Photoswitchable azoheterocycles via coupling of lithiated imidazoles with benzenediazonium salts

Article abstract of DOI:10.1021/jo202688x

In contrast to azobenzenes, heterocyclic azo compounds are less well investigated. Phenylazoimidazoles would be versatile as photodissociable ligands (PDLs) because imidazole is an important donor in coordination chemistry. Here, we present the synthesis of 4- and 5-phenylazoimidazoles via a novel azo-coupling method. 1,2-Protected imidazole is lithiated in the 5-position and coupled with benzenediazonium tetrafluoroborate. Several new phenylazoimidazoles were prepared. They exhibit an excellent switching behavior. Upon irradiation of the trans isomers with UV light, >95% of the cis forms are obtained. Upon heating, a complete transformation back to the trans configuration was achieved. Back switching with visible light, however, is incomplete.

Full text of DOI:10.1021/jo202688x

Article
pubs.acs.org/joc
Photoswitchable Azoheterocycles via Coupling of Lithiated
Imidazoles with Benzenediazonium Salts
Thore Wendler,† Christian Schutt,† Christian Nather,‡ and Rainer Herges*,†
̈
̈
‡
†Otto-Diels-Institute for Organic Chemistry and Institute for Inorganic Chemistry, University of Kiel, Otto-Hahn-Platz 4, Kiel
D-24119, Germany
S
* Supporting Information
ABSTRACT: In contrast to azobenzenes, heterocyclic azo compounds are less
well investigated. Phenylazoimidazoles would be versatile as photodissociable
ligands (PDLs) because imidazole is an important donor in coordination
chemistry. Here, we present the synthesis of 4- and 5-phenylazoimidazoles via a
novel azo-coupling method. 1,2-Protected imidazole is lithiated in the 5-
position and coupled with benzenediazonium tetrafluoroborate. Several new
phenylazoimidazoles were prepared. They exhibit an excellent switching
behavior. Upon irradiation of the trans isomers with UV light, >95% of the cis
forms are obtained. Upon heating, a complete transformation back to the trans
configuration was achieved. Back switching with visible light, however, is incomplete.
INTRODUCTION
■
Azobenzenes are known to undergo photochemical cis/trans-
isomerization and therefore have been extensively used as
molecular switches to achieve a number of dynamic, machine-
like functions.1 While azobenzenes are well investigated,2 the
class of heterocyclic azo compounds has been largely neglected.
Azopyridines have been used as photodissociable ligands
(PDLs).3−7 For instance, 3-azopyridines coordinate to Ni-
porphyrins in their trans configuration forming paramagnetic
complexes. Upon irradiation, the trans-azopyridines undergo
isomerization to the cis-forms, which for sterical reasons
dissociate forming the diamagnetic square planar Ni-porphyrins
Figure 1. Regioisomers of N-substituted azoimidazoles and simplified
without axial ligands (light-driven coordination-induced spin
state switch, LD-CISSS).8−10 Phenylazoimidazoles would be an
binding modes as axial ligands to porphyrins.
interesting class of photoswitchable ligands, particularly of
(1), which are needed for the design of PDLs could not be
PDLs because imidazoles are much stronger donor ligands as
synthesized on a large scale so far. Only traces were found in
compared to pyridines leading to larger association constants
the composition of 2-phenylazoimidazole (2).14,16 The
with transition metals.11
condensation of 4(5)-aminoimidazole with nitrosobenzene
There are three possible regioisomers of azoimidazoles with
(which is one of the standard methods to prepare azobenzenes)
the azo group in the 2, 4, or 5 position (Figure1).
cannot be applied because 4(5)-aminoimidazole is not a stable
It is known that substitution of imidazoles in the 2 position
compound.17,18 For the synthesis of 4(5)-phenylazoimidazoles
we therefore developed an alternative route.
We here report on a convenient synthesis of a number of 5-
severely hinders axial binding to metal porphyrins.12 For
instance, imidazole binds almost 3 orders of magnitude better
to Fe(III)TPP chloride than 2-methylimidazole, whereas
substitution at the 5 position has almost no effect.13 For
substituted imidazoles from 1-dimethylsulfamyolimidazole
(3).19 After the 2-position is blocked with a silyl group the
imidazoles that are substituted in the 4 position, a similar steric
selective activation of the 5-position can be performed with
effect in lowering the binding constant can be expected as in 2-
butyllithium.20,21 Reaction with electrophiles, e.g., alkyl
substituted imidazoles. To develop photodissociative ligands
bromides, yields the corresponding 5-substituted imidazoles.22
with strong axial binding to metal prophyrins we therefore set
Our strategy to synthesize 5-azoimidazoles now is based on the
out to synthesize 5-azophenylimidazoles.
reaction of the 5-lithiated imidazole with diazonium salts as the
There are reports on several 2-substituted 4(5)-phenyl-
azoimidazoles and 2-phenylazoimidazoles which are synthe-
sized by azo coupling of imidazole with benzene diazonium
salts.14,15 However, unsubstituted 4(5)-phenylazoimidazoles
Received: January 9, 2012
Published: March 8, 2012
© 2012 American Chemical Society
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Article
electrophile. To the best of our knowledge, azo compounds
were never prepared via this route. So far, only phenyl Grignard
compounds,23−25 diphenylmercury,26 and triphenylindium27
have been used as the organometallic component in azo-
coupling reactions.
455 nm. The thermal back isomerization (cis to trans form) was
monitored by H NMR.
1
As expected, irradiation of trans-4(5)-phenylazoimidazole (1)
did not lead to a detectable concentration of the cis
configuration at room temperature. A similar behavior was
observed for 2-phenylazoimidazole (2).15 Sinha et al.
demonstrated that the cis form of 2 is indeed formed by
irradiation; however, its lifetime is very short. The cis
configuration isomerizes back to the trans form by a proton-
assisted mechanism via a hydrazone intermediate (for proposed
mechanisms, see the Supporting Information). We suggest an
analogous mechanism for 4(5)-phenylazoimidazole (1). This is
corroborated by low-temperature NMR measurements. Upon
irradiation at −78 °C we observed new signals in the 1H NMR
spectrum that are assigned to the cis-form of 4(5)-phenyl-
azoimidazole (1) and that disappear upon warming the
solution.
RESULTS AND DISCUSSION
■
Synthesis. Although the 2-silyl-protected imidazoles are
isolable it was found to be more convenient to lithiate at the 5-
position in situ with a second equilibrium of butyllithium.20
The 5-lithio derivate was converted to the phenylazoimidazole
by reaction with diazonium benzenetetrafluoroborate (Scheme
1). Deprotection with tetrabutylammonium fluoride leads to 1-
a
Scheme 1. Synthesis of 4- and 5-Phenylazoimidazoles
As opposed to 1, the N-substituted phenylazoimidazoles are
stable in their cis form28 for 5−500 h at room temperature
(Table 1). Conversion from trans to cis is achieved upon
Table 1. Conversion of the Phenylazoimidazoles 4−7 to the
a
Cis Forms
azoimidazole
maximum cis (%)
half lives (h)
4
5
6
7
95
95
96
98
39.7
4.7
320
528
a
The photostationary states are obtained upon irradiation with UV
a
Reagents and conditions: (a) n-BuLi, −78 °C, THF, 30 min, then
light of 365 nm, and the half lives of the cis forms were measured at 25
°C by H NMR.
1
DMTSCl, rt, 16 h; (b) n-BuLi, −78 °C, THF, 30 min, then
+
−
C6H5N2 BF4 , rt, 16 h; (c) TBAF, THF, rt, 2 h; (d) EtOH/HCl 4:1,
80 °C, 3 h; (e) TrCl, acetonitrile, rt, 16 h; (f) NaH, THF, 0 °C to rt,
10 min, then MeI, 0 °C to rt, 3 h; (g) Me2SO4, 20% aq. KOH, rt, 2 h;
(h) MeOTf, CH2Cl2, rt, 16 h, then acetone/water 1:1, rt, 3 h.
irradiation with UV light of 365 nm. The photostationary states
contain between 95 and 98% of the cis isomer. Both were
1
determined by H NMR spectroscopy.
Thus, the switching efficiency for trans−cis isomerization of 7
(98% cis isomer) is superior to parent azobenzene (91%)29 and
azopyridine (91%).6 With a half-life of more than 500 h, cis-5-
phenylazoimidazole 7 is more stable than the cis configurations
of most azobenzenes and azopyridines.
dimethylsulfamoyl-5-phenylazoimidazole (4) in 54% yield over
three steps. A crystal structure determination confirmed the 5-
position of the azo group (see the Supporting Information).
Compound 4 exhibits the desired regiochemistry for coordina-
tion; however, dimethylsulfamoyl is a strong electron-with-
drawing group, which weakens the donor strength of the
imidazole. Therefore, the sulfamoyl group was quantitatively
removed by treatment with acid, giving the 4(5)-phenyl-
azoimidazole (1). To prevent tautomerism between the 4- and
5-phenylazoimidazole, the imidazole ring was N-methylated.
Direct methylation can result in two products, which can be
easily distinguished by HMBC. Methyl iodide leads only to the
unwanted 1-methyl-4-phenylazoimidazole (5) (64% yield).
Dimethyl sulfate as the methylating agent gives mainly 1-
methyl-5-phenylazoimidazole (7), however, in poor yields
(33%), and alongside with the regioisomer 5 (17%). Higher
yields (58%) of pure 7 were obtained by a two pot, three step
procedure. Introduction of a trityl group gives exclusively 1-
trityl-4-phenylazoimidazole (6). Methylation with methyl
triflate and removal of the trityl group leads to the desired
phenylazoimidazole 7.
1
According to H NMR spectra and DFT calculations, the cis
configurations of the phenylazoimidazoles 4−7 exhibit unusual
conformations. In the trans configuration of azoimidazole 7 the
proton at C4 resonates at 7.60 ppm in chloroform-d1 which is
in the expected range (compare 7.14 ppm in the parent
imidazole); however, in the cis form the corresponding signal
appears at 5.44 ppm. Similar upfield shifts were detected in the
cis isomers of the azoimidazoles 4−6 (see the Supporting
Information). DFT calculations (at the B3LYP/6-31G* level of
theory) revealed the reason for these anomalous shifts. In
contrast to the cis isomer of azobenzene where both phenyl
rings are twisted out of planarity by about 56° with respect to
the C−NN−C plane,30 the imidazole ring in cis-7 is coplanar
and the phenyl ring is perfectly orthogonal (90°) with respect
with the C−NN−C plane (Figure 2, top), which prevents
conjugation between the two rings. The proton at C4 of the
imidazole ring protudes into the deshielding range of the
phenyl ring, which gives rise to the upfield shift. The trans
configurations of 4−7 exhibit the expected planar geometries.
The difference in conjugation in the cis and trans isomer of 7 is
well represented by ACID calculations.31,32 The ACID plot
(Figure 2, bottom) reveals a contiguous isosurface of the
To check the applicability of the phenylazoimidazoles as
photodissociable ligands (PDLs), we investigated the switching
behavior of 1 and 4−7. UV spectra were measured, and the
photostationary states were determined at 297, 365, 440, and
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ization) and the n−π* transitions (cis→trans) are clearly
separated. Therefore, the photochemical back switching which
is usually performed with visible light of about 450 nm in
azobenzenes and azopyridines is inefficient in our azoimida-
zoles. The photostationary equilibrium of 7 at 455 nm contains
45% trans and 55% trans at 297 nm. Complete conversion to
the trans configuration is obtained upon heating to 70 °C for
several hours.
CONCLUSION
■
With the 5-phenylazoimidazoles, we present a new class of
photoswitchable azoheterocycles. A novel type of azo coupling,
the reaction of lithiated imidazole with benzene diazonium salts
provides access to these azo compounds. Conversion from the
trans to the cis isomer by irradiation with 365 nm is superior
(95−98%) to most azobenzenes and azopyridines. Moreover,
cis-7 exhibits a very slow thermal back-isomerization to the
trans form.
Figure 2. (Top) DFT (B3LYP/6-31G*) calculated geometries
(dihedral angles are defined each by the four atoms indicated) and
relative energies (kcal mol−1) of the most stable conformations of the
trans and cis configuration of 1-methyl-5-phenylazoimidazole 7.
(Bottom) ACID plots of the structures above (only π electrons are
included, isosurface value 0.023). Critical isosurface values (CIV)
indicate the ACID value at the weakest point in conjugation.
EXPERIMENTAL SECTION
■
1-Dimethylsulfamoyl-5-phenylazoimidazole (4). 1-N,N-Dime-
thylsulfamoylimidazole19 (5.00 g, 28.5 mmol) was dissolved under
nitrogen in dry THF (150 mL) and cooled to −78 °C. Then, n-BuLi
(2.5 M in hexane, 11.4 mL, 28.5 mmol) was injected slowly, and the
solution was stirred at −78 °C for 30 min. Dimethylthexylsilyl chloride
(6.15 mL, 31.3 mmol) was then injected and the cooling bath removed
and stirred at room temperature overnight. The solution was again
cooled to −78 °C, n-BuLi (2.5 M in hexane, 12.5 mL, 31.3 mmol) was
injected slowly, and the mixture was stirred at −78 °C for another 30
min. Benzenediazonium tetrafluoroborate35 (5.47 g, 31.3 mmol) was
added under nitrogen, the cooling bath was removed, and the mixture
was stirred at room temperature overnight. The reaction mixture was
quenched with diluted aq NaHCO3, and the organic layer separated
and dried over MgSO4. The solvent was removed in vacuo to give 12.2
g of a deep red oil, which was used further without purification. The oil
(1.5 g) was stirred for 1 h with tetrabutylammonium fluoride solution
(7.0 mL, 1 M in THF) and a spatula tip of cesium fluoride in THF (10
mL) at room temperature. The solution was treated with diluted aq
NaHCO3 and then extracted with DCM (3 × 50 mL). The organic
layer was separated and dried over MgSO4, and the solvent was
evaporated in vacuo. Purification by chromatography on silica gel
using DCM/EtOAc 9:1 as eluent (Rf = 0.46) gave 4 as a red solid (535
mg, 54%): 1H NMR (600 MHz, CDCl3) δ 8.14 (s, 1H), 7.84 (m, 2H),
7.50 (m, 4H), 2.98 (s, 6H); 13C NMR (125 MHz, CDCl3) δ 152.8,
145.0, 140.9, 131.7, 129.3, 123.0, 118.7, 38.4; HRMS (EI) calcd for
C11H13N5SO2 279.07901, found 279.07892; UV−vis (toluene) λmax (lg
ε) = 340 nm (4.414); mp 91 °C.
density of delocalized electrons including the imidazole ring,
the phenyl ring, and the azo unit, whereas the cis isomer
exhibits a distinctly isolated phenyl ring.
The light-induced switching of conjugation might be
interesting for a number of applications (e.g., coupling of
radical centers33 or control of the donor strength of a
heterocyclic transition metal ligand34). Because of the differ-
ence in conjugation, the π−π* absorptions are well separated in
both isomers (λmax cis-7: 312 nm, trans-7: 362 nm, Figure 3).
4(5)-Phenylazoimidazole (1). 1-Dimethylsulfamoyl-5-phenylazoi-
midazole (4) (300 mg, 1.07 mmol) was stirred for 1 h with EtOH/
concd HCl 4:1 (50 mL) under reflux and neutralized after cooling with
40% KOH (pH ∼10, ice bath). The solution was extracted with
CHCl3 (3 × 50 mL), the combined organic layers were dried over
MgSO4, and the solvent was evaporated in vacuo. Purification by
chromatography on silica gel using EtOAc as eluent (Rf = 0.27) gave 1
Figure 3. Absorption spectra of 1-methyl-5-phenylazoimidazole (7) in
toluene (0.1 mM). Black curve: pure trans-isomer. Red curve:
photostationary equilibrium at 365 nm (98% cis- and 2% trans-isomer).
1
as an orange solid (185 mg, quant): H NMR (500 MHz, CDCl3) δ
7.85 (m, 3H), 7.80 (d, J = 1.0 Hz, 1H), 7.48 (m, 2H), 7.43 (m, 1H);
13C NMR (125 MHz, CDCl3) δ 152.7, 136.3, 130.6, 129.1, 122.5;
HRMS (EI) calcd for C9H8N4 172.07489, found 172.07538; UV−vis
(toluene) λmax (lg ε) = 347 nm (4.335); mp 149 °C.
1-Methyl-4-phenylazoimidazole (5). 4(5)-Phenylazoimidazole
(1) (200 mg, 1.16 mmol) was dissolved in THF (10 mL) and cooled
to 0 °C. Then, 60% sodium hydride in mineral oil (51.0 mg, 1.28
mmol) was added portionwise. The solution was allowed to warm to
room temperature and stirred for 10 min. After the solution was
cooled to 0 °C, methyl iodide (80.0 μL, 1.28 mmol) was added and
the solution stirred for 3 h at room temperature. Water (10.0 mL) was
added and the solution extracted with DCM (3 × 30 mL). The
There is strong absorption of the trans isomer at 365 nm and
very little in the cis form. This is probably the reason for the
very efficient conversion from trans- to cis-7 at this wavelength.
Similar to azobenzenes with electron-donating substituents,2
the π−π* transitions in our azoimidazoles are red-shifted (4:
340 nm, 5, 6: 336 nm, and 7: 362 nm) with respect to the
corresponding transition in parent azobenzene (314 nm) and
partially overlap with the n−π* transitions. This is in contrast
to azobenzenes where the π−π* (used for trans→cis isomer-
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combined organic layers were dried over MgSO4, and the solvent was
evaporated in vacuo. Purification by chromatography on silica gel
using acetone as eluent (Rf = 0.69) gave 5 as an orange solid (140 mg,
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1
65%): H NMR (500 MHz, CDCl3) δ 7.91 (m, 2H), 7.54 (d, J = 1.3
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357.
Hz, 1H), 7.47 (m, 3H), 7.41 (m, 1H) 3.78 (s, 3H); 13C NMR (125
MHz, CDCl3) δ 154.8, 153.0, 138.1, 130.3, 128.9, 122.7, 119.9, 34.1;
MS (EI) m/z 186 (M+), 158, 109; UV−vis (toluene) λmax (lg ε) = 336
nm (4.169); mp 167−170 °C. Anal. Calcd for C10H10N4: C, 64.50; H,
5.41; N, 30.09. Found: C, 64.60; H, 5.57; N, 29.83.
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1-Trityl-4-phenylazoimidazole (6). 4(5)-Phenylazoimidazole
(1) (213 mg, 1.24 mmol), trityl chloride (345 mg, 1.24 mmol), and
triethylamine (172 μL, 1.72 mmol) were heated under reflux in
acetonitrile (20 mL) for 3 h. The solvent was evaporated in vacuo and
the orange precipitate dissolved in CHCl3 (20 mL). The organic layer
was washed with water (2 × 10 mL). The combined organic layers
were dried over MgSO4, and the solvent was evaporated in vacuo. The
orange solid was recrystallized from acetonitrile to give 6 as an orange
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C.; Tuczek, F.; Herges, R. Chem.Eur. J. 2010, 16, 10074−10083.
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1
solid (390 mg, 76%): H NMR (500 MHz, CDCl3) δ 7.88 (m, 2H),
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7.59 (d, J = 1.4 Hz, 1H), 7.55 (d, J = 1.2 Hz, 1H) 7.45 (m, 2H), 7.38
(m, 10H), 7.21 (m, 6H); 13C NMR (500 MHz, CDCl3) δ 151.0,
150.9, 139.7, 137.2, 128.3, 127.7, 126.9, 126.4, 126.3, 120.6, 120.2,
74.3; MS (EI) m/z 414 (M+), 243, 165, 141, 111; UV−vis (toluene)
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C, 81.13; H, 5.35; N, 13.52. Found: C, 81.18; H, 5.61; N, 13.87.
1-Methyl-5-phenylazoimidazole (7). 1-Trityl-4-phenylazoimida-
zole (6) (300 mg, 724 μmol) was dissolved in dry DCM (5 mL), and
methyl triflate (125 μL, 1.09 mmol) was added and stirred at room
temperature for 16 h. Acetone/water 1:1 (20 mL) was added, and
stirring was continued for 3 h. Then, concd aq NaHCO3 (10 mL) was
added, and the organic layer was separated. The aqueous phase was
extracted with DCM (3 × 20 mL), and the combined organic layers
were dried over MgSO4. The solvent was evaporated in vacuo and the
residue purified by chromatography on silica gel using ethyl acetate as
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1
eluent (Rf = 0.20) to give 7 as a thick orange oil (102 mg, 76%): H
NMR (500 MHz, CDCl3) δ 7.83 (m, 2H), 7.66 (s, 1H), 7.59 (s, 1H),
7.49 (m, 2H), 7.45 (m, 1H), 3.98 (s, 3H); 13C NMR (500 MHz,
CDCl3) δ 153.0, 145.3, 140.3, 130.7, 129.1, 123.0, 122.4, 32.5; MS
(EI) m/z 186 (M+), 109; UV−vis (toluene) λmax (lg ε) = 362 nm
(4.411). Anal. Calcd for C10H10N4: C, 64.50; H, 5.41; N, 30.09.
Found: C, 64.53; H, 5.38; N, 30.09.
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ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR, 13C NMR, and UV−vis spectra for compounds 1 and
4−7 and crystallographic data for compound 4 (CIF). This
material is available free of charge via the Internet at http://
pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: rherges@oc.uni-kiel.de.
Notes
■
(33) Tanifuji, N.; Irie, M.; Matsuda, K. J. Am. Chem. Soc. 2005, 127,
13344−13353.
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2004, 234, 261−276.
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1936, 58, 2308−2309.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful for financial support from SFB 677 of the
Deutsche Forschungsgemeinschaft.
■
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