A continuous-flow protocol for light-induced benzylic fluorinations

Article abstract of DOI:10.1021/jo5016757

A continuous-flow protocol for the light-induced fluorination of benzylic compounds is presented. The procedure uses Selectfluor as the fluorine source and xanthone as an inexpensive and commercially available photoorganocatalyst. The flow photoreactor is based on transparent fluorinated ethylene propylene tubing and a household compact fluorescent lamp. The combination of xanthone with black-light irradiation results in very efficient fluorination. Good to excellent isolated yields were obtained for a variety of substrates bearing different functional groups applying residence times below 30 min.

Full text of DOI:10.1021/jo5016757

Note
pubs.acs.org/joc
A Continuous-Flow Protocol for Light-Induced Benzylic Fluorinations
David Cantillo,† Oscar de Frutos,*,‡ Juan A. Rincon,‡ Carlos Mateos,‡ and C. Oliver Kappe*,†
́
†Institute of Chemistry, University of Graz, Heinrichstrasse 28, A-8010 Graz, Austria
‡Centro de Investigacion
́
Lilly S.A., Avda. de la Industria 30, 28108 Alcobendas-Madrid, Spain
S
* Supporting Information
ABSTRACT: A continuous-flow protocol for the light-induced fluorination of
benzylic compounds is presented. The procedure uses Selectfluor as the
fluorine source and xanthone as an inexpensive and commercially available
photoorganocatalyst. The flow photoreactor is based on transparent
fluorinated ethylene propylene tubing and a household compact fluorescent
lamp. The combination of xanthone with black-light irradiation results in very
efficient fluorination. Good to excellent isolated yields were obtained for a
variety of substrates bearing different functional groups applying residence times below 30 min.
ncorporation of fluorine atoms to organic scaffolds has
overirradiation of the solution. Moreover, the optimized
I
become a subject of intense research in the past few years.1
reaction conditions can be readily scaled by a simple scale-
out or numbering up of the reactor.14
Fluorine-enriched compounds typically exhibit enhanced bio-
logical properties and are of considerable importance in the
production of agrochemicals and pharmaceuticals.2 Compared
with the dramatic development of fluorination strategies, direct
selective benzylic fluorination methods have received com-
paratively little attention.3 Introduction of fluorine at benzylic
positions has been typically carried out by halogen exchange,4
electrochemical methods,5 or dehydroxyfluorination of benzyl
alcohols,6 for which two or more reaction steps and harsh,
unselective reagents are required. Recently it has been shown
by the groups of Grooves and Leckta that benzylic C(sp3)−H
bonds can also be selectively fluorinated under mild conditions
using metal catalysts based on manganese,7 iron,8 or copper.9
These reactions typically require several hours at ambient or
slightly elevated temperatures. Several metal-free alternatives
have also been reported,10 using Selectfluor as the fluorine
source and a suitable organocatalyst.11,12 In this context, Chen
and co-workers have recently described a simple and
convenient protocol for visible-light-promoted benzylic fluori-
nations using diaryl ketones such as fluorenone or xanthone as
photocatalysts.12 Notably, these transformations require several
hours to even a few days to reach completion for some
substrates.12 This important drawback and the problems
typically associated with scaling photochemistry13 somewhat
limit the applicability of this method for large-scale
preparations. In this context, we envisaged that continuous-
flow technology combined with a careful optimization of the
light source and reaction parameters would result in an efficient
and practical fluorination protocol. It has been well-
documented that microreactor/continuous-flow technology
can overcome the problems associated with larger-scale
photochemical transformations.14,15 The relatively low reactor
volume and in particular the high surface-to-volume ratio of
capillary flow reactors assures intense and uniform irradiation of
the reaction mixture for a specific time period that is very
precisely controlled by the pump flow rate, thus avoiding
Herein we present a continuous-flow protocol for the light-
induced fluorination of benzylic compounds. Our procedure
utilizes xanthone as an inexpensive and commercially available
photocatalyst and Selectfluor as the fluorine source. With a
black-light household compact fluorescent lamp (CFL) as the
photon source, the process takes places within minutes under
mild conditions. The process has been successfully applied to
several substrates containing benzylic positions, is scalable, and
to the best of our knowledge constitutes the first example of
direct benzylic fluorination under continuous-flow condi-
tions.16,17
Our study commenced with a series of preliminary batch
experiments to determine the most efficient catalyst and light
source combination. For this purpose, the fluorination of
ethylbenzene (1a) was selected as the model reaction (Table
1). All of the reaction mixtures were degassed prior to light
exposure by purging with N2, as the presence of oxygen inhibits
the reaction (entry 1). Three different diaryl ketones (i.e.,
fluorenone, xanthone, and anthraquinone) were tested as
catalysts (entries 2−4). Moderate conversions were observed
after 2 h of light exposure (30 W cool-white CFL) in
acetonitrile, with xanthone providing the best results. Other
common sensitizers such as eosin or rose bengal as well as a
variety of different solvents exhibited very poor performance or
no reaction at all (see Table S1 in the Supporting Information).
Interestingly, increasing the reaction temperature to 40 and 60
°C improved the conversion, although in the latter case several
side products were observed by GC−MS, probably as a result
of product decomposition after 2 h at this temperature. At this
point, we turned our attention to the light source. While
fluorenone has a wide UV−vis absorption band located around
400 nm, xanthone presents a maximum in the near-UV (ca. 340
Received: July 24, 2014
Published: August 21, 2014
© 2014 American Chemical Society
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Note
was located inside a GC oven to enable accurate control of the
temperature (see the Supporting Information for details). Both
the solvent and the reaction mixture were degassed with N2
before being pumped through the system using a commercial
syringe pump (Syrris). Thus, a solution containing 1a as a
model substrate, 1.2 equiv of Selectfluor, and xanthone (5 mol
%) in acetonitrile (0.1 M) was processed using this flow setup.
The temperature was set to 25 °C and the pump flow rate to 1
mL min−1, corresponding to a residence time of 28 min inside
the photoreactor. GC-FID analysis of the crude reaction
mixture revealed that 95% of the substrate had been converted
into the desired 1-fluoroethylbenzene. The selectivity of the
reaction was excellent, and side products were not detected in
significant amounts (the corresponding GC-FID chromatogram
is included as Figure S2 in the Supporting Information).
To demonstrate the general applicability of this continuous-
flow protocol, a series of benzylic compounds 1a−k were
transformed into the corresponding fluorinated derivatives
using the above-described conditions (Figure 2). Notably, we
Table 1. Preliminary Batch Optimization of the Reaction
Conditions for the Light-Induced Fluorination of
Ethylbenzene (1a)
ab
,
equiv of
Selectfluor
conv.
c
T
(°C)
light
source
d
entry
catalyst
(%)
e
1
fluorenone
fluorenone
xanthone
2
<1
53
60
52
85
25
25
25
25
40
60
25
25
25
W
W
W
W
W
W
B
2
3
4
5
6
7
2
2
anthraquinone
xanthone
2
2
f
xanthone
2
89
fluorenone
xanthone
1.2
1.2
1.2
30
92
<1
g
8
h
B
9
xanthone
−
a
Conditions: 0.2 mmol of 1a, 2 mL of MeCN, 5 mol % catalyst.
b
Further optimization experiments are collected in Table S1 in the
c
d
Supporting Information. Measured by GC-FID. W = 30 W cool-
white lamp; B = 25 W black-light lamp. The reaction mixture was not
degassed. Side products were observed by GC-FID. Reaction time =
60 min. The reaction mixture was placed in a sealed vial covered with
e
f
g
h
aluminum foil, degassed, and kept at room temperature for 2 h.
nm). We therefore speculated that using black light (max. 360
nm) for these photocatalysts would have a beneficial effect.
Gratifyingly, this was the case for xanthone (entry 8), and
excellent conversions were obtained even when the reaction
time was decreased to 1 h and the amount of Selectfluor to 1.2
equiv. Apart from the obvious advantages of decreasing the
reagent stoichiometry in terms of economy and waste
generation, under these conditions the reaction mixture was
fully homogeneous, thus facilitating the following translation
into flow. Substitution of the light source had the opposite
effect for fluorenone, and only 30% conversion was observed
after 2 h of irradiation (entry 7). Importantly, in the absence of
light no reaction was observed, and GC-FID analysis of a
reaction mixture that was kept in the dark for 2 h showed no
traces of the fluorinated product (entry 9).
After the most appropriate catalyst and light source for the
fluorination process were established, we decided to move
forward and perform the reaction in flow. The photoreactor
(Figure 1) consisted of transparent fluorinated ethylene
propylene (FEP) tubing (1.6 mm inner diameter) coiled
Figure 2. Continuous-flow visible-light-induced fluorination of
benzylic compounds. Reactor temperatures and isolated yields are
included in parentheses. aThe yields of the volatile compounds 2a and
2c were determined by 19F NMR spectroscopy.
around a glass cylinder (28 mL reactor volume).18
A
commercial black light household compact fluorescent lamp
(105 W) was placed inside the glass cylinder. The photoreactor
were able to keep the pump flow rate (and therefore the
residence time and productivity) constant by tuning the reactor
temperature for each of the substrates. As described above (cf.
Table 1), increased temperatures led to faster fluorinations for
the model reaction, and therefore, unreactive substrates could
be successfully fluorinated simply by increasing the reaction
temperature.19 Thus, a variety of benzylic substrates bearing
halogen, alcohol, carboxylic acid, ester, and ketone functional
groups could be fluorinated (Figure 2) with good to excellent
conversions. The crude reaction mixtures obtained from the
reactor output were diluted with CH2Cl2 and filtered through a
Figure 1. Schematic diagram of the continuous-flow setup employed
for the light-induced fluorination of benzylic compounds.
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plug of silica. Evaporation of the solvent and purification by
column chromatography provided the pure fluorinated
products 2a−k in good to excellent isolated yields.
In summary, we have developed a continuous-flow procedure
for the light-induced selective fluorination of benzylic
compounds. The process is based on the use of Selectfluor as
the fluorine source and xanthone as the photocatalyst. The
photochemical reactor has been designed using readily available
materials such as transparent FEP tubing and a household CFL
lamp. The combination of black light with xanthone as the
catalyst resulted in very efficient fluorination. Less reactive
substrates can also be successfully fluorinated by tuning the
reactor temperature. Thus, several benzylic substrates could be
transformed into their corresponding fluorinated derivatives in
less than 30 min, including the biologically active compounds
ibuprofen methyl ester and celestolide. Product decomposition
in the case of the fluorinated celestolide could be avoided by
installing a second feed immediately after the photoreactor for
in-line dilution with CH2Cl2 and rapid filtration through a plug
of silica. Thus, the reaction mixture could be pumped with a
flow rate of 3 mL min−1 (ca. 9 min residence time) while
obtaining full conversion and an isolated yield of 87%. This
corresponds to a productivity of approximately 0.016 mol h−1.
Motivated by the good conversions and selectivities obtained,
we turned our attention to two biologically active molecules
with more challenging selectivity issues, namely, the anti-
inflammatory ibuprofen methyl ester and the musk compound
celestolide. Ibuprofen methyl ester contains two benzylic
positions, and in principle a mixture of different fluorinated
products could be obtained. Moderate conversions were
achieved at 25 °C. Thus, the reactor temperature was set at
60 °C, and the substrate was processed using the general
protocol described above. Very good selectivity (>90%) was
obtained for the product 2l fluorinated at a single position, and
an isolated yield of 80% was achieved after column
chromatography.
EXPERIMENTAL SECTION
■
1
General Experimental Details. H NMR spectra were recorded
on a 300 MHz instrument. Chemical shifts (δ) are expressed in parts
per million downfield from TMS as an internal standard. The letters s,
d, t, q, and m are used to indicate singlet, doublet, triplet, quadruplet,
and multiplet, respectively. Analytical HPLC analysis was carried out
on a C18 reversed-phase (RP) analytical column (150 mm × 4.6 mm,
particle size 5 mm) at 25 °C using mobile phases A (water/acetonitrile
90:10 (v/v) + 0.1% TFA) and B (MeCN + 0.1% TFA) at a flow rate of
1.5 mL min−1. The following gradient was applied: linear increase from
30% B to 100% B in 8 min, hold at 100% solution B for 2 min. GC-
FID analysis was performed on a standard GC instrument with a flame
ionization detector using an HP5 column (30 m × 0.250 mm × 0.025
mm). After 1 min at 50 °C, the temperature was increased in steps of
25 °C min−1 up to 300 °C and kept at 300 °C for 4 min. The detector
gas for the flame ionization was H2 and compressed air (5.0 quality).
GC−MS monitoring was based on electron impact ionization (70 eV)
using an HP/5MS column (30 m × 0.250 mm × 0.025 μm). After 1
min at 50 °C, the temperature was increased in 25 °C min−1 steps up
to 300 °C and kept at 300 °C for 1 min. The carrier gas was helium,
and the flow rate was 1.0 mL min−1 in constant-flow mode. HPLC-
grade acetonitrile was used in all of the experiments. Selectfluor (>95%
in F+ active, lot no. BCBK5355V) and xanthone (97%, lot no.
06005BOV) were purchased from Aldrich. All other chemicals were
obtained from standard commercial vendors and were used without
any further purification. In all of the flow experiments the lamp, was
turned on at least 15 min prior use to ensure maximum and constant
irradiation, and the reactor solvent container was degassed with a
stream of N2 for 1 h. All of the compounds synthesized herein are
known in the literature. Proof of purity and identity was obtained by
1H, 13C , and 19F NMR spectroscopy and mass spectrometry.
CAUTION: Some of the fluorinated compounds are not stable in
contact with glassware when concentrated and/or decompose in
CDCl3 solution.7b Compounds 2g and 2m were especially sensitive to
contact with glassware. Thus, during purification of these compounds,
the solutions should be evaporated using suitable polymeric materials
(polyethylene, PTFE). Furthermore, chromatography methods should
be kept as short as possible to avoid prolonged contact with silica in all
cases.
General Procedure for Continuous-Flow Benzylic Fluorina-
tion (Figure 2). Selectfluor (850 mg, 2.4 mmol, 1.2 equiv), HPLC-
grade acetonitrile (10 mL), and 10 mL of a 0.01 M stock solution of
xanthone (5 mol %) in the same solvent were placed into a vial
covered with aluminum foil, and the mixture was sonicated in an
ultrasound bath until it was completely homogeneous. The
corresponding substrate 1a−m (2 mmol) was added, and the vial
Celestolide is a common ingredient utilized for fragrance
compositions. Fluorination at 25 °C under our general
conditions to give F-celestolide (2m) was very fast, but the
product then started to decompose within minutes in the
reaction mixture, ultimately leading to a mixture of several
compounds. Interestingly, we observed that quenching the
reaction mixture by dilution with CH2Cl2 followed by filtration
through a plug of silica prevented the decomposition process,
and the product was stable in solution for several days. The
flow setup was accordingly modified by incorporating a feed of
dichloromethane immediately after the photoreactor for inline
dilution. Then the reaction mixture was directly collected from
the output in a filter loaded with silica gel for filtration (for a
detailed description of the modified flow setup, see the
Supporting Information). In this way, the reaction mixture
was quenched within seconds after the desired reaction time
inside the photoreactor. With the modified flow reactor, a
reaction mixture containing celestolide was processed at 25 °C
and a flow rate of 3 mL min−1, corresponding to a residence
time of only ca. 9 min. Full conversion and excellent selectivity
were observed for the reaction, and notably, the fluorinated
product 2m was stable in the obtained crude solution for
further purification. Evaporation of the solvent and purification
by chromatography yielded the desired F-celestolide 2m (88%).
Under the exact same conditions, 100 mL of the reaction
mixture was processed, producing 2.3 g (87% yield) of
fluorinated product, which corresponds to a productivity of
ca. 0.016 mol h−1.
The case of celestolide fluorination is an example high-
lighting the potential of continuous-flow chemistry for
processes where the reaction time must be carefully controlled.
Tuning of the flow rate and/or reactor volume allows very
accurate control of the irradiation time as well as the
application of a rapid in-line quench of the photolyzate, thus
preventing product decomposition. Such reaction control is
difficult to achieve in batch mode, especially when working on a
larger scale, while the continuous-flow protocol can be very
easily scaled as needed simply by running the reactor for longer
periods.
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Note
was capped with a septum. The mixture was degassed with N2 for 10
min, and then the solution was pumped through the reactor at a flow
rate of 1 mL min−1 (3 mL min−1 for 1m). The reaction mixture
collected from the reactor output was diluted with CH2Cl2 and filtered
through a plug of silica, and the solvent was evaporated under reduced
pressure. The crude mixture was then purified by flash column
chromatography using petroleum ether/dichloromethane or petro-
leum ether/ethyl acetate as the eluent.
40.6, 16.1 Hz); MS-EI m/z 164 (30%), 149 (30%), 109 (45%), 104
(70%), 59 (100%).
4-(1-Fluoroethyl)acetophenone (2k). Yield 230 mg, 69%; H
1
NMR (300 MHz, CDCl3) δ 7.99 (d, J = 8.0 Hz, 1H), 7.45 (d, J = 8.3
Hz, 1H), 5.70 (dq, J = 47.6, 6.5 Hz, 1H), 2.63 (s, 2H), 1.66 (dd, J =
24.0, 6.5 Hz, 2H); 13C NMR (75 MHz, CDCl3) δ 197.67, 146.81,
146.55, 136.83, 128.60, 125.15, 125.05, 91.42, 89.16, 26.66, 23.17,
22.83; 19F NMR (282 MHz, CDCl3) δ −171.3 (dq, J = 47.9, 24.0 Hz);
MS-EI m/z 166 (20%), 151 (100%).
Determination of 19F NMR Yields for Volatile Compounds 2a
and 2c. Fluorination of 1a and 1c was carried out using the above-
described general procedure. Subsequently, 2 mmol of fluorobenzene
(internal standard) was added to the crude reaction mixture collected
from the reactor output. A 0.3 mL aliquot of the resulting solution was
diluted in DMSO-d6 and analyzed by 19F NMR spectroscopy.
1-tert-Butyl-4-(fluoromethyl)benzene (2b). Yield 295 mg, 89%;
1H NMR (300 MHz, CDCl3) δ 7.46 (d, J = 9.0 Hz, 2H), 7.36 (d, J =
Methyl 2-(4-(1-Fluoro-2-methylpropyl)phenyl)propanoate
1
(2l). Yield 381 mg, 80%; H NMR (300 MHz, CDCl3) δ 7.35−7.22
(m, 1H), 5.10 (dd, J = 47.0, 6.8 Hz, 1H), 3.75 (dd, J = 14.5, 7.3 Hz,
1H), 3.68 (s, 1H), 2.22−2.04 (m, 1H), 1.52 (d, J = 7.2 Hz, 1H), 1.04
(d, J = 6.7 Hz, 1H), 0.87 (d, J = 6.8 Hz, 1H); 13C NMR (75 MHz,
CDCl3) δ 174.9, 140.3, 138.4, 138.1, 127.3, 126.5, 126.4, 100.2, 97.9,
52.1, 45.2, 34.4, 34.1, 18.6, 18.4, 18.3, 17.6, 17.5; 19F NMR (282 MHz,
CDCl3) δ −179.6 (ddd, J = 47.0, 16.9, 6.0 Hz); MS-EI m/z 238
(20%), 195 (100%), 179 (70%).
9.0 Hz, 2H), 5.47 (s, 1H), 5.30 (s, 1H), 1.37 (s, 1H); 13C NMR (75
MHz, CDCl3) δ 152.0, 152.0, 133.3, 133.1, 127.7, 127.6, 125.6, 125.6,
85.7, 83.5, 34.7, 31.3; 19F NMR (282 MHz, CDCl3) δ −204.3 (t, J =
47.9 Hz); MS-EI m/z 166 (20%), 151 (100%), 123 (40%).
1-Bromo-4-(1-fluoroethyl)benzene (2d). Yield 243 mg, 60%;
1H NMR (300 MHz, CDCl3) δ 7.53 (d, J = 9.0 Hz, 2H), 7.24 (d, J =
9.0 Hz, 2H), 5.60 (dq, J = 48, 6 Hz, 1H), 1.64 (dd, J = 24, 6 Hz, 3H);
13C NMR (75 MHz, CDCl3) δ 140.6, 140.4, 131.6, 127.0, 126.9, 122.1,
91.4, 89.2, 23.0, 22.7; 19F NMR (282 MHz, CDCl3) δ −168.0 (dq, J =
22.6, 45.1 Hz); MS-EI m/z 204 (45%), 202 (50%), 189 (90%), 187
(100%).
4-Acetyl-6-tert-butyl-3-fluoro-1,1-dimethylindane (2m).
1
Yield 462 mg, 88%; H NMR (300 MHz, CDCl3) δ 7.78 (d, J = 1.6
Hz, 1H), 7.44 (t, J = 1.5 Hz, 1H), 6.46 (ddd, J = 53.8, 5.9, 1.4 Hz, 1H),
2.67 (s, 3H), 2.42−2.08 (m, 2H), 1.39 (s, 12H), 1.36 (s, 3H); 13C
NMR (75 MHz, CDCl3) δ 199.7, 155.9, 155.8, 154.2, 154.1, 135.2,
134.9, 134.9, 125.6, 123.6, 94.9, 92.6, 48.5, 48.2, 42.7, 42.7, 35.1, 31.5,
31.4, 29.0, 28.9, 28.6; 19F NMR (282 MHz, CDCl3) δ −158.6 (ddd, J
= 54.3, 33.8, 23.8 Hz); MS-EI m/z 262 (70%), 247 (100%), 227
(35%).
1
3-Fluoro-3-phenylpropanoic Acid (2e). Yield 246 mg, 73%; H
ASSOCIATED CONTENT
* Supporting Information
■
NMR (300 MHz, CDCl3) δ 7.46−7.39 (m, 5H), 5.95 (ddd, J = 45.0,
9.0, 3.0 Hz, 1H), 3.18−2.80 (m, 2H); 13C NMR (75 MHz, CDCl3) δ
176.0, 176.0, 138.4, 138.1, 129.0, 129.0, 128.8, 125.7, 125.6, 91.4, 89.1,
42.4, 42.0; 19F NMR (282 MHz, CDCl3) δ −172.91 (ddd, J = 46.5,
32.5, 13.6 Hz).
S
Supplementary figures and copies of NMR spectra of all
prepared compounds. This material is available free of charge
via the Internet at http://pubs.acs.org.
Ethyl 3-Fluoro-3-phenylpropanoate (2f). Yield 318 mg, 81%;
1H NMR (300 MHz, CDCl3) δ 7.44−7.35 (m, 5H), 5.95 (ddd, J =
46.9, 9.1, 4.2 Hz, 1H), 4.21 (q, J = 7.2 Hz, 2H), 3.11−2.72 (m, 2H),
1.29 (t, J = 7.1 Hz, 3H); 13C NMR (75 MHz, CDCl3) δ 169.69,
169.63, 138.81, 138.55, 128.83, 128.81, 128.65, 125.66, 125.57, 91.79,
89.54, 60.99, 42.70, 42.34, 14.14; 19F NMR (282 MHz, CDCl3) δ
−172.91 (ddd, J = 46.5, 32.5, 13.6 Hz); MS-EI m/z 196 (15%), 167
(10%), 151 (15%), 125 (40%), 122 (40%), 109 (100%).
AUTHOR INFORMATION
Corresponding Authors
*E-mail: o.defrtos@lilly.com.
*E-mail: oliver.kappe@uni-graz.at.
Notes
■
The authors declare no competing financial interest.
1
4-(1-Fluoroethyl)biphenyl (2g). Yield 340 mg, 85%; H NMR
ACKNOWLEDGMENTS
■
(300 MHz, CDCl3) δ 7.66−7.62 (m, 4H), 7.49 (t, J = 9.0 Hz, 4H),
7.39 (t, J = 6.0 Hz, 1H), 5.72 (dq, J = 47.7, 6.4 Hz, 1H), 1.73 (dd, J =
23.8, 6.4 Hz, 3H); 13C NMR (75 MHz, CDCl3) δ 141.3, 141.2, 140.7,
140.6, 140.3, 128.8, 127.5, 127.3, 127.2, 125.8, 125.7, 91.9, 89.7, 23.1,
22.7; 19F NMR (282 MHz, CDCl3) δ −166.6 (dq, J = 47.7, 23.8 Hz);
MS-EI m/z 200 (40%), 185 (100%).
This work was supported by Eli Lilly and Company through the
Lilly Research Award Program (LRAP). The authors gratefully
acknowledge support from NAWI Graz.
REFERENCES
■
4-(1-Fluoroethyl)benzoic Acid (2h). Yield 269 mg, 80%; 1H
NMR (300 MHz, CDCl3) δ 8.16 (d, J = 8.1 Hz, 1H), 7.48 (d, J = 8.4
Hz, 1H), 5.72 (dq, J = 47.6, 6.5 Hz, 1H), 1.68 (dd, J = 24.0, 6.5 Hz,
1H); 13C NMR (75 MHz, CDCl3) δ 171.9, 147.7, 147.4, 130.5, 129.0,
125.1, 125.0, 91.4, 89.2, 23.2, 22.9; 19F NMR (282 MHz, CDCl3) δ
−171.7 (dq, J = 47.9, 24.0 Hz); MS-EI m/z 168 (40%), 153 (60%),
123 (100%).
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̈
1-Chloro-3-fluoro-3-phenylpropane (2i). Yield 244 mg, 71%;
1H NMR (300 MHz, CDCl3) δ 7.52−7.32 (m, 5H), 5.72 (ddd, J =
47.8, 8.9, 3.9 Hz, 1H), 3.78 (ddd, J = 11.0, 8.7, 5.6 Hz, 1H), 3.71−3.53
(m, 1H), 2.62−2.38 (m, 1H), 2.38−2.06 (m, 1H); 13C NMR (75
MHz, CDCl3) δ 139.3, 139.0, 128.7, 125.5, 125.4, 92.4, 90.1, 40.5,
40.4, 40.2, 39.9; 19F NMR (282 MHz, CDCl3) δ −179.4 (ddd, J =
45.6, 31.2, 14.1 Hz); MS-EI m/z 172 (10%), 109 (100%).
4-Fluoro-2-methyl-4-phenylbutan-2-ol (2j). Yield 265 mg,
73%; 1H NMR (300 MHz, CDCl3) δ 7.48−7.33 (m, 5H), 5.82
(ddd, J = 49.0, 10.2, 2.2 Hz, 1H), 2.39−2.14 (m, 1H), 2.07−1.69 (m,
1H), 1.40 (s, 1H), 1.38 (s, 1H); 13C NMR (75 MHz, CDCl3) δ 140.7,
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(19) Although at 60 °C side products had been observed after 2 h of
reaction for the model substrate (Table 1, entry 6), this was not the
case when the combination of xanthone as photocatalyst and black
light was used. In this case, fluorination of ethylbenzene at 60 °C
resulted in excellent conversion and selectivity after 28 min of
irradiation (corresponding to the residence time inside the reactor at 1
mL min−1). Possibly the side reactions previously observed were due
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