## 3 Results and Discussion
First, the amount of **L1** was reduced keeping the loadings of Ru-dimer and base constant and we found that both yield and ee improved when the ratio Ru-dimer:**L1** was 1:2 (compare entries 1-3 in Table 1). Hence, we decided to keep that ratio in the following tests. Next, the loading of base was evaluated and a decrease in the enantioselectivity, in comparison with the results of entry 2, was observed when the amount of potassium hydroxide was either increased to 75 mol% (Table 1, entry 4) or decreased to 25 mol% (Table 1, entry 5). Since the optimum ratio Ru-dimer:**L1**:base seemed to be 1:2:10, it was maintained in further experiments. A different metallic cation in the base did not cause any noticeable effect on the enantioselectivity: lithium and sodium hydroxides gave almost the same ee's as potassium hydroxide, the yield being higher with the latter (compare entries 2, 6 and 7 in Table 1). We tried to reduce the amount of the Ru-dimer to 3 mol%, but this considerably slowed down the transfer hydrogenation process and resulted in a lower yield of **2a**, although the enantioselectivity did not vary (compare entries 2 and 8 in Table 1). The effect of the temperature was also studied. The ee decreased to 60% when the reaction was performed at 50 \({}^{\circ}\)C (Table 1, entry 9). A decrease of the temperature to 0 \({}^{\circ}\)C drastically lengthened the reaction time and, furthermore, the yield decreased to 48% (Table 1, entry 10). However, the enantioselection of the process slightly improved to 78% ee. In order to try to improve the yield, the reaction was repeated and an upward temperature gradient was applied: after stirring for 1 h at 0 \({}^{\circ}\)C, the reaction was allowed to gradually reach room temperature. We were delighted to see that, under these conditions, a 70% yield was obtained with no detriment to the enantioselectivity (Table 1, entry 11). A slight improvement of both yield and enantioselectivity were observed when the initial temperature was -20 \({}^{\circ}\)C instead of 0 \({}^{\circ}\)C (Table 1, entry 12). Since in our previous studies concerning the ATH of _N_-(_tert_-butylsulfonyl)imines [38; 39; 40; 41] we found that potassium _tert_-butoxide was a more convenient base than potassium hydroxide, we decided to evaluate the former in the ATH of phosphinylimine **1a** and we obtained a further improvement, giving 87% yield and 82% ee (Table 1, entry 13). A decrease of the initial temperature to -40 \({}^{\circ}\)C slightly reduced both the yield and the enantiomeric excess (Table 1, entry 14). After having performed this screening, we chose the conditions shown in entry 13 as the optimal ones. The influence of the arene ligand in the ruthenium catalyst was also studied and we found that the use of [RuCl\({}_{2}\)(benzene)]\({}_{2}\) instead of [RuCl\({}_{2}\)(_p_-cymene)]\({}_{2}\) almost completely prevented any asymmetric induction, the ee of the product **2a** being only 8%. Once we had determined the optimum reaction conditions, we decided to test some other chiral \(\beta\)-amino alcohols with the aim of evaluating the influence of the ligand structure on the success of the ATH process. Amino alcohols (1\(R\),2_S_)-norepinephrine **L2**, (1\(R\),2_S_)-2-amino-1,2-diphenylethanol **L3** and prolinols **L4** and **L5** were tested as potential ligands, and the results are summarized in Table 2. The ruthenium complex bearing norepinephrine **L2** was able to reduce imine **1a**, though the results concerning both yield and enantioselectivity were rather poor (Table 2, entry 2). When the ATH reaction was carried out using ligand **L3**, the reduction product was obtained in only 39% yield and with a moderate ee. Therefore, ligands **L2** and **L3** have shown to be clearly inferior to **L1** (compare entries 1 to 3 in Table 2). This drop in enantioselectivity could be due to the fact that (1S,2R)-1-amino-2-indanol **L1** has a more rigid structure than ligands **L2** or **L3**.