Silver-Catalyzed Ring-Opening Strategy for the Synthesis of β- and γ-Fluorinated Ketones

Article abstract of DOI:10.1021/jacs.5b00939

A regioselective synthesis of β- and γ-fluorinated ketones via silver-catalyzed ring opening is described. A variety of β- and γ-fluorinated ketones are efficiently prepared, respectively, from tertiary cyclopropanol and cyclobutanol precursors, providing a straightforward approach for the introduction of a fluorine atom into complex molecules. Preliminary mechanistic studies suggest that a radical-mediated sequential C-C bond cleavage and C-F bond formation pathway is involved.

Full text of DOI:10.1021/jacs.5b00939

Communication
pubs.acs.org/JACS
Silver-Catalyzed Ring-Opening Strategy for the Synthesis of β- and
γ‑Fluorinated Ketones
Huijun Zhao,†,§ Xuefeng Fan,†,§ Jiajia Yu,† and Chen Zhu*,†,‡
†Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science,
Soochow University, 199 Ren-Ai Road, Suzhou, Jiangsu 215123, People’s Republic of China
‡Key Laboratory of Synthetic Chemistry of Natural Substances, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 345 Lingling Road, Shanghai 200032, People’s Republic of China
S
* Supporting Information
Scheme 1. Synthesis of Fluorinated Ketones
ABSTRACT: A regioselective synthesis of β- and γ-
fluorinated ketones via silver-catalyzed ring opening is
described. A variety of β- and γ-fluorinated ketones are
efficiently prepared, respectively, from tertiary cyclo-
propanol and cyclobutanol precursors, providing a
straightforward approach for the introduction of a fluorine
atom into complex molecules. Preliminary mechanistic
studies suggest that a radical-mediated sequential C−C
bond cleavage and C−F bond formation pathway is
involved.
he introduction of a fluorine atom into molecules
Tfrequently renders remarkable changes in their physical,
set out to investigate the project by use of 1-phenylcyclobutanol
1a, which was readily prepared from the addition of aryl Grignard
chemical, and biological properties.1 Thus, the development of
mild and efficient fluorination methods is of considerable
significance in pharmaceuticals, agrochemicals, and material
sciences.2 Recently, great progress has been made in the field of
reagent to cyclobutanone. After thoroughly surveying reaction
conditions (fluorinating agent, solvent, concentration, etc.), we
found that the combination of silver salt and SelectFluor gave the
best fluorination yields at room temperature. In particular,13 (a)
AgBF4 exhibited a better catalytic ability than other silver salts,
such as AgNO3, AgF, and AgOTf; (b) Fluorinating agents other
than SelectFluor significantly decreased the chemical yields; and
(c) a concentrated biphasic solution (0.3 M in DCE/H2O 1:1)
facilitated the transformation.
With the optimized reaction conditions in hand, we turned to
evaluate the generality of this protocol. The transformation has
wide functional group tolerance; both electron-rich and
-deficient tertiary cyclobutanols were compatible, furnishing a
variety of γ-fluorinated ketones (Scheme 2). The model substrate
1a gave the corresponding product 2a in satisfactory isolated
yield. The linear phenylpropyl ketone without fluorination was
identified as the principal byproduct.
Substrates with electron-donating groups, regardless of the
arene substitution (o-, m-, p-), gave rise to the γ-fluoroketones
2b−2f in good yields. Acceptable yields of products 2g and 2h
were achieved when strong electron-withdrawing groups were
present, e.g., OCF3 and CF3. However, the transformation
generally required an increase of silver catalyst and fluorinating
agent, or prolonged reaction times. An unintelligible trend was
observed in the cases of haloaryl substrates 1i−1k; while 1i
readily led to product 2i in a few hours, the conversion of 1j and
2
fluorination of arenes to form Csp −F bonds via transition-metal
catalyzed transformations, in particular, cross-coupling3 and C−
H activation.4 In contrast, direct fluorination on alkyl groups to
3
generate Csp −F bonds is relatively less investigated. This type of
fluorination successfully occurs at special positions, for instance
benzylic fluorination5 and allylic fluorination,6 or via alkene
fluorination.7 The fluorination at nonspecific positions remains a
challenging issue due to the poor regioselectivity.8
The synthesis of fluorinated ketones provides an efficient and
straightforward approach for the introduction of a fluorine atom
into complex molecules. α-Fluorinated ketones are easily
accessed, since the acidic α-H of the ketone can be conveniently
converted to fluorine by use of many electrophilic fluorinating
agents (Scheme 1A).9 However, the practically direct fluorina-
tion to regioselectively synthesize distal (β, γ, δ, etc.) fluorinated
ketones is rarely reported.10 Herein, we report an efficient
synthesis of β- and γ-fluorinated ketones relying on a silver-
catalyzed ring opening strategy (Scheme 1B). The trans-
formation undergoes a sequence of C−C bond cleavage and
C−F bond formation and affords a wide range of fluoro-ketones
with good regioselectivities under mild reaction conditions.
Radical-mediated ring openings with strained cyclic com-
pounds have been long established as a radical clock to determine
the kinetics of free-radical reactions.11,12 We envisioned that this
strategy might be viable for the synthesis of distal fluorinated
ketones from tertiary cycloalkanols. With this idea in mind, we
Received: January 27, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b00939
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Journal of the American Chemical Society
Communication
Encouraged by the results, the ring-opening strategy was
subsequently applied to the fluorination of cyclopropanols. After
a survey of independent reaction parameters, the previously
optimized conditions were slightly modified:14 (a) AgNO3 gave a
better outcome than AgBF4; (b) Chloroform/H2O (0.2 M) was
used instead of DCE/H2O (0.3 M). A variety of cyclopropanols
were assessed (Scheme 3). In general, cyclopropanol demon-
Scheme 2. Synthesis of γ-Fluorinated Ketones from
Cyclobutanols
a b
,
Scheme 3. Synthesis of β-Fluorinated Ketones from
a b
,
Cyclopropanols
a
a
1 (0.3 mmol), AgBF4 (0.06 mmol, 20 mol %), and SelectFluor (0.6
3 (0.2 mmol), AgNO3 (0.04 mmol, 20 mol %), and SelectFluor (0.4
b
b
mmol, 2 equiv) in DCE/H2O (0.5/0.5 mL, 0.3 M) at rt. Yields of
isolated products. 30 mol % AgBF4 and 3 equiv of SelectFluor. 60
°C. 2.5 equiv of SelectFluor. 50 mol % AgBF4 and 3 equiv of
SelectFluor.
mmol, 2 equiv) in CHCl3/H2O (0.5/0.5 mL, 0.2 M) at rt. Yields of
isolated products.
c
d
e
f
strated a higher reactivity than cyclobutanol in terms of reaction
rate. Under the current reaction conditions, both electron-rich
and -deficient 1-arylcyclopropanols smoothly afforded the β-
fluorinated products 4a−4e in good yields within a few hours. In
addition, 1-alkylcyclopropanols with different substituents gave
rise to the corresponding products 4f−4l in useful yields. The
examples of dialkyl substituted cyclopropanol 3m was note-
worthy, as the fluorination product 4m at the tertiary rather than
the secondary carbon was exclusively generated albeit a
prolonged reaction time was used. The fluorination of 3n was
notable because the seven-membered cyclic product 4na via ring
expansion was predominantly formed.
1k took a few days even at elevated temperature. Though less
reactive, aliphatic examples such as benzyl cyclobutanol 1l and n-
hexyl cyclobutanol 1m were also suitable substrates. Remarkably,
heteroaryl cyclobutanols were also tolerated in the reaction,
yielding fluorinated thienyl ketone 2n and pyridyl ketone 2o.
The reaction with 3-substituted cyclobutanol 1p and 1q was
sluggish (72 h) which might be attributed to the increased steric
hindrance from the additional substituent. Ring expansion is
always an intriguing chemical transformation. When alkenyl
substituted cyclobutanol 1r was examined, two isomeric
cyclopentanone 2r were readily obtained. Another interesting
example was the reaction of fused bicyclic 1s which led
regioselectively to the corresponding nine-membered γ-
fluoroketone 2s in useful yield.
Regarding the possible mechanism, three pathways can be
involved (Figure 1). First, in the presence of an electrophilic
fluorinating agent, the four-membered ring might be directly
opened via an anion without a silver catalyst (Path A). An
B
DOI: 10.1021/jacs.5b00939
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Figure 1. Possible mechanistic pathways.
alternative pathway is silver-assisted ring opening (Path B).
Similarly to RhI-catalyzed ring opening of benzocyclobutenol,15
AgI is incorporated with hydroxyl group and inserts into the
proximal C−C bond, forming a C−Ag bond. Simultaneously, AgI
reacts with SelectFluor to generate a AgIII species. The resulting
C−AgIII−F complex undergoes reductive elimination to generate
the product. The other route is a silver-catalyzed radical pathway
(Path C).
To elucidate the mechanism, some control experiments were
performed. In the absence of a silver catalyst, the reaction of 1a
did not furnish the expected fluorination product 2a with or
without the help of DABCO (Scheme 4A).16 This might rule out
Figure 2. Plausible mechanism.
generate F−AgIII complex b. Homolysis of b gives rise to a F−
AgII species and cyclobutyloxy radical c. The alkyl radical d, an
open chain tautomer of c, reacts with the F−AgII species,
eventually forming the product γ-fluoroketone.
In summary, we have developed an efficient synthesis of
fluoroketones via a silver-catalyzed ring-opening strategy. The
transformation exhibits good tolerance for diverse functional
groups. A variety of β- and γ-fluorinated ketones are prepared in
good regioselectivities, respectively, from tertiary cyclopropanols
and cyclobutanols, thus offering versatile building blocks for the
construction of complexly fluorine-incorporated molecules.
Preliminary mechanistic studies suggest a radical-mediated
pathway might be involved. Further applications of the reaction
are ongoing in our laboratory.
Scheme 4. Experiments for Mechanistic Elucidation
ASSOCIATED CONTENT
* Supporting Information
Experimental details, compound characterization data, and NMR
spectra. This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*chzhu@suda.edu.cn
■
Author Contributions
§H.Z. and X.F. contributed equally.
Notes
The authors declare no competing financial interest.
the possibility of Path A. If the reaction goes through Path B via
the sequence of C−C bond insertion/reductive elimination, the
configuration of the fluorinated C-center should be retained.
Otherwise, the preformed configuration at the C-center should
racemize through Path C, the radical pathway. A single
diastereomer of 1t was synthesized and subjected to the standard
reaction conditions (Scheme 4B). It was found that the reaction
did not proceed with retention of configuration, and instead, a
couple of isomers 2ta and 2tb were generated in a 1.5:1 ratio,
which might suggest that a radical pathway was more likely to be
involved in the transformation. To verify the hypothesis, 2 equiv
of TEMPO were added to the reaction (Scheme 4C). The
resulting suppression of 2a formation supports Path C.
ACKNOWLEDGMENTS
■
C.Z. is grateful for the financial support from Soochow
University, the National Natural Science Foundation of China
(Grant No. 21402134), the Natural Science Foundation of
Jiangsu (Grant No. BK20140306), and the Priority Academic
Program Development of Jiangsu Higher Education Institutions
(PAPD). We acknowledge Profs. John Falck and Chuo Chen of
the University of Texas Southwestern Medical Center and Prof.
Xiaoguang Bao of Soochow University for helpful discussions.
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DOI: 10.1021/jacs.5b00939
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