Hydrosilylation of imines

Among titanium compounds, many sit on the border of being organometallic/coordination compounds. One which is of interest in relation to the enhancing effects of F (see Section 9.5.5.2) is the titanium fluoride complex (52), which is highly active for the hydrosilylation of imines (Scheme 33).143... 

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... 

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. 

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. 

Particularly noteworthy is the discovery of a new type of the active catalyst 99,103,104 a crystalline, air-stable yellow-orange solid, which can serve as a highly enantioselective tool in the titanium-catalyzed hydrosilylation of imines. The reaction can be highly stereoselective for both acyclic and cyclic imines under a wide range of hydrogen pressures (Scheme 6-46). 

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... 

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) ... 

An interesting example of the highly enantioselective hydrosilylation of imines has been reported by Buchwald and co-workers. A titanium fluoride complex, (5,5)-(EBTHI)TiF2, catalyzes the formation of silylamines in high yields, with good tolerance of functional groups [Eq. (75)].187... 

High enantioselectivity (ca. 95-99% ee) is observed in this system, better than that revealed in previous reports of the hydrosilylation of imines. The mechanism is as yet unclear however, the authors propose that an active catalyst may be formed by cleavage of the Ti-F bond and generation of a Ti(III) hydride species. Insertion of an imine into the Ti-H bond, followed by a (r-bond metathesis with the silane in a four-centered transition state, may lead to the observed products. Another report on the activity of titanocene complexes as catalysts for the hydrosilylation of aid- and keti-mines also indicates formation of a Ti-H species as catalyst.188 Hydrosilylation proceeds to yield silylamines, with dependence on substitution at nitrogen and on the nature of the ligand bound to the metallocene precursor. 

While hydrosilylation of imines is known to be effected by rhodium catalysts3, nickel catalysts prepared in situ from Ni(0Ac)2 4H20 and thiosemicarbazones are also found to promote the reactions of iV-substituted imines with HSiEt3 in dry DMSO at 35 °C, giving the corresponding secondary amines in excellent yields after basic work-up (equation 77)185. 

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%... 

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... 

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). 

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... 

As discussed in the previous sections, hydrosilylation and hydroamination reactions can be catalyzed by essentially the same catalysts under very similar reaction conditions due to the similarity in their reaction mechanisms. Hence, both reactions can be performed in one synthetic procedure as a one-pot sequence. Although less explored than hydrosilylation of C-C multiple bonds, organolanthanide-catalyzed hydrosilylation of imines is a facile straightforward process [172,173]. Imines, in particular cyclic imines, are readily available via organolanthanide-catalyzed hydroamination of alkynes. Roesky and coworkers have demonstrated that A-silylated saturated heterocycles can be smoothly obtained (38) and (39) utilizing the bis(phosphinoamide)methanide complex 12 (Fig. 8) [57,58]. The higher reactivity of aminoalkynes in the hydroamination process makes this method a valuable alternative to aminoalkene hydroamination. 

Formamides as Lewis-Basic Organocatalysts in Hydrosilylation of Imines... 

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]. 

Discuss the structure of the organocatalysts for hydrosilylation of imines with ClaSiH and identify the key structural features. Suggest other functional groups that can be predicted to act in a similar way. 

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