Expedient one-pot organometallics-free synthesis of tris(2-pyridyl) phosphine from 2-bromopyridine and elemental phosphorus

Article abstract of DOI:10.1016/j.tetlet.2012.03.004

2-Bromopyridine reacts with elemental phosphorus (red or white) in a superbasic KOH/DMSO(H₂O) suspension at 100 °C (for red phosphorus) and 75 °C (for white phosphorus) over 3 h to afford tris(2-pyridyl)phosphine in a 62% yield (from red phosphorus) and a 50% yield (from white phosphorus). Under microwave assistance, the reaction with red phosphorus takes just 20 min to produce tris(2-pyridyl)phosphine in 53% yield. A hitherto unknown complex, [Pd(PPy₃)₂Cl ₂]·CH₂Cl₂, synthesized from tris(2-pyridyl)phosphine and PdCl₂, has the cis-configuration; this is unusual for bis(phosphino)palladium dichloride complexes.

Full text of DOI:10.1016/j.tetlet.2012.03.004

Tetrahedron Letters 53 (2012) 2424–2427
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Expedient one-pot organometallics-free synthesis of tris(2-pyridyl)phosphine
from 2-bromopyridine and elemental phosphorus
Boris A. Trofimov a, , Alexander V. Artem’ev a, Svetlana F. Malysheva a, Nina K. Gusarova a,
⇑
Nataliya A. Belogorlova a, Anastasiya O. Korocheva a, Yuriy V. Gatilov b, Victor I. Mamatyuk b
a A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences, 1 Favorsky Str., 664033 Irkutsk, Russian Federation
b N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences, 9 Lavrentiev Ave., 630090 Novosibirsk, Russian Federation
a r t i c l e i n f o
a b s t r a c t
Article history:
2-Bromopyridine reacts with elemental phosphorus (red or white) in a superbasic KOH/DMSO(H2O) sus-
pension at 100 °C (for red phosphorus) and 75 °C (for white phosphorus) over 3 h to afford tris(2-pyri-
dyl)phosphine in a 62% yield (from red phosphorus) and a 50% yield (from white phosphorus). Under
microwave assistance, the reaction with red phosphorus takes just 20 min to produce tris(2-pyri-
dyl)phosphine in 53% yield. A hitherto unknown complex, [Pd(PPy3)2Cl2]ÁCH2Cl2, synthesized from
tris(2-pyridyl)phosphine and PdCl2, has the cis-configuration; this is unusual for bis(phosphino)palla-
dium dichloride complexes.
Received 21 November 2011
Revised 20 February 2012
Accepted 1 March 2012
Available online 7 March 2012
Keywords:
2-Bromopyridine
Palladium complexes
Phosphorus
Ó 2012 Elsevier Ltd. All rights reserved.
Tris(2-pyridyl)phosphine
Superbasic systems
Over the last decade, pyridylphosphine ligands have attracted
attention for the design of transition metal complexes1 which
demonstrate extraordinary coordination behavior and catalytic
activity.2 The ability of such ligands to coordinate to hard metals
on nitrogen and to a catalytically active transition metal via the
phosphorus atom makes them ideal for constructing metallic and
heterometallic architectures.3 Also, pyridylphosphines have been
shown to be valuable building blocks for supramolecular struc-
tures.4 Moreover, pyridylphosphines were reported as intermedi-
ates for organic synthesis5 and precursors to pharmaceuticals.6
Among the pyridylphosphines, the 2-pyridylphosphines, in partic-
ular, tris(2-pyridyl)phosphine, are of significant interest due to the
neighboring phosphorus and nitrogen atoms, thereby securing a
cooperative chelating interaction with metals.3,7 There have been
an increasing number of applications of tris(2-pyridyl)phosphine
in organic3,8 and organometallic3 chemistry, mainly owing to its
unique structural feature retaining both the phosphorus and nitro-
gen atoms with a versatile coordination potential.3,7 Diverse liga-
tions of tris(2-pyridyl)phosphine have led to numerous novel and
useful complexes,3,9 some of which have been explored as catalysts
for such important industrial reactions as methoxycarbonylation of
alkynes,10 hydroformylation of alkenes,11 ethylene polymeriza-
tion,12 and a Diels–Alder synthesis.13
Commonly, tris(pyridyl)phosphines have been synthesized
from pyridyllithium or pyridylmagnesium reagents that were trea-
ted with phosphorus halides.3a In the organolithium version,14 it
was essential to maintain the low reaction temperature
(À110 °C) and short reaction times to prevent the isomerization
of the parent pyridyllithium compound. In addition, to isolate
tris(pyridyl)phosphines in a yield of about 45%, multiple column
chromatography procedures were required.14 Recently, tris(2-pyr-
idyl)- and tris(3-pyridyl)phosphines were synthesized in 70–75%
yields using the appropriate magnesium reagent (THF, À78 °C).15
Solid–liquid extraction with large quantities of diethylamine was
employed for the isolation of the target phosphines.15 This uncom-
mon isolation protocol was crucial because the presence of hard
cations (such as Mg2+) greatly increases the water-solubility of
these phosphines.15
Herein we report, as a logical extension of our new general
methodology for the phosphorylation of electrophiles using ele-
mental phosphorus,16
a simple and straightforward one-pot
method for the synthesis of tris(2-pyridyl)phosphine (1) which
comprises the direct reaction of the readily available17 2-bromo-
pyridine with elemental phosphorus (both red and white) in a
superbasic KOH/DMSO (H2O) suspension. The reaction was carried
out at 100 °C (for red phosphorus, Pn) or at 75 °C (for white phos-
phorus, P4) over 3 h, the molar ratio 2-bromopyridine/elemental
phosphorus/KOH being 1:2:3, to afford the phosphine 1 in 62%
and 50% yields, for red and white phosphorus, respectively.18 As
shown from the 31P NMR spectra of the reaction mixture, tris(2-
pyridyl)phosphine oxide (2) was also formed in up to 10% yield.
⇑
Corresponding author. Tel.: +7 395242 14 11; fax: +7 395241 93 46.
E-mail address: boris_trofimov@irioch.irk.ru (B.A. Trofimov).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.03.004

B. A. Trofimov et al. / Tetrahedron Letters 53 (2012) 2424–2427
2425
The reaction conditions specified are close to optimal, since
their alteration, as shown in Table 1, did not improve the yield of
phosphine 1. Indeed, altering the temperature and the KOH con-
tent from those shown in Scheme 1 decreases the yield of phos-
phine 1 sharply.
Microwave irradiation (300 W) of the reaction of 2-bromopyri-
dine and red phosphorus in KOH/DMSO suspension (the reactant
ratio being as in entry 10, Table 1) furnished a 53% yield of phos-
phine 1, the synthesis requiring just 20 min.
Notably, 2-chloropyridine, although completely consumed un-
der the above conditions, gave just a 3–5% yield of phosphine 1.
Under comparable conditions, the conversion of 2-iodopyridine
was incomplete (50%) and the reaction mixture contained only a
trace amount of the target phosphine 1.
As far as other halopyridines are concerned, 3-bromopyridine
with red phosphorus was stable under the above phosphorylation
conditions (almost complete recovery), whereas 4-chloropyridine
appeared to be too reactive toward KOH and the expected tris(4-
pyridyl)phosphine was only discernable as a trace amount in the
reaction mixture.
It should be emphasized, that the synthesis of tris(2-pyri-
dyl)phosphine, described here, represents essentially the first effi-
cient transformation of a sp2-C–halogen bond into a sp2-C–P bond
via this new general phosphorylation methodology using elemen-
tal phosphorus-superbase reagents. Despite the great diversity of
electrophiles successfully phosphorylated by this methodology
(sp3-C–halides, alkenes, alkynes, oxiranes),16 only one representa-
tive of sp2-C–halogen electrophiles, namely, 1-bromonaphthalene,
was converted so far by the above protocol into tris(1-naph-
thyl)phosphine in a modest 10–25% yield.19
The synthesis of phosphine 1 is certainly triggered by cleavage
of a P–P bond in the Pn network or in a P4 molecule under the ac-
tion of activated hydroxide generating anionic clusters of the phos-
phide type A, the key nucleophiles, which then attack the
Csp2 —halogen bond (Scheme 2). Such multiatomic nano-P-centered
anions are assumed to possess increased nucleophilicity due to the
effect of the adjacent phosphorus atom. These nucleophilic species
replace bromine in 2-bromopyridine to form the first Csp2 —P bond
(intermediate C), and are still attached to the phosphorus nano-
cluster. Subsequent repetition of the above steps eventually leads
to the final phosphine 1. This rationalization explains satisfactorily,
why only tris(2-pyridyl)phosphine (and a small amount of its
oxide) was formed exclusively and why neither mono- nor bis(2-
pyridyl)phosphines (or phosphine oxides) were detectable in the
reaction mixtures. The augmentation in nucleophilicity of anionic
clusters A upon the insertion of each pyridyl moiety may contrib-
ute to the chemoselectivity of the reaction observed.
The observed difference in reactivity of the halopyridines to-
ward phosphorus-centered nucleophiles, generated in Pn(P4)/
KOH/DMSO suspensions, does not correspond exactly to the reac-
tivity of the sp2-C–halogen bond in the pyridine series and can
be explained by the interference of hard–soft electrophile–nucleo-
phile relationships, that is, by the competition between hydroxide
and diverse phosphorus nucleophiles.
The neutral intermediate B, resulting from the initial P–P-bond
cleavage (Scheme 2), after deprotonation might also participate in
a nucleophilic substitution reaction with 2-bromopyridine, thus, fi-
nally yielding the phosphine oxide 2 (Scheme 3). Phosphine oxide
2 does not originate from the oxidation of phosphine 1, as the reac-
tion of the free phosphine, PH3, in the same KOH/DMSO suspension
affords pure phosphine 1 without any of its oxide 2.
Phosphine 1 demonstrated an unusual ligation toward PdCl2 in
CH2Cl2 solution.20 According to an X-ray diffraction analysis the
complex formed 3 includes a molecule of CH2Cl2 and possessed
the cis-structure (Fig. 1), atypical for PdCl2 complexes. In the Cam-
bridge Structural Database, cis-isomers of bis(phosphino)palla-
dium dichlorides occupy only about 10% of the total number of
these complexes). The Pd atom in complex 3 adopts a distorted
square-planar coordination geometry with the angle between the
Table 1
The influence of the reaction conditions (Scheme 1) on the yield of phosphine 1a
Entry
Temp (°C)
2-PyBr/Pn or P4/KOH molar ratio
Yield of 1b (%)
1
2
3
4
rt
rt
50
50
1:2 (Pn):3
1:2 (P4):3
1:2 (Pn):3
1:2 (P4):3
—
—
Tracesc
20
5
75
1:2 (P4):3
6
7
8
9
75
75
75
75
1:2 (Pn):3
1:2 (Pn):4
1:2 (P4):4
1:2 (Pn):2
39
28
31
19
10
100
1:2 (Pn):3
11
12
100
125
1:2 (Pn):4
1:2 (Pn):3
40
55
a
The reaction time is 3 h.
Yields are given for the isolated product (without conversion of 2-
b
P
P
P
OH
bromopyridine).
+
HO-P
Pn
P
c
Determined by 31P NMR spectroscopy.
P
A
B
KOH/DMSO/(H2O)
+
Pn (P4)
100 oC (Pn), 75 oC (P4), 3 h
N
Br
P
P
A
C
+
P
- Br
N
Br
N
N
N
P
N
C
P
2
N
N
+
O
N
N
P
N
N
1
up to 10%
(
62% (for Pn),
50% (for P4)
31P NMR)
1
Scheme 1. Synthesis of tris(2-pyridyl)phosphine (1) from 2-bromopyridine and
Scheme 2. A possible mechanism for the formation of tris(2-pyridyl)phosphine (1)
elemental phosphorus.
from 2-bromopyridine and elemental phosphorus.

2426
B. A. Trofimov et al. / Tetrahedron Letters 53 (2012) 2424–2427
References and notes
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P
N
Br
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2
Scheme 3. A possible mechanism for the formation of tris(2-pyridyl)phosphine
oxide (2) from 2-bromopyridine and elemental phosphorus.
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Koutsantonis, G. A.; Skelton, B. W.; White, A. H. Dalton Trans. 2007, 4943–4950;
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Chim. Acta 2009, 362, 477–482; (c) Woollard-Shore, J. G.; Holland, J. P.; Jones,
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10. (a) Drent, E.; Arnoldy, P.; Budzelaar, P. H. M. J. Organomet. Chem. 1994, 475, 57–
63; (b) Drent, E.; Arnoldy, P.; Budzelaar, P. H. M. J. Organomet. Chem. 1994, 475,
247–253.
Figure 1. Molecular structure of cis-[Pd(PPy3)2Cl2] CH2Cl2 (3). The dichloromethane
molecule has been omitted for clarity. Selected bond lengths (Å) and angles (°):
Pd1-–P1 2.2530(8), Pd1–P2 2.2539(8), Pd1–Cl1 2.3504(8), Pd1–Cl2 2.3564(8), P–C
1.828(3)–1.844(3), P1–Pd1–P2 96.27(3), Cl1–Pd1–Cl2 91.10(3).
PdP2 and PdCl2 planes being 17.8°. The values of the Pd–P (ca.
2.253 Å) and Pd–C (ca. 2.353 Å) bond lengths are similar to the lit-
erature examples for bis(phosphino)palladium dichloride com-
plexes.21 It should be noted that in complex 3 the intermolecular
11. Kurtev, K.; Ribola, D.; Jones, R. A.; Cole-Hamilton, D. J.; Wilkinson, G. J. Chem.
Soc., Dalton Trans. 1980, 1, 55–58.
12. Karam, A.; Tenia, R.; Martinez, M.; López-Linares, F.; Albano, C.; Diaz-Barrios,
A.; Sánchez, Y.; Catarí, E.; Casas, E.; Pekerar, S.; Albornoz, A. J. Mol. Catal. A:
Chem. 2007, 265, 127–132.
p–p interaction between the pyridine ring is absent (the shortest
intercentroid distance between aromatic rings is 4.263 Å) unlike
the cis-Pd(PPy2Ph)2Cl2 molecule, wherein the minimum intercent-
roid distance between two pyridine rings is 3.400 Å.21a
13. Kuo, C.-Y.; Fuh, Y.-S.; Shiue, J.-Y.; Yu, S. J.; Lee, G.-H.; Peng, S.-M. J. Organomet.
Chem. 1999, 588, 260–267.
14. Bowen, R. J.; Garner, A. C.; Berners-Price, S. J.; Jenkins, I. D.; Sue, R. E. J.
Organomet. Chem. 1998, 554, 181–184.
In conclusion, a novel one-pot organometallics-free synthesis of
tris(2-pyridyl)phosphine in yields of up to 62% from 2-bromopyri-
dine and elemental phosphorus (both red and white) in a super-
base KOH/DMSO (H2O) suspension has been elaborated. Thus,
tris(2-pyridyl)phosphine is now one of the most accessible tertiary
heteroaromatic phosphines and acquires a real chance of finding
diverse applications as a promising ligand, as a building block in
organic synthesis and as a precursor for drug design. This result
contributes to both organophosphorus and pyridine chemistry as
well as to aromatic nucleophilic substitution.
15. Kluwer, A. M.; Ahmad, I.; Reek, J. N. H. Tetrahedron Lett. 2007, 48, 2999–3001.
16. For reviews in this area, see: (a) Trofimov, B. A.; Rakhmatulina, T. N.; Gusarova,
N. K.; Malysheva, S. F. Russ. Chem. Rev. 1991, 60, 1360–1367; (b) Trofimov, B. A.;
Gusarova, N. K. Mendeleev Commun. 2009, 19, 295–302.
17. Craig 2-bromopyridine synthesis. In Comprehensive Organic Name Reactions and
Reagents; Wang, Z., Ed.; John Wiley & Sons: New York, 2010.
18. Procedure for the preparation of tris(2-pyridyl)phosphine (1) from 2-
bromopyridine and elemental phosphorus:
A mixture of 2-bromopyridine
(7.90 g, 50 mmol), red phosphorus (3.10 g, 100 mmol), powdered KOH
0.5H2O (10.00 g, 154 mmol), DMSO (50 mL), and H2O (2 mL) was stirred for
3 h at 100 °C under argon. The mixture was cooled to room temperature,
diluted with H2O (60 mL) and extracted with CHCl3 (3 Â 20 mL). The combined
extract was washed with H2O (3 Â 15 mL) and dried over K2CO3. The solvent
was removed under reduced pressure and the residue washed with cold EtOH
(1 Â 8 mL) and dried in vacuo (1 Torr) to give 2.74 g (62%) of pure phosphine 1
as a microcrystalline powder. Mp 115–116 °C (i-PrOH), Lit. 113 °C.14IR (KBr):
3039, 2961, 2900, 1572, 1558, 1450, 1424, 1413, 1283, 1276, 1147, 1085, 1045,
987, 960, 907, 896, 774, 765, 743, 721, 712, 618, 548, 513, 503, 496, 426, 407,
Acknowledgments
This work was supported by the Russian Foundation for Basic
Research (Grant No. 11-03-00286a) and the President of the Rus-
sian Federation (program for the support of leading scientific
schools, Grant NSh-3230.2010.3).
395 cmÀ1 1H NMR (400.13 MHz, CDCl3): d = 7.18–7.23 (m, 3H, H-5), 7.41 (d,
.
3JHH = 7.0 Hz, 3H, H-3), 7.58–7.64 (m, 3H, H-4), 8.72 (d, 3JHH = 3.70 Hz, 3H, H-6).
1
13C NMR (100.62 MHz, CDCl3): d = 122.5 (C-5), 128.9 (d, JCP = 19.3 Hz, C-2),
3
2
135.6 (d, JCP = 2.6 Hz, C-4), 150.1 (d, JCP = 19.3 Hz, C-3), 161.5 (C-6). 31P NMR
(161.98 MHz, CDCl3): d = À1.86. Anal. Calcd for C15H12N3P: C, 67.92; H, 4.56; N,
15.84; P, 11.68. Found: C, 67.88; H, 4.49; N, 15.80; P, 11.59. According to the
31P NMR data, the content of tris(2-pyridyl)phosphine oxide (2) in the reaction
mixture can reach 10 mol %. It should be noted, that extraction of the reaction
mixture with CHCl3 selectively gives only tris(2-pyridyl)phosphine (1). Tris(2-
pyridyl)phosphine oxide (2) remains completely in the aqueous layer
(according to 31P NMR data).
Supplementary data
Supplementary data (general experimental method, experimen-
tal details, and characterization data for the synthesized com-
pounds) associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.tetlet.2012.03.004.
19. Kuimov, V. A.; Malysheva, S. F.; Gusarova, N. K.; Vakul’skaya, T. I.; Khutsishvili,
S. S.; Trofimov, B. A. Heteroat. Chem. 2011, 22, 198–203.

B. A. Trofimov et al. / Tetrahedron Letters 53 (2012) 2424–2427
2427
20. Procedure for the preparation of cis-dichlorobis[tri(2-pyridyl)phosphino]
palladium(II) dichloromethane solvate (3): To solution of phosphine
18.1320(13) Å,
Å3, Z = 2, dcalc = 1.618 g/cmÀ3 (Mo-K ) = 1.031 mmÀ1, hmax = 30.18°, indepen-
, l
a
a
= 82.166(2)°, b = 81.162(2)°,
c
= 71.870(2)°, V = 1626.81(19)
a
1
(0.557 g, 2.1 mmol) in CH2Cl2 (8 mL), PdCl2 (0.177 g, 1.0 mmol) was added at
ambient temperature under argon. The suspension was stirred at 30–35 °C for
30 min until dissolution of PdCl2 was complete. The hot solution was filtered,
the solvent removed, and the residue precipitated from CHCl3 and then Et2O,
and dried in vacuo (1 Torr) to give complex 3 as a solvate with one molecule of
CH2Cl2. Yellow crystalline powder. Yield 0.71 g (90%). Mp 200–206 °C (CHCl3).
1H NMR (400.13 MHz, DMSO-d6): d = 7.75 (m, 3H, H-5), 8.18 (m, 3H, H-3), 8.50
(m, 3H, H-4), 8.72 (m, H-6). 31P NMR (161.98 MHz, DMSO-d6): d = 31.16 (br s).
Anal. Calcd for C31H26Cl4N6P2Pd: C, 46.97; H, 3.31; N, 10.60; P, 7.81. Found: C,
46.83; H, 3.26; N, 10.49; P, 7.73.
dent reflections 9018 (Rint = 0.0672), data/restraints/parameters: 9018/0/ 393,
indices: R1 = 0.0411 [6995 I > 2 (I)], wR2 (all data) = 0.1166, GOF on
2 = 1.144. The refined data have been deposited at the Cambridge
R
F
r
Crystallographic Data Centre (CCDC 848483).
21. (a) Newkome, G. R.; Evans, D. W.; Fronczek, F. R. Inorg. Chem. 1987, 26, 3500–
3506; (b) Li, S.-L.; Zhang, Z.-Z.; Wu, B.-M.; Mak, T. C. W. Inorg. Chim. Acta 1997,
255, 239–248; (c) Reetz, M. T.; Demuth, R.; Goddard, R. Tetrahedron Lett. 1998,
39, 7089; (d) Autio, J.; Vuoti, S.; Haukka, M.; Pursiainen, J. Inorg. Chim. Acta
2008, 361, 1372–1380; (e) Hayashi, M.; Yamasaki, T.; Kobayashi, Y.; Imai, Y.;
Watanabe, Y. Eur. J. Org. Chem. 2009, 4956–4962.
X-ray crystallographic data for compound 3:
C30H24Cl2N6P2Pd + CH2Cl2,
Mr = 792.72, triclinic, space group P-1, a = 8.9162(6) Å, b = 10.7634(7) Å, c =