Catalytic formation of C(sp3)-F bonds via decarboxylative fluorination with mechanochemically-prepared Ag2O/TiO2 heterogeneous catalysts

Article abstract of DOI:10.1039/c7ra06180c

Mechanochemically-prepared, Ag₂O-containing solid materials, are shown to be efficient heterogeneous catalysts for the synthesis of C(sp³)-F bonds via decarboxylative fluorination. Five catalytic cycles without loss of intrinsic activity could be performed with the optimal catalyst, composed of 1 wt% Ag₂O supported on TiO₂ (P25), despite the challenging conditions. The catalyst is easily prepared from the corresponding oxides in 20 minutes by simple mechanical mixing methods. In addition to ease of separation and re-use, the turnover numbers obtained over the solid catalyst are over one order of magnitude higher than those obtained with the state-of-the-art homogeneous catalyst, AgNO₃, under otherwise identical conditions. To the best of our knowledge, this represents the first true heterogeneous catalyst for the selective formation of C(sp³)-F bonds with electrophilic fluorine donors, representing a major breakthrough in the field of catalytic fluorination.

Full text of DOI:10.1039/c7ra06180c

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3
Catalytic formation of C(sp )–F bonds via
decarboxylative fluorination with
Cite this: RSC Adv., 2017, 7, 30185
mechanochemically-prepared Ag O/TiO
2
2
heterogeneous catalysts†
G. Tarantino, L. Botti, N. Dimitratos and C. Hammond
*
2
Mechanochemically-prepared, Ag O-containing solid materials, are shown to be efficient heterogeneous
3
catalysts for the synthesis of C(sp )–F bonds via decarboxylative fluorination. Five catalytic cycles without
loss of intrinsic activity could be performed with the optimal catalyst, composed of 1 wt% Ag
supported on TiO (P25), despite the challenging conditions. The catalyst is easily prepared from the
2
O
2
corresponding oxides in 20 minutes by simple mechanical mixing methods. In addition to ease of
separation and re-use, the turnover numbers obtained over the solid catalyst are over one order of
Received 3rd June 2017
Accepted 5th June 2017
3
magnitude higher than those obtained with the state-of-the-art homogeneous catalyst, AgNO , under
otherwise identical conditions. To the best of our knowledge, this represents the first true
DOI: 10.1039/c7ra06180c
3
heterogeneous catalyst for the selective formation of C(sp )–F bonds with electrophilic fluorine donors,
rsc.li/rsc-advances
representing a major breakthrough in the field of catalytic fluorination.
Introduction
Fluorinated compounds are widely employed in several indus-
1
Scheme 1 Decarboxylative fluorination of aliphatic carboxylic acids to
yield new C(sp )–F bonds.
trial elds as agrochemicals, pharmaceuticals and materials.
3
1
8
Furthermore, F compounds are widely used as tracers for
2
Positron Emission Tomography. Unfortunately, the synthesis
of uorinated molecules is very difficult and whilst several
5
3
3
1). However, despite the potential of this process, an active and
breakthroughs have been achieved, the synthesis of C(sp )–F
bonds via catalytic uorination remains an immense challenge.
Whilst several recent studies have demonstrated that transition
metals such as iron, copper, silver and palladium react with
electrophilic uorinating reagents such as F-TEDA
reusable heterogeneous catalyst has not yet been developed,
and catalyst activity remains extremely low, with typical turn-
over numbers being below 5 even aer extended periods i.e.
24 h. Accordingly, the development of a highly active, stable
and truly heterogeneous catalyst for this process is the focus of
this study.
<
1
a
3
(
Selectuor®), NFSI or DAST, to yield new C(sp )–F and
2
4
C(sp )–F bonds, several of these methods require stoichio-
metric (i.e. 100 mol%) metal loadings (i.e. metal use is non-
catalytic, hence TON < 1). Moreover, heterogeneous catalysis –
preferred for several process intensication reasons – has not Results and discussion
yet been achieved, likely due to the extremely challenging
conditions, which can easily induce restructuring, leaching and
hence, deactivation, of solid catalysts.
Amongst a range of metal oxide-supported, Ag-containing
catalysts, we rst identied 1 wt% Ag/TiO , prepared by sol
2
6
immobilisation (henceforth denoted 1AgTiO2(SI)), as being
The decarboxylative uorination of aliphatic carboxylic
a suitable catalyst for decarboxylative uorination. The perfor-
mance of the material aer various heat treatment procedures
was rst evaluated for the decarboxylative uorination of 2,2-
dimethylglutaric acid (DMGA, (1a)), a useful model substrate
possessing both a primary and a tertiary carboxylic group
3
acids, catalysed by homogeneous AgNO , represents a powerful
3
way to selectively synthesise new C(sp )–F bonds from readily
available substrates under mild reaction conditions (Scheme
Cardiff Catalysis Institute, Cardiff University School of Chemistry, Main Building,
Park Place, Cardiff, CF10 3AT, UK. E-mail: hammondc4@cardiff.ac.uk; Web: http://
www.blogs.cardiff.ac.uk/hammond
(
Scheme 2). Under conditions optimized from homogeneous
ꢀ
AgNO (20 mol% Ag, 1.0 equivalent K CO , 25 C) higher levels
3
2
3
of activity were observed with the heterogeneous catalyst
following calcination at 350 C (Fig. 1, le). The only product
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c7ra06180c
ꢀ
1
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Scheme
2 Decarboxylative fluorination of 1a to yield 1b with
Selectfluor® as fluorine donor.
Fig. 2 (Left) Solid line: yield of 1b with time without filtering the
catalyst. Dashed line, catalytic activity of the supernatant solution
following filtration of the catalyst at 10 min. Reaction conditions
identical to Fig. 1, left, albeit with 0.23 mmol of K
Ag/TiO2(SI350). (Right) UV-Vis spectra of colloidal Ag(0) after addition
of Selectfluor® and K CO
2 3
CO and 100 mg of
1
2
3
.
be a true catalyst, and not solely a reaction initiator in the same
Fig. 1 (Left) Yield of 1b with time over (blue/circles) 1Ag/TiO2(SI350) and
7
(
red/triangles) AgNO
3
. Reaction conditions: 0.2 mmol of 1a, 0.4 mmol manner as Cu(I) as observed by Pitts et al. TiO -supported Ag is,
2
of Selectfluor®, 0.04 mmol of Ag, 4 mL H
atmosphere, 25 C. (Right) Initial rate with different amounts of 1Ag/
TiO2(SI350). Reaction conditions otherwise identical to left.
2
O, 0.2 mmol K
2
CO
3
, N
2
therefore, the rst truly heterogeneous catalysts for the forma-
tion of C(sp )–F bonds with electrophilic uorine donors.
ꢀ
3
According to previous studies, decarboxylative uorination
requires cationic Ag species for efficient catalysis to be
5
observed. However, sol immobilisation catalysts are known to
observed over the timescale of the reaction by NMR and HPLC
was 4-uoro-4-methylpentanoic acid (4F4MPA (1b), selectivity >
contain a heterogeneous distribution of cationic and metallic
8,9
species following heat treatment. Thus, to understand the
catalytic roles of Ag(0) and Ag(I), we investigated the interaction
between Selectuor® and Ag. Accordingly, a colloidal solution
of Ag(0) was placed into a UV-Vis cuvette, and the Plasmon
95%) conrming the higher reactivity of the tertiary carboxylic
5
group in accordance with previous studies.
Whilst the system exhibited good kinetic behaviour between
0–200 mg loading of 1Ag/TiO2(SI350) (Fig. 1, right) the rate devi-
10
resonance of Ag(0) at 400 nm monitored in situ following
addition of Selectuor® and K CO (Fig. 2, right). This treat-
ment resulted in the disappearance of the Plasmon peak in
30 s. Clearly, any residual Ag(0) species in the solid catalysts
ated from linear above this loading, demonstrating transport
limitations to be present at the highest catalyst loading.
Accordingly, all further studies were performed with 100 mg of
2
3
<
1Ag/TiO2(SI350) (4.64 mol% Ag), which represents the best
compromise between intrinsic activity and overall performance.
To further optimise reaction conditions, the impact of the
loading of base (in a range 0–2 equivalents) on the catalytic
performance of 1Ag/TiO2(SI350) was investigated (ESI Fig. S1†).
An optimal base loading of 1.16 equivalents (K
of 1.16) was identied. Interestingly substantially lower activity
was found in the absence of K CO
2 3
CO /DMGA ratio
2
3
.
The system under study represents a formidable challenge
from a catalyst stability perspective, given that various amounts
of carboxylic groups, uorine sources and also HF (as possible
7
reaction by-product) are present to some extent. Each of these
species are known to cause extensive reorganisation, leaching
8
and hence, permanent deactivation, of solid catalysts. To rule
out any homogeneous contribution to the reaction rate, a hot

ltration experiment at the optimal base loading was performed
8d
(Fig. 2, le). The removal of the solid catalyst from the reaction
Fig. 3 (Left) Yield of 1b against time for 1Ag
squares), 1Ag/TiO2(SI350) (blue circles) and 1Ag O/TiO2(MM) (purple
triangles). Reaction conditions as Fig. 2, left. (Right) Ag leaching (grey
bars) at 10 min and yield of 1b (blue bars) at 60 min over neat Ag O, not
O + TiO , and 1Ag O/TiO2(PM)
2
O/TiO2(PM) (green
mixture clearly terminated the catalytic reaction, demonstrating
catalysis to be truly heterogeneous, and conrming that
potential traces of Ag in solution did not contribute to the
2
2
reaction rate. Full termination of the reaction also implies Ag to supported Ag
2
2
2
.
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are immediately oxidised to Ag(I) upon interaction with presence of solvents or other additives, and in readily available
Selectuor®, and cationic Ag is responsible for catalytic experimental apparatus, presents several synthetic advantages
performance.
Given the requirement for cationic Ag active sites, the immobilisation (Scheme 3).
preparation of materials directly containing Ag(I), and not Ag(0), To further optimize the preparation of 1Ag
over other catalyst preparation methodologies, such as sol
2
O/TiO2(MM),
was targeted. Mechanochemistry, whereby mechanical energy is a series of different milling frequencies, in a range 3–30 Hz, was
employed to facilitate chemical processes, has recently received explored (Fig. 5). Interestingly, increasing the milling frequency
considerable attention as a sustainable and scalable method of results in a decrease in catalytic activity, with 1b yield values
11
catalyst synthesis. In addition, it allows metal oxides such as decreasing from 95% to around 10% over the frequency series.
Ag O to be directly employed as solid precursors of the desired Nevertheless, while metal leaching values below the instrument
2
11c
active phase.
We rst investigated the mechanochemical detectability limits were detected by MP-AES analysis for 1Ag O/
2
synthesis of Ag O/TiO catalysts by simple physical grinding TiO2(MM) milled at higher frequencies (15 Hz, 22.5 and 30 Hz),
2
2
(denoted 1Ag
2
O/TiO2(PM)). Physically grinding mixtures of Ag
2
O
leaching values of 6% and 5% were found for the two lowest
and TiO in a pestle and mortar for 20 min led to a catalyst frequencies (3 Hz and 7.5 Hz, respectively). This demonstrates
2
demonstrating almost identical behaviour to heat-treated 1Ag/ that although low frequencies result in active materials, they do
TiO2(SI350), despite the rapidity of the method and the complete not provide sufficient energy to result in strong metal-support
absence of high temperature heat treatments (Fig. 3, le). interactions nor synthesise a truly heterogeneous catalysts.
Notably, in the absence of the TiO support i.e. when Ag O is Accordingly, all further studies utilised 1Ag O/TiO milled
2(MM)
2
2
2
ground alone, and in the absence of the grinding stage i.e. when at 15 Hz.
Ag O and TiO were placed into the reaction mixture together To fully understand the effect of the different frequencies on
without prior physical contact, high levels of leaching occurred the Ag speciation and the resultant performances of the cata-
Fig. 3, right). Moreover, homogeneous contributions to the lysts, detailed characterization of the mechanochemically-
reaction rate were observed (ESI Fig. S2†). Clearly, obtaining prepared catalysts with Powder X-ray Diffraction (pXRD),
strong interaction between Ag O and TiO is essential. We note Temperature Programmed Reduction (TPR) and X-ray Photo-
that homogeneous catalytic activity with Ag
2
2
(
2
2
2
O has previously electron Spectroscopy (XPS) was performed. XRD analysis (Fig.
12
been reported by the Ritter group.
6, le) reveals that upon increasing the frequency from 3 to
ꢀ
In view of this, and to allow a greater control of the prepa- 30 Hz, the reection at 32.7 , corresponding to Ag O, decreases
2
ꢀ
ration conditions, thus improving the reproducibility of the in intensity, and a new reection at 32.2 , characteristic of AgO,
preparation method (ESI, Fig. S3†), the optimal catalyst appears. As reported in literature, AgO is known to be a mixture
13
composition was re-made by mechanochemical mixing in of Ag(III) and Ag(I) oxides, and will therefore be denoted as
a high frequency milling machine. Mixing Ag O and TiO for 20 Ag herein. In agreement with the trend seen in the XRD
2
2
2 2
O
minutes at 15 Hz led to the formation of a catalyst that was both patterns, the presence of multiple Ag species in the higher
more active and more stable than the sol immobilisation and frequency materials was also observed by TPR (Fig. 6, right). The
physical grinding analogues (the catalyst is denoted 1Ag O/ TPR prole of 1Ag O/TiO
prepared at 3 Hz clearly shows
2
2
2(MM)
ꢀ
TiO2(MM)). For 1Ag O/TiO2(MM), catalysis was also totally only one intense reduction step at 240 C, at temperatures
2
ꢀ
heterogeneous (Fig. 3 (le)), and fully reproducible and stable higher than that found for unsupported Ag O (196 C). We
2
2
over long periods of time (Fig. 4). Preparing an effective and attribute this reduction peak to Ag O interacting with the
reproducible catalyst directly from Ag
2
O so rapidly, without the support material. Upon increasing the milling frequency from
ꢀ
3
Hz to 22.5 Hz, the reduction step at 240 C gradually
decreases, whilst a new reduction feature appears around 160–
ꢀ
1
70 C. This new feature perfectly overlaps with the reduction
temperature measured for unsupported Ag O . The presence of
2
2
two features in these samples indicates the presence of two
Fig. 4 Activity and reproducibility of three different batches of 1Ag
TiO2(MM). Grey bars are percentage yield of 1b at 10 min, blue bars
conversion at 30 min. White and blue/grey bars correspond to the Scheme 3 Graphical representation of classical “wet” preparation
2
O/
activity of 1Ag
Fig. 2, right, 30 C.
2
O/TiO2(MM) aged for 6 months. Reaction conditions as methodologies e.g. sol immobilisation (1), and physical methods e.g.
ꢀ
mechanochemical mixing (2).
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2 2
Fig. 5 Yield of 1b (blue bars) at 30 min with 1% Ag O/TiO milled at
various frequencies. Inset on the top right: Ag leaching values at 10 min
for 1% Ag O/TiO milled at 3 Hz and 7.5 Hz. Reaction conditions as
2
2
Fig. 2, right, room temperature.
Fig. 6 (Left) XRD patterns of 1Ag
2
O/TiO2(MM) prepared at different
milling frequencies (3–30 Hz). (Right) TPR profiles of 1Ag
prepared at different milling frequencies (3–30 Hz).
2
O/TiO2(MM)
distinct Ag species in these materials. At the highest frequency,
only Ag
to Ag
ysis also revealed the presence of multiple AgO
optimal material prepared at 15 Hz (Fig. 7, le). Deconvolution
of the broad asymmetrical peak observed for 1Ag O/TiO
2
O
O
2 2
2
was observed, indicating full transformation of Ag
. Although several of the Ag signals overlap, XPS anal-
species in the
2
O
The general applicability of the heterogeneous catalytic
system in terms of C(sp )–F bond formation was also investi-
x
3
gated with 1Ag O/TiO
milled at 15 Hz as catalyst. These
2
2(MM)
2
2(MM)
studies demonstrated the catalytic system to be versatile and
able to convert even difficult primary carboxylic groups such as
succinic acid (3a, Table 1 entry 3). In all cases, catalytic numbers
of turnovers were observed within a short period of time (#1 h).
into two different components; rstly, a signal at 368.0 eV,
perfectly overlapping with the signal obtained for Ag O;
2
secondly, a signal at lower binding energy (367 eV), assigned in
14
the literature to Ag(III).
A strong inuence of K
2 3
CO loading on both the activity and
Clearly, increasing the energy of the mechanochemical
preparation by increasing the milling frequency results in
a transformation of Ag O to Ag O2, with a distribution of both
2 2
stability of the 1Ag O/TiO2(MM) was also observed with these
2
substrates, such as pivalic acid (2a, Table 1 entry 2) and 2,2-
dimethyl-valeric acid (4a, Table 1 entry 4). In both cases, a two-
fold increase in the TON values, 18.6 and 6.47 respectively, was
oxides being observed at the optimal frequency (15 Hz).
An increase in Ag O formation at higher frequencies
2
2
observed when only 0.58 equivalents of K
2 3
CO were used (Table
appears to correlate with decreased catalytic performance (vide
supra). To verify that the loss in activity observed upon
increasing the milling frequency from 3–15 Hz correlates to the
1
, entries 2 and 4). Optimizing the base : carboxylic acid ratio
for each individual substrate is critical for obtaining the highest
levels of catalytic performance. Substrates not soluble in neat
water, such as 2,2-dimethyl-3-phenylpropionic acid (5a), were
partial conversion of Ag
prepared by milling Ag
2
O into Ag , 1Ag
2
O
2
2 2
O /TiO2(MM) was
2
O
2
with TiO at 15 Hz. As expected,
2
substantially decreased performance was observed (Fig. 7).
Powder X-ray Diffraction (pXRD) analysis of 1Ag O /TiO
2(MM)
2
2
demonstrates that only Ag O is present in this material (ESI
2
2
Fig. S4†).
The loss of activity observed for 1Ag
2
O
2
/TiO2(MM) agrees well
5
with the reaction mechanisms hypothesised in previous works.
According to these studies, the key reaction step involves the
oxidation of a Ag(I)-COOR species by Selectuor®. It is likely
that Ag(III) (present as approximately 50% of the Ag content in
Ag O ) cannot be further oxidized, thus stopping the reaction
2
2
cycle and acting as a “trap”. According to this, it would be ex-
pected that catalysts milled at the highest frequencies (22.5 and
30 Hz) should still display some levels of catalytic performance,
given that Ag contains approximately 50% Ag(I). We
2 2
O
hypothesise that the greater than expected decrease in activity
for these materials may be due to decreased crystallinity of the
support, as evidenced by XRD and BET measurements (ESI
Fig. S5 and Table S1†).
Fig. 7 (Left) XPS analysis of commercial Ag
prepared at 15 Hz. (Right) Yield of 1b against time over 1Ag
2
O and 1Ag
2
O/TiO2(MM)
O/TiO2(MM)
2
(purple triangles), and 1Ag
2
O
2
/TiO2(MM) (blue rhomboids) both
ꢀ
prepared at 15 Hz. Reaction conditions as Fig. 2, right, 30 C.
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19
Table 1 Yields calculated with F NMR using a,a,a-trifluorotoluene levels of reusability, even without any intermediate treatments
and HPLC analysis against authentic standards. Values in parentheses
correspond to total TON including reuses. Reaction conditions:
being performed. Substrates containing additional functional
groups, such as 1a, presented a greater challenge. Indeed, in the
0
mL H
.2 mmol substrate, 0.4 mmol Selectfluor®, 100 mg 1Ag/TiO2(MM), 4
ꢀ
absence of secondary treatments, a large drop in the catalytic
activity of 1Ag O/TiO2(MM) was observed during the second cycle
O, 0.23 mmol K CO , N
2 2 3 2
, 30 C, 0.5 h
2
a
Entry
Product
TON
for 1a decarboyxlative uorination (ESI Fig. S6†). However, even
in these cases, the initial level of performance could be fully
2 3
restored by suspending the catalyst in an aqueous K CO solu-
1
1b
17.8 (34.2 aer 2 cycles)
tion for 60 min between successive cycles (Fig. 8, right). We
hypothesize that removal of residual carboxylic species by
K CO is responsible for improved recyclability, as such species
are well established poisons of metal-based catalysts. It is
therefore clear that the heterogeneous catalyst possesses
promising stability characteristics for intensied operation. For
b
c
2
3
2b
9.6 , 18.6
2
3
d
8a
3b
4b
4.0
b
b,c
b
4
3.25 , 6.47 (14.6 aer 5 cycles)
4a and 1a, TONs over one order of magnitude higher than
previously reported were obtained aer 5 and 2 cycles,
respectively.
c,e
5
5b
3.21
Conclusions
a
TON calculated as “mol (product)/mol (Ag)” for entries 1 and 3–5, and
b
as “mol (2a converted)/mol(Ag)” for entry 2. Reaction time 1 h. Mechanochemically-prepared 1 wt% Ag O/TiO is found to be
2
2
c
e
d
ꢀ
0
.115 mmol K
2
CO
3
.
200 mg 1Ag
2
O/TiO2(MM), 50 C, 2 mL H
2
O.
an efficient and reusable heterogeneous catalyst for the decar-
boxylative uorination of various carboxylic acids in aqueous
ꢀ
2
00 mg 1Ag/TiO2(MM), H
2
O/acetone (2 mL/2 mL), 60 C.
ꢀ
media at 25 C. The mechanochemical mixture presents higher
levels of catalytic activity than the state-of-the-art homogeneous
catalyst, and is also easily recovered and reused over ve
successive catalytic cycles. Good general applicability of the
catalytic system is also observed, with a variety of primary and
tertiary carboxylic acid groups being selectively converted to the
3
also efficiently converted into the corresponding C(sp )–F uo-
rinated compounds (5b, Table 1 entry 5). In this case, a water:-
acetone mixture (1 : 1) was employed as solvent, and TONs
comparable or better to the ones obtained using AgNO were
3
5
observed.
3
desired C(sp )–F products. Spectroscopic characterisation of the
mechanochemically-prepared materials indicates that phase
A major advantage of a heterogeneous catalyst over a homo-
geneous analogue is the ease in which it can be recovered and
reused. Accordingly, we rst explored the reusability of 1Ag O/
2
TiO2(MM) with 4a as substrate. To ensure that true kinetic
comparisons were made, the activity of the catalyst in multiple
cycles was examined at low conversion, so that the intrinsic
transformations between Ag
the optimal milling frequency in terms of activity and stability is
5 Hz.
2 2 2
O and Ag O can occur, and that
1
8
b,d Acknowledgements
activity and reusability of the catalyst was correctly examined.
As can be seen (Fig. 8, le), 1Ag O/TiO
exhibited good
2
2(MM)
CH gratefully acknowledges The Royal Society for research
funding and provision of a University Research Fellowship
(UF140207).
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30190 | RSC Adv., 2017, 7, 30185–30190
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