Hypervalent iodine(III)-mediated regioselective N-acylation of 1,3-disubstituted thioureas

Article abstract of DOI:10.1021/jo702628g

(Chemical Equation Presented) Reaction of asymmetrical 1,3-disubstituted thioureas with diacetoxyiodobenzene (DIB) produces regioselectively N-acetylurea in shorter time. Regioselectivity is dependent on the pK_a's of the amine attached to the t

Full text of DOI:10.1021/jo702628g

Ureas and thioureas are useful synthons for the construction
of heterocyclic compounds.3 N-Acylureas have found important
applications in agrochemicals and pharmaceuticals.4 Dopamine
D2 agonist cabergoline having an N-acyl derivative is an anti-
Parkinson agent.5 These compounds act as interesting semi-
crystalline materials6 and auxiliaries for the synthesis of chiral
cyclic carboxylic acids.7 Derivatives of acylurea have been used
for the allylation of sulfoxides,8a Claisen rearrangement,8b
Diels-Alder reaction,8c,d nucleophilic addition of TMSCN,8e
Michael addition,8f and enantioselective Strecker and Mannich
reactions.8g-i
Hypervalent Iodine(III)-Mediated Regioselective
N-Acylation of 1,3-Disubstituted Thioureas
C. B. Singh, Harisadhan Ghosh, Siva Murru, and
Bhisma K. Patel*
Department of Chemistry, Indian Institute of Technology,
Guwahati, Assam, India
patel@iitg.ernet.in
The reported methods for the synthesis of N-acylurea are by
the reaction of substituted ureas with acyl chlorides or acids at
elevated temperature and reaction of amides with isocyanates
or carbodiimides.9a The reported method produces a non-
regioselective product for unsymmetrical urea, and a regiose-
lective product is produced from symmetrical urea or by a
carbodiimide approach.10 A similar N-acylation of thioureas
using Mn(OAc)3 was disclosed recently by Mu et al.11 In this
paper, we have demonstrated an unprecedented regioseletive
N-acetylation of disubstituted thioureas leading to N-acetyl ureas
using diacetoxyiodobenzene (DIB) as shown in Scheme 1.
In organic chemistry, Mn(OAc)3 has been most commonly
used in the generation of carbon-centered radicals from various
carbonyl compounds and their oxidative addition to alkenes.12
ReceiVed December 10, 2007
Reaction of asymmetrical 1,3-disubstituted thioureas with
diacetoxyiodobenzene (DIB) produces regioselectively N-
acetylurea in shorter time. Regioselectivity is dependent on
the pKa’s of the amine attached to the thiourea moiety with
acylation taking place toward the amine having a lower pKa.
This is the first example of DIB being employed as an
N-acetylating agent. A mechanism for this novel transforma-
tion is also proposed. Mild reaction conditions, shorter
reaction times, high efficiencies, environmentally benign
methods, and facile isolation of the desired product make
the present methodology a most suitable alternative.
(3) (a) Katritzky, A. R.; Kirichenko, N.; Rogovoy, B. V.; Kister, J.; Tao,
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an explosive development in the last two decades, mainly due
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widely recognized as alternatives to highly toxic heavy metal
oxidants such as lead-, mercury-, and thallium-based reagents.1
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of hypervalent iodine reagent occupy an important place in the
realm of natural and synthetic organic chemistry because it has
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10.1021/jo702628g CCC: $40.75 © 2008 American Chemical Society
Published on Web 03/05/2008
2924
J. Org. Chem. 2008, 73, 2924-2927

SCHEME 1. N-Acylation of Urea from Thiourea
SCHEME 3. Proposed Mechanism of Formation of
N-Acylurea
SCHEME 2. Proposed Mechanism of Formation of
N-Acylurea
propionylated product 1a′ rules out any possibility of an
intramolecular mechanism as proposed in Scheme 2.
Alternatively, a mechanism involving the intermediacy of
carbodiimide seems to be a reasonable proposition as shown in
Scheme 3. Reductive â-elimination of λ3-iodane intermediate
(A) with the expulsion of sulfur will produce carbodiimide (C)
(Scheme 3), which on reaction with acetic acid liberated in the
medium would give 2-acetyl-1,3-disubstituted-isourea. Infrared
spectral analysis of the reaction mixture showed a characteristic
peak for carbodiimide group at 2138 cm-1. Further, the isolation
of stable carbodiimide 10a (Table 1) is testimony to this fact
and supports the mechanism proposed in Scheme 3. However,
similar reactions were not successful for ureas, probably due to
the lower acidity of the NH protons in urea and lesser affinity
of oxygen toward iodine compared to sulfur present in thioureas.
Several symmetrical thioureas 2-9 having various substitu-
ents in the phenyl ring gave their corresponding mono N-
acylated ureas 2a-9a within 5 min giving excellent yields of
the products as shown in Table 1, but for uniformity all of the
reactions were allowed to stir for 15 min. When the reaction
was performed with 1,3-bis(2-methoxyphenyl)thiourea 10, no
N-acetylated product was observed, and the only isolated product
obtained was found to have a bis(2-methoxyphenyl)carbodiimide
10a moiety. The stability of carbodiimide 10a can be explained
by the neighboring group participation of the o-methoxy group
from the adjacent phenyl ring as shown in Scheme 4. Aliphatic
thiourea 11 does not undergo N-acylation; this may be due to
difficulty in deprotonating due to the substantial basic character
of cyclohexylamine (pKa 10.66). This observation is consistent
with the N-acylation using Mn(OAc)3.11
Having successfully synthesized a series of N-acylated ureas,
we were interested in regioselective N-acylation of unsym-
metrical thiourea. We have found that the larger the difference
between the pKa’s of the precursors amines in thiourea the
greater the regioselectivity of N-acylation with preferential
acylation taking place toward the amine having lower pKa.
Unsymmetrical thiourea 12 would form an unsymmetrical
carbodiimide as the intermediate. The attack of acetic acid on
unsymmetrical carbodiimide would lead to the protonation
toward the amine having higher pKa unaffecting the imine group
on the other side. The resultant isourea on rearrangement would
yield N-acylated product in regioselective manner. For 1-phenyl-
3-p-tolylthiourea (12), the phenyl side is acylated 60% compared
to p-tolyl side (40%) as evident from the 1H NMR. The
measured pKa’s of aniline and p-methylaniline are 4.61 and 5.08,
respectively, supporting our arguments and the mechanism
involving carbodimide intermediate (Scheme 3). The measured
pKa’s of both p-chloro- (4.15) and p-bromoanilines (3.86) are
However, significant drawbacks to the use of Mn(OAc)3 are
the harsh reaction conditions and its poor solubility in organic
solvents. On the other hand, diacetoxyiodobenzene in the
presence of iodine or under photochemical conditions generates
radicals. This similarity between diacetoxyiodobenzene (DIB)
with Mn(OAc)3 and the N-acetylating ability11 of the latter
reagent prompted us to use the metal-free reagent diacetoxy-
iodobenzene for the preparation of N-acetylurea.
In a typical reaction, an equimolar mixture of 1,3-diphenylth-
iourea 1, triethylamine, and diacetoxyiodobenzene (DIB) was
mixed together in acetonitrile, and the reaction mixture was
stirred at room temperature. The reaction was completed within
5 min giving N-acetylated product 1a in good yield. The
proposed mechanism for the formation of N-acetylated product
is shown in Scheme 2.
The sulfur atom of the 1,3-disubstituted-thiourea attacks on
the thiophilic iodine of PhI(OAc)2 displaying one of the acetate
groups, giving intermediate A, which is then followed by an
intramolecular nucleophilic attack of the carbonyl group of the
acetate on the imine carbon giving sulfur and phenyliodide as
byproducts. Formation of elemental sulfur and phenyl iodide
has actually been confirmed by isolating them. The resultant
2-acetyl-1,3-disubstituted isourea (B) rearranges to 1-acetyl-1,3-
disubstituted-urea 19a as shown in Scheme 2. The structure of
N-acylated product 1a has been confirmed by crystal X-ray
crystallography.13
However, when the reaction of 1 was carried out in the
presence of 1 equiv of propionic acid and an additional 1 equiv
of triethylamine it gave N-acetylated product 1a along with the
formation of propionylated product 1a′ in a ratio of 58:42. In a
second experiment, the reaction was carried out with a 5-fold
excess of propionic acid and triethylamine, the products 1a and
1a′ obtained were in the ratio 17:83. The formation of
(13) Crystallographic data for compound 1a have been deposited at the
Cambridge Crysallographic Data Centre (deposition no. CCDC-669356).
Copies of the data can be obtained free of charge on application to CCDC,
12 Union Road, Cambridge CB2 1EZ, U.K. [fax (int)] (01223 336033;
email: deposit@ccdc.cam.ac.uk].
(14) Crystallographic data for compound 16a have been deposited at the
Cambridge Crysallographic Data Centre (deposition no. CCDC-669355).
Copies of data can be obtained free of charge on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, U.K. [fax (int)] (01223 336033; email:
deposit@ccdc.cam.ac.uk].
J. Org. Chem, Vol. 73, No. 7, 2008 2925

TABLE 1. N-Acylation of Ureas from Thioureasa
TABLE 2. Regioselective N-Acylation of Ureas from Thiourea
a Reactions were monitored by TLC. b Confirmed by IR and 1H and 13
NMR. c Isolated yield.
C
a Reactions were monitored by TLC. b Confirmed by IR and 1H and 13
NMR. c Isolated yield.
C
Table 2. The exclusive regioselective formation of 15a from
unsymmetrical urea 15 is due to both the favorable pKa and the
steric factor of 2,6-dimethylaniline group. Regioselective N-
acylation is also observed in thiourea 16. The N-acylation toward
the aniline side of 16 can be explained by the lower pKa of the
aniline compared to the p-anisidine (5.34), giving 16a as the
only isolable product. The structure of product 16a has been
confirmed by crystal X-ray crystallography.14
SCHEME 4. Proposed Mechanism of Formation of
Carbodiimide
Again, the higher acidic character of the aromatic amine
aniline compared to aliphatic amines such as benzylamine (pKa
9.41), cyclohexylamine (pKa 10.66), and n-butylamine (pKa
10.77) indicates that the acylation is toward the aniline side of
the urea as shown for substrates 17, 18, and 19 giving
regioselective products 17a, 18a, and 19a, respectively.
In conclusion, this paper reports an efficient method for the
synthesis of N-acylated ureas from 1,3-disubstituted thioureas
using environmentally benign reagent diacetoxy iodobenzene
(DIB). For the first time, DIB has been employed as an acylating
agent. We have also found the correlation between the regi-
lower than aniline (4.61); hence, the preferential N-acylation
toward the p-chloro- and p-bromoaniline side in substrates 13
and 14 giving 13a and 14a as the major product as shown in
2926 J. Org. Chem., Vol. 73, No. 7, 2008

Selected Spectral Data. 1-Acetyl-1,3-di-p-tolylurea (2a): mp
138-140 °C; 1H NMR (400 MHz, CDCl3) δ 2.03 (s, 3H), 2.35 (s,
3H), 2.45 (s, 3H), 7.10 (d, 2H, J ) 8.0 Hz), 7.14 (d, 2H, J ) 8.4
Hz), 7.22 (d, 2H, J ) 8.4 Hz), 7.42 (d, 2H, J ) 8.0 Hz), 11.35 (s,
1H); 13CNMR (100 MHz, CDCl3) δ 20.9, 21.3, 26.7, 120.2, 128.8,
129.6, 130.5, 133.6, 135.3, 136.4, 139.1, 152.1, 175.2; IR (KBr)
3176, 3053, 2920, 2872, 1712, 1688, 1597, 1523, 1452, 1310, 1165,
813 cm-1; HRMS (ESI) MH+ found 283.3488, C17H19N2O2 requires
283.3493.
1-Acetyl-1,3-di-(m-methylphenyl)urea (3a): mp 82-84 °C; 1H
NMR (400 MHz CDCl3) δ 2.00 (s, 3H), 2.31 (s, 3H), 2.40 (s, 3H),
6.90 (d, 1H, J ) 7.2 Hz), 7.07 (d, 1H, J ) 7.2 Hz), 7.08 (s, 1H),
7.17 (t, 1H, J ) 7.6 Hz), 7.25 (d, 1H, J ) 7.6 Hz), 7.29 (d, 1H, J
) 8 Hz), 7.37 (t, 1H, J ) 7.6 Hz), 7.46 (s, 1H), 11.40 (s, 1H);
13CNMR (100 MHz, CDCl3) δ 21.3, 21.5, 26.5, 117.2, 120.8, 124.8,
125.9, 128.8, 129.5, 129.8, 137.7, 138.8, 139.8, 152.0, 175.0; IR
(KBr) 3230, 3176, 1727, 1667, 1609, 1548, 1488, 1367, 1322, 1268,
1192, 1177, 1057, 794, 704, 690, 631 cm-1; HRMS (ESI) MH+
found 283.3490, C17H19N2O2 requires 283.3493.
oselectivity and the pKa of the amine. Compared to the existing
arduous methods of synthesis this methodology is superior in
terms of environmental acceptability, simplicity, convenience,
and general applicability.
Experimental Section
General Procedure for the Preparation of N-Acylated Urea
(1a) from Thiourea (1). To a stirred solution of diphenylthiourea
1 (456 mg, 2 mmol) and triethylamine (276 µL, 2 mmol) in
acetonitrile (10 mL) was added DIB (644 mg, 2 mmol) at room
temperature, and the mixture was allowed to stir for 15 min.
Precipitation of sulfur was observed during this period. After
completion of the reaction, solvent was evaporated and admixed
with ethyl acetate (20 mL). The ethyl acetate layer was washed
subsequently with saturated solution of NaHCO3 (5 mL) and 5%
solution of sodium thiosulphate (5 mL), dried over anhydrous Na2-
SO4, concentrated under reduced pressure, and purified over a silica
gel column (hexane/EtOAc, 9:1) to give (419 mg, 92%) of the
product 1a. Compound 1a was recrystallized from a mixture of
EtOAc/hexane (8:2) to give a colorless crystal: mp 100-102 °C;
1H NMR (400 MHz, CDCl3) δ 1.98 (s, 3H), 7.08 (t, 1H, J ) 7.2
Hz), 7.28 (m, 4H), 7.48 (m, 5H), 11.44 (s, 1H); 13CNMR (100 MHz,
CDCl3) δ 26.8, 120.3, 124.3, 129.1, 129.2, 129.3, 129.9, 137.8,
139.0, 152.1, 175.2; IR (KBr) 3230, 2928, 2857, 1722, 1668, 1602,
1544, 1492, 1445, 1166, 1050, 823, 751, 699 cm-1; HRMS (ESI)
MH+ found 255.2946, C15H15 N2O2 requires 255.2957.
1-Acetyl-1,3-di-(o,p-dimethylphenyl)urea (4a): mp 137-
139 °C; 1H NMR (400 MHz, CDCl3) δ 1.96 (s, 3H), 2.20 (s, 3H),
2.28 (s, 3H), 2.34 (s, 3H), 2.36 (s, 3H), 6.99 (m, 2H), 7.10 (m,
3H), 7.91 (d, 1H, J ) 8 Hz), 11.27 (s, 1H); 13CNMR (100 MHz,
CDCl3) δ 17.7, 18.4, 21.1, 21.4, 26.3, 121.5, 127.4, 128.0, 128.4,
128.9, 131.2, 132.3, 133.8, 134.1, 135.6, 136.0, 139.5, 151.7, 175.6;
IR (KBr) 3180, 2916, 1711, 1671, 1595, 1547, 1498, 1444, 1317,
1253, 1180, 1050, 965, 824, 725, 627, 506 cm-1; HRMS (ESI)
MH+ found 311.4021, C19H23N2O2 requires 311.4029.
Preparation of N-Propionylated Urea (1a′) from Thiourea
(1). To a stirred solution of diphenylthiourea 1 (456 mg, 2 mmol),
triethylamine (982 µL, 7 mmol), and propionic acid (374 µL, 5
mmol) in acetonitrile (10 mL) was added DIB (644 mg, 2 mmol)
at room temperature, and the mixture was allowed to stir for 15
min. Precipitation of sulfur was observed during this period. After
completion of the reaction, solvent was evaporated and admixed
with ethyl acetate (20 mL). The ethyl acetate layer was washed
subsequently with saturated solution of NaHCO3 (5 mL) and 5%
solution of sodium thiosulphate (5 mL), dried over anhydrous Na2-
1-Acetyl-1,3-di-(o,o-dimethylphenyl)urea (5a): mp 109-
111 °C; 1H NMR (400 MHz, CDCl3) δ 1.93 (s, 3H), 2.26 (s, 6H),
2.31 (s, 6H), 7.05-7.26 (m, 6H), 10.67 (s, 1H); 13CNMR (100 MHz,
CDCl3) δ 18.0, 18.9, 25.3, 127.2, 128.3, 129.1, 129.2, 134.1, 135.4,
136.2, 137.3, 151.5, 175.4; IR (KBr) 3246, 2916, 1710, 1668, 1494,
1371, 1313, 1260, 1238, 1165, 1033, 777 cm-1; HRMS (ESI) MH+
HRMS (ESI) MH+ found 311.4018, C19H23N2O2 requires 311.4029.
1
SO4, and concentrated under reduced pressure. H NMR analysis
Acknowledgment. B.K.P. acknowledges the support of this
research from DST New Delhi (SR/S1/OC-15/2006) and CSIR
01(1946)/00/EMR-II. H.G, S.M., and C.B.S thank the CSIR for
fellowships. Thanks are due to CIF IIT Guwahati for NMR
spectra and DST FIST for the XRD facility.
of the crude reaction mixture shows the formation of 1,3-diphenyl-
1-propionylurea 1a′ and 1-acetyl-1,3-diphenylurea 1a in a ratio of
83:17. Compound 1a′ was purified over a silica gel column (hexane/
EtOAc, 9:1) to give (376 mg, 70%) of the product 1,3-diphenyl-
1-propionylurea 1a′. Compound 1a′ was recrystallized from a
mixture of EtOAc/hexane (8:2) to give a colorless needle crystal:
1
mp 114-115 °C; H NMR (400 MHz, CDCl3) δ 1.07 (t, 3H, J )
Supporting Information Available: Experimental details and
7.2 Hz), 2.18 (q, 2H, J ) 7.2 Hz), 7.07-7.56 (m, 10H), 11.55 (s,
1H); 13CNMR (100 MHz, CDCl3) δ 8.94, 31.6, 120.3, 124.2, 129.1,
129.2, 129.4, 129.9, 137.9, 138.3, 152.23, 178.4; IR (KBr) 3310,
2929, 1704, 1615, 1540, 1395, 1215, 1174, 1078, 899, 810, 753,
712 cm-1; HRMS (ESI) MH+, found 269.3235, C15H15N2O2
requires 269.3229.
1
full characterization of compounds; IR, H NMR, and 13C NMR
spectra. Crystallographic description (S2), ORTEP view (S3), and
CIF files for compounds 1a and 16a. This material is available
free of charge via the Internet at http://pubs.acs.org.
JO702628G
J. Org. Chem, Vol. 73, No. 7, 2008 2927