Synthesis of novel thermally reversible photochromic axially chiral spirooxazines

Article abstract of DOI:10.1021/ol1014152

(Equation Presented). The Suzuki reactions of diboronic acid 1 and bromo-spirooxazine 2, under N₂ atmosphere and aerobic conditions, gave the dispirooxazine-substituted binaphthyl product 3 and the monospirooxazine-substituted binaphthyl derivative 4, respectively. The thermally reversible photochromic behavior of the target axially chiral spirooxazines 3 was investigated, and both the ring-opening process upon irradiation with 365 nm light and the thermally reverse ring-closing process were fast. These chiral spirooxazines were found to impart their chirality to an achiral liquid crystal host, at low doping levels, to form a self-organized photoresponsive helical superstructure.

Full text of DOI:10.1021/ol1014152

ORGANIC
LETTERS
2
010
Vol. 12, No. 15
552-3555
Synthesis of Novel Thermally Reversible
Photochromic Axially Chiral
Spirooxazines
3
Li-Mei Jin, Yannian Li, Ji Ma, and Quan Li*
Liquid Crystal Institute, Kent State UniVersity, Kent, Ohio 44240
qli1@kent.edu
Received June 18, 2010
ABSTRACT
2
The Suzuki reactions of diboronic acid 1 and bromo-spirooxazine 2, under N atmosphere and aerobic conditions, gave the dispirooxazine-
substituted binaphthyl product 3 and the monospirooxazine-substituted binaphthyl derivative 4, respectively. The thermally reversible
photochromic behavior of the target axially chiral spirooxazines 3 was investigated, and both the ring-opening process upon irradiation with
365 nm light and the thermally reverse ring-closing process were fast. These chiral spirooxazines were found to impart their chirality to an
achiral liquid crystal host, at low doping levels, to form a self-organized photoresponsive helical superstructure.
Photochromic molecules that change color upon irradiation
with UV light have attracted a great deal of interest because
of their potential applications as smart light-driven molecular
switches and devices. Among all the photochromic mol-
ecules, spirooxazines are a particularly interesting family due
to their unique properties such as excellent photofatigue
resistance, strong photocoloration, and fast thermal relax-
self-developing photography, actinometry, displays, filters,
lenses of variable optical density, and photoswitchable
2
sensors.
It is also known that when a chiral molecule is doped in
an achiral nematic liquid crystal (LC), its molecular chirality
can be transferred to the nematic solvent to form a helical
superstructure, i.e., chiral nematic phase, that can reflect light
selectively according to Bragg’s law. The ability of a chiral
dopant to twist the nematic phase is defined as helical
1
ation. The colorless ring-closed spiro form of spirooxazine
can be transformed into the colored ring-opened merocyanine
form upon irradiation with UV light, whereas its reverse
process occurs thermally in the dark or photochemically by
irradiation with visible light. Since the physical and chemical
properties of the two forms are dramatically different, the
thermally reversible photochromic switching has been the
basis for the intelligent materials with applications in three-
dimensional optical memory, photochemical erasable memory,
-1
twisting power ꢀ according to the equation ꢀ ) (pc) , where
p is the pitch length of the helical structure and c is the chiral
dopant concentration. The isomerization of the spirooxazine
upon irradiation with light can be used to control the helical
(
2) (a) Berkovic, G.; Krongauz, V.; Weiss, V. Chem. ReV. 2000, 100,
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Phys. Chem. B 2006, 110, 13804–13811.
(
1) (a) Minkin, V. I. Chem. ReV. 2004, 104, 2751–2776. (b) Kobatake,
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1
10.1021/ol1014152  2010 American Chemical Society
Published on Web 07/15/2010

pitch and photochromic behavior, which would open the door
compound 3aa was obtained. Fortunately, when we opti-
mized the reaction conditions in which Pd(PPh was used
as the catalyst and THF/H O were used as the sovlents, it
3
to many applications. However, to date, there have been
3 4
)
4
only very few reports on chiral spirooxazines, compared
2
with numerous other chiral photoresponsive molecules such
was found that the reaction yield was dramatically increased
if a larger amount of the catalyst was used. For example, a
52% yield for 3aa was achieved when using 30 mol % of
5
6
as chiral azobenzenes, chiral dithienylethenes, chiral spiro-
7
8
pyrans, and chiral fulgides.
To tailor chiral spirooxazines with satisfactory function-
alities for the device performance, e.g., high helical twisting
power and fast reversible thermal relaxation, here we reported
the synthesis of novel axially chiral binaphthyl-based spiroox-
azines 3. The Suzuki reaction was used to construct the target
compound 3. It is established that the 2,2′-bridged binaphthyl
is a more powerful chiral building block compared to the
3 4
the Pd(PPh ) as the catalyst (Table 1, No. 1). On the basis
Table 1. Synthesis of the Dispirooxazine-Substituted Binaphthyl
a
Derivative 3
9
corresponding unbridged one. Thus, diboronic acid precursor
1
a was used to optimize the coupling reaction conditions.
1
0
First, the intermediate 1a was prepared starting from (R)-
binol with a slightly modified procedure according to the
literature with a total yield of 5%. The synthesis of bromo-
spirooxazine precursor 2a and 2b was commenced with the
corresponding 1-(4-bromophenyl)hydrazine in four steps with
a total yield of 40-50% (see Supporting Information). With
the intermediates in hand, the key Suzuki coupling reaction
was then investigated. First, according to a standard Suzuki
11
1
2
reaction condition, under N
a with 2a in a mixed solvent of THF and water at 70 °C,
using K CO as a base and 10 mol % of Pd(PPh as the
2
atmosphere, the treatment of
1
2
3
3 4
)
catalyst, gave the dispirooxazine binaphthyl product 3aa
surprisingly in only a 5% yield, whereas most starting
material 1a was recovered. When using other solvents, such
as THF, toluene, toluene/H
dioxane/H O, the yield of the reaction was still very low.
We also used Pd(OAc) /L where the L is the (2-biphenyl)di-
tert-butylphosphine as the catalyst. Unfortunately, no target
2 2 2
O, benzene/H O, DMF/H O, or
2
2
13
(
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3
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a
Reaction conditions: 0.5 equiv of diboronic acid (1), 1 equiv of aryl
bromide (2), 0.3 equiv of Pd(PPh
1/1, v/v), water (1 mL/mmol K CO
3 4 2 3
) , 10 equiv of K CO , THF-water
b
(
2
3
2
), 70 °C, N , 12 h. Isolated yield.
of the optimized reaction conditions, the other target chiral
spirooxazines 3 were readily prepared. The intermediates
1
4
15
(
6) (a) de Jong, J. J. D.; van Rijn, P.; Tiemersma-Wegeman, T. D.; Lucas,
1b, 1c, and 1d were synthesized starting from (R)- or
S)-binol in 40-65% yield. The reactions of the diboronic
L. N.; Browne, W. R.; Kellogg, R. M.; Uchida, K.; van Esch, J. H.; Feringa,
B. L. Tetrahedron 2008, 64, 8324–8335. (b) Kim, C.; Marshall, K. L.;
Wallace, J. U.; Chen, S. H. J. Mater. Chem 2008, 18, 5592–5598. (c) Irie,
M. Chem. ReV. 2000, 100, 1685–1716. (d) Maly, K. E.; Wand, M. D.;
Lemieux, R. P. J. Am. Chem. Soc. 2002, 124, 7898–7899.
(
acids 1b and 1a with the bromo-spirooxazine precursors 2a
and 2b went smoothly in the presence of 30 mol % of
Pd(PPh ) to give the target chiral spirooxazines 3ba (40%,
3 4
(
7) Zhou, Y. C.; Zhang, D. Q.; Zhang, Y. Z.; Tang, Y. L.; Zhu, D. B.
J. Org. Chem. 2005, 70, 6164–6170.
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Zimmermann, R. J. Org. Chem. 1986, 51, 589–592. (b) Gottarelli, G.; Hibert,
M.; Samori, B.; Solladi e´ , G.; Spada, G. P.; Zimmermann, R. J. Am. Chem.
Soc. 1983, 105, 7318–7321. (c) Rosini, C.; Rosati, I.; Spada, G. P. Chirality
Table 1, No. 2) and 3ab (50%, Table 1, No. 5), respectively.
The reactions of 1c and 1d with 2a gave the 3,3′-dispiroox-
azine-substituted products 3ca and 3da, respectively, with
the yield of 15% (Table 1, No. 3 and 4). The low yield might
result from the steric hindrance.
(
(
1
995, 7, 353–358.
Interestingly, under aerobic conditions, the monospiroox-
azine-substituted binaphthyl derivative 4 was found to be
(
10) Park, J.-W.; Ediger, M. D.; Green, M. M. J. Am. Chem. Soc. 2001,
1
23, 49–56.
(
(
(
11) D u¨ rr, H.; Ma, Y.; Cortellaro, G. Synthesis 1994, 294–298.
12) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457–2483.
13) Barder, T. E.; Walker, S. D.; Martinelli, J. R.; Buchwald, S. L.
(14) Germaneau, R.; Chavignon, R.; Tranchier, J.-P.; Rose-Munch, F.;
Rose, E.; Collot, M.; Duhayon, C. Organometallics 2007, 26, 6139–6149.
(15) Wipf, P.; Jung, J.-K. J. Org. Chem. 2000, 65, 6319–6337.
J. Am. Chem. Soc. 2005, 127, 4685–4696.
Org. Lett., Vol. 12, No. 15, 2010
3553

the main product. For instance, the reaction of 1a with 2a
without the degassing process gave the compound 4aa as
the major product (39%) together with the compound 3aa
as the minor product (18%) (Table 2, No. 1). Similar results
All the axially chiral spirooxazines exhibited the expected
thermally reversible photochromic behavior in both organic
solvent and LC host. Both the ring-opening and the ring-
closing processes are fast (less than 1 min), especially in
the high polar solvents such as THF, DCM, MeOH, and
acetonitrile at room temperature. The transmittance spec-
trometry was used to get the kinetics of the thermal
a
4,18
Table 2. Synthesis of Compound 4
relaxation.
For example, the transmittance changes of a
solution of 3aa in n-heptane were measured after reaching
the photostationary state (PSS) by irradiation with 365 nm
light (Figure 1a). As seen in the figure, the intensity of the
Figure 1. (a) Transmittance spectra changes of 3aa at 25 °C in
-5
n-heptane (6 × 10 M) followed by 365 nm irradiation for 30 s
to reach the PSS state in the dark (purple). The arrow indicates the
spectra changing tendency, and the spectra were recorded every
a
Reaction conditions: 0.5 equiv of boronic acid (1), 1 equiv of aryl
1
s. (b) Thermal relaxation monitored at 585 nm (λmax for the MC
bromide (2), 0.3 equiv of Pd(PPh
1/1, v/v), water (1 mL/mmol K CO ), 70 °C, air, 12 h. Isolated yield.
3 4 2 3
) , 10 equiv of K CO , THF-water
b
form in n-heptane) versus time. It takes about 15 s to thermally
(
2
3
relax back to the initial state (colorless).
were obtained for the reactions of other diboronic acids with
the bromo-spirooxazine (Table 2).
The formation of those spirooxazine binaphthyl derivatives
peak around 585 nm [λmax for the merocyanine (MC) form]
was gradually decreasing by the observation of gradual
increase of the transmittance due to the color-bleaching. By
using these data, the thermal relaxation at 585 nm versus
time is reproduced in Figure 1b. As seen in Figure 1b, the
thermal relaxation process of 3aa in n-heptane is fast. It takes
less than 20 s to be thermally back to the initial state. After
the transmittance was transferred into the absorbance by
using the equation A ) -ln(T), where A is the absorbance
and T is the transmittance, the resultant smooth curve is well
fitted by the monoexponential equation (A ) B exp(-kt) +
3
and 4 is straightforward. In the absence of oxygen, the
reaction of the two boronic acid function groups with the
bromo-spirooxazine under two consecutive Suzuki reaction
processes (oxidative addition of the bromo-spirooxazine with
palladium(0), transmetalation with the boronic acid, then
reductive elimination of the diarylpalladium complex) gave
the dispirooxazine-substituted binaphthyl derivatives 3. On
the other hand, in the presence of oxygen, the oxidation
of the boronic acid to the hydroxyl (-OH) became a
-1
C) with a rate constant of 0.24 s (see Supporting Informa-
1
6
competitive reaction. So, one of the boronic acid groups
in the starting material underwent oxidation, while another
boronic acid group reacted with the bromo-spirooxazine
to give the monospirooxazine-substituted binaphthyl de-
rivative 4.
tion). Representative thermal relaxation rate constants of the
spirooxazine-substituted binaphthyl compounds and the start-
ing bromo-spirooxazines measured in n-heptane and dichlo-
romethane are summarized in the Table 3. As seen in Table
3
, the thermal back reaction of spirooxazines upon irradiation
with UV light is greatly dependent on the medium polarity.
For example, the thermal back time of 3aa in CH Cl is 2 s,
Noticeably, since all the reactions involving the binathphyl
moieties were performed at a temperature below 100 °C,
the loss of the enantiomeric excess of the products was
2
2
whereas it is 15 s in n-heptane. Also, it clearly shows that
the steric and electronic effects dramatically influence the
rates of the ring-closing reaction. The steric repulsion
1
0,17
negligible.
All the structures of those chiral spirooxazines
1
13
are confirmed by H, C NMR, HR-MS, UV-vis, circular
dichroism (CD), as well as elementary analysis (see Sup-
porting Information).
(18) Schaudel, B.; Guermeur, C.; Sanchez, C.; Nakatani, K.; Delaire,
J. A. J. Mater. Chem. 1997, 7, 61–65
19) Photochromism: Molecules and Systems; D u¨ rr, H., Bouas-Laurent,
T. H., Eds.; Elsevier: Amsterdam, 1990
.
(
(
16) Chaicharoenwimolkul, L.; Munmai, A.; Chairam, S.; Tewasekson,
U.; Sapudom, S.; Lakliang, Y.; Somsook, E. Tetrahedron Lett. 2008, 49,
299–7302.
17) (a) Pu, L. Chem. ReV. 1998, 98, 2405–2494. (b) Wang, C.; Zhang,
D.; Zhang, G.; Xiang, J.; Zhu, D. Chem.sEur. J. 2008, 14, 5680–5686
.
(20) (a) Khairutdinov, R. F.; Giertz, K.; Hurst, J. K.; Voloshina, E. N.;
Voloshin, N. A.; Minkin, V. I. J. Am. Chem. Soc. 1998, 120, 12707–12713.
(b) Favaro, G.; Masetti, F.; Mazzucato, U.; Ottavi, G.; Allegrini, P.;
7
(
.
Malatesta, V. J. Chem. Soc., Faraday Trans. 1994, 90, 333–338
.
3554
Org. Lett., Vol. 12, No. 15, 2010

Table 3. Thermal Relaxation Rate Constants and Time for the
Spirooxazines (25 °C)
Table 4. Helical Twisting Powers (ꢀ) of the Chiral
Spirooxazines as Dopants in E7 for the Initial State and the
Photostationary State (PSS) upon Irradiation with 365 nm Light
a
-1
b
k /s
compd in n-heptane in CH
time /s
a
and the Stationary State (SS) after Thermal Relaxation
2
Cl
2
2 2
in n-heptane in CH Cl
ꢀ
M
(µm-1)
2
2
3
3
3
3
a
b
aa
ab
ba
ca
a
0.20
0.04
0.24
0.05
0.27
0.42
0.87
27
96
15
76
10
11
5
22
2
6
3
dopant
initial state
PSS365 nm
SSthermal relaxation
0.18
2.11
0.39
1.74
1.69
3aa
3ab
3ba
3ca
a
87
64
60
47
94
84
48
41
87
64
60
47
3
The data were obtained by transmittance spectra measurement fitting
ꢀ ) 1/(p·c·ee), where p is the pitch of the cholecteric phase, c is the
molar fraction of the dopant in E7, and ee is the enantionmeric excess.
Here the ee is considered as 1.
b
by using the monoexponential equation [A ) B exp(-kt) + C]. The
thermally back to the initial state in the dark after reaching the PSS by the
irradiation with 365 nm light.
chiral dispirooxazines, the bridged binaphthylene derivative
3aa exhibited the largest helical twisting power in E7.
In conclusion, a series of spirooxazines containing an
axially chiral binaphthalene moiety, for the first of their kind,
were synthesized and characterized. The Suzuki coupling
favoring formation of the spiropyran form has been re-
1
9
ported. Electronic effects are usually interpreted in terms
of their influence upon the extent of charge separation in
20
the MC form. The spirophenanthoxazines with three fused
benzene rings are expected to be more efficient to stabilize
the MC form, compared to those spironaphthoxazines which
only have two fused benzene rings. Thus, in these environ-
ments, spirophenanthoxazine 3ab caused rate constant (k)
to decrease about 5-fold, compared to the compound 3aa.
2
reactions under N atmosphere and aerobic conditions gave
the dispirooxazine-substituted binaphthyl product and the
monospirooxazine-substituted binaphthyl derivative, respec-
tively. Both the ring-opening process upon irradiation with
UV light and the thermally reversible ring-closing process
were fast. The thermal relaxation rate is strongly influenced
by the medium polarity as well as the structure of the
spirooxazine. Furthermore, these axially chiral spirooxazines
were found to impart their chirality to an achiral LC host, at
low doping levels, to form a self-organized optically tunable
helical superstructure. This bifunctional system is promising
for future application because the system exhibits excellent
thermally reversible photochromic behavior and chiral induc-
tion capability in liquid crystal hosts.
As expected, doping the chiral spirooxazine 3 in an achiral
LC host can induce the chiral nematic (N*) phase with a
characteristic oily streak texture (see Supporting Informa-
tion). Their helical twisting powers (ꢀ) were measured in a
wedge cell by pitch determination. Some typical results are
given in the Table 4. The bridged chiral spirooxazines
exhibited higher helical twisting power than the correspond-
ing unbridged ones. Interestingly, as for compounds 3aa and
3
ab, which possess two spirooxazine units in the bridged
binaphthyl moiety, the helical twisting power became larger
upon the irradiation with UV light (365 nm), while for other
chiral spirooxazines, the helical twisting power became
smaller under the same condition. This is probably because
of the more rodlike structure in the MC form for the
compounds 3aa and 3ab, compared to that for others.
The fact that the dihedral angle between the two naphthalene
moieties of the bridged compound is smaller than that in
the corresponding unbridged compounds might be one of
the reasons for the formation of the more rodlike structure
in the MC form. It is worth mentioning that, among those
Acknowledgment. The work is supported by the Air Force
Office of Scientific Research (FA9550-09-1-0193 and FA9550-
9-1-254) and the National Science Foundation (IIP
750379).
0
0
Supporting Information Available: Details of the syn-
1
13
thesis, characterization data, copies of H and C NMR
spectra, measurement of the thermal relaxation rate, and
measurement of HTP. This material is available free of
charge via the Internet at http://pubs.acs.org.
OL1014152
Org. Lett., Vol. 12, No. 15, 2010
3555