N-Heterocyclic Carbene-Catalyzed Synthesis of α-Trifluoromethyl Esters

Article abstract of DOI:10.1021/acs.orglett.8b01482

The N-heterocyclic carbene (NHC)-catalyzed trifluoromethylation of α-chloro aldehydes was developed, allowing straightforward access to valuable α-trifluoromethyl ester derivatives. The unique combination of an electrophilic trifluoromethylation reagent with NHC catalysis was the key for the functionalization of a broad range of α-chloro aldehydes, and the products are formed in moderate to good yields. Investigations of the enantioselective version of this reaction afforded the enantioenriched products in moderate yields with good ee values.

Full text of DOI:10.1021/acs.orglett.8b01482

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
N‑Heterocyclic Carbene-Catalyzed Synthesis of α‑Trifluoromethyl
Esters
Fabien Gelat,† Atanu Patra,‡ Xavier Pannecoucke,† Akkattu T. Biju,§ Thomas Poisson,*,†,∥
and Tatiana Besset*,†
†Normandie Univ., INSA Rouen, UNIROUEN, CNRS, COBRA (UMR 6014), 76000 Rouen, France
‡Organic Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune-411008, India
§Department of Organic Chemistry, Indian Institute of Science, Bangalore-560012, India
∥Institut Universitaire de France, 1 rue Descartes, 75231 Paris, France
S
* Supporting Information
ABSTRACT: The N-heterocyclic carbene (NHC)-catalyzed trifluoromethylation of α-chloro aldehydes was developed, allowing
straightforward access to valuable α-trifluoromethyl ester derivatives. The unique combination of an electrophilic
trifluoromethylation reagent with NHC catalysis was the key for the functionalization of a broad range of α-chloro aldehydes,
and the products are formed in moderate to good yields. Investigations of the enantioselective version of this reaction afforded
the enantioenriched products in moderate yields with good ee values.
Scheme 1. State of the Art and the Present Work
rganofluorine chemistry is a research field of constant
Oevolution offering a large variety of fluorinated
molecules.1 As they are key compounds in the agrochemical
and pharmaceutical industries,2 the quest for straightforward
and innovative methodologies to incorporate a fluorine atom or
a fluorinated group is highly demanding, especially related to
the unique features3 brought by fluorinated groups to
molecules. In particular, we turned our attention to the CF3
group,4 a moiety widely present in various relevant compounds.
Nowadays, a broad panel of transformations enables efficient
trifluoromethylation of several classes of compounds of interest.
Although efforts have been dedicated to the introduction of the
CF3 motif onto C(sp2) centers, less attention has been paid to
the functionalization of aliphatic compounds. The creation of a
C(sp3)−CF3 bond, otherwise made from a 1,2-addition on a
carbonyl group derivative,5 is scarce, and the α-trifluoromethy-
lation of carbonyl derivatives remains a synthetic challenge.
Indeed, to date most of the reports have dealt with the α-
trifluoromethylation of ketones,6 while the α-functionalization
of aldehydes7 and imides8 with a CF3 group is restricted to a
handful of reports. With regard to the α-trifluoromethylation of
esters, most of the reports have focused on highly activated
ester derivatives (Scheme 1, eq 1) such as β-keto esters or α-
nitro esters.9 Another strategy relies on the use of
prefunctionalized esters like α-diazo ester derivatives (Scheme
1, eq 2)10 or sensitive ketene silyl acetals (Scheme 1, eq 3).11
Finally, a couple of examples depicted the direct α-
functionalization of esters using either strong base or oxidant
(Scheme 1, eq 4).12 With the aim to develop a straightforward
access to α-trifluoromethylated esters under mild conditions
starting from readily available and bench-stable starting
materials, we envisaged that the judicious combination of
NHC catalysis and organofluorine chemistry will open new
perspectives. Although NHC catalysis appears nowadays as a
powerful synthetic tool to offer efficient and selective
Received: May 10, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01482
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
transformations,13 the use of an electrophilic trifluoromethyla-
tion reagent in such transformations remains unexplored.14
Herein we report the NHC-catalyzed trifluoromethylation
reaction of α-chloro aldehyde derivatives to access α-
trifluoromethyl ester derivatives. At the outset of this study,
the trifluoromethylation of 2-chloro-3-phenylpropanal (1a) as a
model substrate was investigated. At first, Togni’s reagent (A)
was selected as the electrophilic CF3 source, methanol as the
nucleophile, and DIPEA as the base (Table 1). When 1a was
(Table 1, entries 4−8). Then other NHC precatalysts (Table 1,
entries 10 and 11) and electrophilic reagents (Table 1, entries
12 and 13) were evaluated. In the case of B, low conversion was
obtained, while C gave 2a with a slight increase in the reaction
yield to 67%.
Finally, an increase in the reaction temperature to 0 °C along
with a shorter reaction time and the use of A as a reagent gave
the best reaction conditions, leading to 2a in 70% isolated yield
(Table 1, entry 15). Importantly, in the absence of the NHC
catalyst, no product was detected, demonstrating its key role in
the process (Table 1, entry 16). This result precludes an
enolate formation/nucleophilic attack of Togni’s reagent
sequence.
a
Table 1. Optimization of the Reaction Conditions
With these optimized conditions in hand, the scope of the
transformation was investigated, and a panel of esters 2a−y was
synthesized (Scheme 2). We first studied the nature of the
nucleophile. Ethanol, isopropanol, and cyclohexanol turned out
to be compatible, leading to the corresponding esters 2b−d,
albeit in lower yields in the case of iPrOH and CyOH.
Therefore, methanol was selected to pursue the study. It should
be noted that alternatively 2-bromo-3-phenylpropanal might be
used instead of 1a, affording 2a in 45% yield. Then, α-chloro
hydrocinnamaldehyde derivatives bearing aromatic rings
substituted with an electron-donating (OMe, 2e) or electron-
withdrawing group (CF3, 2f) as well as a halogen (Br, 2g) were
functionalized in good yields. The developed reaction was not
restricted to this class of aldehydes. Indeed, homobenzylic-
substituted aldehydes with a phenyl ring (2i) and even a
heterocyclic compound (2j) were trifluoromethylated in good
yields. Moreover, aliphatic α-chloro aldehydes were also
suitable substrates, as compounds 2k−x were obtained in
moderate to high yields. Simple alkyl-substituted aldehyde 1k
was efficiently functionalized in good yield, and the reaction
was successfully scaled up, offering access to 2k in 73% yield on
a 5 mmol scale. Protected alcohols derived from the 2-chloro-3-
hydroxypropanal, 1l and 1m, were suitable substrates.
Furthermore, aliphatic α-chloro aldehydes functionalized at
the γ-position using allyl and benzyl ethers (1n and 1o), ester
(1p), and silyl (1q) groups are well-tolerated under the present
reaction conditions, affording the corresponding α-trifluorome-
thylated esters 2n−q in good yields, thus demonstrating the
versatility of the present reaction. The synthetic utility of this
transformation was further demonstrated through the function-
alization of aliphatic aldehydes 1r−w bearing various functional
groups such as a protected amine, a chlorine atom, cyano and
azide groups, and alkyne and alkene moieties. This excellent
functional group tolerance offered perspectives for postfunc-
tionalization reactions by using these highly valuable trifluor-
omethylated building blocks to build up more complex
molecules. Moreover, the trifluoromethylation of compounds
of interest such as the oleic and stearic acids 2w and 2x was
efficiently achieved. Finally, with this approach, chlorambucil,
an anticancer drug used in the treatment of leukemia, was
trifluoromethylated in an efficient way, since the α-chloro
aldehyde bearing a substituted 4-aminoaryl group at the γ-
position furnished the desired product 2y in 68% yield.
On the basis of the literature data,15 the mechanism shown in
Scheme 3 has been proposed to explain the reaction outcome.
The reaction proceeds via the generation of the nucleophilic
Breslow intermediate I16 resulting from the reaction of the
NHC catalyst with the α-chloro aldehyde 1 in the presence of
DBN. Then, under basic conditions, the dehydrohalogenation
of I leads to the corresponding NHC-bound enolate
b
+
entry NHC catalyst CF3 reagent
base
solvent
yield (%)
1
2
NHC I
NHC I
NHC I
NHC I
NHC I
NHC I
NHC I
NHC I
NHC I
NHC II
NHC III
NHC I
NHC I
NHC I
NHC I
none
A
A
A
A
A
A
A
A
A
A
A
B
C
A
A
A
DIPEA
DIPEA
DIPEA
DIPEA
K2CO3
KHMDS
DMAP
DBU
toluene
THF
21
23
47
50
32
24
6
3
DCE
4
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
5
6
7
8
54
62
6
9
DBN
10
11
12
13
14
15
16
DBN
DBN
5
DBN
2
DBN
67
c
DBN
69
d
e
DBN
72 (70 )
DBN
−
a
Reaction conditions: 1a (0.2 mmol), CF3+ reagent (1.5 equiv), NHC
catalyst (20 mol %), base (2 equiv), MeOH/solvent (1:2, 0.2 M), −20
°C, 14 h, argon. Abbreviations: DIPEA, N,N-diisopropylethylamine;
DBU, 1,8-diazabicyclo[5.4.0]undec-7-ene; DBN, 1,5-
diazabicyclo[4.3.0]non-5-ene; DMAP, 4-(dimethylamino)pyridine.
+
NHC catalysts and CF3 reagents:
b
Yields were determined by 19F NMR analysis using α,α,α-
trifluorotoluene as an internal standard. The reaction was carried
out at 0 °C. The reaction was carried out at 0 °C for 2 h. Isolated
c
d
e
yield.
reacted with A using the carbene generated from NHC I in a
1:2 MeOH/toluene mixture, the corresponding ester 2a was
observed in 21% yield (Table 1, entry 1). With this encouraging
result in hand, a screening of various cosolvents (Table 1,
entries 2−4) revealed that CH2Cl2 was the best one, furnishing
the expected compound in 50% yield (Table 1, entry 4). It is
noteworthy that this catalytic system selectively afforded the
monotrifluoromethylated product, as no α,α-difunctionalized
product was detected. Various bases were then investigated to
showcase their importance in the outcome of the trans-
formation (Table 1, entries 4−9). It turned out that DBN was
the most efficient one, giving 2a in 62% yield (Table 1, entry
9), while other organic or inorganic bases led to lower yields
B
DOI: 10.1021/acs.orglett.8b01482
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 2. Substrate Scope
a
Reaction conditions: 1 (0.5 mmol), Togni reagent A (1.5 equiv), NHC I (20 mol %), DBN (2 equiv), MeOH/CH2Cl2 (1:2; 0.2 M), 0 °C, 2 h,
b
c
argon. Isolated yields are given. The reaction was performed using 2-bromo-3-phenylpropanal instead of 1a. EtOH was used instead of MeOH.
d
e
f
iPrOH was used instead of MeOH. CyOH was used instead of MeOH. The reaction was performed on a 5 mmol scale.
Scheme 3. Proposed Catalytic Cycle
Scheme 4. Postfunctionalization Reactions
Finally, we were interested in the development of a catalytic
enantioselective trifluoromethylation reaction to access enan-
tioenriched α-trifluoromethyl esters (Scheme 5). After
extensive investigations, we found that the chiral catalyst
NHC IV gave the best results in terms of enantioselectivity.
In addition, during the reaction optimization, the base was
carefully chosen to avoid any racemization within the course of
the reaction, and DIPEA was the optimum one. Under the
optimized conditions, α-chloro aldehydes 1a, 1h, and 1r were
readily converted into the corresponding enantioenriched
esters in moderate yields (34−57%) with good ee values (up
to 81%). This method represents a unique NHC-catalyzed
asymmetric reaction to access enantioenriched tertiary α-
trifluoromethylated esters.
In conclusion, we have developed a straightforward NHC-
catalyzed trifluoromethylation of α-chloro aldehydes to access
α-trifluoromethylated esters. The use of an electrophilic
trifluoromethylation reagent as a reaction partner in a NHC-
catalyzed transformation enabled the functionalization of a
panel of α-chloro aldehydes, including complex molecules, in
intermediate II, which reacts with Togni’s reagent A as the
electrophilic CF3 source to provide species III. Finally,
nucleophilic attack at the acylazolium by methanol affords the
desired α-trifluoromethylated product 2 along with regener-
ation of the carbene catalyst.
To demonstrate the synthetic utility of the present
transformation, ester 2k was readily converted into other
functional groups. Under acidic conditions, the ester was
smoothly converted into the corresponding carboxylic acid 3 in
62% yield. Moreover, alcohol 4 was also obtained in 91% yield
after reduction using LiAlH4 (Scheme 4).
C
DOI: 10.1021/acs.orglett.8b01482
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Organic Letters
Letter
a
Scheme 5. Studies of an Asymmetric Version
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01482.
Procedures, characterization data, and 1H, 13C, and
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: tatiana.besset@insa-rouen.fr.
*E-mail: thomas.poisson@insa-rouen.fr.
ORCID
1993, 115, 2156. (g) Wozniak, Ł.; Murphy, J. J.; Melchiorre, P. J. Am.
́
Akkattu T. Biju: 0000-0002-0645-8261
Thomas Poisson: 0000-0002-0503-9620
Tatiana Besset: 0000-0003-4877-5270
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The authors declare no competing financial interest.
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Generous financial support by the Indo-French Centre for the
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D
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