Highly efficient synthesis of aryl and heteroaryl trifluoromethyl ketones via o-iodobenzoic acid (IBX)

Article abstract of DOI:10.1016/j.tetlet.2013.06.045

A two-step process for the synthesis of aryl and heteroaryl trifluoromethyl ketones from the corresponding aldehydes is described. Trifluoromethyl alcohols were prepared from aryl and heteroaryl aldehydes in excellent yields using catalytic amount of K₂CO₃. The trifluoromethyl alcohols were then oxidized conveniently and efficiently by o-iodoxybenzoic acid (IBX) under mild conditions to get the desired functionalized aryl and heteroaryl trifluoromethyl ketones.

Full text of DOI:10.1016/j.tetlet.2013.06.045

Tetrahedron Letters 54 (2013) 4483–4486
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Highly efficient synthesis of aryl and heteroaryl trifluoromethyl
ketones via o-iodobenzoic acid (IBX)
a
Huicheng Cheng a, Yu Pei a, Faqiang Leng a, Jingya Li b, Apeng Liang a,b, Dapeng Zou a,b, , Yangjie Wu ,
⇑
Yusheng Wu b,c,
⇑
a College of Chemistry and Molecular Engineering, Key Laboratory of Chemical Biology and Organic Chemistry of Henan Province, Zhengzhou University, Zhengzhou, Henan 450052,
PR China
b Tetranov Biopharm, LLC, 75 Daxue Road, Zhengzhou, Henan 450052, PR China
c Tetranov International, Inc., 100 Jersey Avenue, Suite A340, New Brunswick, NJ 08901, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A two-step process for the synthesis of aryl and heteroaryl trifluoromethyl ketones from the correspond-
ing aldehydes is described. Trifluoromethyl alcohols were prepared from aryl and heteroaryl aldehydes in
excellent yields using catalytic amount of K2CO3. The trifluoromethyl alcohols were then oxidized conve-
niently and efficiently by o-iodoxybenzoic acid (IBX) under mild conditions to get the desired function-
alized aryl and heteroaryl trifluoromethyl ketones.
Received 11 May 2013
Revised 5 June 2013
Accepted 11 June 2013
Available online 18 June 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Trifluoromethyl ketones
Trifluoromethyl alcohols
o-Iodoxybenzoic acid (IBX)
Oxidation
Introduction
occupy their role as monomers for novel polymeric materials.5 As
a consequence, considerable efforts have been focused on the tri-
The incorporation of the trifluoromethyl group into organic
compounds often leads to the significant modification of their bio-
logical and physiological activities by promoting electrostatic
interactions with targets, improving cellular membrane permeabil-
ity, and increasing robustness toward oxidative metabolism of the
drug.1 Therefore, this strategy has been widely used in the design
and optimization of biologically active molecules in the realm of
medicinal chemistry.2
In recent years, the ubiquitous emergence of trifluoromethyl
ketones (TFMKs) as a typical trifluoromethyl building block in
the pharmaceutical industry has been resulting in an ever-growing
interest in the synthesis of trifluoromethyl ketone derivatives. The
powerful inductive electron-withdrawing effect of the trifluoro-
methyl group renders these ketones exceptionally electrophilic
and the propensity of trifluoromethyl ketones to form stable hy-
drates and related tetrahedral adducts has been exploited in the
design of numerous enzyme inhibitors.3 Trifluoromethyl ketones
are also valuable intermediates for the synthesis of trifluoro-
methyl-substituted heterocycles and other compounds,4 and also
fluoroacetyl of arenes and heteroarenes.
In general, synthesis of TFMKs has been accomplished by vari-
ous methods:6–16 (1) Friedel–Crafts acylation;6 (2) Grignard reac-
tion;7 (3) ortho-lithiation;8 (4) Suzuki–Miyaura cross-coupling
reaction;9 (5) oxidation of trifluoromethyl alcohols;10–14 (6) nucle-
ophilic trifluoromethylation of esters with trifluoromethyl agent;15
(7) other methods16 such as the Wittig reaction with trifluoroace-
tamides,16a,b conversion of trifluoroethyl amines by NBS/DBU
treatment;16c and catalytic aerobic oxidative decarboxylation of
a
-trifluoromethyl-a
-hydroxy acids.16d In the above mentioned
methods, the oxidation of trifluoromethyl alcohols to the corre-
sponding trifluoromethyl ketones seems much more practical
and efficient. However, fluoroalkyl groups are strong electron-
withdrawing substituents; consequently, the oxidation of the tri-
fluoroalcohol to the corresponding ketone has been proved excep-
tionally difficult.17 Classical oxidation procedures such as the use
18
of MnO2,17a,b KMnO4,17c,d and CrO3 are often unsuccessful or
require harsh conditions which are not suitable for some func-
tional groups. Swern and modified Moffat oxidation methodologies
very often fail or poorly succeed in the case of fluoroalkyl alco-
hols,19 despite some described examples.10 The reliable procedure
available in the literature is the oxidation by the Dess–Martin peri-
odinane reagent (DMP).11 However, there are significant draw-
backs of using DMP, including its cost and the fact that spent
⇑
Corresponding authors. Tel.: +86 371 67766865; fax: +86 371 67762133 (D.Z.);
tel.: +1 732 253 7326; fax: +1 732 253 7327 (Y.W.).
E-mail addresses: zdp@zzu.edu.cn (D. Zou), yusheng.wu@tetranovglobal.com
(Y. Wu).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.06.045

4484
H. Cheng et al. / Tetrahedron Letters 54 (2013) 4483–4486
oxidant cannot be easily recovered.11 Therefore, various oxidizing
reagents, such as sodium periodate catalyzed by ruthenium(II)
complex,12a chromium reagents PDC,12b Jones reagent,12b the
equivalence of TMSCF3 and the catalytic amount of K2CO3, extend-
ing the reaction time and screening experimental temperature, this
methodology provided higher yields and no further purification is
required. The results are shown in Table 2 (entries 1–24) and Table
3 (entries 1–11).
polymer-supported
permanganate,12c
the
oxoammonium
salt 4-acetylamino-2,2,6,6-tetramethylpiperidine-1-oxoammoni-
um tetrafluoroborate,11c and sodium 2-iodobenzenesulfonate-
catalytic Oxone13 have been employed for the oxidation of
trifluoromethyl alcohols.
Within the past decade, o-iodoxybenzoic acid (IBX) stands out
for being mild, selective, and environmentally friendly, as it con-
tains no toxic or expensive heavy metals.20a–d Seeking an alterna-
The oxidation condition is also suitable for the synthesized aryl
trifluoromethyl alcohols to give the corresponding trifluoromethyl
ketones in excellent yields. The results are summarized in Table 2
(entries 1–24). This protocol for the use of IBX is insensitive to the
presence of air or moisture and broadly applicable, which is com-
patible with a wide range of functional groups such as methoxy,
ethylenedioxy, phenoxy ethers, dimethylamino, chloro, bromo,
nitro, nitrile, and styrenyl present in the substrate. Virtually most
of the trifluoromethyl alcohols investigated were converted into
the corresponding carbonyl compounds in >90% yields with only
exceptions. Phenol does not withstand the presence of IBX com-
plex and dark colored reaction mixtures were obtained with
<15% isolated yield (Table 2, entry 8). The oxidation of 2.2.2-tri-
tive oxidant for
a-CF3 alcohols, we reasoned that IBX was the
most attractive in terms of practicality and high efficiency. Investi-
gations from our own laboratories have revealed that IBX was an
efficient and practical oxidant for the synthesis of aryl and hetero-
aryl trifluoromethyl ketones.
Results and discussion
fluoro-1-(4-nitrophenyl) ethanol has been performed on
a
According to the previous reports,10a,11a,b,12,21 trifluoromethyl
carbinol 1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-2,2,2-trifluoro-
ethanol (7A) was used as a model substrate to test the captivity
of the oxidants (Table 1). It was found that 7A was oxidized under
PCC conditions to give the desired product 7B with 47% yield.
Swern oxidation gave the desired product 7B in 87% yield. How-
ever, the Swern oxidation requires low temperature, anhydrous
conditions, and generates stench.10,14 Despite the DMP oxidation
gave the desired product 7B in 83% yield, its explosive nature
and high air and moisture sensitivity limited its utility.11a,b It was
found that IBX is superior to Swern, PCC and DMP oxidations.
IBX showed clean, near-quantitative conversion to the correspond-
ing trifluoromethyl ketone, which is the only product detected by
TLC and 1H NMR in the crude reaction mixture. The oxidation of
7A with IBX was remarkably convenient. The reaction mixture
was simply filtered to remove the oxidant and then concentrated
to give the desired product 7B in near-quantitative yield. When
the oxidation was performed on a multiple-gram (52.4 mmol)
scale, there is no observed decrease in the yield. The oxidation pro-
ceeds well with 1.8 equiv of IBX, but an increased reaction rate is
observed with more equiv of the oxidant. The combination of
2.0 equiv IBX in refluxing EtOAc was chosen as the optimum
condition.
90 mmol scale with identical yield and purity (Table 2, entry 19).
The results obtained from this study indicate that the oxidation
of IBX was effective to both electron rich and electron deficient
benzyl alcohols to the corresponding trifluoromethyl ketones in
excellent yields. Interestingly, the reaction time is found to be
dependent on the nature of the substrate. The first step of IBX oxi-
dation is a nucleophilic addition of the oxygen atom of trifluoro-
methyl alcohols to the iodine atom of IBX. Therefore, the
electron-donating substituent on the benzene ring of the trifluoro-
methyl alcohol shows faster oxidation in most cases (Table 2, en-
tries 1–22). Because the steric hindrance of the ortho substituent
on the benzyl alcohol plays a major role, the same substituent on
Table 2
Synthesis of aryl trifluoromethyl alcohols and trifluoromethyl ketones
OH
CF3
(1) IBX, EtOAc, reflux
O
O
(1) TMSCF3, K2CO3 / DMF, rt, 4 h
(2) H3O+, 4 h
Ar
Ar
H
Ar
CF3
(2) Filtration
A
CF3 alcohol
B
CF3 ketone
Entry Ar
Yield of Aa (%) Time (h) Yield of Bb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Ph–
4-CH3–C6H4–
99
98
99
97
99
99
97
94
99
13
12
11
13
18
9
92
98
99
98
A wide range of aryl and heteroaryl trifluoromethyl alcohols
were prepared for these oxidation reactions. K2CO3 was used as a
catalyst in the synthesis of TMS-protected trifluoromethylated
alcohols according to Prakash’s methods.22 By increasing the
4-CH3O–C6H4–
3-CH3O–C6H4
2-CH3O–C6H4–
3,4,5-(CH3O)3–C6H2–
3,4-OCH2CH2O–C6H3
4-HO–C6H4–c
97
99
11
12
14
14
22
24
26
30
20
22
26
20
20
22
26
19
22
18
99e
<15
99
N,N–Me2–C6H3
Table 1
3-BzO-4-CH3O–C6H4– 96e
99e
97
Screening of oxidants for the oxidation of trifluoromethyl alcohol
4-Cl–C6H4–
3-Cl–C6H4–
2-Cl–C6H4–
96
99
98
97
94
90
98
93
99
99
99
98
94
96
OH
CF3
O
98
Oxidant
O
O
O
O
97
CF3
2,4-Cl2–C6H3–
4-Br–C6H4–
3-Br–C6H4–
2-Br–C6H4–
2-F-4-Br–C6H3–
4-NO2–C6H4–
3-NO2–C6H4–
2-NO2–C6H4–
4-CN–C6H4–
(E)-b-Styryl
98d,e
96
Conditions
99
99
7A
7B
94
Entry
Oxidant (equiv)
Reaction conditions
Yielda (%)
99d
96
1
2
3
4
Swern (2.8)b
DMP (3.7)c
PCC (1.5)d
IBX (2.0)e
CH2Cl2, À78 °C, 1.5 h
CH2Cl2, rt, 3 h
CH2Cl2, rt, 7 h
87
83
47
99
96
97d
96
EtOAc, 78 °C, 11 h
a-naphthyl
99
a
Isolated yields, all reactions carried out on 2.14 mmol scale.
Swern: 1.2 equiv of (COCl)2, 2.8 equiv of DMSO, 6.0 equiv of Et3N, CH2Cl2,
b
cTBS-protected 4-hydroxybenzaldehyde.
a
À78 °C.
All reactions carried out on 4.28–33.09 mmol scale.
Isolated yields, all reactions carried out on 2.14–12.84 mmol scale.
The product was hydrate.
c
b
d
e
DMP: 3.7 equiv of DessÀMartin periodinane, 4.0 equiv of NaHCO3, CH2Cl2, rt.
d
e
PCC: 1.5 equiv of PCC, CH2Cl2, rt.
IBX: 2.0 equiv of o-iodoxybenzoic acid, EtOAc, 78 °C.
New compound.

H. Cheng et al. / Tetrahedron Letters 54 (2013) 4483–4486
4485
Table 3
reactions with consistent activity and it is indistinguishable from
the freshly prepared one from o-iodobenzoic acid.
Synthesis of heteroaryl trifluoromethyl alcohols and trifluoromethyl ketones
O
O
(1) IBX, EtOAc; reflux
OH
CF3
(1) TMSCF3, K2CO3 / DMF,rt, 4 h
(2) TBAF,4 h
HetAr
H
(2) Filtration
HetAr
CF3
HetAr
CF3 alcohol
Conclusion
A
CF3 ketone B
In summary, we have developed an efficient and practical syn-
thesis of aryl and heteroaryl trifluoromethyl ketones by oxidation
of trifluoromethyl alcohols with o-iodoxybenzoic acid (IBX),
affording a series of synthetically useful trifluoromethyl ketones.
These reactions demonstrate excellent reactivity, good functional
group tolerance, and high yields. This procedure also allows recy-
cling and reuse of the oxidant. Moreover, such trifluoromethyl
alcohol oxidation can be performed on gram-scale with almost
quantitative yields.
Entry
HetAr
Yield of Aa (%)
Time (h)
15
Yield of Bb (%)
1
2
96d
98d
BnO
Cl
N
96d
14
97c,d
N
Br
3
4
93d
93d
18
16
99c,d
99c,d
Acknowledgments
N
N
Br
Br
We are grateful to the Natural Science Foundation of China
(20772114, 21172200), the Innovation Fund for Outstanding
Scholar of Henan Province (0621001100) Research Program of
Fundamental and Advanced Technology of Henan Province
(122300413203) for financial support.
5
93d
30
94c,d
N
N
CF3
References and notes
6
7
92d
93d
16
18
98d
96d
Br
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