Hantzsch ester derivatives 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]. 

Hydrogenation and reduction of C=N bond. Chiral Bronsted acids possessing a bulky backbone such as VAPOL derivative 9 attract and hold imine molecules in the concave space, and this reasoning has led to successful development of a protocol for the s3mthesis of a-amino acid derivatives from imino precursors by transfer hydrogenation (from Hantzsch ester). ... 

A bio-inspired catalyst comprising a combined organic hydride donor with metal centre has been reported for TH of imines. A previously used catalyst relied on a Hantzsch ester, but the newer derivative benefits from easier preparation and greater accessibility. Yields are high for the Rh complex containing all the components, and more than for the Ir complex or the complex containing the phenanthroline ligand alone (Fig. 38) [122, 123]. 

Au(l)/Br0nsted Acid System Han et al. developed an unprecedented protocol to synthesize tetrahydroquinolines 332 directly from 2-(2-propynyl)aniline derivatives 365 in one pot under relay catalysis of an achiral Au complex 368 and a chiral phosphoric acid 5j [131]. The Au -catalyzed intramolecular hydroamination of 2-(2-propynyl)aniline provided the 1,4-dihydroquinolines 366, followed by isomerization into imine-like 3,4-dihydroquinoliniums 367 with 5j. This active intermediate then underwent asymmetric transfer hydrogenation with Hantzsch ester to produce enantioenriched tetrahydroquinoUne products (Scheme 2.97). 

The first enantioselective imine reduction using Hantzsch esters as hydride donor was carried our by Singh and Batra in Using cysteine-derived catalysts, only moderate,... 

As mentioned. List et al. used TRIP derivative 31 in combination with a Hantzsch ester to perform the first asymmetric, oiganocatalytic reductive amination. Acetophenone was stirred for 9 hours at room temperature with anisi-dine in the presence of 4 A molecular sieves to first form the imine. Subsequently, 5 mol% 31 and 1.4 equivalents... 

In 2006, Rueping et al. applied their successful concept for the enantioselective reduction of imines to quinolines. After modification of the substitution pattern of their BINOL-derived Brpnsted acid catalyst 21, 2-substituted quinolines were reduced with the Hantzsch ester in excellent yields and enantioselectivities (Scheme 32.24). ... 

A new approach to stereoselective transfer hydrogenation of imines was the application of chiral phosphoric acid esters as organocatalysts [50-52]. The mechanism is based on the assumption that the imine is protonated by a chiral Bronsted acid, which acts as the catalyst. The resulting diastereomeric iminium ion pairs, which may be stabilized by hydrogen bonding, react with the Hantzsch dihydropyridine at different rates to give an enantiomerically enriched amine and a pyridine derivative [50-52]. The exact mechanism is still under discussion however computational density functional theory (DFT) studies ]53, 54] suggest a three-point contact model. ... 

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