AgF-mediated fluorinative cross-coupling of two olefins: Facile access to α-CF3 alkenes and β-CF3 ketones

Article abstract of DOI:10.1002/anie.201409705

A AgF-mediated fluorination with a concomitant cross-coupling between a gem-difluoroolefin and a non-fluorinated olefin is reported. This highly efficient method provides facile access to both α-CF₃ alkenes and β-CF₃ ketones, which otherwise remain challenging to be directly prepared. The application of this method is further demonstrated by the synthesis of bioactive isoxazoline derivatives. This approach represents a conceptually novel route to trifluoromethylated compounds that combines the in situ generation of the CF₃ moiety and a C-H functionalization in a single reaction system.

Full text of DOI:10.1002/anie.201409705

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DOI: 10.1002/anie.201409705
Fluorination
AgF-Mediated Fluorinative Cross-Coupling of Two Olefins: Facile
Access to a-CF3 Alkenes and b-CF3 Ketones**
Bing Gao, Yanchuan Zhao, and Jinbo Hu*
Abstract: A AgF-mediated fluorination with a concomitant
cross-coupling between a gem-difluoroolefin and a non-fluo-
rinated olefin is reported. This highly efficient method provides
facile access to both a-CF3 alkenes and b-CF3 ketones, which
otherwise remain challenging to be directly prepared. The
application of this method is further demonstrated by the
synthesis of bioactive isoxazoline derivatives. This approach
represents a conceptually novel route to trifluoromethylated
compounds that combines the in situ generation of the CF3
difluorocarbene with the fluoride anion and CuI to form the
“CuCF3” intermediate and the copper-mediated coupling
with electrophiles are compatible. Compared to direct
trifluoromethylation, this in situ strategy has advantages for
the preparation of [18F]CF3 compounds for PET imaging
because of its simple manipulation procedure and excellent
efficacy.[5d] Nevertheless, novel methods for the synthesis of
trifluoromethylated compounds that are based on similar
concepts have rarely been reported since. One major
challenge is the in situ generation of the CF3 motif as nearly
all CF3 moieties currently used are originally obtained by
fluorination with harsh reaction conditions.[6]
À
moiety and a C H functionalization in a single reaction
system.
T
he trifluoromethyl (CF3) group often significantly alters the
We envisioned that the gem-difluorovinyl group would be
an attractive alternative CF3 precursor. The difluoromethy-
lene carbon atom is electrophilic because of the inductive
effect of the fluorine atoms and electron repulsion between
the double bond and the fluorine atoms, which facilitates the
regioselective nucleophilic fluorination of the gem-difluoro-
methylene carbon atom to afford a CF3 group.[1a,7] In the past,
this a-CF3 carbanion formed from direct fluoride addition was
found to be unstable and would be spontaneously quenched
by proton abstraction, making its further functionalization
difficult.[7] We have recently found that b,b-difluorostyrene
derivatives reacted with AgF to give the fluorinated homo-
coupling products.[8] Preliminary mechanistic studies implied
that an a-CF3-benzylsilver intermediate, rather than the
a-CF3 carbanion, might be generated although its direct
observation failed. Inspired by this result, we settled to
explore the concomitant fluorination and intermolecular
cross-coupling of a gem-difluoroolefin and another reactant
as an alternative route to construct more sophisticated CF3-
containing compounds. In this regard, alkenes could be an
appealing choice owing to their ready availability and
versatile synthetic utility.[9,10] Herein, we disclose our results
on using the gem-difluorovinyl group as a CF3 precursor,
which is fluorinated to give an a-trifluoromethylated inter-
acidity, lipophilicity, metabolic stability, and conformation of
a molecule; molecules with a trifluoromethyl group are
therefore valuable compounds in pharmaceutical chemistry,
agrochemistry, and material science.[1] Accordingly, the incor-
poration of CF3 group(s) into a target molecule is highly
desirable owing to the nearly complete absence of fluorine in
naturally occurring organic molecules.[2] To date, research
efforts have mainly focused on the direct trifluoromethylation
(especially transition-metal-mediated coupling reactions) of
prefunctionalized compounds with a set of nucleophilic,
electrophilic, or radical trifluoromethyl sources.[3] An alter-
native route to trifluoromethylated compounds entails the use
of CF3-containing building blocks without the involvement of
direct C CF3 bond formation.[4] In both cases, the CF3 groups
À
are already present in the starting materials, and chemists
then have to tailor these substrates into the desired com-
pounds. However, there have been rare examples of combin-
ing the in situ construction of the CF3 motif and the
À
functionalization of target molecules by C C bond formation
in a single reaction system. One pioneering work that matches
this criterion was reported by Chen and co-workers, who have
achieved the copper-mediated trifluoromethylation of aryl
halides
with
methyl
fluorosulfonyldifluoroacetate
(FSO2CF2CO2Me).[5a–c] In their system, the reaction of the
mediate; subsequent intermolecular alkenyl C H functional-
À
ization affords a-CF3 alkenes and b-CF3 ketones (Scheme 1).
At the onset, 2-(2,2-difluorovinyl)naphthalene (1a) was
chosen as a model substrate to study its reaction with 1,1-
diphenylethylene (2a). When the reaction was conducted in
pyridine (2.0 mL) at 808C for six hours [1a (0.5 mmol), 2a
(3.0 equiv), AgF(3.0 equiv)], the fluorination and homo-
coupling process of 1a was so rapid that no cross-coupling
products were detected. The solvents were then carefully
screened to circumvent the undesired homo-coupling. For-
tunately, a-CF3 alkene 3aa was formed in 41% (as deter-
mined by 19F NMR spectroscopy), while 49% of 1a did not
react, when using 1-methyl-2-pyrrolidinone (NMP) as the
solvent under similar reaction conditions. Further screening
[*] Dr. B. Gao, Dr. Y. Zhao, Prof. Dr. J. Hu
Key Laboratory of Organofluorine Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
345 Ling-Ling Road, Shanghai, 200032 (China)
E-mail: jinbohu@sioc.ac.cn
[**] Support of our work by the National Basic Research Program of
China (2015CB931900 and 2012CB215500), the NNSFC (21421002,
21372246, and 21302206), the Shanghai QMX program
(13QH1402400), and the Chinese Academy of Sciences is gratefully
acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201409705.
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Table 1: Variation of the gem-difluoroolefin for the preparation of a-CF3
alkenes.[a]
À
Scheme 1. Combining the formation of a CF3 moiety and a C H
functionalization process into a single reaction system.
of the additives, the ratio of the reactions, and the reaction
time resulted in the optimized reaction conditions: 1a
(0.5 mmol), 2a (3.0 equiv), and AgF (3.0 equiv) were heated
at 808C in NMP for 48 hours under N2 atmosphere. Thus
product 3aa was obtained in 74% yield (82% 19F NMR yield)
while reagent 1a was fully consumed (Scheme 2).
[a] Reaction conditions (unless otherwise specified): 1 (0.5 mmol), 2a
(1.5 mmol), AgF (1.5 mmol), NMP (2.0 mL), 808C, 48 h. [b] Reaction
conditions B: 1 (0.5 mmol), 2a (1.5 mmol), AgF (1.5 mmol), AgBF4
(0.25 mmol), NMP (2.0 mL), 808C, 8 h. Tf=trifluoromethanesulfonyl,
Ts =para-toluenesulfonyl.
Scheme 2. Optimized reaction conditions for the preparation of a-CF3
alkenes. [a] Yield determined by 19F NMR spectroscopy: 82%.
Table 2: Variation of the non-fluorinated alkene for the preparation of
a-CF3 alkenes.[a]
As we have recently disclosed an efficient synthetic access
to gem-difluoroolefins,[11] a series of b,b-difluorostyrene
derivatives were subjected to the optimized reaction condi-
tions to undergo the cascade reaction. As shown in Table 1,
this method is compatible with a wide range of functional
groups, including ester, aldehyde, cyano, sulfoxide, sulfone,
and even nitro moieties. Moreover, functional groups that are
commonly employed in conventional cross-coupling reac-
tions, such as OTf (3ja), OTs (3ca, 3pa), and halogen
moieties (Cl, Br, I; 3da, 3ea, 3ma, 3oa), were also tolerated;
the corresponding products should thus be easily modifiable
by further transformations.[12] Notably, substrates with elec-
tron-withdrawing groups on the aromatic rings were generally
more reactive towards AgF, and therefore, the reaction
reached completion within eight hours. Meanwhile, using an
additional 0.5 equivalents of AgBF4 was beneficial to sup-
press the hydrofluorination of electron-deficient gem-
difluoroolefins (1b, 1d–1g, 1i, 1j, 1m–1o, 1q). Naphthalene
derivative 1l was also smoothly converted into product 3la in
65% yield, indicating the little impact of steric hindrance on
the reactivity. It should be mentioned that the fluorination/
homo-coupling process of the gem-difluoroolefins was mini-
mized to < 3% in all cases.
[a] Reaction conditions: 1c (0.5 mmol), 2 (1.5 mmol), AgF (1.5 mmol),
NMP (2.0 mL), 808C, 48 h. The absolute configurations were not
determined.
impact on the outcome of the transformations. For instance,
substrate 2c, bearing a proximal methyl group, provided 3cc
as the Z/E mixture in relatively low yield. Compared to 2e,
which features an electron-deficient p-CF3 group, 2d, with an
electron-rich p-OMe group, was more reactive and gave the
corresponding product in higher yield. A substrate with
a heteroaryl moiety (2h) and a-methylstyrene could also be
converted, albeit less effectively. Monosubstituted alkene 2i
also gave a low yield probably because of its poor reactivity.
Owing to their synthetic diversity and feasibility, carbon-
yl-based synthons with a CF3 substituent have emerged as
important intermediates in chemical synthesis.[13d] In recent
As most of the b,b-difluorostyrene derivatives gave
satisfactory results, we also varied the non-fluorinated
reaction partners 2 (Table 2). Generally, diaryl ethylenes
reacted well with para-tosyl b,b-difluorostyrene 1c, giving the
corresponding products in moderate to good yields. Both the
steric and electronic properties of the aryl substituents had an
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Table 3: Substrate scope of the preparation of b-CF3 ketones.[a]
years, a range of radical, electrophilic, nucleophilic, and
organometallic approaches have been developed to incorpo-
rate the CF3 group into carbonyl compounds.[13,14] Whereas
most of these efforts succeeded in functionalizing the
carbonyl group itself or its a-position, there were only rare
advances in developing methods for the synthesis of b-CF3-
substituted carbonyl compounds.[15] The “direct 1,4-trifluoro-
methylation to conventional a,b-unsaturated ketones such as
chalcone is very tough”;[15b] but this is just one representative
challenge.
Encouraged by our aforementioned results on the syn-
thesis of a-CF3 alkenes, we sought to apply our method to the
preparation of b-CF3 ketones. In this regard, a one-pot
procedure was designed and features 1) a concomitant
fluorination/cross-coupling process of a-methoxystyrene
derivatives with gem-difluoroolefins to give a-CF3 methoxy-
alkenes and 2) the hydrolysis of the newly formed alkenes to
release the target b-CF3 ketones. To our delight, this method
proved to be viable, and the fluorinated cross-coupling
product of a-methoxystyrene 4 underwent rapid hydrolysis
under acidic conditions to afford the desired ketone 5
(Scheme 3).[16]
[a] Reaction conditions (unless otherwise specified): 1 (0.5 mmol), 4
(1.5 mmol), AgF (1.5 mmol), NMP (2.0 mL), 808C, 48 h; then HCl (2m,
2 mL), 608C, 1 h. [b] Reaction conditions B: 1 (0.5 mmol), 4 (1.5 mmol),
AgF (1.5 mmol), AgBF4 (0.25 mmol), NMP (2.0 mL), 808C, 8 h; then HCl
(2m, 2 mL), 608C, 1 h.
Scheme 3. Preparation of b-CF3 ketones through fluorination and
cross-coupling. Optimized reaction conditions: step 1: 1 (0.5 mmol), 4
(3.0 equiv), AgF (3.0 equiv), NMP (2.0 mL), 808C, 48 h; step 2: HCl
(2.0m, 2.0 mL), 608C, 1 h.
Selected examples of the b-CF3 ketone synthesis are
summarized in Table 3. This strategy was applicable to
a variety of gem-difluoroolefins and a-methoxystyrene deriv-
atives under the optimized reaction conditions, giving the
desired products in excellent overall yields, and tolerated
various functional groups. The hydrolysis process was suffi-
ciently mild to leave the ester, tosyl, sulfonyl, nitro, and cyano
groups intact. Electron-deficient a-methoxystyrene deriva-
tives afforded lower yields than electron-rich ones (4b–4d). It
is noteworthy that a heteroaryl substrate, namely 2-(1-
methoxyvinyl)pyridine (4g), could also be converted into
the corresponding ketone with satisfactory yield.
Structurally unique 3,5-diaryl-5-(trifluoromethyl)-2-isoxa-
zoline derivatives, for which more than 20000 variants with
similar skeletons have been reported, have been approved as
useful pest-control reagents. A1443 exhibits excellent anti-
parasitic activity against cat fleas and dog tick.[17] The
stereoselective synthesis of its key precursor 9 could be
readily realized by employing our method in combination
with Shibataꢀs asymmetric cyclization method.[17b] As shown
in Scheme 4, b-CF3 ketone 7 was prepared in 63% yield with
our method and subsequently transformed into intermediate
8 in 69% yield. Thereafter, 8 could be further converted into 9
with excellent yield and high ee. Heterocyclic analogues of
A1443 were also available from the corresponding b-CF3
ketones. For instance, dihydropyrazole 10 was obtained
Scheme 4. Synthesis of bioactive compounds. LDA=lithium diisopro-
pylamide.
(TEMPO = 2,2,6,6-tetramethyl-1-oxylpiperidine).[18] These
results further highlight the practical utility of our method.
To gain mechanistic insights into the present reaction, the
following experiments were performed (Scheme 5). When
3.0 equivalents of TEMPO, a radical scavenger, were added
to the reaction system, the cross-coupling between 1a and 2a
À
from product 5ba after TEMPO-mediated C H oxidation
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otherwise difficult to prepare in a one-pot process. This
reaction represents a conceptually novel route for the syn-
thesis of trifluoromethylated compounds that combines the
in situ formation of a trifluoromethyl moiety from a gem-
À
difluorovinyl group and silver-mediated alkenyl C H func-
tionalization. Compared with the direct trifluoromethylation
or alkylation using CF3-containing reagents, the current
strategy may find potential applications in specific areas, for
example, for the preparation of 18F-labelled CF3 compounds.
Further explorations on the basis of this strategy are under-
way in our laboratory.
Scheme 5. Mechanistic studies.
Received: October 2, 2014
Published online: November 12, 2014
was completely inhibited, and the radical trapping adduct 11
was obtained in 83% yield. Furthermore, when (1-cyclo-
propylvinyl)benzene was subjected to the standard reaction
conditions with 1b and 1c, the homoallylic trifluoromethy-
lated alkenes 13b and 13c, respectively, were isolated as the
major products. They were likely the result of the ring
opening of the cyclopropane motif followed by intramolec-
ular addition of the carbon radical to the phenyl ring.[19]
On the basis of these findings and our previous results,[8]
one plausible mechanistic pathway is proposed in Scheme 6.
The synergetic addition of AgF to the gem-difluoroolefin
Keywords: alkenes · fluorination · fluorine · ketones · silver
.
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