YCl3-Catalyzed Highly Selective Ring Opening of Epoxides by ...

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Nov 10, 2017 - YCl3 efficiently carried out the ring opening of epoxides by amines ... e to nucleophilic attack by amines to produce β-amino alcohols (Scheme 2). ..... asymmetric catalytic versions of this method are under investigation in our ...
catalysts Article

YCl3-Catalyzed Highly Selective Ring Opening of Epoxides by Amines at Room Temperature and under Solvent-Free Conditions Wuttichai Natongchai 1 , Rais Ahmad Khan 2 , Ali Alsalme 2 and Rafik Rajjak Shaikh 1, * 1 2

*

ID

School of Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology (VISTEC), 555 Moo 1, Payupnai, Wangchan, Rayong 21210, Thailand; [email protected] Department of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia; [email protected] (R.A.K.); [email protected] (A.A.) Correspondence: [email protected]; Tel.: +66-33-014-164

Received: 16 October 2017; Accepted: 6 November 2017; Published: 10 November 2017

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. Keywords: epoxides; β-amino alcohols; rare-earth metal catalysis; green chemistry

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 [1–3]. 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 [8–26]. 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.

Catalysts 2017, 7, 340; doi:10.3390/catal7110340

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  Scheme 1. Aminolysis of epoxides for the synthesis of β‐amino alcohols.  Scheme 1. Aminolysis of epoxides for the synthesis of β-amino alcohols. Rare-earth metals including lanthanides, scandium, and yttrium are being increasingly exploited Rare‐earth  metals  including  lanthanides,  scandium,  and  yttrium  are  being  increasingly  in organic synthesis because of their unique reactivity and selectivity [29–37]. Rare-earth metals exploited in organic synthesis because of their unique reactivity and selectivity [29–37]. Rare‐earth  tend to tend  be oxyphilic, in general, exhibit a exhibit  highly Lewis-acidic character. character.  One workOne  involving metals  to  be  oxyphilic,  in  and general,  and  a  highly  Lewis‐acidic  work  rare-earth acid catalysts suchcatalysts  as YCl3 demonstrated highly efficient strategy catalytic  towards involving Lewis rare‐earth  Lewis  acid  such  as  YCl33  ademonstrated  a catalytic highly  efficient  the synthesis of cyclic activation [38]. we envisaged theSimilarly,  Lewis-acidic strategy  towards  the carbonates synthesis via of  epoxide cyclic  carbonates  via  Similarly, epoxide  activation    [38].  we  activation of epoxide by Sc- and Y-based catalysts, thus making epoxide susceptible to nucleophilic envisaged the Lewis‐acidic activation of epoxide by Sc‐ and Y‐based catalysts, thus making epoxide  attack by amines to produce β-amino alcohols (Scheme 2). susceptible to nucleophilic attack by amines to produce β‐amino alcohols (Scheme 2).   

  Scheme 2. Lewis-acidic activation of epoxide by YCl3 . 33.  Scheme 2. Lewis‐acidic activation of epoxide by YCl

In this work, we report a simple, efficient, and environmentally friendly method using a rare-earth In this work, we report a simple, efficient, and environmentally friendly method using a rare‐ metal-based catalyst, YCl3 , for33, for the ring opening of a library of epoxides by various amines under  the ring opening of a library of epoxides by various amines under earth metal‐based catalyst, YCl solvent-free conditions at room temperature. To the best of our knowledge, the ring-opening reaction solvent‐free conditions at room temperature. To the best of our knowledge, the ring‐opening reaction  of epoxide with amines catalyzed by YCl333 has not been reported before.  has not been reported before. of epoxide with amines catalyzed by YCl 2. Results and Discussion 2. Results and Discussion  Initially, we screened the ring-opening reaction of styrene oxide (1a) as the model substrate with 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 aniline (2a) as the model nucleophile at room temperature. The conversion and regioselectivity were  calculated based on 111H‐NMR of crude reaction mixture, and the results are summarized in Table 1.  H-NMR of crude reaction mixture, and the results are summarized in Table 1. calculated based on  In the absence of a catalyst, In the  the In  the  absence  of  a  catalyst, only only traces traces of of corresponding corresponding β-amino β‐amino alcohol alcohol were were observed. observed.  In  presence of 1 mol % YCl as a catalyst, we were delighted to see a more than 90% conversion of presence of 1 mol % YCl333 as a catalyst, we were delighted to see a more than 90% conversion of 1a,  1a, producing both possible regioisomers, 3aa and 4aa (Table 1, entry 2). The result indicates that producing both possible regioisomers, 3aa and 4aa (Table 1, entry 2). The result indicates that the  the reaction proceeded smoothly with high regioselectivity (3aa:4aa, 93:7). After 3 h of reaction, reaction proceeded smoothly with high regioselectivity (3aa:4aa, 93:7). After 3 h of reaction, 1 mol %  1loading of YCl mol % loading of YCl3 gave a 100% conversion of 1a under solvent-free conditions (Table 1, entry 6). 33 gave a 100% conversion of 1a under solvent‐free conditions (Table 1, entry 6). Under  Under identical reaction conditions, ScCl333 gave an 82% conversion of 1a with 92% regioselectivity  gave an 82% conversion of 1a with 92% regioselectivity  identical  reaction  conditions, ScCl towards 3a (Table 1, entry 3). towards 3a (Table 1, entry 3).   We tried a higher catalyst loading (5 mol %) for both YCl333 and ScCl and ScCl333 and found no noticeable  and found no noticeable We tried a higher catalyst loading (5 mol %) for both YCl effect on regioselectivity of the reaction; however, an increase in rate of substrate conversion was noted effect  on  regioselectivity  of  the  reaction;  however,  an  increase  in  rate  of  substrate  conversion  was  (Table 1, entries 4 and 5). Adding the solvent to the reaction had a negative effect on the catalytic noted  (Table  1,  entries  4  and  5).  Adding  the  solvent  to  the  reaction  had  a  negative  effect  on  the  activity of Lewis acids. When CHCl3 was used as the reaction solvent with 1 mol % YCl3 and at room catalytic activity of Lewis acids. When CHCl 33 was used as the reaction solvent with 1 mol % YCl 33 and  temperature, only a 50% conversion of 1a was noted after a 1 h reaction time (Table 1, entry 7), and it at room temperature, only a 50% conversion of 1a was noted after a 1 h reaction time (Table 1, entry  took more than 12 h to achieve full conversion of the epoxide under these conditions. It is worth noting 7), and it took more than 12 h to achieve full conversion of the epoxide under these conditions. It is  that, when 5 mol % YCl3 was used with and without the solvent (CHCl3 ), the aminolysis of 1a was worth noting that, when 5 mol % YCl 33 was used with and without the solvent (CHCl 33), the aminolysis  slightly slower in the presence of a solvent, further confirming that the highest efficiency of YCl3 can of 1a was slightly slower in the presence of a solvent, further confirming that the highest efficiency  be achieved under solvent-free conditions (Table 1, entry 8). Full conversion of 1a was noticed after 2 h of YCl 33 can be achieved under solvent‐free conditions (Table 1, entry 8). Full conversion of 1a was  using 5 mol % YCl3 in CHCl3 , while33 in CHCl under a 33solvent-free condition, 100% conversion was noted after , while under a solvent‐free condition, 100% conversion  noticed after 2 h using 5 mol % YCl

was noted after 1 h. Similarly, a 5 mol % loading of ScCl33 also gave a lower conversion of 1a (88%) in 

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1 h. Similarly, a 5 mol % loading of ScCl3 also gave a lower conversion of 1a (88%) in the presence of the presence of CHCl 3 (Table 1, entry 9) compared to solvent‐free conditions (100%) (Table 1, entry  CHCl5).  (Table 1, entry 9) compared to solvent-free conditions (100%) (Table 1, entry 5). 3 Table 1. Optimization studies for the aminolysis of styrene oxide (1a). Table 1. Optimization studies for the aminolysis of styrene oxide (1a). 

  Sr. No. 1Sr. No.  2 3 4a 5b 6c 7d 8e 9f 10 11 12 13

Catalyst Catalyst  None

Conversion (%) Conversion  NR (%)  90 NR  82 90  100 82  100 100  100 100  50 100  87 50  88 87  82 75 88  traces 82  85 75  traces  e

3aa:4aa Ratio (%) 3aa:4aa Ratio  (%)  ‐  93:7 92:8 93:7  93:7 92:8  92:8 93:7  93:7 92:8  94:6 93:7  94:6 94:6  93:7 94:6  87:13 87:13 93:7  87:13 nd 82:18 87:13 

YClNone  3 1  ScCl 3 3  2  YCl YCl3 3  ScCl3  ScCl3 4 a  YCl3  YCl3 5 b  ScCl YCl3 3  6 c  3  YClYCl 3 7 d  3  ScClYCl 3 8 e  YCl NbCl 5 3  ZrCl ScCl 9 f  4 3  ZnCl 2 5  10  NbCl Nb(OEt) 5 4  11  ZrCl 12  ZnCl a 5 mol % YCl ; b 5 mol % ScCl ; c 3 h; d CHCl as2 reaction solvent; 5 mol % YCl andnd  CHCl3 as reaction solvent; 3 3 3 3 f 5 mol % ScCl and CHCl13  Nb(OEt) 5  No Reaction; 85  82:18  NR = nd = not determined. 3 3 as reaction solvent. a 5 mol % YCl3; b 5 mol % ScCl3; c 3 h; d CHCl3 as reaction solvent; e 5 mol % YCl3 and CHCl3  f 5 mol % ScCl We also compared the catalytic activity of four 3transition metal-based Lewis acid catalysts NbCl5 , as reaction solvent;  3 and CHCl  as reaction solvent. NR = No Reaction; nd =  not determined.  ZrCl4 , ZnCl 2 , 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%, We  after also  compared  the  catalytic  activity  of  four  transition  metal‐based  Lewis  acid  catalysts  respectively, a 1 h reaction time with a decrease in regioselectivity when compared to tested NbCl5, ZrCl4, ZnCl2, and Nb(OEt)5 (Table 1, entries 10–13). When used under solvent‐free conditions  rare-earth based catalysts. The change in counter anion in the case of Nb(OEt)5 did not result in at room temperature, 1 mol % NbCl5, Nb(OEt)5, and ZrCl4 gave conversions of 82%, 85%, and 75%,  any considerable change or enhancement in catalytic activity towards aminolysis. Moreover, to our respectively, after a 1 h reaction time with a decrease in regioselectivity when compared to tested  surprise, ZnCl2 gave almost no conversion under identical reaction conditions, even when reaction rare‐earth based catalysts. The change in counter anion in the case of Nb(OEt) 5 did not result in any  time was prolonged to 5 h. Figure 1 shows the proton nuclear magnetic resonance (1 H-NMR) spectra considerable  change  or  enhancement  in  catalytic  activity  towards  aminolysis.  Moreover,  to  our  for the aminolysis2 gave almost no conversion under identical reaction conditions, even when reaction  reaction of 1a with 2a for all tested catalysts under identical reaction conditions. surprise, ZnCl 1H‐NMR) spectra  time was prolonged to 5 h. Figure 1 shows the proton nuclear magnetic resonance ( Comparatively, YCl3 as a catalyst seemed to be the best choice amongst the tested series of catalysts for the aminolysis reaction of 1a with 2a for all tested catalysts under identical reaction conditions.  considering the higher conversion of epoxide and higher regioselectivity (Figure 1 and Table 1). 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). 

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  1H‐NMR spectra of crude mixtures of regioselective ring‐opening  Figure 1. Comparative analysis of  Figure 1. Comparative analysis of 1 H-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).  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  The regioisomer (3aa) forms predominantly, as can be observed from spectra. The letters a, b, c, and d d indicate the peaks or peak region corresponding to protons on carbons indicated by a, b, c and d in  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 %),  chemical structures of 3aa and 4aa. Reaction conditions: 1a (1 equiv.), 2a (1 equiv.), catalyst (1 mol %), RT, 1 h.  RT, 1 h.

The most convenient and economically friendly reaction conditions—1 mol % YCl3 3, , no solvent,  no solvent, The most convenient and economically friendly reaction conditions—1 mol % YCl and room temperature—were then applied to overnight reactions of structurally diverse epoxides 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 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 the aminolysis of 1a proceeds efficiently with aromatic amines such as aniline (2a), 3‐methoxyaniline  (2b), (2b), and and 4-chloroaniline 4‐chloroaniline (2c) (2c) (Table (Table 2,2, entries entries 1–3), 1–3), giving giving aa 100% 100% conversion conversion ofof 1a 1a with with excellent excellent  regioselectivity, while thethe presence  presence of aof a  strong electron-withdrawing nitronitro  group in 4-nitroaniline (2d) regioselectivity,  while  strong  electron‐withdrawing  group  in  4‐nitroaniline  showed a negating effect on the reaction, giving only a 13% conversion of 1a in an almost equimolar (2d)  showed  a  negating  effect  on  the  reaction,  giving  only  a  13%  conversion  of  1a  in  an  almost  mixture of 3ad and 4ad regioisomers (Table 2, entry 4). equimolar mixture of 3ad and 4ad regioisomers (Table 2, entry 4). 

 

 

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Table 2. Ring‐opening of epoxides with amines catalyzed by YCl Table 2. Ring‐opening of epoxides with amines catalyzed by YCl Table 2. Ring‐opening of epoxides with amines catalyzed by YCl Table 2. Ring-opening of epoxides with amines catalyzed by YCl3.3.3.3.3.3.3. .     3.  Table 2. Ring‐opening of epoxides with amines catalyzed by YCl Table 2. Ring‐opening of epoxides with amines catalyzed by YCl Table 2. Ring‐opening of epoxides with amines catalyzed by YCl

O

R11111 R R R R1 R R R11

1

Table 2. Ring‐opening of epoxides with amines catalyzed by YCl Table 2. Ring‐opening of epoxides with amines catalyzed by YCl3.3.   R333333 R R44444 4 R R OH R R44444 4 R OH R R33NN R R R R R O O OH OH R R R OH R OH R R R44 N 4 3 YCl (1mol%) 4 3 O YCl (1mol%) N N N O 3 O 3 OH R OH R44 R R R R 4 3 + YCl (1mol%) NN OH O 3 333 44 N YCl (1mol%) N YCl YCl (1mol%) O +22 ++ R R ++R4R 33333 (1mol%) OH 1 1 3 44N R R R YCl (1mol%) R N N 3 YCl (1mol%) + + 1 1 OH N 1 33 N 3 R11 1 NN R RN R RR N R222 ++ RR R OH OH 33 R111 1 R R R OH++++ +R N NN H H R R R OH 11R 33 R3 R OH R N R RT, solvent-free N RT, solvent-free solvent-free R 11 H + R R2R 2 R R 2 R R R R H H R22 RT, 2 R 2 H HH R R R 2 RT, solvent-free RT, solvent-free 2 2 R 2 R22 RT, R RT, solvent-free 2 RT,solvent-free solvent-free R R R R R 22 R R22 R R R2R 2 2 1 1 2 44 11 11 33 1 2 2222 3 44 44 33 4 333 4

2a: R 2a: =Ph, R R44444=H =H R33333=Ph, 1a: =Ph, R =H R =Ph, R =H R 1a: R =Ph, R R22222=H =H R11111=Ph, 2a: 1a: 33 44 2a: 2a: =Ph, R =H R =Ph, R =H R 11 22 1a: 1a: 3 =Ph, R =H R =Ph, R =H R 1 3 2a: =Ph, R =H R 2 1a: 1 2 =Ph, R =H R 2a: =Ph, R =H66H R 3 4 1a: =Ph, R =H R 1 2 2b: 3 =3-OMe-C R =H R 2b: =3-OMe-C H55,,, R R44444=H =H R33=Ph, 1b:=Ph, 1b: =CH Cl, R R222=H =H R111=CH Cl, R 2a: R =H RR 1a: 1b: R 222=H R 1b: 2b: =CH Cl, R =H R =3-OMe-C H R 333 44 2b: 2b: =3-OMe-C =H =3-OMe-C H ,4,R R =H R 1b: =CH Cl, R =H R 111 22 =CH R =H R 6H 5,4 66 55 22Cl, 2b: 3 =3-OMe-C H , R =H R 2 1b: 1 2 2b: =CH Cl, R =H R =3-OMe-C H R R 1b: =CH Cl, R =H R 6 5 3 4=H 22R 6 5 1 R 2 2c: 2c: =4-Cl-C H , R =H R =4-Cl-C H , R =H R 3 4 1c: 1c: =Me, R =H R =Me, =H 1 2 6 5 3 4 6 5 3 4 1 2 2b: 2c: =3-OMe-C H , R =H R 1 2 =4-Cl-C H , R =H R 1b: 1c: =CH R22=H =H R 1c: =Me, R =H R 6 2c: =4-Cl-C H ,, R R544=H =H R 33 2c: 2Cl,R =4-Cl-C 1c: R =Me, R =H R 6H 66 2c: 33 44 =4-Cl-C HH55555,H =H R 22 1c: 2c:R =Me, R =H R =4-Cl-C , R5R =H R 1c: =Me, R =H R111111=Me, 6-C 6 2d: 2d: =4-NO H , R =H R =4-NO -C , R =H R 3 4 3 4 1e: , 1e: , =CH =CH =H R R =CH =CH =H R R 2 1 2 2 6 3 4 2 6 5 2 2 3 4 2, R22=H 2c: 1 =CH 2d: =4-NO -C H ,R =H 1e: H666H ,R =H RR =CH R 1c: 1e: =Me, R 2=CH =H R 3=4-Cl-C 2d: 2d: =4-NO =4-NO -C H R =H R 1=CH 1e: 6-C 5 3 R =CH =CH =H R R 2,,,2,2R 22 =4-NO -C R =H R 1e: 111 =CH =CH =H R R 2d:R =4-NO22222-C H55555,,,,R R44=H =H R 1e:R =CH222222Me, =CHR =H 2d: R R22=H 66H 222 =Me 1f: 1f: =CO Me, R =Me R =CO 3 4 1R 1=CO 2 11 222=Me 1f: Me, R 2 2d: =4-NO -C H , R =H R 1e: 1f: , 11 22 1f: =CH =CH =H R R =CO Me, R =Me R =CO Me, R =Me R 2 6 5 2 2 2 2 1f: 1f: RR =CO =CO22Me, Me, RR =Me =Me 1 2 2e= 2e= 2f= 2f= 1f: R1d= =CO Me, R =Me 2e= 2f= 2e= 2e= 2f= 2f= 1d= 2 O O N 2e= N 2f= 2e= 1d= O 2f= 1d= 1d= N O O N N 1d= H 1d= H O O N N H 2e= 2f= H H HH 1d= O N Epoxide  Amine      Epoxide  Amine  Epoxide  Amine       Epoxide  Amine  Epoxide  Amine  H Epoxide  Amine  Epoxide  Amine  Sr.     Sr.  Sr.    Sr.    Sr.    Epoxide Amine Sr.     Sr. 

  Epoxide  Amine  Sr. No.  1  2  No.  1  2  Sr.  No.  No.  1  2  No.    1  1  2  2  No.  No.  No.

No. 

1  1 1

1  1  1 1  1  1  1  1  1 



2  22 

2  1a  1a  1a 1a  1a  1a  1a  1a 

1a 

3  3  3  3  3  3  33 

N N N N N H H N N H H H H H

         Epoxide  Epoxide  Epoxide  Epoxide  Epoxide    Epoxide  Epoxide  Conversio Conversio Conversio Conversio Conversio Epoxide Conversio aa and 3:4  Conversio n   and 3:4  n  Epoxide  a aaaa and 3:4  n   and 3:4  n  n   and 3:4  Conversion aa and 3:4  n  bb   n Ratio   and 3:4  Ratio  b b Conversio b b b and 3:4 Ratio Ratio    Ratio  Ratio  b b   Ratio  Ratio    (%)  (%)  (%) a (%)  n   and 3:4  (%)  (%)  (%)  (%) 

N H

4  4  4  4  4  4  44 



Ratio b  (%) 



100  100  100 100  100  100  100  100  93:7  93:7  93:7 93:7  93:7  93:7  93:7  93:7 

2a  2a  2a 2a  2a  2a  2a  2a          

        

2a 

100  93:7   

  1a  1a 1a  1a  1a  1a  1a  1a 

2  2  2 2  2  2  2  2 

1a 



2b  1a 1a  1a  1a  1a  1a  1a  1a 

3  3 3  3  3  3  3  3 

2b  2b 2b  2b  2b  2b  2b  2b 

2c 2c  2c  2c  2c  2c  2c  2c 

        

        

100  >99:99:99:99:99:99:99:99:99: