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Investigation of Proline Amides and Pyridinium Salts as Catalyst for Direct Aldol Reactions in Water.

Byline: Asli Ozkan, Murat Emrah Mavis, Feray Aydogan and Cigdem Yolacan

: Summary: The catalytic potential of chiral proline amides and pyridinium salts in the aldol condensation in water was investigated. The aldol reactions of acetone with various aromatic aldehydes were carried out in water by using proline amide derivatives and pyridinium salts derived from chiral pyridine derivatives for the first time. The products were obtained in good yields within short reaction times.

Keywords: Aldol reactions, Green chemistry, Homogeneous catalysis, Phase-transfer catalysis, Proline amides, Pyridinium salts.

Introduction

The development of new synthetic methods that are more environmentally benign has been propelled by the growing importance of green chemistry [1]. In this fact, scientists have been aimed to produce safer and environmentally friendly chemicals and processes in recent years. The twelve principles of green chemistry include the use of non- toxic solvents and catalytic processes to minimize the environmental effect of chemical synthesis. From this point, water is an uniquely alternative as a solvent. It is a non-toxic, non-flammable, non- volatile, inexpensive, environmentally benign solvent. Besides, important rate enhancements, unique selectivity and reactivity have been observed in the reactions carried out in water. Consequently, the development of synthetically useful reactions in water is of considerable interest [2]. Unfortunately, its use is limited by the low solubility of organic compounds. One of the most important strategies to overcome this limitation of water is using phase transfer catalysts (PTC). These catalysts are powerful tools to carry out reactions efficiently between reagents dissolved in mutually insoluble aqueous and organic phases. The most important advantages of PTC are simplicity, mild conditions, high-reaction rates, high selectivities, and the use of inexpensive reagents [3]. Another important strategy to fulfill the requirements of green chemistry is the use of organocatalytic processes. Organocatalysts have several advantages over transition-metal catalysts and enzymes. They are usually robust, inexpensive, readily available and non-toxic. Because of their robustness they often do not require demanding reaction conditions like inert gas atmosphere and absolute solvent [4]. Some examples have been reported on the use of organocatalysts in water which is an important approach to the principles of green chemistry [5].The aldol reaction is one of the mostimportant carbon-carbon bond forming reactions in modern organic synthesis as a key step in natural product synthesis and for the rapid access to polyoxygenated compounds. This useful transformation facilitates the construction of larger complex molecules with new stereogenic centers from smaller ones. Thus, asymmetric aldol reaction has been extensively studied and new organocatalysts have been developed successfully [6]. Although, using new environmentally friendly reaction medium has found a lot of interest from the viewpoint of green chemistry, most of the organocatalytic aldol reactions performed in organic solvents such as DMSO, DMF, toluene, chloroform, and there is a few example on using water as solvent [7]. In this paper we would like to investigate catalytic activities of some chiral proline amides and pyridinium salts on aldol reactions in water. Three new chiral proline amide derivatives and one pyridinium salt were synthesized for the first time. These catalysts were successfully used to catalyze direct aldol reaction of acetone with several aromatic aldehyde in water.Results and Discussion

In the first part of our study, three new proline amide derivatives (6, 7 and 8) were prepared by the simple amidation reactions of N-Boc-proline or N-Boc-4-hydroxyproline with L-tert-leucine hydrochloride or N-aminomorpholine in the presence of DCC and DMAP or HOBt and DIC followed by the deprotection of Boc group with TFA (Scheme-1). The structures of these compounds were determined by their spectroscopic data. We started the study by investigation of the aldol reaction of acetone with 4- nitrobenzaldehyde in the presence of catalyst 6 to see the catalytic effect of newly synthesized proline amide derivatives in water. Different catalyst amounts such as 15, 10, 5, 2.5, and 1% were tried and the obtained yields were 81, 83, 72, 51, and 43%, respectively, so 10 mol% catalyst was the best

amount. Then, the aldol reactions of acetone with several aldehydes with or without the catalysts 6, 7 and 8 in water were investigated. The enantiomericexcesses were determined by chiral HPLC. From theresults in the Table-1, it can be seen clearly that thecatalysts, especially catalyst 6, have good catalyticactivity in the direct aldol reaction of acetone with different aldehydes in water and the catalysts 7 and 8 show better asymmetric induction than catalyst 6.This can be explained by the positive effect of lone pair electrons on the oxygen and nitrogen atoms.

Table-1: Comparison of the catalytic activity of catalysts 6, 7 and 8.

###Product###Catalyst

###Yield###Time###ee

###(%)###(h)###(%)

###6###83###3###13

###7###89###5

###8###68###5

###9a [8, 9]###none###24###48###-

###6###61###2###6

###7###62###5###24.1

###8###46###4###9.7

###-

###9b [10]###none###28###48

###6###90###3

###7###41###5###35.1

###8###43###5

###9c [11]###none###20###48###-

###7###84###5###21.5

###8###30###6###30

###9###28###3###48.8

###none###none###48###-

9d [12]

In the second part of our study, the N- alkylpyridinium salts (11 and 12) were synthesizedby the reaction of 1-bromohexane and 1,6-

dibromohexane with chiral pyridine derivatives.Compound 11 was synthesized for the first time in the literature starting from 1-bromohexane and iminopyridine derivative, 10, which was synthesized from the condensation of pyridine-3-carbaldehydewith S-phenylethylamine in 100% yield according to

the literature procedure [13] (Scheme-2). Compound12 was synthesized by the reaction of S-(-)-nicotine with 1,6-dibromohexane [14] (Scheme-3). Thestructures of all compounds were determined by theirspectroscopic data and were in accordance with theliterature data for known compounds.

The catalytic activities of pyridinium salts were also investigated starting with the aldol reaction of acetone with 4-nitrobenzaldehyde. The reactions were carried out in water-reagent/substrate biphasic system without any other organic solvent. Several different bases such as potassium hydroxide, triethylamine, potassium carbonate were tried as base in the presence of catalysts 11 and it was decided that potassium hydroxide was the good base in 10 mol% amount. Then different catalyst amounts such as 15,10, 5, 2.5, and 1% were tried and the obtained yields were 75, 78, 65, 51, and 43%, respectively, so 10 mol% catalyst was the best amount. Under these optimum conditions, the aldol reactions of acetone with various aromatic aldehydes were carried outwith catalysts 11 and 12, and also without catalyst atroom temperature for comparison. The pyridinium salts have good catalytic effect on the aldol condensation of acetone with short reaction times and good to moderate yields as summarised in Table-2. All of the aldol products (9 a-j) were known in the literature and all the spectroscopic and physical data are in full agreement with those reported in the literature. Under these conditions, B-hydroxyketones were major products, only small amounts (10-15% yield) of elimination products were obtained as side product. Asymmetric inductions were expected fromthe chiral pyridinium salts, but optical rotation measurements showed that the products did not show any optical activity.

Table-2: Comparison of the catalytic activity of catalysts 11 and 12.

###Yield (%)###Time (min)

Product###11 12 No catalyst 11 12 No catalyst

###78 83###48###10###5###40

9a [8, 9]

###78 85###56###10###5###15

9c [11]

###60 70###58###12###5###20

9e [8, 15]

###75 65###43###15###5###30

9f [8]

###60 63###41###15###5###25

9g [15]

###77 65###46###15###5###25

9h [8, 9]

###58 70###40###10###5###20

9i [12]

###74 65###68###5###5###15

9j [16]

Experimental

All reagents were of commercial quality and reagent quality solvents were used without further purification. IR spectra were determined on a Perkin Elmer, Spectrum One FT-IR spectrometer. NMR spectra were recorded on Mercury VX-400 MHz and Bruker Avance 3 500 MHz spectrometers. LC-MS spectra were obtained on a Agilent 6460-A LC-Triple Quadruple MS/MS instrument. Column

chromatography was conducted on silica gel 60 (70-230 mesh). TLC was carried out on aluminum sheets precoated with silica gel 60F254 (Merck). Optical rotations were measured on Optical Activity AA-55 polarimeter with a sodium lamp at room temperature. Enantiomeric excesses were measured by a Shimadzu HPLC equipped with SPD-20A detector with Chiralpak AD column. Boc-protected S-proline [17] and 2S,4R-4-hydroxyproline [18] were prepared according to literature procedure.

(S)-tert-Butyl 2-((S)-1-methoxy-3,3-dimethyl-1- oxobutan-2-ylcarbamoyl)pyrrolidine-1-carboxylate(3)Triethylamine (2.0 mmol) was added to the solution of Boc-proline (1.0 mmol) and L-tert-leucinemethyl ester hydrochloride (1.0 mmol) in dry THFand the mixture was stirred at 0 oC for 30 min. After the addition of DCC (1.2 mmol) and DMAP (3.0 mmol), the mixture was stirred at 0 oC for 1 h and then at room temperature for 16 h. The reaction mixture was filtered and THF was evaporated. The residue was dissolved in ethyl acetate and washed with water. The organic layer was dried with MgSO4, condensed in vacuo. The crude product was purified by column chromatography on silica gel eluted by ethyl acetate/n-hexane 1:1. Yellow oil; yield 82.2%;1H NMR (500 MHz, CDCl3): and 0.96 (s, 9H, tert- Leu-C(CH3)3), 1.48 (s, 9H, C(CH3)3), 1.80-1.88 (m,2H, Pro-CH2), 2.16-2.20 (m, 1H, Pro-CH2), 2.36-2.42 (m, 1H, Pro-CH2), 3.31-3.49 (m, 2H, Pro-CH2), 3.71 (s, 3H, OCH3), 4.25-4.46 (m, 2H, Pro-CH, tert-Leu- CH), 7.65 (bs, 1H, tert-Leu-NH) ppm; FTIR (atr): 3333, 2968, 2876, 1740, 1686 cm-1.(2S,4R)-tert-Butyl 4-hydroxy-2-((S)-1-methoxy-3,3- dimethyl-1-oxobutan-2-ylcarbamoyl) pyrrolidine-2- carboxylate (4)

HOBt (1.1 mmol) and DIC (1.1 mmol) were added to the solution of Boc-4-hydroxyproline (0.9 mmol) in dry THF at 0 oC, following the addition of Et3N (2.2 mmol) the mixture was stirred for 15 min. After the addition of L-tert-leucine methyl ester hydrochloride (1.1 mmol), the mixture was stirred at0 oC for 1 h and then at room temperature for 16 h. The reaction mixture was filtered and THF was evaporated. The residue was dissolved in ethylacetate and washed with 1 M HCl, saturated NaHCO3, brine and water. The organic layer was dried with MgSO4, condensed in vacuo. The crude product was purified by column chromatography on silica gel eluted by chloroform/ethyl acetate 1:12. Yellow oil; yield 64%; 1H NMR (400 MHz, CDCl3): and 0.95 (s, 9H, tert-Leu-C(CH3)3), 1.47 (s, 9H, C(CH3)3), 1.97-2.54 (m, 2H, Pro-CH2), 3.47-3.51 (m,2H, Pro-CH2), 3.70 (s, 3H, OCH3), 4.05 (t, 1H, J 6.8 Hz, Pro-CH), 4.34-4.47 (m, 3H, Pro-CH, tert- Leu-CH and OH), 7.61 (bs, 1H, tert-Leu-NH) ppm; FTIR (atr): 3337, 2964, 2936, 2876, 1740, 1668 cm-1.

(S)-tert-Butyl 2-(morpholinocarbamoyl)pyrrolidine-1-carboxylate (5)

Compound 5 was prepared by the same procedure for Compound 4, and purified by column chromatography on silica gel eluted by methanol/ethyl acetate 1:8. Yellow oil; yield 73%;1H NMR (400 MHz, CDCl3): and 1.46 (s, 9H,C(CH3)3), 1.70-2.22 (m, 4H, 2xPro-CH2), 2.80 (bt,4H, Morph-N-CH2), 3.34-3.50 (m, 2H, Pro-CH2),3.80 (bt, 4H, Morph-OCH2), 4.16-4.22 (m, 1H, Pro-CH), 8.0 (bs, 1H, NH) ppm; FTIR (atr): 3224,2966, 2930, 2864, 1691, 1671 cm .General Procedure for Deprotection

To a solution of Compound 3, 4 or 5 (1.0 mmol) in dry CH2Cl2, trifluoroacetic acid (27.0 mmol) solution in dry CH2Cl2 was added dropwise at0 oC under nitrogen. The mixture was stirred at 0 oCfor 1 h and then at room temperature for 2 h. Then2M K2CO3 was added to make the solution basic. The organic layer was separated and washed with water, dried with MgSO4, condensed in vacuo. The products were pure and used without any purification.

(S)-Methyl 3,3-dimethyl-2-((S)-pyrrolidine-2- carboxamido)butanoate (6)

White solid; m.p. 90-91 C; yield 98%; [x]20D -31.9 (c 2.7, CHCl3); 1H NMR (500 MHz, CDCl3): and 0.97 (s, 9H, C(CH3)3), 1.72-1.79 (m, 2H, Pro-CH2), 1.90-1.98 (m, 1H, Pro-CH2), 2.13-2.22 (m,1H, Pro-CH2), 2.96-3.13 (m, 3H, Pro-CH2, Pro-NH),3.72 (s, 3H, OCH3), 3.85 (dd, 1H, J 9.2, 5.0 Hz,Pro-CH), 4.39 (d, 1H, J 9.8 Hz, tert-Leu-CH), 8.21 (d, 1H, J 9.2 Hz, tert-Leu-NH) ppm; ESI(-)-MS: m/z 242.90 [M+]; FTIR (atr): 3345 and 3289, 2968,2956, 2932, 2861, 1728, 1666 cm-1.

(S)-Methyl 2-(((2S,4R)-4-hydroxypyrolidin-2- yl)methylamino)propanoate (7)

Yellow oil; yield 95%; [x]20D -19.5 (c 4.0, CHCl3); 1H NMR (400 MHz, CDCl3): and 0.95(s, 9H, C(CH3)3), 1.90-1.96 (m, 1H, Pro-CH2), 2.03-2.31 (m, 2H, Pro-CH2 ve Pro-NH), 2.80-2.84 (dd,1H, J 12.4, 3.6 Hz, Pro-CH2), 3.06-3.09 (bd, 1H, J 12.4 Hz, Pro-CH2), 3.71 (s, 3H, OCH3), 4.02 (t,1H, J 8.4 Hz, Pro-CH), 4.34-4.37 (m, 2H, Pro-CH,tert-Leu-CH), 4.44 (bs, 1H, OH), 8.30 (bd, 1H, J 8.8 Hz, tert-Leu-NH) ppm; ESI(-)-MS: m/z 258.70 [M+]; FTIR (atr): 3416-3318, 2962, 2873, 1731,1651 cm-1.(S)-N-Morpholinopyrrolidine-2-carboxamide (8)

White solid, m.p. 103-105 C; yield 96%;[x]20

- 36.3 (c 5.5, CHCl3); 1H NMR (400 MHz,CDCl3): and 1.63-1.73 (m, 2H, Pro-CH2), 1.86-1.94(m, 1H, Pro-CH2), 2.08-2.17 (m, 2H, Pro-CH2, Pro-NH), 2.78 (t, 4H, J 4.7 Hz, Morph-N-CH2), 2.83-2.89 (m, 1H, Pro-CH2), 2.95-3.02 (m, 1H, Pro-CH2),3.69-3.76 (m, 1H, Pro-CH), 3.80 (t, 4H, J 5.0 Hz,Morph-OCH2), 8.32 (bs, 1H, Morph-NH) ppm; ESI(-)-MS: m/z 200.20 [M+]; FTIR (atr): 3358,3211, 2957, 2855, 1669 cm-1.

1-Hexyl-3-{[(1S)-1- phenylethyl]iminomethyl}pyridinium bromide (11)

A mixture of 1-bromohexane (1.0 mmol) and compound 10 (1.1 mmol) was heated at 100 oC for 1 h. The resulting mixture was treated with diethyl ether to remove unreacted starting materials. The product was pure and used without any purification. Yellow viscous oil, yield 50%, [x]20 +19.0 (c 15.0, CH3OH); 1H NMR (500 MHz, CDCl3): and 0.94 (t, 3H, J 7.0 Hz, CH3), 1.31-1.49 (m, 6H, CH2), 1.63 (d, 3H, J 6.6 Hz, CH-CH3),2.04-2.12 (m, 2H, N-CH2-CH2), 4.75 (t, 2H, J 6.3Hz, N-CH2), 4.80 (t, 1H, J 6.3 Hz, CH-Ph), 7.26 (t,1H, J 7.3 Hz, Ph), 7.36 (t, 2H, J 7.5 Hz, Ph), 7.48(d, 2H, J 7.3 Hz, Ph), 8.20 (t, 1H, J 6.2 Hz, Pyr),8.74 (s, 1H, CHN), 8.98 (d, 1H, J 8.1 Hz, Pyr),9.13 (d, 1H, J 6.1 Hz, Pyr), 9.45 (s, 1H, Pyr) ppm;13C NMR (125 MHz, CDCl3) and 13.94 (CH3), 22.35 (CH2), 24.45 (CH3), 25.67 (CH2), 31.10 (CH2), 31.90(CH2), 62.25 (N-CH2), 69.93 (N-CH-Ph), 122.66,127.47, 128.41, 128.69, 136.71, 142.95, 143.47,144.35, 146.01 (aromatic carbons), 152.65 (CN)ppm; FTIR (atr): 3027, 2960, 1646, 1451 cm-1.

(S)-N,N'-(Hexan-1,6-diyl)bis{3-[(S)-1- methylprolidin-2-yl]pyridinium} Dibromide (12)

1,6-Dibromohexane (1.0 mmol) and S-(-)- nicotine (2.0 mmol) were refluxed in ethanol (20 mL) for 72 h. The solvent was evaporated under reduced pressure. The crude product was purified by column chromatography on silica gel eluted with CHCl3/ MeOH 6:1 then MeOH. Yellow viscous oil, yield40%, [x]D20 - 49.1 (c 1.14, CH3OH); 1H NMR (400 MHz, CD3OD): and 1.50-1.58 (m, 4H, CH2),1.84-2.16 (m, 10H, CH2), 2.35 (s, 6H, N-CH3), 2.42-2.52 (m, 2H, Pro-CH2), 2.58-2.67 (q, 2H, J 9.2 Hz,Pro-N-CH2), 3.38 (t, 2H, J 8.2 Hz, Pro-N-CH2),3.78 (t, 2H, J 8.2 Hz, Pro-N-CH), 4.72 (t, 4H, J 5.8 Hz, Pyr- N-CH2), 8.12 (t, 2H, J 6.2 Hz, Pyr),8.64 (d, 2H, J 8.2 Hz, Pyr), 9.03 (d, 2H, J 5.8 Hz,Pyr), 9.17 (s, 2H, Pyr) ppm; FTIR (atr): 3033,2935, 2859, 2788, 1629, 1500, 1456, 1373, 1344,1151 cm-1.

General Procedure for Aldol Reaction with Catalysts6, 7 and 8To a stirring mixture of acetone (10.0 mmol) and water (3 mL), Catalyst 6, 7 or 8 (10 mol%) was added. After 30 min, aldehyde (1.0 mmol) was added, and the mixture was stirred at room temperature. The reaction was monitored by TLC, after the completion of the reaction saturated NH4Cl (15 mL) was added to the mixture, and extracted with ethyl acetate. The organic layer was dried over Na2SO4, condensed in vacuo. The crude product was purified by column chromatography on silica gel (n-hexane/EtOAc).

General Procedure for Aldol Reactions withCatalysts 11 and 12

Potassium hydroxide (0.1 mmol) and catalyst 11 or 12 (0.1 mmol) were dissolved in water (20 mL), then aldehyde (1.0 mmol) and acetone (1.0 mmol) were added. The reaction mixture was stirred at 25 C for a period of time long enough to complete the reaction (TLC), and it was extracted with ethyl acetate. The combined organic extracts were dried with MgSO4, and the solvent was removed under reduced pressure. The residue was purified by column chromatography on silica gel (n- hexane/EtOAc).

TR values for some aldol products

4-Hydroxy-4-(4-nitrophenyl)butan-2-one(9a). Daicel Chiralpak AD, n-hexane/iPrOH 98:2, flow rate 0.5 mL/min, 254 nm; retention time:42.0 min (minor) and 43.0 min (major).

4-Hydroxy-4-(3-nitrophenyl)butan-2-one (9b). Daicel Chiralpak AD, n-hexane/ethanol 90:10, flow rate 1.5 mL/min, 210 nm; retention time:16.1 min (major) and 24.0 min (minor).

4-Hydroxy-4-(2-nitrophenyl)butan-2-one(9c). Daicel Chiralpak AD, n-hexane/ethanol 95:5, flow rate 1.0 mL/min, 210 nm; retention time: 8.7 min (major) and 11.1 min (minor).

4-Hydroxy-4-phenybutan-2-one (9d). Daicel Chiralpak AD, hexane/iPrOH 95:5, flow rate 1.0 mL/min, 254 nm; retention time: 15.0 min (major) and 16.6 (minor).

ConclusionIn conclusion, proline amide derivatives and chiral pyridinium salts, which were prepared easily from cheap, commercially available materials, were very effective catalysts in the aldol reactions of acetone with aromatic aldehydes. These compounds were used for aldol reaction as catalyst for the first

time and they have good catalytic effect with short reaction times and good to moderate yields. Although the pyroline amide derivatives showed some asymmetric induction, pyridinium salts did not show any asymmetric induction for this reaction. Using these proline amides and pyridinium salts as catalysts in aldol reaction is also important from the viewpoint of green chemistry, because of using environmentally friendly solvent water.

Acknowledgement

We thank Yildiz Technical UniversityScientific Research Foundation (2011-01-02-KAP-02) for financial support.

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