**2.1. Asymmetric transfer hydrogenation of cyclic imines (dihydroisoquinolines)**

Theoretical work accompanied with the computational study with the application of DFT to investigate the ionic mechanism concept in ATH of 1-methyl-3,4-dihydroisoquinoline resulted in a series of interesting proposals. All calculations, having employed [Ru(Cl)(η6 *p* cymene)(*S, S)-*TsDPEN] as the catalyst, differentiated from the previous computational studies by the application of asymmetric η6 -*p*-cymene ligand instead of typically used symmetric η6 benzene or η6 1,2,3,4,5,6-hexamethylbenzene. The main outcome lied in the calculated structures of transition states, their energy minima followed by the construction of two energy diagrams where the most probable structures have been studied. Furthermore, the resulting high enantioselectivity of the ATH of cyclic imines was explained.

It was assumed that a protonated substrate is attached to the molecule of the catalyst by the hydrogen bond between the hydrogen of the protonated substrate and the oxygen of the sulfonyl amide group of the catalyst and by that transition state is stabilized. Nonbinding interactions between π electrons of aromatic ring of the substrate and hydrogen atoms of *p*cymene (i.e., CH/π interactions [7]), which also stabilize the favorable transition state, play a crucial role in the determination of enantiomeric excess (*ee*). Depending on energetic differences between "favorable" and "not favorable" transition states, geometry of the transition state determines *ee* of the product at the end of the reaction.

Besides, corresponding calculations allowed to suggest the pathway toward ATH of 1 methyl-3,4-dihydroisoquinoline using formic acid/triethylamine azeotrope as the source of hydrogen (**Figure 3**).

**Figure 3.** Suggested scheme of the catalytic cycle in ATH of 1-methyl-3,4 dihydroisoquinoline.

The proposed cycle starts with the transformation of the [Ru(Cl)(η6 *p* cymene)(*S,S)* TsDPEN] complex (**a**) onto ruthenium-hydride (**b**), where this action is accompanied by the release of HCl, further neutralized by the present base (triethylamine), leading to the 16 ecomplex (**c**). In the next step, the complex (**c**) receives a proton from formic acid to form the complex (**d**), followed by the complex (**d**) turning in the presence of formate ion into ruthenium-hydride (**b**) and releasing CO2. Finally, ruthenium-hydride participates in the asymmetric transfer hydrogenation process of the protonated substrate (**1'**). With the transfer of hydrogen from the complex (**b**) to the substrate (**1'**) the catalytic complex turns back to the intermediate (**d**). By this, the whole cycle is closed.

#### **2.2. Asymmetric transfer hydrogenation of acyclic imines (***N***-benzyl-1-phenylethan-1-imin)**

Asymmetric transfer hydrogenation of acyclic imines proceeds rather differently than in the case of cyclic imines such as substituted 3,4-dihydroisoquinolines. ATH of *N* benzyl-1 phenylethan-1-imin using Noyori's based catalyst with (*S, S*) ligand configuration lead to the excess of the product with (*S*)‐configuration, which is in conflict with the results obtained in ATH of cyclic imines. The application of computational methods can help to find out [8] specific differences between both reaction types. It has been pointed out that in opposite to the rigid structure of cyclic imines, acyclic imines are very flexible in conformation and can undergo isomerization between *E* and *Z* isomer. Both of these isomers are capable of an interaction with the catalysts in a different way, ergo this fact has an extended impact on the enantioselectivity of the whole reaction, since both of the isomers are hydrogenated over different transition states. Therefore, in the case of the ATH of cyclic amines as well as in this case the transition state is stabilized by the hydrogen bond between the oxygen group of the catalyst and hydrogen of the protonated acyclic substrate. However, ATH of acyclic imines proceeds *via* the transition state with a different geometry producing the products with opposite geometry than in the case of cyclic imines such as 3,4‐dihydroisoquinolines. Among other issues, this work stated that major *E* isomer is hydrogenated with a high selectivity to the corresponding (*S*)‐isomer. Meanwhile, *Z* isomer is hydrogenated with much lower selectivity that causes decreasing of an overall enantioselectivity of the reaction.
