Imines catalytic hydrosilylation

Micro)titer plate and indicator (dye, fluorescent, etc.) 1. Catalytic electrooxidation of methanol + pH indicator3 2. Catalytic hydrosilylation of alkenes and imines + dyeb... 

Typical Procedures forthe Catalytic Hydrosilylation of Imines... 

The asymmetric catalytic reduction of ketones (R2C=0) and imines (R2C=NR) with certain organohydrosilanes and transition-metal catalysts is named hydrosilylation and has been recognized as a versatile method providing optically active secondary alcohols and primary or secondary amines (Scheme 1) [1]. In this decade, high enantioselectivity over 90% has been realized by several catalytic systems [2,3]. Therefore the hydrosilylation can achieve a sufficient level to be a preparative method for the asymmetric reduction of double bond substrates. In addition, the manipulative feasibility of the catalytic hydrosilylation has played a major role as a probe reaction of asymmetric catalysis, so that the potential of newly designed chiral ligands and catalysts can be continuously scrutinized. 

Part two (section 3) deals with the hydrosilylation of unsaturated carbon-heteroatom bond, mostly 0=0 and 0=N (but also C N, and C=S), as a catalytic method for the reduction of C=0 and C=N bonds—one of the most fundamental transformations in organic chemistry. Catalytic hydrosilylation of prochiral ketones and imines with substituted silanes and siloxanes that can provide (if followed by hydrolysis) convenient access to chiral alcohols and amines, respectively, discussed from the catalytic and synthetic point of view completes this part. 

Catalytic Hydrosilylation of Unsaturated Carbon-Heteroatom Bonds. Hydrosilylation of ketone produces silyl ether, which can be easily converted to alcohol via an additional hydrolysis (deprotection) step (Scheme 28). Analogously, hydrosilylation of imine leads to the formation of silylamine, which can be conveniently transformed to amine. Hydrosilylation of carbonyl (imine) group with subsequent hydrolysis is often referred to as the reduction by silanes. 

A wide range of imines undergoes hydrosilylation with PMHS in the presence of catalytic amounts of butyltris(2-ethylhexanoate)tin (245). JY-Tosyl aldimines and ketimines were efficiently reduced to tosylamides in the presence of catalytic amount of lithium methoxide. Benzaldimines and ketimines undergo efficient hydrosilylation with HSiMe2Ph in the presence of BlCeFsla (246). 

Asymmetric Hydrosilylation of Unsaturated Carbon-Heteroatom Bonds. Asymmetric, catalytic hydrosilylation of prochiral ketones with substituted silanes or siloxanes gives silyl ethers that can be easily hydrolyzed to chiral alcohols. Similarly, prochiral imines undergo asymmetric hydrosilylation to give A-silylamines and, after subsequent hydrolysis, chiral amines (Scheme 32). 

Following the pioneering discovery by Oi and Inoue on the diarylation of imines [43], improved diarylation of imines was reached in NMP using a Ru(II)/KOAc/ PPhs catalytic system for aldimines and Ru(ll)/2 KOAc without PPhs but for longer time for ketimines. The diarylated imines could be reduced into bulky amines by catalytic hydrosilylation with the same Ru(ll) catalyst. Thus the overall reaction could be performed in two steps via sequential Ru(ll) catalysed C-H bond functionalization/hydrosilylation [(Eq. 19)] [84]. 

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... 

Hydrosilylation of imine compounds was also an efficient method to prepare amines. The hydrosilylation product TV-silylamines can readily be desilylated upon methanol or water treatment, yielding the corresponding amines. The amines can be converted to their corresponding amides by subsequent acyl anhydride treatment. The first attempt to hydrogenate prochiral imines with Rh(I) chiral phosphine catalysts was made by Kagan102 and others. These catalysts exhibited low catalytic activity, and only moderate ee was obtained. 

Treatment of this precatalyst with phenylsilane yields a very active catalytic system that can be used for the hydrosilylation of imines. This discovery was initiated by the observed hydrosilylation of imines when phenylsilane was added to Cp2TiF2. It was presumed that it might be possible to break the strong... 

Ti-F bond and generate a Ti-H species when 99 was treated with phenylsilane. The chirality transfer may take place through imine insertion into the Ti-H bond, similar to that in the catalytic hydrogenation process.1000 The reaction can be carried out by the subsequent addition of imines. The corresponding silylated amines can be obtained and further converted to enantiomerically enriched amines upon acid treatment. For example, in the presence of 99, N-methylimine 100 undergoes complete hydrosilylation within 12 hours at room temperature, with 97% ee and up to 5000 turnovers.103... 

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. 

As exemplified by the reactions of Schemes 1 and 4, fluorotitanium compounds could open new possibilities for metal-catalyzed processes. Their fascinating structural diversity [7] as well as further catalytic possibilities in the field of olefin polymerizations [7i, 16] have been put forward by the pioneering work of Roesky, Noltemeyer and co-workers. Similar properties were also exhibited by the analogous zirconium and hafnium compounds [7b,i]. A Zr binaphtholate has already been successfully applied for the enantioselective allylstannylation of aldehydes [2f], Buch-wald and co-workers successfully used a chiral titanocene difluoride as precursor for the corresponding Ti(lII) hydride, a very efficient catalyst for the enantioselective hydrosilylation of imines [17]. 

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). 

Seyferth and Wiseman reacted as-obtained silazanes with catalytic amounts of potassium hydride, KH. They observed dehydrocoupUng of Si-H with N-H units and proposed a mechanism involving silylene-imine Si=N motifs as key species which rapidly add Si-H intermolecularly in hydrosilylation-type reactions. There has been, until now, no experimental proof for the proposed mechanism. Alternatively, a mechanism excluding silylene-imine formation was suggested... 

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]. 

Lanthanide(ll)-imine complexes, obtained by reduction of aromatic ketimines with samarium and ytterbium metal, effectively catalyze the hydrosilylation of imines. The proposed catalytic cycle for the imine hydrosilylation is outlined in Scheme 279.961 1033... 

Optically active alcohols, amines, and alkanes can be prepared by the metal catalyzed asymmetric hydrosilylation of ketones, imines, and olefins [77,94,95]. Several catalytic systems have been successfully demonstrated, such as the asymmetric silylation of aryl ketones with rhodium and Pybox ligands however, there are no industrial processes that use asymmetric hydrosilylation. The asymmetric hydrosilyation of olefins to alkylsilanes (and the corresponding alcohol) can be accomplished with palladium catalysts that contain chiral monophosphines with high enantioselectivities (up to 96% ee) and reasonably good turnovers (S/C = 1000) [96]. Unfortunately, high enantioselectivities are only limited to the asymmetric hydrosilylation of styrene derivatives [97]. Hydrosilylation of simple terminal olefins with palladium catalysts that contain the monophosphine, MeO-MOP (67), can be obtained with enantioselectivities in the range of 94-97% ee and regioselectivities of the branched to normal of the products of 66/43 to 94/ 6 (Scheme 26) [98.99]. 

Since CIgSiH is known to be activated by DMF and other Lewis bases to effect hydrosilylation of imines (Scheme 4.2) [8], it is hardly surprising that chiral formamides, derived from natural amino adds, emerged as prime candidates for the development of an asymmetric variant of this reaction [8]. It was assumed that, if successful, this approach could become an attractive altemative to the existing enzymatic methods for amine production [9] and to complement another organo catalytic protocol, based on the biomimetic reduction with Hantzsch ester, which is being developed in parallel [5]. 

Such imines can be used as ligands for catalytic enantioselective hydrosilylation of ketones (Section D.2.3.1.4) or asymmetric hydroformylation [Section D.l.5.8. which uses (.S)-/V-(2-hy-droxybenzylidene)-l-phenylethylamine (2) " ] or reduced further to chiral amines, e.g.. by sodium borohvdridc4. A convenient one-pot synthesis of such secondary amines uses sodium cyanoborohydride as the reducing agent6. 

A catalytically active chiral hydridotitanium complex obtained from (5,5j-ethylene(T -tetrahydroindenyl)titanium difluoride and PhSiHj hydrosilylates imines under mild conditions with significantly higher substrate catalyst ratios than known methods yield. ... 

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