The Application of New Chiral Ferrocene Ligands in Asymmetric Transfer Hydrogenation of Ketones

Four easily available ferrocenyl chiral ligands have been screened firstly for ruthenium (II)-catalyzed asymmetric transfer hydrogenation of acetophenone with HCOOH/Et3N azeotrope as the hydrogen source. A moderate chemical yield of 1-phenylethanol with 83% ee was obtained when (RC, SFc)-1-(Diphenylphosphino)-2-[1-N-(3-methylpyridin-2-ylmethyl) ethyl] ferrocene (L1) was used. Particularly, both ruthenium and iridium could coordinate with L1 to accomplish the asymmetric reduction of series of aromatic ketones separately. The desired products were achieved with up to 86% ee.


Introduction
Asymmetric transfer hydrogenation (ATH) often has been achieved by the help of the combination of transition metal and chiral ligands [1] [2] [3].In the field, the exploration of new ligands or the new utilization of reported ones always attracted researcher's interest.Ferrocene-based chiral complexes generally played important roles in asymmetric reactions due to ferrocene's highly electron donating property as well as the unique structure [4] [5] [6] [7].However, it should be noticed that most of the ATH catalyzed by chiral ferrocene derived catalysts used isopropanol as the hydrogen donor.Formic acid, as a more effective hydrogen donor for its irreversible kinetic enantioselectivity seldom collocated with ferrocenyl chiral ligands in ATH [8]- [13].Furthermore, as far as transition metal in ATH was concerned, ruthenium and iridium could both be employed in ATH [14] [15] [16] [17].Commonly, different transition metals' coordination with the same chiral ligand provides more possibility for more stereoinduction [18] [19] [20] [21] [22].Therefore, chiral ferrocenyl derivatives' (L 1 -L 4 , Figure 1) combination with the HCOOH hydrogen donor and its integration with two transition metals are both investigated.The results indicated that the ATH of aryl ketone could proceed smoothly at the presence of L 1 -L 4 and formic acid.
Among them, the reaction with L 1 -metal complex existed gave the best chemical yield and optical yield.

General
All reactions involving air-or moisture-sensitive species were finished under N 2 atmosphere.Solvents were dried by standard methods and freshly distilled before use if needed.All other chemicals were used as purchased.L 1 -L 4 were prepared from (R)-Ugi's amine according the published reports [23].NMR spectra were recorded on a Bruker AV-400 spectrometer with TMS as an internal reference.Chiral High Performance Liquid Chromatography (HPLC) analyses were completed with an Agilent 1200 series apparatus and Chiralpak OD-H and OJ-H columns.The configuration of the products was determined by comparison to the literature data.

Results and Discussion
L 1 -L 4 's synthesis could be accomplished according to our published report [23].Once these ligands in hands, a preliminary study should be carrying out.Firstly, we have used L 1 with RuCl 2 (p-cymene) as catalyst while acetophenone (1a) was substrate to optimize the reaction efficiency.Various solvents, different temperatures and the ratios of catalyst/substrate (C/S) were tested.The results were listed in Table 1.
The data in Table 1 showed that the solvent had influence on the experiment.For example, only trace conversion happened in MeOH, CH 2 Cl 2 and THF (Table 1, entry 1-3).Another three solvents (EtOAc, t-BuOMe, DMF) gave better chemical yields and better enantioselectivities.Among them, DMF provided the highest ee and chemical yield (Table 1, entry 1-6).So DMF was chosen as the optimized solvent for further experiments.
In DMF, ATH of 1a were conducted at different temperatures that ranged from r.t. to −40˚C.Compared with r.t., −20˚C was proved to be a suitable reaction temperature with the improved ee value (Table 1, entry 6 vs. 7).However, when the temperature decreased to −40˚C, only trace products could be observed (Table 1, entry 8).So the rest of experiments were completed at −20˚C.Additionally, the change of C/S from 0.5% to 0.25% caused a lower chemical yield and lessened ee (Table 1, entry 9).But the bigger ratio did not mean the improved results.When the ratio was 1%, the yield was raised slightly and the ee value was almost the same as that of 0.5% C/S (Table 1, entry 7 vs. entry 10).
Therefore, the 0.5% of C/S was reasonable for next work.
Under the optimized conditions, the ligands structure effect on the ATH was also screened.The corresponding results were shown in Table 2. L 1 with P, N, N elements and planar chirality gave the best optical selectivity (84% ee) (Table 2, entry 1).L 2 's result (81% ee) was a little worse than L 1 's (Table 2, entry 2).It implied that N atom on pyridine unit of L 1 had positive effect on the stereoinduction and the yield.The behaviors of L 3 , L 4 were both worse.L 3 's racemic result was not surprising for the reason that there was only one stereogenic center in L 3 (Table 2, entry 3).L 4 gave (S)-products with 37% ee (Table 2, entry 4).The configuration of moiety in L 4 is also (S, S).This indicated that the configuration of the product was controlled by the chiral diamine moiety in ligand structure.The chirality of Uig's amine had not apparent contribution to the stereocontrol.Altogether, the planer chiral elements and the P unit on the ferrocene ring were essential for a higher enantioselectivities.As far as the chemical yields were concerned, the four ligands' performance was almost similar.Lastly, L 1 was choosed to be examined for various ketones' ATH.
As Table 3 summarized, for L 1 -Ru or L 1 -Ir catalyst, the reactions of acetophenone derivatives proceed smoothly.The ee values ranged from 66% to 86%.
First, the position of the ring substituents had influence on the enantioselectivity.Then it seems that electronic properties of the ring substituents had no effect on the results because 3-methoxyacetophenone or 3'-(trifluoromethyl) acetophenone almost obtained the paralleled stereoselectivity (Table 3, entry 4, 5).
Furthermore, for halogen substituted substrates, with the increase of atomic number of halogen, the ees of the reaction had a little increase (Table 3, entry

Conclusion
In conclusion, we have demonstrated four chiral ferrocene-based ligands' utilization in ATH of aromatic ketones.Herein, HCOOH/Et 3 N (5:2) system was firstly applied in this kind of reduction with chiral ferrocenyl ligands complexes existed.Both ruthenium and iridium could coordinate with L 1 and could realize the reduction of acetophenone derivatives.Moderate to good enantiomeric excesses and medium isolated yields were achieved.Further studies of these ligands in other catalytic reactions are currently underway.

Table 3 .
The ATH of aromatic ketones catalyzed by L 1 -Ru or L 1 -Ir catalyst.
[6][7][8]d was isolated yields after column chromatography.The ee and configuration of product were determined by chiral HPLC with Chiralpak OD-H or OJ-H.The absolute configuration was determined by comparison of the sign of optical rotation or retention time with 1iterature data[24][25] [26][27].[6][7][8].All the chemical yields reached moderate (55% -68%).In addition, almost no difference existed between the performance of Ru and Ir catalyst in the ATH of various substrates.