Polyfluoroalkylation of 2-aminothiazoles

Article abstract of DOI:10.1016/j.jfluchem.2011.07.005

An efficient, highly selective method for polyfluoroalkylation of 2-aminothiazole derivatives was described. Interestingly, a defluorinated reductive 2-aminothiazole derivative was obtained in moderate yields when 2-aminothiazole was reacted with (CF

Full text of DOI:10.1016/j.jfluchem.2011.07.005

Journal of Fluorine Chemistry 133 (2012) 115–119
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Polyfluoroalkylation of 2-aminothiazoles
*
Qingqing Qi, Qilong Shen, Long Lu
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
An efficient, highly selective method for polyfluoroalkylation of 2-aminothiazole derivatives was
described. Interestingly, a defluorinated reductive 2-aminothiazole derivative was obtained in moderate
yields when 2-aminothiazole was reacted with (CF3)2CFI.
Received 22 April 2011
Received in revised form 6 July 2011
Accepted 7 July 2011
ß 2011 Elsevier B.V. All rights reserved.
Available online 18 July 2011
Dedication to Professor Weiyuan Huang
on the occasion of his 90th birthday.
Keywords:
Polyfluoroalkylation
Sulfinatodehalogenation
2-Aminothiazole
1. Introduction
which is firstly developed by Huang [11] and well-known for
the polyfluoroalkylation of electron-rich arenes, would provide a
2-Aminothiazole is regarded as a privileged structure motif in
medicinal chemistry due to its presence in the numerous
pharmaceuticals [1,2] and agrochemicals [3]. The decoration of
2-aminothiazole is, therefore, of great current interests. In
particular, the incorporation of fluoroalkyl groups into 2-
aminothiazole represents an attractive strategy in this regard
(Scheme 1) [2,4,5], since it is well-known that fluorination of less
active precursors often leads to potent drugs with enhanced
bioavailability, reduced toxicity or improved affinity for the target
receptor [6].
In general, fluoroalkyl-substituted 2-aminothiazoles are syn-
thesized through condensation of thiourea with fluoroalkyl-
substituted synthons [2,5,7,8] or copper-mediated nucleophilic
polyfluoroalkylation of halogen-substituted 2-amino-thiazoles
[9]. However, both of these methods suffer from tedious
procedures for the preparation of fluoroalkyl-substituted syn-
thons or the halogen-substituted 2-aminothiazoles, the incom-
patibility of important functional groups and low yields. Direct
coupling of 2-aminothiazole with polyfluoroalkyl halides under
mild conditions would be more straightforward and efficiency
[10]. We speculated that sulfinatodehalogenation reaction,
highly selective route for the polyfluoroalkylation of 2-ami-
nothiazoles. Herein, we describe a facile procedure for highly
selective polyfluoroalkylation of 2-aminothiazole and its deriva-
tives in good yields. Interestingly, an unexpected reduction of sp3
C–F bond instead of the polyfluoroalkylation of aminotiazole
occurred when (CF3)2CFI was used.
2. Results and discussion
Reactions of 2-aminothiazole with nonafluoro-1-iodobutane
under standard sulfinatodehalogenation conditions (Na2S2O4/
NaHCO3, CH3CN/H2O (4:1), 5–10 8C) gave the coupled products
in 80% yield with perfect selectivity at 5-position of 2-
aminothiazole (Table 1, entry 3). The reaction was not sensitive
to the concentration of the reactants and the acetonitrile/water
ratio only slightly affected the yields of the products. Table 1
summarized the reactions of
a variety of substituted 2-
aminothiazoles with polyfluoroalkyl iodides. Reactions of
2-aminothiazoles with CF3I, C2F5I, n-C4F9I, ClC4F8I and
IC2F4OC2F4SO2F occurred to give the corresponding 5-polyfluor-
oalkyl-2-aminothiazoles in good to excellent yields (Table 1,
entries 1-5). 4-Aryl-substituted 2-aminothiazoles also reacted
with perfluorobutyl iodides to give the desired products in good
yields (Table 1, entries 6–14). Substrates with chloride, nitro or
hydroxy group on phenyl ring were compatible under these
conditions (Table 1, entries 9–11, 14).
* Corresponding author. Tel.: +86 21 54925199; fax: +86 21 64166128.
E-mail address: lulong@sioc.ac.cn (L. Lu).
0022-1139/$ – see front matter ß 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2011.07.005

116
Q. Qi et al. / Journal of Fluorine Chemistry 133 (2012) 115–119
CF3
CF3
CF3
S
O
S
O
S
Cl
N
N
N
H
O
S
N
N
N
H
N
H
Cl
O
N
CF3
Cl
tBu
P2Y1 receptor inhibitor
PCT 2006078621
Herbicide antidotes
EP335831
Calcium Channel blockers
US 2003199523
Scheme 1. Example of drug candidates with 2-amino-5-trifluoromethylthiazole moiety.
Reaction of 2-aminothiazole with CF3Br, a much cheaper
operation may be applied to the reaction conducted on a 0.2 mol
scale (Eq. (1)).
trifluoromethyl resource than CF3I, also proceeded smoothly to
give 5-trifluoromethyl-2-aminothiazole in 61% yield when the
reaction was conducted in an autoclave at 80 8C. The same
Na S O
2 4
2
N
S
N
S
Na HPO 12H O
2
4
2
H N
2
+
H N
2
CF Br
3
(1)
CH CN/H O
3
2
H
CF
3
61%
Table 1
20 g
90 g
20.5 g
Polyfluoroalkylation of substituted 2-aminothiazoles under sulfinatodehalogena-
tion conditions.a
Interestingly, when (CF3)2CFI was used under standard
sulfinatodehalogenation conditions, unexpected C–F bond reduced
products 3 instead of the perfluoroalkylated derivative were
isolated. A number of other N-substituted 2-aminothiazoles also
reacted with (CF3)2CFI to afford the reductive defluorinated
products in good yields (Table 2, entries 1–5) [8]. Reactions of
4-alkyl or aryl-substituted 2-aminothiazoles, however, gave the
1.0 equiv. Na S O
2
2
4
3
R
R
R
2
2
H
H
N
S
N
S
1.0 equiv. NaHCO
+
R I
f
N
N
CH CN/H O (4:1)
3
2
R
1
R
1
H
f
o
5-10 C
b
Entry
Product
Yield (%)
CF
R =
67
60
80
85
1
2
3
4
5
N
3
f
Table 2
CF CF
H N
2
3
Reductive polyfluoroalkylation of substituted 2-aminothiazoles under sulfinato-
dehalogenation conditions.a
2
S
C F
4
9
8
4
R
f
C F Cl
4
2.0 equiv. Na S O
C F OC F SO F
2
2
2
4
3
72
R
R
2
4
2
R
R
3
R
R
R
1
3
1
N
S
N
S
2.0 equiv. NaHCO
N
N
(CF ) CF
3 2
+
CH CN/H O (3:1)
3
2
6
7
8
9
H
Me
OMe
Cl
61
58
63
75
R =
H
CH(CF )
3 2
2
2
N
S
o
d
d
d
0.1 M, 5-10
product
C
H N
2
b
entry
1
yield (%)
C F
4
9
9
N
S
H N
2
78
CH(CF )
3 2
e
NO
N
S
2
10
76
N
S
H N
2
78
74
78
2
3
HN
N
C F
4
CH(CF )
3 2
Ph
H
N
S
N
N
N
d
d
11
12
R = Cl
68
86
S
C F
4
CH(CF )
9
9
9
3 2
CF
3
R
H
N
Ph
N
H
N
S
4
5
S
CH(CF )
N
N
3 2
d
13
14
90
78
C F
4
OH
H
N
N
S
Ph
60
H
N
S
Cl
CH(CF )
3 2
N
Cl
C F
4
67
79
85
R
6
7
8
CH
R =
N
S
3
CH OH
2
OH
H N
2
c
Ph
CF(CF )
3 2
aReaction conditions: 1 (1.0 mmol), 1.5 equiv. of RfI in the presence of 1.0 equiv. of
Na2S2O4 and 1.0 equiv. of NaHCO3 in 1.5 mL of CH3CN/H2O (4/1) at 5–10 8C.
bIsolated yields.
cCF3Br was used and the reaction was conducted in an autoclave at 80 8C for 2 h.
dDMF/H2O (4:1) was used as solvent.
aReaction conditions: 1 (1.0 mmol), 1.5 equiv. of RfI in the presence of 2.0 equiv. of
Na2S2O4 and 2.0 equiv. of NaHCO3 in 10.0 mL of CH3CN/H2O (3/1) at 5–10 8C.
bIsolated yields.
cDMF/H2O (4:1) was used as solvent.
eReaction conducted at 0.05 M.

Q. Qi et al. / Journal of Fluorine Chemistry 133 (2012) 115–119
117
2-
heterocycles that are important for pharmaceuticals and agro-
chemicals are ongoing in our laboratory.
2SO
S O
2
2
4
2SO
R
R I
f
SO
+
+
I
+
2
f
2
4. Experimental
I
N
S
N
S
N
S
H
R
H
R
R I
f
4.1. General information
R
+
H N
H N
2
f
H N
2
2
f
f
f
All solvents were purified by standard method. 1H, 13C, and 19
F
H
R
R
H
NMR spectra were recorded on 300 or 400 MHz, 100 MHz, and
282 MHz spectrometer, respectively. 1H NMR and 13C NMR
chemical shifts were determined relative to an internal standard
TMS at d
0.0 and 19F NMR chemical shifts were determined relative
N
N
-HI
-HF
H N
2
H N
2
if R = alkyl or aryl
F
S
CF
S
3
CF
3
F C
3a
3
to CFCl3 as an external standard. All reactions were monitored by
TLC or 19F NMR. Flash column chromatography was carried out
using 300–400 mesh silica gel at medium pressure. Melting points
were determined on a SGW X-4 apparatus and were uncorrected.
F C
3
N
N
S
H O
2e
H
H N
2
H N
2
H
H
CF
S
CF
Chemical shifts (d) are reported in ppm, and coupling constants (J)
3
are in Hertz (Hz). The following abbreviations were used to explain
the multiplicities: s = singlet, d = doublet, t = triplet, q = quartet,
m = multiplet, br = broad.
3
F C
3
F C
3
3b
3
Scheme 2. Proposed mechanism for the formation of reductive defluorinated
product 3.
4.2. Typical procedure for the polyfluoroalkylation of 2-
aminothiazoles under sulfinato-dehalogenation conditions
perfluoroalkylated products in good yields. Increasing the amount
of Na2S2O4 or longer reaction time did not lead to the reductive
defluorinated products. This result is important because recently
heptafluoroisopropyl substituted aniline derivatives showed high
insecticidal effect on diamondback moth [12].
Sodium dithionite (1.0 mmol) was added in one portion to a
mixture of 2-aminothiazole (1.0 mmol), NaHCO3 (1.0 mmol) and
polyfluoroalky iodide (1.5 mmol) in acetonitrile/water (4:1/v:v) at
5–10 8C under Ar atmosphere. The mixture was stirred until
complete conversion of starting material as indicated by TLC
analysis. Acetonitrile was removed under reduced pressure and
water (5 mL) was added. The mixture was extracted with ethyl
acetate (4.0 mL Â 3) and the combined organic layers were dried
over Na2SO4. The solvent was removed under vacuum and the
residue was purified by column chromatography on silica gel
(petroleum/ethyl acetate) to give the product.
While defluorination of polyfluoroalkyl-substituted arenes
followed by nucleophilic attack from water have been well
documented in the literature [13], very few reductive defluorina-
tions of polyfluoroalkyl-substituted arenes under mild conditions
have been reported previously [14]. We then studied the reaction
in more detail. Careful monitoring of the reaciton by TLC and 19F
NMR spectroscopy revealed that perfluoroalkylation product 3a
was formed as the only product after 30 min. However, compound
3a was not stable under sulfinatodehalogenation conditions and
slowly converted to compound 3 within 3 h. Compound 3a, which
was not stable on silica gel or neutral aluminum oxide, was
purified by extraction with diethyl ether and drying over Na2SO4,
followed by removing the solvent under vacuum [15]. Treatment of
isolated 3a with Na2S2O4/NaHCO3 in CH3CN/H2O (4:1) at 5–10 8C
leads to 3 quantitatively. The yield of the same reaction under
oxygen atmosphere was much lower.
4.2.1. 5-(Trifluoromethyl)thiazol-2-amine [7] (Table 1, entry 1)
The general procedure conducted with sodium dithionite
(174 mg, 1.0 mmol), 2-aminothiazole (100 mg, 1.0 mmol), NaHCO3
(84 mg, 1.0 mmol) and CF3I (294 mg, 1.5 mmol) in 5.2 mL
acetonitrile and 1.3 mL water gave 112 mg (67%) of the product
as a light yellow oil. 1H NMR (300 MHz, CDCl3/TMS)
5.82 (br, 2H). 19F NMR (282 MHz, CDCl3)
À54.90 (s, 3F).
d 7.38 (s, 1H),
d
4.2.2. 5-(Perfluoroethyl)thiazol-2-amine (Table 1, entry 2)
Based on these observations, we postulate that compound 3a
was formed initially under sulfinatodehalogenation conditions
(Scheme 2). Intramolecular elimination of hydrogen fluoride from
3a, followed by two-electron reduction in the presence of Na2S2O4
gave an anionic intermediate 3b. Protonation of intermediate 3b
afforded compound 3. In the presence of oxygen, the two-electron
oxidation process was prohibited. When 4-alkyl or aryl-substi-
tuted 2-aminothiazoles were used, elimination of hydrogen
fluoride was much slower possibly due to the steric hindrance
between the trifluoromethyl group and the 4-substituent group
(Scheme 2).
The general procedure conducted with sodium dithionite
(174 mg, 1.0 mmol), 2-aminothiazole (100 mg, 1.0 mmol), NaHCO3
(84 mg, 1.0 mmol) and CF3CF2I (294 mg, 1.5 mmol) in 5.2 mL
acetonitrile and 1.3 mL water gave 131 mg (60%) of the product as
a light yellow solid. Mp: 26–27 8C. 1H NMR (300 MHz, CDCl3/TMS)
d
7.40 (s, 1H), 5.45 (br, 2H). 19F NMR (282 MHz, CDCl3)
d
À83.87 (m,
3F), À103.30 (m, 2F). 13C NMR (100 MHz, CDCl3)
d 171.7, 141.8 (t,
J = 6.6 Hz), 123.0–109.1. MS (ESI positive ion): 218.9 (M+1); HRMS
(ESI) calcd for C5H4F5N2S+1 (M+H): 219.00099. Found: 219.00145.
IR (thin film)
d 3297, 3166, 1622, 1552, 1506, 1347, 1289, 1208,
1121, 1076, 936, 747 cmÀ1
.
4.3. Preparation of 5-(trifluoromethyl)thiazol-2-amine from CF3Br
3. Conclusion
Sodium dithionite (70 g, 0.4 mol), 2-aminothiazole (20 g,
0.2 mol) and Na2HPO4Á12H2O (76 g, 0.1 mol) in 150 mL acetoni-
trile/180 mL water was added into a 1.0 L autoclave, and then it
was sealed and evacuated followed with the introduction of CF3Br
(90 g, 0.6 mmol). The reaction mixture was heated to 80 8C for 3 h.
The work up was the same as that aforementioned to give 5-
(trifluoromethyl)thiazol-2-amine in 61% yield.
In summary, we have developed a highly selective protocol for
the polyfluoroalkylation of 2-aminothiazole derivatives in good
yields under mild conditions. Careful control of the reaction
conditions enables us to obtain either reductive defluorinated 2-
aminothiazole derivatives or polyfluoroalkylated products when
(CF3)2CFI was used. Expansion of the scope of the reaction to other

118
Q. Qi et al. / Journal of Fluorine Chemistry 133 (2012) 115–119
4.4. Characterization of 5-(perfluoropropan-2-yl)thiazol-2-amine
(thin film) 3450, 3281, 3135, 2939, 1630, 1537, 1509, 1312, 1218,
1095, 1041, 949, 913, 721 cmÀ1
.
Sodium dithionite (261 mg, 1.5 mmol) in one portion was
added to a mixture of 2-aminothiazole (100 mg, 1.0 mmol),
NaHCO3 (252 mg, 3.0 mmol) and (CF3)2CFI (443 mg, 1.5 mmol)
in 5.2 mL acetonitrile and 1.3 mL water at 5–10 8C under argon. The
mixture was stirred until complete conversion of starting material
as indicated by TLC analysis (within 30 min). Acetonitrile was
removed under reduced pressure (Caution: the temperature of the
water bath of the rotvap maintained below 15 8C. Otherwise,
higher temperature resulted in the decomposition of the product).
The mixture was extracted with diethyl ether (4.0 mL Â 1) and the
organic layers were dried over Na2SO4. The solvent was removed
under vacuum to give a white solid in 76% yield, 204 mg. The white
solid was pure as indicated by 1H NMR spectroscopy and Mass
Spectroscopy. Note: The title product was stable for months when
stored at -20 8C. 5-(perfluoropropan-2-yl)thiazol-2-amine: White
4.5.3. 5-(Perfluoropropan-2-yl)-4-phenylthiazol-2-amine (Table 2,
entry 8)
The general procedure conducted with sodium dithionite (2.0 g,
11.4 mmol), 2-aminothiazole (1.0 g, 5.7 mmol), NaHCO3 (960 mg,
11.4 mmol) and (CF3)2CFI (2.52 g, 8.6 mmol) in 20.0 mL DMF and
10.0 mL water gave 1.66 g (85%) of the product as a white solid.
Mp: 132–133 8C. 1H NMR (300 MHz, CDCl3/TMS)
d 7.37 (m, 5H),
5.70 (br, 2H). 19F NMR (282 MHz, CDCl3)
d
À76.10 (d, J = 10.4 Hz,
6F), À174.33 (m, 1F). 13C NMR (100 MHz, CDCl3)
d 168.4, 156.2,
135.0 (d, J = 2.2 Hz), 129.0 (d, J = 2.9 Hz), 128.6, 127.7, 120.3 (qd,
J = 285.1, 27.2 Hz, CF3), 102.0 (d, J = 22.6 Hz), 89.4–93.4 (m). MS
(ESI positive ion): 345.0 (M+1); HRMS (ESI) calcd for C12H8F7N2S+1
(M+H): 345.0291. Found: 345.0302. IR (thin film) 3461, 3279,
3097, 2970, 1631, 1527, 1298, 1206, 1159, 1068, 969, 934, 922,
solid, isolated yield: 76%. 1H NMR (300 MHz, CDCl3/TMS)
1H), 5.44 (br, 2H). 19F NMR (282 MHz, CDCl3)
d
7.33 (s,
777, 754, 723, 706, 663, 596 cmÀ1
.
d
À75.93 (d,
J = 9.3 Hz, 6F), À167.60 (m, 1F). MS (ESI positive ion): 269.0 (M+1);
HRMS (ESI) calcd for C6H4F7N2S+1 (M+H): 268.9978. Found:
268.9977.
4.6. Reductive defluorination of 4-methyl-5-(perfluoropropan-2-
yl)thiazol-2-amine
A solution of sodium dithionite (123 mg, 0.7 mmol) and 4-
4.5. Typical procedure of the reductive polyfluoroalkylation of
substituted 2-aminothiazoles
methyl-5-(perfluoropropan-2-yl)thiazol-2-amine
(100 mg,
0.4 mmol) in 1.2 mL acetonitrile and 1.2 mL water was heated
to 60 8C under argon. The mixture was stirred until complete
conversion of starting material as indicated by TLC analysis.
Acetonitrile was removed under reduced pressure and water
(5 mL) was added. The mixture was extracted with ethyl acetate
(4.0 mL Â 3) and the combined organic layers were dried over
Na2SO4. The solvent was removed under vacuum and the residue
was purified by column chromatography on silica gel (petroleum/
ethyl acetate) to give 55 mg (60%) of the product as a white solid. 5-
Sodium dithionite (2.0 mmol) was added in one portion to a
mixture of 2-aminothiazole (1.0 mmol), NaHCO3 (2.0 mmol) and
polyfluoroalky iodide (1.5 mmol) in 10.0 mL acetonitrile/water
(3:1/v:v) at 5–10 8C under Ar atmosphere. The mixture was stirred
until complete conversion of starting material as indicated by TLC
or 19F NMR analysis. Acetonitrile was removed under reduced
pressure and water (5 mL) was added. The mixture was extracted
with ethyl acetate (4.0 mL Â 3) and the combined organic layers
were dried over Na2SO4. The solvent was removed under vacuum
and the residue was purified by column chromatography on silica
gel (petroleum/ethyl acetate) to give the product.
(1,1,1,3,3,3-
hexafluoropropan-2-yl)-4-methylthiazol-2-amine.
5.61 (br, 2H),
Mp: 104–105 8C. 1H NMR (300 MHz, CDCl3/TMS)
d
4.30 (m, J = 7.8 Hz, 1H), 2.20 (s, 3H). 19F NMR (282 MHz, CDCl3)
d
À66.70 (d, J = 7.3 Hz, 6F). 13C NMR (100 MHz, CDCl3)
d 168.3, 151.3,
122.6 (q, J = 281.4 Hz), 102.9, 47.8 (septet, J = 31.0 Hz), 14.98. MS
(ESI positive ion): 265.0 (M+1); HRMS (ESI) calcd for C7H7F6N2S+1
(M+H): 265.0229. Found: 265.0227. IR (thin film) 3469, 3231,
3080, 2927, 1613, 1574, 1533, 1509, 1348, 1331, 1271, 1224, 1203,
4.5.1. 5-(1,1,1,3,3,3-Hexafluoropropan-2-yl)thiazol-2-amine (Table
2, entry 1)
The general procedure conducted with sodium dithionite
(348 mg, 2.0 mmol), 2-aminothiazole (100 mg, 1.0 mmol), NaHCO3
(168 mg, 2.0 mmol) and (CF3)2CFI (443 mg, 1.5 mmol) in 7.5 mL
acetonitrile and 2.5 mL water gave 194 mg (78%) of the product as
1171, 1091, 965, 897, 867, 734, 695, 622, 530, 449 cmÀ1
.
Acknowledgments
a white solid. Mp 67–68 8C. 1H NMR (300 MHz, CDCl3/TMS)
d 7.13
(s, 1H), 5.23 (br, 2H), 4.16–4.32 (Septet, J = 7.9 Hz, 1H). 19F NMR
The authors gratefully acknowledge the financial support from
Special Fund for Agro-scientific Research in the Public Interest
(201103007), the National Key Technologies R&D Program
(2011BAE06B05), National Basic Research Program of China
(2010CB126103), the Key Program of National Natural Science
Foundation of China (21032006) and SIOC startup fund for financial
support. We also thank Mr. Yanchuan Zhao for helpful discussion.
(282 MHz, CDCl3)
CDCl3)
J = 31.0 Hz, –CH–). MS (ESI positive ion): 250.9 (M+1); HRMS (ESI)
calcd for C6H5F6N2S+1 (M+H): 251.0078. Found: 251.0072. IR (thin
film) 3468, 3287, 3122, 2678, 1630, 1556, 1528, 1513, 1353, 1278,
d
À67.31 (d, J = 7.9 Hz, 1F). 13C NMR (100 MHz,
d
170.3, 142.3, 122.3 (q, J = 210.9 Hz, CF3), À109.7, 48.2 (t,
1233, 1188, 1149, 1095, 1050, 902, 863, 738, 692, 533 cmÀ1
.
4.5.2. (2-Amino-5-(perfluoropropan-2-yl)thiazol-4-yl)methanol
(Table 2, entry 7)
Appendix A. Supplementary data
The general procedure conducted with sodium dithionite
(134 mg, 0.8 mmol), 2-aminothiazole (50 mg, 0.4 mmol), NaHCO3
(65 mg, 0.8 mmol) and (CF3)2CFI (171 mg, 0.6 mmol) in 3.0 mL
acetonitrile and 1.0 mL water gave 90 mg (79%) of the product as a
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jfluchem.2011.07.005.
white solid. Mp: 110–111 8C. 1H NMR (300 MHz, DMSO-d6/TMS)
7.60 (s, 2H, –NH2), 5.08 (t, J = 6.0 Hz, 1 H, –OH), 4.29 (dd, J = 2.1,
d
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d
À75.70 (d,
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