Organic Dye-Catalyzed Atom Transfer Radical Addition-Elimination (ATRE) Reaction for the Synthesis of Perfluoroalkylated Alkenes

Article abstract of DOI:10.1021/acs.orglett.7b01952

An atom transfer radical addition elimination (ATRE) reaction of terminal alkenes with perfluoroalkyl halides under visible light is described. The photoredox catalysis with Eosin Y provides perfluoroalkenes in good yields. The reaction has been utilized

Full text of DOI:10.1021/acs.orglett.7b01952

Letter
pubs.acs.org/OrgLett
Organic Dye-Catalyzed Atom Transfer Radical Addition−Elimination
(ATRE) Reaction for the Synthesis of Perfluoroalkylated Alkenes
Deo Prakash Tiwari, Saumya Dabral, Jian Wen, Jan Wiesenthal, Steven Terhorst, and Carsten Bolm*
Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, D-52074 Aachen, Germany
S
* Supporting Information
ABSTRACT: An atom transfer radical addition elimination (ATRE)
reaction of terminal alkenes with perfluoroalkyl halides under visible light
is described. The photoredox catalysis with Eosin Y provides
perfluoroalkenes in good yields. The reaction has been utilized for the
late stage perfluoroalkenylation of an estrone-derived alkene.
ntroduction of fluorine into organic compounds has been
Scheme 1. Perfluoroalkylation of Compounds with Carbon−
I
one of the major areas of research in recent years.1 Owing to
Carbon Multiple Bonds
their tendency to alter the lipophilicity, metabolic stability, and
electronic properties, fluorinated organic compounds have been
widely used in medicinal chemistry,2 crop science,3 and several
other industrial functions.4 Perfluoroalkylation5−11 is one of the
most popular methods to incorporate fluorine into organic
compounds. Since the first report by Kharasch and co-
workers5a,b in 1945, atom transfer radical addition (ATRA)5
has extensively been utilized for perfluoroalkylation of organic
compounds.6 ATRA-based approaches often require peroxide,7a
Na2S2O4,7b,d Et3B,7c metal catalysts,7e,j,m or UV light8 as the
radical initiators. Recently, photoredox catalysis9 has emerged
as an alternative to the traditional initiator-promoted radical
reactions. The past decade witnessed exponential progress in
visible-light-promoted perfluoroalkylation reactions.10,11 Even
after undergoing a significant renaissance in recent years,
photoredox-catalyzed perfluoroalkylation encounters several
issues to be addressed such as (1) employment of very costly
Ru/Ir polypyridyl metal complexes as photoredox catalysts; (2)
use of the several additives, which could promote side
reactions; and (3) a narrow photocatalyst scope for different
kinds of reactions and perfluoroalkylating reagents.
The high price and tedium involved in the synthesis of
traditional Ru/Ir-photocatalysts prompted researchers to un-
cover several alternative photocatalysts such as TiO2,8f
Cu(dap)2Cl,10h,i EDA-complexes,10e photosensitizers,10j and
organic dyes.11
We were interested in developing a one-step route for the
synthesis of perfluoroalkenes12 directly from alkenes through an
atom transfer radical addition elimination (ATRE)13 reaction
utilizing organic dye-promoted photoredox catalysis (Scheme
1).
For our initial studies, we performed the reaction of 4-phenyl
but-1-ene (1a) and C4F9I (2a, 3.0 equiv) employing Eosin Y (5
mol %) as the photocatalyst under visible light from a 16 W
white fluorescent lamp under argon. The results are
summarized in Table 1.
In THF and water, using Cs2CO3 as the base, only addition
occurred, and iodoperfluoroalkane 4 was obtained as the sole
product in 66% and 63% yield, respectively (Table 1, entries 1
and 2). In MeCN, however, the desired 1-perfluoroalkene 3a
(45%) was indeed generated, albeit still in combination with 4
(13%) (Table 1, entry 3). The desired 3a was formed in 72%
yield as the sole product when the reaction was performed in
DMF (Table 1, entry 4). Switching to dimethylacetamide
(DMAc) as the solvent significantly improved the yield of 3a
(95%), and also in this case 4 remained undetected (Table 1,
entry 5). Reduction in the amount of the perfluoroalkyl iodide
from the standard 3 equiv to 1 equiv of 2a lowered the yield of
3a to 75% (Table 1, entry 6). In general, the transformation
was highly dependent on additives, and weaker bases such as
ATRA of perfluoroalkyl halides to alkenes and alkynes leads
to the generation of halo perfluoroalkanes A (-enes, C)
(Scheme 1, eqs 1 and 2). If a proton source is present,
perfluorolalkanes (-enes) are the observed products. Often the
solvent and additives determine which product will be
generated. We hypothesized that a suitable combination of
base and solvent could generate perfluoroalkenes14 through
ATRE directly from terminal alkenes (Scheme 1, eq 3).
Received: June 26, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01952
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Table 1. Optimization Studies
Scheme 2. Substrate Scope
b
c
entry
base
photocatalyst
solvent
3a (%) (E/Z) 4 (%)
1
2
3
4
5
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2HPO4
DIPEA
TBD
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Rose Bengal
Rhodamine B
Bn2NH
−
THF
−
−
66
63
13
−
−
−
30
75
29
−
−
−
water
CH3CN
DMF
45 (3.4:1)
72 (4.5:1)
95 (5:1)
75 (5:1)
41 (3.2:1)
34 (−)
71 (3.7:1)
72 (4.3:1)
55 (3.4:1)
75 (3.1:1)
50 (−)
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
d
6
e
7
8
9
10
DBU
fg
11 ,
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
fg
12 ,
13
14
−
−
−
49 (4.4:1)
h
15
Eosin Y
22 (−)
a
Reaction conditions: 1a (0.38 mmol), C4F9I (1.14 mmol, 3.0 equiv),
Eosin Y (5 mol %), Cs2CO3 (2.0 equiv), and DMAc (3.8 mL) at rt for
b
48 h in visible light under argon. In parentheses, E/Z ratios as
c
determined by 1H NMR spectroscopy. Ratio of 3a and 4 as
determined by 1H NMR using 1,3,5-trimethoxybenzene (TMB) as the
d
e
internal standard. Use of 1.0 equiv of C4F9I. 16% of 1a remain
f
g
unreacted. 1H NMR yield using TMB as the internal standard. Yields
based on unreacted 1a. Without light source; 10% of 1a remained
unreacted, and the yield of 3a was determined by H NMR using
h
1
1,3,5-trimethoxybenzene (TMB) as the internal standard.
a
1
K2HPO4 and DIPEA promoted formation of mixtures of both
3a and 4 (Table 1, entries 7 and 8).
In parentheses, E/Z ratios as determined by H NMR spectroscopy.
b
c
On a 1.5 mmol scale: 88% of 3a (E:Z = 4:1). Formation of the E
d
isomer exclusively. N.D. = not determined (due to its volatility).
Unlike other photoredox-catalyzed perfluoroalkylation re-
actions, DBU10b and TBD were less effective although
complete conversion of the alkene 1a was observed (Table 1,
entries 9 and 10). Other organic dyes (Rose Bengal and
Rhodamine B) were inferior in terms of yield when compared
to Eosin Y (Table 1, entries 11 and 12). The recently reported
dibenzyl amine-catalyzed perfluoroalkyation10k was less effi-
cient, producing 3a in 50% yield only (Table 1, entry 13).
When the reaction was performed without a photocatalyst,
the yield of 3a was only 49% (Table 1, entry 14).15 In the
absence of the light source, 3a was obtained in 22% yield and
10% of 1a remained unreacted (Table 1, entry 15).
Next, the scope of the photocatalysis was examined. The
results are shown in Scheme 2. Reacting the corresponding
alkenes with perfluoroalkyl iodides C3F7I, i-C3F7I C4F9I, C6F13I,
and C8F17I under the aforementioned optimized conditions
afforded perfluoroalkenes 3a−x in good yields. Functional
groups had only a minor effect on the reaction outcome. On a
1.5 mmol scale, the reaction of 4-phenyl but-1-ene (1a) with
C4F9I (2a) provided 3a in 88% yield (E/Z = 4:1).
Several observations are noteworthy: The first one relates to
the formation of iodo-substituted perfluoroalkene 3l. Starting
from sulfonate 1l it was formed by a multistep pathway
involving an initial ATRE reaction followed by a subsequent
SN2 substitution with in situ generated iodide. Second, whereas
internal olefins produced complex mixtures of regioisomers and
E/Z-isomers (not shown), 2,2-disubstituted alkenes coupled
well as illustrated by the formation of 3o in 76% yield. Reacting
styrene, representing an aryl-substituted alkene, with C4F9I
afforded perfluoroalkene 3x in 52% yield. Compared to the
other transformations, this yield was only moderate, presum-
ably due to the high volatility of perfluorinated 3x. Finally, the
analogous reaction with estrone-derived alkene 1y led to
product 3y in 95% yield, confirming the pronounced functional
group tolerance of the process, which might prove useful for
late-stage functionalizations of complex natural products of
drug-like molecules.16
B
DOI: 10.1021/acs.orglett.7b01952
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Organic Letters
Letter
Next, we examined if the same conditions could also be
applied for difluoroalkylations of alkenes. Upon reaction of the
corresponding alkenes 1 with CF2Br2 (5a),10d bromodifluoro-
alkenes 6a−c were generated in yields ranging from 65% to
80% (Scheme 3). Similarly, ICF2CO2Et (5b)10b reacted with
perfluoroalkyl halides. It utilizes visible light and inexpensive
organic dyes as photocatalysts. Various functional groups on
the alkene are tolerated, and the process is applicable for late-
stage perfluoroalkenylations of complex and diversely sub-
stituted molecules.
ASSOCIATED CONTENT
* Supporting Information
Scheme 3. Organic Dye-Catalyzed Synthesis of
Difluoroalkenes
■
a
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b01952.
General experimental procedure and characterization
details (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: Carsten.Bolm@oc.rwth-aachen.de.
ORCID
Carsten Bolm: 0000-0001-9415-9917
Notes
The authors declare no competing financial interest.
a
In parentheses, E/Z ratios as determined by 19F NMR spectroscopy.
Instead of Eosin Y, Rhodamine B was used as photocatalyst.
b
ACKNOWLEDGMENTS
■
D.P.T. acknowledges the Alexander von Humboldt Foundation
for a postdoctoral fellowship. S.D. thanks the European Union
for support (Marie Curie ITN “SuBiCat” PITN-GA-2013-
607044), and J.W. is grateful to the China Scholarship Council
for a predoctoral stipend.
alkenes to give 6d and 6e in 68% and 73% yield, respectively.
For the reactions involving 5b as a perfluoroalkyl halide,
Rhodamine B was found to be superior over Eosin Y as a
photocatalyst.11g
Scheme 4 shows a plausible mechanism for the visible light
promoted ATRE reaction. First, photoexcited Eosin Y (EY) is
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