Copper-Catalysed Suzuki-Miyaura Cross-Coupling of Highly Fluorinated Aryl Boronate Esters with Aryl Iodides and Bromides and Fluoroarene?Arene π-Stacking Interactions in the Products

Article abstract of DOI:10.1002/cctc.201901220

A combination of copper iodide and phenanthroline as the ligand is an efficient catalyst for Suzuki-Miyaura cross-coupling of highly fluorinated boronate esters (aryl?Bpin) with aryl iodides and bromides to generate fluorinated biaryls in good to excellent yields. This method represents a nice alternative to traditional cross-coupling methods which require palladium catalysts and stoichiometric amounts of silver oxide. We note that π???π stacking interactions dominate the molecular packing in the partly fluorinated biaryl crystals investigated herein. They are present either between the arene and perfluoroarene, or solely between arenes or perfluoroarenes, respectively.

Full text of DOI:10.1002/cctc.201901220

Accepted Article
Title: Copper-Catalysed Suzuki-Miyaura Cross-Coupling of Highly
Fluorinated Aryl Boronate Esters with Aryl Iodides and Bromides;
Fluoroarene-arene π-Stacking Interactions in the Products
Authors: Udo Radius, Yudha P. Budiman, Alexandra Friedrich, and
Todd B. Marder
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ChemCatChem
10.1002/cctc.201901220
Copper-Catalysed Suzuki-Miyaura Cross-Coupling of
Highly Fluorinated Aryl Boronate Esters with Aryl Iodides and Bromides and
Fluoroarene-arene p-Stacking Interactions in the Products
ꢀ
[
a,b]
[a]
[a]*
[a]*
Yudha P. Budiman, Alexandra Friedrich, Udo Radius, Todd B. Marder
[
a,b]
[a]
[a]*
Yudha P. Budiman, Dr. Alexandra Friedrich, Prof. Dr. Udo Radius, Prof. Dr. Todd B.
[
a]*
Marder
[
a]
Institut für Anorganische Chemie, and Institute for Sustainable Chemistry & Catalysis with Boron,
Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany.
[
b]
Department of Chemistry, Faculty of Mathematics and Natural Sciences,
Universitas Padjadjaran, 45363 Jatinangor, Indonesia.
Corresponding Authors:
*
u.radius@uni-wuerzburg.de, *todd.marder@uni-wuerzburg.de
Abstract
A combination of copper iodide and phenanthroline as the ligand is an efficient catalyst for Suzuki-
Miyaura cross-coupling of highly fluorinated boronate esters (aryl-Bpin) with aryl iodides and
bromides to generate fluorinated biaryls in good to excellent yields. This method represents a nice
alternative to traditional cross-coupling methods which require palladium catalysts and stoichiometric
amounts of silver oxide. We note that π×××π stacking interactions dominate the molecular packing in
the partly fluorinated biaryl crystals investigated herein. They are present either between the arene
and perfluoroarene, or solely between arenes or perfluoroarenes, respectively.
ꢀ
1ꢀ
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10.1002/cctc.201901220
Introduction
The conversion of highly fluorinated aromatics into fluorinated arylborononate esters is desirable as
[1]
arylboronate esters are useful building blocks for organic synthesis. We recently reported the
successful defluoroborylation of polyfluorinated aromatics using an NHC nickel complex (NHC = N-
Heterocyclic Carbene) as a catalyst and the diboron(4) compound B
2
pin (pin = pinacolato) as the
2
[
2]
boron source. Having a good source of fluorinated arylboronates in hand, we were interested to
explore the chemistry of these electron poor aromatics, especially in Suzuki-Miyaura cross-coupling
reactions which are employed in various fields, from the synthesis of natural products to materials
[
3]
chemistry, including large-scale production. Applications of polyfluorobiphenyls, are quite
[
4]
[5]
numerous including medicinal chemistry, organic light emitting diodes, electron-transport
[
6]
[7]
[8]
materials,
crystal engineering,
metal−organic frameworks (MOFs),
and supramolecular
[9]
chemistry.
However, Suzuki-Miyaura cross-coupling of highly fluorinated boronate esters, especially
pentafluorophenyl boronates is highly challenging under typical palladium catalysed Suzuki-Miyaura
[
10]
conditions, as the transfer of C
6
F
5
to the palladium centre in the transmetalation step is usually
In many cases, stoichiometric amounts of costly silver oxide (Ag O) were added in
addition to the palladium catalyst to enhance the transmetalation step and thus to obtain the desired
[
11c]
inefficient.
2
[
11]
coupling product in fair to good yield. In 1987, it was initially reported by Kishi et al. that Ag O
2
can accelerate the rate of palladium-catalysed Suzuki-Miyaura cross-coupling of alkenylboronic acids
with alkenyl iodides with the relative rate being 30 times faster than that using common bases such as
[
12]
KOH.
Inspired by that, in 2002, Adonin et al. used 1.2 equivalents of Ag O to enhance the
2
efficiency of Pd(OAc)
2
/PPh /K
3 2
CO
3
to catalyse the Suzuki-Miyaura cross-coupling of
[
11f]
[11g]
[11c]
C
6
F
5
B(OMe)
an effective method for coupling of C
for coupling with aryl bromides using Pd
equivalents of Ag O. In 2005, Adonin et al. extended their previous studies
K with aryl bromides instead of iodides employing Pd(OAc) /P(t-Bu)
3
Li
or C
6
F
5
BF
3
K
with aryl iodides in toluene. In 2005, Korenaga et al.
B(OH) with aryl iodides using Pd(PPh
(dba) /P(t-Bu) /CsF in DMF, in both cases requiring 1.2
reported
6
F
5
2
3
) /CsF in DMF and
4
2
3
3
[
11g]
2
to the coupling of
C
6
F
5
BF
3
2
3
/K
2
CO
3
in toluene, but
ꢀ
2ꢀ
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ChemCatChem
10.1002/cctc.201901220
[
11b]
this was still only effective in the presence of 1.2 equivalents of Ag
that Ag
species, which shows higher reactivity in the transmetallation step with aryl boronates. The reaction
of trans-[Pd(PEt (C )I] with Ag O in toluene–water, for example, generates the complex trans-
Pd(PEt (C )(OH)], which undergoes transmetalation with the boronic acid 4-
MeOC B(OH)
problem, and this works without additional Ag
2
O.
Osakada et al. reported
2
O has the ability to replace the halide ligand of the catalyst to generate an hydroxy-palladium
3
)
2
6
F
5
2
[
3
)
2
F
6 5
[
13]
6
H
4
2
.
In 2010, Buchwald et al. reported the precatalyst XPhos-Pd-G2 to solve the
O to catalyse the coupling of C B(OH) with aryl
However, this
2
6
F
5
2
[
11h]
chlorides, bromides, and triflates but, interestingly, did not work for aryl iodides.
palladium catalyst is quite expensive or at least requires a multistep synthesis.
Scheme 1. Recent challenges for Pd-catalysed Suzuki-Miyaura cross-coupling to achieve polyfluorinated
biaryls
ꢀ
[13]
Previously, Osakada et al.
Pd(C )(PEt I] in the presence of Ag
of the stable intermediate trans-[Pd(C
showed that 2,4,6-trifluorophenyl-B(OH)
2
reacts with trans-
O, but the reaction stops with generation
)(PEt ] as reductive elimination was not
Pd(Ar)(Ar¢)], if both Ar and Ar¢ are
highly electron deficient, the reductive elimination step becomes much more difficult, as the Pd−Ar
[
6
F
5
3
)
2
2
O in toluene and H
2
6
F
5
)(2,4,6-C
6
F H
3 2
3 2
)
observed for this complex. Thus, for palladium complexes [L
2
[14]
bonds are strong. Those reports show the current challenge for the palladiumcatalysed Suzuki-
Miyaura cross-coupling of C -boronates with fluorinated aryl halides (Ar -X), especially if the C-X
6
F
5
F
bond flanked by two C-F bonds.
ꢀ
3ꢀ
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Polyfluorinated biaryls can be synthesised via Suzuki-Miyaura cross-coupling of polyfluorinated aryl
[
10d]
boronic acid esters and polyfluorinated aryl iodides, as reported by Bulfield and Huber
using
palladium catalysts. They employed both fluoroaryl boronate and fluoroaryl halide substrates as
coupling partners using a combination of palladium sources and various phosphine ligands. Although
this reaction works in some cases, they had to optimise each reaction separately for the corresponding
aryl boronate and aryl halide, using different types of expensive phosphine ligands. These reactions
all required long reaction times (over 60 hours) and a procedure that would works for C
was not developed.
6
F B(OH)
5
2
[
15]
Recently, research to replace precious metal catalysts with cheaper and Earth-abundant metals as
[
16]
well as metals of lower toxicity has attracted much attention. Several groups have developed Cu(I)
[
17]
catalysts for Suzuki-Miyaura cross-coupling reactions. For example, Li et al. have developed a
copper/DABCO ligand catalyst system, but the reaction, however, does not work for sterically
[
17c,d]
hindered and electron-deficient aryl boronic acids.
Brown et al. reported that a combination of
CuCl with xantphos can effectively catalyse Suzuki-Miyaura cross-coupling of aryl iodides with aryl
[
17a]
boronic acid neopentylglycol ester (Bneop)
and later using Cy
3
PCuCl catalyst for a cross-coupling
[
17h]
[17e]
of aryl-Bpin with heteroaryl bromides.
Other systems involve the use of copper nanoclusters
or
[
17f]
copper powder in polyethylene glycol solvents.
Giri et al. reported an efficient system employing
CuI and (o-(di-tert-butylphosphino)-N,N-dimethylaniline (PN) as the ligand for efficient Suzuki-
[
17b]
Miyaura cross-coupling of aryl-Bneop with aryl iodides
and extended with electron deficient aryl
However, to the best of our knowledge, there are no reports of copper-catalysed Suzuki-
Miyaura cross-coupling reaction of electron deficient, highly fluorinated aryl boronate esters, and
[17h]
bromides.
[
17f,g,h]
only a few examples of the use of aryl bromides in Cu-catalysed Suzuki-Miyaura reactions.
On
the other hand, the optimised conditions for Cu-catalysed Suzuki-Miyaura cross-coupling employing
[
17a]
aryl-Bpin and aryl iodide as coupling partners is still challenging, as Brown, et al.
and Giri, et
[17b]
al.
reported that their optimised methods to employ aryl-Bpin instead of aryl-Bneop only affording
fair yields.
ꢀ
4ꢀ
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Herein, we report Suzuki-Miyaura cross-coupling of aryl iodides and bromides with highly
fluorinated arylboronate esters (Ar -Bpin), catalysed by phenanthroline-ligated copper complexes.
F
Notably, Cu(I)-catalysed Suzuki-Miyaura cross-coupling of C
6
5
F Bpin does not require the addition of
silver oxide to achieve quantitative yields.
Result and Discussion
We began our investigation using the most electron deficient compound, C
6
F
5
Bpin (1a), which was
[
18]
synthesised via an Ir-catalysed C-H borylation reaction. Coupling of C
2a) to give 2,3,4,5,6-pentafluoro biphenyl (3a) was chosen as a model reaction. Giri at al.
the mechanism of Cu(I)-catalysed Suzuki-Miyaura cross-coupling and showed that, after the
formation of [(PN)CuI] , the addition of CsF led to the formation of [(PN)CuF] , which then yielded
(PN)CuPh] after transmetallation with an aryl-Bneop reagent. Inspired by that work, we screened
Cu(I) salts with different ligands, bases and solvents.
6
F
5
Bpin with phenyl iodide
[17g]
(
studied
2
2
[
As Giri et al. demonstrated that the combination of CuI and (o-(di-tert-butylphosphino)-N,N-
dimethylaniline (PN), CsF, in DMF/dioxane solvent at 130 °C was efficient for the Suzuki-Miyaura
[
17b,g]
cross-coupling of aryl-Bneop with aryl iodide,
we initially tried to employ these conditions to
cross-couple C Bpin with phenyl iodide, but obtained only a 5% isolated yield of the biaryl product
6 5
F
after 18 h (Table 1, entry 1). Changing the PN-ligand to N-only-based chelating ligands such as 2,2¢-
bipyridine (bpy) and 4,4¢-di-tert-butyl-2,2¢-bipyridyl (dtbpy) (Table 1, entry 2 and 3) led to better
yields (42% and 57%, respectively), while phenanthroline emerged as the best ligand for our purpose.
The reaction of C
6
5
F Bpin with 1.5 equivalents of iodobenzene in DMF at 130 °C in the presence of 10
mol% CuI, 10 mol% phenanthroline, and 2 equiv. of CsF afforded the cross-coupling product in an
excellent yield of 99% after workup (Table 1, entry 4). The use of monodentate
4-(dimethylamino)pyridine (DMAP) instead of phenanthroline led to much poorer activity (21%
yield, Table 1, entry 5).
ꢀ
5ꢀ
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10.1002/cctc.201901220
[a]
Table 1. Suzuki-Miyaura cross-oupling of C
6
5
F Bpin with PhI
[
b]
Entry Cu catalyst
Ligand
PN
Base
CsF
CsF
CsF
Solvent
DMF/dioxane
DMF
Time Yield (%)
1
2
3
CuI
CuI
CuI
18
24
24
5
bpy
42
57
dtbpy
DMF
99
[
c]
4
CuI
phenanthroline
CsF
DMF
18
99
[
d]
48
5
6
7
8
9
CuI
CuCl
CuBr
CuI
DMAP
CsF
CsF
CsF
KF
DMF
DMF
DMF
DMF
DMF
DMF
Toluene
THF
24
24
24
24
24
24
24
24
24
24
24
18
21
55
36
68
29
3
phenanthroline
phenanthroline
phenanthroline
phenanthroline
phenanthroline
phenanthroline
phenanthroline
-
CuI
NMe
4
F
1
0
1
2
3
4
5
6
CuI
K
3
PO
4
[
e]
1
1
1
1
1
1
CuI
CsF
CsF
CsF
-
0
[
f]
CuI
0
CuI
DMF
DMF
DMF
DMF
26
0
CuI
phenanthroline
phenanthroline
-
-
CsF
CsF
0
(Phen)CuI
94
[
a]
General conditions: 1a (0.4 mmol), 2a (0.6 mmol), Cu catalyst (10 mol%), ligand (10 mol%), base (2 equiv.),
solvent (3 mL), under argon. [b] Isolated yield after flash chromatography. [c]
C
6
F
5
BF
K instead of C
F
Bpin. [d]
3
6
5
T = 100 °C. [e] T = 110 °C. [f] T = 60 °C.
CuCl and CuBr, in place of CuI, were also tested but led to a decrease of the isolated yields to 55%
and 36%, respectively (Table 1, entry 6 & 7). It is important to note that hydroxide and alkoxide bases
must be avoided as the para-carbon atom of C
6
6
F Bpin is susceptible to nucleophilic attack by these
ꢀ
6ꢀ
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10.1002/cctc.201901220
[
17]
bases and they can replace the para-fluoro-substituent on the perfluorinated boronate substrate.
Thus, we examined fluorides and phosphates as bases and found that CsF gave the best results.
[
11c]
Similarly, Korenaga et al.
coupling of C B(OH) with aryl halides using a Pd-catalyst. Using KF instead of CsF also gave a
good yield of 68% (Table 1, entry 7), whereas NMe F afforded the product in only 29% yield (Table
, entry 8). Non-fluoride bases such as K PO resulted in a poor yield of 3 % (Table 1, entry 10).
observed that CsF gave the highest yield for the Suzuki-Miyaura cross-
6
F
5
2
4
1
3
4
Toluene and THF were ineffective solvents at temperatures close to their boiling points (Table 1,
entry 11 and 12). It is interesting to note that without any additional ligand, the copper catalyst still
gave a 26% yield (Table 1, entry 13) and was more active than the system employing PN as the ligand
(Table 1, entry 1). The absence of either CsF or CuI resulted in no product formation, indicating that
both base and catalyst are required. A combination of CuI and phenanthroline generates
[
20]
[
(Phen)CuI] as using preformed [(Phen)CuI] gave an excellent yield (Table 1, entry 16). CsF reacts
[
15b]
as a nucleophile and exchanges the halide ligand at Cu(I) to generate a CuF complex,
which reacts
more readily with the aryl boronate ester in the transmetalation step and thus accelerates the transfer
[
10c,15b]
of the aryl group to the metal.
CuI gave better result than CuBr or CuCl probably because the
[
19]
low bond energy led to a more efficient anion exchange with CsF. It is also known that the reaction
[
20]
of [(phen)CuI] and CsF in DMF gives [(phen)CuF].
CuI/phenanthroline, and 2 equiv. of CsF, in DMF at 130 °C emerged as the ideal conditions for the
Suzuki-Miyaura cross-coupling of C Bpin with aryl iodide. The use of organotrifluoroborates is
attractive as these are inexpensive and more stable towards air and moisture than organoboronate
Thus a combination of 10 mol% of
F
6 5
[
21]
[11b,f]
substrates.
found that this also produced C
Scheme 2. Cross-coupling reaction of C
Thus, we also employed C
–C in an almost quantitative yield of 99% (Table 1, entry 4).
BPin with PhBr[a]
6
F
5
BF
3
K
for the Suzuki-Miyaura cross-coupling and
6
F
5
6
H
5
F
6 5
ꢀ
7ꢀ
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10.1002/cctc.201901220
[
a]
Conditions: C
6
F
5
BPin (0.4 mmol), PhBr (0.6 mmol), CsF (2 equiv.), CuI/phenanthroline (30 mol%),
DMF/toluene (1:1, 4 mL), 140 °C, 36 h, under argon, isolated yield was reported.
While the use of aryl bromides and aryl boronate substrates for Cu-catalysed Suzuki-Miyaura cross-
[
17g]
coupling has been reported,
substrates was found to be difficult.
Bpin in a mixed solvent system such as DMF : toluene (1:1) by increasing the loading of
CuI/phenanthroline to 30 mol%, generating the cross-coupling product in 87 % yield (Scheme 2).
the reaction of aryl bromides with electron-deficient aryl boronate
[
17g]
We found that phenyl bromide was effective in reactions with
C
F
6 5
ꢀ
8ꢀ
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Table 2. Cu-catalysed cross-coupling of C
6
F
Bpin with Ar-X (X = I or Br)[a]
5
Entry
1
Ar-X
Product
Yield (%)[b]
90
2b
3b
2
88
2c
3c
3
4
67
88
3d
2d
3e
2e
5
6
7
95
91
71
2
f
3f
2g
3g
2h
3h
8
9
86
91
93
77
2
i
3
i
2j
3j
1
1
0
1
2k
3
k
3l
2l
1
2
57 (80)[c]
3m
2m
1
1
3
4
75
78
3n
2n
3o
2o
[
a]
Standard conditions: C
6
5
F Bpin (0.4 mmol), Ar-X (0.6 mmol), CuI/phenanthroline (10 mol% if X = I; 30
mol% if for X = Br), CsF (0.8 mmol), DMF (3 mL, if X = I) or DMF/toluene (1:1, 4 mL, if X = Br), T (130 °C
if X = I; 140 °C if X = Br), t (18 h if X = I; 36 h if X = Br), under argon. [b] Isolated yield after flash
chromatography. [c] CuI/phenanthroline (50 mol%)
Having determined the optimal conditions for both phenyl iodides and bromides, we next investigated
the scope of the reaction of C
6
5
F Bpin with different aryl iodides and bromides (Table 2). Aryl iodides
ꢀ
9
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10.1002/cctc.201901220
bearing electron donating groups (2b, 2c, 2e, 2f, 2g) at meta or para positions gave good to excellent
yields (88-95%), whereas sterically more hindered substrates such as 2-methylphenyl iodide (2d), 2-
methoxyphenyl iodide (2h), and mesityl bromide (2m) gave fair to moderate yields (57-71%). Aryl
iodides bearing electron withdrawing substituents, such as methyl 4-iodobenzoate (2i),
4-iodotrifluorotoluene (2j), and 4-fluorophenyl iodide (2k), gave very good to excellent yield (86-93
F F
%). In palladium catalysis, if both Ar -Bpin and Ar -X have two ortho-fluoro substituents, reductive
[13]
elimination has been reported to be difficult
(Scheme 3), but in our case, 1,3-difluoro-2-
iodobenzene (2l) also provided a good yield (77 %). Napththalene and anthracene derivatives are
[22]
interesting for application in blue organic light-emitting diodes (OLED). Thus, we employed 2-
bromonaphthalene (2n) and 9-bromoanthracene (2o) in our reaction, and these substrates generated 75
%
and 78 % isolated yields of the corresponding perfluoro phenyl naphthalene and anthracene
products, respectively.
Furthermore, we varied the number of fluoro subtituents on the aryl-Bpin reagent in coupling
reactions with phenyl iodide or bromide. Thus, reaction of other fluorinated aryl-Bpin such as 2,6-
difluorophenyl-Bpin (4a), 2,3- or 2,5-difluorophenyl-Bpin (4b, 4c), 2,3,4-trifluorophenyl-Bpin (4d),
2,3,4,5-tetrafluorophenyl-Bpin (4e), can be coupled with an aryl iodide affording good to excellent
yields (Table 3, entries 1–5). In term of coupling with an aryl bromide, 2,6-difluorophenyl-Bpin (4a)
proceeded nicely to give the coupling product in 85 % yield (Table 3. entry 1). However, if the
aryl-Bpin has one ortho-fluoro substituent, 50 mol% of CuI/phenanthroline is required to couple
nicely with aryl bromide (Table 3, entries 2-5). Another type of electron deficient aryl boronate ester,
namely 3,5-bis(trifluoromethyl)phenyl-Bpin (4f), also coupled nicely with phenyl iodide, but its
coupling with phenyl bromide proved difficult (Table 3, entry 6). On the other hand, electron-rich 3,5-
dimethylphenyl-Bpin (4g) coupled well with phenyl iodide, but not viable for phenyl bromide (Table
3, entry 7), indicating that in this system, aryl bromides only couple efficiently with electronic
deficient aryl-Bpin compounds. However, our results show that aryl iodides can be coupled not only
with electron deficient aryl-Bpin compounds but also with electron rich ones.
ꢀ
10ꢀ
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Table 3. Cu-catalysed cross-coupling of various fluorinated and nonfluorinated aryl-Bpin with Ph-X (X=
I or Br)[a]
Entry
1
Ar-Bpin
Product and Yield (%)[b]
4a
5a
X: I (99), Br (85)
2
3
4b
5b
[c]
X: I (95), Br (59)(80)
4c
5c
[c]
X: I (94), Br (73)
4
5
5
d
4d
[c]
X: I (93), Br (54)(70)
4e
5e
[c]
X: I (81), Br (42)
6
7
4f
5f
X: I (92), Br (35)
4g
5g
X: I (95), Br (trace)
[
a]
Standard conditions: Ar-Bpin (0.4 mmol), Ph-X (0.6 mmol), CuI/phenanthroline (10 mol% if X = I; 30 mol%
if X = Br), CsF (0.8 mmol), DMF (3 mL, if X = I) or DMF/toluene (1:1, 4 mL, if X = Br), T (130 °C if X = I;
40 °C if X = Br), t (18 h if X = I; 36 h if X = Br), under argon. [b] Isolated yield after flash chromatography. [c]
1
CuI/phenanthroline (50 mol%).
Notably, when 2,4,6-trifluorophenyl-Bpin (4h) and 2,3,5,6-tetrafluorophenyl-Bpin (4i) were
employed, besides good yields of the expected coupling products, we also observed C-H arylation
byproducts generated in small (<10 %) amounts after C-Bpin arylation occured (Table 4, entries 1 and
2). It is known that the high acidity of the C-H bonds flanked by two C-F groups can be used in direct
[
23]
arylation. However, our results show that even though both carbons are electron deficient, the C-
Bpin group is more reactive than the C-H moiety as an arylation target.
ꢀ
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Table 4. Cross-coupling of 2,4,6-F
3
C
6
H
2
Bpin and 2,3,5,6-F
4
C
HBpin with PhX (X = I or Br)[a]
6
[
b]
Entry
1
Ar -Bpin
F
Product and Yield (%)
5
h
4
h
5
h
¢¢
5h
¢
X: I (8 %),
Br (5 %)
X: I (75 %),
Br (70 %)
X: I or Br (trace)
2
4
i
5i
5i
¢
X: I (74 %),
Br (69%)
X: I (10%),
Br (9%)
[
a]
Standard conditions: 0.4 mmol of Ar
mol% if X = Br), CsF (0.8 mmol), DMF (3 mL, if X = I) or DMF/toluene (1:1, 4 mL, if X = Br), T (130 °C if X
I; 140 °C if X = Br), t (18 h if X = I; 36 h if X = Br), under argon. [b] Isolated yield after flash
F
-Bpin, 0.4 mmol of PhX, CuI/phenanthroline (10 mol% if X = I or 30
=
chromatography.
Generally, aryl halides bearing electron-withdrawing groups are more reactive and give better yields
compared to aryl halides bearing electron-donating groups. We compared the reactivities of electron-
poor and electron-rich aryl halides by reacting a 1:1 mixture of 4-trifluoromethyliodobenzene and 4-
iodotoluene with C
6
5
F Bpin (Scheme 2). Trifluoromethylphenyl iodide was more reactive, resulting in
a 3 : 1 ratio of the trifluoromethyl to methyl-substrated pentafluorobiphenyl compunds.
Scheme 3. Comparison of the reactivities of an electron poor and an electron rich aryl iodide[a]
[
a]
Conditions: C
6
5
F Bpin (0.1 mmol), 4-iodotoluene (0.2 mmol), 4-iodobenzotrifluoride (0.2 mmol), CuI (0.01
mmol), 1,10-phenanthroline (0.01 mmol), CsF (0.2 mmol), and DMF (1.0 mL) at 130 °C, 18 h.
These conditions can be used to prepare polyfluorinated biaryl products on a gram scale. Thus, the
coupling of C
6
F
5
Bpin with 1,2,3-trifluoro-5-iodobenzene (3o) was conducted without any difficulty
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using the standard conditions to provide a 98% yield of the unsymmetrical octafluoro biphenyl
product (Scheme 4).
Scheme 4. Gram scale reaction[a]
F
F
F
F
F
F
F
F
F
F
F
1
0 mol % CuI/ phenanthroline
CsF, DMF
F
Bpin +
I
F
F
F
o
130 C, 18 h
F
2
a
2p
3p
1.47 g, 98 %)
[b]
(
[
a]
Reaction conditions: C
6
F
5
Bpin (1.47 g, 5 mmol), 1,2,3-trifluoro-5-iodobenzene (1.80 g, 7 mmol), (95 mg,
0
.50 mmol, 10 mol%), phenanthroline (90 mg, 0.50 mmol, 10 mol%), CsF (1.52 g, 10.0 mmol, 2 equiv), and
DMF (30 mL), 130 °C, 18 h, under argon. [b] Isolated yield after column chromatography.
Molecular and crystal structures: Intermolecular π×××π stacking interactions
The crystal structures of the polyfluorinated biaryls 3d, 3n, 3o, and 5d were analysed using single-
crystal X-ray diffraction. A comparison of the molecular geometries of these compounds in their
crystal structures (Figure 1) shows a small influence of the steric demand of the hydrogenated aryl
group in the vicinity of the C–C bonds joining the rings and, hence, of the repulsion between both
groups of the biaryl units on their geometries. The central C–C bond is in the range 1.483(4) –
[24]
1
.495(2) Å (Table 5) which is typical of biphenyl compounds. It is slightly longer in compounds 3d
and 3o than in 3n and 5d, although only within 1–2 su. The twist between the aryl moieties of the
biaryl is slightly stronger in compounds 3d and 3o (61.66(5) and 64.28(5)°) than in 3n and 5d
(51.23(15) and 49.76(7)°) (Table 5). These small differences are likely due to the substitution at the
ortho position of the non-fluorinated phenyl ring. In 3d, a methyl group is bonded at the ortho
position and in 3o, the central phenyl ring of the anthracene moiety is bonded to the fluorinated
phenyl ring. This increases the bulkiness of these aryl moieties in close vicinity to the central C–C
bond and, hence, to the respective fluorinated phenyl rings. Large twist angles are also reported in the
[
25a]
bulky biaryl compounds with a pentafluorophenyl group bonded to benzo[h]quinoline (67°),
in
[
25b]
9,10-bis(pentafluorophenyl)anthracene (68°),
and in 5-perfluorophenyl-11-phenyltetracene
[
25c]
(72°).
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Figure 1. The solid-state molecular structures of 3d, 3n, 3o, and 5d determined by single-crystal X-ray
diffraction at 100 K. All ellipsoids are drawn at the 50% probability level, and H atoms are omitted for clarity.
Colours: white (carbon), green (fluorine).
Table 5. Selected bond lengths (Å) and angles (°) of 3d, 3n, 3o, and 5d.
3
d
3n
3o
5d
C
Aryl(H)–CAryl(F)
1.495(2)
61.66(5)
1.484(7)
51.23(15)
1.490(2)
64.28(5)
1.483(4)
49.76(7)
Ð Aryl(H)-Aryl(F)
Particularly interesting in the crystal structure analyses are the intermolecular interactions and, hence,
molecular packing in these compounds. The presence of both fluorinated and nonfluorinated aryl
groups leads to the formation of opposite multipoles of these moieties due to the differences in
electronegativity of hydrogen and fluorine atoms with respect to the carbon atoms. This often results
in attractive multipole forces between the aromatic and perfluoroaromatic groups, also called the
arene-perfluoroarene interaction, and, hence, in face-to-face π-stacking with mean interplanar
[
26]
distances between 3.3 and 3.6 Å. This type of interaction is mostly found in co-crystals of arenes
[
26c,27]
and perfluoroarenes, which form highly oriented, π-stacked systems.
However, also self-
complementary compounds that contain both perfluorinated and nonfluorinated aryl groups, such as
2,3,4,5,6-pentafluorobiphenyl and 1-pentafluorophenyl-2-phenylacetylene, form arene-perfluoroarene
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[
27d,28a]
interactions.
This is also the case in the polyfluorinated biaryl compounds 3n and 5d in which
the arene-perfluoroarene interaction determines the packing of the molecules (Table 6).
Table 6. Aryl···aryl (p···π) distances (Å) in crystals of 3d, 3n, 3o, and 5d at 100 K: centroid-centroid distance,
interplanar separation, and offset shift.
Compound
Aryl···Aryl
Centroid-
Interplanar separation
Offset shift [a]
centroid distance
3
3
d
n
Aryl(F)···Aryl(F)
Aryl(F)···Aryl(H)
3.9121(15)
3.4489(16)
3.398(5) / 3.473(4)
3.408(4) / 3.354(5)
3.4625(18)
1.846(2)
1.753(7) / 1.599(6)
1.605(6) / 1.714(7)
1.879(2)
3
3
.823(3)
.767(3)
3
.9397(18)
.2171(19)
3.696(2)
3
5
o
Aryl(H)···Aryl(H)
4
3.4043(18)
2.489(2)
d
Aryl(F/H)···Aryl(H)
3.380(2) / 3.336(2)
1.494(4) / 1.591(4)
[
a]
The offset shift, also called inter-centroid shift, is the distance within a plane of an aryl ring between the
centroid of the respective aryl ring and the intersection point with the normal to the plane through the centroid of
the other aryl ring.
In compound 3n the biaryls form columns of offset face-to-face π-stacked naphthalene and
perfluorophenyl moieties along the b axis. Due to the large twist angle of ca. 51° of the biaryl, the
columnar stacks are cross-like formed (Figure 2). Similar cross-like columnar stacks are observed in
2,3,4,5,6-pentafluorobiphenyl, 1,2,4,5-tetrafluoro-3-phenyl-6(trifluoromethyl)benzene, and 4¢-bromo-
2,3,5,6-tetrafluorobiphenyl-4-carbonitrile, which show biphenyl twist angles of 52, 50, and 41°,
[
28]
respectively.
In compound 5d only part of one phenyl ring is fluorinated. Still, the arene-
perfluoroarene interaction leads to p-stacking between phenyl and 2,3,4-trifluoro phenyl rings.
However, the phenyl and partly fluorinated phenyl rings are p-stacked with different neighboring
molecules. Hence, they do not form a columnar arrangement but instead, zig-zag like chains along the
c axis (Figure 3). The packing of the molecules in compound 3d follows a similar pattern as in 5d
(Figure 4). However, the type of π×××π interactions is different as there is no arene-perfluoroarene
interaction present. Instead, neighboring perfluorophenyl moieties are π-stacked via perfluoroarene-
perfluoroarene interactions and neighboring tolyl moieties are offset-stacked forming zig-zag chains
along the a axis.
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Figure 2. Crystal structure of 3n projected along (left) an axis close to the a axis and (right) along the b axis, at
00 K. Perfluorophenyl and naphthalene moieties are p-stacked along the b axis via arene-perfluoroarene
interactions. The intramolecular angle between the planes of the perfluorophenyl and naphthalene moieties is
1° leading to the formation of cross-like stacks (right). All ellipsoids are drawn at the 50% probability level,
1
5
and H atoms are omitted for clarity. Colours: white (carbon), green (fluorine).
Figure 3. Crystal structure of 5d projected along the b axis, at 100 K. Phenyl and partly fluorinated phenyl rings
are p-stacked with different neighboring molecules via the arene-perfluoroarene interaction. All ellipsoids are
drawn at the 50% probability level, and H atoms are omitted for clarity. Colours: white (carbon), green
(fluorine). Close C×××C contacts are shown in red.
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Figure 4. Crystal structure of 3d projected along the b axis, at 100 K. Neighboring perfluorophenyl moieties are
p-stacked, while tolyl moieties are offset-stacked. All ellipsoids are drawn at the 50% probability level, and H
atoms are omitted for clarity. Colours: white (carbon), green (fluorine). Close C×××C contacts are shown in red.
Finally, in compound 3o the intermolecular packing is dominated by π-stacking arene-arene
interactions between the anthracene moieties, which form stacks along the a axis (Table 6). Similarly,
offset π×××π stacking is observed between the tetracene moieties of the compound 5-perfluorophenyl-
[
25c]
11-phenyltetracene.
9,10-bis(pentafluorophenyl)anthracene was reported as a host system with
[25b]
benzene or 1,4-dioxane guest molecules.
In these structures, no π×××π interactions are observed, but
C–F×××π interactions are dominant for the molecular arrangement in the crystal structures. In contrast,
the bulky 5-pentafluorophenyl-benzo[h]quinoline exhibits both arene-perfluoroarene and arene-arene
[
25a]
π-stacking interactions in the crystal structure.
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Figure 5. Crystal structure of 3o projected along the b axis, at 100 K. Anthracene moieties are p-stacked along
the a axis. All ellipsoids are drawn at the 50% probability level, and H atoms are omitted for clarity. Colours:
white (carbon), green (fluorine).
Conclusion
In summary, a combination of copper (I) iodide with phenanthroline as the ligand is an efficient
catalyst for Suzuki-Miyaura cross-coupling reactions of electron deficient C
6
5
F Bpin with aryl iodides
and bromides in up to quantitative yield. Thus, the reaction proceeds using a non-toxic and
inexpensive Earth-abundant metal catalyst, replacing the traditional palladium catalysts which require
large amounts of silver oxide as an additive. This reaction is also viable for cross-coupling a wide
range of fluorinated phenyl boronic acid pinacol esters with aryl iodides or bromides. Notably, for
aryl iodides, it can be used not only for coupling with electron deficient fluoroaryl boronates, but also
for electron rich aryl boronates giving excellent yields.
A diverse range of π×××π stacking interactions is observed in the partly perfluorinated biaryl
compounds investigated herein, ranging from arene-perfluoroarene interactions (3n, 5d) to arene-
arene (3o) and perfluoroarene-perfluoroarene (3d) interactions. Other applications of highly
fluorinated aryl boronate substrates are under investigation in our laboratory.
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Experimental Section
General procedure for the coupling reactions: unless otherwise noted, inside a glovebox, fluorinated
phenyl boronic acid pinacol ester (0.4 mmol), the arylhalide (Ar-X) (0.6 mmol), CuI (10 mol% if X =
iodide; 30–50 mol% if X = bromide), phenanthroline (10 mol% if X = iodide; 30–50 mol% if X =
bromide), CsF (0.8 mmol, 2 equiv), were added to a Schlenk flask that equipped with a stirring bar.
The flask was capped and taken out of the glovebox. Solvent (DMF 3 mL if X = iodide; 1/1 mixture
of DMF and toluene, 4 mL if X = bromide) were added under an argon atmosphere using a Schlenk
vacuum line. The reaction was heated and stirred at 130 °C for 18 h if X = iodide or at 140 °C for 36 h
if X = bromide. After cooling to room temperature, the resulting mixture was extracted with ethyl
acetate (3 x 20 mL). The organic phase was dried over MgSO , filtred, and concentrated in vacuo. The
4
residue was finally purified by flash column chromatography on silica gel (hexane). After
concentrating the fractions containing the product, the residue was dried under reduced pressure to
yield the pure product. Full experimental details as well as characterization data and spectra of the
products are provided in the Supporting Information.
Crystallographic details
Crystal data collection and processing parameters are given in the Supporting Information. CCDC-
1917134 (3d), 1917135 (3n), 1917136 (3o), and 1917137 (5d) contain the supplementary
crystallographic data for this paper. These data are provided free of charge by The Cambridge
Crystallographic Data Centre.
Acknowledgment
Y.P.B. thanks the Indonesia Endowment Fund for Education (LPDP) for a Ph.D scholarship. T.B.M.
and U.R. thank the Julius-Maximilians-Universität Würzburg and the Deutsche
Forschungsgemeinschaft (DFG) for support. We thank AllyChem Co. Ltd. for a generous gift of
B
2
pin .
2
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Conflict of interest
The authors declare no conflict of interest.
Keywords: homogeneous catalysis; boron; boronate; fluorine; fluoroarene
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ChemCatChem
10.1002/cctc.201901220
Graphics and Entry for TOC
Fn
O
B
O
I
Fn
R
Cu /phenanthroline
n = 0 - 5
+
CsF
R
DMF or DMF/toluene (1:1)
o
1
30 - 140 C,
X
24 - 36 h
X = I or Br
25 examples up to 99 % yield
Moving to a cheap, Earth-abundant, and less toxic metal catalyst. We introduce an
alternative for Suzuki-Miyaura cross-coupling reactions of challenging substrates, namely
highly fluorinated aryl-boronate esters, with aryl iodides or bromides using
I
Cu /phenanthroline as the catalyst precursor to generate fluorinated biaryls in good to
excellent yields. Crystals of the products show a diverse range of interesting π-stacking
interactions.
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