Control of the equilibrium between 2-phosphinoazobenzenes and inner phosphonium salts by heat, solvent, acid, and photoirradiation

Article abstract of DOI:10.1246/bcsj.20110259

Several 2-phosphinoazobenzenes, which are in equilibrium with inner phosphonium salts, were synthesized. Effects of substituents, solvents, and acidic additives on their equilibria are described. Thermodynamic parameters of the equilibria in various solvents suggest that the acceptor character of the solvents is mainly responsible for the solvent effects. Addition of phenols changed the equilibria depending on their acidity. Substituents at the 4- and 4′-positions of the azobenzene also affected the equilibrium constants, which shifted the equilibrium toward the phosphonium salt in the order of their electron-withdrawing ability. Photoisomerization of the 2-phosphinoazobenzenes bearing electron-donating substituents at the 4- and 4′-positions, which shifted the equilibrium toward the 2-phosphinoazobenzene, proceeded successfully. While the phosphonium salt in equilibrium with the (E)-isomer of the 2-phosphinoazobenzene was protonated by perchlorophenol, the (Z)-isomer did not react with a proton source because it could not take on the form of an inner phosphonium salt. Thus, the properties and reactivity of the inner phosphonium salts in equilibrium with the phosphines can be successfully controlled by photoirradiation.

Full text of DOI:10.1246/bcsj.20110259

110
Bull. Chem. Soc. Jpn. Vol. 85, No. 1, 110-123 (2012)
© 2012 The Chemical Society of Japan
Control of the Equilibrium between 2-Phosphinoazobenzenes and
Inner Phosphonium Salts by Heat, Solvent, Acid, and Photoirradiation
Masaki Yamamura, Naokazu Kano,* and Takayuki Kawashima*,³
Department of Chemistry, Graduate School of Science, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
Received August 26, 2011; E-mail: kano@chem.s.u-tokyo.ac.jp, Takayuki.Kawashima@gakushuin.ac.jp
Several 2-phosphinoazobenzenes, which are in equilibrium with inner phosphonium salts, were synthesized. Effects
of substituents, solvents, and acidic additives on their equilibria are described. Thermodynamic parameters of the
equilibria in various solvents suggest that the acceptor character of the solvents is mainly responsible for the solvent
effects. Addition of phenols changed the equilibria depending on their acidity. Substituents at the 4- and 4¤-positions of
the azobenzene also affected the equilibrium constants, which shifted the equilibrium toward the phosphonium salt in the
order of their electron-withdrawing ability. Photoisomerization of the 2-phosphinoazobenzenes bearing electron-donating
substituents at the 4- and 4¤-positions, which shifted the equilibrium toward the 2-phosphinoazobenzene, proceeded
successfully. While the phosphonium salt in equilibrium with the (E)-isomer of the 2-phosphinoazobenzene was
protonated by perchlorophenol, the (Z)-isomer did not react with a proton source because it could not take on the form of
an inner phosphonium salt. Thus, the properties and reactivity of the inner phosphonium salts in equilibrium with the
phosphines can be successfully controlled by photoirradiation.
Phosphorus compounds usually take the trivalent-tricoordi-
nation numbers of silicon or boron and the reactivity of the
compounds by photoisomerization of the azo group. In these
cases, an azo group operates as a nucleophile toward an
electrophilic silicon or boron atom. An azobenzene is also
expected to work as an electrophile because the azo group is
known to react with an alkyllithium.17 Introduction of an
azobenzene unit as an intramolecular electrophile to a
phosphine may provide the phosphorus atom with a coordi-
nation mode different from that of silicon and boron atoms. The
azo group of azobenzenes bearing a phosphino group at the
2-position is expected to work as an electrophile toward the
phosphorus atom, and the coordination number of the phospho-
rus atom would be changed by external stimuli (Chart 1,
nated, pentavalent-tetracoordinated, or pentavalent-pentacoor-
dinated state, and they show different properties and reactivities
depending on the coordination number or atomic valence.
Tertiary phosphines usually serve as soft bases and react with
electrophiles such as alkyl halides and activated double-bond
compounds to give quaternary phosphonium salts,1 which are
involved as important intermediates in many organic reactions
such as the Wittig reaction,2 the Mitsunobu reaction3 and
reactions using the Appel reagent.4 Changing the coordination
mode of phosphorus moieties results in a drastic change in their
properties, which are applicable to molecular sensors and
molecular switches.5
Many equilibrium systems between trivalent-tricoordinated
phosphorus compounds and pentavalent phosphorus com-
pounds are known.1 One of the simplest examples is an intra-
molecular proton transfer from phosphinous acids to secondary
phosphine oxides (Chart 1, type A),6 and another example is
the tautomerization between a trivalent (hydroxyalkyl)phos-
phine and an alkoxyhydrophosphorane (Chart 1, type B). In
the latter case, the equilibrium is controlled by factors such as
the acidity of the inserting alcohol moiety,7,8 rigid molecular
frameworks,9,10 and solvents.11 In addition, Dyer et al. have
reported such a system as type C using pyridyl-N-phosphino-
imine,12 and its potential as an application of the system to
materials13 and transition-metal complexes.14 However, equi-
librium systems of type-C have been quite limited.
We previously reported the synthesis of 2-silylazobenzenes15
and 2-borylazobenzenes16 and the control of both the coordi-
³ Present address: Faculty of Science, Gakushuin University,
1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588
Chart 1.
Published on the web January 7, 2012; doi:10.1246/bcsj.20110259

M. Yamamura et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 1 (2012)
111
type C). The reversible conversion between a phosphine and a
phosphonium salt by external stimuli would lead to develop
novel molecular switches based on the change in the properties
and reactivity of the phosphorus atom. However, they have
never been reported before our work, although some azoben-
zenes bearing a pentavalent phosphorus atom at the 2-position
have been synthesized.18 We previously reported the synthesis
of a 2-phosphinoazobenzene and its equilibrium with an inner
phosphonium salt as a preliminary communication.19 In this
paper, we report synthetic methods for various derivatives of 2-
phosphinoazobenzene and the effects of substituents, solvents,
and proton sources on their equilibrium constants. We describe
how the properties of the azobenzene moiety as a chromophore
and the reactivity of the phosphorus functional groups were
changed based on control of the equilibrium. We also report
control of the phosphonium salt character by the photoisom-
erization of an (E)-2-phosphinoazobenzene.
Desulfurization of (E)-2a-2j with tributylphosphine (1.2
equiv) in toluene at 100 °C successfully gave 2-(diphenyphos-
phino)azobenzenes (E)-6a-6j (Scheme 4). Quantitative forma-
1
tion of (E)-6a-6j was confirmed by H and 31P NMR spec-
troscopy. These compounds are highly sensitive toward
moisture.
31P NMR Spectra. In contrast to the crystal structure of
2-(diphenyphosphino)azobenzene ((E)-6a), which showed no
interaction between the phosphorus atom and the azo group,19
the properties of (E)-6a in solution are very different from
those of usual triarylphosphines; in the 31P NMR spectra of (E)-
6a in toluene, only one singlet (¤P ¹9.2) was observed above
60 °C, while the signal was broadened between 20 and ¹60 °C
and split into two singlets (¤P ¹10.3 and 30.1) below ¹60 °C
(see Supporting Information). The signal at lower field is
assigned to the inner phosphonium salt 7a in equilibrium with
(E)-6a (Scheme 4).19 The equilibrium constants, K1 = [7a]/
[(E)-6a], were calculated from the ratio of 7a to (E)-6a, and
van’t Hoff plots of K1 at each temperature gave a good linear
Results and Discussion
Synthesis. 2-(Diphenylphosphino)azobenzenes could be
synthesized by neither reactions of the 2-lithioazobenzenes
with chlorodiphenylphosphine,20 nor reduction of 2-(diphenyl-
phosphinoyl)azobenzenes with trichlorosilane.21,22 However,
they were successfully obtained by desulfurization of the
corresponding phosphine sulfides that were prepared by the
following three methods. First, successive treatment of 2-
iodoazobenzenes (E)-1a-1d23 with butyllithium (1 equiv) and
diphenylphosphinothioyl chloride (1 equiv) in Et2O or THF at
¹105 °C gave phosphine sulfides (E)-2a-2d (Scheme 1). In
contrast to (E)-1a-1d, the same reaction of (E)-1e bearing a
chloro substituent at the 4¤-position of the azobenzene moiety
failed.
Second, subsequent reactions of reduction of (E)-1e-1i,
palladium-catalyzed coupling reactions, and sulfurization gave
the corresponding phosphine sulfides 3e-3i, which were
oxidized with pyridinium tribromide to afford phosphine
sulfides (E)-2e-2i bearing an azobenzene moiety, respectively
(Scheme 2).21
Scheme 2.
Third, the reactions of a diazonium salt prepared from 2-
(diphenylphosphinoyl)aniline (4) with in situ generated sodium
phenoxide gave (E)-5 (Scheme 3). 4¤-Hydroxyazobenzene (E)-
5 was converted to 4¤-methoxyazobenzene (E)-5¤ by treatment
with methyl iodide in alkaline solution, and its reaction with
Lawesson’s reagent gave the corresponding phosphine sulfide
(E)-2j.
Scheme 1.
Scheme 3.

112
Bull. Chem. Soc. Jpn. Vol. 85, No. 1 (2012)
2-Diphenylphosphinoazobenzenes
R1
R1
n-Bu3P
K1
(E)-2a−2j
N
N
P
P
N N
Ph
Ph
toluene
(quant)
Ph
Ph
R2
R2
(E)-6a−6j
7a−7j
Scheme 4.
Table 1. Equilibrium Constants and Thermodynamic
Parameters of the Equilibrium between 2-(Diphenylphos-
phino)azobenzene (E)-6a and Inner Phosphonium Salt 7a
in Various Solvents
Figure 1. Correlation between enthalpy change (¦H°) and
acceptor number (AN).
the phosphonium salt 7a provides it with the role as a donor.
Meanwhile, the absolute value of the entropy change, ¦S°, is
smaller in THF than in toluene, which is due to a polar solvent
effect as described in the following section.
To clarify the solvent effect on ¦S° in addition to that on
¦H°, the equilibrium constants and thermodynamic parameters
for all synthesized 2-phosphinoazobenzenes (E)-6a-6j were
evaluated in both toluene and THF, which have almost the
same AN but very different polarity, as shown in Table 2. The
equilibrium constants, K1, of (E)-6a-6j and absolute values of
the entropy change (¦S°) are obviously larger and smaller in
THF than in toluene, respectively. The difference in the entropy
term (T¦S°) of (E)-6a between toluene and THF corresponds
a)
d)
Solvent
K1
¦H°b)
¦S°c)
¾
ANe)
r
f)
f)
n-hexane
Et3N
Et2O
toluene
THF
AcOMe
CD2Cl2
CDCl3
<0.01
0.19
0.76
4.00
8.25
11.9
150
®
®
1.88
0.0
1.4
3.9
8.2
8.0
10.7
20.4
23.1
¹11.9(0.2)h) ¹83(1)h) 2.42
¹14.1(0.2)h) ¹84(1)h) 4.20
¹17.3(0.3)h) ¹88(2)h) 2.38
¹16.2(0.2)h) ¹76(1)h) 7.58
¹17.4(0.3)h) ¹80(2)h) 6.68
¹20.9(0.3)h) ¹80(2)h) 8.93
¹1
to 2.1 kJ mol at ¹100 °C, which is not a negligible amount.
g)
g)
>150
®
®
4.81
The negative entropy change, ¦S°, results from disorder of an
ordering of solvent molecules induced by long-range inter-
actions such as dipole/dipole, dipole/induced-dipole, and
induced-dipole/induced-dipole with the inner phosphonium
salt 7a, which has a large dipole moment. Because polar
solvents are more structured than nonpolar solvents owing to
their high dipole moment and relative permittivity,25 the
ordering of solvent molecules is disordered more in polar
solvents by the abundant existence of the inner phosphonium
salt 7a, resulting in a less negative ¦S° in more polar solvents,
THF, AcOMe, and CD2Cl2 than in Et3N and Et2O (Table 1).
a) At ¹100 °C. b) kJ mol¹1. c) J mol¹1 K¹1. d) Relative
permittivity at 25 °C. e) Acceptor number. f) Too small
equilibrium constant to be observed. g) Too large equilibrium
constant to be observed. h) Numbers in parentheses represent
standard errors.
relationship. Enthalpy and entropy changes were obtained from
the temperature dependence of the equilibrium constants
K1 (¦H° = ¹17.3 kJ mol¹1, ¦S° = ¹88 J mol¹1 K¹1). In the
following sections, the enthalpy and entropy changes for the
equilibrium between 6a-6j and 7a-7j were similarly estimated
under various conditions to clarify the effects of solvents,
substituents, and acids on the equilibrium between the
phosphine and the phosphonium salt.
Solvent Effect. The equilibria between 6a-6j and 7a-7j are
expected to depend on the solvent because of the polarized
structures of zwitterions 7a-7j. There was no relationship
between the polarity of the solvent, ¾r, and the enthalpy change,
¦H°, but ¦H° did correlate well with the electrophilic
character of the solvent as exemplified by their correlation
with acceptor number (AN) (Table 1 and Figure 1).24 Namely,
as the AN of the solvent increases, ¦H° becomes more
negative. In n-hexane, whose AN is zero, the phosphonium salt
was not observed at all even at ¹100 °C. Thermal stabilization
of the phosphonium salt in a solvent with a larger AN indicates
the roles of the solvent and the phosphonium salt as an acceptor
and a donor, respectively. The negatively charged nitrogen in
Substituent Effect.
The variable temperature (VT)
31P NMR spectra of (E)-6a-6j measured in toluene and THF
showed a drastic substituent effect for the 4- and 4¤-positions of
the azobenzene moiety on the equilibrium constants (Table 2).
The K1 values of 2-phosphinoazobenzenes (E)-6a, -6b, -6e, and
-6j increased in the order of the electron-withdrawing character
of the substituent at the 4¤-position (Cl > H > Me > OMe).
Substituent effects at the 4-position were compared with those
at the 4¤-position based on the K1 value of (E)-6a. 4-Methyl-2-
(diphenylphosphino)azobenzene ((E)-6c) had a much smaller
K1 than that of the 4¤-methyl derivative (E)-6b. In contrast, the
4-chloro derivative (E)-6f had a much larger K1 than that of
(E)-6e. Taking into consideration the contribution of resonance
structures of (E)-6a-6j and 7a-7j (Scheme 5), the substituent
effect is explained as follows: when R1 or R2 are electron-
donating groups such as a methyl group, (E)-6a-6j are
stabilized due to a positive charge at the 4- and 4¤-positions

M. Yamamura et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 1 (2012)
113
Table 2. Equilibrium Constants and Thermodynamic Parameters of the Equilibrium between 2-(Diphenylphosphino)-
azobenzenes (E)-6a-6j and Inner Phosphonium Salts 7a-7j in Toluene and THF Solutions
In toluene
¦H°b)
In THF
¦H°b),c)
(E)-6
R1
R2
¦S°c)
K1
¦S°b),d)
a)
a)
K1
(E)-6a
(E)-6b
(E)-6c
(E)-6d
(E)-6e
(E)-6f
(E)-6g
(E)-6h
(E)-6i
(E)-6j
H
Me
H
Me
Cl
H
Cl
Me
Cl
H
H
4.00
0.46
0.01
<0.01
4.38
16.7
0.11
1.95
18.0
<0.01
¹17.3(0.3)g)
¹88(2)g)
8.25
0.90
0.16
0.02
16.1
57.3
0.43
6.17
>100
¹16.2(0.3)g)
¹76(1)g)
¹12.5(0.3)g)
¹78(2)g)
¹13.1(0.1)g)
¹77(1)g)
e)
e)
e)
e)
Me
Me
H
Cl
Me
Cl
Cl
H
®
®
®
®
®
®
e)
e)
e)
e)
®
®
¹17.7(0.4)g)
¹19.1(0.3)g)
¹89(2)g)
¹87(2)g)
¹17.5(0.2)g)
¹18.9(0.4)g)
¹13.9(0.4)g)
¹16.1(0.1)g)
¹20.8(0.5)g)
¹78(1)g)
¹77(2)g)
¹87(2)g)
¹80(1)g)
¹81(3)g)
e)
e)
®
¹15.7(0.2)g)
¹19.1(0.2)g)
®
¹85(1)g)
¹86(2)g)
e)
e)
f)
f)
f)
OMe
®
®
®
®
®
a) At ¹100 °C. b) in THF. c) kJ mol¹1. d) J mol¹1 K¹1. e) Too small equilibrium constant to be observed. f) Not
measured. g) Numbers in parentheses represent standard errors.
R1
R1
R1
P
N N
P
N N
P
N N
Ph
Ph
Ph
Ph
Ph
Ph
R2
R2
R2
(E)-6a−6j
(E)-6′a−6′j
(E)-6″a−6″j
K1
R1
R1
R1
N N
N N
P
N N
P
P
Ph
Ph
Ph
Ph
Ph
Ph
R2
R2
R2
7a−7j
7′a−7′j
7″a−7″j
Scheme 5.
in canonical forms (E)-6¤a-6¤j and (E)-6¤¤a-6¤¤j, respectively,
resulting in a decrease in the K1 value. Similarly, electron-
withdrawing groups such as a chloro group destabilize (E)-6a-
6j, increasing the K1 value. Contrary to (E)-6a-6j, 7a-7j have
canonical structures 7¤a-7¤j with a negative charge at the 4-
position, which make the substituents effects at the 4-position
large, while those at the 4¤-position negligible. Therefore,
the substituent effects at the 4- and 4¤-positions are mainly
determined by the stability of 7a and 6a, respectively. Ylide
structures 7¤¤a-7¤¤j are also considered to contribute to the
resonance structures of the phosphonium salts 7a-7j and to the
larger substitution effect at the 4-position.26
mechanism for the hydrolysis, similar to the Mitsunobu
¹
reaction,3 involves the formation of 7a¢H+¢HO by pro-
tonation of 7a, which is an equilibrium isomer of (E)-6a,
nucleophilic attack of hydroxide giving hydroxyphosphorane
9a, and irreversible P-N bond cleavage giving 10a. Finally,
intramolecular proton transfer in 10a gives 8a.
Reactions of (E)-6a with other protic molecules whose
conjugate bases have weak nucleophilicity would form similar
intermediates. However, the reactions are expected to be
reversible by freezing the nucleophilic attack of the conjugate
base on the phosphorus atom or the irreversible step via 10a.
One equivalent of 4-tert-butylphenol was added as an acidic
additive to a THF solution of (E)-6a, and its 31P NMR spectra
were measured at several temperatures to observe the inter-
mediate (Figure 2). Besides the signals of (E)-6a and 7a, one
Effects of Acidic Additives. We have previously reported
the reaction of phosphine (E)-6a with water to form phosphine
oxide 8a quantitatively (Scheme 6).19 A plausible reaction

114
Bull. Chem. Soc. Jpn. Vol. 85, No. 1 (2012)
2-Diphenylphosphinoazobenzenes
Scheme 7.
Similarly, (E)-6a was treated with other phenols28 either
2,4,6-trichlorophenol or perchlorophenol in THF and the
31P NMR spectra were observed. In both cases of the phenols,
whose conjugate bases have lower nucleophilicity than that of
4-tert-butylphenol, the signal due to pentacoordinated species
was not observed at ¹60 °C. Instead, in the case of 2,4,6-
trichlorophenol, only two lower field signals were observed
at ¤P 40.1 and ¹10.6 ppm and the signal at ¤P 40.1 shifted
downfield compared with the signal (¤P 31.2) observed in the
case of 4-tert-butylphenol, indicating that contribution of
Scheme 6.
7a¢H+¢ArO in equilibrium between 7a and 7a¢H+¢ArO is
larger in the case of 2,4,6-trichlorophenol than 4-tert-butyl-
phenol. Moreover, in the case of more acidic perchlorophenol,
the 31P NMR spectrum showed only a singlet, which can be
¹
¹
¹
assigned to 7a¢H+¢ArO , judging from the chemical shift (¤P
44.0) similar to that of triphenyl[2-(2-phenylhydrazino)phen-
yl]phosphonium perchlorate29 (¤P 45.0). These results indicate
that nucleophilic attack of the aryloxide on the phosphorus atom
does not proceed at all, or that the equilibrium between 7a¢
¹
H+¢ArO and 11a almost completely inclines toward 7a¢H+¢
¹
ArO . As mentioned above, the signal of (E)-6a completely
diminished upon the addition of perchlorophenol. Therefore,
the equilibrium control was achieved by the addition of acid.
The crystal structure of the protonated phosphonium salt
7a¢H+¢C6Cl5O was successfully determined by X-ray crys-
¹
Figure 2. VT-31P NMR of (E)-6a in THF after addition of
one equivalent of 4-t-BuC6H4OH (Small peaks are the
signals due to negligible impurities).
tallographic analysis. ORTEP drawings are shown in Figure 3.
It shows tetracoordinated and tetrahedral structure around the
phosphorus atom with an N-P bond in contrast to tricoordi-
nated structure of (E)-6a.19 It is a good evidence for existence
of the equilibrium between the phosphine bearing the azoben-
zene moiety and the phosphonium salt. The N1-N2
(1.436(5) ¡) and N1-P1 (1.657(3) ¡) bond lengths fall within
the range of usually observed N-N and N-P single bond
lengths. The N1 atom (357.5°) shows almost planar geometry.
A similar planarity of the nitrogen atom was shown in the
N-N-P phosphonium salt isolated as an intermediate of the
Mitsunobu reaction.30 The N1-N2 bond and two benzene rings
attached to this bond are coplanar.
UV-vis Spectra. 2-Phosphinoazobenzene (E)-6a showed
thermochromism in solution. The color of a toluene solution of
(E)-6a is red at 100 °C based on the azobenzene unit and black
at ¹78 °C based on the zwitterion 7a, reflecting the thermal
change in the equilibrium ratio of (E)-6a and 7a (Figure 4).19
Although the UV-vis spectra of (E)-6a depend on temperature,
the difference in the long wavelength absorption based on the
zwitterion 7a is too small to be observed owing to the small
equilibrium constant even in the presence of phenol. Therefore,
4-tert-butylphenol was added to the THF solution of chloro
derivative (E)-6e to increase the equilibrium constant K1
singlet was observed at high field (¤P ¹64.3) at low temperature.
This chemical shift is reasonable for a pentacoordinated
phosphorus compound,27 suggesting the formation of phos-
phorane 11a (Scheme 7). This VT NMR result indicates that
the reaction proceeds via reversible processes which involve
¹
(E)-6a, 7a, 7a¢H+¢ArO , and 11a, in contrast to the irreversible
reaction with water. Taking into consideration that three
signals were observed independently, there are two possibilities.
(1) The rates of interconversions between (E)-6a and 7a
¹
and between 7a and 7a¢H+¢ArO are slow and that between
¹
7a¢H+¢ArO and 11a is fast on an NMR time scale. (2)
The rates of interconversions between (E)-6a and 7a and
¹
between 7a¢H+¢ArO and 11a are slow and that between
¹
7a and 7a¢H+¢ArO is fast. In the former case, the equilibration
¹
between 7a¢H+¢ArO and 11a must incline to 11a, whereas
¹
the equilibration between 7a and 7a¢H+¢ArO inclines
to 7a in the latter case, because the chemical shifts of observed
signals (¤P 31.2 and ¹64.3) are close to those of inner
phosphonium salt 7a and pentacoordinated species 11a,
respectively.

M. Yamamura et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 1 (2012)
115
a)
Figure 4. The color change of (E)-6a depending on the
temperature and addition of perchlorophenol.
b)
Figure 5. The UV-vis spectra of (E)-6e in a THF solution
(5 mM) (a) black line: no additive, (b) red line: addition of
one equivalent of 4-tert-butylphenol, and (c) blue line:
addition of one equivalent of perchlorophenol.
Figure 3. ORTEP drawings of protonated phosphonium
¹
salt 7a¢H+¢C6Cl5O (thermal ellipsoid plot with 50%
probability). Hydrogen atoms on benzene rings were
omitted. a) Side view, b) top view. Selected bond lengths
(¡), angles (deg), and torsion angles (deg): P1-N1,
1.657(3); P1-C1, 1.764(4); P1-C2, 1.803(4); P1-C3,
1.814(4); N1-N2, 1.436(5); P1-N1, 1.657(3); N1-C4,
1.436(5); N2-C5, 1.406(5); C1-P1-C2, 114.80(18); C1-
P1-C3, 113.62(17); C2-P1-C3, 109.23(17); N1-P1-C1,
HOMO and HOMO¹1 are nonbonding orbitals of the nitrogen
atom conjugated with the ³ orbitals of the benzene rings (nN).
The TD-DFT calculation yielded singlet excitation energies
for (E)-6a, 7a, and 7a¢H+ (Table 3). All three excitations
for (E)-6a correspond to transitions from occupied orbitals
(HOMO, HOMO¹1, and HOMO¹2) to the LUMO, whose
excitation energies are almost the same as those of the
unsubstituted azobenzene. Excitations of 7a, unlike those of
(E)-6a, correspond to transitions from the HOMO to unoccu-
pied orbitals that consist mainly of antibonding orbitals of
the phosphorus. Considering that the nitrogen and phosphorus
atoms take on negative and positive charges, respectively,
these transition states are charge transfer excitations from the
negatively charged nitrogen atom to the positively charged
phosphorus moiety. Because a charge transfer excitation is
generally broadened by the orientation of solvents or other
factors, the long-wavelength excitations of 7a become broad-
ened so that 7a absorbs a wide range of visible light, which
is consistent with the black color of 7a. The excitation energies
of 7a¢H+ are much larger than those of 7a because the energy
difference between the frontier orbitals of 7a¢H+ is larger
than that in 7a. Actually, the color of the THF solution is
changed from black to pale yellow by protonation of inner
phosphonium salt 7a (Figure 4). It is interesting to point
out that the control of the equilibrium is reflected in the color
change.
91.77(18);
N1-P1-C2,
112.48(17);
N1-P1-C3,
114.17(16); C4-N1-N2-C5, 176.3(3); C6-C4-N1-N2,
168.0(3); C1-C5-N2-N1, 10.3(4).
between (E)-6e and 7e, which strengthened the absorption in
the long-wavelength region, 500-700 nm (Figure 5). Converse-
ly, when a stronger acid, perchlorophenol, was added, the
absorption dramatically weakened. This result is apparently
produced by the protonation of the nitrogen atom and is
consistent with the change in color.
To clarify the electronic properties of phosphine (E)-6a,
inner phosphonium salt 7a, and protonated phosphonium
salt 7a¢H+, molecular orbitals and vertical transition energies
were calculated at the B3PW91/6-311+G(d,p)// B3PW91/
6-31G(d,p) level (Figure 6).31 The LUMO, HOMO¹1, and
HOMO¹2 of phosphine (E)-6a are predominantly ³*, n, and ³
orbitals of the azobenzene moiety, respectively, and the HOMO
is a lone pair on the phosphino group. Both the LUMOs of the
phosphonium salts 7a and 7a¢H+ consist of the antibonding
orbitals of the P-N bond (·*) delocalized to the ³ orbital of
phenyl groups bound to the phosphorus atom and both the

116
Bull. Chem. Soc. Jpn. Vol. 85, No. 1 (2012)
2-Diphenylphosphinoazobenzenes
Scheme 8.
phosphonium salt 7 on the photoreaction. Actually, 2-phos-
phinoazobenzenes bearing an electron-donating group showed
higher yields of (Z)-6 as the equilibrium constant between (E)-6
and 7 decreased. The detailed substituent effect on photo-
isomerization is not obvious, but it is significant that photo-
isomerization was successfully accomplished by tuning the
substituents.
2-Phosphinoazobenzene (Z)-6d was reversibly isomerized to
(E)-6d either by irradiation (- = 445 nm, E/Z = 55/45 at PSS)
¹1
or by heat in the dark with a rate constant of 5.3 © 10¹5 s
at 300 K in benzene-d6 (Scheme 8). After addition of one
equivalent of perchlorophenol to the mixture of (Z)-6d in
benzene-d6, (Z)-6d decayed with an apparent first-order rate
Figure 6. Optimized structures and molecular orbitals of
(E)-6a (left), 7a (center), and 7a¢H+ (right) calculated at
the B3PW91/6-311+G(d,p)// B3PW91/6-31G(d,p) level.
Orange, blue, gray, and white atoms represent phosphorus,
nitrogen, carbon, and hydrogen, respectively. Energy level
of each orbital is shown in parenthesis (eV).
¹1
constant (d[(Z)-6d]/dt) of 7.9 © 10¹5 s to give 7d¢H+ in the
dark at 300 K. The rate constant for conversion of analogous
¹1
(E)-6a to 7a was estimated to be 5 © 104 s at the same
temperature by using dynamic NMR analysis of the 31P NMR
spectra. Considering the large difference in the rate constants,
the rate-determining step for conversion of (Z)-6d to 7d was
thermoisomerization of an azobenzene unit. Photoirradiation
(- = 445 nm) of the mixture of (Z)-6d and perchlorophenol
accelerated the formation of 7d¢H+. These results indicate that
the conversion of (Z)-6d to 7d¢H+ is interpreted as proceeding
via (E)-6d. A small acceleration of thermal isomerization by
an acid is the result of protonation of the nitrogen atom of
the (Z)-isomer. Thus, it was found that the protonation of
(E)-6d to 7d¢H+ could be interrupted as photoisomerization of
(E)-6d to (Z)-6d, and photo- or thermal isomerization of (Z)-6d
to (E)-6d immediately resumes the conversion to 7d¢H+.
Furthermore, the reaction of 7d¢H+ with 4-(N,N-dimethyl-
amino)pyridine (4-Me2NPy) regenerated (E)-6d, the starting
state of this system.
Table 3. Selected Calculated Singlet Excitation Energies for
(E)-6a, 7a, and 7a¢H+
Excitation
wavelength/nm strength
Oscillator
Transition
(E)-6a S1 HOMO ¼ LUMO
524
0.0044
0.0585
0.5646
0.0238
0.0174
0.0015
0.0136
0.0115
0.0407
S2 HOMO¹1 ¼ LUMO 439
S3 HOMO¹2 ¼ LUMO 341
7a
S1 HOMO ¼ LUMO
682
S2 HOMO ¼ LUMO+1 560
S3 HOMO ¼ LUMO+2 474
7a¢H+ S1 HOMO ¼ LUMO
372
S2 HOMO ¼ LUMO+1 320
S3 HOMO ¼ LUMO+2 300
Photoisomerization.
Photoisomerization of (E)-6b and
Conclusion
(E)-6d proceeded to produce (Z)-6b (E/Z = 73/27 at the
photostationary state (PSS)) and (Z)-6d (E/Z = 38/62 at PSS),
respectively, by irradiation (- = 360 nm) of benzene-d6 solu-
tion for one hour, while photoirradiation of (E)-6a resulted in
no reaction (Scheme 8).19 The difference in the photoreactivity
depending on the substituents suggests the influence of inner
Several 2-phosphinoazobenzenes, which are in equilibrium
with the inner phosphonium salts, have been synthesized. The
temperature dependence of equilibrium constants and the
thermodynamic parameters of the equilibria were obtained by
VT-NMR experiments, which clarified the effects of solvents,

M. Yamamura et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 1 (2012)
117
additives, and substituents on the equilibrium. Thermodynamic
parameters of the equilibria in various solvents show that the
main factor of the solvent effect is the acceptor character of the
solvent. Similar to the solvent effect, phenols as an additive
changed the equilibria depending on the acidity of the phenols.
In addition, a strong acid such as perchlorophenol protonated
the inner phosphonium salts, causing the drastic color change
from black to pale yellow. Accordingly, 2-phosphinoazoben-
zenes showed a color change in response to the presence of an
acid and its acidity.
The substituent effects at the 4- and 4¤-positions of an
azobenzene unit on the equilibrium played an important role in
the photoisomerization of the phosphinoazobenzenes, which
successfully proceeded only by introducing electron-donating
substituents at the 4- and 4¤-positions that decreased the
equilibrium constant K1. While the (E)-azobenzene was
protonated by perchlorophenol, the (Z)-azobenzene was not
protonated because the latter had no equilibrium with the inner
phosphonium salt. That is, the properties and reactivity of the
inner phosphonium salts in equilibrium with the phosphines
can be successfully controlled by photoirradiation.
CDCl3): ¤ 20.93 (CH3), 103.21 (CI), 116.78 (CH), 123.36 (CH),
129.05 (CH), 129.67 (CH), 131.19 (CH), 140.07 (CH), 143.00
(CMe), 149.11 (CN), 152.23 (CN); HRMS (FAB, positive):
m/z calcd for C13H11N2I [M]+ 321.9967, found: 321.9959.
1
(E)-1d: H NMR (500 MHz, CDCl3): ¤ 2.37 (s, 3H), 2.44
3
3
(s, 3H), 7.21 (t, JHH = 8.0 Hz, 1H), 7.31 (d, JHH = 8.0 Hz,
2H), 7.54 (d, 3JHH = 8.0 Hz, 1H), 7.85 (s, 1H), 7.87 (d, 3JHH
=
8.0 Hz, 2H); 13C{1H} NMR (126 MHz, CDCl3): ¤ 20.90 (CH3),
21.61 (CH3), 102.91 (CI), 116.75 (CH), 123.34 (CH), 129.63
(CH), 129.69 (CH), 139.98 (CH), 141.79 (CMe), 142.63
(CMe), 149.17 (CN), 150.42 (CN); HRMS (FAB, positive):
m/z calcd for C14H13N2I [M]+ 336.0123, found: 336.0126.
3
(E)-1e: 1H NMR (500 MHz, CDCl3): ¤ 7.17 (td, JHH
=
=
=
4
7.5 Hz, 4JHH = 1.5 Hz, 1H), 7.42 (td, 3JHH = 7.5 Hz, JHH
3
1.5 Hz, 1H), 7.50 (d, 3JHH = 8.0 Hz, 2H), 7.62 (dd, JHH
7.5 Hz, 4JHH = 1.5 Hz, 1H), 7.93 (d, 3JHH = 8.0 Hz, 2H),
8.03 (dd, 3JHH = 7.5 Hz, 4JHH = 1.5 Hz, 1H); 13C{1H} NMR
(126 MHz, CDCl3): ¤ 102.80 (CI), 117.18 (CH), 124.66
(CH), 128.79 (CH), 129.34 (CH), 132.35 (CH), 137.05
(CCl), 139.79 (CH), 150.49 (CN), 150.90 (CN); HRMS
(FAB, positive): m/z calcd for C12H8N2ClI [M]+ 341.9421,
The equilibria between the 2-phosphinoazobenzenes and the
inner phosphonium salts are changed by various external
stimuli such as heat, solvent, an acidic additive, and photo-
irradiation. In addition, the properties and reactivity are
controlled based on the change in the equilibria. These
multiresponsive changes of the phosphine-phosphonium equi-
librium are expected to be applied to masking of phosphine
ligands for transition-metal catalytic reactions and control of
reactions of phosphonium salts.
found: 341.9416.
3
(E)-1f: 1H NMR (500 MHz, CDCl3): ¤ 7.41 (dd, JHH
=
=
8.0 Hz, 4JHH = 1.5 Hz, 1H), 7.50-7.55 (m, 3H), 7.59 (d, 3JHH
8.0 Hz, 1H), 7.97 (dd, 3JHH = 8.0 Hz, 4JHH = 1.5 Hz, 2H), 8.02
(d, 4JHH = 1.5 Hz, 1H); 13C{1H} NMR (126 MHz, CDCl3):
¤ 102.80 (CI), 117.75 (CH), 123.55 (CH), 129.12 (CH),
129.14 (CH), 131.75 (CH), 137.32 (CCl), 139.00 (CH), 149.69
(CN), 150.04 (CN); HRMS (FAB, positive): m/z calcd for
C12H8N2ClI [M]+ 341.9421, found: 341.9422.
1
(E)-1g: H NMR (500 MHz, CDCl3): ¤ 2.38 (s, 3H), 7.23
Experimental
(dd, 3JHH = 8.0 Hz, 4JHH = 1.5 Hz, 1H), 7.48 (d, JHH
=
=
3
3
Materials. 2-Iodo-4-methylaniline and 4-chloro-2-iodoani-
line were prepared quantitatively by a procedure similar to a
reported one.32
8.0 Hz, 2H), 7.55 (d, 3JHH = 8.0 Hz, 1H), 7.86 (d, JHH
1.5 Hz, 1H), 7.91 (d, 3JHH = 8.0 Hz, 2H); 13C{1H} NMR
(126 MHz, CDCl3): ¤ 20.83 (CH3), 103.60 (CI), 116.74 (CH),
124.58 (CH), 129.31 (CH), 129.69 (CH), 137.11 (CCl), 140.15
(CH), 143.48 (CMe), 148.87 (CN), 150.58 (CN); HRMS (FAB,
positive): m/z calcd for C13H10N2ClI [M]+ 355.9577, found:
355.9561.
General Procedure of the Synthesis of 2-Iodoazobenzenes
(E)-1a-1i.23
To an acetic acid solution of 2-iodoaniline
was added nitrosobenzene, which was prepared from aniline,
hydrogen peroxide, and MoO3 and aq. KOH solution according
to a literature method33 and collected by filtration without any
purification. After the reaction mixture was stirred at 100 °C for
12 h, 2-iodoazobenzene ((E)-1a) was purified similarly to the
literature procedure. Similarly, (E)-1b-1i were synthesized, and
the yields of (E)-1a-1i are as follows: (E)-1a, 73%; (E)-1b,
43%; (E)-1c, 40%; (E)-1d, 31%; (E)-1e, 29%; (E)-1f, 26%;
(E)-1g, 37%; (E)-1h, 37%; (E)-1i, 38%.
1
(E)-1h: H NMR (500 MHz, CDCl3): ¤ 2.44 (s, 3H), 7.32
(d, 3JHH = 8.0 Hz, 2H), 7.39 (dd, 3JHH = 8.0 Hz, 4JHH = 1.5 Hz,
3
3
1H), 7.57 (d, JHH = 8.0 Hz, 1H), 7.87 (d, JHH = 8.0 Hz, 2H),
8.00 (d, 3JHH = 1.5 Hz, 1H); 13C{1H} NMR (126 MHz, CDCl3):
¤ 21.68 (CH3), 102.54 (CI), 117.72 (CH), 123.54 (CH), 129.07
(CH), 129.81 (CH), 136.95 (CCl), 138.91 (CH), 142.52 (CMe),
149.74 (CN), 150.25 (CN); HRMS (FAB, positive): m/z calcd
1
(E)-1b: H NMR (500 MHz, CDCl3): ¤ 2.44 (s, 3H), 7.14
for C13H10N2ClI [M]+ 355.9577, found: 355.9580.
(td, 3JHH = 7.5 Hz, 4JHH = 1.5 Hz, 1H), 7.32 (d, 3JHH = 8.0
(E)-1i: 1H NMR (500 MHz, CDCl3): ¤ 7.40 (dd, JHH
=
3
Hz, 2H), 7.41 (td, 3JHH = 7.5 Hz, 4JHH = 1.5 Hz, 1H), 7.61
8.0 Hz, JHH = 1.5 Hz, 1H), 7.50 (d, JHH = 8.0 Hz, 2H), 7.59
(d, 3JHH = 8.0 Hz, 1H), 7.92 (d, 3JHH = 8.0 Hz, 2H), 8.02
4
3
3
(dd, 3JHH = 7.5 Hz, 4JHH = 1.5 Hz, 1H), 7.89 (d, JHH
=
8.0 Hz, 2H), 8.01 (td, 3JHH = 7.5 Hz, 4JHH = 1.5 Hz, 1H);
13C{1H} NMR (126 MHz, CDCl3): ¤ 21.65 (CH3), 102.19 (CI),
117.26 (CH), 123.48 (CH), 128.80 (CH), 129.76 (CH), 129.80
(CH), 131.81 (CH), 142.19 (CMe), 150.40 (CN), 151.26 (CN);
HRMS (FAB, positive): m/z calcd for C13H11N2I [M]+
321.9967, found: 321.9954.
(d, JHH = 1.5 Hz, 1H); 13C{1H} NMR (126 MHz, CDCl3): ¤
4
103.12 (CI), 117.69 (CH), 124.74 (CH), 129.14 (CH), 129.43
(CH), 137.65 (CCl), 137.77 (CCl), 139.09 (CH), 149.45 (CN),
150.37 (CN); HRMS (FAB, positive): m/z calcd for
C12H7N2Cl2I [M]+ 375.9031, found: 375.9036.
Synthesis of (E)-2-(Diphenylphosphinothioyl)azoben-
zenes (E)-2a-2d (Lithiation). To an Et2O solution (30 mL)
of 2-iodoazobenzene ((E)-1a) (1.00 g, 3.24 mmol) was rapidly
added n-BuLi (1.6 M in hexane, 2.1 mL, 3.36 mmol) at
(E)-1c: 1H NMR (500 MHz, CDCl3): ¤ 2.38 (s, 3H), 7.23 (d,
3JHH = 7.5 Hz, 1H), 7.49-7.58 (m, 4H), 7.86 (d, 4JHH = 1.5 Hz,
3
1H), 7.97 (d, JHH = 8.0 Hz, 2H); 13C{1H} NMR (126 MHz,

118
Bull. Chem. Soc. Jpn. Vol. 85, No. 1 (2012)
2-Diphenylphosphinoazobenzenes
1
¹105 °C. After the reaction solution was stirred at ¹105 °C
further for 5 min, diphenylphosphinothioyl chloride prepared
from chlorodiphenylphosphine (0.68 mL, 3.56 mmol) and
elemental sulfur (114 mg, 3.56 mmol) were added to the
solution. The reaction mixture was stirred at r.t. for 3 h.
After evaporation of the solvent, recrystallization of the crude
solid from ethanol gave (E)-2a (1.15 g, 89%). Compounds
(E)-2b-2d were similarly synthesized using THF as the
solvent, and the yields of (E)-2a-2d are shown in Scheme 1.
(E)-2a: mp 133-135 °C; 1H NMR (500 MHz, CDCl3): ¤
(d, JCP = 1.8 Hz, CN), 149.46 (s, CN); 31P NMR (162 MHz,
CDCl3): ¤ 42.98 (s); Anal. Calcd for C26H23N2PS: C, 73.22; H,
5.44; N, 6.57%. Found: C, 73.01; H, 5.51; N, 6.40%.
Synthesis of 2-(Diphenylphosphinothioyl)hydrazoben-
zenes 3e-3i (Pd-Catalyzed Coupling Reaction). According
to the synthetic procedures of 2-(diphenylphosphinothioyl)-
hydrazobenzene (3a),21 3e-3i were synthesized similarly. A
mixture of (E)-1e (1.00 g, 2.90 mmol) and hydrazine mono-
hydrate (2 mL) in EtOH (30 mL) was stirred vigorously at
80 °C for 5 h in air. Addition of water, extraction with CHCl3
and evaporation of the solvent gave a crude solid of 2-
iodohydrazobenzene. The crude solid was dissolved in toluene
(20 mL), and the solution was treated with PdCl2 (10 mg,
56 ¯mol), diphenylphosphine (0.62 mL, 3.6 mmol), and tri-
ethylamine (1 mL, 7 mmol). After the reaction solution was
stirred at 100 °C for 12 h, elemental sulfur (0.20 g, 6.25 mmol)
was added to the reaction mixture. After the reaction, solution
was stirred at r.t. for 30 min, and addition of water, extraction
with CHCl3 and evaporation of the solvent gave a crude oil.
The crude oil was separated by silica gel chromatography
(eluent: CHCl3) and recrystallization from ethanol/CHCl3 gave
a colorless solid of 3e (0.93 g, 74%). The yields of 3e-3i are
shown in Scheme 2.
3e: H NMR (500 MHz, CDCl3): ¤ 5.55 (brs, 1H), 6.42 (d,
3JHH = 8.0 Hz, 2H), 6.67-6.70 (m, 2H), 7.00 (d, 3JHH = 8.0 Hz,
2H), 7.15 (t, JHH = 8.0 Hz, 1H), 7.34 (t, JHH = 8.0 Hz, 1H),
7.47-7.52 (m, 4H), 7.56-7.60 (m, 2H), 7.76 (dd, JHP =
14.0 Hz, 3JHH = 7.5 Hz, 4H), 8.30 (brs, 1H); 13C{1H} NMR
(126 MHz, CDCl3): ¤ 112.28 (d, JCP = 7.4 Hz, CH), 112.89 (d,
1JCP = 85.8 Hz, CP), 113.48 (s, CH), 118.25 (d, JCP = 12.3 Hz,
CH), 124.25 (s, CCl), 128.59 (d, JCP = 12.4 Hz, CH), 128.89
(s, CH), 131.10 (d, 1JCP = 85.0 Hz, CP), 131.83 (d, JCP
3.3 Hz, CH), 132.36 (d, JCP = 10.7 Hz, CH), 132.63 (d, JCP
9.1 Hz, CH), 133.45 (s, CH), 146.55 (s, CN), 151.62 (d, JCP
4.9 Hz, CN); 31P NMR (162 MHz, CDCl3): ¤ 39.26 (s); HRMS
(FAB, positive): m/z calcd for C24H20N2PSCl [M]+ 434.0773,
found: 434.0769.
3f: H NMR (500 MHz, CDCl3): ¤ 5.56 (brs, 1H), 6.49 (d,
3JHH = 7.5 Hz, 2H), 6.62 (dd, JHP = 15.0 Hz, JHH = 2.5 Hz,
1H), 6.77 (d, JHH = 7.5 Hz, 1H), 7.08 (t, JHH = 7.5 Hz, 2H),
3
7.28-7.30 (m, 4H), 7.33-7.42 (m, 7H), 7.50 (t, JHH = 7.5 Hz,
1H), 7.63 (t, 3JHH = 7.5 Hz, 1H), 7.76 (dd, 3JHH = 7.5 Hz,
J
HP = 4.8 Hz, 1H), 7.80 (dd, 3JHH = 7.5 Hz, 4JHH = 1.5 Hz,
3
3
4
1H), 7.87 (ddd, JHP = 15.0 Hz, JHH = 7.5 Hz, JHH = 1.5 Hz,
4H); 13C{1H} NMR (126 MHz, CDCl3): ¤ 115.76 (d, JCP
=
7.2 Hz, CH), 123.62 (s, CH), 128.25 (d, JCP = 13.2 Hz, CH),
128.74 (s, CH), 130.58 (d, JCP = 12.0 Hz, CH), 130.00 (d,
1
J
CP = 2.6 Hz, CH), 131.40 (s, CH), 131.84 (d, JCP = 81.8 Hz,
CP), 131.89 (d, JCP = 10.8 Hz, CH), 132.93 (d, JCP = 2.4 Hz,
CH), 133.82 (d, 1JCP = 87.7 Hz, CP), 134.39 (d, JCP = 10.0 Hz,
CH), 152.16 (d, JCP = 1.2 Hz, CN), 152.27 (s, CN); 31P NMR
(162 MHz, CDCl3): ¤ 42.8 (s); Anal. Calcd for C24H19N2PS: C,
72.34; H, 4.81; C, 7.03%. Found: C, 72.09; H, 4.93; N, 6.73%.
(E)-2b: mp 199-200 °C; 1H NMR (500 MHz, CDCl3): ¤ 2.36
1
3
3
(s, 3H), 7.08 (d, JHH = 8.0 Hz, 2H), 7.16 (d, JHH = 8.0 Hz,
2H), 7.32-7.41 (m, 6H), 7.49 (t, 3JHH = 7.5 Hz, 1H), 7.62
3
3
3
3
3
(t, JHH = 7.5 Hz, 1H), 7.75 (dd, JHH = 7.5 Hz, JHP = 4.8 Hz,
3
4
1H), 7.80 (dd, JHH = 7.5 Hz, JHH = 1.5 Hz, 1H), 7.86 (dd,
3JHP = 15.5 Hz, 3JHH = 7.5 Hz, 4H); 13C{1H} NMR (126 MHz,
CDCl3): ¤ 21.51 (s, CH3), 115.66 (d, JCP = 7.5 Hz, CH), 123.57
(s, CH), 128.12 (d, JCP = 13.2 Hz, CH), 129.33 (s, CH), 130.19
(d, JCP = 12.4 Hz, CH), 130.83 (s, CH), 131.79 (d, JCP
=
=
=
=
1
10.8 Hz, CH), 132.14 (d, JCP = 80.9 Hz, CP), 132.80 (s, CH),
133.89 (d, 1JCP = 87.9 Hz, CP), 134.33 (d, JCP = 10.0 Hz, CH),
142.01 (s, CH), 150.44 (s, CN), 152.13 (s, CN); 31P NMR
(162 MHz, CDCl3): ¤ 42.81 (s); Anal. Calcd for C25H21N2PS: C,
72.79; H, 5.13; C, 6.79%. Found: C, 72.64; H, 5.13; N, 6.57%.
(E)-2c: mp 183-185 °C; 1H NMR (500 MHz, CDCl3): ¤ 2.41
1
3
3
3
4
(s, 3H), 7.19 (d, JHH = 8.0 Hz, 2H), 7.27 (d, JHH = 8.0 Hz,
2H), 7.32-7.44 (m, 8H), 7.69 (dd, 3JHH = 7.5 Hz, JHP = 4.8 Hz,
1H), 7.78 (d, JHP = 15.0 Hz, 1H), 7.84 (dd, 3JHP = 13.5 Hz,
3JHH = 7.5 Hz, 4H); 13C{1H} NMR (126 MHz, CDCl3): ¤ 21.42
(s, CH3), 115.34 (d, JCP = 7.5 Hz, CH), 123.13 (s, CH), 127.91
(d, JCP = 13.2 Hz, CH), 128.39 (s, CH), 130.61 (s, CH), 130.82
3
3
3
3
3
7.19 (dd, JHH = 8.0 Hz, JHP = 6.0 Hz, 1H), 7.27 (dd, JHH
8.0 Hz, 4JHH = 2.5 Hz, 1H), 7.52 (td, 3JHH = 7.5 Hz, JHP
3.0 Hz, 4H), 7.60 (td, JHH = 7.5 Hz, JHP = 2.0 Hz, 2H), 7.77
(dd, 3JHP = 13.5 Hz, 3JHH = 7.5 Hz, 4H), 8.25 (brs, 1H);
13C{1H} NMR (126 MHz, CDCl3): ¤ 112.10 (s, CH), 114.02
(d, JCP = 8.3 Hz, CH), 114.08 (d, JCP = 84.6 Hz, CP), 119.77
(s, CH), 122.60 (d, JCP = 15.7 Hz, CCl), 128.61 (d, JCP
12.6 Hz, CH), 128.91 (s, CH), 130.11 (d, JCP = 85.4 Hz, CP),
131.49 (d, JCP = 10.0 Hz, CH), 132.00 (s, CH), 132.12 (d,
=
=
4
3
5
1
(s, CH), 131.41 (d, JCP = 80.9 Hz, CP), 131.49 (d, JCP = 10.8
Hz, CH), 133.39 (s, CH), 133.83 (d, 1JCP = 88.3 Hz, CP),
134.72 (d, JCP = 10.0 Hz, CH), 141.09 (d, JCP = 13.2 Hz,
CMe), 149.77 (d, JCP = 3.2 Hz, CN), 151.93 (s, CN); 31P NMR
(162 MHz, CDCl3): ¤ 43.01 (s); Anal. Calcd for C24H19N2PS: C,
72.79; H, 5.13; N, 6.79%. Found: C, 72.65; H, 5.24; N, 6.64%.
(E)-2d: mp 190-192 °C; 1H NMR (500 MHz, CDCl3): ¤ 2.35
(s, 3H), 2.41 (s, 3H), 7.04-7.09 (m, 4H), 7.31-7.43 (m, 7H),
7.68 (dd, 3JHH = 7.5 Hz, JHP = 5.0 Hz, 1H), 7.80-7.87 (m, 5H);
13C{1H} NMR (126 MHz, CDCl3): ¤ 20.76 (s, CH3), 20.81 (s,
CH3), 114.65 (d, JCP = 6.6 Hz, CH), 122.50 (s, CH), 127.25 (d,
1
=
1
J
CP = 10.7 Hz, CH), 133.05 (s, CH), 147.36 (s, CN), 150.34
(d, JCP = 4.1 Hz, CN); 31P NMR (162 MHz, CDCl3): ¤ 39.11
(s); HRMS (FAB, positive): m/z calcd for C24H20N2PSCl [M]+
434.0773, found: 434.0789.
1
3g: H NMR (500 MHz, CDCl3): ¤ 2.09 (s, 3H), 5.54 (brs,
1H), 6.42 (d, 3JHH = 8.0 Hz, 2H), 6.46 (dd, 3JHP = 15.0 Hz,
4JHH = 1.5 Hz, 1H), 7.00 (d, 3JHH = 8.0 Hz, 2H), 7.04 (dd,
3JHH = 7.5 Hz, JHP = 6.0 Hz, 1H), 7.15 (d, 3JHH = 7.5 Hz, 1H),
J
J
CP = 13.2 Hz, CH), 128.49 (s, CH), 130.00 (s, CH), 130.74 (d,
CP = 10.7 Hz, CH), 130.99 (d, 1JCP = 80.1 Hz, CP), 132.78 (s,
CH), 133.21 (d, 1JCP = 87.4 Hz, CP), 133.89 (d, JCP = 10.0 Hz,
CH), 139.98 (d, JCP = 13.6 Hz, CMe), 140.71 (s, CMe), 149.05
7.50 (td, JHH = 7.5 Hz, JHP = 2.8 Hz, 4H), 7.57 (td, JHH
7.5 Hz, JHP = 2.0 Hz, 2H), 7.77 (dd, JHP = 13.5 Hz, JHH
=
=
3
3
3
3

M. Yamamura et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 1 (2012)
119
7.5 Hz, 4H), 8.13 (brs, 1H); 13C{1H} NMR (126 MHz, CDCl3):
¤ 20.48 (s, CH3), 112.53 (d, JCP = 8.3 Hz, CH), 112.91
(d, 1JCP = 85.0 Hz, CP), 113.41 (s, CH), 123.97 (s, CCl),
127.44 (d, JCP = 12.3 Hz, CMe), 128.51 (d, JCP = 13.2 Hz,
CH), 130.64 (d, JCP = 12.4 Hz, CH), 130.91 (d, JCP
3.3 Hz, CH), 131.68 (d, JCP = 10.7 Hz, CH), 132.70 (d, JCP
=
=
1.7 Hz, CH), 133.01 (d, 1JCP = 81.7 Hz, CP), 133.47 (d,
1JCP = 86.6 Hz, CP), 133.90 (d, JCP = 10.0 Hz, CH), 137.16
(s, CCl), 150.48 (s, CN), 151.76 (d, JCP = 3.3 Hz, CN);
31P NMR (162 MHz, CDCl3): ¤ 42.50 (s); Anal. Calcd
for C24H18N2PSCl: C, 66.59; H, 4.19; N, 6.47%. Found: C,
66.37; H, 4.30; N, 6.28%.
1
CH), 128.79 (s, CH), 131.15 (d, JCP = 85.0 Hz, CP), 131.75
(d, JCP = 2.5 Hz, CH), 132.32 (d, JCP = 10.7 Hz, CH), 132.63
(d, JCP = 8.0 Hz, CH), 134.09 (s, CH), 146.72 (s, CN), 149.34
(d, JCP = 5.0 Hz, CN); 31P NMR (162 MHz, CDCl3): ¤ 39.26
(s); HRMS (FAB, positive): m/z calcd for C25H22N2PSCl [M]+
448.0930, found: 448.0909.
(E)-2f: mp 165-167 °C; 1H NMR (500 MHz, CDCl3): ¤ 7.23
3
3
(d, JHH = 8.0 Hz, 2H), 7.29 (t, JHH = 8.0 Hz, 2H), 7.35-7.43
(m, 7H), 7.58 (dd, 3JHH = 8.0 Hz, JHP = 2.5 Hz, 1H), 7.74 (dd,
3JHH = 8.0 Hz, JHP = 5.0 Hz, 1H), 7.84 (dd, 3JHP = 13.5 Hz,
3JHH = 7.5 Hz, 4H); 13C{1H} NMR (126 MHz, CDCl3): ¤
117.25 (d, JCP = 7.4 Hz, CH), 123.61 (s, CH), 128.33 (d,
1
3h: H NMR (500 MHz, CDCl3): ¤ 2.21 (s, 3H), 5.47 (brs,
1H), 6.40 (d, 3JHH = 8.0 Hz, 2H), 6.60 (dd, 3JHP = 15.0 Hz,
4JHH = 1.5 Hz, 1H), 6.88 (d, 3JHH = 8.0 Hz, 2H), 7.20
(d, 3JHH = 7.5 Hz, 1H), 7.51 (td, 3JHH = 7.5 Hz, JHP
=
3
2.8 Hz, 4H), 7.59 (td, JHH = 7.5 Hz, JHP = 2.0 Hz, 2H), 7.76
(dd, 3JHP = 13.5 Hz, 3JHH = 7.5 Hz, 4H), 8.20 (brs, 1H);
13C{1H} NMR (126 MHz, CDCl3): ¤ 20.52 (s, CH3), 112.42
JCP = 13.2 Hz, CH), 128.74 (s, CH), 131.19 (d, JCP = 2.5 Hz,
CH), 131.59 (s, CH), 131.77 (d, JCP = 11.6 Hz, CH), 132.94
1
(d, JCP = 2.0 Hz, CH), 133.22 (d, JCP = 88.7 Hz, CP), 134.09
1
(s, CH), 114.15 (d, JCP = 84.1 Hz, CP), 114.20 (d, JCP = 8.2
(d, JCP = 11.6 Hz, CH), 134.68 (d, 1JCP = 78.0 Hz, CP), 136.89
(d, JCP = 15.7, CCl), 150.22 (d, JCP = 3.3 Hz, CN), 152.08
(s, CN); 31P NMR (162 MHz, CDCl3): ¤ 42.61 (s); Anal. Calcd
for C24H18N2PSCl: C, 66.59; H, 4.19; N, 6.47%. Found: C,
66.52; H, 4.29; N, 6.36%.
Hz, CH), 122.67 (d, JCP = 13.7 Hz, CCl), 128.73 (d, JCP
=
12.3 Hz, CH), 129.31 (s, CMe), 129.57 (s, CH), 130.55
(d, 1JCP = 85.8 Hz, CP), 131.57 (d, JCP = 10.1 Hz, CH), 132.08
(d, JCP = 3.3 Hz, CH), 132.32 (d, JCP = 11.6 Hz, CH), 133.20
(s, CH), 145.18 (s, CN), 150.71 (d, JCP = 5.0 Hz, CN);
31P NMR (162 MHz, CDCl3): ¤ 38.79 (s); HRMS (FAB,
positive): m/z calcd for C25H22N2PSCl [M]+ 448.0930, found:
448.0894.
(E)-2g: mp 202-203 °C; 1H NMR (500 MHz, CDCl3): ¤ 2.30
3
3
(s, 3H), 7.14 (d, JHH = 8.0 Hz, 2H), 7.23 (d, JHH = 8.0 Hz,
2H), 7.36-7.45 (m, 7H), 7.62 (d, 3JHP = 15.5 Hz, 1H), 7.69 (dd,
3JHH = 8.0 Hz, JHP = 5.0 Hz, 1H), 7.84 (dd, 3JHP = 13.5 Hz,
3JHH = 7.5 Hz, 4H); 13C{1H} NMR (126 MHz, CDCl3): ¤ 20.70
(s, CH3), 115.56 (d, JCP = 6.6 Hz, CH), 124.63 (s, CH), 128.20
1
3i: H NMR (500 MHz, CDCl3): ¤ 5.56 (brs, 1H), 6.39 (d,
3
4
3JHH = 8.0 Hz, 2H), 6.62 (dd, JHP = 15.0 Hz, JHH = 1.5 Hz,
1H), 7.01 (d, 3JHH = 8.0 Hz, 2H), 7.14 (dd, 3JHH = 7.5 Hz,
HP = 5.0 Hz, 1H), 7.28 (dd, 3JHH = 7.5 Hz, 4JHH = 1.5 Hz,
(d, JCP = 12.4 Hz, CH), 128.92 (s, CH), 130.93 (d, JCP
3.3 Hz, CH), 131.78 (d, JCP = 10.7 Hz, CH), 132.69 (d, 1JCP
=
=
=
J
1H), 7.53 (td, 3JHH = 7.5 Hz, JHP = 2.8 Hz, 4H), 7.61 (td,
3JHH = 7.5 Hz, JHP = 2.0 Hz, 2H), 7.74 (dd, 3JHP = 13.5 Hz,
3JHH = 7.5 Hz, 4H), 8.27 (brs, 1H); 13C{1H} NMR (126 MHz,
CDCl3): ¤ 113.43 (s, CH), 113.97 (d, JCP = 7.8 Hz, CH),
114.52 (d, 1JCP = 83.3 Hz, CP), 122.99 (d, JCP = 15.7 Hz,
CCl), 124.37 (s, CCl), 128.72 (d, JCP = 13.2 Hz, CH), 128.83
80.0 Hz, CP), 133.54 (d, JCP = 2.5 Hz, CH), 133.95 (d, JCP
1
85.8 Hz, CP), 134.86 (d, JCP = 10.0 Hz, CH), 137.00 (s, CCl),
141.68 (d, JCP = 12.4 Hz, CMe), 149.90 (d, JCP = 3.4 Hz, CN),
150.64 (s, CN); 31P NMR (162 MHz, CDCl3): ¤ 42.75 (s);
Anal. Calcd for C25H20N2PSCl: C, 67.18; H, 4.51; N, 6.27%.
Found: C, 66.91; H, 4.53; N, 6.03%.
(s, CH), 130.23 (d, 1JCP = 86.8 Hz, CP), 131.55 (d, JCP
10.0 Hz, CH), 132.12 (d, JCP = 2.5 Hz, CH), 132.22 (d, JCP
=
=
(E)-2h: mp 208-210 °C; 1H NMR (500 MHz, CDCl3): ¤ 2.35
3
3
(s, 3H), 7.08 (d, JHH = 8.0 Hz, 2H), 7.12 (d, JHH = 8.0 Hz,
10.7 Hz, CH), 133.13 (s, CH), 146.17 (s, CN), 150.07 (d,
2H), 7.34-7.45 (m, 6H), 7.57 (ddd, 3JHH = 8.0 Hz, JHP
2.5 Hz, 4JHH = 1.5 Hz, 1H), 7.73 (dd, 3JHH = 8.0 Hz, JHP
=
=
J
CP = 5.0 Hz, CN); 31P NMR (162 MHz, CDCl3): ¤ 38.70 (s);
HRMS (FAB, positive): m/z calcd for C24H19N2PS35Cl2
[M]+ 468.0384, C24H19N2PS35Cl37Cl [M]+ 470.0354, found:
468.0366, 470.0352.
5.0 Hz, 1H), 7.81-7.90 (m, 5H); 13C{1H} NMR (126 MHz,
CDCl3): ¤ 21.57 (s, CH3), 117.21 (d, JCP = 7.4 Hz, CH),
123.63 (s, CH), 128.28 (d, JCP = 13.2 Hz, CH), 129.43 (s, CH),
131.13 (d, JCP = 2.4 Hz, CH), 131.73 (d, JCP = 11.5 Hz, CH),
132.88 (s, CH), 133.29 (d, 1JCP = 85.8 Hz, CP), 134.11 (d,
General Procedure of the Synthesis of (E)-2-(Diphenyl-
phosphinothioyl)azobenzenes (E)-2e-2i. To a CHCl3 solu-
tion (10 mL) of 3e (0.20 g, 0.43 mmol) was added an EtOH
solution (5 mL) of pyridinium tribromide (0.15 g, 0.46 mmol),
and the reaction solution was stirred at r.t. for 30 min. After the
solvent was evaporated, recrystallization from EtOH and
washing with EtOH gave an orange solid of (E)-2e (0.14 g,
74%). Similarly, (E)-2f-2i were synthesized. The yields of
(E)-2f-2i are shown in Scheme 2.
J
J
CP = 11.6 Hz, CH), 134.24 (d, 1JCP = 77.6 Hz, CP), 136.51 (d,
CP = 13.3 Hz, CCl), 142.40 (s, CMe), 150.27 (d, JCP = 3.3 Hz,
CN), 150.33 (s, CN); 31P NMR (162 MHz, CDCl3): ¤ 42.53 (s);
Anal. Calcd for C25H20N2PSCl: C, 67.18; H, 4.51; N, 6.27%.
Found: C, 67.27; H, 4.77; N, 6.11%.
(E)-2i: mp 223-224 °C; 1H NMR (500 MHz, CDCl3):
¤ 7.19 (t, 3JHH = 8.0 Hz, 2H), 7.26 (t, 3JHH = 8.0 Hz, 2H),
7.38-7.45 (m, 6H), 7.56 (d, 3JHH = 8.0 Hz, 1H), 7.68 (dd,
(E)-2e: mp 158-160 °C; 1H NMR (500 MHz, CDCl3): ¤
3
4
3
7.21-7.27 (m, 4H), 7.35-7.44 (m, 6H), 7.48 (t, JHH = 8.0 Hz,
3JHP = 15.0 Hz, JHH = 1.5 Hz, 1H), 7.74 (dd, JHH = 8.0 Hz,
1H), 7.63 (t, 3JHH = 8.0 Hz, 1H), 7.66 (dd, 3JHP = 15.5
J
HP = 5.0 Hz, 1H), 7.84 (dd, 3JHP = 13.5 Hz, 3JHH = 7.5 Hz,
4H); 13C{1H} NMR (126 MHz, CDCl3): ¤ 117.25 (d, JCP
=
Hz, 3JHH = 7.5 Hz, 1H), 7.76 (dd, 3JHH = 7.5 Hz, JHP
=
3.5 Hz, 1H), 7.85 (dd, 3JHP = 13.2 Hz, 3JHH = 7.5 Hz, 4H);
13C{1H} NMR (126 MHz, CDCl3): ¤ 115.59 (d, JCP = 6.6 Hz,
CH), 124.70 (s, CH), 128.11 (d, JCP = 12.4 Hz, CH), 128.83 (s,
7.4 Hz, CH), 124.86 (s, CH), 128.40 (d, JCP = 13.2 Hz, CH),
129.07 (s, CH), 131.31 (d, JCP = 3.3 Hz, CH), 131.78 (d,
J
CP = 10.7 Hz, CH), 132.91 (d, JCP = 2.5 Hz, CH), 133.08

120
Bull. Chem. Soc. Jpn. Vol. 85, No. 1 (2012)
2-Diphenylphosphinoazobenzenes
3
(d, 1JCP = 88.3 Hz, CP), 133.91 (d, JCP = 10.7 Hz, CH), 135.22
(d, 1JCP = 78.4 Hz, CP), 137.12 (d, JCP = 13.2 Hz, CCl),
137.66 (s, CCl), 150.10 (d, JCP = 2.5 Hz, CN), 159.53 (s,
CN); 31P NMR (162 MHz, CDCl3): ¤ 42.89 (s); Anal. Calcd
for C24H17N2PSCl2: C, 61.68; H, 3.67; N, 5.99%. Found: C,
61.50; H, 3.80; N, 5.85%.
6H), 7.45 (ttd, JHH = 7.5 Hz, JHP = 2.5 Hz, J = 1.5 Hz, 1H),
7.62 (tt, 3JHH = 7.5 Hz, J = 1.5 Hz, 1H), 7.74 (ddd, JHH
=
=
=
3
3
7.5 Hz, JHP = 6.0 Hz, J = 1.5 Hz, 1H), 7.79 (ddd, JHP
15.0 Hz, 3JHH = 7.5 Hz, J = 1.5 Hz, 1H), 7.86 (ddd, JHP
3
13.5 Hz, JHH = 7.5 Hz, J = 1.5 Hz, 4H); 13C{1H} NMR (126
MHz, CDCl3): ¤ 55.44 (s, CH3), 113.76 (s, CH), 115.60 (d,
Synthesis of (E)-2-(Diphenylphosphinothioyl)azobenzene
JCP = 7.4 Hz, CH), 125.53 (s, CH), 128.07 (d, JCP = 13.2 Hz,
((E)-2j) (Diazo Coupling Reaction).
To a 3 M aq. HCl
CH), 129.75 (d, JCP = 12.4 Hz, CH), 130.76 (d, JCP = 2.4 Hz,
solution (20 mL) of 2-(diphenylphosphinoyl)aniline (4)34
(1.00 g, 3.41 mmol) was slowly added an aqueous solution
(5 mL) of NaNO2 (0.40 g, 5.8 mmol) at 0 °C. After the reaction
solution was stirred further at 0 °C for 10 min, phenol (1.0 g,
11 mmol) was added to the solution. After addition of aq.
NaOH, neutralization with aq. HCl, extraction with CHCl3,
evaporation of the solvent, and washing of the residue with
ethanol gave orange solid of (E)-5 (1.31 g, 96%). To a 1/1
THF and 1 M aq. NaOH solution (30 mL) of (E)-5 (1.41 g,
3.41 mmol) at r.t. was added MeI (2.0 g, 32 mmol). After the
reaction solution was stirred at r.t. further for 24 h, extraction
with AcOEt, evaporation of the solvent, and recrystallization of
the residue from EtOH/hexane gave orange solids of (E)-5¤
(1.39 g, 99%). A toluene (10 mL) solution of phosphine oxide
(E)-5¤ (417 mg, 1.01 mmol) and Lawesson’s reagent (418 mg,
1.03 mmol) was refluxed for 7 h. After evaporation of the
solvent, the crude oil was separated by silica gel chromatog-
raphy (eluent: CHCl3). Evaporation of the eluate gave orange
solids of (E)-2j (199 mg, 46%). Compound (E)-2j was used for
the next reaction without further purification.
CH), 131.69 (d, JCP = 10.7 Hz, CH), 131.76 (d, JCP = 81.6
1
Hz, CP), 132.74 (s, CH), 133.85 (d, 1JCP = 87.4 Hz, CP),
134.12 (d, JCP = 10.0 Hz, CH), 146.76 (s, CN), 152.18 (d,
J
CP = 3.3 Hz, CN), 162.17 (s, CO); 31P NMR (162 MHz,
CDCl3): ¤ 42.8 (s).
Synthesis of (E)-2-(Diphenylphosphino)azobenzenes (E)-
6a-6j. To a toluene solution (5 mL) of phosphine sulfide (E)-
2a (200 mg, 0.50 mmol) was added tributylphosphine (150 ¯L,
0.60 mmol) and the reaction solution was degassed and sealed.
Afterwards the solution was heated at 100 °C for 48 h. Quan-
titative formation of (E)-6a was confirmed by 1H and 31P NMR
spectroscopy. After evaporation of the solvent, recrystallization
from benzene gave (E)-6a (146 mg, 79%) as moisture sensitive
red crystals. Similarly, (E)-6b-6j were synthesized.
1
(E)-6a: mp 110-111 °C; H NMR (500 MHz, C6D6, r.t.): ¤
3
6.91 (t, J = 7.5 Hz, 1H), 6.96-7.04 (m, 11H), 7.37 (td, JHH
=
=
4
7.5 Hz, JHP = 2.7 Hz, 4H), 7.77 (dd, 3JHH = 7.5 Hz, JHH
1.5 Hz, 2H), 7.85 (dd, 3JHH = 7.5 Hz, 3JHP = 3.8 Hz, 1H);
13C{1H} NMR (126 MHz, C6D6, 60 °C): ¤ 117.71 (s, CH),
122.94 (s, CH), 128.36 (d, JCP = 7.4 Hz, CH), 128.50 (s, CH),
128.70 (s, CH), 129.08 (s, CH), 129.88 (s, CH), 130.33 (s, CH),
133.30 (s, CH), 134.15 (d, JCP = 20.7 Hz, CH), 136.23 (d,
1JCP = 15.2 Hz, CP), 138.13 (d, 1JCP = 9.5 Hz, CP), 152.69
(s, CN), 154.46 (d, JCP = 13.7 Hz, CN); 31P NMR (162 MHz,
C6D6, 60 °C): ¤ ¹9.2 (s); HRMS (FAB): m/z calcd for
C24H19N2P [M]+ 366.1286, found: 366.1301.
(E)-5: mp (dec.) 276-278 °C; 1H NMR (500 MHz, CDCl3): ¤
6.81 (d, 3JHH = 7.5 Hz, 2H), 7.18-7.25 (m, 3H), 7.36 (td,
3JHH = 7.5 Hz, JHH = 1.5 Hz, 1H), 7.40-7.43 (m, 3H), 7.49 (t,
3JHH = 7.5 Hz, 2H), 7.66 (t, 3JHH = 7.5 Hz, 4H), 7.71 (dd,
3JHP = 12.4 Hz, JHH = 7.5 Hz, 4H), 7.85 (dd, 3JHH = 7.5 Hz,
J
HP = 4.0 Hz, 1H), 12.93 (brs, 1H); 13C{1H} NMR (126 MHz,
CDCl3): ¤ 115.93 (d, JCP = 7.2 Hz, CH), 116.10 (s, CH), 125.68
1
(E)-6b: mp 118-120 °C; H NMR (500 MHz, C6D6, r.t.): ¤
(s, CH), 128.45 (d, JCP = 12.6 Hz, CH), 129.10 (d, JCP
=
1.98 (s, 3H), 6.89 (d, J = 8.0 Hz, 2H), 6.97 (d, J = 7.5 Hz, 1H),
7.03-7.20 (m, 8H), 7.51 (td, 3JHH = 7.5 Hz, JHP = 2.2 Hz, 4H),
7.79 (d, J = 8.0 Hz, 2H), 8.00 (dd, 3JHH = 8.0 Hz, JHP = 5.2 Hz,
1H); 13C{1H} NMR (126 MHz, C6D6, 60 °C): ¤ 21.36 (s, CH3),
118.01 (s, CH), 123.45 (s, CH), 128.65 (d, JCP = 6.2 Hz, CH),
128.77 (s, CH), 129.33 (s, CH), 129.77 (s, CH), 130.37 (s, CH),
133.77 (s, CH), 134.58 (d, JCP = 20.9 Hz, CH), 137.60 (d,
13.2 Hz, CH), 130.40 (d, 1JCP = 103.5 Hz, CP), 131.52 (d,
1
J
CP = 10.1 Hz, CH), 131.77 (s, CH), 132.62 (d, JCP = 108.6
Hz, CP), 133.80 (s, CH), 134.08 (d, JCP = 11.0 Hz, CH), 145.92
(s, CN), 154.27 (s, CN), 162.11 (s, CO); 31P NMR (162 MHz,
CDCl3): ¤ 32.7 (s); Anal. Calcd for C24H19N2PO2: C, 72.35; H,
4.81; N, 7.03%. Found: C, 72.08; H, 4.96; N, 6.79%.
1
1
(E)-5¤: H NMR (500 MHz, CDCl3): ¤ 3.84 (s, 3H), 6.77 (d,
1JCP = 18.5 Hz, CP), 138.94 (d, JCP = 11.7 Hz, CP), 141.28
3JHH = 8.0 Hz, 2H), 7.21 (d, 3JHH = 8.0 Hz, 2H), 7.33-7.38 (m,
(s, CMe), 151.56 (s, CN), 154.89 (d, JCP = 13.7 Hz, CN);
31P NMR (162 MHz, C6D6, 60 °C): ¤ ¹9.9 (s); HRMS (FAB):
m/z calcd for C25H21N2P [M]+ 380.1442, found: 380.1453.
3
4H), 7.41-7.45 (m, 2H), 7.55 (tt, JHH = 7.5 Hz, J = 1.5 Hz,
3
3
1H), 7.65 (t, JHH = 7.5 Hz, 1H), 7.74 (ddd, JHP = 13.0 Hz,
3JHH = 7.5 Hz, J = 1.5 Hz, 4H), 7.78 (td, 3JHH = 7.5 Hz, JHP
=
(E)-6c: 1H NMR (500 MHz, C6D6, r.t.): ¤ 1.92 (s, 3H),
5.0 Hz, 1H), 8.07 (ddd, 3JHP = 13.2 Hz, 3JHH = 7.5 Hz, J =
1.5 Hz, 1H); 13C{1H} NMR (126 MHz, CDCl3): ¤ 55.28 (s,
CH3), 113.64 (s, CH), 115.12 (d, JCP = 7.4 Hz, CH), 125.17 (s,
CH), 127.92 (d, JCP = 12.6 Hz, CH), 129.78 (d, JCP = 11.8 Hz,
CH), 130.99 (d, JCP = 2.5 Hz, CH), 131.31 (d, JCP = 10.0 Hz,
6.96-7.04 (m, 4H), 7.05-7.12 (m, 7H), 7.55 (td, JHH
=
=
3
4
8.0 Hz, JHP = 2.0 Hz, 4H), 7.84 (dd, 3JHH = 8.0 Hz, JHH
1.5 Hz, 2H), 8.00 (dd, 3JHH = 8.0 Hz, JHP = 4.0 Hz, 1H);
13C{1H} NMR (126 MHz, C6D6, r.t.): ¤ 21.47 (s, CH3),
118.02 (s, CH), 123.31 (s, CH), 128.70 (d, JCP = 7.6 Hz,
CH), 128.74 (s, CH), 129.10 (s, CH), 130.32 (s, CH), 130.76
(s, CH), 134.16 (s, CH), 134.53 (d, JCP = 20.0 Hz, CH), 138.09
(d, 1JCP = 15.7 Hz, CP), 138.73 (d, 1JCP = 10.7 Hz, CP),
141.32 (s, CMe), 152.44 (d, JCP = 13.2 Hz, CN), 152.97 (s,
CN); 31P NMR (162 MHz, C6D6, r.t.): ¤ ¹9.7 (s).
1
CH), 131.34 (d, JCP = 99.5 Hz, CP), 132.85 (d, JCP = 2.5 Hz,
CH), 133.69 (d, 1JCP = 106.9 Hz, CP), 134.07 (d, JCP = 7.4 Hz,
CH), 146.44 (s, CN), 152.44 (d, JCP = 4.2 Hz, CN), 162.04 (s,
CO); 31P NMR (162 MHz, CDCl3): ¤ 28.2 (s); HRMS (FAB):
m/z calcd for C25H21N2O2P [M]+ 412.1341, found: 412.1316.
(E)-2j: 1H NMR (500 MHz, CDCl3): ¤ 3.84 (s, 3H), 6.78 (d,
3JHH = 8.0 Hz, 2H), 7.26 (d, 3JHH = 8.0 Hz, 2H), 7.33-7.41 (m,
(E)-6d: mp 184-187 °C; 1H NMR (500 MHz, C6D6, r.t.):
3
¤ 1.93 (s, 3H), 1.99 (s, 3H), 6.91 (d, JHH = 8.0 Hz, 2H), 6.98

M. Yamamura et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 1 (2012)
121
3
3
(d, JHH = 8.0 Hz, 1H), 7.02 (d, JHP = 4.5 Hz, 1H), 7.05-7.13
31P NMR (162 MHz, C6D6, 60 °C): ¤ ¹9.1 (s); HRMS (FAB):
m/z calcd for C25H20N2PCl [M]+ 414.1053, found: 414.1076.
3
(m, 6H), 7.52 (td, JHH = 8.0 Hz, JHP = 2.0 Hz, 4H), 7.82 (d,
3JHH = 8.0 Hz, 2H), 8.04 (dd, 3JHH = 8.0 Hz, JHP = 4.5 Hz,
1H); 13C{1H} NMR (126 MHz, C6D6, 60 °C): ¤ 21.17 (s, CH3),
21.32 (s, CH3), 117.84 (s, CH), 123.42 (s, CH), 128.67 (s, CH),
128.70 (d, JCP = 6.8 Hz, CH), 129.85 (s, CH), 130.32 (s, CH),
134.19 (s, CH), 134.59 (d, JCP = 21.1 Hz, CH), 138.29 (d,
1JCP = 18.2 Hz, CP), 138.97 (d, 1JCP = 12.0 Hz, CP), 141.17 (s,
(E)-6i: H NMR (500 MHz, C6D6, 60 °C): ¤ 7.03 (d, JHH
8.0 Hz, 2H), 7.05-7.09 (m, 6H), 7.13 (dd, 3JHH = 8.0 Hz,
HH = 1.5 Hz, 1H), 7.15 (dd, JHP = 5.5 Hz, JHH = 1.5 Hz, 1H),
=
1
3
J
3
3
7.40 (dd, JHH = 8.0 Hz, JHP = 2.0 Hz, 4H), 7.48 (d, JHH
=
8.0 Hz, 2H), 7.61 (dd, 3JHH = 8.0 Hz, JHP = 4.0 Hz, 1H);
13C{1H} NMR (126 MHz, C6D6, 60 °C): ¤ 118.85 (s, CH),
124.24 (s, CH), 128.93 (d, JCP = 7.4 Hz, CH), 129.40 (s, CH),
129.43 (s, CH), 129.83 (s, CH), 132.81 (d, JCP = 2.5 Hz, CH),
134.28 (d, JCP = 20.6 Hz, CH), 136.15 (brs, CP), 136.64 (s,
CCl), 136.69 (s, CCl), 137.01 (brs, CP), 150.60 (s, CN), 152.51
(d, JCP = 14.0 Hz, CN); 31P NMR (162 MHz, C6D6, 60 °C): ¤
¹7.0 (brs); HRMS (FAB): m/z calcd for C24H17N2P35Cl2 [M]+
434.0506, found: 434.0477, calcd for C24H17N2P35Cl37Cl [M]+
436.0477, found: 436.0487.
CMe), 141.22 (s, CMe), 151.36 (s, CN), 152.54 (d, JCP
=
13.2 Hz, CN); 31P NMR (162 MHz, C6D6, 60 °C): ¤ ¹9.7 (s);
HRMS (FAB): m/z calcd for C26H23N2P [M]+ 394.1599,
found: 394.1581.
1
3
(E)-6e: H NMR (500 MHz, C6D6, r.t.): ¤ 6.94 (t, JHH
=
7.5 Hz, 1H), 6.99 (d, 3JHH = 8.0 Hz, 2H), 7.05-7.14 (m,
8H), 7.47 (td, 3JHH = 7.5 Hz, JHP = 2.0 Hz, 4H), 7.54 (d,
3JHH = 8.0 Hz, 2H), 7.88 (dd, 3JHH = 7.5 Hz,
JHP = 3.5
Hz, 1H); 13C{1H} NMR (126 MHz, C6D6, 60 °C): ¤ 117.95
(s, CH), 124.45 (s, CH), 128.55 (d, JCP = 8.7 Hz, CH),
128.90 (s, CH), 129.37 (s, CH), 130.55 (s, CH), 131.83
(E)-6j: H NMR (500 MHz, C6D6, r.t.): ¤ 3.16 (s, 3H), 6.64
1
3
3
(d, JHH = 8.0 Hz, 2H), 6.98 (t, JHH = 7.5 Hz, 1H), 7.04-7.11
3
(m, 6H), 7.14-7.17 (m, 2H), 7.53 (dd, JHH = 8.0 Hz, JHP
=
=
(s, CH), 132.52 (d, 1JCP = 9.6 Hz, CP), 133.73 (s, CH), 134.50
2.0 Hz, 4H), 7.87 (d, 3JHH = 8.0 Hz, 2H), 8.04 (dd, JHH
3
1
(d, JCP = 20.7 Hz, CH), 136.91 (s, CCl), 138.38 (d, JCP
=
8.0 Hz, JHP = 4.0 Hz, 1H); 31P NMR (162 MHz, C6D6, r.t.): ¤
¹9.4 (s); HRMS (FAB): m/z calcd for C25H21N2OP [M]+
396.1391, found: 369.1401.
9.3 Hz, CP), 151.35 (s, CN), 154.62 (d, JCP = 13.7 Hz,
CN); 31P NMR (162 MHz, C6D6, 60 °C): ¤ ¹9.0 (s); HRMS
(FAB): m/z calcd for C24H18N2PCl [M]+ 400.0896, found:
400.0892.
(E)-6f: H NMR (500 MHz, C6D6, 60 °C): ¤ 7.03-7.07 (m,
6H), 7.08-7.13 (m, 3H), 7.14-7.18 (m, 2H), 7.42 (td, JHH
8.0 Hz, JHP = 1.6 Hz, 4H), 7.66 (dd, 3JHH = 8.0 Hz, JHP
4.5 Hz, 1H), 7.73 (dd, 3JHH = 8.0 Hz, 4JHH = 1.5 Hz, 2H);
13C{1H} NMR (126 MHz, C6D6, 60 °C): ¤ 119.07 (s, CH),
123.15 (s, CH), 128.88 (d, JCP = 8.3 Hz, CH), 129.11 (s, CH),
129.32 (s, CH), 129.79 (s, CH), 130.62 (s, CH), 132.81 (d,
31P NMR Spectra of (E)-2-(Diphenylphosphino)azoben-
zenes (E)-6a-6j. The solutions of (E)-6 (0.10 M, 0.5 mL) in
several solvents were put into each NMR tube and they were
degassed and sealed. Their VT-31P NMR spectra were measured
using tributylphosphine (¤P = ¹31.8) as an external standard.
The equilibrium constants, K1 = [7]/[(E)-6] were calculated
from the integral ratios of (E)-6 and 7 in the VT-31P NMR
spectra using 45° pulses with non-decoupling mode and
sufficiently long relaxation time from 5 to 150 s, when the
ratio became constant.35 All the VT-31P NMR spectra using (E)-
6a are shown in the Supporting Information. When the signals
of (E)-6 and 7 were broadened, some signals due to impurities
were also observed, but these signals were negligible at
¹100 °C, suggesting no effect on the equilibration. A van’t
Hoff analysis was performed using K1 value below ¹60 °C
because the signals were broadened above the temperature. All
the van’t Hoff plots are shown in the Supporting Information.
All the data of the equilibrium constants are summarized in
Tables 1 and 2 in the text. A first-order kinetics for an exchange
reaction between (E)-6a and 7a was performed using dynamic
NMR for a two-site system between 40 and ¹60 °C. The rate
constants were obtained assuming that the chemical shifts of
(E)-6a and 7a did not change in the temperature range.
Reaction of (E)-2-(Diphenylphosphino)azobenzene ((E)-
6a) with Phenols. To a solution of (E)-6a (50 ¯mol) in THF
(0.5 mL) was added a phenol (50 ¯mol) and the VT-31P NMR of
the reaction mixture was measured.
1
3
=
=
J
CP = 2.5 Hz, CH), 134.32 (d, JCP = 20.6 Hz, CH), 135.46 (brs,
CP), 135.88 (s, CCl), 136.91 (d, 1JCP = 4.1 Hz, CP), 152.39 (s,
CN), 152.76 (d, JCP = 14.0 Hz, CN); 31P NMR (162 MHz,
C6D6, 60 °C): ¤ ¹7.3 (s); HRMS (FAB): m/z calcd for
C24H18N2PCl [M]+ 400.0896, found: 400.0887.
(E)-6g: 1H NMR (500 MHz, C6D6, r.t.): ¤ 1.92 (s, 3H), 6.96-
7.06 (m, 4H), 7.07-7.12 (m, 6H), 7.50-7.57 (m, 6H), 7.94 (dd,
3JHH = 8.0 Hz, JHP = 4.0 Hz, 1H); 13C{1H} NMR (126 MHz,
C6D6, r.t.): ¤ 21.48 (s, CH3), 117.78 (s, CH), 124.46 (s, CH),
128.75 (d, JCP = 7.5 Hz, CH), 128.84 (s, CH), 129.35 (s, CH),
130.33 (s, CH), 133.81 (d, JCP = 23.5 Hz, CP), 134.18 (s, CH),
1
134.50 (d, JCP = 20.6 Hz, CH), 136.69 (s, CCl), 138.57 (d,
1JCP = 10.7 Hz, CP), 141.71 (s, CMe), 151.22 (s, CN), 152.20
(d, JCP = 13.2 Hz, CN); 31P NMR (162 MHz, C6D6, r.t.): ¤ ¹9.5
(s); HRMS (FAB): m/z calcd for C25H20N2P [M]+ 414.1053,
found: 414.1016.
1
(E)-6h: H NMR (500 MHz, C6D6, 60 °C): ¤ 2.03 (s, 3H),
6.92 (d, 3JHH = 8.0 Hz, 2H), 7.03-7.11 (m, 6H), 7.15 (dd,
3JHH = 8.0 Hz, JHH = 1.5 Hz, 1H), 7.18 (dd, 3JHH = 8.0 Hz,
Isomerization of (E)-4,4¤-Dimethyl-2-(diphenylphos-
phino)azobenzene ((E)-6d). Benzene-d6 solution (25 mM,
0.5 mL) of (E)-6d was irradiated with a high-pressure mercury
lamp through a colored glass filter (- = 360 nm) for 1 h at
room temperature. In the VT-31P NMR spectra, a new sharp
singlet was observed at ¤P ¹16.0 which was assigned to (Z)-6d.
The E:Z ratios of the product for the photoisomerization were
determined based on the integral of the 1H NMR spectra of the
reaction solution after it reached the photostationary state. The
3
J
HP = 2.0 Hz, 1H), 7.44 (td, JHH = 8.0 Hz, JHP = 1.6 Hz, 4H),
3
3
7.70 (d, JHH = 8.0 Hz, 2H), 7.72 (dd, JHH = 8.0 Hz, JHP
4.0 Hz, 1H); 13C{1H} NMR (126 MHz, C6D6, 60 °C): ¤ 22.98
(s, CH3), 118.93 (s, CH), 123.35 (s, CH), 128.75 (d, JCP
7.5 Hz, CH), 129.02 (s, CH), 129.58 (s, CH), 129.76 (s, CH),
133.01 (s, CH), 134.32 (d, JCP = 20.7 Hz, CH), 135.48 (brs,
CP), 136.47 (s, CCl), 137.48 (d, 1JCP = 9.1 Hz, CP), 141.43 (s,
CMe), 150.98 (s, CN), 152.62 (d, JCP = 13.2 Hz, CN);
=
=

122
Bull. Chem. Soc. Jpn. Vol. 85, No. 1 (2012)
2-Diphenylphosphinoazobenzenes
Theoretical Calculations. All the calculations were
performed using Gaussian 03 program (Gaussian Inc., Pittsburg,
PA, 2003).31 Full geometry optimizations were performed at
the density functional theory (B3PW91)38 using basis sets
6-31G(d,p). The lowest excited states were calculated by
the time-dependent DFT39 (B3PW91) method using basis sets
6-311G+(d,p).
ratio of (E)-6d:(Z)-6d was determined to be 38:62. The reaction
solution of (Z)-6d was irradiated again with the lamp through
another colored glass filter (- = 445 nm) for 2 h. The ratio of
(E)-6d:(Z)-6d was determined to be 55:45. The reaction
solution was kept at 300 K in the dark, and ratio of (Z)-6d
was determined by 1H NMR spectroscopy to calculate the
rate constant of thermal isomerization from (Z)-6d to (E)-6d
(5.3 © 10¹5 s¹1).
Reaction of (Z)-4,4¤-Dimethyl-2-(diphenylphosphino)azo-
benzene ((Z)-6d) with Perchlorophenol. To a benzene-d6
solution (25 mM, 0.5 mL) of the 38:62 mixture of (E)-6d and
(Z)-6d in an NMR tube was added perchlorophenol (3.5 mg,
13 ¯mol). The reaction solution was kept at 300 K in the
We thank Tosoh Finechem Corp. for gifts of alkyllithiums.
This work was partially supported by Grants-in-Aid for Global
COE Program, Chemistry Innovation and for Scientific
Research Nos. 15105001 (T.K.), 16033215 (T.K.), 1611512
(M.Y.), 22108508 (N.K.), and 22685005 (N.K.) from Ministry
of Education, Culture, Sports, Science and Technology, Japan,
and Japan Society for Promotion of Science. NK thanks
Dainippon Ink and Chemical Inc. for financial support.
1
dark. Ratio of (Z)-6d to 7d¢H+ was determined by H NMR
spectroscopy to calculate the rate constant (7.9 © 10¹5 s¹1) of
the decay of (Z)-6d to form 7d¢H+ for 150 min. The similarly
prepared reaction solution of (Z)-6d with perchlorophenol was
irradiated with a high-pressure mercury lamp through a colored
glass filter (- = 445 nm) at 300 K to form quantitatively 7d¢H+
for 30 min. Addition of 4-(N,N-dimethylamino)pyridine to the
solution of 7d¢H+ gave (E)-6d immediately and quantitatively.
Reaction of (E)-2-(Diphenylphosphino)azobenzene ((E)-
Supporting Information
VT-NMR spectral data, details of kinetics studies, and
theoretical calculations. This material is available free of
charge on the Web at: http://www.csj.jp/journals/bcsj/.
6a) with Perchlorophenol.
To a THF solution (0.5 mL)
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analysis clearly shows larger negative charge on the carbon atom at
the ¡-position of the phosphorus atom, suggesting the contribution
of a ylide structure 7¤¤a. A detail is shown in the Supporting
Information.
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27 R. R. Holmes, Pentacoordinated Phosphorus: Structure
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