Azobenzene-tethered bis(trityl alcohol) as a photoswitchable cooperative acid catalyst for Morita-Baylis-Hillman reactions

Article abstract of DOI:10.1002/chem.201201383

Incorporation of an azobenzene core into tethered bis(trityl alcohol) allows the photoswitchable arrangement of the two trityl alcohol units through photoisomerization of azobenzene. The differently arranged trityl alcohol units change their cooperative function to reflect the positional relationships, and thus, the activity as a cooperative acid can be controlled by light stimuli (see figure). Copyright

Full text of DOI:10.1002/chem.201201383

COMMUNICATION
DOI: 10.1002/chem.201201383
Azobenzene-Tethered Bis(Trityl Alcohol) as a Photoswitchable Cooperative
Acid Catalyst for Morita–Baylis–Hillman Reactions
Tatsushi Imahori,*[a] Ryo Yamaguchi,[b] and Seiji Kurihara[b]
Manipulation of functions at the molecular level by exter-
nal stimuli is a key controlling mechanism in nature, as ex-
emplified by the allosteric control of enzymes. To realize ad-
vanced functions in man-made materials, various functional
molecules whose functions can be controlled and operated
by suitable stimuli have been developed.[1,2] Among them,
photoswitchable catalysts are very attractive.[3] As well as
being potentially clean, light can readily provide diverse
non-invasive stimuli by selecting the wavelength, and this
allows precise control of catalyst reactivities by light stimu-
li.[3] In the reactions promoted by the photoactivated cata-
lysts, light stimuli are amplified in the catalytic cycle, and
clean light energy is efficiently translated into chemical ac-
tions.[4] Photoswitchable catalysts are considered to hold
great promise for realizing various advanced functions, such
as a multi-catalyst system,[5] on account of their switching
ability.[6] Despite such attractive features, however, relatively
few developments have been made in the area of photo-
switchable catalysts.[3] Although a few general photoswitcha-
ble catalysts have been reported in recent years,[7] the over-
all scope remains small. Herein, we report on a new photo-
switchable acid catalyst, azobenzene-tethered bis(trityl alco-
hol).[8] This catalyst switches its catalytic activity as a cooper-
ative acid based on the photoinduced dynamic molecular
motion like a molecular machine.[1] A stimuli-responsive
and highly efficient Morita–Baylis–Hillman (MBH) reaction
has been demonstrated by using this photomechanical, new
cooperative acid catalyst.
Manipulating this cooperative function by using light stimuli
can directly induce photoresponsivity. Then, we envisaged
the incorporation of a photoresponsive core, which provides
reversible dynamic molecular motion by photoisomeriza-
tion, into the bifunctional catalyst (Figure 1). Because the
Figure 1. Concept of photoswitchable cooperative catalyst on the basis of
photoinduced reversible dynamic molecular motion.
positional relationships of the constituent functional units
greatly affect the cooperative function of a bifunctional cat-
alyst,[9] reversible photoisomerization of the photoresponsive
core could be used to switch the catalyst reactivity by chang-
ing the spatial arrangement of the functional units.
When we started to develop such a photomechanical co-
operative catalyst, only a few related catalysts had been re-
ported.[10] These previously reported catalysts[10a,b] are cate-
gorized as photoswitchable templates that perform the pho-
tomechanical cooperative function based on host–guest
chemistry.[3] Thus, the scope of the applicable transforma-
tions was limited. We designed an azobenzene-tethered bis-
(trityl alcohol) 1 (Figure 2). Based on our ongoing studies of
To develop useful and widely applicable photoswitchable
catalysts, we focused our attention on bifunctional catalysts,
whose activity is attributed to the cooperative function of
two functional units. Bifunctional catalysts are an important
class of catalyst in organic synthesis because they can dem-
onstrate advanced catalyses on account of their multifunc-
tionality.[9] The cooperative function mechanism of a bifunc-
tional catalyst appears suitable for photoswitchable catalysts.
[a] Prof. Dr. T. Imahori
Priority Organization for Innovation and Excellence
Kumamoto University (Japan)
E-mail: imahori@kumamoto-u.ac.jp
[b] R. Yamaguchi, Prof. Dr. S. Kurihara
Department of Applied Chemistry and Biochemistry
and Graduate School of Science and Technology
Kumamoto University, 2-39-1 Kurokami, Chuo-ku
Kumamoto 860-8555 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201383.
Figure 2. Design of the photoswitchable cooperative acid catalyst.
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
1
&
ÞÞ
These are not the final page numbers!

azobenzene-based functional materials,[11] we selected azo-
benzene as the photoresponsive core. The distinct, reversible
structural changes in the photoisomerization of azobenzene
(trans/cis) should be beneficial for switching the arrange-
ment of functional units as the core.[12] Bis(trityl alcohol),
which can function as a cooperative acid catalyst (hydrogen-
bonding catalyst),[13] was chosen as the functional units. We
assumed that the cis-azobenzene form of 1 would locate its
two acidic trityl alcohol units in proximity to each other in
order to avoid the steric hindrance of substituent
X
(Figure 2), and then, their cooperative function, such as an
intramolecular hydrogen bonding between the two hydroxyl
groups to enhance the acidity of one trityl alcohol unit[13,14]
or a certain proximity effect, could be induced. On the
other hand, the separately placed two trityl alcohol units in
trans-1 could not induce the cooperative function. This pho-
toswitchable cooperative function would demonstrate a pho-
toswitchable catalyst activity. Very recently, Wang and Fer-
inga reported an excellent photoswitchable bifunctional cat-
alyst with a similar design concept that, remarkably, switches
the stereochemistry, too, via thermal isomerization.[7b]
The designed catalyst 1a (X=Me, Ar=Ph) was synthe-
sized from commercially available methyl 3-amino-4-methyl-
benzoate (Scheme 1). The azobenzene core was first con-
structed according to the reported procedure via the forma-
Figure 3. Changes in UV/Vis absorption spectra of 1a upon photoirradia-
tion in CHCl3.
Scheme 1. Synthesis of azobenzene-tethered bis(trityl alcohol) 1a.
tion of diazonium salt to afford diester 2.[15] Then, two
equivalents of PhMgBr were added to each methoxy car-
bonyl group to afford the desired bis(trityl alcohol) 1a. The
photoresponsive structural changes in 1a were then evaluat-
ed through spectral studies. The changes in the UV/Vis ab-
sorption spectra of 1a upon photoirradiation are shown in
Figure 3 (a: UV light irradiation on the solution of 1a
(trans-1a/cis-1a=93:7 (1H NMR) in CHCl3), b: subsequent
visible light irradiation).[16] The photoresponsivity in the ab-
sorption spectra indicates reversible photoisomerization of
the azobenzene core of 1a between the trans- and cis-
forms.[16,17]
The transition of 1a was investigated in detail by
1H NMR spectroscopy.[16] The irradiation of UV light
(365 nm, 30 mWcm2) on the solution of 1a (trans/cis=85:15
(1H NMR), 1.0ꢁ10À1 m in [D8]THF) induced a characteristic
upfield shift of a doublet peak with a small coupling con-
Figure 4. Changes in 1H NMR spectrum upon photoisomerization in
[D8]THF and proposed structure of cis-azobenzene core of cis-1a.
cause the overlapped protons in the helical structure of cis-
azobenzene are shown at higher field than normal aromatic
protons in the 1H NMR spectra owning to the shielding
effect of the overlapping benzene ring,[7a,18] the observed up-
field shift is attributed to photoisomerization of trans-1a to
the cis-isomer which is overlapped at the 6- and 6’-positions
(Figure 4). This spectrum of cis-1a supports the expected
structure in which the two trityl alcohol units are located
inside the cis-azobenzene nucleus (Figure 2). Other major
isomers of cis-1a were not observed in the NMR spectrum.
The composition of the cis-isomer increased with the irradi-
ation of UV light and reached trans/cis=5:95 at the photo-
stationary state (after irradiation for 80 min). Then, the irra-
diation of visible light (>420 nm, 361 mWcm2) on the solu-
!
stant (Figure 4, ). This signal is assigned as the 6- and 6’-
position protons of the azobenzene core (Scheme 1). Be-
&
2
&
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Photoswitchable Acid Catalysts
COMMUNICATION
tion led to a reversal, and the trans-isomer recovered to
trans/cis=48:52 (saturated after irradiation for 60 min). The
photoinduced reversible dynamic motion of 1a was support-
ed by these spectral analyses.[16] Additionally, the isomeriza-
tion of cis-1a to the trans-isomer can be completed (trans/
cis= >99:<1) via thermal isomerization (1508C, 24 h).[16]
Having already investigated the photoresponsive reversi-
ble dynamic motion of the azobenzene-tethered bis(trityl al-
cohol) 1a, we next investigated its ability as a photoswitcha-
ble cooperative acid catalyst. We first tried hetero Diels–
Alder reaction that the related biaryl-tethered bis(trityl al-
cohol) catalysts can promote,[13] but neither cis- nor trans-1a
notably accelerated the reaction. Instead, it was interestingly
found that a catalytic amount of 1a can promote the phos-
phine-catalyzed Morita–Baylis–Hillman (MBH) reaction in
response to light stimuli (Figure 5). A trityl alcohol-cata-
the MBH reaction. The reaction with 20 mol% of quasi-cis-
1a (trans/cis=5:95 (1H NMR)) generated by UV-irradiation
in THF provided product 5 in 78% yield.[22] The result of
extrapolation to 20 mol% of pure cis-1a indicated that, if
pure cis-1a is used, the yield would increase to 81%.[23]
A
clear photoswitchable catalyst activity was demonstrated.
To evaluate the proposed photoswitchable cooperative
function of the two trityl alcohol units in 1a, we then inves-
tigated the catalyst activity of trityl alcohol (6), the function-
al unit of 1a (Figure 5). The reaction with 40 mol% of 6,
the same amount of trityl alcohol units as in 1a, afforded 5
in 36% yield. This result suggests that the catalyst acitivity
of trans-1a (37%) is probably attributable to the independ-
ent function of the each trityl alcohol unit. Thus, the differ-
ence between the catalyst activities of the cis- and the trans-
isomers of 1a in the MBH reaction arises from additional
acceleration effect(s) in cis-1a associated with the structure.
A certain cooperative function of the two proximal trityl al-
cohol units in the cis-azobenzene form would provide the
potent acceleration.[24]
The photoswitchable catalyst was modified by changing
the aryl groups of the trityl alcohol units (Table 1, Figure 6).
Azobenzene-tethered bis(trityl alcohol) derivatives 1b
Figure 6. Structures of azobenzene-tethered bis(trityl alcohol) catalysts
(Ar=1-naphthyl) and 1c (Ar=4-fluoro-3,5-dimethylphenyl)
were prepared by adding appropriate aryl-metal reagents to
diester 2.[16] For 1b, bulky 1-naphthyl group was selected to
fix the two functional hydroxyl groups in the active cis-azo-
benzene form.[25] We expected that the fixation of the hy-
droxyl groups at suitable positions would induce potent cat-
alyst activity. Furthermore, 1c was designed to increase the
acidity of the functional hydroxyl groups in order to en-
hance the cooperative function. An electron-withdrawing
Figure 5. Photoswitchable catalyst activity of 1a in MBH reaction.
lyzed MBH reaction has never been reported previously.[19]
Among the reported acid-catalyzed MBH reactions,[20] we
chose the reaction of 2-cyclopenten-1-one (3) and 3-phenyl-
propanal (4) with 20 mol% of PnBu3 in THF at 208C for 2 h
as the standard reaction condi-
tions to evaluate the photores-
Table 1. Evaluation of photoreswitchable catalyst activities of 1a–c.
ponsive catalyst activity of 1a.
The reaction without 1a, that is,
the background reaction pro-
ceeded in 26.5% yield. With
20 mol% of trans-1a (trans/
cis= >99:<1 (1H NMR)) pre-
pared by the photo(visible
light)- and thermal isomeriza-
tions, the reaction was acceler-
ated slightly (37% yield), but
good catalytic activity was not
obtained.[21] In contrast, cis-1a
[b]
Catalyst
[mol%, trans/cis]
Yield of 5
[%]
Acceleration
[%]
TONA[a]
TONArel
ACHTUNGTRENNUNG(ON/OFF)
none
26.5
26.8
51
52.3
31
52
55.7
53
–
0.3
–
–
–
trans-1a (5, >99:<1)
quasi-cis-1a (5, 5:95)
cis-1a (5, 0:100)[c]
trans-1b (5, >99:1)
quasi-cis-1b (5, 15:85)
cis-1a (5, 0:100)[c]
quasi-trans-1c (3, >99:>1)
quasi-cis-1c (3, 15:85)
0.06
4.9
5.15
0.9
5.1
5.8
8.8
11
24.5
25.8
4.5
25.5
29.2
26.5
34.5
82
85.9
–
5.7
6.5
–
61
1.3
[a] TONA=Turnover number of the catalyst for the acceleration. These values were calculated from following
equations: TONA=acceleration (%)/catalyst loading (mol%). [b] TONArel (ON/OFF)=TONAcis/TONAtrans
showed distinct acceleration of [c] Extrapolated to pure cis-catalyst.[23]
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
3
&
ÞÞ
These are not the final page numbers!

T. Imahori et al.
fluoro group was introduced to the aryl groups at the 4-posi-
tions for this purpose; in addition, two methyl groups were
introduced at the 3,5-positions to lend bulkiness for fixation.
The activities of these catalysts in the MBH reaction are
summarized in Table 1. To accurately evaluate the switching
abilities of the catalysts (1a–c), the reactions were per-
formed with low loading of each catalyst under the standard
conditions, and the results at an intermediate stage of the re-
action were investigated.[26] An approximately 86-fold differ-
ence in catalyst activity is expected between the ON- and
the OFF-states of 1a on the basis of each turnover number
of the catalyst for the accelerations (TONA).[23] The 1-naph-
thyl-containing catalyst 1b showed worse switching ability
(ca. 6.5-fold) although the catalyst activity of the ON-state is
slightly better than that of 1a.[23] The catalyst 1c induced
greater accelerations of the MBH reaction. However, the
switching ability was low because of the unexpectedly high
catalyst activity of the OFF-state trans-isomer.[27] Conse-
quently, 1a achieved the best photoswitching ability.
Although the detailed photoswitchable cooperative func-
tion of the two trityl alcohol units in azobenzene-tethered
bis(trityl alcohol) catalysts remains unclear at this stage of
our investigations, some insights have been obtained. We
first assumed the intramolecular hydrogen-bonding between
the two hydroxyl groups to enhance the acidity of one trityl
alcohol unit as the cooperative function,[14,28] which was pro-
posed in the related biaryl-tethered bis(trityl alcohol) cata-
lyst for hetero Diels–Alder reaction.[13] However, cis-1a did
not function as a catalyst for the hetero Diels–Alder reac-
tion. To estimate effect of the intramolecular hydrogen-
bonding in azobenene-tethered bis(trityl alcohol) catalysts,
we investigated catalyst activities of hydrogen-bonding bis(-
trityl alcohol)s in the MBH reaction (Scheme 2). We chose
bis(trityl alcohol) derivatives 7 and 8 as the hydrogen-bond-
function of the proximal two trityl alcohol units should exist
in the cis-azobenene-tethered bis(trityl alcohol) catalysts.
Further mechanistic studies to elucidate the photoswitchable
catalyst activity of this new cooperative acid catalyst in
greater detail are currently underway.
Finally, we evaluated potential of the azobenzene-teth-
ered bis(trityl alcohol) catalysts by comparing the catalyst
activity with that of a representative organo acid catalyst for
MBH reaction. We chose (S)-1,1’-bi(2-naphthol) ((S)-
BINOL) as the standard of comparison.[20] When (S)-
BINOL (20 mol%) was used under the standard conditions,
the MBH reaction proceeded in 61% yield (average of two
trials, Scheme 2). Thus, cis-1a has more superior catalyst ac-
tivity (81%) than (S)-BINOL in the MBH reaction. A
highly reactive novel organo acid catalyst for the MBH reac-
tion has been developed by present photomechanical coop-
erative system.[19]
In summary, we have developed azobenzene-tethered
bis(trityl alcohol) as a photoswitchable cooperative acid cat-
alyst, which demonstrated photoswitchable catalyst activity
in the MBH reaction.[8] The switching ability is based on the
photoinduced reversible dynamic molecular motion of cata-
lyst associated with the photoisomerization of the azoben-
zene core, and the differently arranged two trityl alcohol
units on the resulting isomeric catalysts would switch the
catalyst activity by changing their cooperative function.
Some insights into the photoswitchable catalyst activity sug-
gest a different cooperative function of the two trityl alcohol
units than that of the related biaryl-tethered bis(trityl alco-
hol) catalysts.[13] Further modification and expansion of the
scope of this photoswitchable, new cooperative acid catalyst
as well as detailed mechanistic studies are currently under-
way. Furthermore, it should be noted that this new coopera-
tive acid catalyst has demonstrated a new catalysis of MBH
reaction and improved the efficiency. We believe that dy-
namic photomechanical catalysts can potentially demon-
strate new reactivities, new efficiencies, and new reaction
controls on the basis of the specific reaction field created by
the photoinduced dynamic molecular motion. Our future
studies will focus on developing smart chemical transforma-
tions using dynamic photomechanical catalysts, which open
up new possibilities in organic synthesis.[5]
Scheme 2. Effect of the intramolecular hydrogen-bonding in bis(trityl al-
cohol) catalysts and comparison with (S)-BINOL.
Acknowledgements
This work was supported by the Program of Institutional Reforms for
Creating a Personnel Support System for Creative Young Researchers
from Kumamoto University. This work was performed under the Cooper-
ative Research Program: Network Joint Research Center for Materials
and Devices (Institute for Materials Chemistry and Engineering, Kyushu
University). We thank Prof. Ryo Irie (Kumamoto University), Prof. Ma-
sanobu Uchiyama (The University of Tokyo and RIKEN), Dr. Kazuto
Takaishi (RIKEN), and Prof. Takuji Hatakeyama (Kyoto University) for
their valuable comments and suggestions.
ing catalysts, in which or in whose close derivative the intra-
molecular hydrogen-bonding was observed.[29,30] The MBH
reaction with 20 mol% of 7 proceeded in 47% yield and
a small acceleration (11%) was observed from the reaction
with 40 mol% of trityl alcohol (6, 36% yield). But, 8
showed almost same catalyst activity as 6. These results sug-
gest that the intramolecular hydrogen bonding in bis(trityl
alcohol) catalysts is not enough to gain potent catalyst activ-
ity in the MBH reaction like cis-1a. Other main cooperative
Keywords: acid catalysts · azo compounds · cooperative
catalysts · Morita–Baylis–Hillman reaction · photochemistry
&
4
&
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Photoswitchable Acid Catalysts
COMMUNICATION
Mandolini, J. Am. Chem. Soc. 2003, 125, 2224–2227. Although de-
tails of the catalyst activity are unclear, an azobenzene-tethered bis-
copper complex with a similar cooperative function mechanism has
been reported for photoswitchable DNA cleavage, see: c) H. Pra-
kash, A. Shodai, H. Yasui, H. Sakurai, S. Hirota, Inorg. Chem. 2008,
47, 5045–5047. In addition, a photoswitchable cooperative catalyst
that switches enantioselectivity has been developed, see: d) D. Sud,
T. B. Norsten, N. R. Branda, Angew. Chem. 2005, 117, 2055–2057;
Angew. Chem. Int. Ed. 2005, 44, 2019–2021.
[1] a) Molecular Switches (Ed.: B. L. Feringa), Wiley-VCH, Weinheim,
2001; b) V. Balzani, M. Venturi, A. Credi, Molecular Devices and
Machines, Wiley-VCH, Weinheim, 2008; c) E. R. Kay, D. A. Leigh,
F. Zerbetto, Angew. Chem. 2007, 119, 72–196; Angew. Chem. Int.
Ed. 2007, 46, 72–191; d) O. Sato, J. Tao, Y.-Z. Zhang, Angew. Chem.
2007, 119, 2200–2236; Angew. Chem. Int. Ed. 2007, 46, 2152–2187;
e) A. P. R. Johnston, G. K. Such, F. Caruso, Angew. Chem. 2010, 122,
2723–2725; Angew. Chem. Int. Ed. 2010, 49, 2664–2666; f) Special
Issue: “Photochromism” M. Irie, Chem. Rev. 2000, 100, 1683–1890;
g) I. Willner, S. Rubin, Angew. Chem. 1996, 108, 419–439; Angew.
Chem. Int. Ed. Engl. 1996, 35, 367–385; h) G. Mayer, A. Heckel,
Angew. Chem. 2006, 118, 5020–5042; Angew. Chem. Int. Ed. 2006,
45, 4900–4921; i) P. Gorostiza, E. Y. Isacoff, Science 2008, 322, 395–
399; j) K. Kinbara, T. Muraoka, T. Aida, Org. Biomol. Chem. 2008,
6, 1871–1876.
[11] a) A. Kausar, H. Nagano, Y. Kuwahara, T. Ogata, S. Kurihara,
Chem. Eur. J. 2011, 17, 508–515; b) A. Kausar, H. Nagano, T.
Ogata, T. Nonaka, S. Kurihara, Angew. Chem. 2009, 121, 2178–
2181; Angew. Chem. Int. Ed. 2009, 48, 2144–2147; c) Md. Z. Alam,
T. Yoshioka, T. Ogata, T. Nonaka, S. Kurihara, Chem. Eur. J. 2007,
13, 2641–2647; d) T. Yoshioka, T. Ogata, T. Nonaka, M. Moritsugu,
S.-N. Kim, S. Kurihara, Ad. Mater. 2005, 17, 1226–1229.
[2] For selected recent reports of stimuli-responsive functional materi-
als, see: a) H. Maeda, Y. Bando, K. Shimomura, I. Yamada, M.
Naito, K. Nobusawa, H. Tsumatori, T. Kawai, J. Am. Chem. Soc.
2011, 133, 9266–9269; b) J. Wang, L. Hou, W. R. Browne, B. L. Fer-
inga, J. Am. Chem. Soc. 2011, 133, 8162–8164; c) E. Kim, D. Kim,
H. Jung, J. Lee, S. Paul, N. Selvapalam, Y. Yang, N. Lim, C. G. Park,
K. Kim, Angew. Chem. 2010, 122, 4507–4510; Angew. Chem. Int.
Ed. 2010, 49, 4405–4408; d) H. J. Yoon, J. Kuwabara, J.-H. Kim,
C. A. Mirkin, Science 2010, 330, 66–69; e) H. Komatsu, S. Matsumo-
to, S. Tamaru, K. Kaneko, M. Ikeda, I. Hamachi, J. Am. Chem. Soc.
2009, 131, 5580–5585; f) V. Blanco, A. Carlone, K. D. Hanni, D. A.
Leigh, B. Lewandowski, Angew. Chem. 2012, 124, 5256–5259;
Angew. Chem. Int. Ed. 2012, 51, 5166–5169.
[12] The related photoswitchable templates also use azobenzene as the
photoresponsive core, see refs. [10a,b].
[13] Biaryl-tethered bis(trityl alcohol)s were reported as a hydrogen-
bonding catalyst for hetero Diels–Alder reactions, see; A. K. Unni,
N. Takenaka, H. Yamamoto, V. H. Rawal, J. Am. Chem. Soc. 2005,
127, 1336–1337.
[14] Activation of a Brønsted acid by another Brønsted acid through hy-
drogen-bonding interaction is defined as a Brønsted acid-assisted
Brønsted acid (BBA), which is a common activation method for
Brønsted acid catalysts, see: H. Yamamoto, K. Futatsugi, Angew.
Chem. 2005, 117, 1958–1977; Angew. Chem. Int. Ed. 2005, 44, 1924–
1942. Actually, enhanced acidities of TADDOL (a,a,a’,a’-tetraaryl-
2,2-dimethyl-1,3-dioxolane-4,5-dimethanol) derivatives compared
with the corresponding mono-methanols have been observed, see: P.
Christ, A. G. Lindsay, S. S. Vormittag, J.-M. Neudçrfl, A. Berkessel,
A. C. OꢄDonogue, Chem. Eur. J. 2011, 17, 8524–8528.
[3] For reviews on photoswitchable catalysts, see: R. S. Stoll, S. Hecht,
Angew. Chem. 2010, 122, 5176–5200; Angew. Chem. Int. Ed. 2010,
49, 5054–5075.
[4] A photoswitchable catalyst that switches the catalyst activity and/or
reactivity in response to light stimuli is different from a photocatalyst
in this point. The photocatalyst harvests and converts light into
chemical energy, but it does not amplify light. For photocatalysts,
see: a) V. N. Parmon, Catal. Today 1997, 39, 137–144; b) A. J. Ess-
wein, D. G. Nocera, Chem. Rev. 2007, 107, 4022–4047; c) G. Palm-
isano, V. Augugliaro, M. Pagliaro, L. Palmisano, Chem. Commun.
2007, 3425–3437.
[5] The potential of stimuli-responsive switchable catalysts for multica-
talyst systems toward one-pot multistep synthesis has been men-
tioned, see: H. Birch, Chemistry World, News, 01 Oct. 2010. See also
ref. [2f].
[6] For applications of photoinduced catalysts, see: a) S. A. MacDonald,
C. G. Willson, J. M. J. Frꢂchet, Acc. Chem. Res. 1994, 27, 151–158;
b) D. Wang, K. Wurst, W. Knolle, U. Decker, L. Prager, S. Naumov,
M. R. Buchmeiser, Angew. Chem. 2008, 120, 3311–3314; Angew.
Chem. Int. Ed. 2008, 47, 3267–3270.
[7] a) M. V. Peters, R. S. Stoll, A. Kꢃhn, S. Hecht, Angew. Chem. 2008,
120, 6056–6060; Angew. Chem. Int. Ed. 2008, 47, 5968–5972; b) J.
Wang, B. L. Feringa, Science 2011, 331, 1429–1432; c) O. B. Berry-
man, A. C. Sather, A. Lledo, J. Rebek, Jr., Angew. Chem. 2011, 123,
9572–9575; Angew. Chem. Int. Ed. 2011, 50, 9400–9403.
[15] T. Kawasaki, M. Tokuhiro, N. Kimizuka, T. Kunitake, J. Am. Chem.
Soc. 2001, 123, 6792–6800.
[16] Details are reported in the Supporting Information.
[17] Photochromism: Molecules and Systems (Eds.: H. Dꢃrr, H. Bouas-
Laurent), Elsevier, Amsterdam, 2003.
[18] The overlapped protons in helicenes also show similar higher-field
signals in 1H NMR spectra, see: a) R. Irie, A. Tanoue, S. Urakawa,
T. Imahori, K. Igawa, T. Matsumoto, K. Tomooka, S. Kikuta, T.
Uchida, T. Katsuki, Chem. Lett. 2011, 40, 1343–1345; b) S. K. Col-
lins, A. Grandbois, M. P. Vachon, J. Cꢅtꢂ, Angew. Chem. 2006, 118,
2989–2992; Angew. Chem. Int. Ed. 2006, 45, 2923–2926; c) Y. Kita-
hara, K. Tanaka, Chem. Commun. 2002, 932–933; d) H. Wynberg,
M. B. Groen, H. Schadenberg, J. Org. Chem. 1971, 36, 2797–2809.
[19] The potential of trityl alcohol derivatives as a catalyst for MBH re-
action will be reported in due course.
[20] a) Y. M. A. Yamada, S. Ikegami, Tetrahedron Lett. 2000, 41, 2165–
2169; b) N. T. McDougal, S. E. Schaus, J. Am. Chem. Soc. 2003, 125,
12094–12095.
[21] The composition of isomers of the utilized trans-1a (trans-1a/cis-
1a= >99:<1) would remain unchanged during the MBH reaction.
Isomerization of trans-1a to the cis-isomer was not observed in the
1H NMR spectrum in [D8]THF at 208C for 2 h. For details, see the
Supporting Information.
[22] cis-1a would hardly isomerize to the trans-isomer during the MBH
reaction. The composition of isomers of the utilized quasi-cis-1a
changed only slightly in [D8]THF at 208C for 2 h (0 min: trans-1a/
cis-1a=3:97, 2 h: trans-1a/cis-1a=3.3:96.7). For details, see the Sup-
porting Information.
[23] The acceleration abilities of pure catalysts were approximately ex-
trapolated on a pro-rate basis of the observed accelerations of the
MBH reactions based on the composition of isomers of the utilized
catalyst. For details, see the Supporting Information.
[24] Although we cannot rule out changes in the intrinsic acidity of the
trityl alcohol units associated with the structural changes of the cata-
lyst in the photoisomerization, 1H NMR spectra of cis- and trans-1a
showed similar chemical shifts of the hydroxyl groups (see the Sup-
porting Information). Rather, cis-1a showed lower chemical shift
[8] A part of the results of this communication were utilized in a patent
application: Japanese patent application No. 2011-115326.
[9] a) Bifunctional Molecular Catalysis T. Ikariya, M. Shibasaki, Top.
Organomet. Chem. 2011, 37, 1–210; b) P. I. Dalko, L. Moisan,
Angew. Chem. 2004, 116, 5248–5286; Angew. Chem. Int. Ed. 2004,
43, 5138–5175; c) M. Shibasaki, M. Kanai, K.-I. Funabashi, Chem.
Commun. 2002, 1989–1999; d) T. Ooi, M. Takahashi, M. Yamada,
E. Tayama, K. Omoto, K. Maruoka, J. Am. Chem. Soc. 2004, 126,
1150–1160. Our recent reports on bifunctional cooperative catalyst
systems, see: e) C. H. Cheon, T. Imahori, H. Yamamoto, Chem.
Commun. 2010, 46, 6980–6982; f) T. Imahori, H. Ojima, Y. Yoshi-
mura, H. Takahata, Chem. Eur. J. 2008, 14, 10762–10771.
[10] For photoswitchable cooperative catalysts, see: a) F. Wꢃrthner, J.
Rebek, Jr., Angew. Chem. 1995, 107, 503–505; Angew. Chem. Int.
Ed. Engl. 1995, 34, 446–448; b) R. Cacciapaglia, S. Di Stefano, L.
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
5
&
ÞÞ
These are not the final page numbers!

T. Imahori et al.
than the trans-isomer and weaker acidity was implied. Thus, the
clear photoswitchable catalyst activity of 1a would not be derived
from the difference in the intrinsic acidy of the isomeric catalysts.
[25] a) A. N. Thadani, A. R. Stankovic, V. H. Rawal, Pros. Natl. Acad.
Soc. 2004, 101, 5846–5850; b) Y. Huang, A. K. Unni, A. N. Thadani,
V. H. Rawal, Nature 2003, 424, 146–146.
[26] Because the reaction slows down in the last stage, the catalyst activi-
ties of 1a–c were investigated at the intermediate stage to obtain ad-
equate switching abilities. In the initial stage, it is difficult to esti-
mate the OFF-state activities.
acidity (see ref. [24] and the Supporting Information), the optimized
structure of cis-1a by DFT calculations using the B3LYP/6–31G**
basis set (Spartan 08 for Macintosh, Wavefunction) indicated the in-
tramolecular hydrogen-bonding between the two hydroxyl groups.
We assumed the transient intramolecular hydrogen-bonding. For de-
tails of DFT calculations, see the Supporting Information.
[29] For the intramolecular hydrogen-bonding in a close derivative of 7,
see; K. A. Carey, W. Clegg, M. R. J. Elsegood, B. T. Golding, H.
Maskill, Acta Crystallogr. E 2003, 59, o263–o265.
[30] For the intramolecular hydrogen-bonding in 8, see: F. Toda, A. Kai,
R. Toyotaka, W.-H. Yip, T. C. W. Mak, Chem. Lett. 1989, 18, 1921–
1924.
[27] The extrapolations to pure catalysts were not performed for 1c be-
cause of the poor photoswitching ability in the acceleration of the
MBH reaction.
[28] Although chemical shifts of the hydroxyl groups in the 1H NMR
spectra of cis-azobenzene catalysts did not support the enhanced
Received: April 24, 2012
Published online: && &&, 0000
&
6
&
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Photoswitchable Acid Catalysts
COMMUNICATION
Photoswitchable Catalysts
T. Imahori,* R. Yamaguchi,
S. Kurihara .................... &&&& — &&&&
Azobenzene-Tethered Bis(Trityl
Alcohol) as a Photoswitchable Cooper-
ative Acid Catalyst for Morita–Baylis–
Hillman Reactions
Incorporation of an azobenzene core
into tethered bis(trityl alcohol) allows
the photoswitchable arrangement of
the two trityl alcohol units through
photoisomerization of azobenzene.
The differently arranged trityl alcohol
units change their cooperative function
to reflect the positional relationships,
and thus, the activity as a cooperative
acid can be controlled by light stimuli
(see figure).
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
7
&
ÞÞ
These are not the final page numbers!