Iridium and phosphine promoted C-F bond activation: The C-S cross-coupling of aryl fluorides with diaryl disulfides to synthesize thioethers

Article abstract of DOI:10.1016/j.tetlet.2015.09.133

Carbon-fluorine bond is the strongest known single bond to carbon and proved very difficult to cleave. An iridium and phosphine promoted C-F bond activation was developed, for the first time achieving the C-S cross-coupling reaction of disulfides with aryl fluorides using iridium complex. The corresponding monoarylthiolation products were obtained at moderate to good yields. Thus, it represents a new method for the synthesis of aryl sulfides through C-F bond activation.

Full text of DOI:10.1016/j.tetlet.2015.09.133

Tetrahedron Letters xxx (2015) xxx–xxx
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Tetrahedron Letters
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Iridium and phosphine promoted C–F bond activation: the C–S
cross-coupling of aryl fluorides with diaryl disulfides to synthesize
thioethers
⇑
Liang Li, Hongyan Miao, Yuqiang Ding
The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, Jiangsu Province, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Carbon–fluorine bond is the strongest known single bond to carbon and proved very difficult to cleave. An
iridium and phosphine promoted C–F bond activation was developed, for the first time achieving the C–S
cross-coupling reaction of disulfides with aryl fluorides using iridium complex. The corresponding
monoarylthiolation products were obtained at moderate to good yields. Thus, it represents a new method
for the synthesis of aryl sulfides through C–F bond activation.
Received 17 August 2015
Revised 25 September 2015
Accepted 28 September 2015
Available online xxxx
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
C–F activation
C–S cross-coupling
Iridium and phosphine promoted
Thioethers
Transition metal catalyzed carbon–halogen bond activation and
transformation into carbon–heteroatom bond has been considered
a crucial aspect of organic synthesis in recent decades.1 Among car-
bon–halogen bond, carbon–fluorine bond is the strongest known
single bond to carbon and has proved very difficult to cleave,2
the low reactivity of oxidative addition is attributable to the
strength of the carbon–fluorine bond (154 kcal/mol for C6F6).3
Despite the aforementioned difficulties, scientists have devoted
great efforts to this area. Hitherto, several kinds of transition
metals such as, Pd, Pt, Ni, Cu, Rh, Ti, Zr, Sm, Au have been proved
to be effective to C–F bond activation and also achieved their
catalytic C–F activations.4,5 Among these examples, one of the
strategies is to add activators involving Si, P, or B elements to make
C–F bond activation easier.6 Since Milstein reported the first exam-
ple of M–F/P–R exchange reaction of metal–fluoride complexes,7
several scientists (Macgregor, McGrady, Perutz, Li, et al.) have
published their results in this area.8–10 For example, Macgregor
et al. explored the phosphine-assisted C–F activation by Pt, Ir, Pd,
Rh complexes.9 McGrady reported Ni-mediated C–F activation of
pentafluoropyridine.10 Li realized C–F activation of hexafluoro-
propene using cobalt and nickel complexes.10d Johnson described
the reaction of Ni(PEt3)2 synthon with polyfluorinated pyridines
and explored the reaction mechanism.10e
Recently, we have shown that the stoichiometric C–F bond acti-
vation of a chloro-bridged iridium(III) dimer bearing dfppy (2,4-di-
fluorophenylpyridine) as a ligand leads to the formation of a
heteroleptic cyclometalated iridium(III) fluorophenyl pyridine
complex.11 Having investigated the mechanism of this reaction,
we were aware of the C–F bond activation was realized by the
oxidative addition of iridium to the C–F bond. Therefore, it remains
to be seen whether the amount of iridium complex involved in a
C–F bond activation reaction can be reduced from a stoichiometric
level to a catalytic level, or whether iridium complex can be used
as a catalyst to realize the activation and even the transformation
of C–F bond. Considering the availability of organic sulfur
compounds and the significance of C–S bond formation, it is
worthwhile to investigate the reaction between organofluorine
compounds and organosulfur compounds. In this Letter, as a con-
tinuation of our previous iridium-involved C–F bond activation,
we report on iridium promoted C–F bond activation and C–S bond
cross-coupling reaction associated with phosphorus reagents by
forming P–F bond and accordingly we propose a possible avenue
for this process (Scheme 1).10b
We started with the coupling reaction of p-fluoronitrobenzene
(1a) with bis(p-methoxyphenyl)disulfide (2a) in order to investi-
gate the effect of iridium complex, phosphine and additive on
the reaction. Arylthiolation product 3a was obtained at a moderate
yield (61%) when using IrH(CO)(PPh3)3 (2 mol%) dppBz
(dppBz = 1,2-bis(diphenylphosphino)benzene, 4 mol%) and PPh3
⇑
Corresponding author. Tel.: +86 510 85326290; fax: +86 510 85917763.
E-mail address: yding@jiangnan.edu.cn (Y. Ding).
http://dx.doi.org/10.1016/j.tetlet.2015.09.133
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Li, L.; et al. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.09.133

2
L. Li et al. / Tetrahedron Letters xxx (2015) xxx–xxx
Table 2
Screen reaction conditions of additives
F
LnIr PR3
O2N
F
S
1a
IrH(CO)(PPh3)3
dppBz, Additive
S
+
Scheme 1. The strategy of Ir-promoted C–F bond activation by forming P–F bond.
PhCl, 6 h
130oC
OMe
O2N
2a
MeO
2
3a
(1 equiv) in PhCl at 130 °C under an atmosphere of nitrogen
(Table 1, entry 6) and no by-products like phosphine sulfides or
thioethers were found. The yield of the desired product was lower
than this when either of these reagents was absent. These results
showed us that iridium complex, phosphine and additive were
essential for getting a higher yield. And we considered the additive
played an important role in the cleavage of C–F bond by serving as
a fluorine-capture reagent and excess ligand to prohibit the
quenching of catalyst by the product.
Entrya
Additive
Yieldb (%)
1
2
3
4
5
6
7
Ph3P
61
85
79
32
62
<5
76
nBu3P
Cy3P
(p-MeC6H4)3P
(p-MeOC6H4)3P
dppf
dppe
a
Reaction conditions: 1a (2.4 equiv), 2a (0.25 mmol, 1.0 equiv), IrH(CO)(PPh3)3
(2%), dppBz (4%) and additive (1.0 equiv) in PhCl (2 mL).
Next, the screening reaction conditions of additive was carried
out for a better yield. The results are shown in Table 2. Arylthiola-
tion product 3a was obtained at a highest yield (85%) when using
IrH(CO)(PPh3)3 (2 mol%) dppBz (4 mol%) and nBu3 P (1 equiv) in
PhCl at 130 °C under an atmosphere of nitrogen (Table 2, entry
2). Thus, nBu3P turned out to be the most suitable one.
b
Isolated yields based on 2a was reported.
Table 3
Scope of the thiolation of fluorobenzene with diaryl disulfides
Ir complex,
phosphine,
addtive
Having delineated the optimal reaction conditions for the thio-
lation of fluorobenzene with disulfides to obtain the arylthiolation
products through C–F bond activation (Table 2, entry 2), we
applied them to a variety of substrates in order to determine the
scope and limitations of the method. However, by changing
4-fluoronitrobenzene into 4-fluorobenzonitrile, the yield of desired
product 3c decreased dramatically into only 8%. So various iridium
complexes were examined, it was exciting to find that product 3c
was obtained in higher yields (88%; Table 3) when IrH(CO)(PPh3)3
was replaced by [Ir(COD)Cl]2 with addition of Cs2CO3 using
dioxane as solvent. Cs2CO3 is crucial for this reaction to proceed
for the reason that in the absence of Cs2CO3 the yields of the
desired product were lower. As reported by some published
articles12, Cs2CO3 may play a role of a promoter in the activation
of C–F bond especially for those fluorobenzenes which could not
undergo this process easily. PhCl was changed into dioxane serving
as solvent for the convenience of removal during the purification of
product. Yields listed in Tables 3 and 4 showed us that dioxane
could tolerate most substrates.
S
F
H3CO
S
2
R
solvent,
reflux,
time
H3CO
R
1
3
2a
S
S
H3CO
NO2
H3CO
H3CO
O2N
3a, 85%a
3b, 95%a
S
S
NC
H3CO
CN
CN
3c, 88%b
3d, 83%b
S
S
Ph
H3CO
H3CO
C
H3CO
H3CO
O
3e, 32%b
3f, 86%b
As depicted in Table 3, electron withdrawing groups on the phe-
nyl ring of fluorobenzene were beneficial to the reaction (80–95%
yield). Moreover, for the m-nitrile substrate, a monoarylthiolation
product was obtained at a lower yield. This was probably due to
the increased electron density at the meta position of such a
S
S
OCH3
< 5%
< 5%
a Yield of 3a, 3b was obtained using method in entry 2 of Table 2.
b Yield of 3c–3f were obtained under following conditions: 1 (3.0 equiv),
2a (0.25 mmol, 1.0 equiv), [Ir(COD)Cl]2 (2%), dppBz (4%), nBu3P (1.0 equiv)
and Cs2CO3 (3.0 equiv) in refluxed dioxane (2 mL) for 20 h. All yields listed
in table were isolated yields based on 2a.
Table 1
Screen reaction conditions
O2N
F
S
1a
Ir complex
S
Phosphine, Additive
+
substrate. While for the reaction involving fluorobenzene and
fluorobenzene with electron donating group like methoxy as
starting materials, the desired product could hardly be detected
even though various methods involving different iridium com-
plexes, phosphines, solvent with higher temperatures and longer
reaction times have been tried. These results showed us the
obvious influence of electron withdrawing groups on this reaction.
Given the optimal conditions with which to obtain the arylthi-
olation product by C–F bond activation, we then examined the sub-
strate scope with respect to disulfides. As shown in Table 4, a series
of electron-donating groups in disulfides (such as p-tolyl, p-chlor-
ophenyl and n-butyl) afforded the corresponding arylthiolation
products 4a–f at moderate to good yields (Table 4).
PhCl, 6 h
130oC
OMe
O2N
2a
MeO
2
3a
Entrya
Ir complex/phosphine/additive
Yieldb (%)
1
2
3
4
5
6
—/—/—
—/dppBz/—
—/—/PPh3
—/dppBz/PPh3
IrH(CO)(PPh3)3/—/—
IrH(CO)(PPh3)3/dppBz/PPh3
0
23
34
42
<5
61
a
Reaction conditions: 1a (2.4 equiv), 2a (0.25 mmol, 1.0 equiv), IrH(CO)(PPh3)3
(2%), dppBz (4%) and additive (1.0 equiv) in PhCl (2 mL).
b
Isolated yields based on 2a was reported.
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L. Li et al. / Tetrahedron Letters xxx (2015) xxx–xxx
3
Table 4
the Fundamental Research Funds for the Central Universities
(JUSRP51513), MOE & SAFEA for the 111 Project (B13025) and
Jiangsu Province Ordinary University Postgraduate Scientific
Research Innovation Projects (KYLX15_1155).
Scope of the thiolation of fluorobenzene with diaryl disulfides
[Ir(COD)Cl]2
dppBz
NO2
NO2
nBu3P
F
R
S
R
2
S
Dioxane
reflux
20 h
Supplementary data
2
4a-4f
1
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.09.
133. These data include MOL files and InChiKeys of the most
important compounds described in this article.
NO2
S
NO2
CH3
Cl
S
4a, 81%a
4b, 92%a
References and notes
NO2
O2N
CH3
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Zn, AcOH, EtOH
r.t, 3 h
S
S
MeO
MeO
N
3d
5, yield: 87%
OH
OH
CHO
S
OMe
6, yield: 67%
Scheme 2. The derivatization reactions.
AcOH, EtOH, reflux, 15 h
This methodology of iridium-promoted C–F activation provides
a convenient means to synthesize monoarylthiolation products
from very cheap starting materials, such as fluorobenzene. For
example, one of a series of useful ligands of titanium complexes
exhibiting high activity in the polymerization of ethylene13 was
easily synthesized from the C–F activation product 3d through
the steps mentioned hereunder (Scheme 2).
In conclusion, we have demonstrated a novel iridium and phos-
phine promoted cross-coupling of aryl fluoride with diaryl disul-
fides by C–F bond activation. This reaction proceeds under mild
conditions and tolerates some functional groups, and the reaction
products are strongly influenced by substrates. Moreover, this is
a new methodology for the construction of aryl sulfides from fluo-
rinated organics. Further work seeks to investigate competitive
processes between C–H bond and C–F bond cleavage under oxida-
tive iridium complex catalysis.
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Acknowledgments
We gratefully acknowledge financial support of this work by
the National Natural Science Foundation of China (21371080),
Please cite this article in press as: Li, L.; et al. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.09.133

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