Triarylimidazole redox catalysts: Electrochemical analysis and empirical correlations

Article abstract of DOI:10.1021/jo302309m

A series of triarylimidazoles was synthesized and characterized electrochemically. The synthetic route is general, providing a pathway to 30 redox mediators that exhibit a > 700 mV range of accessible potentials. Most of the triarylimidazoles display three oxidation peaks where the first redox couple is quasi-reversible. The electronic character of the substituents affects the oxidation potential. This is exemplified by a linear correlation between the first oxidation potential and the sum of the Hammett σ⁺ substituent constants, as well as with a series of calculated ionization potentials. We close by putting forward a rule of thumb stating that for a given mediator, the upper limit of accessible potentials can be extended by at least 500 mV beyond the largest recorded value. A rationale, the conditions under which the rule is likely to apply, and an example are provided.

Full text of DOI:10.1021/jo302309m

Article
pubs.acs.org/joc
Triarylimidazole Redox Catalysts: Electrochemical Analysis and
Empirical Correlations
Ni-tao Zhang,† Cheng-chu Zeng,†,* Chiu Marco Lam,‡ Randi K. Gbur,‡ and R. Daniel Little*,‡
†College of Life Science & Bioengineering, Beijing University of Technology, Beijing 100124, China
‡Department of Chemistry & Biochemistry, University of California, Santa Barbara, Santa Barbara, California 93106, United States
S
* Supporting Information
ABSTRACT: A series of triarylimidazoles was synthesized and
characterized electrochemically. The synthetic route is general,
providing a pathway to 30 redox mediators that exhibit a > 700
mV range of accessible potentials. Most of the triarylimidazoles
display three oxidation peaks where the first redox couple is
quasi-reversible. The electronic character of the substituents
affects the oxidation potential. This is exemplified by a linear
correlation between the first oxidation potential and the sum of
the Hammett σ+ substituent constants, as well as with a series of
calculated ionization potentials. We close by putting forward a
rule of thumb stating that for a given mediator, the upper limit of
accessible potentials can be extended by at least 500 mV beyond
the largest recorded value. A rationale, the conditions under
which the rule is likely to apply, and an example are provided.
processes. In addition, the opportunity to achieve reagent-based
selectivity arises, with the mediator serving as the reagent in the
role of a homogeneous electron transfer catalyst. As such, one
can imagine taking advantage of the structure of the mediator
to select between different environments within a substrate in
order to achieve site selectivity between two electrophores
possessing very similar oxidation/reduction potentials.
Earlier this year we described the synthesis and character-
ization of five members of a new class of organic redox
mediators based on the triarylimidazole scaffold, M.3 These
substances proved to be stable and easy to synthesize and
provided access to a 370 mV window of potentials. They also
served as effective mediators for the oxidation of electron-rich
benzylic alcohols and ethers, converting each to the
corresponding carbonyl compounds in the manner portrayed
in Scheme 2.
INTRODUCTION
■
A redox mediator can be likened onto a photochemical
sensitizer; the latter transfers energy and spin, while a mediator
transfers charge.1 In contrast with direct oxidation and
reduction, which take place at an electrode, mediated processes
involve homogeneous electron transfer.2 Several features render
the use of a mediator particularly attractive. Consider for
example, the mediated oxidation of a substrate, Sub, to form a
product, P as portrayed in Scheme 1. Now, imagine that the
Scheme 1. Avoiding an Impasse and Catalysis
conversion is initiated by a thermodynamically unfavorable
electron transfer equilibrium between the mediator, M, in its
oxidized form, and the substrate (eq 2). This apparent
thermodynamic impasse can be overcome provided there is a
followup reaction that shifts the equilibrium away from it (eq
3). In this manner, it is possible to use a mediator to achieve
the oxidation (reduction) of a substrate at an electrode
potential that is less than that required for the direct process.
Consequently, the necessary input energy is reduced. This,
coupled with the fact that the mediator serves as a catalyst,
constitutes two of the attractive features of redox-mediated
Given these promising results, coupled with the prospect that
like the widely used triarylamines,4 the triarylimidazoles might
find applicability as light emitting devices and/or in lithium ion
Special Issue: Howard Zimmerman Memorial Issue
Received: October 23, 2012
Published: November 27, 2012
© 2012 American Chemical Society
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Scheme 2. Indirect Anodic Oxidation Mediated by 2-(4-Bromophenyl)-1-methyl-4,5-diphenyl-1H-imidazole
Scheme 3. Synthesis of Triarylimidazoles
Table 1. Peak Potential, EOX1, of the First Oxidation Peak for Three Series of Triarylimidazoles
a
The value in parentheses corresponds to oxidation potential of the correspondingly substituted triarylamine (data from ref 8).
battery research,5 we elected to expand the scope of our
investigation. Herein we (1) describe the synthesis and
electrochemical characterization of a series of 30 triarylimida-
zoles, (2) demonstrate the generality of the synthetic route, (3)
address how the nature of substituents affect the oxidation
potential of the mediator, and (4) correlate redox properties
with the molecular structures. In addition, we reveal the
existence of a linear correlation between the first oxidation
potential of the redox catalysts and both Hammett σ+
substituent constants as well as the calculated ionization
potentials.6 These correlations allow one to predict the
potential for systems yet to be synthesized and will assist in
determining which catalyst may be best suited to a particular
need.
presence of a catalytic amount of sodium dihydrogen
phosphate, are heated to 150 °C in a thick-walled tube fitted
with a screw-on Teflon top.7 The reaction proceeds smoothly
to afford the desired triarylimidazoles in excellent yields
(generally >85%).
Voltammetry. The electrochemical properties of the
triarylimidazoles were studied using cyclic voltammetry and
the results are summarized in Table 1. Samples were dissolved
in a solution of acetonitrile and dichloromethane (4:1 by
volume) containing 0.2 M LiClO4 as the supporting electrolyte.
A glassy carbon disk served as the working electrode and a Pt
wire the counter electrode. The oxidation potentials, measured
at the peak are designated as EOX1, and are reported relative to
Ag/0.1 M AgCl. With one exception (structure 3ai), all 30
imidazoles exhibit three oxidation peaks and one cathodic peak,
the latter being associated with the first of the three peaks (see
Supporting Information). Fortunately, the first redox couple is
quasi-reversible with the current being slightly smaller during
the reduction scan. Thus, the initially formed cation radicals
display good stability on the time scale of the CV measurement.
RESULTS AND DISCUSSION
■
Synthesis. The synthetic route used to access the mediators
is shown in Scheme 3. It follows a known procedure wherein an
aqueous solution consisting of a mixture of a benzil (1), an
aldehyde (2), methylamine and ammonium acetate in the
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The data in Table 1 is organized into three columns and ten
rows. In each column, R2 is held constant, namely, H for 3aa−
3aj, OMe for 3ba−3bj, and Br for 3ca−3cj, while R1 varies,
becoming progressively more electron withdrawing as one
reads down a column. For the family 3aa−3aj, EOX1 spans from
0.77 for 3aa (R1 = 4-N(CH3)2) to 1.37 V for 3aj (R1 = 4-NO2),
a range of 600 mV. In the case of redox catalyst 3ae where R1 is
H, its oxidation potential is 1.23 V. When R1 is -N(Me)2, OMe
or Me, the potentials shift in accord with the electron donating
character of the substituent, from 0.77 V (3aa), to 1.14 V
(3ab), to 1.22 V (3ad), respectively. In contrast, the oxidation
occurs at more positive potentials when R1 is electron
withdrawing. For example, the oxidation potentials of 3ag
and 3ai increase to 1.29 and 1.38 V when R1 is either F or CF3,
respectively.
correspond to the geometry-optimized structures. This accords
with the philosophy that in contrast with an electronic
excitation, the time scale of a voltammetric experiment is
significantly longer, thereby giving the ion radical time to relax
its geometry to that of an energy minimum. This approach is
similar to that previously adopted by Fry and co-workers when
they developed empirical relationships for triarylamines.8 The
calculated IP was then assumed to be the difference in energy
between that of the neutral and ion radical forms, each in
acetonitrile. Table 2 summarizes the data, and Figures 1 and 2
illustrate the excellent correlations (R2 = 0.976 for Σσ+ and
0.998 for IP) as well as the resulting empirical relationships.
Table 2. Summary of Data Used to Correlate Potential with
Calculated IP and Σσ+
Similar trends are observed for the family of redox catalysts
3ba−3bj (R2 = OMe) and 3ca−3cj (R2 = Br). For the former,
the oxidation potentials span 400 mV, changing from 0.71 to
1.11 V, while for 3ca−3cj the range is 620 mV, spanning from
1
calculated IP
(eV)
Eox (V, vs Ag/
structure
R1
R2
Σσ+
AgCl)
3bc
3bf
3ac
3af
3cf
OMe
Br
OMe
OMe
H
−2.34
−1.41
−0.78
+0.15
+0.45
4.64
4.77
4.87
5.07
5.17
0.96
1.05
1.14
1.28
1.37
1
0.82 to 1.44 V. Once again, we see that the Eox values are
OMe
Br
affected qualitatively by the electronic character of R1, namely,
the more electron donating the substituent(s), the easier the
substrate is to oxidize.
H
Br
Br
1
The Eox values of the redox catalysts are also dependent
upon the electronic character of R2 when values are compared
for the same R1 groups. For example, the oxidation potential of
3aa (R1 = N(CH3)2) is 0.77 V, and rises to 0.82 V when an
inductively withdrawing Br is introduced, and decreases to 0.71
when OMe is present. Similarly, when R1 = H, the oxidation
potentials changes from 1.23 for 3ae (R2 = H), to 1.02 V (3be;
R2 = OMe) to 1.30 V (3ce, R2 = Br).
Interestingly, the slope associated with each correlation is
small, namely, 0.144 for Σσ+ and 0.770 for IP (note Figure 1).
We were surprised to discover that the slopes are also small for
the triarylamines and also to note the very similar relationships
between the observed potential and calculated IP, it being
−3.190 + 0.789(IP) for the triarylamines, and −2.614 +
0.770(IP) for the imidazoles. Thus, neither framework
responds dramatically to substituent effects, though each
responds in the anticipated manner with the more electron
rich systems being easiest to oxidize. Undoubtedly the small
slopes are due, at least in part, to the fact that both frameworks
must distort to avoid steric interactions, thereby reducing
conjugation and therefore, the ability of the substituent to
express its character (note Figure 2). The aryl units at C-2 and
C-5, positioned closest to the N-methyl group of the imidazole,
twist most severely. The torsion angle between the imidazole
core and the Ph unit at C-5, for example, is 120°. In addition,
interaction between the vicinal aryl groups located at C-4 and
C-5 also leads to deviation from planarity. Both distortions are
shown clearly in the space-filling rendition of the X-ray
structure for 3ac (R1 = OMe, R2 = H), portrayed below.
Concluding Remarks: a Rule of Thumb and a Critique.
The triarylimidazoles are particularly easy to synthesize, leading
to a reasonably broad spectrum of structures. Those described
allow access to a range of potentials, from 0.71 to 1.44 V, a
difference of 730 mV, and nearly double the range that was
exhibited by triarylimidazoles prior to this investigation.3 The
range is wide enough to allow one to oxidize a host of common
functional groups. We suggest that the operational range is even
larger based upon the following “rule of thumb”, a rule that is
intended to serve as a useful guideline.4a,13 The rule states that
for a mediated electron transfer, the upper limit of accessible
potentials can be extended by at least 500 mV, a value that is
founded in our work (vide infra) and that of others, most
The inductively withdrawing character of Br is clearly evident
when one compares the data for the series of redox catalysts
3ae-3af-3ce-3cf. Each member of the series has one more p-Br
group than the one following it in the list, and the potentials
increase steadily (1.23, 1.28, 1.30 and 1.37 V). It is worth
noting that structure 3cf is the triarylimidazole analog of tris(4-
bromophenyl)amine, probably the most frequently used of the
triarylamine redox mediators and in this regard, it is of interest
to compare their oxidation potentials: for 3cf the value is 1.37
while for tris(4-bromophenyl)amine, it is 0.78.8 Finally, we
observe that the potentials for the triarylimidazoles are
uniformly larger than those observed for the corresponding
triarylamine when the substituents are the same. Compare, for
example, the values listed in parentheses in Table 1 for the
1
triphenylimidazole 3ae (EOX = 1.23) with triphenylamine
1
(0.54), trimethoxyimidazole 3bc (EOX = 0.96) with its amine
1
counterpart (0.25), and tribromoimidazole 3cf (EOX = 1.37)
with the tribromoamine (0.78).
Computational Analysis and Empirical Relationships.
Five of the imidazoles, 3bc, 3bf, 3ac, 3af, and 3cf, were
examined to determine whether there was an empirical
relationship between the first oxidation potential, Eox1, and
(a) the sum of Hammett σ+ values for the substituents
appended to the three aromatic rings, and/or (b) the calculated
ionization potentials (IP).9 Should they exist, the correlations
would provide a convenient means by which to predict the
potential for systems yet to be synthesized and also to assess
how effectively these systems respond to changes in their
electronic character.8,10,11
IPs were calculated using the B3LYP hybrid functional and a
6-31+G(d) basis set, with acetonitrile as the solvent for both
the neutral and cation forms.12 The cation radical energies
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1
Figure 1. (a) Graph of observed potential vs Σσ+ for systems 3bc, 3bf, 3ac, 3af, and 3cf. Eox = 0.144(Σσ+) + 1.273 (R2 = 0.976). (b) Graph of
1
observed potential vs calculated IP for systems 3bc, 3bf, 3ac, 3af, and 3cf. Eox = 0.770(IP) − 2.614 (R2 = 0.998).
using tris(4-bromophenyl)amine as the mediator, a substance
that is ∼520 mV easier to oxidize than the substrate. Yet, when
the operating potential is set at that of the mediator, the
rearrangement occurs smoothly to deliver 5 in a 66% isolated
yield.
While the range of accessible potentials is good, the
significant distortion from planarity presents a problem, one
that undoubtedly limits the effectiveness of substituents to
influence potential. One way to obviate this problem would be
Figure 2. Twisting of the aryl groups out of planarity reduces
interaction with substituents.
to link the ortho carbons of the aromatics positioned at C-4 and
C-5 to generate the fused framework 6.15 Whether the
anticipated improvement in behavior/properties will be
manifested remains to be seen.
notably the late Eberhard Steckhan.13,14 The implication is that
one can use the mediator to oxidize (reduce) a substrate whose
oxidation (reduction) potential exceeds that of the mediator by
500 mV, perhaps more, depending upon the equilibrium
constant for the electron transfer and the rate of the followup
reaction that drains the unfavorable equilibrium (note Scheme
1).13
The examples shown in Scheme 4 illustrate the rule of thumb
in action. In the first instance,3 the mediated oxidation of p-
Scheme 4. Two Examples of the “Rule of Thumb” in Action
EXPERIMENTAL SECTION
General Information. All solvents were of commercial quality and
■
1
were dried and purified by conventional methods. The H NMR and
13C NMR spectra were obtained using either a 300 or 400 MHz
spectrometer in solvent (CDCl3) with TMS as internal reference. Data
for structures 3ac, 3af, 3bc, 3bf, 3cf have been reported previously.3
High resolution mass spectra were obtained using a time-of-Flight
(TOF) mass spectrometer fitted with an electron ionization (EI)
source.
General Procedure for the Synthesis of Triarylimidazoles. A
mixture of the benzil of choice (5 mmol), methylamine (5 mmol; or
another amine of interest), aldehyde (5 mmol), ammonium acetate (5
mmol) and NaH2PO4 (1.5 mmol) was added to a thick-walled test
tube with a screw-on Teflon top. The reaction mixture was heated to
150 °C and maintained at the temperature for 2−5 h; the reaction
mixture was stirred throughout. Then the reaction mixture was cooled
methoxybenzyl alcohol was carried out at the potential of the
mediator. Nevertheless, it occurs smoothly despite the fact that
the triarylimidazole 3af is 260 mV easier to oxidize than the
substrate. The second example, stemming from our previous
exploration of the use of triarylamine mediators in synthesis,14b
exemplifies the rearrangement of the strained hydrocarbon 4
to room temperature. Acetone was added to dissolve the mixture and
the undissolved residue was removed by filtration. After evaporation of
the solvent under reduced pressure, the resulting solid residue was
recrystallized from acetone−water to obtain pure products 3.
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N,N-Dimethyl-4-(1-methyl-4,5-diphenyl-1H-imidazol-2-yl)-
aniline (3aa):16 325.2 mg, 92% yield; brown powder, mp: 227−229
2-(3,4-Dimethoxyphenyl)-4,5-bis(4-methoxyphenyl)-1-
methyl-1H-imidazole (3bb): 347.0 mg, 86% yield; yellow powder,
mp: 125−126 °C; IR (KBr): ν 3436, 2932, 1611, 1519, 1494 cm−1; 1H
NMR (400 MHz, CDCl3): δ 3.47 (s, 3H), 3.77 (s, 3H), 3.88 (s, 3H),
3.94 (s, 3H), 3.98 (s, 3H), 6.77 (d, J = 8.8 Hz, 2H), 6.95−6.97 (m,
1H), 7.00 (d, J = 8.8 Hz, 2H), 7.19−7.21 (m, 1H), 7.31−7.33 (m,
3H), 7.49 (d, J = 8.8 Hz, 2H); 13C NMR (100 MHz, CDCl3): δ 33.0,
55.1, 55.3, 56.0, 56.1, 110.9, 112.6, 113.5, 114.5, 121.4, 123.5, 123.9,
127.6, 128.0, 129.1, 132.2, 137.1, 147.4, 149.1, 149.5, 158.2, 159.7.
HREI-MS (m/z) calcd. for C26H27N2O4 (M + H) 431.1971, found
431.1962.
1
°C; H NMR (400 MHz, CDCl3): δ 3.02 (s, 6H), 3.49 (s, 3H), 6.80
(d, J = 8.8 Hz, 2H), 7.11−7.15 (m, 1H), 7.19−7.22 (m, 2H), 7.40−
7.48 (m, 5H), 7.56 (d, J = 7.2 Hz, 2H), 7.63 (d, J = 8.8 Hz, 2H).
2-(3,4-Dimethoxyphenyl)-1-methyl-4,5-diphenyl-1H-imida-
zole (3ab): 314.9 mg, 85% yield; white needles, mp: 197−198 °C; IR
1
(KBr): ν 3436, 2955, 1602, 1586, 1528,1322 cm−1; H NMR (400
MHz, CDCl3): δ 3.52 (s, 3H), 3.96 (s, 3H), 4.00 (s, 3H), 6.98 (d, J =
8.4 Hz, 1H), 7.16 (t, J = 6.8 Hz,, 1H), 7.21−7.7.25 (m, 3H), 7.37 (s,
1H), 7.42−7.58 (m, 7H); 13C NMR (100 MHz, CDCl3): δ 33.2, 55.9,
56.0, 110.8, 112.5, 121.5, 123.7, 126.3, 127.0, 128.1, 128.5, 129.0,
130.3, 130.9, 131.3, 134.6, 137.4, 147.9, 149.0, 149.6. HRESI-MS (m/
z) calcd. for C24H23N2O2 (M + H) 371.1760, found 371.1749.
1-Methyl-4,5-diphenyl-2-(p-tolyl)-1H-imidazole (3ad):17
292.0 mg, 90% yield; white needles, mp: 211−212 °C; 1H NMR
(400 MHz, CDCl3): δ 2.42 (s, 3H), 3.50 (s, 3H), 7.12−7.16 (m, 1H),
7.19−7.23 (m, 2H), 7.30 (d, J = 8.0 Hz, 2H), 7.40−7.48 (m, 5H),
7.54−7.56 (m, 2H), 7.64 (d, J = 8.0 Hz, 2H).
4,5-Bis(4-methoxyphenyl)-1-methyl-2-(p-tolyl)-1H-imida-
zole (3bd): 342.2 mg, 89% yield; yellow needles, mp: 129−131 °C; IR
1
(KBr): ν 3435, 2919, 1614, 1517, 1495, 1247 cm−1; H NMR (400
MHz, CDCl3): δ 2.42 (s, 3H), 3.46 (s, 3H), 3.76 (s, 3H), 3.88 (s, 3H),
6.77 (d, J = 8.8 Hz, 2H), 7.00 (d, J = 8.4 Hz, 2H), 7.29 (d, J = 8.0 Hz,
2H), 7.32 (d, J = 8.8 Hz, 2H), 7.49 (d, J = 8.8 Hz, 2H), 7.62 (d, J = 8.0
Hz, 2H); 13C NMR (100 MHz, CDCl3): δ 21.4, 33.0, 55.2, 55.3, 113.5,
114.5, 123.5, 127.6, 128.0, 128.2, 128.9, 129.1, 129.2, 132.2, 137.2,
138.5, 147.5, 158.1, 159.7. HREI-MS (m/z) calcd. for C25H24N2O2
(M) 384.1838, found 384.1850.
1-Methyl-2,4,5-triphenyl-1H-imidazole (3ae):13 276.2 mg, 89%
yield; white needle, mp: 143−145 °C; 1H NMR (400 MHz, CDCl3): δ
3.51 (s, 3H), 7.15−7.17 (m, 1H), 7.20−7.24 (m, 2H), 7.40−7.52 (m,
8H), 7.54−7.57 (m, 2H), 7.75−7.77 (m, 2H).
4,5-Bis(4-methoxyphenyl)-1-methyl-2-phenyl-1H-imidazole
1
(3be): 326.0 mg, 88% yield; white needles, mp: 158−161 °C; H
NMR (300 MHz, CDCl3): δ 3.48 (s, 3H), 3.76 (s, 3H), 3.87 (s, 3H),
6.77 (d, J = 9.0 Hz, 2H), 7.00 (d, J = 8.7 Hz, 2H), 7.31 (d, J = 8.7 Hz,
2H), 7.40−7.51 (m, 5H), 7.73−7.75 (m, 2H).
2-(4-Fluorophenyl)-1-methyl-4,5-diphenyl-1H-imidazole
(3ag): 279.1 mg, 85% yield; yellow powder, mp: 158−160 °C; IR
1
(KBr): ν 3440, 2919, 2850, 1603, 1528, 1467, 1225 cm−1; H NMR
2-(4-Fluorophenyl)-4,5-bis(4-methoxyphenyl)-1-methyl-1H-
imidazole (3bg): 314.6 mg, 81% yield; yellow powder, mp: 148−150
(400 MHz, CDCl3): δ 3.49 (s, 3H), 7.13−7.23 (m, 5H), 7.41 (d, J =
7.6 Hz, 2H), 7.45−7.54 (m, 5H), 7.71−7.75 (m, 2H); 13C NMR (100
MHz, CDCl3): δ 33.1, 115.6 (d, JC−F = 22.0 Hz), 126.4, 126.9, 127.1
1
°C; IR (KBr): ν 3435, 2938, 2837, 1612, 1519, 1495, 1248 cm−1; H
NMR (400 MHz, CDCl3): δ 3.46 (s, 3H), 3.77 (s, 3H), 3.88 (s, 3H),
6.77 (d, J = 8.8 Hz, 2H), 7.00 (d, J = 8.8 Hz, 2H), 7.18 (t, J = 8.8 Hz,
2H), 7.31 (d, J = 8.8 Hz, 2H), 7.48 (d, J = 8.8 Hz, 2H), 7.72 (dd, J =
8.4, 5.2 Hz, 2H); 13C NMR (100 MHz, CDCl3): δ 33.0, 55.2, 55.3,
113.6, 114.5, 115.6 (d, JC−F = 22.0 Hz), 123.3, 127.2, 127.4, 128.0,
129.3, 130.9 (d, JC−F = 8.0 Hz), 132.2, 137.3, 146.4, 158.2, 159.8, 163.0
(d, JC−F = 248.0 Hz). HREI-MS (m/z) calcd. for C24H21N2O2F (M)
388.1587, found 388.1585.
2-(3,4-Difluorophenyl)-4,5-bis(4-methoxyphenyl)-1-methyl-
1H-imidazole (3bh): 357.6 mg, 88% yield; yellow powder, mp: 135−
137 °C; IR (KBr): ν 3436, 2928, 2838, 1609, 1519, 1495, 1247 cm−1;
1H NMR (400 MHz, CDCl3): δ 3.48 (s, 3H), 3.77 (s, 3H), 3.88 (s,
(d, JC−F = 3.0 Hz), 128.1, 128.7, 129.1, 130.5, 130.8, 130.9 (d, JC−F
=
9.0 Hz), 131.1, 134.5, 137.7, 146.9, 163.0 (d, JC−F = 247.0 Hz). HREI-
MS (m/z) calcd. for C22H17N2F (M) 328.1376, found 328.1359, calcd.
for C22H16N2F (M − H) 327.1298, found 327.1302.
2-(3,4-Difluorophenyl)-1-methyl-4,5-diphenyl-1H-imidazole
(3ah): 287.5 mg, 83% yield; white powder, mp: 130−131 °C; IR
1
(KBr): ν 3434, 2918, 2850, 1603, 1535, 1501, 1484, 1225 cm−1; H
NMR (400 MHz, CDCl3): δ 3.51 (s, 3H), 7.15−7.24 (m, 3H), 7.28−
7.33 (m, 1H), 7.39−7.41 (m, 2H), 7.48−7.53 (m, 6H), 7.60−7.64 (m,
1H); 13C NMR (100 MHz, CDCl3): δ 33.1, 117.6 (d, JC−F = 17.0 Hz),
118.3 (d, JC−F = 18.0 Hz), 125.1, 125.2 (dd, JC−F = 6.0, 4.0 Hz), 126.5,
126.9, 127.9−128.0 (m), 128.2, 128.8, 129.1, 130.8, 130.9, 134.3,
3H), 6.77 (d, J = 8.4 Hz, 2H), 7.01 (d, J = 8.8 Hz, 2H), 7.24−7.32 (m,
3H), 7.45−7.46 (m, 3H), 7.57−7.62 (m, 1H); 13C NMR (100 MHz,
CDCl3): δ 33.0, 55.2, 55.3, 113.6, 114.6, 117.5 (d, JC−F = 18.0 Hz),
118.1 (d, JC−F = 19.0 Hz), 123.0, 125.1 (dd, JC−F = 3.0, 3.0 Hz), 127.2,
138.0, 145.7, 150.3 (dd, JC−F = 250.0, 15.0 Hz), 150.7 (dd, JC−F
=
252.0, 16.0 Hz). HREI-MS (m/z) calcd. for C22H16N2F2 (M)
346.1282, found 346.1273, calcd. for C22H15N2F2 (M − H)
345.1203, found 345.1213.
128.0, 128.0−128.2 (m), 129.8, 132.1, 137.7, 145.2, 150.3 (d, JC−F
=
1-Methyl-4,5-diphenyl-2-(4-(trifluoromethyl)phenyl)-1H-
234.0 Hz), 150.6 (dd, JC−F = 234.0, 10.0 Hz), 158.3, 159.9; HRESI-MS
(m/z) calcd. for C24H21N2O2F2 (M + H) 407.1571, found 407.1563.
4,5-Bis(4-methoxyphenyl)-1-methyl-2-(4-(trifluoromethyl)-
phenyl)-1H-imidazole (3bi): 368.3 mg, 84% yield; yellow powder,
mp: 141−144 °C; IR (KBr): ν 3436, 2949, 2837, 1616, 1517, 1328
cm−1; 1H NMR (400 MHz, CDCl3): δ 3.52 (s, 3H), 3.77 (s, 3H), 3.88
(s, 3H), 6.78 (d, J = 8.8 Hz, 2H), 7.01 (d, J = 8.4 Hz, 2H), 7.32 (d, J =
8.4 Hz, 2H), 7.48 (d, J = 8.8 Hz, 2H), 7.74 (d, J = 8.4 Hz, 2H), 7.89
(d, J = 8.0 Hz, 2H); 13C NMR (100 MHz, CDCl3): δ 33.2, 55.2, 55.3,
113.6, 114.3, 114.6, 122.8, 125.5 (q, JC−F = 4.0 Hz), 127.0, 128.1,
129.2, 130.2, 132.1, 132.4, 134.3, 137.9, 145.8, 158.4, 159.9; HRESI-
MS (m/z) calcd. for C25H22N2O2F3 (M + H) 439.1633, found
439.1621.
imidazole (3ai): 321.6 mg, 85% yield; white powder, mp: 147−148
1
°C; IR (KBr): ν 3432, 2923, 1616, 1324, 1160 cm−1; H NMR (400
MHz, CDCl3): δ 3.55 (s, 3H), 7.15−7.25 (m, 3H), 7.41−7.43 (m,
2H), 7.46−7.55 (m, 5H), 7.76 (d, J = 8.0 Hz, 2H), 7.92 (d, J = 8.0 Hz,
2H); 13C NMR (100 MHz, CDCl3): δ 33.3, 125.6 (q, JC−F = 4.0 Hz),
126.6, 126.9, 128.2, 128.8, 129.1, 129.2, 130.7, 130.8, 130.9, 131.2,
134.3, 134.4, 138.3, 146.3; HREI-MS (m/z) calcd. for C23H17N2F3
(M) 378.1344, found 378.1330, calcd. for C23H16N2F3 (M − H)
377.1266, found 377.1274.
1-Methyl-2-(4-nitrophenyl)-4,5-diphenyl-1H-imidazole
(3aj):18 316.3 mg, 89% yield; golden-colored powder, mp: 199−200
°C; 1H NMR (400 MHz, CDCl3): δ 3.59 (s, 3H), 7.18−7.24 (m, 3H),
7.40−7.43 (m, 2H), 7.49−7.54 (m, 5H), 7.99 (d, J = 8.8 Hz, 2H), 8.37
(d, J = 8.8 Hz, 2H).
4-(4,5-Bis(4-methoxyphenyl)-1-methyl-1H-imidazol-2-yl)-
N,N-dimethylaniline (3ba): 372.2 mg, 90% yield; yellow powder,
mp: 153−154 °C; IR (KBr): ν 3436, 2920, 1612, 1563, 1518, 1247
cm−1; 1H NMR (400 MHz, CDCl3): δ 3.02 (s, 6H), 3.46 (s, 3H), 3.76
(s, 3H), 3.87 (s, 3H), 6.76 (d, J = 8.8 Hz, 2H), 6.80 (d, J = 8.8 Hz,
2H), 6.99 (d, J = 8.8 Hz, 2H), 7.31 (d, J = 8.4 Hz, 2H), 7.50 (d, J = 8.4
Hz, 2H), 7.60 (d, J = 8.4 Hz, 2H); 13C NMR (100 MHz, CDCl3): δ
33.0, 40.4, 55.1, 55.3, 112.0, 113.4, 114.4, 118.9, 123.8, 127.9, 128.0,
128.7, 129.9, 132.2, 136.9, 148.2, 150.5, 158.0, 159.5. HRESI-MS (m/
z) calcd. for C26H28N3O2 (M + H) 414.2182, found 414.2177.
4,5-Bis(4-methoxyphenyl)-1-methyl-2-(4-nitrophenyl)-1H-
imidazole (3bj):19 361.4 mg, 87% yield; orange powder, mp: 123−
1
125 °C; H NMR (400 MHz, CDCl3): δ 3.57 (s, 3H), 3.78 (s, 3H),
3.89 (s, 3H), 6.79 (d, J = 8.8 Hz, 2H), 7.02 (d, J = 8.8 Hz, 2H), 7.32
(d, J = 8.8 Hz, 2H), 7.48 (d, J = 8.0 Hz, 2H), 7.97 (d, J = 8.8 Hz, 2H),
8.35 (d, J = 8.8 Hz, 2H).
4-(4,5-Bis(4-bromophenyl)-1-methyl-1H-imidazol-2-yl)-N,N-
dimethylaniline (3ca): 460.1 mg, 90% yield; yellow powder, mp:
212−214 °C; IR (KBr): ν 3434, 2920, 2838, 1612, 1543, 1487, 1443
cm−1; 1H NMR (400 MHz, CDCl3): δ 3.03 (s, 6H), 3.48 (s, 3H), 6.80
(d, J = 8.8 Hz, 2H), 7.26 (d, J = 8.4 Hz, 2H), 7.34 (d, J = 8.4 Hz, 2H),
7.40 (d, J = 8.4 Hz, 2H), 7.58 (d, J = 8.4 Hz, 2H), 7.60 (d, J = 8.0 Hz,
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2H); 13C NMR (100 MHz, CDCl3): δ 33.3, 40.3, 112.0, 118.0, 120.2,
122.9, 128.6, 128.8, 129.9, 130.2, 131.2, 132.3, 132.4, 133.7, 149.3,
150.7; HREI-MS (m/z) calcd. for C24H21N3Br2 (M) 509.0102, found
509.0089, calcd. for C24H20N3Br2 (M − H) 508.0024, found 508.0006.
4,5-Bis(4-bromophenyl)-2-(3,4-dimethoxyphenyl)-1-methyl-
1H-imidazole (3cb): 449.0 mg, 85% yield; white powder, mp: 176−
2H), 7.64 (d, J = 8.4 Hz, 2H), 7.77 (d, J = 8.0 Hz, 2H), 7.88 (d, J = 8.0
Hz, 2H); 13C NMR (100 MHz, CDCl3): δ 33.4, 120.8, 122.6, 123.5,
125.3, 125.6 (q, JC−F = 3.0 Hz), 128.5, 129.2, 130.1, 130.7, 131.1,
131.4, 132.3, 132.6, 132.8, 137.5, 146.8; HREI-MS (m/z) calcd. for
C23H15N2Br2F3 (M) 533.9554, found 533.9541, calcd. for
C23H14N2Br2F3 (M − H) 532.9476, found 532.9478.
4,5-Bis(4-bromophenyl)-1-methyl-2-(4-nitrophenyl)-1H-imi-
dazole (3cj): 456.7 mg, 89% yield; yellow powder, mp: 131−133 °C;
IR (KBr): ν 3433, 2920, 1637, 1522, 1344 cm−1; 1H NMR (400 MHz,
CDCl3): δ 3.59 (s, 3H), 7.27 (d, J = 8.0 Hz, 2H), 7.38 (s, 4H), 7.66 (d,
J = 8.0 Hz, 2H), 7.98 (d, J = 8.4 Hz, 2H), 8.36 (d, J = 8.4 Hz, 2H); 13C
NMR (100 MHz, CDCl3): δ 33.6, 121.0, 123.7, 124.0, 128.5, 129.0,
129.4, 131.0, 131.5, 132.2, 132.7, 132.8, 136.6, 138.2, 146.0, 147.7.
HREI-MS (m/z) calcd. for C22H15N3O2Br2 (M) 510.9531, found
510.9534.
Cyclic Voltammetry Measurements. Cyclic voltammograms
were measured using a 273A Potentiostat/Galvanostat (Princeton
Applied Research) equipped with an electrochemical analysis software,
using a conventional three-electrode cell. The working electrode was a
glassy carbon disk electrode (ca. ϕ = 3 mm). The auxiliary and
reference electrodes for these studies corresponded to a Pt wire and
Ag/AgCl (in 3 M KCl), respectively; LiClO4 (0.2 mol L−1) in a
solution of acetonitrile and dichloromethane (4:1 by volume) was
used as the supporting electrolyte system. The concentration of each
triarylimidazole was 1 mmol L−1.
Calculations. Calculations were performed using the Spartan ’08
software package.12 As indicated in the text, the quantities used to
calculate IP values refer to energy minimized structures for both the
neutral and cation radical forms, in acetonitrile as the solvent. The
B3LYP hybrid functional was used along with the 6-31+G(d) basis set.
1
178 °C; IR (KBr): ν 3466, 2921, 1631, 1531, 1488, 1224 cm−1; H
NMR (400 MHz, CDCl3): δ 3.50 (s, 3H), 3.95 (s, 3H), 3.98 (s, 3H),
6.97 (d, J = 8.4 Hz, 1H), 7.19−7.21 (m, 1H), 7.27 (d, J = 8.0 Hz, 2H),
7.31 (s, 1H), 7.35 (d, J = 8.4 Hz, 2H), 7.40 (d, J = 8.4 Hz, 2H), 7.62
(d, J = 8.4 Hz, 2H); 13C NMR (100 MHz, CDCl3): δ 33.3, 56.0, 56.1,
110.9, 112.5, 120.4, 121.5, 123.1, 128.6, 129.2, 129.8, 131.3, 132.3,
132.5, 133.3, 136.8, 148.4, 149.1, 149.8. HRESI-MS (m/z) calcd. for
C24H21N2O2Br2 (M + H) 526.9970, found 526.9954.
4,5-Bis(4-bromophenyl)-2-(4-methoxyphenyl)-1-methyl-1H-
imidazole (3cc): 413.5 mg, 83% yield; white powder, mp: 145−147
°C; IR (KBr): ν 3434, 2953, 2924, 1612, 1577, 1531, 1479, 1402, 838
cm−1; 1H NMR (400 MHz, CDCl3): δ 3.47 (s, 3H), 3.87 (s, 3H), 7.02
(d, J = 8.8 Hz, 2H), 7.26 (d, J = 8.4 Hz, 2H), 7.34 (d, J = 8.4 Hz, 2H),
7.39 (d, J = 8.8 Hz, 2H), 7.62 (d, J = 8.4 Hz, 2H), 7.64 (d, J = 8.8 Hz,
2H); 13C NMR (100 MHz, CDCl3): δ 33.2, 55.4, 114.1, 120.4, 123.0,
123.1, 128.5, 129.1, 129.9, 130.4, 131.3, 132.3, 132.5, 133.4, 136.8,
148.4, 160.2. HREI-MS (m/z) calcd. for C23H18N2OBr2 (M)
495.9786, found 495.9786.
4,5-Bis(4-bromophenyl)-1-methyl-2-(p-tolyl)-1H-imidazole
(3cd): 400.2 mg, 83% yield; white powder, mp: 189−190 °C; IR
1
(KBr): ν 3433, 2919, 1638, 1478, 1391, 833 cm−1; H NMR (400
MHz, CDCl3): δ 2.42 (s, 3H), 3.49 (s, 3H), 7.27 (d, J = 8.4 Hz, 2H),
7.30 (d, J = 8.0 Hz, 2H), 7.35 (d, J = 8.8 Hz, 2H), 7.40 (d, J = 8.8 Hz,
2H), 7.59−7.63 (m, 4H); 13C NMR (100 MHz, CDCl3): δ 21.4, 33.2,
120.4, 123.1, 127.6, 128.6, 128.9, 129.2, 129.4, 129.9, 131.3, 132.3,
132.5, 133.4, 137.0, 139.0, 148.6. HREI-MS (m/z) calcd. for
C23H18N2Br2 (M) 479.9837, found 479.9831.
ASSOCIATED CONTENT
■
4,5-Bis(4-bromophenyl)-1-methyl-2-phenyl-1H-imidazole
S
* Supporting Information
(3ce): 412.0 mg, 88% yield; white powder, mp: 201−203 °C; IR
1
(KBr): ν 3435, 3061, 2920, 1494, 1474, 837 cm−1; H NMR (300
Crystallographic data (CIF) and thermal ellipsoid plot for
structure 3ac, a table of redox data for 30 imidazole mediators,
CV curves for redox catalysts 3ag, 3bc and 3ch and their quasi-
MHz, CDCl3): δ 3.50 (s, 3H), 7.27 (d, J = 8.4 Hz, 2H), 7.35 (d, J = 8.7
Hz, 2H), 7.40 (d, J = 8.7 Hz, 2H), 7.45−7.53 (m, 3H), 7.63 (d, J = 8.4
Hz, 2H), 7.70−7.73 (m, 2H); 13C NMR (100 MHz, CDCl3): δ 33.2,
120.5, 123.2, 128.6, 128.7, 129.0, 129.4, 129.8, 130.5, 131.3, 132.3,
132.5, 133.3, 137.1, 148.5. HRESI-MS (m/z) calcd. for C22H16N2Br2
(M) 465.9680, found 465.9689.
1
reversible first oxidation peak, H and 13C NMR spectral data
for imidazole redox catalysts. This material is available free of
charge via the Internet at http://pubs.acs.org.
4,5-Bis(4-bromophenyl)-2-(4-fluorophenyl)-1-methyl-1H-
imidazole (3cg): 393.8 mg, 81% yield; white powder, mp: 172−174
AUTHOR INFORMATION
1
°C; IR (KBr): ν 3450, 2919, 1639, 1479, 1447, 833 cm−1; H NMR
■
(400 MHz, CDCl3): δ 3.48 (s, 3H), 7.20 (d, J = 8.4 Hz, 1H), 7.22 (d, J
= 8.8 Hz, 1H), 7.26 (d, J = 8.4 Hz, 2H), 7.35 (d, J = 8.8 Hz, 2H), 7.39
(d, J = 8.8 Hz, 2H), 7.63 (d, J = 8.4 Hz, 2H), 7.70 (d, J = 8.4 Hz, 1H),
7.72 (d, J = 8.4 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ 33.2, 115.8
(d, JC−F = 21.0 Hz), 120.5, 123.3, 126.8, 128.5, 129.4, 129.7, 130.9 (d,
JC−F = 4.0 Hz), 131.3, 132.3, 132.5, 133.2, 137.1, 147.5, 163.2 (d, JC−F
= 248.0 Hz). HREI-MS (m/z) calcd. for C22H15N2Br2F (M) 483.9586,
found 483.9580, calcd. for C22H14N2Br2F (M − H) 482.9508, found
482.9509.
Corresponding Author
*zengcc@bjut.edu.cn; little@chem.ucsb.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Grants from the National Natural
Science Foundation of China (No. 21272021), Beijing Natural
Science Foundation (No. 7112008), the National Key
Technology R&D Program (2011BAD23B01), the National
Basic Research Program of China (No. 2009CB930200) and a
fund to C.-C.Z. from the Beijing City Education Committee
(KM201010005009). R.D.L., C.M.L. and R.K.G. thank Amgen
and Clorox for donations in support of education, R.D.L.
thanks the UCSB Academic Senate Council on Research and
Instructional Resources, and R.K.G. and R.D.L. thank the
National Science Foundation PIRE-ECCI Program for a
fellowship to R.K.G. (NSF PIRE-ECCI Fellow). We are
grateful to Dr. Guang Wu (UCSB) for the x-ray analysis of
structure 3ac, and Dr. James Pavlovich (UCSB) for performing
the mass spectral analyses.
4,5-Bis(4-bromophenyl)-2-(3,4-difluorophenyl)-1-methyl-
1H-imidazole (3ch): 438.6 mg, 87% yield; white powder, mp: 144−
146 °C; IR (KBr): ν 3449, 1604, 1568, 1533, 1484, 1321, 835 cm−1;
1H NMR (400 MHz, CDCl3): δ 3.50 (s, 3H), 7.24−7.33 (m, 3H),
7.34−7.39 (m, 4H), 7.45−7.47 (m, 1H), 7.56−7.60 (m, 1H), 7.62−
7.64 (m, 2H); 13C NMR (100 MHz, CDCl3): δ 33.2, 117.7 (dd, JC−F
=
14.0, 5.0 Hz), 118.3 (d, JC−F = 18 Hz, 2.0 Hz), 120.7, 123.4, 125.2 (t,
JC−F = 5.0 Hz), 127.0−127.6 (m), 128.5, 129.4, 129.8, 131.4, 132.3,
132.6, 133.0, 137.3, 146.3, 150.3 (dd, JC−F = 250.0 Hz, 15.0 Hz),
150.9(dd, JC−F = 251.0 Hz, 14.0 Hz). HRESI-MS (m/z) calcd. for
C22H14N2Br2F2 (M) 501.9492, found 501.9485.
4,5-Bis(4-bromophenyl)-1-methyl-2-[4-(trifluoromethyl)-
phenyl]-1H-imidazole (3ci): 450.4 mg, 84% yield; white powder,
mp: 154−156 °C; IR (KBr): ν 3450, 2924, 1619, 1496, 1327, 1167,
1
1134, 1073, 832 cm−1; H NMR (400 MHz, CDCl3): δ 3.54 (s, 3H),
7.27 (d, J = 8.0 Hz, 2H), 7.36 (d, J = 8.8 Hz, 2H), 7.39 (d, J = 8.8 Hz,
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Article
eliminated but frequently redox catalysts can be applied with potentials
which are 600 mV or in some cases even up to 1 Volt lower than the
electrode potentials of the substrates.” Given this comment, the reader
will recognize that our “500 mV rule of thumb” is conservative, and
deliberately so.
DEDICATION
■
We dedicate this manuscript to the memory of R.D.L.’s
wonderful mentor and friend, Howard E. Zimmerman.
(14) (a) Miranda, J.; Wade, C.; Little, R. D. J. Org. Chem. 2005, 70,
8017−8026. (b) Park, Y. S.; Little, R. D. J. Org. Chem. 2008, 73, 6807−
6815. (c) Simonet, J.; Pilard, J.-F. Electrogenerated Reagents. In
Organic Electrochemistry, 4th ed.; Lund, H., Hammerich, O., Eds.;
Dekker: New York, 2001; Chapter 29.
(15) We suspect, but have not proven, that the in situ formation of
structures like 6, generated via an intramolecular Friedel−Crafts-like
cyclization of the initially formed cation radical may be responsible for
the fact that in some cases, the first redox couple is quasi- rather than
fully reversible.
REFERENCES
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(9) Strictly speaking, the calculated values do not correspond to
ionization potentials (IP). By definition, IP refers to the gas phase and
to a vertical excitation where the geometry does not change. In
contrast, the values we have calculated are based upon geometry-
optimized structures and solvent-corrected energies. We have chosen
this approach because of the vastly different time scales associated with
a vertical excitation and a voltammetric measurement.
(10) The correlation of redox potential with substituent constants is,
of course, not new; an excellent discussion is found in: Zuman, P.
Substituent Effects in Organic Polarography; Plenum Press: New York,
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(11) Davis, A. P.; Fry, A. J. J. Phys. Chem. A 2010, 114, 12299−12304.
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(13) We are not claiming that our rule of thumb constitutes an
original contribution. Our intent is simply to draw attention to one of
the very attractive features associated the use of mediators. We are well
aware of the pioneering contributions of Steckhan; see reference 4a for
example, as well as: Steckhan, E. Organic syntheses with electro-
chemically regenerable redox systems. In Topics in Current Chemistry,
Electrochemistry I; Steckhan, E., Ed.; Springer: Berlin, 1987; Vol. 142.
There, he wrote: “It is very exciting that not only overpotentials can be
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