Formation of a cyclic bis(iminophosphorane) from a 2,2′- bis(phosphino)azobenzene via N=N bond cleavage

Article abstract of DOI:10.1002/hc.20680

An eight-member cyclic bis(iminophosphorane) was synthesized by treatment of 2,2′-bis(phosphino)azobenzene with pentachlorophenol via N=N bond cleavage of the azobenzene. The crystal structure of the cyclic bis(iminophosphorane) was determined by the X-ra

Full text of DOI:10.1002/hc.20680

Heteroatom Chemistry
Volume 22, Number 3/4, 2011
Formation of a Cyclic Bis(iminophosphorane)
ꢀ
from a 2,2 -Bis(phosphino)azobenzene via
=
N N Bond Cleavage
Masaki Yamamura, Naokazu Kano and Takayuki Kawashima
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan
Received 25 August 2010; revised 7 October 2010
the Staudinger reaction [2] of an azide with a phos-
ABSTRACT: An eight-member cyclic bis(iminophos-
phine or a phosphite. Considering the explosive po-
tential of several azide reagents and sodium azide,
a precursor of many organic azides, other nitrogen-
containing precursors are desired for the synthesis
phorane) was synthesized by treatment of 2,2ꢀ-
bis(phosphino)azobenzene with pentachlorophenol
=
via N N bond cleavage of the azobenzene. The crys-
tal structure of the cyclic bis(iminophosphorane) was
=
of iminophosphoranes. Azobenzene is a stable N N
determined by the X-ray crystallographic analysis. A
double-bond compound and is considered as a can-
study on the reaction mechanism showed that both
didate for such nitrogen-containing precursors be-
protonation of the azo moiety and intramolecular nu-
cause its combination with 2 equiv of phosphine
cleophilic attacks of the phosphino groups played a key
is expected to form two iminophosphoranes. How-
role in the formation of the bis(iminophosphorane)
ever, azobenzene has rarely been converted to an
from the azo compound bearing two phosphino
=
iminophosphorane because bond fission of the N N
ꢁ
C
groups. 2011 Wiley Periodicals, Inc. Heteroatom Chem
double bond by a phosphorus reagent is difficult
22:301–306, 2011; View this article online at wileyonlineli-
brary.com. DOI 10.1002/hc.20680
[3]. In contrast, we previously reported the syn-
thesis of 2-phosphinoazobenzene 1 and its unique
properties and reactivity based on an equilibrium
with inner phosphonium salt 2 [4] that corresponds
INTRODUCTION
to the betaine intermediate of the Mitsunobu re-
action (Scheme 1) [5]. The formation of 2 encour-
aged us to study reactivities of an azobenzene bear-
ing two phosphino groups; in particular, its double
Iminophosphoranes are useful both as reagents for
aza-Wittig reactions and as ligands for transition
metal complexes [1]. They are usually prepared by
=
P N bond formation reaction via the N N bond fis-
sion reaction. We report here the synthesis of 2,2ꢀ-
bis(phosphino)azobenzene, and its reactivity to form
a cyclic bis(iminophosphorane).
Dedicated to Prof. Kin-ya Akiba on the occasion of his 75th
birthday.
Correspondence to: Takayuki Kawashima; e-mail: Takayuki
.Kawashima@gakushuin.ac.jp.
Contract grant sponsor: Grants-in-Aid for Global COE Program,
Chemistry Innovation, and for Scientific Research from Ministry
of Education, Culture, Sports, Science, and Technology, Japan.
Contact grant numbers: 15105001 (TK), 16033215 (TK),
1611512 (MY), 22108508 (NK), and 22685005 (NK).
Contract grant sponsor: Japan Society for Promotion of Sci-
ence.
RESULTS AND DISCUSSION
4,4ꢀ-Dibutyl-2,2ꢀ-diiodoazobenzene (3) [6] was lithi-
ated with butyllithium (2.2 equiv), and then the re-
action mixture was treated with diphenylthiophos-
phinoyl chloride (2.3 equiv) to give the correspond-
ing 2,2ꢀ-bis(diphenylthiophosphinoyl)azobenzene 4
Contract grant sponsor: Dainippon Ink and Chemical Inc. (NK).
c
ꢀ
2011 Wiley Periodicals, Inc.
301

302 Yamamura, Kano, and Kawashima
SCHEME 1
FIGURE 2 ORTEP drawing (50% probability) of 8. Hydrogen
˚
atoms are omitted for clarity. Selected bond lengths (A), an-
gles (◦), and torsion angles (◦): P1 N1*, 1.5650(15); P1 C1,
1.8034(18); P1 C2, 1.8097(18); P1 C3, 1.8146(17);
C4 N1, 1.394(2); N1* P1 C1, 115.81(8); N1* P1 C2,
115.96(8); N1* P1 C3, 107.63(8); C1 P1 C2, 106.29(8);
C1 P1 C3, 105.28(8); C2 P1 C3, 104.83(8); P1 C1 C4,
117.80(12);
C1 C4 N1,
121.68(15);
C4 N1 P1*,
128.82(12); C1 C4 N1 P1*, 83.30(19).
FIGURE 1 ORTEP drawing (50% probability) of 5. Hydro-
˚
gen atoms are omitted for clarity. Selected bond lengths (A),
angles (◦), and torsion angles (◦): P1 C1, 1.8419(15);
P1 C2, 1.8353(18); P1 C3, 1.8309(16); N1 N2,
1.2556(16); P2 C6, 1.8437(15); P2 C7, 1.8292 (17);
P2 C8, 1.8342(15); C6 P2 C7, 103.35(6); C6 P2 C8,
100.38(7); C7 P2 C8, 101.79(7); C1 C4 N1 N2,
166.06(11); C4 N1 N2 C5, 176.97(10) ; N1 N2 C5 C6,
178.88(11).
were almost the same as those of triphenylphosphine
and azobenzene, respectively. These results show no
interaction between the phosphorus atom and the
azo group of 5 in the crystalline state.
Azobenzene 5 is moisture sensitive, and hydroly-
sis of 5 in THF gave hydrazobenzene 6 quantitatively
(Scheme 3), indicating the existence of an equilib-
rium with 7 in the solution state, similarly to 1 [4].
Addition of pentachlorophenol (1.0 equiv) to a so-
lution of 5 in toluene resulted in a color change
from red of the azo group to pale yellow. After heat-
ing at 100◦C for 6 h and the usual workup, cyclic
iminophosphorane 8 was obtained in 84% yield as a
colorless solid (Scheme 4). The structure of 8 was
identified by 1H, 13C, and 31P NMR spectroscopy,
MS, and X-ray analysis. The crystal structure of 8 is
shown in Fig. 2. The molecular structure of 8 has a
(72%) (Scheme 2). Desulfurization of 4 with hexa-
methylphosphorous triamide (excess) in toluene at
100◦C in a sealed tube and successive recrystalliza-
tion from toluene/hexane gave red crystals of 2,2ꢀ-
bis(diphenylphosphino)azobenzene 5 (74%). X-ray
crystallographic analysis of 5 revealed an almost
Ci symmetric structure with a planar azobenzene
moiety (Fig. 1). The C P C bond angles (100.38◦–
◦
˚
=
103.37 ) and the N N bond length (1.2556(16) A)
SCHEME 2
Heteroatom Chemistry DOI 10.1002/hc

ꢀ
=
Cyclic Bis(iminophosphorane) from a 2,2 -Bis(phosphino)azobenzene via N N Bond Cleavage 303
SCHEME 3
SCHEME 4
center of symmetry (Ci symmetry). The interatomic
the reaction of 8 with benzaldehyde in C6D6 at 60◦C
for 48 h resulted in only little conversion (<10%),
indicating the low reactivity of 8.
˚
=
N· · ·N distance was 3.2 A, clearly showing the N N
bond cleavage of the azo group. The short P N dis-
˚
To the best of our knowledge, there have been
only a few reports of formation of an iminophos-
phorane by the reaction of a phosphine with an azo
compound instead of azides [3,8]. Reactions of al-
lenylphosphonates with dialkyl azodicarboxylate in
the presence of triphenylphosphine were reported
to give imonophosphoranes. Detailed mechanism
is, however, not described there [8]. To investigate
the reaction mechanism, the VT-31P NMR spectra of
the reaction solution of 5 after the addition of pen-
tachlorophenol were measured and it was found that
two singlets appeared at −15 and 48 ppm, which can
tances 1.5650(15) A are almost the same as the P N
bond lengths of usual iminophosphoranes [7] also
implying P N bond formation. The central eight-
membered ring has a chair-like conformation. De-
spite Ci symmetry in the crystal structure, the four
phenyl groups bound to the phosphorus atoms are
1
equivalent in the H and 13C NMR spectra in C6D6
because of the fast conformational change of the
eight-membered ring. While many iminophospho-
ranes such as PhN=PPh3 usually undergo the aza-
Wittig reaction with aldehydes to give the corre-
sponding imine and phosphine oxide quantitatively,
Heteroatom Chemistry DOI 10.1002/hc

304 Yamamura, Kano, and Kawashima
be assigned to the signals due to a phosphine and
a phosphonium salt, respectively. Considering that
protonation of 1 formed a nitrogen-protonated inter-
mediate via equilibrium with 2 [4], these results in-
dicate the formation of intermediate 7·H+ by proto-
nation with pentachlorophenol. Attempted isolation
of the intermediate 7·H+ failed because of its insta-
bility. Furthermore when no acid or the less acidic
p-ButC6H4OH was used, the signal due to the phos-
phonium salt was not observed at all, even at low
temperature, indicating the presence of the acidity-
dependent equilibration of 5 with 7·H+. In addition,
heating 5 for 30 days under no acid or less acidic
conditions resulted in no formation of 8. These re-
sults indicate the formation of iminophosphorane 8
via 7·H+.
Mass spectra were recorded with a JEOL JMX-SX
102 mass spectrometer. Elemental analyses were
performed by the Microanalytical Laboratory of
Department of Chemistry, Faculty of Science, The
University of Tokyo. 2,2ꢀ-Diiodoazobenzene 3 was
prepared according to a procedure mentioned in the
literature [6].
Synthesis of 2,2ꢀ-Bis(diphenylthiophosphinoyl)-
azobenzene 4. To a THF solution (150 mL) of 2,2ꢀ-
diiodoazobenzene 3 (1.00 g, 1.83 mmol) at −105◦C,
n-BuLi (1.6 M in hexane, 2.5 mL, 4.0 mmol) was
added rapidly. After the reaction solution was stirred
further at −105◦C for 5 min, diphenylthiophosphi-
noyl chloride prepared from chlorodiphenylphos-
phine (0.80 mL, 4.20 mmol) and elemental sulfur
(134 mg, 4.20 mmol) were added and the reaction
mixture was stirred at room temperature for 3 h. Af-
ter evaporation of the solvent, separation by silica-
gel chromatography (eluent: CHCl3) gave red crys-
tals of 4 (958 mg, 72%).
A plausible reaction mechanism is suggested
as follows: first,7·H+ is formed by the protona-
tion of 7 equilibrated with 5. Then the phosphine
moiety of 7·H+ attacks the nitrogen atom of the
NH group to form iminophosphorane 8·H+ with
an aminophosphonium moiety via the N N bond
cleavage, which undergoes deprotonation to give
8 finally (Scheme 4). A similar intermolecular re-
action of triphenylphosphine and the protonated
phosphonium salt 2·H+ that was prepared from 2-
phosphinoazobenzene 1 resulted in recovery of the
starting materials. These results suggest that in-
tramolecular nucleophilic attack of the phosphino
4: 1H NMR (500 MHz, CDCl3) δ 0.88 (t, J =
7.5 Hz, 6H), 1.29 (sext, J = 7.5 Hz, 4H), 1.51 (quint,
J = 7.5 Hz, 4H), 2.56 (t, J = 7.5 Hz, 4H), 6.41 (dd,
J = 7.5 Hz, J = 5.0 Hz, 2H), 7.07 (d, J = 7.5 Hz,
2H), 7.30 (dd, J = 7.5 Hz, J = 3.0 Hz, 8H), 7.36–7.38
(m, 4H), 7.61 (dd, J = 16.0 Hz, J = 1.5 Hz, 2H), 7.72
1
(dd, J = 13.7 Hz, J = 7.5 Hz, 8H); 13C{ H} NMR
(126 MHz, CDCl3) δ 13.91 (s, CH3), 22.27 (s, CH2),
32.96 (s, CH2), 35.49 (s, CH2), 116.53 (d, JCP = 7.2 Hz,
=
group and formation of the P N bonds are the driv-
ing forces in the present reaction.
CH), 128.12 (d, JCP = 12.9 Hz, CH), 130.91 (d, JCP =
In summary, we have shown the synthesis of 2,2ꢀ-
bis(phosphino)azobenzene and its treatment with an
acid to form the cyclic bis(iminophosphorane). Bond
2.4 Hz, CH), 131.72 (d, JCP = 10.8 Hz, CH), 132.31 (d,
JCP = 80.3 Hz, CP), 132.73 (s, CH), 134.15 (d, JCP
=
88.1 Hz, CP), 134.25 (d, JCP = 10.3 Hz, CH), 146.57
(d, JCP = 10.8 Hz, CBun), 149.85 (d, JCP = 2.8 Hz,
=
fission of the N N double bond of the azobenzene
1
derivative is an interesting approach for the synthe-
sis of the iminophosphoranes although it is limited
to this azobenzene at present. These results suggest
the necessity of both an acid and an intramolecular
CN); 31P{ H} NMR (162 MHz, CDCl3) δ 43.5 (s).
Synthesis of 2,2ꢀ-Bis(diphenyphosphino)azoben-
zene 5. To a toluene solution (0.5 mL) of phosphine
sulfide 4 (100 mg, 138 μmol), hexamethylphospho-
rous triamide (0.10 mL, 5.45 mmol) was added and
the reaction solution was sealed. After the solu-
tion was heated at 100◦C for 24 h, evaporation of
the solvent and recrystallization of the residue from
toluene/hexane gave red single crystals of 5 (67.7 mg,
74%).
=
nucleophile for N N double bond cleavage via an
inner phosphonium salt.
EXPERIMENTAL
General Procedure
All chemicals were reagent grade and used with-
out further purification. Solvents were purified be-
fore use. All reactions were carried out under ar-
gon atmosphere unless otherwise noted. The 1H
NMR (500 MHz), 13C NMR (126 MHz), and 31P
NMR (162 MHz) spectra were measured at 293
K with a JEOL A500 spectrometer using the sig-
nals of residual nondeuterated solvents and tri-
butylphosphine (δP = −31.8) as internal standards.
5: 1H NMR (500 MHz, C6D6) δ 0.75 (t, J = 7.5 Hz,
6H), 1.05 (sext, J = 7.5 Hz, 4H), 1.48–1.53 (m, 4H),
2.24 (t, J = 7.5 Hz, 4H), 6.88 (dd, J = 7.5 Hz, J =
4.8 Hz, 2H), 6.89 (s, 2H), 7.04–7.12 (m, 12H), 7.51
(td, J = 7.5 Hz, J = 1.5 Hz, 8H), 7.54 (dd, J = 7.5 Hz,
1
J = 3.6 Hz, 2H); 13C{ H} NMR (126 MHz, C6D6)
δ 13.99 (s, CH3), 22.41 (s, CH2), 33.30 (s, CH2), 35.72
(s, CH2), 116.61 (s, CH), 128.66 (d, JCP = 14.9 Hz,
CH), 128.72 (s, CH), 129.74 (s, CBun), 133.17
Heteroatom Chemistry DOI 10.1002/hc

ꢀ
=
Cyclic Bis(iminophosphorane) from a 2,2 -Bis(phosphino)azobenzene via N N Bond Cleavage 305
(s, CH), 134.53 (d, JCP = 20.6 Hz, CH), 138.80 (d,
87.2 Hz, CP), 127.92–128.01 (m, CH), 128.35–128.51
(m, CH), 130.16 (s, CH), 131.63–131.70 (m, CH),
131.88–131.97 (m, CBun), 133.40 (s, CH), 133.62
(d, JCP = 114.8 Hz, CP), 134.51–134.56 (m, CH),
JCP = 12.4 Hz, CP), 139.27 (d, JCP = 17.3 Hz, CP),
1
146.21 (s, CH), 152.59 (d, JCP = 14.0 Hz, CN); 31P{ H}
NMR (162 MHz, C6D6) δ −12.2 (s).
1
156.45 (s, CN); 31P{ H} NMR (162 MHz, CDCl3)
Reaction of 2,2ꢀ-Bis(diphenyphosphino)azoben-
zene 5 with Water. To a THF solution of 5 (10 mg,
15 μmol), a 1:1 mixture of THF and water was added.
After the reaction solution was stirred at room tem-
perature for 1 min, the solvent was evaporated.
Quantitative formation of colorless solid of 6 was
confirmed by the 1H and 31P NMR spectra in CDCl3.
6: 1H NMR (500 MHz, CDCl3) δ 0.80 (t, J =
7.5 Hz, 3H), 0.81 (t, J = 7.5 Hz, 3H), 1.15–1.22
(m, 4H), 1.29–1.38 (m, 4H), 2.32 (t, J = 7.5 Hz,
2H), 2.33 (t, J = 7.5 Hz, 2H), 6.36 (brs, 1H), 6.48
(dd, J = 8.0 Hz, J = 5.0 Hz, 1H), 6.54 (dd, J =
8.0 Hz, J = 1.5 Hz, 1H), 6.56 (d, J = 1.5 Hz, 1H),
6.70 (dd, J = 8.0 Hz, J = 5.0 Hz, 1H), 6.82 (dd,
J = 8.0 Hz, J = 1.5 Hz, 1H), 7.00 (d, J = 8.0 Hz,
1H), 7.29–7.36 (m, 10H), 7.44–7.52 (m, 4H), 7.53–
7.61 (m, 2H), 7.62–7.68 (m, 4H), 8.22 (brs, 8H);
δ 9.9 (s). HRMS (FAB) m/z calcd for C44H44N2P2
[M]+ 662.2980, found: 662.3000. Elemental analysis
calc. for C44H44N2P2·H2O: C, 77.63; H, 6.81; N, 4.11.
Found: C, 77.24; H, 6.69; N, 3.80%.
Treatment
of
2,2ꢀ-Bis(diphenylphosphino)
azobenzene 5 with Phenols. To a solution of (E)-6
(12.6 mg, 20 μmol) in toluene (0.5 mL), pen-
tachlorophenol or 4-tert-butylphenol (3.1 mg, 20
μmol) was added and the VT-31P NMR spectra of
the reaction mixture were measured.
X-ray Crystallographic Analysis
X-ray diffraction data were collected on
a
Rigaku Mercury CCD diffractometer with graphite-
monochromated Mo Kα adiation. The structures of
5 and 8 were solved by direct methods (SIR97)
and expanded using Fourier techniques [9]. The
non-hydrogen atoms were refined anisotropically,
and hydrogen atoms were refined isotropically with
SHELX-97 [10]. Crystal data for 5: C44H44N2P2, tri-
1
13C{ H} NMR (126 MHz, CDCl3) δ 13.77 (s, CH3),
13.80 (s, CH3), 21.83 (s, CH2), 21.89 (s, CH2), 33.34
(s, CH2), 33.41 (s, CH2), 34.22 (s, CH2), 34.42 (s,
CH2), 111.14 (d, JCP = 103.7 Hz, CP), 111.34 (d, JCP
= 3.3 Hz, CH), 112.14 (d, JCP = 8.3 Hz, CH), 117.88
(d, JCP = 10.8 Hz, CP), 128.26 (d, JCP = 11.6 Hz, CH),
128.32 (d, JCP = 6.6 Hz, CH), 128.55 (s, CH), 130.35
(s, CH), 131.28 (d, JCP = 12.4 Hz, CH), 131.75 (d,
JCP = 2.5 Hz, CH), 131.88 (d, JCP = 10.0 Hz, CH),
¯
clinic, P1, a = 12.699(6), b = 12.795(6), c = 12.953(6)
◦
˚
A, α = 104.787(4), β = 94.500(4), γ = 113.723(4)
3
˚
V = 1823.7(16) A , MW = 662.75, Z = 2, D = 1.207
g/cm3, 11556 measured, 6224 independent, GOF =
1.041, R (I > 2σ(I)) = 0.0335, wR (all data)
=
1
2
132.07 (d, JCP = 103.7 Hz, CP), 132.24 (d, JCP
=
0.0921. Crystal data for 8, monoclinic, P21/n,
10.8 Hz, CH), 133.20 (d, JCP = 2.5 Hz, CH), 133.37
(s, CH), 133.46 (d, JCP = 3.3 Hz, CH), 133.56 (s,
CH), 135.11 (d, JCP = 8.3 Hz, CBun), 148.63 (d,
JCP = 13.5 Hz, CBun), 151.91 (s, CN), 151.94 (d,
˚
a = 10.999(4), b = 8.703(3), c = 18.993(8) A, β =
◦
3
˚
93.5044(13) , V = 1814.6(12) A , MW = 662.75, Z =
2, D = 1.213 g/cm3, 11254 measured, 3191 indepen-
dent, GOF = 1.064, R (I > 2σ(I)) = 0.0398, wR (all
1
JCP = 3.8 Hz, CN); 31P{ H} NMR (162 MHz, CDCl3)
1
2
data) = 0.1130. CCDC 781725 (5) and 781726 (8)
contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/conts/retrieving.html (or
from the Cambridge Crystallographic Data Centre,
12, Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033; e-mail: deposit@ccdc.cam.ac.uk).
δ −20.9 (s), 36.0 (s). HRMS (FAB) m/z calcd for
C44H46N2OP2 [M]+ 680.3085, found: 680.3080.
Synthesis of Cyclic Bis(iminophosphorane) 8.
To a toluene solution (0.5 mL) of phosphine 5 (12.6
mg, 19 μmol), pentachlorophenol (5.3 mg, 20 μmol)
was added and the reaction solution was sealed. Af-
ter the solution was heated at 100◦C for 6 h, evap-
oration of the solvent followed by separation using
GPC (eluent: chloroform) gave colorless crystals of
8 (10.8 mg, 84%).
ACKNOWLEDGMENTS
We thank Tosoh Finechem Corp. for gifts of alkyl-
lithiums.
8: 1H NMR (500 MHz, CDCl3) δ 0.80 (t, J =
7.5 Hz, 6H), 1.22 (sext, J = 7.5 Hz, 4H), 1.35
(quint, J = 7.5 Hz, 4H), 2.30 (t, J = 7.5 Hz, 4H),
6.43 (d, J = 13.5 Hz, 2H), 7.16 (s, 4H), 7.21–
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