Metal-Catalyzed Strecker-Type Reaction

Metal catalysis has been successfully applied in one-pot Strecker reaction using hafnium [93], iron [67], and zirconium catalysts [94]. However, most of the methods involve the use of strong acidic conditions, expensive reagents, long reaction times, harsh conditions, fast hydrolysis, and tedious work-up leading to the generation of a large amount of 

SCHEME 10.31 Strecker reaction under solvent-free conditions. 

As mentioned in Section 10.2.3, the application of Ga(OTf)3 as an effective and reusable catalyst for the Strecker reaction has been demonstrated [62]. This is a simple and alternative route to the stepwise methodologies for the Strecker reaction of ketones. Moreover, the use of fluorinated ketones is a significant aspect of this procedure due to the increase interest in organofluorated compounds. 

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Sigman and Jacobsen reported the first example of a metal-catalyzed enantioselective Strecker-type reaction using a chiral Alnl-salen complex (salen = N,N -bis(salicyhdene)-ethylenediamine dianion) [4]. A variety of N-allylimines 4 were evaluated in the reaction catalyzed by complex 5 to give products 6, which were isolated as trifluoroacetamides in good yields and moderate-to-excellent enantioselectivities (Scheme 3). Substituted arylimines 4 were the best substrates, while alkyl-substituted imines afforded products with considerably lower ee values. Jacobsen and co-workers also reported that non-metal Schiff base catalysts 8 and 9 proved to be effective in the Strecker reaction of imines 7 with hydrogen cyanide to afford trifluoroacetamides 10 after reaction with trifluoroacetic anhydride, since the free amines were not stable to chromatography (Scheme 4) [5]. 

The assymetric Strecker reaction of diverse imines, including aldimines as well as ketoimines, with HCN or TMSCN provides a direct access to various unnatural and natural amino acids in high enantiomeric excesses, using soluble or resin-linked non-metal Schiff bases the corresponding chiral catalysts are obtained and optimized by parallel combinatorial library synthesis [93]. A rather general asymmetric Strecker-type synthesis of various imines and a, 9-unsaturated derivatives is catalyzed by chiral bifunctional Lewis acid-Lewis base aluminum-containing complexes [94]. When chiral (salen)Al(III) complexes are employed for the hydrocyanation of aromatic substituted imines, excellent yields and enatio-selectivities are obtained [94]. 

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