Efficient cu-catalyzed atom transfer radical addition reactions of fluoroalkylsulfonyl chlorides with electron-deficient alkenes induced by visible light

Article abstract of DOI:10.1002/anie.201412199

Fluoroalkylsulfonyl chlorides, R_fSO₂Cl, in which R_f=CF₃, C₄F₉, CF₂H, CH₂F, and CH₂CF₃, are used as a source of fluorinated radicals to add fluoroalkyl groups to electron-deficient, unsaturated carbonyl compounds. Photochemical conditions, using Cu mediation, are used to produce the respective α-chloro-β-fluoroalkylcarbonyl products in excellent yields through an atom transfer radical addition (ATRA) process. Facile nucleophilic replacement of the α-chloro substituent is shown to lead to further diverse functionalization of the products.

Full text of DOI:10.1002/anie.201412199

Angewandte
Chemie
DOI: 10.1002/anie.201412199
Photoredox Catalysis
Efficient Cu-catalyzed Atom Transfer Radical Addition Reactions of
Fluoroalkylsulfonyl Chlorides with Electron-deficient Alkenes Induced
by Visible Light**
Xiao-Jun Tang and William R. Dolbier, Jr.*
Abstract: Fluoroalkylsulfonyl chlorides, RfSO2Cl, in which
Rf = CF3, C4F9, CF2H, CH2F, and CH2CF3, are used as a source
of fluorinated radicals to add fluoroalkyl groups to electron-
deficient, unsaturated carbonyl compounds. Photochemical
conditions, using Cu mediation, are used to produce the
respective a-chloro-b-fluoroalkylcarbonyl products in excel-
lent yields through an atom transfer radical addition (ATRA)
process. Facile nucleophilic replacement of the a-chloro
substituent is shown to lead to further diverse functionalization
of the products.
of the fluorinated radical to the electron-deficient alkene
(Scheme 1). Thus the rate of abstraction of the halogen atom
from RfX or RfSO2Cl by the electrophilic radical intermediate
is diminished, chain propagation is inhibited, and the ATRA
T
he unique physical and chemical properties of fluoroor-
ganic compounds often bestow enhanced biological proper-
ties upon them compared to their nonfluorinated analogues.
Therefore, fluorine-containing compounds continue to
receive particular attention in the agrochemical, pharmaceut-
ical, and material science industries.[1] Of the many ways to
introduce fluorine-containing substituents into molecules,
additions of fluorinated radicals to alkenes, alkynes, and
arenes remain among the most direct and useful methods.[2] In
particular, atom transfer radical addition (ATRA) reactions
of RfX (X = I, Br) to electron-rich alkenes is one such well-
demonstrated methodology, which has been utilized fre-
quently in recent decades. In these reactions the Rf radical is
usually generated thermally by use of catalytic amounts of
low-valent metals[3] or radical initiators such as Na2S2O4,[4]
azobisisobutyronitrile (AIBN),[5] benzoyl peroxide (BPO),[6]
Et3B/O2,[7] or by use of reductive photocatalysts[8] to afford 1:1
adducts in high yields. Thus far, such methods have not
proved useful for additions to electron-deficient alkenes, the
use of which resulted in low conversion, selectivity, and yield,
along with the formation of undesired byproducts and
oligomers.[9] Recent attempts to improve yields of reactions
of RfI with electron-deficient alkenes by the use of UV light
have been reported, but a large excess of RfI and highly
diluted reaction conditions were required.[10] In addition,
elimination[10] and reduced products and oligomers[11] were
commonly observed as significant byproducts. Such unsatis-
factory results can be largely attributed to the electrophilic
nature of the radical intermediate that is formed by addition
Scheme 1. ATRA reaction of the fluorinated radical with electron-
deficient alkene.
process is diverted to form alternative products. In addition,
the impact of the adjacent electron-withdrawing group makes
it energetically unfavorable for the radical to be oxidized to
a corresponding carbocation that could be trapped by halide
ions.
Since the first FeCl2-catalyzed ATRA reaction of CCl4
with an electron-deficient alkene, acrylonitrile, was reported
in 1956,[12] transition-metal (Ru[13] and Cu[14])-catalyzed
ATRA/ATRP reactions of polychloroalkanes or activated
alkyl halides with electron-rich or electron-deficient alkenes
have become widely used in organic synthesis and polymer-
ization. Thus far, due to the poor reducing ability of Ru or Cu
species, fluoroalkyl halides have not proved very effective in
such reactions, with useful reactions being mainly limited to
easily reduced polyhalo, fluorinated alkanes, with temper-
atures as high as 1408C being required in some cases.[15] In
1989, a Ru-catalyzed ATRA reaction using CF3SO2Cl was
reported using ethyl acrylate as substrate to obtain a moderate
yield of the desired 1:1 adduct.[16] However, because of the
relatively poor reducing ability of RuCl2(PPh3)3, this protocol
required the use of a large excess of ethyl acrylate (5 equiv)
along with high temperatures (1208C) in benzene.
[*] Dr. X.-J. Tang, Prof. Dr. W. R. Dolbier, Jr.
Department of Chemistry, University of Florida
PO Box 117200, Gainesville, Florida, 32611-7200 (USA)
E-mail: wrd@chem.ufl.edu
Over the last few years, photoredox catalysis has received
much attention due to the excellent reducing ability of
photocatalysts in the excited state.[17] Recently, we reported
a general method of generating fluorinated radicals such as
[**] Support of this research by a grant from Syngenta Crop Protection is
gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201412199.
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CF2H, CFH2, and CF3CH2 from their corresponding fluo-
roalkylsulfonyl chlorides using photoredox catalysis under
mild conditions.[18] On the basis of that work, we came to
believe that photoinduced ATRA reactions of fluoroalkyl-
sulfonyl chlorides with electron-deficient alkenes might be
possible.
Using the reaction of CF3SO2Cl with N-methyl-N-phenyl-
acrylamide (2a) as model reaction for exploratory experi-
ments, we did not find reaction conditions that produce
a significant yield of addition product when using either
Ru(phen)3Cl2 or Ru(bpy)3Cl2 as catalysts (Table 1, entries 1–
mediating the transfer of the Cl atom.[21] Nevertheless, the
poor redox potentials of commonly used Cu catalysts[22] made
them unattractive as potential catalysts for reducing fluo-
roalkylsulfonyl chlorides. Recently, however, Cu(dap)2Cl was
reported to be an efficient photocatalyst in the reaction of
alkyl halides with electron-rich alkenes.[23] We thus speculated
that Cu(dap)2Cl, with its excellent reductive ability in the
excited state (E1/2II/I* = À1.42 V vs. SCE in MeCN), combined
with good Cl transfer properties, might be a good choice for
initiating and propagating the ATRA reaction of fluoroal-
kylsulfonyl chlorides with electron-deficient alkenes. Indeed,
the reaction of a,b-unsaturated amide 2a with CF3SO2Cl in
MeCN resulted in formation of the desired adduct 3aa in
57% yield (Table 1, entry 9). Yields improved significantly
when DCM was used as the solvent instead of MeCN
(Table 1, entry 11). Lastly, the addition of 20 mol%
K2HPO4, led to a virtually quantitative yield of 3aa in
either DCM or DCE (Table 1, entries 12 and 13).
Table 1: Exploratory experiments for reaction of CF3SO2Cl with N-methyl-
N-phenylacrylamide.
Yield[a]
Using these optimized conditions, reactions of CF3SO2Cl
with various electron-deficient alkenes catalyzed by Cu-
(dap)2Cl under visible light were examined. As seen in
Table 2, the preparation of a-chloro-b-trifluoromethyl
amides, esters, and ketones were obtained from the respective
unsaturated carbonyl substrates in moderate to excellent
yields. The reactivity of a-substituted a,b-unsaturated amide
3ac was comparable to that of the unsubstituted 3ab.
However, when the substrate was substituted at the b
position, as in 3ae, yields were reduced significantly. In
particular, excellent yields of 1:1 adducts were obtained from
Entry
Catalyst
Solvent
T
1
2
3
4
5
6
7
8
1 mol% Ru(phen)3Cl2
1 mol% Ru(bpy)3Cl2
1 mol% Ru(phen)3Cl2
1 mol% Ru(phen)3Cl2
50 mol% AIBN
50 mol% DLP
1 mol% Ir(ppy)3
1 mol% Ir(ppy)3
0.5 mol% Cu(dap)2Cl
0.5 mol% Cu(dap)2Cl
0.5 mol% Cu(dap)2Cl
0.5 mol% Cu(dap)2Cl
0.5 mol% Cu(dap)2Cl
MeCN
MeCN
MeCN
MeCN
DCE
RT
RT
RT
RT
808C
808C
RT
RT
RT
RT
RT
RT
RT
trace
trace
trace[b]
trace[c]
trace
DCE
trace
MeCN
MeCN
MeCN
DMF
DCM
DCM
DCE
44%
43%[b]
57%
9
10
11
12
13
trace
83%
98%(94%[d])[b]
98%(96%[d])[b]
Table 2: Reactions of CF3SO2Cl with electron-deficient alkenes.
[a] All reactions were run with 0.3 mmol 2a and 0.6 mmol CF3SO2Cl (1a)
in 2 mL solvent for overnight under visible light except entries 5 and 6,
and yields were determined by 19F NMR spectroscopy using PhCF3 as the
internal standard. [b] 20 mol% K2HPO4 was used as the additive.
[c] 100 mol% K2HPO4 was used as the additive. [d] The yields of isolated
product 3aa were based on 2a. bpy=2,2’-bipyridyl, dap=2,9-bis(4-
methoxyphenyl)-1,10-phenanthroline, DCE=1,2-dichloroethane,
DCM=dichloromethane, DMF=dimethylformamide, phen=1,10-phe-
nanthroline, ppy=2-phenylpyridinato-C2,N.
4), in spite of the fact that these catalysts had been effective in
the ATRA reaction of CF3SO2Cl with electron-rich
alkenes.[19] Also, not unexpectedly, simple thermally induced
radical initiators, such as AIBN and dilauroyl peroxide (DLP)
were also found to be ineffective in promoting the desired
reaction (Table 1, entries 5 and 6), although we have recently
found that similar reactions of fluoroalkylsulfonyl chlorides
with electron-rich alkenes produced 1:1 adducts in high yields
when using DLP as the radical initiator.[20]
We were encouraged, however, to find that the use of
Ir(ppy)3 in place of the Ru catalysts led to a 43–44% yield of
addition product (Table 1, entries 7 and 8). This result
probably indicated that IrIV was a better mediative source
of the Cl atom than was RuIII.
Among all metal catalysts used in ATRA or atom transfer
radical polymerization (ATRP) reactions, Cu species have
been the most popular because of their effectiveness in
[a] Yields of isolated products were based on electron-deficient alkenes.
[b] The reaction was run for 36 h; the yield as determined by 19F NMR
spectroscopy was 61% with a d.r.=5:1. However, only one diastereo-
isomer was isolated successfully. [c] The reaction was run in DCM; the
yield was determined by 19F NMR spectroscopy. [d] The d.r. was 1:10 in
the crude mixture, but was inverted and modified to 1.4:1 after
purification by column chromatography on silica gel. [e] 1 equiv of
K2HPO4 was used.
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well-known, easily polymerizable acrylonitrile and n-butyl
acrylate (3af, 3ai). Likewise, comparison of the yields of
esters 3ag–3ai indicated that the reactivity of phenyl acrylate
was greater than those of the alkyl acrylates. As for the N-
arylmethacrylamide substrates 2v or 2w, which we have used
previously in tandem radical cyclization studies,[18] ATRA
adducts 3av and 3aw were still observed as the main products
in 78% and 92% yields, respectively. It should be noted that
CF3Cl (19F NMR: À28 ppm) was detected as a byproduct in
all of these reactions. Its formation can be attributed to
H2CFSO2Cl and CF3CH2SO2Cl underwent reaction with
a,b-unsaturated amides, esters, and ketones to produce 1:1
adducts in good yields when the reaction temperature was
increased to 1108C (3cb–3cj and 3 db–3dj). At higher
temperatures, the rate of abstraction of the chlorine atom
from the fluoroalkylsulfonyl chlorides or CuII species by the
fluorinated radicals increased, which decreased the overall
yields of the 1:1 adducts. Due to the importance of the CF2H
group, our examination of the scope of substrates used in
reactions with HCF2SO2Cl was more extensive. In most cases,
excellent yields of 1:1 adducts bearing CF2H were obtained in
reactions with electron-deficient alkenes at 1008C. Substitu-
a
competitive abstraction of the chlorine atom from
CF3SO2Cl or the CuII species by the intermediate trifluoro-
methyl radical. In addition, because the reduction potential of
CF3SO2Cl is much more positive than that of the resulting
product, excess of the former can serve to prevent the latter
from further reduction or polymerization.
À
À
À
À
À
ents such as CN, Ac, Br, OMe, and NO2 on phenyl
were well-tolerated in these reactions. Notably, the reaction
of a,b-unsaturated carboxylic acid (2s) with HCF2SO2Cl led
to the desired adduct in 92% yield. Furthermore, use of a,b-
unsaturated sulfone and phosphonate substrates, 2t and 2u,
afforded the corresponding products in 92% and 88% yield,
respectively.
This reaction was then extended to the use of other
fluoroalkylsulfonyl chlorides, with the results being shown in
Table 3. As expected, reactions of easily reduced C4F9SO2Cl
with a,b-unsaturated amide, ester, and ketone gave the
corresponding 1:1 adducts (3bb, 3bg, and 3bj) in excellent
yields at room temperature. However, the reduction poten-
tials of HCF2SO2Cl, H2CFSO2Cl, and CF3CH2SO2Cl are more
negative than those of CF3SO2Cl and C4F9SO2Cl, and there-
fore their reactions required higher temperatures.
Finally, expanded synthetic applications were explored
using a,b-unsaturated amide 2x. The reaction of 2x with
HCF2SO2Cl gave 3ex in 95% yield on a 1 gram scale in the
À
presence of 0.2 mol% Cu(dap)2Cl. The C Cl bond of 3ex is
À
much more reactive than a simple secondary C Cl bond, and
we have found it to be useful for performing further synthetic
elaborations. As seen in Scheme 2, 3ex undergoes many
useful transformations such as reduction, elimination, and
Table 3: Reactions of RfSO2Cl with electron-deficient alkenes.
Scheme 2. Further transformations of 3ex. i) NaN3, DMSO, 758C, 3 h;
ii) Zn, HOAc, RT, 3 h; iii) piperidine (neat), 458C, 6 h; iv) PhSO2Na,
DMSO, 808C, 45 min; v) DBU, toluene, reflux, 12 h; vi) NaCN, DMSO,
808C, 3 h. DBU=1,8-diazabicyclo[5.4.0]undec-7-ene, DMSO=dime-
thylsulfoxide.
substitution by N3, CN, amine, and sulfinate in high yields
(4exa–4exf).
In conclusion, general and highly effective, Cu-catalyzed
ATRA reactions of fluoroalkylsulfonyl chlorides with elec-
tron-deficient alkenes have been achieved. This reaction can
be applied to a variety of fluoroalkylsulfonyl chlorides,
including CF3SO2Cl, C4F9SO2Cl, CF2HSO2Cl, CH2FSO2Cl,
and CF3CH2SO2Cl, in reaction with various electron-deficient
[a] Yields of isolated products were based on electron-deficient alkenes.
[b] Reactions were run at room temperature. [c] Reactions were run at
1108C.
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Received: December 19, 2014
Published online: && &&, &&&&
Keywords: atom transfer radical addition reactions · copper ·
.
fluorine · photoredox catalysis · radicals
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Photoredox Catalysis
Fluorinated radicals are generated under
Cu catalysis from RfSO2Cl. They are used
for atom transfer radical addition reac-
tions with electron-deficient alkenes such
as a,b-unsaturated ketones, amides,
esters, carboxylic acids, sulfone, and
phosphonate. ISET=induced single
electron transfer.
X.-J. Tang, W. R. Dolbier*
&&&& — &&&&
Efficient Cu-catalyzed Atom Transfer
Radical Addition Reactions of
Fluoroalkylsulfonyl Chlorides with
Electron-deficient Alkenes Induced by
Visible Light
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