One-pot synthetic route to a class of polydental pyridylphosphines

Article abstract of DOI:10.1081/SCC-200064921

A one-pot synthetic route is presented for convenient preparation of di-2-pyridylphenylphosphine (Py₂PPh, 1), di[(2-pyridyl) phenylphosphino]methane (DPyPM, 2), and two new, long-chain polydental pyridylphosphines: di[2-(6-diphenylphosphino)pyridyl]phenylphosphine (P ₃N₂, 3) and bis[2-(6-diphenylphosphinopyridyl) phenylphosphino]methane (P₄N₂, 4). Copyright Taylor & Francis, Inc.

Full text of DOI:10.1081/SCC-200064921

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One‐Pot Synthetic Route
to a Class of Polydental
Pyridylphosphines
Tianle Zhang a , Yu Qin a , Deyou Wu a , Rong Zhou a
, Xiaowei Yi a & Changlin Liu a
a Department of Chemistry, Huazhong University of
Science and Technology, Wuhan, Hubei, China
Version of record first published: 16 Aug 2006.
To cite this article: Tianle Zhang, Yu Qin, Deyou Wu, Rong Zhou, Xiaowei
Yi & Changlin Liu (2005): One‐Pot Synthetic Route to a Class of Polydental
Pyridylphosphines, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 35:14, 1889-1895
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Synthetic Communicationsw, 35: 1889–1895, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1081/SCC-200064921
One-Pot Synthetic Route to a Class
of Polydental Pyridylphosphines
Tianle Zhang, Yu Qin, Deyou Wu, Rong Zhou, Xiaowei Yi,
and Changlin Liu
Department of Chemistry, Huazhong University of Science and
Technology, Wuhan, Hubei, China
Abstract: A one-pot synthetic route is presented for convenient preparation of
di-2-pyridylphenylphosphine (Py2PPh, 1), di[(2-pyridyl)phenylphosphino]methane
(DPyPM, 2), and two new, long-chain polydental pyridylphosphines: di[2-(6-diphenyl-
phosphino)pyridyl]phenylphosphine (P3N2, 3) and bis[2-(6-diphenylphosphinopyri-
dyl)phenylphosphino]methane (P4N2, 4).
Keywords: Polydental, pyridylphosphine
INTRODUCTION
In the past decade, the design and synthesis of polydental organic ligands
for the construction of linear metal chain complexes have attracted consider-
able attention because of the unique polynuclear metal–metal bonding,
properties, and potential application as molecular wires and in catalysis.[1,2]
Compared to oligo-a-pyridylamines,[3,4] pyridylphosphines are able to
stabilize transition metals at low oxidation states, and are especially useful
to construct hetero polynuclear complexes to study interactions among differ-
ent metals based upon different properties for phosphorous and nitrogen
atoms.[5–14] To date, several pyridylphosphines have been reported, but
related synthetic routes were complicated and time-consuming, for example,
Received in Japan December 16, 2004
Address correspondence to Tianle Zhang, Department of Chemistry, Huazhong
University of Science and Technology, Wuhan, Hubei 430074, China. Fax: þ86-27-
87543632; E-mail: tlzhang@mail.hust.edu.cn
1889

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T. Zhang et al.
di[(2-pyridyl)phenylphosphino]methane (DPyPM), first prepared by
Budzelaar in 1990.[15] Here, a one-pot synthetic route to pyridylphosphines
is presented, leading to convenient preparation of Py2PPh 1 and DPyPM
2, and two long-chain pyridylphosphines, 3 (P3N2) and 4 (P4N2), in high
yields.
A direct reaction of lithium phosphinidide (PhPLi2) with 2 equiv.
2-chloropyridine afforded Py2PPh 1 in 83% yield (Scheme 1). In a similar
way, pentadental pyridylphosphine 3 was synthesized using 2-chloro-6-diphe-
nylphosphinopyridine 5 as a starting material without 2-chloropyridine.
To synthesize 3, first 5 was prepared by adding a dilute Ph2PLi solution
in THF to a toluene solution containing excess 2,6-dichloropyridine,
followed by sublimation to remove the unreacted 2,6-dichloropyridine
(Scheme 2). PhPLi2 reacted with 2 equiv. 5 to give pyridylphosphine 3
(P3N2) in mild yield (60.5%) (Scheme 3) after purification by silica column
chromatograph using CH2Cl2/hexane as eluent and recrystallization from
CH2Cl2/hexane.
DPyPM 2 was prepared in 67.6% yield first by a reaction of PhPHLi with
1 equiv. 2-chloropyridine to give PhPyPH and subsequently with 0.5 equiv.
CH2Br2 after a treatment of PhPyPH with n-BuLi (Scheme 4). In this way,
hexadental pyridylphosphine 4 (P4N2) was also synthesized in 73.9% yield
using 5 as a starting material without 2-chloropyridine (Scheme 5). An
addition of equiv. amount n-butyllithium to PhPH2 afforded PhPHLi, which
reacted with 1 equiv. 5 to give 2-(6-diphenylphino)pyridyl-phenylphosphine
(Ph2PPyPhPH). Then, to Ph2PPyPhPLi solution in situ generated with
1 equiv. n-BuLi was added 0.5 equiv. CH2Br2 to obtain 4. During the course
of the reaction, the color changes were clearly indicated the completion of
the reactions. Therefore, there was no need to separate the intermediate
product as the literature method calls for. It is interesting to note that
1
from the H NMR spectrum of 4, two multiplets belonging to CH2 group
are clearly observed at 3.35–3.42 and 3.56–3.62 ppm, indicating that there
are two diastereomers for 4 as expected (Scheme 6), and the ratio of the dia-
stereomers was evaluated from integration approximately to be 1:2.3. We
were unable separate two isomers.
In summary, we present a method for convenient synthesis of polydental
pyridylphosphines. Studies to explore their low-valent metal chain complexes
are actively underway in this laboratory.
Scheme 1. One-pot synthesis of 1.

Polydental Pyridylphosphines
1891
Scheme 2. One-pot synthesis of 5.
EXPERIMENTAL
All synthetic procedures were performed under an atmosphere of dry argon
using Schlenk techniques. Solvents were dried and distilled by standard
methods. The chemicals and reagents were obtained commercially and used
with further purification. Phenylphosphine (PhPH2) and diphenylphosphine
(Ph2PH) were prepared according to literature methods.[16] 1H NMR spectra
were recorded on a Varian mercury-Vx 300 MHz instrument using
tetramethylsilane as an internal standard. 31P NMR spectra were recorded
on a Varian mercury-Vx 600 MHz spectrometer that operates at 242.8 MHz;
chemical shifts are quoted relative to H3PO4 (external, 31P). Mass spectra
were measured on a Finnigan LCQ DETA XPplus spectrometer.
Preparation of 2-Chloro-6-diphenylphosphinopyridine 5
To a THF solution (10 mL) of Ph2PH (1 mL, 5.7 mmol) was added dropwise
n-BuLi (2.50 mol/L, 5.7 mmol) at 08C. The red solution was left to stir for
more than 30 min after warming to room temperature, and was diluted with
25 mL of Et2O. Then, the solution was added slowly to a toluene solution
(40 mL) of 2,6-dichloropyridine (2.55 g, 17 mmol); the color immediately
changed from yellow to colorless. The reaction mixture was left to stir
overnight to give a white suspension. The solvent was removed and the
resulting mixture was extracted with CH2Cl2. The LiCl residue was
discarded, and the filtrate was reduced to give a white mixture. The
unreacted 2,6-dichloropyridine was removed through sublimation; pure 5
was obtained in high yields as white powder (1.5 g, 87.7%). Mp 748C;
1H NMR (300M, CDCl3) d 7.106 (d, J ¼ 8.1 Hz, 1H), 6.847 (J ¼ 7.8Hz, 1H),
7.28–7.67 (m, PhH, 4-PyH, 11H); 31P NMR (242.8M, CDCl3) d 22.463.
Scheme 3. One-pot synthesis of pentadental pyridylphosphine 3.

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Scheme 4. One-pot synthesis of 2.
Preparation of Di[2-(6-diphenylphosphino)pyridyl]
phenylphosphine 3 (P3N2)
To a THF solution (10 mL) of PhPH2 (0.37 mL, 3.36 mmol) was added
dropwise n-BuLi (2.50 mol/L, 6.72 mmol) at 08C. The resulting yellow
mixture was left to stir for more than 30 min after warming to room tempera-
ture, and then the solvent was removed in vacuo, leaving a yellow solid, to
which 15 mL of benzene was added. While adding 5 (2.02 g, 6.78 mmol) in
10 mL of benzene, the color immediately changed from yellow to dark red,
and the yellow solid was dissolved slowly. The reaction mixture was left
stirring overnight to give a pale yellow solution and white precipitate. After
an addition of 15 mL of distilled water, the mixture was extracted with
CH2Cl2 and dried with anhydrous Na2SO4. The solvent was removed to
afford a sticky pale yellow solid. The solid was purified by silica column chro-
matograph and recrystallization from CH2Cl2/hexane, and then was dried in
vacuo to give a pure 3 as a pale yellow solid, 1.30 g (60.5%). Mp 94–968C;
1H NMR (300M, CDCl3) d 6.99 (d, J ¼ 7.2 Hz, 2H), 7.09 (d, J ¼ 8.1 Hz,
2H), 7.28–7.67 (m, 27H); 31P NMR (242.8M, CDCl3) d 22.520, 19.640;
MS m/e 633.6 [M þ H]þ.
Scheme 5. One-pot synthesis of hexadental pyridylphosphine 4.

Polydental Pyridylphosphines
1893
Scheme 6. Two diastereomers of 4.
Preparation of Di[(2-pyridyl)phenylphosphino]methane 2 (DPyPM)
To a THF solution (10 mL) of PhPH2 (1 mL, 9.09 mmol) was added dropwise
n-BuLi (2.50 mol/L, 9.09 mmol) at 08C. The resulting yellow solution was left
to stir for about 30 min after warming to room temperature. This was then
added dropwise to a THF solution (4 mL) of 2-chloropyridine (0.86 mL,
9 mmol), resulting in a dark red solution, which was kept at 408C overnight
to give a yellow solution. While adding n-BuLi (2.50 mol/L, 9.09 mmol)
with an ice/salt bath, the mixture turned dark red immediately. It was
stirred for more than 30 min at room temperature; then a THF solution
(3 mL) of CH2Br2 (0.32 mL, 4.5 mmol) was added slowly with an ice/salt
bath. The mixture was left stirring overnight to give a yellow solution. The
volume of solution was reduced to about 5 mL and then treated with 20 mL
of 4 mol/L NaOH solution. The product was extracted with 20 mL of
CH2Cl2 three times, and the solvent was stripped to give the crude product,
which was purified by silica column chromatograph to afford 2 as a sticky
1
yellow solid, 1.21 g (69.7%). Mp 37–398C; H NMR (300 MHz, CDCl3)
d 2.681–3.273 (m, –CH2–, 2H), 7.099–8.755 (m, Ph, Py, 18H).
Preparation of bis[2-(6-Diphenylphosphinopyridyl)
phenylphosphino]methane 4 (P4N2)
To a THF solution (5 mL) of PhPH2 (0.55 mL, 5.00 mmol) was added
dropwise n-BuLi (2.50 mol/L, 5.00 mmol) at 08C. The resulting yellow
solution was left to stir for about 30 min after warming to room temperature.
This was then added dropwise to a THF suspension (5 mL) of 5 (1.50 g,
5.04 mmol), resulting in a dark red solution, which was kept at 408C
overnight to give a yellow solution. While adding n-BuLi (2.50 mol/L,
5.00 mmol) with an ice/salt bath, the mixture turned dark red immediately
and was stirred for more than 30 min at room temperature. Then a THF
solution (2 mL) of CH2Br2 (0.18 mL, 2.50 mmol) was added slowly with an
ice/salt bath. The mixture was left stirring overnight to give a yellow
solution. The volume of solution was reduced to about 5 mL, and then

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T. Zhang et al.
treated with 20 mL of 4 mol/L NaOH solution. The product was extracted
with 20 mL of CH2Cl2 three times, and the solvent was stripped to give the
crude product, which was purified by silica column chromatograph to afford
4 as a sticky yellow solid, 1.39 g (73.9%). Mp 57–608C; 1H NMR
(300 MHz, CDCl3) d 3.35–3.42, 3.57–3.62 (m, –CH2–, 2H), 7.15–7.85
(m, Ph, Py, 36H); 31P NMR (242.8M, CDCl3) d 22.750, 22.066, 19.769,
22.052; MS m/e 755.3 [M þ H]þ.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the funding of Huazhong University of
Science and Technology.
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