Heterogeneously Catalyzed Hydrogenation of Imines

The production of enantiopure amines has received growing attention in recent years because chiral amines have great potential as building blocks in drug 

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.  

Recently, a new approach was pursued combining organocatalysis with heterogeneous catalysis [11]. In this case, molecular hydrogen is used as hydrogen source and supported noble metal catalysts for the activation of hydrogen. The implementation of chirahty is affected by a suitable modifier such as chiral phosphoric add esters (P-add) ]55]. 

The investigated imines show characteristic v(C=N) bands in the ATR-FTIR as well as in the Raman spectra, the exact position of which is listed in Table 3.3. 

The position of the v(C=N) bands differs depending on the different substituents at the nitrogen atom of the N-benzylidene ring. Typical modes of the aromatic rings appear around 1595/1575cm as well as 1495/1485/1458cm. The respective ATR-FTIR and Raman spectra of the hydrogenation products show less prominent bands with essentially lower intensities ]11]. 

---

ATR-FTIR/UV-vis/Raman spectroscopy has been applied for mechanistic studies of the heterogeneously catalyzed hydrogenation of imines and of formal [3+3] cyclocondensation reactions catalyzed by Lewis adds [11, 33]. In the latter reaction, different products were obtained dependent on the Lewis acid used. It could be elucidated by in situ spectroscopic investigations that the intermediate complex formed between the ketene acetal component and the Lewis acid (an acid-catalyzed rearrangement reaction occurs in the case of trimethylsilyltrifluoromethanesulfonate (TM SOTf), forming a bischelate complex with TiCl4) controls the subsequent attack of the second reactant in the form of a diene, and consequently, the mechanism of product formation [33]. 

---