Regioselective acetylate of 1,3-disubstituted selenoureas promoted by recyclable ion-supported hypervalent iodine(III) reagent

Article abstract of DOI:10.1080/10426507.2013.797418

Abstract A mild, efficient, and environmentally friendly reaction of 1,3-disubstituted selenoureas with a recyclable ion-supported hypervalent iodine(III) reagent produces regioselectively N-acetylureas. This is the first example of ion-supported hypervalent iodine reagent [dibmim]⁺[BF4] ^-being employed as an N-acetylating agent. Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.

Full text of DOI:10.1080/10426507.2013.797418

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Regioselective acetylate of 1,3-disubstituted
selenoureas promoted by recyclable ion-supported
hypervalent iodine(III) reagent
Yuanyuan Xie a & Haixuan Pan a
a Key Laboratory for Green Pharmaceutical Technologies and Related Equipment of Ministry
of Education, College of Pharmaceutical Sciences , Zhejiang University of Technology ,
Hangzhou , 310014 , P.R. China
Accepted author version posted online: 26 Jul 2013.
To cite this article: Phosphorus, Sulfur, and Silicon and the Related Elements (2013): Regioselective acetylate of 1,3-
disubstituted selenoureas promoted by recyclable ion-supported hypervalent iodine(III) reagent, Phosphorus, Sulfur, and
Silicon and the Related Elements, DOI: 10.1080/10426507.2013.797418
To link to this article: http://dx.doi.org/10.1080/10426507.2013.797418
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Regioselective acetylate of 1,3-disubstituted selenoureas promoted by recyclable
ion-supported hypervalent iodine(III) reagent
Yuanyuan Xie* and Haixuan Pan
Key Laboratory for Green Pharmaceutical Technologies and Related Equipment of
Ministry of Education, College of Pharmaceutical Sciences, Zhejiang University of
Technology, Hangzhou 310014, P.R. China
E-mail: pharmlab@zjut.edu.cn
Abstract A mild, efficient and environmentally friendly reaction of 1,3-disubstituted
selenoureas with a recyclable ion-supported hypervalent iodine(III) reagent produces
regioselectively N-acetylureas. This is the first example of ion-supported hypervalent
iodine reagent [dibmim]+[BF4]– being employed as an N-acetylating agent.
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Keywords hypervalent iodine reagent; 1,3-disubstituted selenoureas; N-acetylureas;
N-acetylation; recyclable
INTRODUCTION
Various organic reactions can be performed using hypervalent iodine(III) reagents, with
the accompanying formation of more stable monovalent iodine compounds as the driving
force.1 Despite the environment-friendly advantages of the hypervalent iodine(III)
reagent, the coproduction and separation of stoichiometric amounts of iodoarene
byproduct are accompanying problems.2 Therefore, the demands are encouraging
chemists to search for other types of recyclable alternatives to hypervalent iodine(III)
reagents, which can facilitate the separation of the iodoarene byproduct from the
reaction mixture as well as the reuse of the reagents.3
In the last two decades, there has been an explosion of interest in ionic liquids.4 Ionic
liquids are used as environmentally benign reaction media, which represent interesting
properties such as high thermal stability, negligible vapor pressure, high loading capacity
and easy recyclability. Herein we report ion-supported hypervalent iodine(III) reagent
1-(4-diacetoxyiodobenzyl)-3-methylimidazolium tetrafluoroborate ([dibmim]+[BF4]–) as
recyclable alternative to hypervalent iodine(III) reagent, which can be recovered and
reused after oxidation with peracetic acid.5
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Ureas and selenoureas are widely used in the synthesis of heterocyclic compounds. The
N-acylureas have useful applications in pharmaceutical and agrochemical.6 For example,
dopamine D2 agonist Cabergoline having an N-acyl derivative was launched in 1993 as
an anti Parkinson’s agent.7 Numerous methods are available to accomplish the
synthesis of N-acetylureas. Direct acylation of symmetrical ureas is a conventional route
for the preparation of N-acylureas using acyl chlorides, acids, ester and acetic anhydride
as efficient N-acetylating agents.8 The reactions of acids with carbodiimides or
carbodiimides chloride are broadly used in N-acetylureas synthesis.9 In addition, Wiley
reported the reaction of amides with isocyanates to form N-acetylureas.10 However, the
unsymmetrical ureas produce non-regioselective products, and symmetrical ureas
produce regioselective products in the reported methods. The Mn(OAc)3-mediated
N-acetylation can be used to produce single regioselective N-acetylureas from
symmetrical or asymmetrical thioureas.11 A similar N-acetylation of thioureas using
diacetoxyiodobenzene (DIB) was disclosed recently by Singh.12 Herein, we wish to
report the regioseletive N-acetylation of disubstituted selenoureas to N-acetylureas using
the [dibmim]+[BF4]– under mild conditions (Scheme 1). This is the first example of
[dibmim]+[BF4]– being employed as an N-acetylating agent.
[Insert Scheme 1]
RESULTS AND DISCUSSION
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1,3-Diphenylselenourea was chosen as starting materials to optimize the reaction
conditions (Table 1). The N-acetylation was performed at room temperature in
acetonitrile using an equimolar [dibmim]+[BF4]– and triethylamie. The reaction was
completed within 30 min to afford the N-acetylated product in good yields. Then other
solvents such as toluene, acetic acid, ethyl acetate, dichloromethane and methanol were
tested, but longer time was needed to finish the reaction. Furthermore, different reaction
temperatures were also attempted and the yields were not improved.
[Insert Table 1]
After the optimization of reaction conditions, the scope of this reaction was explored.
Several symmetrical selenoureas having various substituents in the phenyl ring were
investigated and the corresponding mono N-acetylated ureas were obtained. The
reaction results are listed in Table 2. Both electron-donating and electron-withdrawing
groups on 1,3-diphenylselenoureas were well tolerated in this reaction. Furthermore
1,3-diphenylselenoureas with electron-donating groups gave better reaction yields than
1,3-diphenylselenoureas with electron-withdrawing groups. But 3,5-dimethylphenyl
selenourea 1h and 2,4,6-trimethylphenyl selenourea 1i gave relatively lower yields and
needed longer time due to the steric factor effect.
[Insert Table 2]
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Encouraged by above results, we proceeded to discuss the regioselctive N-acetylation of
unsymmetrical selenoureas (Table 3). We found that the results were agreeable with the
symmetrical selenoureas.
[Insert Table 3]
A possible mechanism for the regioselective N-acetylation is proposed in Scheme 2. The
selenium atom of 1,3-disubstituted selenourea 1 attacks the iodine atom of
[dibmim]+[BF4]– 3 in place of one of the AcO− giving intermediate 4. And the resulting
AcO− abstracts a hydrogen from the intermediate 4 which would undergo reductive
β-elimination following the expulsion of by-products selenium 7 and compound 8 to
produce carbodiimide 5. Then 5 reacts with the liberated acetic acid to generate
compound 6, and the final N-acetylated ureas 2 would be obtained after
rearrangement.12
[Insert Scheme 2]
The pKa’s and the steric effect of the precursor amines were important in the
regioselectivity of N-acetylation.12 An unsymmetrical carbodiimide was the intermediate
of the reaction. Acetic acid attacked unsymmetrical carbodiimide that produced isourea
which have higher pKa after protonation toward the amine unaffecting the imine group on
the other side. The regioselective N-acetylated product was obtained via rearrangement
of consequential isourea. The difference of pKa between the precursor amines have
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important influence on the regioselectivity of N-acetylation leading to preferential
acetylation toward the amine having lower pKa. The regioselective formation of 2p from
unsymmetrical selenourea 1p could be explained by the higher pKa of the
cyclohexylamine (10.57) comparing to the aniline (4.61), so the unsymmetrical
N-acetylurea was the only product. Otherwise, the pKa of naphthylamine (4.21) and
2,4,6-trimethylaniline (4.74) is similar to aniline (4.61), but the 2q and 2r were the
exclusive regioselective product mainly due to the steric factor of 2,4,6-trimethylphenyl
group and naphthyl group.
When the reaction was complete, the recovery of the by-products selenium 7 and
ion-supported iodobenzene 8 was very easy. After completion of the reaction, the
precipitate selenium 7 could be easily obtained by filtering and reused for the preparation
of additional selenoureas. The filtrate was evaporated under vacuum, then the residue
was dissolved in dichloromethane and precipitated by adding hexane. The ion-supported
iodobenzene 8 was obtained through filtration and reused after oxidation with peracetic
acid to form [dibmim]+[BF4]– 3.
CONCLUSION
In summary, we have developed a regioselective, mild, efficient and environmentally
friendly N-acetylation reaction using a recyclable ion-supported hypervalent iodine
reagent [dibmim]+[BF4]–. This is the first example [dibmim]+[BF4]– being employed as an
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N-acetylating agent. Meanwhile [dibmim]+[BF4]– and selenium can be readily recovered
and reused to avoid pollution problems.
EXPERIMENTAL
1H NMR and 13C NMR spectra were measured on a Varian 400 (400 MHz) spectrometer
(chemical shifts in δ ppm) using TMS as internal standard. Mass spectra (ESI-MS) were
determined on a Thermo Finnigan LCQ-Advantage. High resolution mass spectra
(ESI-HRMS) were determined on an Agilent 6210 TOF instrument. IR Spectra were
determined on a Nicolet Avatar-370 spectrometer in KBr (ν in cm−1). Melting points were
measured on a Büchi B-540 capillary melting point apparatus. Element Analyses were
characterized by a ThermoFinnigan Flash EA 1112 instrument. A complete set of
characterization data and sample 1H and 13C NMR spectra (Figures S 1 and S 2) can be
found in the Supplemental Materials.
General procedure for the Preparation of 1-acetyl-1,3-diphenylurea 2a
A mixture of 1,3-diphenylselenourea (1a, 0.275g, 1mmol) and triethylamine (0.101g,
1mmol) in acetonitrile (10 mL) was added [dibmim]+[BF4]– (0.505g, 1mmol), and the
mixture was allowed to stir for 30 min at room temperature. Precipitation of solid
selenium was observed during this period. After the reaction was complete (monitored
by TLC, 30min), the mixture was filtered to separate solid selenium (0.075g, 95%yield).
Then the filtrate was evaporated under vacuum, and the residue was dissolved in
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dichloromethane(1mL) and precipitated by adding hexane (20mL). The ion-supported
iodobenzene (0.330g, 85%yield) was obtained through filtration and the filtrate was
evaporated. The residue was purified by column chromatography on silica gel
(hexane/EtOAc=10:1) to give 1-acetyl-1,3-diphenylurea(2a, 0.236g, 93%yield): m.p.
103-105℃; IR(KBr): νmax = 3230, 3185, 3065, 1721, 1667, 1545, 963, 902, 754, 700 cm-1;
1H NMR (400 MHz, CDCl3): δ= 11.41 (s, 1H), 7.54-7.42 (m, 5H), 7.31-7.26 (m, 4H), 7.07
(t, J = 7.8 Hz, 1H), 2.00(s, 3H); 13C NMR (100 MHz, CDCl3): δ= 174.7, 151.6, 138.6,
137.4, 129.6, 128.9, 128.8, 128.8, 123.9, 120.0, 26.7; ESI-MS: m/z=255[M+H]+.
1-Acetyl-1,3-di-(o-ethylphenyl)urea (2b): White solid; White solid; m.p. 78-80℃; Anal.
Calcd for C19H22N2O2: C, 73.52; H, 7.14; N, 9.03; found: C, 73.73; H, 7.20; N, 9.05;
R(KBr): νmax = 3127, 3064, 2970, 2879, 1709, 1670, 1546, 1058, 991, 756 cm-1; 1H NMR
(400 MHz, CDCl3): δ = 11.30 (s, 1H), 7.97 (dd, J = 8.0, 0.8 Hz, 1H), 7.35-7.32(m, 2H),
7.28-7.23(m, 1H), 7.16-7.09 (m, 3H), 7.01 (td, J = 7.2, 1.2 Hz, 1H), 2.72 (q, J = 7.5 Hz,
13
2H), 2.65-2.49 (m, 2H), 1.95 (s, 3H), 1.30(t, J = 7.2Hz, 3H), 1.24(t, J = 7.6Hz, 3H); C
NMR (100 MHz, CDCl3): δ = 175.1, 151.5, 141.3, 137.2, 135.4, 133.7, 129.3, 129.2,
129.0, 128.3, 127.1, 126.4, 124.3, 121.7, 26.2, 24.9, 23.8, 14.1, 13.8; ESI-HRMS: m/z
calcd for C19H22N2NaO2 [M+Na]+:333.1573; found: 333.1576.
1-Acetyl-1,3-di-(p-fluorophenyl)urea (2g): White solid; m.p. 125-128℃; IR(KBr): νmax
=
3190, 3070, 1709, 1612, 1552, 1505, 1405, 1372, 1325, 1267, 1173, 1101, 1053, 825,
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753, 622 cm-1; 1H NMR (400 MHz, CDCl3): δ= 11.33 (s, 1H), 7.46(dd, J = 4.8Hz, 8.8Hz,
2H), 7.26-7.23(m, 2H), 7.17(t, J = 8.0Hz, 2H), 6.99(t, J = 8.4Hz, 2H), 2.01 (s, 2H); 13C
1
1
NMR (100 MHz, CDCl3): δ = 174.6, 162.3(d, JCF = 248 Hz), 159.1(d, JCF = 241 Hz),
3
3
2
151.7, 134.4 133.3, 130.6(d, JCF = 9Hz), 121.7(d, JCF = 8Hz), 116.6(d, JCF = 22 Hz),
2
115.4 (d, J CF= 23Hz), 26.5; ESI-HRMS: m/z calcd for C15H12F2N2NaO2 [M+Na]+:
313.0759; found: 313.0751.
1-Acetyl-1,3-di-(m,m-dimethylphenyl)urea (2h): White solid; m.p. 142-145℃; IR(KBr):
νmax = 3139, 3084, 3012, 2922, 1716, 1666, 1560, 1458, 1365, 1293, 1178, 1053, 839,
747, 706, 641 cm-1; 1H NMR (400 MHz, CDCl3): δ =11.32 (s, 1H), 7.19(s, 2H), 7.04(s,
1H), 6.86 (s, 2H), 6.71 (s, 1H), 2.34 (s, 6H), 2.26 (s, 6H), 2.00 (s, 3H); 13C NMR (100
MHz, CDCl3): δ=175.1, 152.2, 139.7, 138.9, 138.7, 137.8, 130.9, 126.7, 125.9, 118.0,
26.9, 21.8, 21.6; ESI-HRMS: m/z calcd for C19H22N2NaO2 [M+Na]+: 333.1573; found:
333.1573.
1-Acetyl-1,3-di-(o,o,p-trimethylphenyl)urea (2i): White solid; m.p. 115-118℃; IR(KBr):
νmax = 3162, 3067, 2912, 1720, 1664, 1610, 1552, 1441, 1175, 1053, 842, 735, 701, 620
cm-1; 1H NMR (400 MHz, CDCl3): δ= 10.45 (s, 1H), 6.89(s, 2H), 6.82(s, 2H), 2.27 (s, 3H),
2.23 (s, 9H), 2.17 (s, 6H), 1.91 (s, 3H). 13C NMR (100 MHz, CDCl3): δ= 174.9, 151.2,
138.4, 136.1, 135.3, 134.5, 134.3, 131.1, 129.4, 128.5, 25.1, 21.1, 20.9, 18.6, 17.7;
ESI-HRMS: m/z calcd for C21H26N2NaO2 [M+Na]+: 361.1886; found: 361.1888.
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1-Acetyl-1-phenyl-3-o-ethylphenylurea (2j) + 1-Acetyl-1- o-ethylphenyl-3-phenylurea (2j')
(20:80): 1H NMR (400 MHz, CDCl3): IR(KBr): νmax = 3178, 3056, 2972, 2874, 1716, 1663,
1594, 1552, 1266, 1169, 1053, 898, 755, 629 cm-1; 1H NMR (400 MHz, CDCl3): δ = 11.46
(s, 1H), 11.38 (s, 1H), 8.03(d, J = 8.0Hz, 1H), 7.54(d, J = 8.4Hz, 2H), 7.50-7.37(m, 5H),
7.32-7.27(m, 5H), 7.20-7.14(m, 3H), 7.09-7.04(m, 2H),2.73 (q, J =7.6, 2H), 2.65-2.49 (m,
2H), 2.01 (s, 3H), 1.94 (s, 3H), 1.31 (t, J = 7.6Hz, 3H), 1.24 (t, J = 7.6 Hz, 3H); 13C NMR
(100 MHz, CDCl3): δ= 174.9, 151.8, 151.2, 141.3, 138.7, 137.5, 137.0, 135.3, 133.5,
129.5, 129.3, 129.1, 128.9, 128.80, 128.75, 128.3, 127.1, 126.4, 124.2, 123.9, 121.4,
119.9, 26.6, 26.2, 24.8, 23.7, 14.1, 13.7; ESI-HRMS: m/z calcd for C17H18N2NaO2
[M+Na]+: 305.1260; found: 305.1261.
1-Acetyl-1-phenyl-3-m-methylphenylurea (2k) + 1-Acetyl-1- m-methylphenyl-3-
phenylurea (2k') (78:22): IR(KBr): νmax = 3146, 3073, 1719, 1664, 1594, 1547, 1271,
1190, 1059, 761,701 cm-1; 1H NMR (400 MHz, CDCl3): δ= 11.43 (s, 1H), 11.37 (s, 1H),
7.53 (d, J = 7.6 Hz, 2H), 7.50-7.43(m, 4H), 7.36 (t, J = 7.6 Hz, 1H), 7.31-7.23 (m, 5H),
7.17 (t, J = 7.6 Hz, 1H), 7.09-7.05 (m,, 4H), 6.89 (d, J = 7.6 Hz, 1H), 2.39 (s, 3H), 2.31 (s,
3H), 2.00 (s, 3H), 1.99 (s, 3H); 13C NMR (100 MHz, CDCl3): δ= 174.7, 174.6, 151.7,
151.6, 139.6, 138.6, 138.5, 137.5, 137.3, 129.6, 129.5, 129.3 128.8, 128.7, 128.5, 125.7,
124.6, 123.8, 120.6, 119.9, 117.0, 26.5, 21.5, 21.3. ESI-HRMS: m/z calcd for
C16H16N2NaO2 [M+Na]+: 291.1104; found: 291.1107.
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1-Acetyl-1-phenyl-3-m,m-dimethylphenylurea
(2o)
+
1-Acetyl-1-m,m-dimethylphenyl-3-phenylurea (2o') (62:38): IR(KBr): νmax = 3162, 2912,
1720, 1664, 1610, 1552, 1252, 1175, 1053, 842, 735, 701, 620 cm-1; 1H NMR (400 MHz,
CDCl3): δ= 11.43 (s, 1H), 11.30 (s, 1H), 7.53 (d, J = 7.6 Hz, 2H), 7.49-7.42 (m, 4H),
7.30-7.24 (m, 3H), 7.18 (s, 3H), 7.08-7.05 (m, 2H), 6.87 (s, 1H), 6.72 (s, 1H), 2.35 (s, 6H),
13
2.27 (s, 6H), 2.01 (s, 3H), 1.98 (s, 3H); C NMR (100 MHz, CDCl3): δ= 174.9, 174.6,
151.8, 151.6, 139.3, 138.7, 138.4, 137.6, 137.3, 130.6, 129.5, 128.8,128.7, 126.3, 125.6,
123.8, 120.0, 117.7, 26.6, 26.5, 21.4, 21.3; ESI-HRMS: m/z calcd for C17H18N2NaO2
[M+Na]+: 305.1260; found: 305.1260.
1-Acetyl-1-phenyl-3-naphthylurea (2r): White solid; m.p. 107-110℃; IR(KBr): νmax
=
3164, 3060, 1716, 1666, 1595, 1552, 1319, 1185, 1046, 775, 645 cm-1; 1H NMR (400
MHz, CDCl3): δ= 11.50 (s, 1H), 7.89-7.84 (m, 2H), 7.74 (d, J = 7.6Hz, 1H), 7.52-7.45 (m,
5H), 7.39(d,J=7.2Hz, 1H), 7.25-7.17 (m, 2H),7.02 (t, J = 7.2 Hz, 1H), 1.89 (s, 3H); 13C
NMR (100MHz, CDCl3): δ= 175.3, 151.4, 137.6, 135.0, 134.3, 130.6, 129.6, 128.8,
128.5, 127.7, 127.0, 126.6, 125.4, 123.9, 121.3, 120.0, 25.9; ESI-HRMS: m/z calcd for
C19H16N2NaO2 [M+Na]+: 327.1104; found: 327.1105.
1-Acetyl-1-phenyl-3-o,o,p-trimethylurea (2q): White solid; m.p. 114-115℃; IR(KBr):νmax
=
3132, 3045, 2915, 1708, 1668, 1596, 1553, 1446, 1290, 1225, 1177, 1021, 746, 627
cm-1; 1H NMR (400 MHz, CDCl3): δ= 11.47 (s, 1H), 7.54 (d, J=8.0Hz, 2H), 7.29 (t, J = 8.0
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Hz, 2H), 7.07 (t, J = 7.2 Hz, 1H), 6.97 (s, 2H), 2.31 (s, 3H), 2.17 (s, 6H), 1.91 (s, 3H); 13C
NMR (100 MHz, CDCl3): δ= 175.0, 150.6, 138.7, 137.7, 135.3, 134.1, 129.5, 128.8,
123.8, 119.9, 25.2, 21.1, 17.8; ESI-HRMS: m/z calcd for C18H20N2NaO2 [M+Na]+:
319.1417; found: 319.1416.
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