hydrosilylation of imines catalyzed by chiral formamides.

Asymmetric hydrosilylation of imines followed by hydrolysis of the JV-silyl-amine intermediate yields chiral amines. The Kagan group hydrosilylated a series of prochiral imines with both polymethylhydrogensiloxane and diphenyl ... 

Asymmetric Hydrosilylations of Imines with the (+) -DIOP Catdysf... 

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... 

Asymmetric hydrosilylation of imines affords synthetically useful optically active sec-amines, but study on this subject is still quite limited [8 c]. Uemura and co-workers applied their Rh(I)-2 catalytic system to two imines [6]. A good result was obtained from N-phenyl-l-phenylpropanimine (up to 53% ee), but the reaction with the M-benzyl analogue gave low selectivity (up to 11 % ee). 

For the asymmetric hydrosilylation of imine derivatives, no improvements have been reported after 1990. Until the late 1980s, for example, the imines were reduced in the middle range of enantioselectivity around 60% ee. The imines II and 12 (Fig. 10) were converted to the corresponding secondary amines A1 and A2 in 65% ee and 66% ee, respectively, with Rh-DIOP catalysts and diphenylsilane [48]. 

Table 3. Asymmetric hydrosilylation of imines with the chiral titanium catalyst (S,S)-(EBT-HI)Tip2 (T4)...

In the 1990s, no improvement has been reported for the asymmetric hydrosilylation of imines (Scheme 2.3) by using chiral Rh-catalysts. Imines 59 and 60 were reduced with 2 by using DIOP-Rh catalysts to the corresponding secondary amines 61 and 62 with 65% ee and 66% ee, respectively [43]. Chiral Ru-catalyst, however, achieved higher enantioselectivity in the asymmetric reduction of a particular imine (see below). 

Asymmetric hydrosilylation of the cyclic imine 17 (Approach A) was precedented on simpler substrates by Buchwald but the method requires an expensive and highly air-sensitive chiral htanocene catalyst (Scheme 8.5) [6]. 

While it is beyond the scope of this chapter to cover the asymmetric hydrosilylation of ketones and imines in any detail, a number of the more catalytically active ML combinations will be mentioned here. A full review of the area has recently appeared.138 Asymmetric hydrosilylation of carbonyl groups is usually performed with rhodium or titanium catalysts bearing chelating N- or P-based ligands. Representative results for some of the most active Rh/L combinations (Scheme 32) for addition of Si H to acetophenone are given in Table 11. 

Preparation of enantiomerically pare secondary amines by catalytic asymmetric hydrogenation or hydrosilylation of imines is as important as the preparation of alcohols from ketones. However, asymmetric hydrogenation of prochiral ON double bonds has received relatively less attention despite the obvious preparative potential of this process.98... 

ASYMMETRIC HYDROSILYLATION OF KETONES AND IMINES WITH RH AND RU CATALYSTS... 

Asymmetric hydrosilylation of prochiral carbonyl compounds, imines, alkenes and 1,3-dienes has been extensively studied and continues to be one of the most important subjects in the hydrosilylation reactions. This topic has been reviewed at each stage of its development as a useful synthetic method based on asymmetric catalytic processes1,3,187-189. In the last decade, however, substantial progress has been made in the efficiency of this reaction. Accordingly, this section summarizes the recent advances in this reaction. 

Asymmetric hydrosilylation of prochiral imine IV-oxides (nitrones) (209-211) catalyzed by R Cl SH—)-tolbinap]2(NEt3) with H2SiPh2 gives the corresponding /V,/V-disLibslihilcd hydroxylamines (212-214) with high enantiomeric purity (equations 82-84)230. 

Chiral amines can be prepared by asymmetric hydrogenation, transfer hydrogenation and hydrosilylation of imines. The piperidine 137 with 98% ee was obtained by highly efficient asymmetric hydrogenation of the cyclic imines 136 catalysed by the Ti catalyst 61 [82]. Pyrrolidine 139 with 99% ee was obtained in 34% after 50%... 

Recent advances in the asymmetric hydrosilylation of ketones and imines have been reviewed.276... 

Michael-aldol reaction as an alternative to the Morita-Baylis-Hillman reaction 14 recent results in conjugate addition of nitroalkanes to electron-poor alkenes 15 asymmetric cyclopropanation of chiral (l-phosphoryl)vinyl sulfoxides 16 synthetic methodology using tertiary phosphines as nucleophilic catalysts in combination with allenoates or 2-alkynoates 17 recent advances in the transition metal-catalysed asymmetric hydrosilylation of ketones, imines, and electrophilic C=C bonds 18 Michael additions catalysed by transition metals and lanthanide species 19 recent progress in asymmetric organocatalysis, including the aldol reaction, Mannich reaction, Michael addition, cycloadditions, allylation, epoxidation, and phase-transfer catalysis 20 and nucleophilic phosphine organocatalysis.21... 

Diselenides as Chiral Ligands for Asymmetric Hydrosilylation of Ketones and Imines... 

Asymmetric hydrosilylation of prochiral carbonyl compounds, alkenes, 1,3-dienes, and imines has been extensively studied and remains one of the most important subjects in the field. This reaction is strongly affected by the nature of the catalyst (metal, type of chiral ligand) and the substrate as well as the reaction conditions (solvent, temperature, etc.). In recent years, many papers have been published on asymmetric hydrosilylation, describing new catalytic systems (mainly new optically active ligands) and new synthetic applications of the reaction [4, 24]. 

Hydrosilylation of unsaturated organic molecules is an attractive organic reaction. Asymmetric hydrosilylation of prochiral ketones or imines provides effective routes to optically active secondary alcohols or chiral amines (Scheme 756). These asymmetric processes can be catalyzed by titanium derivatives. The ( A ebthi difluoro titanium complex has been synthesized from the corresponding chloro compound.1659 This compound results in a very active system for the highly enantioselective hydrosilylation of acyclic and cyclic imines and asymmetric hydrosilylation reactions of ketones including aromatic ketones.1661,1666,1926-1929 An analogous l,l -binaphth-2,2 -diolato complex catalyzes the enantioselective hydrosilylation of ketones.1... 

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