A C3v-symmetric triphosphine ligand derived from trindane skeleton: synthesis, inclusion of C60, and catalytic activity of its Pd complex

Article abstract of DOI:10.1016/j.tetlet.2015.08.069

With the aim to design a multidentate C_3v-symmetric trindane-based trisphosphine ligand 3 for Suzuki-Miyaura cross-coupling of 3-bromothiophene with phenylboronic acid, we observed the ability of this ligand to form an inclusion complex with buckyball C₆₀. Along with its catalytic activity, the pyramidal inversion at phosphorous atoms of 3 and the formation of 3@C₆₀ were investigated by ¹H NMR, ³¹P{¹H} NMR and DFT methods.

Full text of DOI:10.1016/j.tetlet.2015.08.069

Tetrahedron Letters xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A C -symmetric triphosphine ligand derived from trindane skeleton:
3
v
synthesis, inclusion of C , and catalytic activity of its Pd complex
6
0
a
a,b
a
c
a,
⇑
Dong Seob Lim , Suban K. Sahoo , Chan Sik Cho , Yang Kim , Heung-Jin Choi
a
Department of Applied Chemistry, Kyungpook National University, Daegu, 702-701, South Korea
Department of Applied Chemistry, SV National Institute of Technology, Surat, Gujrat 395007, India
Department of Chemistry & Advanced Materials, Kosin University, 194 Wachi-Ro, Yeongdo-gu, Busan 606-701, South Korea
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
With the aim to design a multidentate C3v-symmetric trindane-based trisphosphine ligand 3 for Suzuki–
Received 16 July 2015
Revised 20 August 2015
Accepted 25 August 2015
Available online xxxx
Miyaura cross-coupling of 3-bromothiophene with phenylboronic acid, we observed the ability of this
ligand to form an inclusion complex with buckyball C60. Along with its catalytic activity, the pyramidal
inversion at phosphorous atoms of 3 and the formation of 3@C60 were investigated by 1H NMR,
31
P
1
{
H} NMR and DFT methods.
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
Trisphosphine ligand
Suzuki–Miyaura cross-coupling
Inclusion complexation
C
60
3
1P NMR
Phosphine ligands are well known in chemistry because of their
fascinating coordination behavior with various transition metal
ions and wide industrial applications as catalysts.1 . The use of
phosphines as supporting ligands for cross-coupling reactions gen-
erating carbon–carbon bonds, in particular the Pd-catalyzed
Suzuki–Miyaura reaction of aryl halides with organoboron
reagents for the synthesis of biaryls, is well known due to several
advantages such as mild reaction conditions, high tolerance toward
reactive functional groups, commercial availability of various boro-
nic acids, and ease of separation of the products from the boron-
containing byproducts.3 These advantages have made Suzuki–
Miyaura coupling (SMC) a reaction of choice in the large-scale syn-
thesis of pharmaceutical compounds, fine chemicals, and poly-
mers. Recent results with multidentate phosphines having a
preorganized structure known to stabilize the palladium catalyst
multifunctional ligands design using the rigid tripodal framework
1
4–16
trindane,
herein, we introduced a novel C3v symmetric
,2
trisphosphine ligand to enrich the ligand library of SMC reactions.
Further, the ability to encapsulate C60 within the cavity formed by
the trisphosphine of 3 was examined by NMR and DFT methods.
The synthesis of trisphosphine ligand 3 was presented in
Scheme 1. The synthesis of starting compound 1, that is, benzyl
chloride derivative of trindane was carried out by following our
1
4
reported method. Because of the slow reactivity of the nucle-
ophilic substitution reaction to get the intermediate compound 2,
the starting compound 1 was first heated at 100 °C for 30 min
directly in neat ethyl diphenylphosphinite and then the mixture
was refluxed in toluene for another 2 h. Further purification by col-
umn chromatography gave 2 in 95% yield. Reduction of 2 with
–6
2
PhSiHCl in THF was carried out to afford the desired trisphosphine
7–9
and also improved the catalytic activity.
The present work is
ligand 3 under nitrogen environment. It was observed that the
alkyl phosphine product in THF sensitively reacted with oxygen
upon contact with air and was oxidized back to phosphine oxide.
Therefore, without any much exposure to the surrounding, the
reaction solvent and volatile by-products were removed by vac-
uum distillation at high temperature and then vacuum-dried to
get the desired trisphosphine 3 quantitatively.
an extension of this approach to catalyst improvement.
Moreover, the development of molecular clips having conju-
gated aromatic side arms with complementary cavity to wrap
buckyballs like C60 through pi-pi interactions has gained an
immense interest in the field of nanoscience and technology after
the first experimental evidence of stable inclusion complex of C60
was reported in 2007.1
0–13
As a part of our ongoing research on
The molecular structures of 2 and 3 were established by various
1
13
31
spectroscopic data ( H NMR, C NMR, P NMR, and HR-MS). From
1
H NMR, the appearance of two sets of doublets for 2 (d 3.18 and
2.80 ppm, J = 15.7 Hz) (Fig. S1a) and 3 (d 3.15 and 2.77 ppm,
J = 15.8 Hz) due to the two diastereotopic benzylic protons of
2
⇑
Corresponding author. Tel.: +82 53 950 5582; fax: +82 53 950 6594.
E-mail address: choihj@knu.ac.kr (H.-J. Choi).
2
http://dx.doi.org/10.1016/j.tetlet.2015.08.069
040-4039/Ó 2015 Elsevier Ltd. All rights reserved.
0
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2
D. S. Lim et al. / Tetrahedron Letters xxx (2015) xxx–xxx
O
O
Ph P
PPh2
Ph2P
O
2PPh
O
Cl
Cl
Cl
2
O
PPh2
PPh2
(a)
(b)
O
O
O
O
OEt
OEt
OEt
OEt
OEt
OEt
O
O
O
OEt
2
OEt
OEt
1
3
2 2
Scheme 1. Synthesis of trisphosphine ligand 3. Reagents and conditions: (a) (i) Ph POEt, 100 °C, 30 min and (ii) toluene, reflux, 2 h; (b) PhSiHCl , THF, reflux, 18 h.
the trindane skeleton and the singlet for the methylene groups of
benzyl groups at 2.95 and 2.90 ppm, respectively, confirmed the
at equilibrium compared to the phosphine oxide 2 (Fig. S1). The
pyramidal inversion of phosphine also affects the chemical shift
3
1
31
1
C3v-symmetrical conformation of the trindane derivatives. In
P
of the proton decoupled P{ H} NMR spectrum of 3 recorded at
1
NMR, the phosphine oxide 2 and reduced phosphine 3 gave only
one kind of singlet respectively at d 31.2 and d 4.7 ppm indicative
of the structural purity of the compounds.
various temperatures (Fig. 2). Similar to the H NMR observation,
the phosphine peak at 4.7 ppm (297 K) was shifted upfield to
À6 ppm (323 K) upon increasing the temperature. The broad sin-
glet at various temperatures indicates that the symmetry of the
molecule is maintained over a wide temperature.
The rigid C3v symmetric trindane derived trisphosphine with
multi-aromatic systems and preorganized cavity is expected to
The trisphosphine ligand 3 along with the two commercial
phosphines, that is, triphenylphosphine (4) and 1,2-bis(diphenyl-
phosphino) (5) were applied as a ligand for the Pd-catalyzed
Suzuki–Miyaura cross-coupling of 3-bromothiophene with phenyl-
boronic acid (Table 1). The prolonged reaction time of 40 h was
kept for all the three phosphines to evaluate the catalytic activity
with their overall turnover. As summarized in Table 1, the Pd-cat-
alyzed reaction with trisphosphine ligand 3 showed better activity
on the cross-coupling reaction compared to monophosphine and
diphosphine. The preorganized structure of 3 with multiphosphine
groups along with the cooperative effect increases the catalytic
activity.
act as a tripodal molecular clip to encapsulate fullerenes like C60
To get a preliminary information on the inclusion complexation
behavior of 3, a saturated solution of C60 in CDCl (solubility of
60 in chloroform: 0.16 mg/mL ) was treated with 3 (10 mM),
.
3
1
7
C
3
1
1
and the P{ H} NMR spectrum was recorded at different temper-
atures (Fig. 3). At 297 K, the free ligand 3 showed a small broad
peak at 4.7 ppm. In the presence of C60, a new sharp multiplet at
42.5 ppm appeared due to the formation of an inclusion complex
In 1H NMR of 3 (Fig. 1) recorded at room temperature, we
observed a broad peak for the methylene group of the phosphine
possibly due to the pyramidal inversion slower than the NMR time
scale, instead well-defined all sharp peaks due to the correspond-
ing phosphine oxide 2. The broad singlet at 4.6 ppm of the methy-
lene group of phosphine at 297 K was shifted upfield upon
increasing the temperature due to the conformation rotation at
phosphine without any significant changes in the overall C3v sym-
metry of 3. When the temperature increased to 323 K, the pyrami-
dal inversion on phosphine is getting faster and therefore resulted
a broad doublet at 3.5 ppm for the methylene protons coupled with
with C60 through CH–p/p–p interactions. With the rise in temper-
ature from 297 K to 323 K, the peak characterized for the 3@C60
was shifted upfield from 42.5 ppm to 29.7 ppm. Such a shift indi-
cates the effect of pyramidal inversion as well as the participation
of the phosphine group in the complex formation. At 323 K, a sharp
peak at À8 ppm was also observed similar to the free ligand 3
indicative of the temperature dependent partial reverse dissocia-
tion of 3@C60 due to the inversion of 3 at the phosphorous atom.
The inclusion of C60 within the cavity created by the phosphine
group of 3 was also further complemented with the dynamic 1
H
NMR (Fig. 4). The broad singlet at 4.6 ppm of the methylene group
of free triphosphine 3 at 297 K was shifted upfield to 3.95 ppm in
the presence of C60. The upfield shift of methylene peak was con-
tinued further upon increasing the temperature, which clearly
indicates the effect of pyramidal inversion on the formation of
inclusion of between C60 and 3. At a higher temperature of 323 K,
the methylene peak splits into two different peaks at 3.65 and
2
phosphorous nuclei ( JHP = 13 Hz). The possible pyramidal inver-
1
3
sion of phosphine group of 3 was clearly identified in the
NMR (Fig. S2). The peaks observed between 128 and 136 ppm of
were comparatively broad rather than the specific chemical shift
C
3
owing to the presence of variety of molecular inversion conformers
3
.35 ppm due to the equilibrium observed between the complexed
Table 1
3@C60 and free triphosphine 3. The pyramidal inversion of 3 is
active in both uncomplexed and complexed conditions which can
also be understood from the appearance of broad peaks for the aro-
matic protons between d 8.1–6.8 ppm (Fig. S3). However, the aro-
matic proton peaks of free triphosphine 3 were shifted to the
upfield region significantly with the rise in temperature indicating
faster pyramidal inversion. But such a significant upfield shift of
aromatic protons peaks of 3 was not observed after the addition
of C60 attributed to the formation of 3@C60 where the fullerene is
expected to wrap within the tripodal cavity through multiple
Palladium-catalyzed cross-coupling reaction of 3-bromothiophene (1 mmol) with
phenylboronic acid (1.5 mmol) with phosphine ligands 3, 4, and 5
Br
[
Pd(C H )Cl] /ligand
3 5 2
+
B(OH)
2
S
S
2 3
K CO (2 eq), toluene, reflux
(
1 mmol)
(1.5 mmol)
*
Entry Pd catalyst (
l
mol) Ligand (lmol) Reaction time (h) Yield (%)
1
2
3
4
0.4
0.2
0.2
0.2
0
20
40
40
40
8
15
33
86
CH–p/p–p interactions.
5 (0.2)
4 (0.4)
3 (0.2)
To gain further insights into the structure of 3@C60, we
performed the density functional theory (DFT) calculations by
⁄
*
applying the B3LYP exchange–correlation functional with 6-31G
3 5 2
Reaction conditions: [Pd(C H )Cl] as palladium catalyst in toluene (5 mL) at
reflux, yields determined by gas chromatography.
basis sets. All calculations were performed by using the Spartan’14
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D. S. Lim et al. / Tetrahedron Letters xxx (2015) xxx–xxx
3
Figure 1. 1H NMR spectra of 3 at various temperatures in CDCl
(*: THF).
3
Figure 2. 31P{ H} NMR spectra of 3 at various temperatures in CDCl
1
3
.
software package. The energy minimized structure of 3 showed a
3v-symmetry at the trindane framework, but the symmetry was
C
distorted at the phosphine groups possibly due to the flexibility
and/or the pyramidal inversion (Fig. 5a). Upon introduction of
Figure 3. 31P{ H} NMR spectra of 3 (10 mM) with C60 at various temperatures (in
1
C60, the three appended arms of 3 rearranged themselves to create
CDCl ).
3
a C3v-symmetry tripodal cavity of diameter ꢀ12.1 Å (Fig. 5b and c).
It is well known that the size-fitting is the important factor for the
formation of host–guest inclusion complex. The obtained diameter
is close to the recommended diameter of 13 Å in order to host
The position of C60 within 3 was ascertained by analyzing the
closest bond distances between the host and guest (Fig. 6a). The
C60 was 3.632 Å above the plane of trindane platform whereas a
distance of 3.891 Å was maintained from the trindane benzene
18
fullerenes like C60 or C70
.
Also, the computed 3@C60 structure
(Fig. 5) clearly supported the ability of 3 to recognize C60
.
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4
D. S. Lim et al. / Tetrahedron Letters xxx (2015) xxx–xxx
Figure 4. 1H NMR spectra of 3 (10 mM) with C60 at various temperatures in CDCl
(*: THF).
3
⁄
Figure 5. DFT (B3LYP/6-31G ) computed molecule structure of trisphosphine ligand 3 (a) and the two views of host–guest complex with 3@C60 (b and c).
Figure 6. (a) Some important bond lengths and (b) the molecular electrostatic potential map (MEP) of 3@C60
.
rings, which indicates that the guest C60 is located within the cen-
ter of the host cavity. Also, the obtained distances are within the
fifteen CH–p interactions. These supramolecular interactions
induced an internal charge transfer between C60 and 3, which
range for the possible
p–p
stacking interactions.19 In addition,
can be confirmed from the molecular electrostatic potential map
2
0
we also observed that the guest C60 wrapped with the help of
(MEP) of 3@C60 (Fig. 6b).
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D. S. Lim et al. / Tetrahedron Letters xxx (2015) xxx–xxx
5
In conclusion, we have introduced a novel trisphosphine ligand
derived from the rigid trindane tripodal framework in excellent
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0. The MEP was drawn by showing the most negative region by red color whereas
the positive region by blue color, and then the color was extrapolated within
red and blue to show the other regions.
References and notes
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4
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