Lewis Acid Catalyzed Synthesis of α-Trifluoromethyl Esters and Lactones by Electrophilic Trifluoromethylation

Article abstract of DOI:10.1021/acs.orglett.5b03088

An electrophilic trifluoromethylation of ketene silyl acetals (KSAs) by hypervalent iodine reagents 1 and 2 has been developed. The reaction proceeds under very mild conditions in the presence of a catalytic amount of trimethylsilyl bis(trifluoromethanesulfonyl)imide (up to 2.5 mol %) as a Lewis acid providing a direct access to a variety of secondary, tertiary, and quaternary α-trifluoromethyl esters and lactones in high yield (up to 98%).

Full text of DOI:10.1021/acs.orglett.5b03088

Letter
pubs.acs.org/OrgLett
Lewis Acid Catalyzed Synthesis of α‑Trifluoromethyl Esters and
Lactones by Electrophilic Trifluoromethylation
Dmitry Katayev, Vaclav Matousek, Raffael Koller, and Antonio Togni*
́
̌
Department of Chemistry and Applied Biosciences, Swiss Federal Institute of Technology, ETH Zurich, Vladimir-Prelog-Weg 2,
CH-8093 Zurich, Switzerland
S
* Supporting Information
ABSTRACT: An electrophilic trifluoromethylation of ketene
silyl acetals (KSAs) by hypervalent iodine reagents 1 and 2 has
been developed. The reaction proceeds under very mild
conditions in the presence of a catalytic amount of
trimethylsilyl bis(trifluoromethanesulfonyl)imide (up to 2.5
mol %) as a Lewis acid providing a direct access to a variety of
secondary, tertiary, and quaternary α-trifluoromethyl esters
and lactones in high yield (up to 98%).
he introduction of a trifluoromethyl group at the enolizable
position of carbonyl compounds significantly alters their
synthesis of α-CF3-β-keto esters under phase-transfer cataly-
sis10c,14 and its enantioselective version,15 the related copper-
catalyzed synthesis of α-CF3-α-nitro esters,14a,c,16 and the
recently developed synthesis of α-CF3 compounds from silyl
enol ethers13a,14a,b,16a and silyl ketene imines17 under copper and
vanadium catalysis, respectively. The advantages of using
reagents 1 and/or 2 in organic synthesis over other CF3 sources
is that they are easily accessible, are stable crystalline solids, and
can be handled in air.10a Therefore, the development of new
methods for the generation of trifluoromethylated all-carbon
quaternary centers using hypervalent iodine-CF3 reagents 1 and
2 is essential.
For the aforementioned reasons, we targeted the synthesis of
α-trifluoromethylated derivatives of carboxylic acids. Preliminary
experiments demonstrated that the in situ generated lithium
enolate of butyl isobutyrate provided only trace amounts of the
desired product when reacted with reagent 1 or 2. Subsequently,
we turned our attention to ketene silyl acetals (KSAs), which
have been broadly investigated as nucleophiles for the formation
of α-carbonyl quaternary carbon centers.18 Their trifluorome-
thylation with CF3I has previously been reported; however, a
large excess of CF3I had to be used and only a narrow substrate
scope and moderate yields were reported.11g,12a We hypothe-
sized that KSAs might be suitable to react with hypervalent
iodine reagent 1 or 2 in the absence of any metal catalysts and
strong bases. Herein, we report an operationally simple and
highly efficient method for the α-trifluoromethylation of
substituted ester- and lactone-derived ketene silyl acetals by
reagents 1 and 2 in the presence of catalytic amounts of
trimethylsilyl triflimide (TMSNTf2) (1−2.5 mol %) or under
catalyst-free conditions.
T
physicochemical properties.1 For example quaternary α-trifluor-
omethylated amino acids differ from their natural counterparts in
a variety of parameters, including lower pKa values of the carboxyl
and amino moieties,2 higher lipophilicity,2b,3 resistance toward
enzymatic proteolysis,4 distinct conformational behavior,5 and a
weaker propensity to accept hydrogen bonds.6 In addition, α-
trifluoromethylated carbonyl compounds can serve as versatile
precursors for the synthesis of α-trifluoromethylated carboxylic
acids, α-trifluoromethylated alcohols and amines, etc.7
Currently, there is a relatively limited number of methods to
construct a trifluoromethylated quaternary carbon adjacent to a
carbonyl center.8 To this end, there are generally two
approaches. The classical approach relies on a radical
trifluoromethylation of nucleophilic metal enolates or enolate
surrogates such as enamines or silyl enol ethers with
trifluoromethyl iodide. Conceptually newer methods employ
electrophilic trifluoromethylating reagents, in particular trifluor-
omethyl diarylchalcogenium salts8f,9 and hypervalent iodine-
(III)-CF3 reagents 1 and 2, which were introduced by our group
in 2006 (Figure 1).10
Figure 1. Hypervalent iodine-based reagents 1 and 2.
The introduction of the trifluoromethyl unit through enolate
derivatives using gaseous trifluoromethyl iodide was, for instance,
promoted by organoboronic esters11 or initiated by visible-light
photocatalysts12 or by transition-metal catalysts.13 However,
methods for the formation of a quaternary carbon center by
hypervalent iodine-CF3 reagents 1 and 2 are still very scarce. To
date, the only reported transformations of this type are the
In order to examine the feasibility of such a transformation, we
chose KSA 3a as a model substrate for the reaction with reagent 1
Received: October 25, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03088
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Organic Letters
Letter
a
Table 1. Development of the Catalytic Trifluoromethylation
of 3a
Scheme 1. Trifluoromethylation of KSAs: Ester Scope
a
b
entry
reagent
additive (mol %)
T [°C]
rt
4a [%]
1
1 or 2
−
<5
22
2
1
2
2
2
2
2
2
2
2
2
2
2
2
Zn(NTf2)2 (10)
Zn(NTf2)2 (10)
Zn(NTf2)2 (10)
Zn(NTf2)2 (10)
Zn(NTf2)2 (10)
Zn(NTf2)2 (2.5)
HNTf2 (2.5)
rt
3
rt
39
4
0
58
5
−20
60
6
−78 to rt
−78 to rt
−78 to rt
−78 to rt
−78 to rt
−78 to rt
−78 to rt
−78 to rt
−78 to rt
66
7
75
8
69
9
LiNTf2 (2.5)
47
10
11
12
AgNTf2 (2.5)
CuNTf2 (2.5)
TMSNTf2 (2.5)
TMSNTf2 (2.5)
TMSNTf2 (2.5)
65
52
85
a
c
d
13
c
96(94)
54
Reaction conditions: 3 (1.5 mmol, 0.05 M in CH2Cl2), 2 (1.0 mmol),
TMSNTf2 (1.0 mol %), −78 °C to rt, Ar atmosphere, 19 h. Yields of
isolated products are given.
14
a
Reaction conditions: 3a (0.3 mmol, 0.3 M in CH2Cl2), 1 or 2 (0.2
mmol), additive (0.5−10 mol %), −78 °C to rt, Ar atmosphere, 19 h.
b
Yields were determined by 19F NMR using benzotrifluoride as an
to slightly decreased yields, presumably due to the increased
steric bulk at the reactive carbon center. Furthermore, the
presence of a terminal alkene substituent in KSA 3j was also well
tolerated under the reaction conditions and the corresponding α-
trifluoromethylated ester was formed in 75% yield. The fact that
the terminal olefin remained intact during the reaction is a key
benefit of our approach over previously developed trifluor-
omethylation methods of carbonyl compounds.
c
d
internal standard. 0.05 M in CH2Cl2. Yield of isolated product.
and 2 (Table 1). The direct trifluoromethylation of 3a using
either 1 or 2 in CH2Cl2 at room temperature gave only trace
amounts of the product (Table 1, entry 1). In this case an excess
of KSA (1.5 equiv) was used. The addition of catalytic amounts
of Zn(NTf2)2 (10 mol %), a known activator of reagent 1,17,19 to
the reaction mixture led to modest yields of 4a. Reagent 2
demonstrated slightly better reactivity, and 4a was formed in 39%
yield compared to the corresponding reaction with 1, where only
22% was observed (Table 1, entries 2−3). Therefore, subsequent
reactions were conducted using reagent 2. Reducing the catalyst
loading to 2.5 mol % and the reaction temperature to −78 °C
improved the yield of 4a to 75% (Table 1, entries 4−7). To
further enhance the reactivity of reagent 2, we tested various
triflimides as additives including HNTf2, AgNTf2, LiNTf2, and
Cu(NTf2)2 (Table 1, entries 8−12). Among them, trimethylsilyl
triflimide proved to be a particularly efficient additive and
increased the yield of 4a to 85% (Table 1, entry 12). Dilution of
the reaction mixture improved the reactivity, and after 19 h the
desired product was obtained in 94% isolated yield (Table 1,
entry 13). Reducing the catalyst loading to 0.5 mol % led to lower
conversion of the starting material (Table 1, entry 14). Further
screening of solvents and variation of the molar ratio of 3a and 2
did not significantly improve the yield (see the Supporting
Information).
We next explored the reaction scope of other carbonyl
compounds and focused on KSAs derived from lactones
(Scheme 2). In contrast to ester-derived KSAs, their synthesis
a
Scheme 2. Trifluoromethylation of KSAs: Lactone Scope
With this trifluoromethylation protocol in hand, we next
examined the reaction scope with various substituted KSAs,
which were easily prepared by the lithiation of the corresponding
esters followed by trapping with TMSCl. The results are
summarized in Scheme 1. KSAs bearing different alkoxy groups
such as octyloxy- (3b), ethyloxy- (3f), phenyloxy- (3h), and 2-
phenylethoxy- (4i) demonstrated excellent reactivity toward
reagent 2 under the optimized reaction conditions giving the
corresponding quaternary α-CF3-substituted esters in high
yields. Equally, KSAs bearing one or no alkyl substituents at
the carbon atom in α-position gave the corresponding products
in excellent yields (4b and 4c). Replacement of the methyl
groups at the α-carbon atom of KSA 3a by larger substituents led
a
Reaction conditions: 5 (1.3 mmol, 1.5 M in CH2Cl2), 2 (1.0 mmol),
TMSNTf2 (1.0 mol %), −78 °C to rt, Ar atmosphere, 19 h. Yields of
b
c
isolated products are given. 94% 19F NMR yield. In the absence of
TMSNTf2.
B
DOI: 10.1021/acs.orglett.5b03088
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Letter
was more straightforward, as the isolated crude products were
sufficiently pure to be used directly for the subsequent
trifluoromethylation step (see the Supporting Information).
Preliminary experiments using KSA 5a as a model substrate
showed that its trifluoromethylation with reagent 2 takes place
even without a catalyst providing the corresponding α-
trifluoromethylated lactone 6a in 72% yield. However, only a
few other lactone-derived KSAs gave satisfactory trifluorome-
thylation without TMSNTf2 catalysis (5a, 5c, 5d, 5l, 5o). We
assume that a high level of trifluoromethylation efficiency
achieved for KSA 5a under catalyst-free reaction conditions is the
result of a combination of the low steric requirements of the
substrate and its high electron density. Addition of a catalytic
amount of TMSNTf2 (1 mol %) to the reaction mixture
significantly improved the reactivity as shown in Scheme 2.
Furthermore, reducing the amount of lactone-derived KSAs from
1.5 to 1.3 equiv did not significantly affect the yield of products.
KSAs derived from γ-butyrolactone with a variety of α-alkyl
substituents such as benzyl (5c), cyclohexyl (5d), ethyl (5g),
isopropyl (5h), tert-butyl (5i), and cyclopropyl (5j) showed
excellent reactivity delivering the corresponding α-trifluorome-
thylated lactones in good to high yields. The reaction tolerates
steric bulk in the α-position, and 5d and 5i reacted with 2 to give
6d and 6i in high yields. In the case of α-phenyl substituted KSA
5b, the low yield of the corresponding product might be
explained by the depleted electron density of the double bond
due to conjugation. KSAs with α-alkyl substituents bearing a
terminal olefin, 5e and 5f, were also tolerated under the
developed reaction conditions. Finally, various α-trifluoromethy-
lated six- (6k−l, 6n−o) and seven-membered (6m) lactones can
be accessed in excellent yields by this approach demonstrating its
broad applicability.
Scheme 3. Plausible Catalytic Cycle for the
Trifluoromethylation of KSAs by Reagent 1
DET: dissociative electron transfer. SET: single electron transfer.
a
SET by the substrate (KSA) in the beginning of the catalytic cycle.
transformations, often observed with electron-rich and sterically
nondemanding lactone-derived KSAs, we assume that the
reaction takes place via initial generation of a charge-transfer
complex (CTC) between the substrate and reagent 2. The
following transfer of a silyl group leads to the formation of a
species similar to A, which further liberates CF3 radicals.
To illustrate the practical utility of this protocol, we have
performed the synthesis of quaternary α-CF3 lactone 6l from 10
mmol of the corresponding ketene silyl acetal 5l in 96% yield.
Lactones are versatile precursors in organic synthesis,21 and
hence product 6l was subjected to different transformations
(Scheme 4). Refluxing 6l in a solution of isopropanol and thionyl
Further investigations were performed to obtain insight into
the mechanism of the trifluoromethylation of KSAs. The
presence of an excess of radical acceptors, such as 2,2,6,6-
tetramethylpiperidine-N-oxyl (TEMPO) or styrene in the
reaction mixture of 3b, completely suppressed product formation
(see the Supporting Information). These findings suggest that
radical species are likely reactive intermediates in the trans-
formation.
a
Scheme 4. Follow-up Functionalizations of 6l
Based on the above results, we propose the following
mechanism for this trifluoromethylation reaction (Scheme 3).
In the case of the Lewis acid catalyzed transformation we assume
that TMSNTf2 activates reagent 2 to form intermediate A. In the
presence of the KSA substrate, intermediate A participates
further in a single electron transfer (SET) to give a neutral radical
species B. This highly reactive intermediate, upon thermally
induced internal dissociative electron transfer (DET), leads to
the formation of CF3 radicals. The feasibility of such mechanistic
pathways has been recently demonstrated in our group by ab
initio molecular dynamics simulations, in particular metady-
namics.20 The liberation of a CF3 radical species accounts for an
activation barrier of as little as 2.8 kcal mol−1, starting from the
protonated form of reagent 2. In the next step of the catalytic
cycle, the electrophilic trifluoromethyl radical rapidly reacts with
the substrate to give intermediate C, which subsequently takes
part in a second SET with intermediate A, restoring the active
radical species B and forming the silyloxycarbenium species D.
The latter will rapidly undergo desilylation in the presence of the
a
Reaction conditions: (a) iPrOH, SOCl2, reflux, 24 h; (b) 2-
chloroaniline, DCE, AlCl3, 0 °C to rt, 4.5 h; (c) THF, LiAlH4, 0
°C, 2 h.
chloride yielded quaternary α-CF3 ester 7 in almost quantitative
98% yield. The reaction with an amine, such as 2-chloroaniline, in
the presence of aluminum chloride gave α-CF3 amide 8 in 89%
yield. Reduction of lactone 6l using LiAlH4 produced β-CF3
alcohol 9 again in nearly quantitative yield. These simple
functionalizations of lactone 6l give access to more complex
molecules containing quaternary substituted centers next to an
ester, amide, or alcohol functional group, scaffolds with potential
applications in medicinal chemistry22 and agrochemistry.23
In conclusion, we have developed a simple and efficient
protocol for the trifluoromethylation of ketene silyl acetals by
¯
triflimide anion (NTf2 ) to form the corresponding trifluor-
omethylated product and regenerate the active catalyst
TMSNTf2. Alternatively, intermediate D can directly silylate
another molecule of reagent 2. In the case of noncatalyzed
C
DOI: 10.1021/acs.orglett.5b03088
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Cvengros,
̌
J.; Santschi, N.; Battaglia, P.; Togni, A. Tetrahedron 2010, 66,
hypervalent iodine reagents, which proceeds under Lewis acid
catalysis leading to the formation of various quaternary α-
trifluoromethylated esters and lactones in good to excellent
yields. We have also demonstrated that the reaction can be
conducted without a catalyst for lactone-derived KSAs. The
reaction could be realized on gram scale without a decrease in
yield. The synthetic utility of the method was demonstrated by
the transformation of an α-trifluoromethylated lactone into
synthetically useful organofluorine building blocks such as β-CF3
alcohols and α-CF3 esters and amides which are difficult to
synthesize by other methods. This reaction concept opens up a
wealth of opportunities for the development of new efficient
transformations using hypervalent iodine reagents 1 and 2.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b03088.
(14) (a) Kieltsch, I.; Eisenberger, P.; Stanek, K.; Togni, A. Chimia
2008, 62, 260. (b) Eisenberger, P. Diss. ETH No. 17371, Zurich, 2007.
(c) Kieltsch, I.; Eisenberger, P.; Togni, A. Angew. Chem., Int. Ed. 2007,
Experimental procedures and detailed characterization
data of all new compounds (PDF)
̈
46, 754.
(15) Deng, Q.-H.; Wadepohl, H.; Gade, L. H. J. Am. Chem. Soc. 2012,
134, 10769.
AUTHOR INFORMATION
Corresponding Author
■
(16) (a) Koller, R. Diss. ETH No, 192919, Zurich, 2010. (b) Kieltsch, I.
̈
*E-mail: atogni@ethz.ch.
Notes
Diss. ETH No. 17990, Zurich, 2008.
̈
(17) Fruh, N.; Togni, A. Angew. Chem., Int. Ed. 2014, 53, 10813.
̈
(18) For representative publications, see: (a) Inamoto, Y.; Kaga, Y.;
Nishimoto, Y.; Yasuda, M.; Baba, A. Chem. - Eur. J. 2014, 20, 11664.
(b) Ran, R.-Q.; He, J.; Xiu, S.-D.; Wang, K.-B.; Li, C.-Y. Org. Lett. 2014,
16, 3704. (c) Yanai, H.; Ishii, N.; Matsumoto, T.; Taguchi, T. Asian J.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by ETH Zurich. D.K. thanks the Swiss
National Science Foundation (SNSF) for a fellowship.
■
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