Thermal and photocontrol of the equilibrium between a 2-phosphinoazobenzene and an inner phosphonium salt

Article abstract of DOI:10.1021/ja054293d

The reversible conversion between a phosphine and a phosphonium salt has been achieved by external stimuli of light and heat. Two 2-phosphinoazobenzenes were successfully synthesized by desulfurization of the corresponding phosphine sulfides. One of the p

Full text of DOI:10.1021/ja054293d

Published on Web 08/06/2005
Thermal and Photocontrol of the Equilibrium between a
2-Phosphinoazobenzene and an Inner Phosphonium Salt
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 June 29, 2005; E-mail: takayuki@chem.s.u-tokyo.ac.jp
Tertiary phosphines usually serve as soft bases and play an
important role in catalytic reactions as ligands for transition metals.
Quaternary phosphonium salts are involved as intermediates in
many reactions, such as the Arbuzov reaction,1 the Mitsunobu
reaction,2 and reactions with the Appel reagent.3 Many phosphonium
salts have been synthesized by a nucleophilic attack of tertiary
phosphines on electrophiles, such as alkyl halides and activated
double bonds.4 If the reversible conversion between a phosphine
and a phosphonium salt is possible by external stimuli, such as
light and heat, the spectral properties and reactivity of each
phosphorus compound can be controlled without addition of an
external reagent. We previously reported the synthesis of 2-silyl-
azobenzene derivatives and the control of both the coordination
number of silicon and the reactivity of the compound by photoi-
somerization of the azo group.5 In these cases, an azo group and a
silicon atom operate as a nucleophile and an electrophile, respec-
tively. A phosphine bearing a double bond, such as an azo group,
in the vicinity of the phosphorus atom is expected to produce a
phosphonium salt generated from an azo group as an electrophile
and a phosphine as a nucleophile and to be controlled by
photoisomerization.6 We report here the synthesis of 2-phos-
phinoazobenzenes and their equilibrium with an inner phosphonium
salt, as well as the photocontrol of the character as a phosphonium
salt.
Figure 1. ORTEP drawing of (E)-3a with thermal ellipsoid plot (50%
probability). Selected bond lengths (Å) and bond angles (deg) for (E)-3a:
P1-C1, 1.844(2); P1-C2, 1.838(2); P1-C3, 1.842(2); N1-N2, 1.263(2);
C1-P1-C2, 102.44(7); C1-P1-C3, 100.30(7); C2-P1-C3, 101.43(8).
Successive treatment of 2-iodoazobenzenes7 (E)-1a and (E)-1b
with butyllithium (1 equiv) and diphenylthiophosphinoyl chloride
(1 equiv) in Et2O at -105 °C gave phosphine sulfides (E)-2a (89%)
and (E)-2b (61%), respectively (Scheme 1). Desulfurization of (E)-
Figure 2. VT 31P NMR spectra of (E)-3a and 4a in toluene-d8 solution.
-10.3 and 30.1) at -100 °C (Figure 2). The signal at high field
(δP -10.3), whose chemical shift is close to that of diphenyl[2-
(phenylimino)phenyl]phosphine11 (δP -13.0), is assigned to the
triarylphosphine (E)-3a. The signal at low field (δP 30.1) is
reasonably assigned to the inner phosphonium salt 4a, considering
that the chemical shift of 4a was calculated to be δP 27.2 by a
gauge-including atomic orbital calculation at the B3LYP/6-31G-
(d,p)//B3LYP/6-31G(d,p) level.12,13 The phosphonium salt 4a is
considered to be formed by intramolecular nucleophilic attack of
the phosphorus atom on the azo group in (E)-3a, and (E)-3a and
4a are in equilibrium in solution. A van’t Hoff plot for equilibrium
constants calculated from the ratio of 4a to (E)-3a at each
temperature gave a good linear relationship, proving an equilibrium
system between the two species (E)-3a and 4a.14 van’t Hoff plots
can be used to derive enthalpy and entropy changes for the [4a]/
[(E)-3a] equilibrium. The enthalpy change in toluene (∆H° ) -17.3
( 0.3 kJ/mol) and in THF (∆H° ) -16.2 ( 0.3 kJ/mol) are almost
the same, while the entropy change in toluene (∆S° ) -88 ( 2
J/mol‚K) is much more negative than that in THF (∆S° ) -76 (
1 J/mol‚K). The ratio of 4a to (E)-3a in toluene (equilibrium
constant: Keq ) 4.00 at -100 °C) is lower than that in THF (Keq
) 8.52 at -100 °C) because zwitterion 4a, which has a large dipole
moment, orders the nonpolar solvent, toluene, much more than the
polar solvent, THF.15
Scheme 1
2a and (E)-2b with tributylphosphine (1 equiv) in toluene at 100
°C in a sealed tube and successive recrystallization from toluene/
hexane gave red crystals of 2-(diphenylphosphino)azobenzenes (E)-
3a (79%) and (E)-3b (90%), respectively. X-ray crystallographic
analysis of triarylphosphine (E)-3a revealed that the bond angles
(100.30-102.44°) around the phosphorus atom of (E)-3a were
slightly smaller than those in triphenylphosphine (103°) (Figure
1).8,9 The NdN bond length (1.263(2) Å) is almost as long as that
in unsubstituted azobenzene (1.247 Å).10 These results show no
interaction between the phosphorus atom and the azo group in the
crystalline state.
In the 31P NMR spectra of (E)-3a in toluene-d8, only one singlet
(δP -9.2) was observed above 60 °C, while the signal was
broadened between 20 and -60 °C and split into two singlets (δP
9
11954
J. AM. CHEM. SOC. 2005, 127, 11954-11955
10.1021/ja054293d CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Thermochromism was observed in solution, reflecting the thermal
change of the equilibrium ratio. The toluene solution showed a red
color at 100 °C based on the azobenzene unit of (E)-3a and black
color at -78 °C, which was ascribed to the charge transfer excitation
of the zwitterion 4a. In the UV-vis spectra of (E)-3a, the absorption
maximum at 320 nm is assigned to the π f π* transition of an
azo group. The lower the temperature, the weaker the absorption
at λmax 320 nm and the stronger the absorptions at λmax 350 and
380 nm. The results of the UV-vis spectra are consistent with the
thermochromism.
on the azo group. (Z)-3b was thermally isomerized to (E)-3b with
a rate constant of 3.1 × 10-4 s-1 at 296 K in benzene-d6. Addition
of water to the mixture of (E)-3b and (Z)-3b in benzene-d6 resulted
in quantitative conversion of (E)-3b to hydrazine 7b via phospho-
nium salt 4b and no reaction of (Z)-3b after 1 min. (Z)-3b decayed
with a rate constant of 1.7 × 10-3 s-1 to give hydrazine 7b at 296
K. Considering the reaction rate, the conversion of (Z)-3b proceeded
via isomerization to (E)-3b. Photoisomerization of 3b causes
switching of the unique reactivity of phosphonium salt.
In summary, we have synthesized azobenzenes (E)-3a and (E)-
3b bearing a diphenylphosphino group at the 2-position, which are
in equilibrium with inner phosphonium salts 4a and 4b. The
equilibrium constant changed depending on the temperature and
the solvent, and thermochromism was observed in solution. (E)-3a
shows the usual reactivity of a triarylphosphine, while 4a was
hydrolyzed to give hydrazinophenylphosphine oxide 7a by a
mechanism similar to the Mitsunobu reaction. Photoisomerization
of (E)-3b to (Z)-3b changed the reactivity toward water. Such
phosphines in equilibrium with inner phosphonium salts are
expected to be useful for controlling organic reactions by taking
advantage of the photoisomerization of the azobenzene moiety.
Reaction of the mixture of (E)-3a and 4a with an excess amount
of elemental selenium gave phosphine selenide (E)-5 in good yield
(90%), and reaction with borane-dimethyl sulfide (1 equiv) gave
the corresponding phosphine borane (E)-6 (76%) (Scheme 2).
Scheme 2
Acknowledgment. We thank Tosoh Finechem Corp. for gifts
of alkyllithiums. This work was partially supported by Grants-in-
Aid for 21st Century COE Program for Frontiers in Fundamental
Chemistry and for Scientific Research Nos. 14740395 (N.K.),
15105001 (T.K.), 16033215 (T.K.), and 1611512 (M.Y.) from
Ministry of Education, Culture, Sports, Science and Technology,
Japan, and Japan Society for Promotion of Science, and Dainippon
Ink and Chemicals, Inc.
Phosphine (E)-3a shows the usual reactivity of a triarylphosphine,
although (E)-3a is in equilibrium with phosphonium salt 4a.
Conversely, the reaction with water gave phosphine oxide 7a
quantitatively. In the formation of 7a, the phosphine moiety of (E)-
3a was oxidized to a phosphine oxide, and the azo group of (E)-3a
was reduced to a hydrazine. These conversions show a similarity
between the present reaction and the Mitsunobu reaction, suggesting
that inner phosphonium salt 4a behaves as an intermediate in the
Mitsunobu reaction.2,16 Therefore, the reaction mechanism for
hydrolysis is described as follows. The negatively charged nitrogen
in phosphonium salt 4a that is in equilibrium with (E)-3a is
reversibly protonated, and subsequent substitution at phosphorus
of 8 with hydroxide ion gives amido anion 9, which undergoes
internal proton transfer from the hydroxyphosphonium group to
give hydrazine 7a.
Supporting Information Available: Synthetic procedure and
spectral data of (E)-2a, (E)-2b, (E)-3a, (E)-3b, (E)-5, (E)-6, 7a, and
7b, complete ref 12, and X-ray crystallographic file in CIF format for
(E)-3a. This material is available free of charge via the Internet at http://
pubs.acs.org.
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Although photoisomerization of (E)-3a did not proceed at all,
that of (E)-3b bearing a methyl group at the 4′-position of
azobenzene to produce (Z)-3b (27%) was achieved by irradiation
(λ ) 360 nm) in benzene-d6 for 1 h (Scheme 3). In the 31P NMR
spectra of (Z)-3b in toluene-d8, one singlet (δP -16.0) of (Z)-3b
was not broadened between 20 and -100 °C, in contrast to (E)-
3b.14 This result suggests that there is no equilibrium between the
phosphine and an inner phosphonium salt in (Z)-3b because the
geometry of (Z)-3b prohibits a nucleophilic attack of the phosphine
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b ) 10.218(4) Å, c ) 10.504(6) Å, â ) 92.803(2)°, V ) 1901(2) Å3, Z
) 4, MW ) 366.38, Dc ) 1.280 g‚cm-3, T ) 120 K, R1 (I > 2σ(I)) )
0.0363, wR2 (all data) ) 0.0889, GOF(F2) ) 1.062.
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Scheme 3
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