Photoswitchable organocatalysis: Using light to modulate the catalytic activities of N-heterocyclic carbenes

Article abstract of DOI:10.1021/ja304067k

A 4,5-dithienylimidazolium salt was found to undergo electrocyclic isomerization upon exposure to UV radiation (λ_irr = 313 nm) under neutral and basic conditions; subsequent exposure to visible light reversed the reaction. Under ambient light and in the presence of base, the imidazolium species catalyzed transesterifications as well as amidations in a manner similar to those of previously reported N-heterocyclic carbene precatalysts. However, upon UV irradiation to effect the aforementioned photocyclization, the rate of the transesterification reaction between vinyl acetate and allyl alcohol was significantly attenuated (k_vis/UV = 12.5), as was the rate of the condensation of ethyl acetate with aminoethanol (k_vis/UV = 100). The rates of these reactions were successfully toggled between fast and slow states by alternating exposure to visible and UV light, respectively, thus demonstrating a rare example of a photoswitchable catalyst that operates via photomodulation of its electronic structure.

Full text of DOI:10.1021/ja304067k

Article
pubs.acs.org/JACS
Photoswitchable Organocatalysis: Using Light To Modulate the
Catalytic Activities of N‑Heterocyclic Carbenes
Bethany M. Neilson and Christopher W. Bielawski*
Department of Chemistry and Biochemistry, The University of Texas, Austin, Texas 78712, United States
S
* Supporting Information
ABSTRACT: A 4,5-dithienylimidazolium salt was found to
undergo electrocyclic isomerization upon exposure to UV
radiation (λirr = 313 nm) under neutral and basic conditions;
subsequent exposure to visible light reversed the reaction.
Under ambient light and in the presence of base, the
imidazolium species catalyzed transesterifications as well as
amidations in a manner similar to those of previously reported
N-heterocyclic carbene precatalysts. However, upon UV
irradiation to effect the aforementioned photocyclization, the
rate of the transesterification reaction between vinyl acetate and allyl alcohol was significantly attenuated (kvis/UV = 12.5), as was
the rate of the condensation of ethyl acetate with aminoethanol (kvis/UV = 100). The rates of these reactions were successfully
toggled between fast and slow states by alternating exposure to visible and UV light, respectively, thus demonstrating a rare
example of a photoswitchable catalyst that operates via photomodulation of its electronic structure.
based catalysts.10 Herein, we report a DAE-annulated NHC
organocatalyst and demonstrate that its activity may be
Photoswitchable catalysis is a burgeoning field of study that
switched through the remote photomodulation of electronic
utilizes photochemical processes to alter the courses of
structure.
INTRODUCTION
■
chemically catalyzed transformations.1 Although many
known2 photochromic moieties may be reversibly switched
RESULTS AND DISCUSSION
between states that feature different steric and/or electronic
properties,3 examples of using photochromism to modulate
catalytic reactions are scarce.4 Following Ueno’s seminal report
of an azobenzene-capped β-cyclodextrin which was used to
modulate the hydrolysis of p-nitrophenyl acetate in 1981
(Figure 1a),4e Cacciapaglia and Mandolini described a photo-
tunable “butterfly” crown ether that facilitated the ethanolysis
of anilides (Figure 1b).4f Hecht subsequently reported an
elegantly designed piperidine that enabled photoswitchable
control over the Henry reaction (Figure 1c).4a−c In all of these
and other examples, however, the catalytic activity was
modulated through reversible steric shielding that resulted
from a photochemically induced isomerization reaction.4
Indeed, as recently noted by Hecht, “no example of successful
reactivity switching by a photochrome-mediated electronic
modulation of a catalyst’s active site has been described to
date.”1 Catalysts with photoswitchable electronic structures are
expected to be broadly applicable while enabling precise control
over intrinsic chemo- and/or regioselectivities.
To realize such a photoswitchable catalyst, we were drawn to
the photochromic diarylethenes (DAEs),5 which were recently
shown by our group6 and others7 to alter the electron density at
the C2 position of imidazolium salts and related N-heterocyclic
carbene (NHC)8 adducts. As NHCs and their complexes are
known to catalyze a variety of useful synthetic transformations,9
we envisioned that such photoinduced changes in electronic
properties may be used to modulate the activities of NHC-
■
Considering that the previously reported photochromic DAE-
annulated NHC adducts and precursors required high energy
radiation (λirr ≤ 297 nm) to undergo cyclization,6,7 we targeted
a derivative that would isomerize under relatively mild
conditions. As shown in Scheme 1, the NHC precursor
1·HPF6 features phenyl substituents at the 5- and 5′-positions
of the thiophene rings, which extend the conjugation length of
the dithienyl backbone, and therefore was expected to undergo
electrocyclic ring-closure upon exposure to relatively low
energy radiation.
The synthesis of the open form of the aforementioned salt
(1o·HPF6) is summarized in Scheme 2. Acylation of
commercially available 2-methyl-5-phenylthiophene with acetic
anhydride and tin(IV) chloride afforded 3-acetyl-2-methyl-5-
phenylthiophene 2, which was then oxidized with selenium
dioxide. The resulting glyoxal monohydrate 3 was coupled with
1 equiv of 2-methyl-5-phenylthiophene in the presence of
tin(IV) chloride to give known11 α-hydroxy ketone 4.
Oxidation of 4 with copper(II) acetate and ammonium nitrate
in refluxing acetic acid afforded diketone 5, which was
formylatively cyclized to give imidazole 6. Finally, methylation
with iodomethane under basic conditions followed by anion
metathesis12 afforded imidazolium salt 1o·HPF6, as evidenced
Received: May 2, 2012
Published: July 19, 2012
© 2012 American Chemical Society
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Figure 1. Representative examples of photoswitchable catalysts: (a) Ueno et al. reported a photoswitchable azobenzene-capped β-cyclodextrin in
which the reversible E → Z isomerization resulted in a deeper cavity and enhanced substrate binding affinity, leading to faster hydrolysis.4e (b) The
reversible E → Z isomerization of a phototunable crown ether facilitated the light-gated ethanolysis of anilide derivatives.4f (c) An azobenzene-
annulated piperidine catalyzed the Henry reaction at a significantly faster rate in the Z form compared to the more stable E isomer due to steric
shielding of the basic site.4a.
reported 4,5-dithienyl N-heterocycles.6,7a As a result, exposing a
Scheme 1. Photochromism of NHC Precursor 1·HPF6
solution of 1o·HPF6 in benzene to relatively low energy
radiation (λirr = 313 nm vs 280 nm6) resulted in a color change
from pale yellow to bright blue. Concomitant with this color
change, a decrease in the intensity of the absorption band
centered at 292 nm and the appearance of a new band at 670
nm were observed (Figure 2a). The spectroscopic changes
reached a steady state after 240 s of UV exposure and reflected
1
in part by the appearance of diagnostic H (δ = 8.6 ppm) and
13C (δ = 142 ppm; CDCl3) NMR signals, which corresponded
to the proton and carbon nuclei at the C2 position, respectively
(see the Supporting Information).
The UV−vis spectra recorded for 1o·HPF6 dissolved in
benzene, tetrahydrofuran, or acetonitrile exhibited intense
absorption bands between 275 and 325 nm, which were
assigned to the n → π* and π → π* transitions of the N-
heterocycle and thiophene systems, respectively. Due to the
phenyl groups in the 5- and 5′-positions of the thiophene
moieties, the absorption spectrum of 1o·HPF6 was bath-
ochromically shifted when compared to those of previously
an 88% conversion of 1o·HPF6 to its ring-closed isomer
1c·HPF6 upon measurement (see Scheme 1).5,13a Moreover, an
isosbestic point was observed at 309 nm, which indicated that
the cyclization proceeded without appreciable side reactions.
Subsequent irradiation of the UV exposed solutions with visible
light (λirr > 500 nm) resulted in the attenuation of the broad,
low energy absorption bands. After 300 s of irradiation, the
UV−vis spectrum of 1o·HPF6 was nearly completely restored
(>95% conversion),13a which suggested to us that the ring-
closed product 1c·HPF6 reverted back to the starting material
(Figure 2b).
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Scheme 2. Synthesis of 1o·HPF6
Figure 3. Plots of the percent of starting materials which have
converted to products versus time for the condensation of vinyl acetate
and allyl alcohol catalyzed by 1 (prepared in situ from 1·HPF6 and
KOtBu) in THF. The reactions were monitored over time by GC
using n-octane as an internal standard. (a) Two reactions were run
concurrently with one vessel exposed to UV irradiation (λirr = 313 nm)
for 1 h prior to substrate addition (blue diamonds) and one kept
under ambient light (red squares). (b) A single reaction vessel was
exposed to UV irradiation (λirr = 313 nm) for 1 h prior to substrate
addition. The contents of the vessel were then stirred under UV light
Figure 2. (a) UV−vis spectral changes of 1o·HPF6 in C6H6 upon UV
irradiation (λirr = 313 nm). (b) UV−vis spectrum in C6H6 of 1o·HPF6,
the photostationary state (PSS) reached after UV irradiation of
1o·HPF6 for 240 s, and spectral changes of the PSS upon visible
irradiation (λirr > 500 nm). (c) UV−vis spectral changes of 1o
(generated in situ by treatment of 1o·HPF6 with 1.0 equiv of
NaHMDS) in C6H6 upon UV irradiation (λirr = 313 nm). (d) UV−vis
spectrum in C6H6 of 1o, the photostationary state (PSS) reached after
UV irradiation of 1o for 120 s, and spectral changes of the PSS upon
visible irradiation (λirr > 500 nm). In all cases, the initial concentration
of 1o was 4 × 10−5 M and the spectra were recorded after irradiation
for the indicated amount of time. The arrows indicate the evolution of
the spectral changes over time.
for 30 min (blue diamonds) prior to exposure to visible light (λirr
>
500 nm) (red squares).
focused on investigating the photochemical behavior of
1o·HPF6 under basic conditions. Although the free NHC 1o
was not isolable, its formation was observed in situ by NMR
spectroscopy upon treatment of 1o·HPF6 with 1 equiv of
KOtBu or NaHMDS, as evidenced by the loss of the
1
The photocyclizations were further confirmed by H NMR
1
spectroscopy. After irradiation of 1o·HPF6 in CD3CN at 313
nm for 45 min ([1o·HPF6]0 = 1 × 10−3 M), the signals
assigned to the thiophene protons shifted upfield from δ 7.3
ppm to 6.9 ppm, and a significant upfield shift of the signal
assigned to the proton at the C2 position in 1o·HPF6 from 8.6
ppm to 8.1 ppm was observed (see Figure S20 of the
Supporting Information). Integration of these signals revealed
that 81% of 1o·HPF6 converted to 1c·HPF6. Exposure of the
UV treated solution to ambient light for 4 h reversed the
aforementioned chemical shifts (>95% conversion).
imidazolium H NMR signal at 8.82 ppm and the appearance
of a 13C NMR signal assigned to the carbenoid nucleus at 201.9
ppm (C6D6). Similar to that of its imidazolium precursor, the
UV−vis spectrum of 1o exhibited an intense absorption
centered at 291 nm in benzene. Upon UV irradiation, a
decrease in the intensity of this band was observed concomitant
with the appearance of new bands at 378 and 554 nm, as well as
an isosbestic point at 321 nm (Figure 2c). Measurement of
these signals revealed that >99% of 1o converted to its ring-
closed isomer 1c after 120 s of UV irradiation.13b As further
1
Considering that catalytically active NHCs may be generated
by exposing imidazolium salts to base, subsequent efforts
confirmation of the forward cyclization reaction, the H NMR
signals assigned to the thiophene protons shifted upfield from δ
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Having demonstrated that 1o·HPF6 and 1o underwent
similar reversible photocyclizations, our attention shifted
toward exploring the photoinduced isomerizations as a means
to modulate catalytic activity. Our attention was directed
toward transesterification and amidation reactions, as imidazo-
lium salts in the presence of base have been shown to catalyze
these useful transformations.14 Moreover, Nolan and others
have shown that the catalytic activities observed in various
condensation reactions are sensitive to changes in the electronic
structure of the NHC organocatalyst.14c
In a preliminary experiment, 1o was found to promote the
transesterification of allyl alcohol and vinyl acetate (Figure 3).
A 35% conversion15 to the expected ester product was observed
Figure 4. Plots of the percent of starting materials which have
converted to products versus time for the condensation of ethyl
acetate and 2-aminoethanol catalyzed by 1 (prepared in situ from
1·HPF6 and NaHMDS) in 3:1 C6H6/THF (v/v). The reactions were
monitored over time by GC, using n-octane as an internal standard.
(a) Two reactions were run concurrently with one vessel exposed to
UV irradiation (λirr = 313 nm) for 1 h prior to substrate addition (blue
diamonds) and one kept under ambient light (red squares). (b) A
single reaction was allowed to proceed in ambient light for 2 h (red
1
after 1 h by H NMR spectroscopy upon treatment of a THF
solution containing equimolar quantities of the aforementioned
alcohol and ester starting materials (initial concentration of
each: 0.1 M) with 1o (1 mol %; prepared in situ by treating
1o·HPF6 with 1 equiv of KOtBu) at room temperature. To
determine if exposure to UV irradiation would influence the
aforementioned condensation reaction, a freshly prepared
solution of 1o in THF ([1o]0 = 1 × 10−3 M) was divided in
half: one-half was placed in a quartz cuvette sealed with a
Teflon-lined septum cap, and the other half was transferred to a
flask and sealed with a rubber septum. The solution in the
quartz cuvette was then subjected to UV irradiation (λirr = 313
nm) for 1 h while the other solution was kept in ambient light
over the same period of time, after which equimolar amounts of
vinyl acetate and allyl alcohol were added to each reaction
vessel separately. Aliquots were then periodically removed from
each mixture, diluted with methanol to quench the reaction,
and analyzed by gas chromatography using n-octane as an
internal standard. Inspection of these data revealed that the
reaction exposed to ambient light proceeded with a second
order rate constant, kvis, of 5 1 × 10−4 mol−1·s−1 whereas the
reaction that had been subjected to UV irradiation exhibited
squares) and then subjected to UV irradiation (blue diamonds) (λirr
=
313 nm) for 1 h and kept in the dark for a further 3 h prior to exposure
to visible light (λirr > 500 nm) (red squares). For experiments where
the reaction was switched over different time scales, see the Supporting
Information.
only negligible conversion to product (<3% by GC; kUV = 4
1
× 10−5 mol−1·s−1). The 10-fold decrease in reactivity suggested
to us that photocyclization occurred upon UV irradiation and
the corresponding changes in the electronic structure of 1
significantly attenuated its catalytic activity (Figure 3a).16
Moreover, the differences in reactivity were consistent with
Nolan’s observation that imidazol-2-ylidene catalysts were more
active than their relatively electron deficient imidazolin-2-
ylidene analogues in similar transesterification reactions.17
Since 1o and 1c may be interconverted by exposure to light
of different wavelengths, we sought to explore the potential of
switching the catalytic activity over the course of a
condensation reaction. When vinyl acetate and allyl alcohol
were added to a THF solution of 1c, no conversion was
observed by GC for at least 30 min while the reaction vessel
was irradiated with UV light. However, the rate of product
formation significantly increased (Figure 3b) upon subsequent
exposure to visible light (λirr > 500 nm), consistent with the
formation of the catalytically active 1o from 1c. Unfortunately,
multiple switching cycles were precluded by the relatively slow
photocyclization and condensation kinetics.15 Regardless, the
result constituted the first example of using light to activate a
latent NHC-based organocatalyst.1,18
Figure 5. Quantitative 13C NMR spectra collected sequentially in
C6D6 over the course of the following experiment: A sample of (a) 1o
labeled at the C2 position with a 13C atom (i.e., 1o*; [1o*]0 = 1.0 ×
10−2 M) (b) was treated with equimolar quantities of 2-aminoethanol
and ethyl acetate and (c) then exposed to UV irradiation for 1 h (λirr
=
313 nm; [1o*]0 = 2.5 × 10−3 M) and finally (d) irradiated with visible
light for an additional 2 h (λirr > 500 nm, [1o*]0 = 1.0 × 10−2 M). See
text for additional details.
7.0 ppm to 6.7 ppm after irradiation of 1o in C6D6 at 313 nm
([1o]0 = 1 × 10−3 M), with the conversion of 1o to 1c reaching
71% after 1 h of irradiation. Additionally, a new 13C NMR
resonance was observed at δ 66.6 ppm, after irradiation of 1o in
C6D6, and assigned to the sp3 carbon nuclei adjacent to the
sulfur atoms in 1c. Subsequent irradiation with visible light for
300 s reversed the observed UV−vis spectral changes, and the
spectrum of 1o was nearly restored (85% conversion),13b
indicating that 1c had undergone photocycloreversion (Figure
2d). Together, these results suggested to us that 1 underwent a
reversible photochemical process analogous to its precursor
1·HPF6.
Next, efforts were directed toward exploring photoswitchable
amidation reactions, which were envisioned to be better suited
for photoswitching as they generally proceed with consistently
higher conversions and longer reaction times than analogous
transesterification reactions.14e Initial studies showed that when
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Scheme 3. Proposed Mechanism of Photoswitchable NHC Catalyzed Condensation Reactions
ethyl acetate and 2-aminoethanol were added to 1o (2.5 mol %;
prepared in situ from 1o·HPF6 and 0.9 equiv of NaHMDS), a
Since low concentrations facilitated the photocyclization
reactions, an analogue of 1o·HPF6 that was isotopically labeled
with a 13C atom at the C2 position was used. The labeled
precatalyst, 1o·HPF6*, was synthesized via an analogous route
to that employed for 1o·HPF6, with the exception that 13CH2O
was used in the formylative cyclization step (see the Supporting
Information). Upon treatment of 1o·HPF6* with 1 equiv of
NaHMDS in C6D6, a 13C NMR signal was observed at 201.9
ppm (Figure 5a), as expected8,9 for the in situ formation of the
free NHC 1o*. The addition of equimolar quantities of 2-
aminoethanol and ethyl acetate to 1o* (2.5 mol %) in C6D6
resulted in the formation of an imidazolium species,21 as
evidenced by the upfield shift of the C2 13C NMR signal from
201.9 ppm to 138.6 ppm (Figure 5b). UV irradiation of the
mixture ([1o*]0 = 2.5 × 10−3 M; λirr = 313 nm) for 1 h resulted
in a further upfield shift of the signal assigned to the C2 atom
(Figure 5c). The new resonance was observed at 93.3 ppm and
identified as an NHC-alcohol adduct,22,23 the formation of
which may have been facilitated by the decreased electron
density at the C2 position caused by the photocyclization
reaction. Visible light irradiation reversed these spectroscopic
changes and caused the signal at 93.3 ppm to shift downfield to
138.6 ppm, consistent with reversion to the imidazolium
species (Figure 5d).24 In combination with the results from the
catalysis experiments described above, the 13C NMR data
suggested to us that the resting state of the active catalyst was
an imidazolium species, which converted to a NHC-alcohol
adduct upon UV irradiation and effectively suspended the
catalytic cycle (Scheme 3). Moreover, the restoration in activity
observed upon visible light irradiation demonstrated that the
photocyclized adduct may be converted back to an imidazolium
species that then re-engages the catalytic cycle.25 Together,
these results support the conclusion that the catalyst was
reversibly switched with high fidelity between active and
inactive adducts via photoinduced changes in electronic
structure. Moreover, given that the transesterification reactions
discussed above involve analogous alcohol and ester substrates,
a similar photoswitching mechanism may be operative.
1
61% conversion to the expected amide was observed by H
NMR spectroscopy after 21 h. Using similar comparative
kinetics experiments as described above for the transester-
ification reactions, the aforementioned amidation reaction
performed under ambient light proceeded with a second
order rate constant of kvis = 5 4 × 10−4 mol−1·s−1 whereas an
analogous reaction exposed to UV light was relatively slow (kUV
= 5 4 × 10−6 mol−1·s−1; kvis/UV = 100); see Figure 4a.19 The
disparate rates enabled the photomodulation of the reaction
kinetics over the course of a single amidation reaction. As
shown in Figure 4b, after exposure to ambient light for 2 h (kvis
= 6 5 × 10−4 mol−1·s−1), the vessel containing an identical
mixture to that described above was subjected to UV irradiation
for 1 h, which effectively stopped the reaction. After a further 3
h in the dark, during which no conversion of starting material
to product was observed, exposure to visible light (λirr > 500
nm) resulted in a significant restoration of the catalytic activity
(k = 1.3 0.5 × 10−4 mol−1·s−1). The initial reaction rate was
not fully restored, likely due to photochemical fatigue of the
catalyst upon prolonged UV irradiation. However, since only a
minor portion of the catalyst underwent decomposition,20 it
was possible to switch the catalytic activity over multiple time
scales. A reaction duplicate to that described above was kept
under ambient light for 30 min before being exposed to UV
irradiation for 30 min, which effectively stopped the reaction.
The reaction conversion remained stagnant while the reaction
vessel was kept in the dark for a further 1 h; however, the
catalytic activity was restored upon subsequent exposure to
visible light (see Figure S1 of the Supporting Information).
Collectively, these results suggested to us that the photo-
cyclization process intrinsic to 1 was responsible for the
changes in catalytic activity, which was consistent with our
previous observation that the dithienylethene photocyclization
decreases the electron density at the C2 position of the NHC.6
Furthermore, since we have previously shown that the change
in the steric properties of related photochromic NHCs upon
photocyclization is minimal,6 these results demonstrate that
photoinduced changes in electronic structure may be used to
reversibly tune the activity of a catalytic species.
CONCLUSIONS
■
In summary, we report a rare26 example of a photoswitchable
catalyst that operates via the remote photomodulation of its
electronic structure. By incorporating a dithienylethene moiety
into the backbone of a NHC precursor, the activity of the
To gain additional insight into the photoswitchable NHC
reactivity, the identities of the active and inactive adducts of the
amidation catalyst were probed using 13C NMR spectroscopy.
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to UV or visible light. The rates of NHC-catalyzed trans-
esterification and amidation reactions were attenuated by 1 to 2
orders of magnitude upon exposure to UV light, while
subsequent exposure to visible light restored the catalytic
activity. Given the vast array of reactions catalyzed by NHCs
and NHC-supported metal complexes, the ability to remotely
modulate catalyst electronic properties using light is expected
to endow a broad range of catalysts with the ability to finely
tune intrinsic chemo-, regio-, and stereoselectivities. Such
catalysts are envisioned to find applications that range from the
preparation of polymeric materials with sophisticated micro-
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ASSOCIATED CONTENT
* Supporting Information
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Marion, N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612−3676. (f) Droge,
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O.; Henseler, A. Chem. Rev. 2007, 107, 5606−5655. (c) Marion, N.;
Detailed experimental procedures, additional spectral data, and
kinetic analyses. This material is available free of charge via the
Internet at http://pubs.acs.org.
́
Díez-Gonzalez, S.; Nolan, S. P. Angew. Chem., Int. Ed. 2007, 46, 2988−
AUTHOR INFORMATION
Corresponding Author
bielawski@cm.utexas.edu
■
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Notes
The authors declare no competing financial interest.
(10) For studies on tuning the electron donating properties of NHCs
as a means to influence catalytic reactions, see: (a) Khramov, D. M.;
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ACKNOWLEDGMENTS
■
This material is based upon work supported in part by the U.S.
Army Research Laboratory under Grant Number W911NF-09-
1-0446. Additional support from the National Science
Foundation (CHE-0645563) and the Robert A. Welch
Foundation (F-1621) is also gratefully acknowledged. We
thank Steve Sorey for his help with the quantitative 13C NMR
experiments.
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Journal of the American Chemical Society
Article
(16) When a control reaction was performed in the absence of 1o
under ambient light, a background reaction was observed with the
KOtBu acting as the catalyst (31% conversion after 1 h). However,
when an analogous reaction was exposed to UV light, no significant
reduction of catalytic activity was observed (26% conversion after 1 h),
indicating that UV irradiation alone did not impede the reaction.
Similarly, UV irradiation did not reduce the catalytic activity when 1,3-
dimesitylimidazolinylidene (SIMes), an NHC-based organocatalyst
which lacks a photochromic moiety, was used in lieu of 1o (conversion
after 1 h: 39% under ambient light; 58% under UV light).
decomposition may account for the inability to fully restore the initial
reaction rate after irradiation.
(26) During the preparation of this manuscript, an elegant example of
photomodulating deuterium exchange reactions using a pyridinium
catalyst electronically linked to a DAE scaffold was reported; see:
Wilson, D.; Branda, N. R. Angew. Chem., Int. Ed. 2012, 51, 5431−5434.
NOTE ADDED AFTER ASAP PUBLICATION
Part b of Figure 4 was incorrect in the version published ASAP
July 19, 2012. The correct Figure 4 reposted July 24, 2012.
■
(17) Nolan reported that unsaturated NHCs were more active
transesterification catalysts than their saturated analogues; see ref 14c.
While no formal change in unsaturation occurs upon conversion of 1o
to 1c, the disruption of the endocyclic double bond in 1o upon
photocyclization has a similar effect on the nucleophilicity of the
NHC, which was reflected by the reduction in reaction rate observed
under UV irradiation and in accord with the previous demonstration
that the electron donating ability of the NHC was reduced upon
photocyclization; see ref 6.
(18) For other examples of light-activated latent catalysts, see:
(a) Sun, X.; Gao, J. P.; Wang, Z. Y. J. Am. Chem. Soc. 2008, 130, 8130−
8131. (b) Ben-Asuly, A.; Aharoni, A.; Diesendruck, C. E.; Vidavsky, Y.;
Goldberg, I.; Straub, B. F.; Lemcoff, N. G. Organometallics 2009, 28,
4652−4655. (c) Hafner, A.; Muhlengach, A.; van der Schaaf, P. A.
Angew. Chem., Int. Ed. 1997, 36, 2121−2124. (d) Keitz, B. K.; Grubbs,
R. H. J. Am. Chem. Soc. 2009, 131, 2038−2039. (e) Wang, D.; Wurst,
K.; Knolle, W.; Decker, U.; Prager, L.; Naumov, S.; Buchmeiser, M. R.
Angew. Chem., Int. Ed. 2008, 47, 3267−3270.
(19) When a control reaction was performed in the absence of 1o, a
background reaction was observed with NaHMDS acting as the
catalyst (44% conversion after 4 h) under ambient light. However, no
attenuation of catalytic activity was observed under UV light (53%
conversion after 4 h). When imidazolium salts without photochromic
moieties, such as 1,3-dimesitylimidazolium chloride or 1,3-dimesityli-
midazolinium chloride, were used the precatalyst, no significant
conversion to product was observed under ambient light or UV
irradiation after 4 h.
(20) Decomposition (29%) of 1c was observed by UV−vis
spectroscopy upon prolonged UV irradiation (>0.5 h, [1o]0 = 4 ×
10−5 M); see Figure S7.
(21) The observed results were consistent with the formation of an
imidazolium alkoxide intermediate, as proposed in a previous report of
NHC-catalyzed amidation reactions; see ref 14e.
(22) (a) Trnka, T. M.; Morgan, J. P.; Sanford, M. S.; Wilhelm, T. E.;
Scholl, M.; Choi, T.-L.; Ding, S.; Day, M. W.; Grubbs, R. H. J. Am.
Chem. Soc. 2003, 125, 2546−2558. (b) Csihony, S.; Culkin, D. A.;
Sentman, A. C.; Dove, A. P.; Waymouth, R. M.; Hedrick, J. L. J. Am.
Chem. Soc. 2005, 127, 9079−9084.
(23) When a stoichiometric quantity of 2-aminoethanol was added to
1o* (generated in situ), a signal assigned to the C2 nucleus of the
NHC was observed at 165 ppm and in agreement with a proton
transfer reaction. Upon UV irradiation, the signal shifted upfield to 102
ppm, which was consistent with the formation of a NHC-alcohol
adduct. Visible light irradiation reversed the spectral changes, which
suggested to us that the decreased electron density at the C2 position
upon photocyclization resulted in the reversible formation of a NHC-
alcohol adduct; see the Supporting Information.
(24) UV irradiation of 1o* was performed in 4 mL of C6D6 in a
quartz cuvette ([1o*]0 = 2.5 × 10−3 M). Prior to NMR analysis, the
solution was concentrated under reduced pressure to a volume of
approximately 1 mL ([1o*] = 1.0 × 10−2 M). The visible light
irradiation was carried out directly on the concentrated sample in the
NMR tube.
(25) Using quantitative 13C NMR analysis of 1o* versus p-xylene as
an internal standard, a small amount (13%) of the catalyst was
observed to decompose to insoluble byproduct(s) after UV irradiation
([1o*]0 = 2.5 × 10−3 M; λirr = 313 nm for 1 h) followed by visible
irradiation (λirr > 500 nm for 2 h) ([1c*] = 1.0 × 10−2 M). The
12699
dx.doi.org/10.1021/ja304067k | J. Am. Chem. Soc. 2012, 134, 12693−12699