Suzuki coupling of cyclopropylboronic acid with aryl halides catalyzed by a palladium-tetraphosphine complex

Article abstract of DOI:10.1080/00397910500330833

The tetraphosphine all-cis-1,2,3,4-tetrakis(diphenylphosphinomethyl)- cyclopentane (Tedicyp) in combination with [Pd(C₃H ₅)Cl]₂ affords a very efficient catalyst for the coupling of cyclopropylboronic acid with aryl bromides and aryl chlorides. Higher reactions rates were observed with aryl bromides than with aryl chlorides; however, even in the presence of 1-0.4% of catalyst, a few aryl chlorides gave the coupling products in good yields. A wide variety of substituents such as alkyl, methoxy, trifluoromethyl, acetyl, benzoyl, formyl, carboxylate, nitro, and nitrile on the aryl halides are tolerated. The coupling reaction of sterically very congested aryl bromides such as bromomesitylene or 2,4,6-triisopropylbromobenzene also proceeds in good yields. Copyright Taylor & Francis LLC.

Full text of DOI:10.1080/00397910500330833

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Suzuki Coupling of Cyclopropylboronic
Acid With Aryl Halides Catalyzed by a
Palladium–Tetraphosphine Complex
Mhamed Lemhadri a , Henri Doucet a & Maurice Santelli a
a UMR 6180 CNRS and Université d'Aix‐Marseille III, Laboratoire de Synthèse
Organique, Faculté des Sciences de Saint Jérôme, Université d'Aix‐Marseille
III , Marseille Cedex, France
Published online: 21 Aug 2006.
To cite this article: Mhamed Lemhadri , Henri Doucet & Maurice Santelli (2006) Suzuki Coupling of
Cyclopropylboronic Acid With Aryl Halides Catalyzed by a Palladium–Tetraphosphine Complex, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 36:1,
121-128, DOI: 10.1080/00397910500330833
To link to this article: http://dx.doi.org/10.1080/00397910500330833
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Synthetic Communicationsw, 36: 121–128, 2006
Copyright # Taylor & Francis LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500330833
Suzuki Coupling of Cyclopropylboronic
Acid With Aryl Halides Catalyzed by a
Palladium–Tetraphosphine Complex
Mhamed Lemhadri, Henri Doucet, and Maurice Santelli
´
UMR 6180 CNRS and Universite d’Aix-Marseille III, Laboratoire de
´ ˆ
` ´
Synthese Organique, Faculte des Sciences de Saint Jerome, Universite
´
d’Aix-Marseille III, Marseille Cedex, France
Abstract: The tetraphosphine all-cis-1,2,3,4-tetrakis(diphenylphosphinomethyl)-
cyclopentane (Tedicyp) in combination with [Pd(C3H5)Cl]2 affords a very efficient
catalyst for the coupling of cyclopropylboronic acid with aryl bromides and aryl
chlorides. Higher reactions rates were observed with aryl bromides than with aryl
chlorides; however, even in the presence of 1–0.4% of catalyst, a few aryl chlorides
gave the coupling products in good yields. A wide variety of substituents such as
alkyl, methoxy, trifluoromethyl, acetyl, benzoyl, formyl, carboxylate, nitro, and
nitrile on the aryl halides are tolerated. The coupling reaction of sterically very
congested aryl bromides such as bromomesitylene or 2,4,6-triisopropylbromobenzene
also proceeds in good yields.
Keywords: Aryl halide, catalysis, cyclopropylboronic acid, palladium, tetraphosphine
The palladium-catalyzed Suzuki reaction is one of the most powerful methods
for the formation of C–C bonds.[1] In recent years, several thermally stable
palladium catalysts have been successfully used for this reaction, but most
Received in the UK May 23, 2005
Address correspondence to Henri Doucet, UMR 6180 CNRS and Universite d’Aix-
` ´
Marseille III, Laboratoire de Synthese Organique, Faculte des Sciences de Saint
´
´ ˆ
´
Jerome, Universite d’Aix-Marseille III, Avenue Escadrille Nomandie-Niemen,
13397 Marseille Cedex 20, France. Fax: (33) 4 91 98 38 65; E-mail: henri.doucet@
univ.u-3mrs.fr
121

122
M. Lemhadri, H. Doucet, and M. Santelli
of these catalysts have not been tested for synthesis of arylcyclopropanes by
coupling of aryl halides with cyclopropylboronic acid. The most popular
ligand for the reaction of cyclopropylboronic acid derivatives with aryl
halides is triphenylphosphine.[2–4] However, the catalyst formed by associ-
ation of this ligand with palladium complexes is not very efficient in terms
of the ratio of substrate/catalyst. We found only three examples of
formation of arylcyclopropanes via the Suzuki reaction using ligands other
than triphenylphosphine. The best results were obtained with PCy3 as ligand
with PdOAc2; however, with this catalytic system, 10% of ligand and 5% of
palladium were used, and moreover very low conversions were obtained
with aryl chlorides.[5] Two other monodentate ligands have also been used
successfully for the coupling of cyclopropylboronic acids derivatives with
aryl iodides or vinyl triflates: PtBu3 (10 mol%)[6] and PAs3 (20 mol%).[7]
However, to our knowledge, low-catalyst-loading Suzuki reactions using
cyclopropylboronic acid with aryl chlorides or bromides have not been
reported.
To obtain a highly stable palladium catalyst, we have prepared the tetra-
phosphine ligand, all-cis-1,2,3,4-tetrakis(diphenylphosphinomethyl)cyclo-
pentane or Tedicyp[8] (Figure 1). We have already reported several results
obtained for Suzuki cross-coupling with arylboronic acids using Tedicyp as
ligand.[9] We have also reported recently the results obtained for the
coupling of primary alkylboronic acids with several arylhalides.[10] To
further establish the requirements for a successful Suzuki reaction with our
catalyst, we herein report on the reaction of secondary alkylboronic acid
with aryl halides.
For this study, based on our previous results,[10] xylene was chosen as the
solvent and potassium carbonate as the base. The reactions were performed at
1308C, under argon, in the presence of a ratio 1/2 of [Pd(C3H5)Cl]2/Tedicyp
as catalyst.
First, we studied the influence of the substituents on the aryl bromide on
the reaction rate for the coupling with cyclopropylboronic acid (Scheme 1,
Table 1). We observed that in most cases the reactions performed with this
secondary alkylboronic acid proceed very smoothly. Quite similar reactions
rates were observed in the presence of electron-poor and electron-rich aryl
bromides. For example, turnover numbers (TONs) of 320–6100 were
obtained in the presence of 4-bromoacetophenone, 4-bromobenzaldehyde,
4-bromobenzophenone, 4-bromonitrobenzene, or 4-trifluoromethylbromo-
benzene (Table 1, entries 1–12). With electron-rich aryl bromides such as
Figure 1. Structure of Tedicyp.

Cyclopropylboronic Acid
123
Scheme 1.
4-tbutylbromobenzene, 4-bromotoluene, or 4-bromoanisole TONs of 920,
940, and 280 were obtained respectively (Table 1, entries 13–16, 19, and
20). These results seem to indicate that, with this catalyst, the rate-limiting
step for the coupling of cyclopropylboronic acid with aryl bromides is not
the oxidative addition of the aryl bromide to palladium.
Then, we studied the influence of the presence of ortho substituents on the
aryl bromides on the reaction rate. Similar reactions rates were obtained with
the ortho-substituted 2-bromobenzaldehyde and with the 4-bromobenzal-
dehyde (Table 1, entries 21 and 22). We also observed that the coupling of
1-bromonaphthalene with cyclopropylboronic acid proceeds in the presence
of 0.1% catalyst, in 100% conversion (Table 1, entries 23 and 24). Next,
we tried to evaluate the difference of reaction rate between mono- and
di-ortho-substituted aryl bromides, and we observed that even very hindered
aryl bromides could be coupled efficiently with cyclopropylboronic acid.
For example, with 2,4,6-trimethylbromobenzene and 2,4,6-triisopropyl-
bromobenzene the aryl cyclopropanes were obtained in 73 and 92% conver-
sion, respectively, in the presence of 0.1 and 1% catalyst (Table 1, entries
25–27).
Then, we investigated this reaction in the presence of heteroaryl
bromides. With 4-bromopyridine, 3-bromoquinoline, and 4-bromoisoquino-
line, the expected adducts were obtained in good yields in the presence of
1–0.1% catalyst (Table 1, entries 28–33).
Finally, the reactivity of five aryl chlorides was studied (Scheme 1,
Table 2). In the presence of electron-poor aryl chlorides such as 4-chloro-
acetophenone, 4-chlorobenzaldehyde, 4-chlorobenzonitrile, or 4-chloronitro-
benzene, the corresponding arylcyclopropanes were obtained in good
yields using 1–0.4% catalyst (Table 2, entries 1–6). The reaction also
proceeds nicely with the heteroaryl chloride 2-chloroquinoline (Table 2,
entries 7 and 8).
These results prompted us to investigate the cross-coupling of a few
other secondary alkylboronic acids. However, in all cases with isopropyl-,
cyclopentyl-, cyclohexyl-, or cyclooctylboronic acids and iodobenzene or
4-bromobenzophenone as coupling partners, very disappointing results were
obtained. The expected adducts were not observed even in the presence of
1–5% catalyst. In some cases, the dimerization of the aryl halides was
obtained. We had previously observed that high reactions rates for the

124
M. Lemhadri, H. Doucet, and M. Santelli
Table 1. Palladium-catalyzed coupling reactions of aryl bromides with cyclopropyl-
boronic acid (Scheme 1)a
Entry
Aryl bromide
Product Ratio substrate/catalyst
Yieldb
1
2
4-Bromoacetophenone
4-Bromoacetophenone
Methyl 4-bromobenzoate
Methyl 4-bromobenzoate
4-Bromobenzaldehyde
4-Bromobenzophenone
4-Bromobenzophenone
4-Bromobenzonitrile
1
1
1000
10,000
250
100 (94)
61
3
2
100 (95)
90
4
2
1000
1000
250
5
3
100 (95)
100 (93)
56
6
4
7
4
1000
1000
250
8
5
98 (92)
100 (94)
58
9
4-Bromonitrobenzene
4-Bromonitrobenzene
4-Trifluoromethylbromobenzene
4-Trifluoromethylbromobenzene
4-tButylbromobenzene
4-tButylbromobenzene
4-Bromotoluene
6
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
6
1000
250
7
100
7
1000
250
32
8
100
92 (88)
100
8
1000
250
9
4-Bromotoluene
9
1000
250
94
6-Methoxy-2-bromonaphthalene
6-Methoxy-2-bromonaphthalene
4-Bromoanisole
10
10
11
11
12
12
13
13
14
14
15
16
16
17
17
18
18
19
20
100 (94)
80
1000
250
100 (96)
28
4-Bromoanisole
1000
250
2-Bromobenzaldehyde
2-Bromobenzaldehyde
1-Bromonaphthalene
100 (90)
36
1000
250
100
1-Bromonaphthalene
1000
250
100 (93)
100 (91)
73
2,4,6-Trimethylbromobenzene
2,4,6-Trimethylbromobenzene
2,4,6-Triisopropylbromobenzene
4-Bromopyridine hydrochloride
4-Bromopyridine hydrochloride
3-Bromoquinoline
1000
100
92 (82)
100 (92)c
54c
100
250
250
100 (93)
37
3-Bromoquinoline
1000
100
4-Bromoisoquinoline
100 (93)
60
4-Bromoisoquinoline
250
5-Bromo-2-furaldehyde
1,4-Dibromobenzene
100
100
88 (84)
100d
aConditions: [ClPd(C3H5)]2/Tedicyp ¼ 1 : 2, aryl bromide: 1 mmol, cyclopropyl-
boronic acid: 2 mmol, K2CO3: 2 mmol, xylene, 1308C, 20 h, argon, GC and NMR yields.
bYields in parentheses are isolated.
cK2CO3: 3 mmol.
dCyclopropylboronic acid: 3 mmol, K2CO3: 3 mmol.
coupling of several primary alkylboronic acids with aryl halides could be
obtained.[10] We had also observed that isopropenylboronic acid or but-1-
en-2-ylboronic acid can be coupled very efficiently with several aryl
bromides.[11] The difference of reactivity among these secondary alkylboronic

Cyclopropylboronic Acid
125
Table 2. Palladium-catalyzed coupling reaction of aryl chlorides with cyclopropyl-
boronic acid (Scheme 1)a
Ratio substrate/
catalyst
Entry
Aryl bromide
Product
Yieldb
1
2
3
4
5
6
7
8
4-Chloroacetophenone
4-Chlorobenzaldehyde
4-Chlorobenzaldehyde
4-Chlorobenzonitrile
4-Chlorobenzonitrile
4-Chloronitrobenzene
2-Chloroquinoline
1
3
100
100
250
100
250
100
100
250
81 (77)
100 (91)
26
3
5
100 (90)
42
5
6
60 (56)
100 (83)
70
21
21
2-Chloroquinoline
aConditions: [ClPd(C3H5)]2/Tedicyp ¼ 1 : 2, aryl chloride: 1 mmol, cyclopropyl-
boronic acid: 2 mmol, K2CO3: 2 mmol, xylene, 1308C, 20 h, argon, GC and NMR
yields.
bYields in parentheses are isolated.
acids, cyclopropylboronic acid, and primary alkylboronic acids probably
comes from both the steric hindrance and the electronic properties of alkyl-
boronic acids.
CONCLUSION
The use of the tetradentate ligand Tedicyp associated to a palladium complex
provides a convenient catalyst for the coupling reaction of cyclopro-
pylboronic acid with substituted aryl bromides and aryl chlorides. Both the
steric hindrance and the electronic properties of secondary alkylboronic
acids seem to have an effect on the reaction. No coupling product was
obtained with isopropyl-, cyclopentyl-, cyclohexyl-, or cyclooctylboronic
acids and iodobenzene or 4-bromobenzophenone. The complex seems to
possess a fine balance of steric and electronic properties that generally
allow fast catalytic processes with cyclopropylboronic acid. This secondary
alkylboronic acid gave the expected arylcyclopropyl adducts in good
yields in the presence of 0.4–0.1% catalyst in most cases, even
with sterically congested aryl bromides. As expected, lower TONs were
observed for the coupling in the presence of aryl chlorides, but these
substrates gave the arylcyclopropyl adducts in satisfactory yields with only
1–0.4% catalyst. We believe that this system compares favorably with the
other catalysts that have been reported for this reaction. Because
of the high price of palladium, the practical advantage of such low-
catalyst-loading reactions can become increasingly important for industrial
processes.

126
M. Lemhadri, H. Doucet, and M. Santelli
EXPERIMENTAL
General
Xylene, analytical grade, was not distilled before use. Potassium carbonate
99þ was used. All reactions were run under argon using vacuum lines in
Schlenk tubes in oven-dried glassware. The reactions were followed by GC
and NMR for high-boiling-point substrates and by GC for low-boiling-point
substrates. 1H (300 MHz) and 13C (75 MHz) spectra were recorded in
CDCl3 solutions. Chemical shift (d) are reported in ppm relative to CDCl3.
Flash chromatographies were performed on silica gel (230–400 mesh).
Preparation of the Pd-Tedicyp Catalyst[8]
An oven-dried 40-mL Schlenk tube equipped with a magnetic stirring bar,
under argon atmosphere, was charged with [Pd(h3-C3H5)Cl]2 (4.2 mg,
11.6 mmol) and Tedicyp (20 mg, 23.2 mmol). Anhydrous DMF (2.5 mL) was
added, and then the solution was stirred at room temperature for 10 min.
General Procedure
As a typical experiment, the reaction of aryl halide (1 mmol), cyclopropyl-
boronic acid (0.172 g, 2 mmol), and K2CO3 (0.276 g, 2 mmol) at 1308C
during 20 h in xylene (2 mL) in the presence of cis-1,2,3,4-tetrakis(diphenyl-
phosphinomethyl)cyclopentane–1/2[PdCl(C3H5)]2 complex under argon
affords the corresponding products after addition of water, extraction with
ether or dichloromethane, separation, drying (MgSO4), evaporation, and puri-
fication by chromatography on silica gel.
1-Acetyl-4-cyclopropylbenzene (1). 1H NMR (CDCl3, 300 MHz):
d ¼ 7.83 (d, J ¼ 7.2 Hz, 2H), 7.10 (d, J ¼ 7.2 Hz, 2H), 2.55 (s, 3H), 1.93
(m, 1H), 1.05 (m, 2H), 0.77 (m, 2H).
4-Cyclopropylbenzoic acid methyl ester (2). 1H NMR (CDCl3,
300 MHz): d ¼ 7.90 (d, J ¼ 8.3 Hz, 2H), 7.07 (d, J ¼ 8.3 Hz, 2H), 3.88
(s, 3H), 1.91 (m, 1H), 1.05 (m, 2H), 0.77 (m, 2H).
4-Cyclopropylbenzaldehyde (3). 1H NMR (CDCl3, 300 MHz): d ¼ 9.93
(s, 1H), 7.74 (d, J ¼ 8.3 Hz, 2H), 7.17 (d, J ¼ 8.3 Hz, 2H), 1.95 (m, 1H), 1.07
(m, 2H), 0.79 (m, 2H).
1-Benzoyl-4-cyclopropylbenzene (4). 1H NMR (CDCl3, 300 MHz):
d ¼ 7.76 (d, J ¼ 8.3 Hz, 2H), 7.70 (d, J ¼ 8.3 Hz, 2H), 7.56 (t, J ¼ 7.3 Hz,
1H), 7.47 (d, J ¼ 7.3 Hz, 2H), 7.13 (d, J ¼ 8.3 Hz, 2H), 1.96 (m, 1H), 1.07
(m, 2H), 0.80 (m, 2H). 13C NMR (75 MHz, CDCl3): d ¼ 196.2, 149.7,
137.9, 134.6, 132.0, 130.3, 129.8, 128.1, 125.2, 15.7, 10.3. Anal. calcd. for
C16H14O: C, 86.45; H, 6.35. Found: C, 86.61; H, 6.18.

Cyclopropylboronic Acid
127
1
4-Cyclopropylbenzonitrile (5). H NMR (CDCl3, 300 MHz): d ¼ 7.50
(d, J ¼ 8.3 Hz, 2H), 7.11 (d, J ¼ 8.3 Hz, 2H), 1.91 (m, 1H), 1.07 (m, 2H),
0.76 (m, 2H).
1-Cyclopropyl-4-nitrobenzene (6). 1H NMR (CDCl3, 300 MHz):
d ¼ 8.10 (d, J ¼ 8.8 Hz, 2H), 7.14 (d, J ¼ 8.8 Hz, 2H), 1.99 (m, 1H), 1.13
(m, 2H), 0.81 (m, 2H).
1-Cyclopropyl-4-trifluoromethylbenzene (7). 1H NMR (CDCl3, 300MHz):
d ¼ 7.58 (d, J ¼ 8.1 Hz, 2H), 7.14 (d, J ¼ 8.1 Hz, 2H), 1.93 (m, 1H), 1.03
(m, 2H), 0.73 (m, 2H).
1
1-tert-Butyl-4-cyclopropylbenzene (8). H NMR (CDCl3, 300 MHz):
d ¼ 7.48 (d, J ¼ 8.0 Hz, 2H), 7.20 (d, J ¼ 8.0 Hz, 2H), 2.03 (m, 1H), 1.49
(s, 9H), 1.09 (m, 2H), 0.85 (m, 2H).
1-Cyclopropyl-4-methylbenzene (9). 1H NMR (CDCl3, 300 MHz):
d ¼ 7.03–6.95 (m, 4H), 2.35 (s, 3H), 1.85 (m, 1H), 0.93 (m, 2H), 0.64
(m, 2H).
2-Cyclopropyl-6-methoxynaphthalene (10). 1H NMR (CDCl3,
300 MHz): d ¼ 7.64 (m, 2H), 7.47 (s, 1H), 7.20–7.10 (m, 3H), 3.91 (s, 3H),
2.03 (m, 1H), 0.99 (m, 2H), 0.79 (m, 2H). 13C NMR (CDCl3, 75 MHz):
d ¼ 156.9, 139.0, 132.8, 129.0, 128.7, 126.7, 125.2, 123.7, 118.7, 105.6,
55.2, 15.4, 8.8. Anal. calcd. for C14H14O: C, 84.81; H, 7.12. Found: C,
84.97; H, 6.99.
1
1-Cyclopropyl-4-methoxybenzene (11). H NMR (CDCl3, 300 MHz):
d ¼ 7.05 (d, J ¼ 8.0 Hz, 2H), 6.83 (d, J ¼ 8.1 Hz, 2H), 3.80 (s, 3H), 1.88
(m, 1H), 0.91 (m, 2H), 0.64 (m, 2H).
2-Cyclopropylbenzaldehyde (12). 1H NMR (CDCl3, 300 MHz):
d ¼ 10.61 (s, 1H), 7.82 (dd, J ¼ 7.7 and 1.5 Hz, 1H), 7.48 (td, J ¼ 7.5 and
1.5 Hz, 1H), 7.32 (t, J ¼ 7.3 Hz, 1H), 7.15 (d, J ¼ 7.3, 1H), 2.64 (m, 1H),
1.08 (m, 2H), 0.80 (m, 2H).
1-Cyclopropylnaphthalene (13). 1H NMR (CDCl3, 300 MHz): d ¼ 8.40
(d, J ¼ 8.3 Hz, 1H), 7.80–6.90 (m, 6H), 1.80 (m, 1H), 1.05 (m, 2H), 0.78
(m, 2H).
2-Cyclopropyl-1,3,5-trimethylbenzene(14). 1H NMR(CDCl3, 300 MHz):
d ¼ 6.82 (s, 2H), 1.65 (m, 1H), 0.97 (m, 2H), 0.52 (m, 2H).
2-Cyclopropyl-1,3,5-triisopropylbenzene (15). 1H NMR (CDCl3,
300 MHz): d ¼ 6.96 (s, 2H), 3.86 (sept, J ¼ 7.1, 2H), 2.87 (sept, 1H),
1.66 (m, 1H), 1.22 (m, 18H), 1.04 (m, 2H), 0.57 (m, 2H). 13C NMR
(CDCl3, 75 MHz): d ¼ 149.4, 146.6, 133.9, 120.9, 34.0, 28.8, 24.5, 24.0,
10.3, 8.4. Anal. calcd. for C18H28: C, 88.45; H, 11.55. Found: C, 88.61;
H, 11.50.
4-Cyclopropylpyridine (16). 1H NMR (CDCl3, 300 MHz): d ¼ 8.04
(d, J ¼ 7.5 Hz, 2H), 7.14 (d, J ¼ 7.5 Hz, 2H), 1.97 (m, 1H), 1.18 (m, 2H),
0.90 (m, 2H).
1
3-Cyclopropylquinoline (17). H NMR (CDCl3, 300 MHz): d ¼ 8.79
(s, 1H), 8.05 (d, J ¼ 8.5 Hz, 1H), 7.70 (m, 2H), 7.62 (t, J ¼ 8.3 Hz, 1H),
7.48 (t, J ¼ 8.5 Hz, 1H), 2.07 (m, 1H), 1.08 (m, 2H), 0.85 (m, 2H).

128
M. Lemhadri, H. Doucet, and M. Santelli
4-Cyclopropylisoquinoline (18). 1H NMR (CDCl3, 300 MHz): d ¼ 9.13
(s, 1H), 8.32 (m, 2H), 7.96 (d, J ¼ 8.1 Hz, 1H), 7.75 (td, J ¼ 6.9 and 1.3 Hz,
1H), 7.61 (td, J ¼ 6.9 and 1.3 Hz, 1H), 2.24 (m, 1H), 1.09 (m, 2H), 0.80
(m, 2H).
5-Cyclopropyl-furan-2-carbaldehyde (19). 1H NMR (CDCl3, 300 MHz):
d ¼ 9.44 (s, 1H), 7.15 (d, J ¼ 3.6 Hz, 1H), 6.17 (d, J ¼ 3.6 Hz, 1H), 1.98
(m, 1H), 1.00 (m, 4H).
1
1,4-Dicyclopropylbenzene (20). H NMR (CDCl3, 300 MHz): d ¼ 6.97
(s, 4H), 1.84 (m, 2H), 0.92 (m, 4H), 0.65 (m, 4H).
1
2-Cyclopropylquinoline (21). H NMR (CDCl3, 300 MHz): d ¼ 8.00
(m, 2H), 7.75 (d, J ¼ 8.5 Hz, 1H), 7.65 (m, 1H), 7.44 (t, J ¼ 7.6 Hz, 1H),
7.15 (d, J ¼ 8.4 Hz, 1H), 2.24 (m, 1H), 1.10 (m, 4H).
ACKNOWLEDGMENTS
We thank the CNRS for providing financial support.
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