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Article

YCl3-Catalyzed Highly Selective Ring Opening of Epoxides by Amines at Room Temperature and under Solvent-Free Conditions

by
Wuttichai Natongchai
1,
Rais Ahmad Khan
2,
Ali Alsalme
2 and
Rafik Rajjak Shaikh
1,*
1
School of Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology (VISTEC), 555 Moo 1, Payupnai, Wangchan, Rayong 21210, Thailand
2
Department of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia
*
Author to whom correspondence should be addressed.
Catalysts 2017, 7(11), 340; https://doi.org/10.3390/catal7110340
Submission received: 16 October 2017 / Revised: 29 October 2017 / Accepted: 6 November 2017 / Published: 10 November 2017
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Abstract

:
A simple, efficient, and environmentally benign approach for the synthesis of β-amino alcohols is herein described. YCl3 efficiently carried out the ring opening of epoxides by amines to produce β-amino alcohols under solvent-free conditions at room temperature. This catalytic approach is very effective, with several aromatic and aliphatic oxiranes and amines. A mere 1 mol % concentration of YCl3 is enough to deliver β-amino alcohols in good to excellent yields with high regioselectivity.

Graphical Abstract

1. Introduction

The β-amino alcohols are categorized as very useful chemical compounds due to their presence in numerous natural products, medicinally important molecules and ligands or chiral auxiliaries [1,2,3]. Other applications of these versatile compounds include their use as intermediates/precursors in the synthesis of cosmetics and daily-use products such as perfumes, hair dyes, and photo developers [4]. Moreover, β-amino alcohols are very useful precursors towards structurally complex and intriguing chemical compounds such as (multi-)cyclic organic compounds which can be obtained through rather facile chemical transformations or through synthesizing metalloorganic compounds via complexation with a metal center [5,6].
The simplest approach to the β-amino alcohols is the ring opening of epoxides with amines (Scheme 1) [7]. This approach is usually met with some limitations such as kinetically slow reactions that may be due to inefficient activation of epoxides and low regioselectivity. To overcome these issues, many new catalytic methods have been introduced using various catalysts, with aims to enhance the electrophilicity of epoxides through metallic coordination or H-bonding [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26]. The N-alkylation of amines using cyclic carbonates is also an attractive approach to molecules under discussion [27,28]. Many methods for the synthesis of β-amino alcohols from epoxides or cyclic carbonates require a high reaction temperature, dry reaction conditions, and high catalyst loadings. The preparation of β-amino alcohols under solvent-free conditions and at room temperature is desirable and important due to environmental considerations.
Rare-earth metals including lanthanides, scandium, and yttrium are being increasingly exploited in organic synthesis because of their unique reactivity and selectivity [29,30,31,32,33,34,35,36,37]. Rare-earth metals tend to be oxyphilic, in general, and exhibit a highly Lewis-acidic character. One work involving rare-earth Lewis acid catalysts such as YCl3 demonstrated a highly efficient catalytic strategy towards the synthesis of cyclic carbonates via epoxide activation [38]. Similarly, we envisaged the Lewis-acidic activation of epoxide by Sc- and Y-based catalysts, thus making epoxide susceptible to nucleophilic attack by amines to produce β-amino alcohols (Scheme 2).
In this work, we report a simple, efficient, and environmentally friendly method using a rare-earth metal-based catalyst, YCl3, for the ring opening of a library of epoxides by various amines under solvent-free conditions at room temperature. To the best of our knowledge, the ring-opening reaction of epoxide with amines catalyzed by YCl3 has not been reported before.

2. Results and Discussion

Initially, we screened the ring-opening reaction of styrene oxide (1a) as the model substrate with aniline (2a) as the model nucleophile at room temperature. The conversion and regioselectivity were calculated based on 1H-NMR of crude reaction mixture, and the results are summarized in Table 1. In the absence of a catalyst, only traces of corresponding β-amino alcohol were observed. In the presence of 1 mol % YCl3 as a catalyst, we were delighted to see a more than 90% conversion of 1a, producing both possible regioisomers, 3aa and 4aa (Table 1, entry 2). The result indicates that the reaction proceeded smoothly with high regioselectivity (3aa:4aa, 93:7). After 3 h of reaction, 1 mol % loading of YCl3 gave a 100% conversion of 1a under solvent-free conditions (Table 1, entry 6). Under identical reaction conditions, ScCl3 gave an 82% conversion of 1a with 92% regioselectivity towards 3a (Table 1, entry 3).
We tried a higher catalyst loading (5 mol %) for both YCl3 and ScCl3 and found no noticeable effect on regioselectivity of the reaction; however, an increase in rate of substrate conversion was noted (Table 1, entries 4 and 5). Adding the solvent to the reaction had a negative effect on the catalytic activity of Lewis acids. When CHCl3 was used as the reaction solvent with 1 mol % YCl3 and at room temperature, only a 50% conversion of 1a was noted after a 1 h reaction time (Table 1, entry 7), and it took more than 12 h to achieve full conversion of the epoxide under these conditions. It is worth noting that, when 5 mol % YCl3 was used with and without the solvent (CHCl3), the aminolysis of 1a was slightly slower in the presence of a solvent, further confirming that the highest efficiency of YCl3 can be achieved under solvent-free conditions (Table 1, entry 8). Full conversion of 1a was noticed after 2 h using 5 mol % YCl3 in CHCl3, while under a solvent-free condition, 100% conversion was noted after 1 h. Similarly, a 5 mol % loading of ScCl3 also gave a lower conversion of 1a (88%) in the presence of CHCl3 (Table 1, entry 9) compared to solvent-free conditions (100%) (Table 1, entry 5).
We also compared the catalytic activity of four transition metal-based Lewis acid catalysts NbCl5, ZrCl4, ZnCl2, and Nb(OEt)5 (Table 1, entries 10–13). When used under solvent-free conditions at room temperature, 1 mol % NbCl5, Nb(OEt)5, and ZrCl4 gave conversions of 82%, 85%, and 75%, respectively, after a 1 h reaction time with a decrease in regioselectivity when compared to tested rare-earth based catalysts. The change in counter anion in the case of Nb(OEt)5 did not result in any considerable change or enhancement in catalytic activity towards aminolysis. Moreover, to our surprise, ZnCl2 gave almost no conversion under identical reaction conditions, even when reaction time was prolonged to 5 h. Figure 1 shows the proton nuclear magnetic resonance (1H-NMR) spectra for the aminolysis reaction of 1a with 2a for all tested catalysts under identical reaction conditions. Comparatively, YCl3 as a catalyst seemed to be the best choice amongst the tested series of catalysts considering the higher conversion of epoxide and higher regioselectivity (Figure 1 and Table 1).
The most convenient and economically friendly reaction conditions—1 mol % YCl3, no solvent, and room temperature—were then applied to overnight reactions of structurally diverse epoxides and various amines as nucleophiles. The results are summarized in Table 2. The results showed that the aminolysis of 1a proceeds efficiently with aromatic amines such as aniline (2a), 3-methoxyaniline (2b), and 4-chloroaniline (2c) (Table 2, entries 1–3), giving a 100% conversion of 1a with excellent regioselectivity, while the presence of a strong electron-withdrawing nitro group in 4-nitroaniline (2d) showed a negating effect on the reaction, giving only a 13% conversion of 1a in an almost equimolar mixture of 3ad and 4ad regioisomers (Table 2, entry 4).
We also carried out aminolysis of epichlorohydrin (1b), an interesting molecule with applications in polymer chemistry with different amines. When reacted with 2a and 2c, it showed full conversion and moderate results with respect to regioselectivity (Table 2, entries 5 and 7). On the other hand, reaction with 2b resulted in poor conversion of 1b and lower regioselectivity (Table 2, entry 6). Unlike 1a, 1b gave 4 as a major regioisomer. This shift in regioselectivity can be attributed to the nucleophilic attack of amine at less hindered site of the epoxide ring (SN2 mechanism). Propylene oxide (1c) was also successfully tested for YCl3-catalyzed aminolysis with 2a to give 100% conversion (Table 1, entry 7). We also investigated the preparation of β-amino alcohols from a cyclic epoxide, cyclohexene oxide (1d) using 2a as nucleophilic amine; the corresponding reaction gave full conversion of 1d (Table 2, entry 9). The reaction of 1a with indole (2e), a very commonly studied heterocyclic amine due to its structural features and biological importance [39,40,41,42,43,44], gave only a 30% conversion of 1a (Table 2, entry 5) but, interestingly, 91% regioselectivity (Table 2, entry 10). Since the most reactive position of 2e is C3, the C3-attack is favored to give 3ae and 4ae.
Butadiene monoxide (1e) is an important epoxide substrate considering its current and potential applications in polymer chemistry [45,46]. When subjected to aminolysis with 2a and indoline (2f), it gave 75% and 100% conversions, respectively (Table 2, entries 11 and 12) with moderate regioselectivity. The resultant β-amino alcohols could be used to develop novel polymeric materials with numerous applications through simple polymerization processes. To further test the potential of our catalytic approach, we tried a novel epoxide, 1e, as a challenging substrate bearing electron-withdrawing ester functionality. Interestingly, aminolysis of 1e with 2a gave full conversion of 1e, further establishing the potential of rare-earth based catalysts for aminolysis of epoxides. Moreover, 1e gave 4ea as the only product, indicating the nucleophilic attack of 1a at less hindered sites, similarly following the trend of 1b bearing a chloromethyl substituent.
Finally, we compared the activity of our present catalytic system with that of some of the previously reported catalysts used for the synthesis of β-amino alcohols under solvent-free conditions and at room temperature. These results are summarized in Table 3 for the reaction of 1a and 2a as the model reaction. These results indicate that some of these reactions involved the use of complex catalysts for the activation of epoxide and higher catalyst loading, and some systems required longer reaction time. Our rare-earth based metal catalyst (YCl3), however, demonstrates better catalytic efficiency for the synthesis of β-amino alcohols.
The plausible mechanism of the reaction involves the activation of the epoxide ring by Lewis-acidic and oxyphilic YCl3 (Scheme 3). The catalyst activates the epoxide ring through interaction with the oxygen atom of the epoxide, making the epoxide ring more vulnerable to nucleophilic attack by amines. Interestingly, the nucleophilic attack was found to be regioselective for most of the substrates.
The nucleophilic attack at sterically hindered site indicates the dominance of the electronic effect (SN1-type mechanism) over the steric effect due to the possible stabilization of intermediate carbocation in resonance with substituents such as the phenyl ring or the double bond. On the other hand, the results obtained for 1b and 1e indicate that the nucleophilic attack occurs at less hindered site of the epoxide ring (SN2 mechanism).

3. Materials and Methods

All chemicals were obtained from commercial vendors and used without further purification. Styrene oxide was purchased from TCI Chemicals Co. Ltd., Tokyo, Japan. 1,2-propylene oxide, cyclohexene oxide epichlorohydrin, 4-nitroaniline and 4-chloroaniline were purchased from Merck, Darmstadt, Germany. Butadiene monoxide was purchased from Alfa Aesar, Ward Hill, MA, USA. Aniline was purchased from Panreac AppliChem, Darmstadt, Germany. Indole, indoline and 3-methoxyaniline were purchased from Acros Organics, Geel, Belgium.
All reactions were performed in vials. The reaction progress was routinely monitored by 1H-NMR and analytical thin-layer chromatography (TLC) using a pre-coated silica gel glass plates. The products were identified and analyzed using IR, 1H-NMR, and 13C-NMR. The IR spectra were recorded on a Frontier FT-IR spectrometer (PerkinElmer Inc., Waltham, MA, USA). The 1H-NMR and 13C-NMR spectra were recorded on a Bruker AscendTM 600 MHz spectrometer (Bruker Co., Billerica, MA, USA) using CDCl3 as a reference solvent. The mass analysis data was obtained from Compact mass spectrometer (Bruker Co., Billerica, MA, USA).

3.1. General Procedure

In a general procedure, epoxide (1 mmol) and amine (1 mmol) were placed in a vial equipped with a magnetic stirrer and plastic cap. The mixture was stirred at room temperature in the presence of YCl3 (1 mol %) without solvent. After the reaction, the crude mixture was purified by column chromatography using silica gel as stationary phase. All products were identified by spectroscopic studies and by comparison with the previously described physical and spectroscopic data of the β-amino alcohols.

3.2. Spectroscopic Data

All β-amino alcohols are known except for 3ab, 3ea, 3ef, 4ef, and 4fa, which are new compounds, whose spectroscopic data are given below.
2-((3-methoxyphenyl)amino)-2-phenylethan-1-ol (3ab, Table 2, Entry 2). 1H-NMR (600 MHz, CDCl3): δ = 3.57 (s, 3H), 3.61 (m, 1H), 3.79 (dd, J = 11.1, 3.8 Hz, 1H) 4.37, (t, J = 5.4 Hz, 1H), 6.02 (s, 1H), 6.09 (d, J = 8.0 Hz, 1H), 6.14 (d, J = 8.2 Hz, 1H), 6.90 (t, J = 8.1 Hz, 1H), 7.15 (t, J = 6.9 Hz, 1H), 7.22 (m, 4H). 13C-NMR (MHz, CDCl3): δ = 55.2, 60.1, 67.4, 100.1, 103.2, 107.1, 126.9, 127.8, 129.0, 130.1, 140.3, 148.9, 160.8. MS (APCI): m/z calculated for [M + 1]+: 244.1332 Found: 244.1323 FT-IR (ATR): 3394, 1609, 1598, 1492, 1450, 1304, 1210, 1036, 908, 827, 751, 729, 701, 687.
1-(phenylamino)but-3-en-2-ol (3ea, Table 2, Entry 11). 1H-NMR (600 MHz, CDCl3): δ = 3.62 (dd, J = 10.7, 6.3 Hz, 1H), 3.77 (dd, J = 10.8, 3.8 Hz, 1H), 4.02 (d, J = 4.0 Hz, 1H), 5.24 (d, J = 10.4 Hz, 1H), 5.31 (d, J = 17.2 Hz, 1H), 5.80 (m, 1H), 6.65 (d, J = 7.8Hz, 2H), 6.72 (t, J = 7.1 Hz, 1H), 7.16 (t, J = 7.5 Hz, 2H). 13C-NMR (MHz, CDCl3): δ = 57.9, 65.1, 70.7, 114.1, 117.6, 118.3, 129.4, 136.6, 147.5. MS (APCI): m/z calculated for C10H14NO [M + 1]+: 164.0169 Found: 164.1064 FT-IR (ATR): 3388, 1598, 1500, 1433, 1416, 1316, 1067, 1028, 992, 922, 872, 746, 690.
1-(indolin-1-yl)but-3-en-2-ol (3ef, Table 2, Entry 12). 1H-NMR (600 MHz, CDCl3): δ = 3.02 (m, 4H), 3.17 (q, J = 8.0 Hz, 1H), 3.29 (q, J = 8.8 Hz, 1H), 3.53 (q, J = 8.0 Hz, 1H), 4.37 (s, 1H), 5.22 (d, J = 10.5 Hz, 1H), 5.39 (d, J = 17.2 Hz, 1H), 5.92 (m, 1H), 6.55 (d, J = 7.9 Hz, 1H), 6.70 (t, J = 7.3 Hz, 1H), 7.08 (dd, J = 18.0, 7.6 Hz, 2H). 13C-NMR (MHz, CDCl3): δ = 29.0, 54.9, 57.4, 70.7, 107.7, 116.4, 118.7, 124.8, 126.9, 127.6, 138.3, 152.8. MS (APCI): m/z calculated for [M + 1]+: 190.1226 Found: 190.1226 FT-IR (ATR): 3399, 1603, 1489, 1268, 1235, 1050, 992, 922, 743, 715.
2-(indolin-1-yl)but-3-en-1-ol (4ef, Table 2, Entry 12). 1H-NMR (600 MHz, CDCl3): δ = 2.98 (m, 2H), 3.35 (q, J = 9.4 Hz, 1H), 3.45 (q, J = 6.0 Hz, 1H), 3.80 (m, 2H), 4.19 (q, J = 7.02 Hz, 1H) 5.25 (dd, J = 20.0, 14.1 Hz, 2H), 5.77 (m, 1H), 6.53 (d, J = 7.9 Hz, 1H), 6.67 (t, J = 7.3 Hz, 1H), 7.06 (m, 2H). 13C-NMR (MHz, CDCl3): δ = 28.6, 47.4, 60.2, 62.2, 108.2, 119.5, 124.8, 124.5, 130.5, 132.5, 151.4. MS (APCI): m/z calculated for [M + 1]+: 190.1226 Found: 190.1222 FT-IR (ATR): 3380, 1606, 1486, 1461, 1254, 1050, 1025, 922, 743, 712.
Methyl 3-hydroxy-2-(phenylamino)butanoate (3fa, Table 2, Entry 13). 1H-NMR (600 MHz, CDCl3): δ = 1.08 (d, J = 6.5 Hz, 3H), 3.82 (s, 3H), 3.93 (m, 1H), 4.42 (d, J = 1.3 Hz, 1H), 6.66 (d, J = 8.2 Hz, 2H), 6.72 (t, J = 7.3 Hz, 1H), 7.17 (t, J = 7.7 Hz, 1H). 13C-NMR (MHz, CDCl3): δ = 14.4, 51.5, 52.9, 71.6, 114.3, 118.4, 129.6, 146.7, 174.3. MS (APCI): m/z calculated for [M + 1]+: 210.1124 Found: 210.1122 FT-IR (ATR): 3388, 1732, 1601, 1500, 1433, 1254, 1212, 1126, 1075, 1017, 749, 690.

4. Conclusions

In summary, we have demonstrated a new, simple, highly efficient, and environmentally friendly method for the synthesis of β-amino alcohols using a rare-earth metal catalyst, YCl3. This catalytic strategy provides β-amino alcohols with high selectivity and in good to excellent yields under solvent-free conditions at room temperature. Various epoxides and amines could be successfully employed under these conditions to give corresponding β-amino alcohols. Mechanistic studies and asymmetric catalytic versions of this method are under investigation in our laboratory. We also have commenced studies for further expansion of our approach to carry out novel transformations based on rare-earth metal catalysis; new findings will be reported in due time.

Acknowledgments

R.R.S. is thankful to National Research Council of Thailand for an NRCT Foreign Researcher award. R.R.S., and W.N. are thankful to Vidyasirimedhi Institute of Science and Technology for research facilities and financial support for this work. R.A.K. and A.A. extend their appreciation to the Deanship of Scientific Research at King Saud University for funding this work through research group no. RG-1438-006.

Author Contributions

R.R.S. conceived and designed the experiments; W.N. performed the experiments and provided the spectroscopic data in the paper; R.R.S. and W.N. analyzed the data; R.A.K. and A.A. gave suggestions while the paper was written. R.R.S. wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 07 00340 sch001 550
Scheme 1. Aminolysis of epoxides for the synthesis of β-amino alcohols.
Scheme 1. Aminolysis of epoxides for the synthesis of β-amino alcohols.
Catalysts 07 00340 sch001
Catalysts 07 00340 sch002 550
Scheme 2. Lewis-acidic activation of epoxide by YCl3.
Scheme 2. Lewis-acidic activation of epoxide by YCl3.
Catalysts 07 00340 sch002
Catalysts 07 00340 g001 550
Figure 1. Comparative analysis of 1H-NMR spectra of crude mixtures of regioselective ring-opening reaction of styrene oxide (1a) with aniline (2a) using different metal catalysts to give (3aa) and (4aa). The regioisomer (3aa) forms predominantly, as can be observed from spectra. The letters a, b, c, and d indicate the peaks or peak region corresponding to protons on carbons indicated by a, b, c and d in chemical structures of 3aa and 4aa. Reaction conditions: 1a (1 equiv.), 2a (1 equiv.), catalyst (1 mol %), RT, 1 h.
Figure 1. Comparative analysis of 1H-NMR spectra of crude mixtures of regioselective ring-opening reaction of styrene oxide (1a) with aniline (2a) using different metal catalysts to give (3aa) and (4aa). The regioisomer (3aa) forms predominantly, as can be observed from spectra. The letters a, b, c, and d indicate the peaks or peak region corresponding to protons on carbons indicated by a, b, c and d in chemical structures of 3aa and 4aa. Reaction conditions: 1a (1 equiv.), 2a (1 equiv.), catalyst (1 mol %), RT, 1 h.
Catalysts 07 00340 g001
Catalysts 07 00340 sch003 550
Scheme 3. A plausible mechanism for the ring-opening reactions of epoxides with amines catalyzed by YCl3.
Scheme 3. A plausible mechanism for the ring-opening reactions of epoxides with amines catalyzed by YCl3.
Catalysts 07 00340 sch003
Table
Table 1. Optimization studies for the aminolysis of styrene oxide (1a).
Table 1. Optimization studies for the aminolysis of styrene oxide (1a).
Catalysts 07 00340 i001
Sr. No.CatalystConversion (%)3aa:4aa Ratio (%)
1NoneNR-
2YCl39093:7
3ScCl38292:8
4 aYCl310093:7
5 bScCl310092:8
6 cYCl310093:7
7 dYCl35094:6
8 eYCl38794:6
9 fScCl38893:7
10NbCl58287:13
11ZrCl47587:13
12ZnCl2tracesnd
13Nb(OEt)58582:18
a 5 mol % YCl3; b 5 mol % ScCl3; c 3 h; d CHCl3 as reaction solvent; e 5 mol % YCl3 and CHCl3 as reaction solvent; f 5 mol % ScCl3 and CHCl3 as reaction solvent. NR = No Reaction; nd = not determined.
Table
Table 2. Ring-opening of epoxides with amines catalyzed by YCl3..
Table 2. Ring-opening of epoxides with amines catalyzed by YCl3..
Catalysts 07 00340 i002
EpoxideAmine Epoxide Conversion a and 3:4 Ratio b (%)
Sr.
No.1234
11a2a Catalysts 07 00340 i003 Catalysts 07 00340 i004100
93:7
21a2b Catalysts 07 00340 i005 Catalysts 07 00340 i006100
>99:<1
31a2c Catalysts 07 00340 i007 Catalysts 07 00340 i008100
98:2
41a2d Catalysts 07 00340 i009 Catalysts 07 00340 i01013
50:50
51b2a Catalysts 07 00340 i011 Catalysts 07 00340 i012100
19:81
61b2b Catalysts 07 00340 i013 Catalysts 07 00340 i01470
<1:>99
71b2c Catalysts 07 00340 i015 Catalysts 07 00340 i016100
2:98
81c2a Catalysts 07 00340 i017 Catalysts 07 00340 i018100
70:30
91d2a Catalysts 07 00340 i019100
-
101a2e Catalysts 07 00340 i020 Catalysts 07 00340 i02130
93:7
111e2a Catalysts 07 00340 i022 Catalysts 07 00340 i02375
83:17
121e2f Catalysts 07 00340 i024 Catalysts 07 00340 i025100
82:18
131f2a Catalysts 07 00340 i026 Catalysts 07 00340 i027100
0/100
a,b Based on 1H-NMR of crude reaction mixture.
Table
Table 3. Data regarding the solvent-free aminolysis of styrene oxide (1a) with aniline (2a).
Table 3. Data regarding the solvent-free aminolysis of styrene oxide (1a) with aniline (2a).
Sr. No.CatalystCat. Loading (mol %),
Temperature (°C)/Time (h)		Conversion (%)Ref.
1(Et4N)2[MnL2Cl]1
25/4		99[47]
2[Co(L1)Cd(OH2)2(NO3)]·H2O1
r.t./4		98[48]
3[(L2(H2))Zn(NO3)2]2
30/4		98[49]
4Nano Fe3O410
r.t./20		83[50]
5YCl31
25/3		100This work

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Natongchai, W.; Khan, R.A.; Alsalme, A.; Shaikh, R.R. YCl3-Catalyzed Highly Selective Ring Opening of Epoxides by Amines at Room Temperature and under Solvent-Free Conditions. Catalysts 2017, 7, 340. https://doi.org/10.3390/catal7110340

AMA Style

Natongchai W, Khan RA, Alsalme A, Shaikh RR. YCl3-Catalyzed Highly Selective Ring Opening of Epoxides by Amines at Room Temperature and under Solvent-Free Conditions. Catalysts. 2017; 7(11):340. https://doi.org/10.3390/catal7110340

Chicago/Turabian Style

Natongchai, Wuttichai, Rais Ahmad Khan, Ali Alsalme, and Rafik Rajjak Shaikh. 2017. "YCl3-Catalyzed Highly Selective Ring Opening of Epoxides by Amines at Room Temperature and under Solvent-Free Conditions" Catalysts 7, no. 11: 340. https://doi.org/10.3390/catal7110340

APA Style

Natongchai, W., Khan, R. A., Alsalme, A., & Shaikh, R. R. (2017). YCl3-Catalyzed Highly Selective Ring Opening of Epoxides by Amines at Room Temperature and under Solvent-Free Conditions. Catalysts, 7(11), 340. https://doi.org/10.3390/catal7110340

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