Modulating the Lewis acidity of boron using a photoswitch

Article abstract of DOI:10.1002/anie.200800869

(Chemical Equation Presented). Light turns the Lewis acid on: The Lewis acidity of a boron atom integrated into a cyclic dithienylethene photoswitch is modulated by light: 1a has low Lewis acidity since the p orbital of the boron center is partially occupied by delocalized π electrons, whereas the rearrangement of the p electrons in 1b reduces the electron density at the boron center and turns the Lewis acid on .

Full text of DOI:10.1002/anie.200800869

Communications
DOI: 10.1002/anie.200800869
Photoswitches
Modulating the Lewis Acidity of Boron Using a Photoswitch**
Vincent Lemieux, M. Daniel Spantulescu, Kim K. Baldridge, and Neil R. Branda*
Tricoordinate organoboron compounds are versatile Lewis
acids used as catalysts or reagents for important organic
transformations,[1] and commercially as co-catalysts in metal-
locene-mediated olefin polymerization[2] and as catalytic
curing agents for epoxy resins.[3] The Lewis acidity of boron
also imparts unique properties to new p-electronic materials
for use in sensing, electron-transport and other materials
science applications.[4] Integrating an external stimulus to
regulate the Lewis acidity in boron-containing compounds
offers a means to control chemical processes that are
catalyzed by these versatile chemical species and modulate
the behavior of functional materials containing them. This
integration is the focus of the studies described herein.
Light is a particularly effective stimulus to spatially and
temporally trigger changes in structure and function of
molecules and materials. This can be achieved reversibly by
inducing the reactions of photochromic compounds between
their two isomers, each of which have unique steric and
electronic properties.[5] Photoresponsive dithienylcyclopen-
tenes (DTCPs) are especially appealing systems because they
tend to undergo thermally irreversible ring-closing and ring-
opening reactions when irradiated with UV and visible light,
respectively, often with a high degree of fatigue resistance
(Scheme 1).[6] There are a few examples describing how the
electronic and geometric changes that accompany the photo-
reactions of DTCP can be used to regulate chemical reactivity
and catalysis.[7] However, these compounds exhibit only small
observable effects on the rate of catalysis,[7d–e] and in some
cases, the presence of the reactive substrate significantly
reduces the photoactivity of DTCP.[7a–c] We have demon-
strated that more dramatic changes in how each of the
photoisomers behave in chemical reactions can be achieved
by taking advantage of the photoinduced rearrangement of
the “pi” bond in the central 5-membered ring of the DTCP.[8]
In a well-designed system, the electronic changes localized
within the central cyclopentene ring are more significant than
the often too subtle electronic and steric differences between
the thiophene heterocycles in the ring-open and ring-closed
DTCP isomers. This report describes one such example.
The central 5-membered ring in compound 1a is a 1,3,2-
dioxaborole system in which the Lewis acidity of the boron
atom can be significantly and reversibly modulated using two
different wavelengths of light.[9] The 1,3,2-dioxaborole in this
ring-open isomer is a planar, conjugated system of over-
lapping p orbitals containing 4n+2 p electrons. It is therefore
expected to have significant aromatic character[10] and a low
Lewis acidity due to the p orbital of the boron atom being
partially occupied by the delocalized p electrons. Irradiation
with UV light triggers the cyclization of isomer 1a to generate
1b. Now the borate group is cross-conjugated with the linearly
conjugated p backbone of the rest of the molecule. This
rearrangement of p electrons should reduce the amount of
electron density at the boron center and turn the Lewis acid
“on”. The system can be turned “off” again using visible light
to reverse the cyclization reaction and regenerate the
aromatic ring system.
Computational investigations performed on simplified
versions of isomers 1a and 1b (the three phenyl rings have
been removed in 1a’ and 1b’) estimate that the ring-open
form is considerably lower in energy than its ring-closed
counterpart, with an energetic preference of 19.0 kcalmolÀ1,
calculated at the M06-2X/DZ(2d,p) level of theory. As
anticipated, the molecular orbitals of 1a’ are part of a
conjugated p-orbital system that includes delocalization
within the dioxaborole ring. A comparison of the lowest
unoccupied molecular orbitals shows there is orbital density
on the boron atom only in isomer 1b’ (Figure 1), an initial
indication that there should be a difference in the Lewis acid
nature between the two isomers. Other calculated values
support the prediction, including the ionization potentials for
1a’ and 1b’ (calculated to be 6.90 and 6.03eV, respectively),
the difference in charge distribution on the boron established
with a variety of different analyses,[11] and calculations on the
reduced forms of the two isomers, which show the energetic
preference for the ring-open isomer over the ring-closed
isomer drops to only a few kcalmolÀ1.
Scheme 1. Reversible photocyclization reaction of a DTCP.
[*] V. Lemieux, M. D. Spantulescu, Prof. N. R. Branda
4D LABS, Department of Chemistry
Simon Fraser University
8888 University Drive, Burnaby, BCV5A 1S6 (Canada)
Fax: (+1)778-782-8061
E-mail: nbranda@sfu.ca
Prof. K. K. Baldridge
Organic Chemistry Institute
University of Zürich
Winterthurerstrasse 190, 8050 Zürich (Switzerland)
[**] This work was supported by the Natural Sciences and Engineering
Research Council of Canada, the Canada Research Chair Program,
Simon Fraser University and the University of Zürich. K.K.B. would
like to acknowledge the Swiss National Science Foundation for
support of this work, and Donald Truhlar for enabling the use of the
M06-2X functional recently developed but not yet in the public
domain.
Computational structures and properties of the ring-open
and ring-closed isomers, 1a and 1b, are very similar to those
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Chemie
1b.[16] The visual demonstration of this photoreaction is the
change in color of the solution from colorless to purple due to
the formation of the extended p-conjugated backbone
created in the ring-closed isomer. The corresponding UV/
Vis absorption spectra show trends typically observed for
ring-closing reactions of dithienylethene derivatives (Fig-
ure 2a). The high-energy bands in the spectra become less
intense as a broad band centered at 535 nm appears. At a
concentration of 3.3 ” 10À5 m (in [D6]benzene), a photosta-
tionary state is reached within 12 seconds, containing 81% of
the ring-closed isomer as measured by 1H NMR spectroscopy.
The ring-closed isomer is stable at room temperature and the
ring-opening reaction is only induced by irradiating the
solution of 1b with visible light (greater than 434 nm),[15]
which regenerates the original UV/Vis spectrum correspond-
ing to 1a.
The first experimental evidence supporting the hypothesis
set forth in the introduction of this report is provided by
Figure 1. a) Synthesis and reversible ring closing of dithienylethene 1a.
The dioxaborole ring in the ring-open isomer, 1a, is expected to have
greater aromatic character than the ring in the ring-closed counterpart,
1b. b) The calculated lowest unoccupied molecular orbitals of a
simplified version of each isomer show that only 1b’ has orbital
density on the boron atom.
already described for the simplified versions, with the former
isomer being preferred over the latter by 18.0 kcalmolÀ1 at
the M06-2X/DZ(2d,p) level of theory. The molecular orbitals
and the charge distribution maps of 1a and 1b also show
similar features to those of the simplified versions. The
ionization potentials in 1a and 1b are slightly lower, at 5.95
and 4.72 eV, respectively. The reduced forms of compounds
1a and 1b bring the ring-closed isomer lower in energy than
the ring-open form by approximately 5 kcalmolÀ1.
Compound 1a is prepared by condensing hydroxyketone
3 and phenylboronic acid as illustrated in Figure 1. A
compound similar to 3 has been reported previously,[12]
however, a new method was developed to improve the
synthesis. When the known aldehyde 2[13] is treated with
Figure 2. a) Changes in the UV/Vis absorption spectra of a benzene
solution (3.3”10À5 m) of 1a as it is irradiated with 312 nm light until
the photostationary state is reached. b) 1H NMR chemical shift corre-
sponding to proton Ha before and after a [D6]benzene solution
(1.0”10À3 m) of 1a is irradiated with 312 nm light (until a 1:1 mixture
of 1a and 1b is reached), after pyridine (pyr) is added and after
irradiation with >434 nm light to regenerate 1a. c) Change in the
3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium
chloride,
the hydroxyketone 3 is produced. Although the product is
only generated in relatively small amounts, the major com-
pound isolated is the starting aldehyde, which can be
resubjected to the reaction conditions. The result is a
convenient way to prepare large amounts of compound 3.
As is common with dioxaborole derivatives, solutions of
1a are easily oxidized in air to yield the diketone version of 3
as the deborylation product.[10] In comparison, anhydrous and
degassed solutions are highly stable. 11B NMR spectroscopy
of 1a in [D6]benzene shows a chemical shift of 31.4 ppm
relative to BF3OEt2, a result that is in agreement with a planar
tricoordinated boron center with a vacant pz orbital.[14]
Irradiating a benzene solution (anhydrous and degassed)
of compound 1a with 312 nm light[15] triggers the photo-
cyclization reaction and generates the ring-closed isomer
*
^
chemical shift of proton Ha in 1a ( ) and 1b ( ) as [D6]benzene
solutions (1.0”10À3 m) are treated with pyridine. The calculated
chemical shifts are also shown (”) for 1b. d) Chemical shift when a
similar solution of 1a is irradiated with UV light, treated with pyridine
and irradiated with alternating visible and UV light. UV (312 nm)
irradiation periods are 10 min generating 40–45% of 1b. Visible
(>434 nm) irradiation periods are 5–7 min to regenerate 100% of the
ring-open isomer. e) Typical changes in the UV/Vis absorption spectra
of a benzene solution (3.2”10À5 m) of 1b as it is irradiated with
*
^
>434 nm light in the presence ( ) and absence ( ) of pyridine, and
their corresponding first order decay fit (solid lines) with k=0.031 and
0.11 sÀ1, respectively.[11]
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Communications
comparing the peaks observed in the 1H NMR spectra
enhance aromaticity). In the absence of pyridine, 1b can
adopt a planar geometry. Similar kinetics and hypsochromic
shifts are observed when pyridine is used as the solvent,
supporting the conclusion that the photochemistry is not
significantly hindered in the 1b·pyridine complex.
In summary, we have demonstrated that the Lewis acidity
of a boron atom integrated into a photochromic backbone can
be modulated using light of the appropriate wavelength. This
ability to regulate the Lewis acidity could enable the control
of chemical processes requiring a Lewis acid as an activator,
reagent, or catalyst, a possibility that we are currently
investigating.
corresponding to the phenyl-ring protons directly adjacent
to the boron atom (Ha in Figure 2) in isomers 1a and 1b. The
upfield shift ( ꢀ 0.1 ppm) observed for these peaks as ring-
closing is induced (Figure 2b) is in agreement with the loss in
aromatic character of the dioxaborole unit as 1a is converted
to 1b. This shift can be expected for protons that were
originally lying within the combined deshielding regions of
two ring systems having aromatic character (the benzene and
the dioxaborole in 1a) but now feel the effect of only one (the
benzene in 1b).
The relative Lewis acidity of the ring-open and ring-closed
photoisomers 1a and 1b can be evaluated by measuring their
respective binding affinities to a Lewis base such as pyridine.
Figure 2b and c show the changes in the peaks in the 1H NMR
spectra corresponding to protons Ha as [D6]benzene solutions
of each photoisomer are treated with pyridine.[17] There are
minimal recordable spectral changes in the case of ring-open
1a even after 10 equivalents of pyridine are added to the
solution. On the other hand, the peaks for the ring-closed
isomer are significantly affected by the presence of pyridine
and shift by more than 0.2 ppm upfield from their original
positions. The changes in chemical shift for 1b can be fit to a
1:1 binding model giving an association constant of (7.0 Æ
Received: February 22, 2008
Published online: June 2, 2008
Keywords: boron · dithienylethenes · Lewis acids ·
.
molecular switches · photochromism
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.
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corresponds to the ring-open isomer, illustrating the release
of the pyridine from the complex. As shown in Figure 2d,
alternatively irradiating the same solution with UVand visible
light toggles the system between 1b·pyridine and 1a + pyr-
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addition of 5 equivalents of pyridine.[11] Moreover, the rate
constant for the ring-opening reaction in the presence and
absence of pyridine was evaluated to be (0.029 Æ 0.006) sÀ1
and (0.13 Æ 0.02) sÀ1, respectively (Figure 2e), showing that
the photochemical reaction is approximately 4 times slower in
the presence of the Lewis base. We attribute this rate
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in the dioxaborole contributes to the driving force for the
ring-opening reaction. When isomer 1b is bound to pyridine,
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[9] Although the photoswitching of the Lewis acidity of a catechol-
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plagued by severe limitations such as slow photoisomerization
(2 h to reach the photostationary state) and low conversion (only
51% of the photoisomeric product is formed). These limitations,
as well as the thermal instability often observed for azobenzene
derivatives, are not usually suffered by dithienylethene deriva-
tives. Another limitation is based on the fact that the operation
of the azobenzene system relies on changing the coordination
number at the boron center using a photoresponsive ligand. This
implies that if a Lewis base stronger than the azobenzene is
present, it will displace the photoswitch.
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[11] See Supporting Information for details.
carried out using the light of a 300 W halogen photo-optic source
passed through a 434 nm cutoff filter to eliminate higher energy
light.
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M. M. Krayushkin, Chem. Heterocycl. Compd. 2001, 37, 85 – 90.
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[14] R. G. Kidd in NMR of Newly Accessible Nuclei, Vol. 2 (Ed.: P.
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[15] All ring-closing reactions were carried out using the light source
from a lamp used for visualizing TLC plates at 312 nm (Spectro-
line E series, 470 WcmÀ2). The ring-opening reactions were
[16] It was immediately observed that the ring-closed photoisomer
1b is significantly more sensitive to oxidation than its ring-open
counterpart. All irradiation studies were performed in anhy-
drous and oxygen-free atmosphere.
[17] A 1:1 mixture of 1a and 1b was used to monitor the changes in
chemical shift for the ring-closed isomer. This mixture was
obtained by irradiating a solution of 1a with 312 nm light for
10 min.
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