Direct C-F bond formation using photoredox catalysis

Article abstract of DOI:10.1021/ja412083f

We have developed the first example of a photoredox catalytic method for the formation of carbon-fluorine (C-F) bonds. The mechanism has been studied using transient absorption spectroscopy and involves a key single-electron transfer from the ³MLCT (triplet metal-to-ligand charge transfer) state of Ru(bpy)₃²⁺ to Selectfluor. Not only does this represent a new reaction for photoredox catalysis, but the mild reaction conditions and use of visible light also make it a practical improvement over previously developed UV-mediated decarboxylative fluorinations.

Full text of DOI:10.1021/ja412083f

Article
pubs.acs.org/JACS
Direct C−F Bond Formation Using Photoredox Catalysis
Montserrat Rueda-Becerril,†,§ Olivier Mahe,‡,§ Myriam Drouin,‡ Marek B. Majewski,† Julian G. West,†
́
Michael O. Wolf,† Glenn M. Sammis,*,† and Jean-Francois Paquin*,‡
̧
†Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, V6T 1Z1, Canada
‡Canada Research Chair in Organic and Medicinal Chemistry, CGCC, PROTEO, Dep
artement de Chimie, Universite Laval, 1045
Avenue de la Medecine, Pavillon Alexandre-Vachon, Quebec, Quebec, G1V 0A6, Canada
* Supporting Information
́
́
́
́
́
S
ABSTRACT: We have developed the first example of a photoredox
catalytic method for the formation of carbon−fluorine (C−F) bonds.
The mechanism has been studied using transient absorption spectros-
3
copy and involves a key single-electron transfer from the MLCT
2+
(triplet metal-to-ligand charge transfer) state of Ru(bpy)3 to
Selectfluor. Not only does this represent a new reaction for photoredox
catalysis, but the mild reaction conditions and use of visible light also
make it a practical improvement over previously developed UV-
mediated decarboxylative fluorinations.
We recently reported that stable electrophilic fluorine sources,
INTRODUCTION
■
such as Selectfluor, can transfer fluorine to alkyl radicals.11,12
Selectfluor may complicate photocatalytic transformations
because it is an oxidant and has the potential to interfere with
the redox catalytic cycle.
Photoredox catalytic transformations have a rich history in
organic synthesis starting with early work by Kellogg in the
1970s.1,2 The reports of MacMillan3 and Yoon4 in 2008 have led
to a dramatic increase in the number of outstanding
contributions in this field.5 All of this work has culminated in
the development of numerous effective synthetic methodologies,
particularly for the formation of carbon−carbon (C−C),
carbon−hydrogen (C−H), and various carbon−heteroatom
(C−Het) bonds.5 Despite both the significant interest in
photoredox catalysis and the pharmaceutical, agrochemical,
and radiochemical importance of incorporating fluorine into
molecules,6 there have been no reports to date of the direct
photoredox catalytic formation of carbon−fluorine (C−F)
bonds (Figure 1). The only reports of fluorine incorporation7
using photoredox catalysis focus on adding trifluoromethyl
groups via C−C bond formation.8
To test whether Selectfluor is compatible with common
photoredox catalysts, we first needed to identify an appropriate
substrate that has a suitably high oxidation potential such that it
cannot be directly oxidized by Selectfluor13 but can be oxidized
with the assistance of a photocatalyst. We decided to investigate
the photofluorodecarboxylation of aryloxyacetic derivatives (1,
Scheme 1).14 These substrates are ideal because they cannot be
oxidized with Selectfluor unless they are first excited with 300 nm
light. In the visible region needed to access the photoexcited
catalyst (400−500 nm), neither the aryloxyacetic derivatives,
such as 1a, nor Selectfluor absorbs light (Figure 2).
Scheme 1. Photodecarboxylation of Aryloxyacetic Acid
Derivatives and a Proposed Photoredox Catalyzed Variant
The absence of photoredox catalytic methods for C−F bond
formation is likely due to the paucity of known fluorine atom
transfer reagents. For years, the few reagents that were available
either required specialized handling protocols, such as F2 or
hypofluorites,9 or are powerful oxidants, such as XeF2.10
Figure 1. Bonds that can be formed using ruthenium and iridium-based
photoredox catalysts.
Received: December 3, 2013
Published: January 17, 2014
© 2014 American Chemical Society
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While this initial study clearly demonstrates that Ru(bpy)3Cl2
effectively promotes the decarboxylative fluorination of phenoxy-
acetic acid (1a), there are still several important mechanistic
questions that need to be addressed to determine if this new C−F
bond forming methodology is a catalytic photoredox process as
2+
proposed. The reaction begins with the excitation of Ru(bpy)3
to afford a singlet excited state that rapidly undergoes an
intersystem crossing to the key triplet excited state, *3[Ru-
(bpy)2(bpy•−)]2+ (Figure 3). At this stage, there are three
Figure 2. Absorption spectra of phenoxyacetic acid 1a (H2O),
Selectfluor (H2O), and [Ru(bpy)3][PF6]2 (1:1 CH3CN/H2O).
An investigation of C−F bond formation using photoredox
catalysis not only is an excellent opportunity to study the
compatibility of Selectfluor with metal-based photoredox
catalysts but also may provide a practical new method that
represents an improvement over our recently reported light-
mediated fluorination methodology (Scheme 1).14 The presence
of a photocatalyst that can be activated using visible light allows
for the use of a lower energy light rather than near-UV light.
Furthermore, our previous methodology relies on direct
excitation of the substrate and any significant structural change
necessitates changing the light source. A photoredox catalytic
system has the significant advantage of requiring only one light
source.
Figure 3. Oxidative, reductive, and energy transfer pathways from
excited intermediate *3Ru(bpy)2(bpy•−)2+.
possible mechanistic pathways.15 The first option is that
*3[Ru(bpy)2(bpy•−)]2+ undergoes a SET to reduce Selectfluor
3+
and form Ru(bpy)3 (pathway 1). Alternatively, *3[Ru-
(bpy)2(bpy•−)]2+ may oxidize phenoxyacetic acid to initiate
+
the decarboxylation and concomitantly form reduced Ru(bpy)3
RESULTS AND DISCUSSION
We began our investigations with phenoxyacetic acid (1a, Table
1). In the absence of a photoredox catalyst, 1a is unreactive when
(pathway 2). The third mechanistic possibility is that the catalyst
may simply act as a photosensitizer by transferring energy from
*3[Ru(bpy)2(bpy•−)]2+ to phenoxyacetic acid (pathway 3),
which would then intersect with the mechanism previously
proposed.14
■
Table 1. Control Experiments for Catalytic Photoredox
Decarboxylative Fluorinations
To differentiate between the three mechanistic possibilities,
we utilized transient absorption (TA) spectroscopy. TA
spectroscopy provides a direct method to monitor the excitation
and oxidation states of the ruthenium catalyst, including
16
2+
Ru(bpy)3 and *3[Ru(bpy)2(bpy•−)]2+. If a difference is
observed in the TA spectrum when phenoxyacetic acid is added,
then the catalyst likely undergoes either reduction (Figure 3,
pathway 2) or energy transfer (pathway 3). Similarly, if a
difference occurs with the addition of Selectfluor, the catalyst
likely undergoes oxidation (pathway 1).
a
b
c
entry
light source
catalyst (mol %)
NMR yield (%)
1
2
3
500 W lamp
0
1
1
0
0
d
none
During our initial TA spectroscopic experiments, it was
observed that a small amount of precipitate was formed when the
decarboxylative fluorination was carried out with Ru(bpy)3Cl2 in
water. This led to light scattering and complicated data analysis.
500 W lamp
84
a
Conditions: 0.1 mmol phenoxyacetic acid, 3.5 equiv of Selectfluor,
b
1.5 equiv of NaOH, 0.1 M H2O. The lamp was positioned 30 cm
from the reaction flask. See Supporting Information for details about
−
This problem was circumvented by using the PF6 salt of
c
the lamp. The NMR yield was determined using 1,3,5-trimethox-
Ru(bpy)32+ in a 1:1 mixture of H2O/CH3CN. The yield of the
decarboxylative fluorination of 1a in 1:1 H2O/CH3CN was the
same regardless of which catalyst was utilized (Scheme 2).
TA spectroscopy of Ru(bpy)3(PF6)2 (Figure 4a) is consistent
with previous reports,16 with a depletion of the band at 450 nm,
which correlates to Ru(bpy)32+ (Figure 2, Ru(bpy)3PF6), and an
increase of the band at 375 nm, which correlates to the
3[Ru(bpy)2(bpy•−)]2+. In the presence of phenoxyacetic acid
(1a), the excited state difference spectrum of Ru(bpy)3(PF6)2
(Figure 4b) is identical to that of Ru(bpy)3(PF6)2 (Figure 4a),
which qualitatively suggests that there is no interaction between
the catalyst and phenoxyacetic acid and is not consistent with
d
ybenzene as the internal standard. The reaction was covered in
aluminum foil. All other conditions were the same, including the
proximity from the active light source to maintain a comparable
reaction temperature.
visible light is used (entry 1). Furthermore, when a catalytic
amount of Ru(bpy)3Cl2 is added to the reaction mixture, there is
no reaction in the absence of light (entry 2). Gratifyingly,
exposure of phenoxyacetic acid and a catalytic amount of
Ru(bpy)3Cl2 to light leads to a high conversion to
fluorodecarboxylated product 2a in only 1 h (entry 3).
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Scheme 2. Photodecarboxylative Fluorination with
Ru(bpy)3(PF6)2 and Ru(bpy)3Cl2
Figure 5. Oxidation and decarboxylative fluorination of phenoxyacetic
acid.
oxidation of phenoxyacetic acid,14 an oxidant in solution
performs a direct oxidation of the substrate.20−22 Once oxidized,
4a undergoes decarboxylation to 5a/6a and fluorination to afford
the desired fluoromethoxy ether, 2a.14
With proof-of-concept for the first use of photoredox catalysis
for the direct formation of C−F bonds, we next investigated
whether this is a practical improvement. Systematic optimization
of the reaction conditions indicated that optimal yields could be
obtained using 1 mol % of Ru(bpy)3Cl2, 3.5 equiv of Selectfluor,
and 1.5 equiv of NaOH in either H2O or H2O/CH3CN mixtures
depending on substrate solubility. The reaction was typically
irradiated with a visible-light-emitting 500 W lamp for 1 h.23 The
base is useful to increase the solubility of the substrate but was
not required.24
These optimized conditions led to the photoredox decarbox-
ylative fluorination of a range of aryloxyacetic acid derivatives in
good to excellent yields (Table 2). p-Phenylaryloxy rings
performed comparably to o-phenyl derivatives (entries 2, 3).
Increasing the alkyl substitution in the ortho-position of the
aryloxy ring (entries 4−6) also did not significantly impact the
yield of fluorinated product. Electron-withdrawing substituents
on the aryloxy rings, such as fluorine (1g, entry 7) or bromine
(1h, entry 8), led to a slight decrease in yield compared to
phenoxyacetic acid (entry 1). Pyridyl derivative 1i was
successfully fluorinated in good yield (entry 9). This method-
ology is also applicable to double decarboxylative fluorinations,
as illustrated by entry 10.
These new visible light decarboxylative fluorination conditions
provide access to a broader scope of substrates compared to our
previously developed UV light-promoted methodology.14
Substrates that can successfully be fluorinated using UV light
generally perform comparably under these new conditions. For
example, under the UV light conditions, substrates 1a, 1d, 1g,
and 1h afforded yields of 84%, 83%, 94%, and 60%, respectively.
However, substrates that were not fluorinated in good yields
using UV light, such as 1b and 1f,25 can be accessed using these
visible light conditions. We next explored the decarboxylative
fluorination in a more complex substrate derived from the
naturally occurring hormone estrone (1k), which has the
potential to enolize under the basic reaction conditions (Scheme
3). Indeed, treatment of 1k under either our UV14 or visible light
standard basic reaction conditions led to multiple fluorination
products. Application of base-free conditions to substrate 1k
afforded the desired product (2k) in 51% yield (Scheme 4).
Figure 4. Excited state difference spectra of (a) Ru(bpy)3(PF6)2, (b)
Ru(bpy)3(PF6)2 (0.11 mM) in the presence of 2-phenoxyacetic acid (1a,
2.6 mM), and (c) Ru(bpy)3(PF6)2 (0.11 mM) in the presence of
Selectfluor (2.6 mM) in 1:1 H2O/CH3CN (λex = 450 nm).
pathways 2 and 3 (Figure 3).17 Conversely, the excited state
difference spectrum of Ru(bpy)3(PF6)2 in the presence of an
excess of Selectfluor (Figure 4c) shows the growth of a new
absorption band centered at 450 nm 18 concomitant with the
decay of the band attributed to the bpy anion. This new band was
not found to decay over the course of the time regimes studied (t
≤ 10 μs).19 These findings are most consistent with pathway 1
(Figure 3), in which the catalyst is first oxidized by Selectfluor.
After the electron transfer in pathway 1 (Figure 3), the
decarboxylative fluorination likely proceeds according to Figure
5. Instead of the previously proposed direct excitation and
CONCLUSION
■
We have successfully developed the first example of direct C−F
bond formation using a Ru(bpy)3Cl2 photocatalyst with
Selectfluor. Mechanistically, the reaction proceeds through a
photoredox pathway involving a SET from the 3MLCT state of
the ruthenium catalyst to Selectfluor. This forms the key oxidant
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Table 2. Substrate Scope for the Catalytic Photoredox
Decarboxylative Fluorination
Scheme 3. Attempted Photodecarboxylative Fluorination
with Estrone Derivative 1k
Scheme 4. Base-Free Photodecarboxylative Fluorination
Conditions
bond formation using photoredox catalysis, but the methodology
also is applicable to a wide range of substrates and the use of
visible light represents a practical improvement over UV light
mediated alternatives.
ASSOCIATED CONTENT
■
S
* Supporting Information
Full experimental details and 1H, 13C, and 19F NMR spectra. This
material is available free of charge via the Internet at http://pubs.
acs.org.
AUTHOR INFORMATION
Corresponding Authors
gsammis@chem.ubc.ca
■
jean-francois.paquin@chm.ulaval.ca
Author Contributions
§M.R.-B. and O.M. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the University of British Columbia
■
(UBC), the Universite
Engineering Research Council of Canada (NSERC), the Canada
Research Chair program, a doctoral fellowship from Consejo
́
Laval, the Natural Sciences and
́
Nacional de Ciencia y Tecnologia (CONACyT) to M.R.-B., the
Canada Foundation for Innovation, and FQRNT Centre in
Green Chemistry and Catalysis (CGCC). The authors thank
Jannu R. Casanova-Moreno and Prof. Dan Bizzotto at UBC
Chemistry Department for their assistance with the Cyclic
Voltammetry measurements, as well as LASIR at UBC for the TA
spectroscopy facilities.
a
Isolated yield (NMR yield). NMR yields obtained by 1H NMR
spectroscopy using trimethoxybenzene as an internal standard.
b
Conditions: 3.5 equiv of Selectfluor, 1.5 equiv of NaOH, H2O.
c
The product is volatile and is challenging to isolate in high yield.
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■
d
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f
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