An improved and efficient synthesis of pinene based bipyridyldiols and bipyridine

Article abstract of DOI:10.1016/j.tetlet.2016.03.075

An improved and efficient synthesis of pinene based two bipyridyldiols and bipyridine is reported. For the first time, the sealed tube-pressure reaction of pinene based pyridone with phosphoryl chloride produced an excellent yield (95%) of pinene based 2-chloropyridine, which renders synthesizing pinene based bipyridyldiols a highly inexpensive and high yielding process. Moreover, highly effective reaction condition was developed for homocoupling of chloropyridine with Ni(0) that afforded pinene based bipyridine in a high yield (84%). These newly demonstrated sealed tube-pressure chlorination and homocoupling reaction of chloropyridine afford extremely effect route for the synthesis of pinene based bipyridine.

Full text of DOI:10.1016/j.tetlet.2016.03.075

Tetrahedron Letters xxx (2016) xxx–xxx
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Tetrahedron Letters
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An improved and efficient synthesis of pinene based bipyridyldiols
and bipyridine
a
a,b,
⇑
Ramalingam Boobalan , Chinpiao Chen
a
Department of Chemistry, National Dong Hwa University, Soufeng, Hualien 974, Taiwan
Department of Nursing, Tzu Chi University of Science and Technology, Hualien 970, Taiwan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
An improved and efficient synthesis of pinene based two bipyridyldiols and bipyridine is reported. For the
first time, the sealed tube-pressure reaction of pinene based pyridone with phosphoryl chloride produced
an excellent yield (95%) of pinene based 2-chloropyridine, which renders synthesizing pinene based
bipyridyldiols a highly inexpensive and high yielding process. Moreover, highly effective reaction condi-
tion was developed for homocoupling of chloropyridine with Ni(0) that afforded pinene based bipyridine
in a high yield (84%). These newly demonstrated sealed tube-pressure chlorination and homocoupling
reaction of chloropyridine afford extremely effect route for the synthesis of pinene based bipyridine.
Ó 2016 Elsevier Ltd. All rights reserved.
Received 17 January 2016
Revised 21 March 2016
Accepted 23 March 2016
Available online xxxx
Keywords:
Bipyridyldiol
Michael addition
Kröhnke annulation
Ni(0)-homocoupling
Phosphorous oxychloride
Introduction
N
N
In the past three decades, numerous developments in the field
of asymmetric synthesis have facilitated the synthesis of numerous
enantioenriched compounds for organic chemists. Many chiral
1
: R=Me
: R=Ph
OH HO
2
1
R
R
R R
ligands or catalysts have been used for stereoselective reactions;
they are mostly either directly available from natural sources or
Figure 1. Structure of pinene-bipyridyldiol.
synthesized from naturally available chiral starting materials
and 15%.2e Therefore, to explore the application of these ligands
to various asymmetric transformation reactions, designing an
efficient synthetic procedure is necessary.
through chiral pool synthesis.1 Although many catalytic systems
are available for each enantioselective and diastereoselective reac-
tion, a new catalyst is still required in asymmetric synthesis for
advancing the field. We studied the application of a-pinene derived
The synthesis and application of modified pinene derived bipyr-
3
idine ligands have been studied mainly by Zelewsky et al., Malkov
O,N,N,O-tetradentate-bipyridyldiol ligands 1 and 2 (Fig. 1) to vari-
4
ous stereoselective reactions2 such as diethylzinc addition to
et al., and our group. The structural modification of pinene derived
bipyridine predominantly involved simple pinene based bipyridine
prochiral aldehydes, Strecker type trimethylsilyl cyanide addition
to prochiral aldehydes and imines, epoxide ring opening by a thiol
nucleophile, and Nozaki–Hiyama–Kishi allylation.
3
as the main intermediate, which was subsequently modified into
a tetradentate ligand through nucleophilic addition to ketone, as
well as into alkylated bidentate ligand through nucleophilic substi-
tution (Scheme 2).
Although a bis-nucleophilic substitution reaction with primary
alkyl halide was a facile process, the presence of high steric hin-
drance in the constrained pinene based bipyridine 3 rendered the
second nucleophilic addition reaction that hindered and lowered
the yield process. Consequently, only 14% (ligand 1) and 15%
Although pinene based bipyridyldiol has been investigated for
almost a decade, among the two bipyridyldiols, 1 generates the
highest chiral induction (up to 99% ee) for the chromium catalyzed
2
e
Nozaki–Hiyama–Kishi allylation of a wide range of aldehydes.
Although ligands 1 and 2 are promising ligands, they have a lack
of an efficient synthetic route; for instance, among its synthetic
reactions (Scheme 1), the yields of the final step was only 14%
(
ligand 2) of yields were achieved in the second nucleophilic addi-
tion reaction (Scheme 1).
Among the reported methods, Malkov’s synthetic procedure4d is
the most concise (five steps) for preparing bidentate pinene based
⇑
Corresponding author. Tel.: +886 3 863 3597; fax: +886 3 863 0475.
E-mail address: chinpiao@mail.ndhu.edu.tw (C. Chen).
http://dx.doi.org/10.1016/j.tetlet.2016.03.075
040-4039/Ó 2016 Elsevier Ltd. All rights reserved.
0
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2
R. Boobalan, C. Chen / Tetrahedron Letters xxx (2016) xxx–xxx
I2, Py
I
N
R
R
R
O
R
O
O
O
Ref.2a,e
steps
N
N
N
N
OH
LDA
N
N
OH
8
LDA
CO
1
Double
Kröhnke
annulation
route
O , hv
R
2
R
2
CO
OH
R
2
1
2
14% yield
15% yield
R
TPP
Ac O
O
2
3
Scheme 1. Reported our synthetic double Kröhnke annulation route for 1 and 2.
produced pyridinium salt 55a in an excellent yield (92–95%). To
Zelewsky, Malkov
overcome the low yield (43%) in the Kröhnke annulations of Mal-
LDA, R-I
N
N
4d
kov’s method,
pyridone
we then design a new route to synthesize
Nucleophilic
substitution
7
that one pot base-catalyzed Michael addition
followed by an acid-catalyzed cyclization. Thus, the reaction of
pyridinium salt 5 with pinocarvone 6 resulted in pyridone 7 in
R
R
N
N
6
7
our group
6
0–65% yield. The yield was consistent regardless of the batch size
LDA, R CO
2
N
N
of the reaction. The utilized pyridinium salt 5 has an advantage
Nucleophilic
addition
over Malkov’s pyridinium salt [1-(2-ethoxy-2-oxoethyl)pyridin-1-
OH
HO
4d
ium bromide] by possessing an amide group that presented in
R
R
3
R
R
pyridone 7; consequently, it obviates the use of an external
nitrogen source (ammonium acetate) required for the Kröhnke
annulation approach.
Scheme 2. Modification of simple pinene based bipyridine 3 into di- or tetraden-
tate ligands.
With pyridone 7 in hand, we then optimized the reaction con-
dition for conversion of pyridone 7 to chloropyridine 8. In the liter-
bipyridine intermediate 3. This method involves a Ni/Zn mediated
homocoupling reaction of pinene based pyridine-2-triflate for
constructing a bipyridine motif, which reduces the synthetic route
compared with Zelewsky’s route that involves sequential double
Kröhnke annulations (similar to our method ). Although Malkov’s
procedure is concise, it has a limitation of applying Kröhnke
annulations, which engender poor yield (43%), and use highly
expensive triflic anhydride. Therefore, we intended to modify this
process for improving synthetic process of 3 and performed a
nucleophilic addition reaction before homocoupling to realize an
efficient route for making pinene based bipyridyldiols. Herein, we
report an efficient and concise synthesis of 1, 2, and 3.
ature, a similar conversion is demonstrated for numerous simple
achiral pyridone moieties under various reaction conditions,8
which are shown in Scheme 4. Although this reaction was reported
to be extremely facile; initially, chloropyridine 8 was obtained only
in very low yield (30À40%) by all aforementioned reaction condi-
tions. Similar yields (30–40%) were obtained by Zelewsky and co-
2a
3d
4d
workers
and Malkov et al.
of similar chiral
for the bromination and
pyridones, respectively.
chlorination
Consequently, Malkov et al. used highly expensive triflic
anhydride instead of an inexpensive chlorinating agent
phosphoryl chloride (POCl ); triflic anhydride enabled them to
3
achieve the corresponding triflate product in an extremely high
yield.
Results and discussion
We envisioned that a pressure reaction could overcome this
drawback. As our assumption, when the chlorination was per-
Synthesis of pinene based bipyridyldiol 1 and 2
formed with neat POCl in a sealed tube at 130–140 °C, chloropy-
3
ridine 8 was obtained in an extremely high yield (89–95%). This
Scheme 3 shows the developed synthetic path for making 1 and
. The synthesis was commenced with the reaction of 2-chloroac-
etamide 4 with pyridine in acetone under reflux conditions that
remarkable sealed tube reaction was reproducible even at 7-g
scale, however, when this reaction was performed using round
bottom flask-heating method, the reaction was very sluggish and
2
Pyridine
O
6
POCl
3
O
Acetone
O
Cl
N
Cl
NH
2
NH
2
1N NaOH,
MeOH,
NH
89-95%
Reflux
seal tube
4
92-95%
AcOH
7
5
O
6
0-65%
R
R
R
NiCl , PPh
R
n-BuLi, DIPA
2
3
N
OH
R
+
R
N
ketone
THF
N
HO
N
HO
Zn, DMF
OH
N
R
8
Cl
Cl
, R=Me, 67%
a, R=Ph, 51%
1a, R=Me
9
9
R
1, R=Me, 65% 2a, R=Ph
2, R=Ph,40%
2
Scheme 3. Synthesis of a-pinene derived C -symmetric tetradentate bipyridyldiol 1 and 2.
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R. Boobalan, C. Chen / Tetrahedron Letters xxx (2016) xxx–xxx
3
POCl3 neat
°
reflux or 80 C
POCl3
catalytic. base
Ph
β
neat, reflux
Hα Hβ
base: Py or Et
POCl3
2
NPh
Phα
NH
C
10
O
OH
N
catalytic. base
Benzene, reflux
N
Cl
base: Py or Et NPh
2
Cl
3 5
POCl , PCl
neat, reflux
N
OH
POCl , PCl
Figure 3. X-ray crystal structure of 9a (CCDC 1463133) and its chem draw
3
5
catalytic .Py
structure.
neat, reflux
NCS/PPh3
Dioxane, reflux
of acetone. When highly dry acetone, obtained from pyrolysis of
the sodium iodide-acetone adduct, was used, the yield improved
to 80%. Gratifyingly, the nucleophilic reaction of 8 with more bulky
ketone, benzophenone, also produced the corresponding product
Scheme 4. Methods for conversion of pyridone/hydroxypyridine to chloropyridine.
9
a in 51% yield. This reaction was successful only using highly
dried benzophenone (dried over P in vacuum desiccator for
5 h). Interestingly, H NMR of compound 9a (Fig. 2) showed an
produced 8 in low yield (ca. 30%). After the effective chlorination
2 5
O
optimization, the next challenge was purchasing POCl
has been banned by the United Nations in Taiwan. Therefore, POCl
was synthesized from PCl through ozonolysis (Scheme 5).
3
, which
1
1
3
unusual up-field chemical shift (d) for a proton at À0.11 ppm (d,
1
3
J = 10.28 Hz) compared to H NMR of compound 9 that showed
Having chloropyridine 8 in hand, next, the nucleophilic addition
reaction of 8 was performed by reacting it with lithium diisopropy-
lamide followed by acetone to produce hydroxypropylchloropy-
ridine 9 (Scheme 3). 9 was obtained in moderate yield (67–70%).
The yield of this reaction was mainly dependent on the dryness
similar doublet at 1.34 ppm. To identify the cause for up-field
chemical shift, X-ray structure was obtained for single crystal of
9
a (Fig. 3) that revealed one of C10 proton (H
one of the phenyl group (Ph ). As in NMR, proton resides above
aromatic ring more shielded by ring current of k-bond compared
to lateral proton. The same ring current of phenyl group (Ph
b
) resided just above
b
b
)
O
P
Cl
might cause an up-field shift of C10
current region.
H
b
hence H
b
is above the ring
Cl
P
O3
Cl
Cl
Cl
Cl
DCM
78 °C
Finally, the obtained hydroxypropylchloropyridine
9
and
-
hydroxydiphenylmethylchloropyridine 9a underwent Ni(0) medi-
ated homocoupling reaction, as reported earlier.4 For this
d
Scheme 5. Synthesis of phosphorous oxychloride.
Figure 2. 1H NMR of compound 9a.
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4
R. Boobalan, C. Chen / Tetrahedron Letters xxx (2016) xxx–xxx
Figure 4. X-ray crystal structure of pinene based bipyridyldiol 1 (CCDC 1441174).
purpose, nickel dichloride (NiCl
2
) was reduced with zinc (Zn) and
series of optimization reactions, we realized that the reaction
triphenylphosphine (PPh
3
) in a polar aprotic solvent dime-
was highly sensitive to moisture and air.
thylformamide (DMF) in an argon atmosphere to generate reddish
tetrakis(triphenylphosphine)nickel(0), which was then reacted
with 9 and 9a to afford the desired tetradentate ligands 1 and 2 in
yields of 65% and 40%, respectively. The structure of obtained
ligands was confirmed through X-ray crystallography of 1 (Fig. 4).
Gratifyingly, generating the blood red Ni(0) complex by adding
zinc through a side-arm solid addition adaptor to the pre-heated
blue solution of anhydrous NiCl and PPh in DMF at 60 °C in an
2 3
argon atmosphere followed by adding a degassed DMF solution
of chloropyridine 8 produced the desired product 3 in an optimal
yield (84%) with a low amount of a reduced product, pinene based
pyridine 10 (Scheme 6). A notable observation of this reaction dur-
ing the optimization was that the generation of Ni(0) complex by
Synthesis of pinene based bipyridine 3
In addition to the successful synthesis of 1, we intended to have a
concise, efficient, and inexpensive synthetic route for obtaining
pinene based bipyridine 3. Although Malkov et al. synthesized 3
through the Ni(0) mediated homocoupling reaction of pyridine tri-
flate, their synthetic route had limitations: (1) they used highly
expensive triflic anhydride for the synthesis of starting material,
3 2
the premixing of all solid starting materials (Zn, PPh , and NiCl )
in degassed DMF at room temperature in an argon atmosphere,
then addition of chloropyridine 8 at 60 °C produced ligand 3 only
in 60% yield. Therefore, the late addition of zinc to the preheated
NiCl -PPh mixture was determined to be crucial for the success
2 3
of this homocoupling reaction.
2
-pyridine triflate; (2) the synthetic procedure utilized expensive
Ph P) NiCl instead of inexpensive NiCl for the homocoupling
reaction [although they used NiCl for synthesizing similar bipyri-
dine ligands, (Ph P) NiCl was used particularly in the preparation
of 3]; (3) the yield of homocoupling reaction was only average (51%).
To overcome these limitations, we optimized conditions for the
Ni(0) mediated homocoupling reaction of chloropyridine 8 by
(
3
2
2
2
Conclusion
2
3
2
2
An efficient and concise route to synthesize O,N,N,O-tetraden-
tate bipyridyldiol 1 and 2 was developed. The key features of the
developed method are as follows: (1) compared with Kröhnke
annulation, the Michael addition-cyclization reaction afforded
pinene based pyridone 7 in a high yield. (2) an exceptionally high
reacting it with Ni(0) generated from anhydrous NiCl
Á6H O, Zn, and PPh in DMF. For more than 50 years, several studies
investigated the Ni(0) mediated aryl halide coupling reaction. Ni
II) salts are used stoichiometrically or substoichiometrically for
2 2
or NiCl -
2
3
3
yielding POCl sealed tube-pressure reaction was used for chlori-
9
nating pyridone 7. (3) a novel concept was developed, and it
involved the nucleophile addition at an early stage to generate half
part of desired bipyridyldiol, followed by homocoupling, thus
enhancing the overall yield and reducing the synthetic steps for
ligands 1 and 2. (4) The structure of synthesized ligands and penul-
timate intermediates were confirmed by X-ray crystallography
especially X-ray structure of intermediate 9a is explained the most
(
this dimerization reaction. Various additives, such as tetraalkylam-
monium iodide, potassium iodide, and pyridine, have improved the
yield of the homocoupling reaction of aryl halides. However,
despite adding these additives, the yield of pinene based bipyridine
3
we obtained was only 40–50% yield. The major byproduct of this
1
reaction is a reduction product 10 (Scheme 6). According to our
probable reason for up-field chemical shift of a proton in H NMR
of 9a and 2. Moreover, an efficient and inexpensive synthetic route
was developed for N,N-bidentate pinene based bipyridine 3. Com-
pared with the previous reported method, our methodology
applied inexpensive starting materials; for instance, chloroac-
etamide is cheaper than ethyl bromoacetate and POCl is consider-
3
ably cheaper than triflic anhydride. The overall yield for the
1
.NiCl
2
, PPh
3
°
DMF, 60 C
N
N
+
°
2
. Zn, 60 C, 2h
N
N
3
. , DMF, 24h
8
synthesis of the bipyridine ligand 3 was 41À49%.
8
Cl
3 steps from 4
overall yield of
9-59%)
10
(
X-ray study
3
(84%)
4
The final atomic coordinates and crystallographic data (exclud-
ing structure factors) for compound 1 have been deposited at the
Scheme 6. Concise, efficient, and inexpensive synthesis of pinene based N,N-
bidentate bipyridine 3.
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R. Boobalan, C. Chen / Tetrahedron Letters xxx (2016) xxx–xxx
5
Cambridge Crystallographic Data Centre (CCDC) and are available
on request upon quoting the deposition numbers CCDC 1441174.
These data can be obtained free of charge from CCDC through
www.ccdc.cam.ac.uk/data_request/cif.
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Please cite this article in press as: Boobalan, R.; Chen, C. Tetrahedron Lett. (2016), http://dx.doi.org/10.1016/j.tetlet.2016.03.075