2,2,2-Trifluoroethylation of Styrenes with Concomitant Introduction of a Hydroxyl Group from Molecular Oxygen by Photoredox Catalysis Activated by Visible Light

Article abstract of DOI:10.1021/acs.orglett.5b02177

The visible-light-induced photoredox difunctionalization reactions of styrenes with 1,1,1-trifluoro-2-iodoethane under an oxygen atmosphere in the presence of water give γ-trifluoromethyl alcohols. In this radical reaction, the oxygen atom in the product originates from molecular oxygen, and water is shown to be important to promote the reaction.

Full text of DOI:10.1021/acs.orglett.5b02177

Letter
pubs.acs.org/OrgLett
2,2,2-Trifluoroethylation of Styrenes with Concomitant Introduction
of a Hydroxyl Group from Molecular Oxygen by Photoredox
Catalysis Activated by Visible Light
Lun Li,† Meiwei Huang,‡ Chao Liu,† Ji-Chang Xiao,† Qing-Yun Chen,*,† Yong Guo,*,†
and Zhi-Gang Zhao‡
†Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling
Road, Shanghai 200032, P.R. China
‡College of Chemistry and Environment Protection Engineering, Southwest University for Nationalities, Chengdu 610041, P. R.
China
S
* Supporting Information
ABSTRACT: The visible-light-induced photoredox difunctionalization
reactions of styrenes with 1,1,1-trifluoro-2-iodoethane under an oxygen
atmosphere in the presence of water give γ-trifluoromethyl alcohols. In
this radical reaction, the oxygen atom in the product originates from
molecular oxygen, and water is shown to be important to promote the reaction.
Scheme 1. Methodologies to CF3CH2-Containing Molecules
by Direct 2,2,2-Trifluoroethylation
ynthetic methodologies of CF3-containing compounds have
Sattracted attention because of the broad applications of such
compounds in fluorinated agrochemicals, pharmaceuticals, and
functional materials.1 Molecules containing a 2,2,2-trifluoroethyl
group can be synthesized from benzyl, allylic, and propargyl
halides or trifluoroacetate,2 and also from terminal alkenes3 with
multiple trifluoromethylation reagents, such as Chen’s reagent
(methyl fluorosulfonyldifluoroacetate), Ruppert−Prakash re-
agent ((trifluoromethyl)trimethylsilane), Togni reagent (1-
(trifluoromethyl)-1,2-benziodoxol-3(1H)-one and trifluoro-
methyl 1,3-dihydro-3,3-dimethyl-1,2-benziodoxole), and Ume-
moto’s reagent (S-(trifluoromethyl)dibenzothiophenium salts).
The first direct 2,2,2-trifluoroethylation on iodobenzene, despite
the low yield, was reported by McLoughlin and Thrower in
1969.4 To date, the direct 2,2,2-trifluoroethylation has been
categorized into C(sp2-vinyl)−CH2CF3,5 C(sp2-aryl)−
CH2CF3,6 C(sp)−CH2CF3,7 and C(sp3)−CH2CF3 formation-
s
5a,8 through CF3CH2 radical or metal-mediated cross-coupling
processes (Scheme 1). The amount of research on 2,2,2-
trifluoroethylation has been far less than that on trifluorome-
thylation.
Difunctionalization of a carbon−carbon double bond is a
powerful synthetic strategy for compounds with various
functional groups.9 Dioxygen, as a clean oxygen source, has
been used in the difunctionalization of a C−C double bond.10,11
Recently, a variety of organometallic complexes, which could be
activated by visible light, have been widely applied in the
development of new organic reactions.12 Owing to the electron-
withdrawing ability of the fluorine atom, polyfluorinated
substrates usually exhibit a higher redox potential than their
nonfluorinated analogues and thus tend to be utilized in
photoredox synthesis activated by visible light.13 Among the
visible-light-promoted fluoroalkylation reactions are the difunc-
tionalization reactions.14−22 However, to the best of our
knowledge, visible-light-induced hydroxytrifluoroethylation of a
double bond has never been reported.
Within our investigations on the organic transformation at the
neighboring carbon to a CF3 group,23 we were interested in
activating 1,1,1-trifluoro-2-iodoethane (CF3CH2I, 1) to a radical
Received: July 27, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b02177
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Letter
using visible-light-induced photoredox catalysis. We have already
reported the sulfinatodehalogenation of 1 with alkyl-substituted
alkenes and that this reaction cannot be applied to styrenes.5a
Herein, by using a visible-light-activation system, we develop a
difunctionalization method by the concomitant introduction of a
hydroxyl group and a 2,2,2-trifluoroethyl group to each end of a
double bond in styrenes (Scheme 1).
We proposed that the moisture in air might contribute to the
difference between the two reactions. To confirm the role of
water in this reaction, we examined the effect of water content on
the difunctionalization in 0.2 mmol scale of 5a in 4 mL of
CH3CN (Figure 1, other conditions as in Scheme 3). In an air
The reaction of styrene 5a with 1 was carried out in the
presence of a catalytic amount of photoredox catalyst fac-
Ir(ppy)3 and excessive N,N-diisopropylethylamine (Hunig’s
̈
base) upon irradiation of visible light using acetonitrile as
solvent under a nitrogen atmosphere at room temperature for 24
h. Although there are several new products after completion of
the reaction, as identified by 19F NMR spectroscopy, we were
able to isolate 6a as a major product in 66% yield. Compound 6a
is a 1:1 mixture of two diastereomers, which can be partially
separated by repeat chromatography on silica gel. Other
photoredox catalysts, such as Ru(bpy)3(PF6)2, Ir-
(ppy)2(dtbbpy), and Ir(dF(CF3)ppy)2(dtbbpy)PF6, were exam-
ined and provided lower yields of products as determined by
identification of the reaction mixtures by 19F NMR spectroscopy.
We proposed that 6a was formed from the dimerization of
benzyl radical Int I, which resulted from the addition of the 2,2,2-
trifluoroethyl radical to styrene 5a (Scheme 2). The dimerization
Figure 1. Effect of water content on hydroxytrifluoroethylation. (a) The
yields were determined by 1H NMR spectroscopy with additional
CH2Br2 as an internal standard.
atmosphere, the yield reached 50% in the presence of 120 μL of
water. In an oxygen atmosphere, 500 μL of water was needed to
enhance the yield to 77%. These experiments suggest that water
can promote hydroxytrifluoroethyl difunctionalization.
Scheme 2. Photoredox Reaction under a Nitrogen
Atmosphere
Under the optimal conditions, we investigated the scope of
alkenes (Scheme 4). The presence of an electron-donating (8c−
e) or halogen (8f−k) substituent on the aromatic ring was well-
tolerated under the optimized conditions. For the α-CH3-
substituted styrene (8k−n), we could also obtain the
difunctionalized products. In the reaction with 5a, there were
several sides products, and one of the side products, though in a
small amount, was 6a. In the reaction with 1-(prop-1-en-2-
yl)naphthalene (5n), 59% of 5n was recovered. No dimerization
was found. The reaction conditions cannot be applied to aliphatic
alkene and alkyne. Using this method, the hydroxyl and
trifluoroethyl group can be introduced in one step, which is
difficult to obtain by the conventional strategy. The hydroxyl
group is widely present in physiologically active molecules, and
the CF3 group is known to be able to enhance lipophilicity,
binding selectivity, and metabolic stability. This approach to the
γ-trifluoromethyl alcohols will be useful.
showed that the benzyl radical was stable and had a long lifetime.
Molecular oxygen is usually used as a radical scavenger. To
suppress the dimerization of the benzyl radical, we carried out the
reaction under air or in an oxygen atmosphere. Unexpectedly,
two special products were observed, which kindled our interest in
investigating the reaction (Scheme 3). The reaction under an
Scheme 3. Photoredox Reaction under an Air or Oxygen
a
Atmosphere
A preliminary investigation of the reaction mechanism
suggested that the reaction most likely involved a 2,2,2-
trifluoroethyl radical. The yield of the target product (8a) was
reduced from 77% to 0% when the reaction was conducted under
the optimized conditions in the presence of TEMPO, hydro-
quinone, and 1,4-dinitrobenzene (Scheme 5a). Furthermore,
a
Conditions: 5a (0.2 mmol), 1 (0.6 mmol), fac-Ir(ppy)3 (0.002
i
TEMPO−CH2CF3 was detected in the reaction mixture by 19
F
mmol), and Pr2NEt (0.6 mmol) in acetonitrile (4 mL) under a
molecular oxygen or air atmosphere upon irradiation of 24 W
NMR spectroscopy and GC−MS. These experiments prove that
the reaction is an SET/radical process. A radical clock
experiment was also conducted, which confirmed the existence
of a trifluoroethyl radical in the reaction mixture (Scheme 5b). In
the experiment, the normal product 10 was formed with a yield of
isolated product of 33%. Additionally, we isolated the ring-
opened product with an iodine substituent (11) but not with an
hydroxyl group (12). Under the standard conditions, when we
replaced H2O by H218O, we isolated the product 8a and no 18O-
labeled product was detected. This means that the oxygen atom
in the product 8a does not originate from H2O (Scheme 5c). We
also carried out the reaction under an 18O2 atmosphere and
fluorescence lamp, rt, 48 h.
oxygen atmosphere gave 2,2,2-trifluoroethyl alcohol, which was
identified by comparison with 7. Further tosylation of 7 to give
CF3CH2OTs (7′) confirmed the structure. In the reaction under
an air atmosphere, in addition to 2,2,2-trifluoroethyl alcohol,
another alcohol 8a (about 20% determined by GC−MS) was
separated and identified by multiple analytical methods. Most of
the 4-phenylstyrene 5a was recovered, and the dimerization was
suppressed.
B
DOI: 10.1021/acs.orglett.5b02177
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Letter
a,b
obtained product 14 with a mass of 282. This confirms that the
oxygen atom in product 8a originates from molecular oxygen.
Based on these experimental results, we propose that the
trifluoroethyl difunctionalization of alkenes proceeds by the
mechanism shown in Scheme 6. First, fac-Ir(III) (ppy)3
Scheme 4. Scope of Styrenes
Scheme 6. Proposed Mechanism
undergoes metal-to-ligand charge transfer (MLCT) irradiated
by visible light, producing a redox-active photoexcited state, fac-
Ir(IV)(ppy)2(ppy•−). iPr2NEt acts as a reductive quencher, and
the fac-Ir(IV) (ppy)2(ppy•−) accepts an electron from iPr2NEt to
generate the fac-Ir(III)(ppy)2(ppy•−) species, which has a high
reducing ability. Single-electron reduction of CF3CH2I by fac-
Ir(III)(ppy)2(ppy•−) yields a radical anion and then generates a
•
2,2,2-trifluoroethyl radical (CF3CH2 ). If the reaction is carried
a
Reaction conditions: alkenes (0.2 mmol), 1 (0.6 mmol), fac-Ir(ppy)3
(0.001 or 0.002 mmol), iPr2NEt (0.6 mmol), MeCN (4 mL), and H2O
(0.5 mL) under a molecular oxygen atmosphere, irradiation with a 24
out under an O2 atmosphere, the trifluoroethyl radical can be
captured by molecular oxygen to produce CF3CH2OH. In
another pathway, the trifluoroethyl radical can react with styrene
to give benzyl radical Int I, which yields the hydroxyl products 8
promoted by water in the presence of molecular oxygen. If the
reaction is carried out under a N2 atmosphere, the benzyl radical
Int I couples with itself to give the dimerization product 6a.
In conclusion, we have developed a visible-light-induced
method for the hydroxytrifluoroethylation of styrenes under a
molecular oxygen atmosphere in the presence of water. In this
radical reaction, the oxygen atom in the product originates from
molecular oxygen. However, the presence of H2O is very
important for the reaction. Without the presence of water, the
product is 2,2,2-trifluoroethanol, and no difunctionalized
product is observed. This reaction can be applied to styrenes
possessing electron-donating and halogen substituents at the aryl
rings and also with substituents at the β-positions of double
bonds.
b
W fluorescence lamp, rt, 48 h. Isolated yields are shown.
Scheme 5. Mechanistic Experiments
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b02177.
Experimental procedures, characterization and NMR
spectra of new compounds (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: chenqy@sioc.ac.cn.
*E-mail: yguo@sioc.ac.cn.
C
DOI: 10.1021/acs.orglett.5b02177
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Letter
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Support of our work by National Basic Research Program of
China (973 Program) (No. 2012CB821600), National Natural
Science Foundation of China (Nos. 21421002, 21032006,
21172241), and the Chinese Academy of Sciences is gratefully
acknowledged.
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