Asymmetric reductive amination of carbonyl compounds was carried out using a novel class of aliphatic quarternary ammonium based chiral ionic liquid. S-(+)-2,3-dihydroxy-N,N,N-tributylpropanaminum bromide chiral ionic liquid has been synthesized, characterized and used for asymmetric reductive amination of carbonyl compounds in the presence of sodium borohydride. These preliminary results are encouraging and advocate dual role of novel ionic liquid as a medium and reducing agent for proficient conversion of ketones to amines, however, reductive amination reaction needs to be established for other substituents.
Functionalized chiral amines are considered as building blocks for the construction of pharmaceuticals [1,2] and agrochemicals. Asymmetric reductive amination of carbonyl compounds [
Keeping in view of the responsibility of a chemist in designing, the ecofriendly solvents to carry out the reactions, we have undertaken the synthesis of novel task specific chiral ionic liquid. Herein we report synthesis, characterization and applications of novel aliphatic quarternary ammonium based chiral ionic liquid. The reaction of S-(+)-3-chloro-1,-2-propanediol and tri-n-butylamine followed by reduction is the key step in designing a new chiral ionic liquid, which plays a dual role as a solvent and a catalyst for reductive amination reaction.
Two novel chiral ionic liquids were prepared and found to be efficient in promoting the reductive amination of carbonyl compounds with aromatic amines to afford chiral secondary amines. These reactions are highly stereoselective in nature. The first ionic liquid, S-(+)-2- 3-dihydroxy-N,N,N-tributylpropanaminum chloride (1) was obtained by the reaction of tributylamine and S-(+)- 3-chloro-1,-2-propanediol at 120˚C for 6 h (Scheme 1). S-(+)-2-3-dihydroxy-N,N,N-tributylpropanaminum chloride ionic liquid is soluble in chloroform, dichloromethane, acetone and ethanol. It is insoluble in water, diethyl ether and n-hexane. The resulting chiral ionic liquid is very stable at room temperature. The other ionic liquid, S-(+)-3-(tributylamino)-1-hydroxy-2-propoxyborohydride (2) was prepared by the reaction of 1 with sodium borohydride in water (Scheme 2).
In a typical procedure of the asymmetric reductive amination of aniline and 4-fluoro-propeophenone in ionic liquid, 1 was carried out to standardize the reaction procedure (Scheme 3). Interestingly, the reaction afforded in 72% yield over a period of 30 minutes. The same reaction was carried out in the presence of ionic liquid 2 at room temperature for ~30 minutes to secure 60% yield of the same product. In the present investigation, an extremely efficient method has been developed for the synthesis of novel chiral amine, 4-fluoropropeophenylN-benzyl-3-(2-amino-4-thiazolyl) coumarin using one pot, three component reaction using ionic liquids 1 and 2 as efficient reusable catalyst. This method has several merits such as high stereo selectivity, cost efficiency, taskspecific, and simple work up and usage in the synthesis of complex amines.
Chemicals and solvents used in the experiment were commercially available. Homogeneity/purity of all the
products was assayed by thin-layer chromatography (TLC) on alumina-coated plates (Merck). Product samples in methanol (MeOH) were loaded on TLC plates and developed in CHCl3: MeOH (9.5:0.5, v/v). When slight impurities were detected by iodine vapor/UV light visualization, compounds were further purified by chromatography on silica gel columns (100 - 200 mesh size, CDH). Purity of the compounds was finally re-checked by TLC. Infrared (IR) spectra were recorded in KBr medium using a Perkin-Elmer Fourier Transform-IR spectrometer, whereas 1H and 13C nuclear magnetic resonance (NMR) spectra were recorded in CDCl3 medium on a JNM ECX-400P (JEOL, USA) spectrometer with tetramethylsilane (TMS) as internal reference. Chemical shifts are expressed in ppm (δ-scale) and coupling constants (J) in Hz. Splitting patterns are described as singlet (s), doublet (d), triplet (t), quartet (q) and multiplet (m).
In a round bottomed 50 cm3 flask equipped with a condenser and magnetic stirrer, 5 g tributylamine (0.02 mole) and 3 g S-(+)-3-Chloro-1, 2-propanediol (0.02 mole) were mixed and heated at 120˚C for 12 h. The reaction mixture was washed with 2 × 30 cm3 diethyl ether. The drying of organic layer under vacuum afforded 1 in 70% yield as a pure oily liquid. 1H NMR (400 MHz, DMSO-d6, 25˚C, ppm): δ 0.90 (q, 9H), 1.28 (m, 6H, 2’CH2), 1.61 (m, 6H, 2’CH2), 3.34 (m, 6H, CH2-N), 4.75 (b, 1H, 2’OH), 4.00 (b, 1H, 1’OH), 3.28 (m, 1H, CH-N), 2.92 (m, 1H, CH-N), 3.62 (m, 2H, 1’CH2), 3.40 (m, 1H, *CH). 13C NMR (100 MHz, CDCl3, 25˚C, ppm): δ 77.3, 77.0, 76.7, 59.5, 23.8, 19.6 and 13.7.
In a round bottomed 100 cm3 flask, 1 g of compound 1 (0.003 mole), 40 cm3 H2O and 0.14 g of sodium borohydride (0.0037 mole) were mixed at room temperature. The contents were stirred rapidly for 2 min and extracted with CH2Cl2. The organic layer was dried over Na2SO4 and the solvent was evaporated under reduced pressure. After drying under vacuum for 2 h, the product 2 was obtained in 65% yield as a colorless oily liquid. 1H NMR (400 MHz, CDCl3, 25˚C, ppm): δ 0.93 (t, 9H), 1.32 (m 12H), 2.33 (m, 0.98H), 2.57 (m, 0.82H), 3.37 (m, 5.92H), 3.41 (m, 3H); 13C NMR (400 MHz, CDCl3, 25˚C, ppm): δ 77.3, 77.0, 76.7, 59.5, 23.8, 19.6, and 13.7.
4-fluoropropeophenone (0.192 g, 0.0012 mole) and aniline (0.07 g, 0.0008 mole) are mixed in 0.25 g (0.0008 mol) ionic liquid 1 at room temperature for 30 minutes followed by the addition of NaBH4. At the end of the reaction, diethyl ether was added. The organic layer was separated, washed with water, dried (Na2SO4), and filtered. The filtrate was evaporated to afford the product. Crystallization from ethanol afforded 1.68 g (72%) 4-fluorobenzylaniline. 1H NMR (400 MHz, DMSO-d6, 25˚C): δ 6.65-7.27 (m, 9H, Ar-H), 6.70 (merged 1H, NH), 0.86 (t, 3H), 1.75 (m, 2H), 4.10 (m, 1H, *CH).
3-(2-amino-4-thiazolyl) Coumarin (0.39 g, 0.0016 mole) and 4-fluoropropeophenone (0.48 g, 0.0032 mole) are mixed in 0.25 g (0.0008 mole) of ionic liquid 1 at room temperature for 90 minutes at 70˚C followed by the addition of NaBH4. After completion of the reaction, the reaction mixture was extracted with ethyl acetate. The extract was dried over anhydrous Na2SO4 and evaporated to afford the product 0.68 g (75.6%). 1H NMR (400 MHz, DMSO-d6, 25˚C, ppm): δ 7.00-7.72 (m, 9H, Ar-H), 6.21 (S,1H), 7.75 (merged, m, 1H, NH), 4.57 (t,1H, *CH), 0.88 (t, 3H), 1.77(m, 2H, 2’H).
In conclusion, we have synthesized novel aliphatic quarternary ammonium based chiral ionic liquids for catalysis of asymmetric reductive amination. The present investigation reports a novel procedure to synthesize chiral amines using novel chiral ionic liquids which are considerably faster with cleaner technology under neutral conditions. As a testimony to the ILs efficiency, these chiral ionic liquids could be easily recycled and reused for three times. Our current study proved that the newly synthesized chiral ionic liquids could be used as solvent, highly efficient and reusable organocatalyst. The results and observations in this study could also be useful in designing of a new type of aliphatic quarternary ammonium based IL with improved stereoselectivity. These preliminary results are encouraging, however, the catalytic efficiency of these new ILs needs to be performed for various reductive amination reactions.
The author gratefully acknowledges UGC for giving financial support under major research project scheme and Department of Chemistry, University of Delhi for NMR & IR spectral analysis. The author also acknowledges Prof. R. K. Bansal, Emeritus Professor, Department of Chemistry, The IIS University, Jaipur for his guidance in drawing the results.