The Suzuki-Miyaura reaction as a tool for modification of phenoxyl-nitroxyl radicals of the 4: H -imidazole N -oxide series

Article abstract of DOI:10.1039/c8ra05103h

2-(3,5-Di-tert-butyl-4-hydroxyphenyl)-5-(4-iodophenyl)-4,4-dimethyl-4H-imidazole 3-oxide reacts with phenylboronic acid and its substituted derivatives in a cross-coupling reaction of the Suzuki-Miyaura type to form 5-biphenyl derivatives of 4H-imidazole-N-oxide. Interaction of the same compound with B₂(pin)₂ in the presence of PdCl₂(PPh₃)₂ proceeds through the formation of intermediate 1,3,2-dioxoborolane and leads to the product of homocoupling: biphenyl-bis(imidazole). Oxidation of the resultant imidazoles with lead dioxide quantitatively yields stable conjugated phenoxyl-nitroxyl mono- and diradicals, which are of interest as electroactive paramagnetic materials. The crystal structure of the monoradical, 2,6-di-tert-butyl-4-[1-oxido-4-(biphenyl-4-yl)-5,5-dimethyl-1H-imidazole-2(5H)-ylidene]cyclohex-2,5-dienone, its magnetic susceptibility, EPR spectra of the obtained hybrid radicals in solution, and cyclic voltammetry characteristics of 4H-imidazoles were studied.

Full text of DOI:10.1039/c8ra05103h

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The Suzuki–Miyaura reaction as a tool for
modification of phenoxyl-nitroxyl radicals of the
Cite this: RSC Adv., 2018, 8, 26099
4
H-imidazole N-oxide series†
a
b
a
bc
Yury A. Ten,
Oleg G. Salnikov, Svetlana A. Amitina,
Dmitri V. Stass,
ab
ab
ab
d
Tatyana V. Rybalova,
Evgeny A. Mostovich
Maxim S. Kazantsev,
and Dmitrii G. Mazhukin
Artem S. Bogomyakov,
ab
*
2-(3,5-Di-tert-butyl-4-hydroxyphenyl)-5-(4-iodophenyl)-4,4-dimethyl-4H-imidazole 3-oxide reacts with
phenylboronic acid and its substituted derivatives in a cross-coupling reaction of the Suzuki–Miyaura
type to form 5-biphenyl derivatives of 4H-imidazole-N-oxide. Interaction of the same compound with
2 2 2 3 2
B (pin) in the presence of PdCl (PPh ) proceeds through the formation of intermediate 1,3,2-
dioxoborolane and leads to the product of homocoupling: biphenyl-bis(imidazole). Oxidation of the
resultant imidazoles with lead dioxide quantitatively yields stable conjugated phenoxyl-nitroxyl mono-
and diradicals, which are of interest as electroactive paramagnetic materials. The crystal structure of the
monoradical, 2,6-di-tert-butyl-4-[1-oxido-4-(biphenyl-4-yl)-5,5-dimethyl-1H-imidazole-2(5H)-ylidene]
cyclohex-2,5-dienone, its magnetic susceptibility, EPR spectra of the obtained hybrid radicals in solution,
and cyclic voltammetry characteristics of 4H-imidazoles were studied.
Received 13th June 2018
Accepted 13th July 2018
DOI: 10.1039/c8ra05103h
rsc.li/rsc-advances
9
oxide series (Chart 1), we turned our attention to the possibility
of modifying the para-haloaryl group at the 5th position of the
The cross-coupling reaction of the Suzuki–Miyaura type can be heterocycle using Pd-catalyzed cross-coupling reactions, in
undoubtedly considered as one of the fundamental methods for particular, the Suzuki–Miyaura process.
Introduction
1
constructing aromatic and heteroaromatic ensembles in the
On the one hand, the introduction of an electron-
synthesis of biologically active compounds for pharmaceutical withdrawing type of substituent possessing an ꢀM-effect into
2
chemistry as well as in the chemistry of new materials (prep- the para-position of the 5-aryl moiety should lead to the addi-
3
aration of components of optoelectronic devices and liquid- tional stabilization of the radical center owing to further
4
crystalline semiconducting oligomers ) and in organic photo- distribution of the electron density of an unpaired electron over
5
voltaics. Recent research on the Suzuki–Miyaura reaction the conjugated system. On the other hand, further functional-
includes extensive studies of various substrates involved in this ization of hybrid radicals means developing their practical
process (for example, low-reactive aryl chlorides, aryl uorides,
6
or tosylates), a wide range of reagents (boronic acids and their
7
derivatives), and several generations of palladium catalysts
8
with different ligand environments.
Continuing the research on the reactivity of new stable
radicals, i.e., hybrid phenoxyl-nitroxides of the 4H-imidazole N-
a
NN Vorozhtsov Novosibirsk Institute of Organic Chemistry SB RAS,
Lavrentyeva Ave., Novosibirsk, 630090, Russia, d-mazhukin@yandex.ru
9 Acad.
b
Novosibirsk State University, 2 Pirogova Str., Novosibirsk, 630090, Russia
c
Voevodsky Institute of Chemical Kinetics and Combustion SB RAS, 3 Institutskaya Str.,
Novosibirsk, 630090, Russia
d
International Tomography Center SB RAS, 3A Institutskaya Str., Novosibirsk, 630090,
Russia
†
Electronic supplementary information (ESI) available: Synthetic procedures,
analytical and spectroscopic data for all new compounds. Experimental and
simulated ESR spectra for the new radicals, CV data for 4H-imidazole N-oxides.
CCDC 1564701. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c8ra05103h
Chart 1 Examples of stable hybrid radicals: 4H-imidazole N-oxide
based phenoxyl-nitroxides (PN); 1,2,3-dithiazolo-1,2,4-thiadiazinyls
(DTA-TDA).
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applicability because of creation of new promising dyadic and radical with a mild selective reagent whose redox potential
triadic molecular systems. cannot change the paramagnetic status of the substrate. In this
While developing fundamentally new smart devices based respect, a study on Pd-catalyzed C–C cross-coupling reactions
on fully organic materials, researchers have become more (particularly, the Suzuki–Miyaura process) involving various
interested in the design and construction of dyads and triads of types of persistent radicals was carried out here under the
DA (donor–acceptor) or ADA (acceptor–donor–acceptor) types, conditions of both methods.
where the group containing a stable radical moiety plays the
For example, thermally unstable nitroxyl radicals stabilized
10
role of the acceptor. These systems can be considered molec- by nitroxyl group conjugation with the benzene nucleus (t-
ular switches (transforming from a neutral state to a zwitter- butylaryl, tetrahydroquinolinyl, [3,1]-benzoxazinyl, and dihy-
ionic or diradical form in solution under the inuence of droacridinyl type) were introduced into the Suzuki–Miyaura
external stimuli, i.e., temperature, pressure, or light), and some reaction in the form of their stable precursors, namely,
11
of them can have properties of conductors in a solid form.
secondary amines. Aer formation of the cross-coupling prod-
Typical examples of such dyads and triads are shown in Chart 2. ucts, the latter were transformed into target nitroxides with m-
13
As for stable hybrid radicals, only a few representatives of CPBA in a yield of 37–70%. At the same time, more robust
this type of paramagnetics were synthesized by scientists in the iodoarene derivatives of the benzotriazinyl radical (Blatter's
1
4a–c
14d
14e
last decade. For instance, resonance-stabilized planar 1,2,3- radical),
dithiazolo-1,2,4-thiadiazinyl (Chart 1) and related radicals
verdazyl radical, and iso-indolinoxyls readily
12a
12b
reacted in cross-coupling with either arylboronic acids or aryl-
possess highly delocalized and easily tuned spin distributions. triolborate salts, which in contrast to parent acids possess
Therefore, the wide range of physical and chemical properties greater reactivity. Depending on the number of iodoarene
of thiazyl-based radicals make them useful building blocks for groups in these radicals, mono-, bis-, and tris-cross-coupling
magnetic and conductive materials as well as coordinating products were obtained without losing a radical center. The
ligands.
yields ranged from 97% for the mono-cross-coupling product to
Depending on the kinetic stability of radical types, there are 67% for bis- and 52% for the tris-phenyl-substituted compound.
two fundamentally different methods for their functionaliza- As to the cyclic nitroxide based on an imidazole nucleus, to the
tion. The rst method represents the interaction of their best of our knowledge, there is only one report about the
diamagnetic precursors directly along a reactive group with Suzuki–Miyaura reaction involving nitronyl nitroxyl radicals
various nucleophilic or electrophilic reagents, and the radical containing an ortho(meta)-iodophenyl group at the C-2 atom of
center is generated at the nal stage, when the product is the heterocycle. The authors explained a rather low yield of
treated with an oxidizing agent (or an excess of an electrophilic cross-coupling products in this reaction (40–43% for the ortho-
reagent). The second method consists of treating the stable iodophenyl derivative and 44% for the meta-iodophenyl deriv-
ative) by competitive destruction of a nitronyl nitroxide occur-
ring at the elevated temperature under typical conditions of the
ꢁ
15
Suzuki process (80 C, 2 h).
Accordingly, it was interesting to nd out how hybrid
phenoxyl-nitroxide radicals would behave under the conditions
of the Suzuki–Miyaura reaction and whether they would be
sufficiently stable to retain the paramagnetic center. Besides,
Pd-catalyzed cross-coupling reactions involving 5-(haloaryl)-4H-
imidazole derivatives and their N-oxides are not mentioned in
any databases.
Results and discussion
To conduct cross-coupling experiments, the para-iodoarene
substituted derivative of 4H-imidazole N-oxide was chosen as
a model substrate because solely iodo derivatives show the
greatest reactivity among other haloaryl compounds and
produce quantitative yields of biphenyls under the classical
Suzuki reaction conditions.
A diamagnetic precursor of the hybrid radical, key 5-(p-
iodophenyl)substituted 4H-imidazole 3-oxide 1, was synthe-
sized in 6 stages based on Friedel–Cras acylation of an iodo-
benzene excess with iso-butyric acid chloride followed by
bromination of 4-iodo-iso-butyrophenone 2, treatment of
bromo compound 3 with hydroxylamine, and hydrolysis of the
formed 2-hydroxylamine oxime 4 by boiling with concentrated
hydrobromic acid. Condensation of the obtained salt of 2-
Chart 2 Some electroactive and photoactive dyads and triads based
10i
10d,f
on stable radicals: oxophenalyloxyl (A), phenoxyl-imidazolyl (B),
10e,g
10b
di-tert-butyl-ortho-semiquinonyl (C),
and trityl (D).
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dioxide excess (the second method is preferable because the
yield of 4H-imidazole N-oxide is quantitative in this case) gave
compound 1 (Scheme 1). Hybrid phenoxyl nitroxyl radical 8 was
formed with a yield of more than 90% when imidazole 1 was
treated with a threefold excess of PbO .
2
At rst, this radical 8 was introduced into the typical Suzuki
cross-coupling reaction under standard classical conditions
(
arylboronic acid 9d, Pd[(PPh
3
)
4
] (5 mol%), K
2
CO
3
, a two-phase
ꢁ
system (toluene–water), 110 C, argon atmosphere). Heating the
mixture for 24 h caused the solution to lose its dark brown color
typical for the conjugated radical, then disappearance of the
EPR signal and formation of a single product, bright yellow
diamagnetic 5-biphenyl-4H-imidazole N-oxide 10d, in a quanti-
tative yield. Oxidation of the obtained compound with lead
dioxide in chloroform smoothly led to corresponding stable
phenoxyl-nitroxyl radical 11d. Thereaer, its immediate
precursor, phenolic imidazole 1, was introduced into the cross-
coupling reaction to reduce the total number of steps necessary
to synthesize new biphenyl derivatives of the hybrid radical.
Thus, compound 1 was involved in the Suzuki–Miyaura reaction
with various arylboronic acids possessing both donor and
acceptor groups (Scheme 2). Regardless of the nature of
substituents in the reagent and its steric features, conversion of
imidazole 1 was complete, and cross-coupling products 10a–
d were obtained in good yields (66–85%). Apparently, the
presence of a sterically hindered phenolic group in the
substrate did not inhibit the cross-coupling process, nor did it
have a signicant effect on the reaction yields. It should be
noted that transformation of biphenyl derivatives of 10a–d into
ꢁ
Scheme 1 Reagents and conditions: (i) AlCl
3
, 45 C, 15 h; (ii) Br
OH ꢂ HCl, NaOH, MeOH/
OAc, MeOH, rt, 6 h; (vi) phenoxyl-nitroxide radicals starts even when their solutions are
2 2
, Et O/
dioxane, room temperature (rt), 2 h; (iii) NH
2
ꢁ
2
H O, 65 C, 7 h; (iv) 48% HBr, D, 2 h; (v) NH
4
2 3 2 3
MnO , CHCl , rt, 7 h; and (vii) PbO , CHCl , rt, 2 h.
kept in ambient air, whereas oxidation of 4H-imidazoles 10a–
d with lead dioxide ends within a few hours, and hybrid radicals
11a–d form quantitatively (Scheme 2).
hydroxylamino ketone 5 with 4-hydroxy-3,5-di-tert-butylbenzal-
Cyclic voltammograms of the 4H-imidazoles 10a–d are pre-
dehyde 6 in the presence of ammonium acetate led to 2,4-diaryl- sented in Fig. 1. All the compounds show similar redox behavior
,5-dihydroimidazole 7 with a good yield, and oxidation of 7 consisting of irreversible oxidation and reduction features.
with air oxygen in the presence of Cu(OAc) or a manganese Apparently, oxidation of starting phenol-imidazole 10 proceeds
2
2
Scheme 2 The Suzuki cross-coupling reaction of 4H-imidazole N-oxide 1 with substituted arylboronic acids and the synthesis of biphenyl
derivatives of hybrid phenoxyl-nitroxide.
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Fig. 3 Molecular structure (an ORTEP diagram with 30% ellipsoid
Fig.
imidazoles 10a–d in a CH
1
Cyclic voltammograms for cross-coupling products, 4H- probability) and atom numbering of hybrid phenoxyl-nitroxide 11a.
+
CN solution. Potentials calibrated vs. Fc/Fc . Selected bond lengths ( A˚ ): N1–O1 1.270(3), N1–C1 1.354(3), N2–C1
3
1
1
1
.395(3), N2–C2 1.296(3), C2–C3 1.511(4), N1–C3 1.502(3), C1–C6
.409(3), C6–C7 1.420(3), C7–C8 1.3573, C8–C9 1.477(4), C9–O2
via irreversible step-by-step formation of galvinoxyl radical I,
ꢁ
.246(3), C2–C12 1.464(3), and C15–C18 1.489(4). Valence angles ( ):
quinoid cation II, and dication radical III, respectively (Scheme N1–C1–N2 109.6(2), C1–N1–C3 110.2(2), C1–N2–C2 109.3(2), and
ꢁ
S1 in ESI†). The reduction step consists only of an irreversible C2–C3–N1 98.1(2). Torsion angles ( ): N1–C1–C6–C7 ꢀ6.6(4), N2–
C2–C12–C13 ꢀ1.5(4), and C14–C15–C18–C19 8.0(5).
peak, which corresponds to the formation of anion-radical IV.
Oxidation and reduction peaks are listed in Table S1, ESI.†
EPR spectra of all the obtained hybrid radicals contain a set
of 21 lines, characterized by a triplet form consisting of 7 lines
in each segment due to the interaction of the unpaired electron
with two nonequivalent nitrogen nuclei (with the isotropic
hyperne splitting (hfs) constants A 1 ¼ 0.541 mT and A 2 ¼
analysis (Fig. 3). The best solvent for these hybrid radicals
seemed to be dipolar aprotic acetonitrile having a high dielec-
tric constant. It should be noted that if solutions of radicals are
kept long in a nonpolar solvent at room temperature, then these
radicals partially decompose.
N
N
0
.061 mT, respectively) and with two nonequivalent protons in
the molecular quinone fragment (hfs constants A
1 ¼ 0.161
mT, A
Among notable features of the molecular structure of radical
H
˚
1
1a are characteristic lengths of the C–O bond (1.246(3) A) and
H
2 ¼ 0.149 mT) (Fig. 2 and S2–S4 in ESI†). The hfs
C–C bond connecting imidazole and quinoid rings (1.409(3) A˚ ).
constants for “distant” protons of the biphenyl moiety are
negligible, indicating the absence of noticeable spin density
delocalization on the aryl substituent at the C-4 position of the
heterocycle.
The rst one is equal to the same C–O bond of 2,4,6-tri-tert-
16
butylphenoxyl radical and very slightly differs from that of the
˚
C–O bond in related biradicaloids (1.252 A (ref. 17a) and 1.241
17c
˚
A, respectively) but signicantly differs from C]O bond
We succeeded in growing suitable single crystals of unsub-
stituted biphenyl derivative 11a for X-ray crystallographic
0
0
0
length in diamagnetic 3,5,3 ,5 -tetra-tert-butyl-4,4 -diphenoqui-
none (1.228 A˚ ). The second one is the typical “one-and-half”
bond with the length close to that of the “bridge bond” C]C
18
˚
19
(
1.408 A) of conjugated phenoxyl radical galvinoxyl and the
˚
C]C bond (1.403 A) between phenoxyl and heterocyclic moie-
17c
ties in biradicaloid QBT. The data above suggest that a radical
molecule should exist mostly in the phenoxyl form in a solid
state. Although the imidazole and both biphenyl rings are plane
within the margin of experimental error, the quinoid ring is
˚
slightly distorted [ꢃ0.040(3) A], with C9 and O2 atoms deviating
˚
from the quinoid ring plane by 0.051(3) and 0.186(2) A,
respectively. The tetracyclic frame of 11a is virtually at; for
example, dihedral angles between imidazole and adjacent rings
ꢁ
ꢁ
are 5.8 for the phenoxyl ring and 1.2 for an aryl unit of
biphenyl. The torsion angle between ring planes in the biphenyl
ꢁ
moiety is 7.2 . Such planarity of the tetracyclic moiety is
appropriate for formation of 0D supramolecular unity, namely
a dimer of molecules, via an interaction of plane p-systems
(Fig. 4 and Table S2 in ESI†) through “head-to-head” stacking.
The molecular dimers in turn interact with the nearest ones via
weak C17–H/O2 hydrogen bonds (Table S2 in ESI†) binding
them along two directions into layers parallel to plane (1, 0, ꢀ2).
Fig. 2 An ESR spectrum of 4-biphenyl-substituted hybrid radical 11a,
recorded at 20 C in a degassed toluene solution; the black curve is the
experimental spectrum, the red curve is its mathematical recon-
ꢁ
struction; giso ¼ 2.0059, A
1 ¼ 0.541 mT, A 2 ¼ 0.061 mT, A
1 ¼ 0.161 There are no short interatomic distances between the layers.
N N H
mT, A
H
2 ¼ 0.149 mT.
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linker; they are of interest as weakly antiferromagnetically
coupled spin-dimers with a singlet ground state that can be
21
turned into a triplet state in a magnetic eld.
To obtain the desired diradical based on imidazole 1, we
tried to employ the recently developed palladium-catalyzed
version of the Ullmann-type reaction, which proceeds with
iodoarene derivatives without phosphine ligands in the pres-
ence of palladium and potassium acetates under aerobic
22
conditions. Nevertheless, under reductive coupling conditions
described by those authors (10 mol% Pd(OAc) , 5 equiv. of
2
ꢁ
AcOK, Me CO, 100 C), imidazole 1 remained unchanged even
2
aer 24 h heating in a sealed ampoule.
Fig. 4 The pair of molecules of compound 11a associated by p–p
interactions.
Earlier, Higuchi and Kurth proposed practical one-pot
synthesis of dimeric polyheterocycles, bis(terpyridines), which
involves forming the corresponding boronic ester from 4 -(para-
0
bromophenyl) terpyridine via the Miyaura reaction, followed by
Assessment of magnetic features of biphenyl radical 11a the Suzuki reaction of the appropriate bromo derivative under
showed that at 300 K, meff was 1.73 m , in good agreement with specially selected and controlled conditions (temperature, the
the theoretical spin value m (Fig. 5) for one paramagnetic solvent, base, quantity of the boron reagent, and Pd catalyst).
center with spin S ¼ 1/2 for a g-factor equal to 2. Intermolecular We tested a similar tandem process involving tricyclic iodo
exchange interactions are weak.
derivative 1 under conditions that allowed us to avoid the use of
B
23
B
Having such a reactive substrate as imidazole 1, we were strong bases. Indeed, the interaction of two equivalents of 4H-
tempted to try it in the self-coupling reaction to obtain imidazole 1 with bis(pinacolato)diboron 12 and potassium
a symmetric biphenyl compound bearing 4H-imidazole moie- acetate as
ties at positions C-4 and C-4 , i.e., to carry out a reaction of the triphenylphosphine)palladium in dimethylformamide (DMF)
a
base in the presence of dichlorobis(-
0
ꢁ
following type: 2Ar–I + {M} / Ar–Ar + MI
2
. This approach would with heating to 110–115 C resulted in formation of a dark
allow us to synthesize another interesting type of a para- orange precipitate of homocoupling product 13 (Scheme 3).
magnetic substance, namely, a symmetrical hybrid diradical, This reaction is likely to proceed with the formation of an
which has not been known previously. Similar compounds, for intermediate (2-aryldioxaborolane 14) which then reacts with an
example, molecules formed by quinonoidal residues conjugated excess of substrate 1, leading to biphenyl derivative 13. Indeed,
with donor bridging fragments, switch to the biradicaloid state 1,3,2-dioxoborolane 14 was isolated as the dominant product in
17
under the inuence of external stimuli (e.g., temperature). As the temperature-controlled process via the interaction of 4H-
reported recently, they could substantially increase the power imidazole 1 with a slight excess of B (pin) . Its structure was
2
2
conversion efficiency of organic photovoltaic systems if used in proved unambiguously by a combination of elemental analysis
20
1
13
11
small quantities (0.3–0.6 wt%) as additives. There is another data and H, C, and B NMR spectra.
class of stable diradicals, bis(nitronyl nitroxides), and their
paramagnetic nuclei interact with each other through a rigid p- conventional organic solvents (alcohols, halocarbons, DMF, or
DMSO); therefore, it can be isolated from the reaction mixture
quantitatively. To separate the precipitated product from
palladium compounds and inorganic impurities, we performed
the Soxhlet hot extraction by boiling the precipitate with chlo-
roform. Again, heating compound 13 in an oxygen atmosphere
resulted in staining the solution with a dark orange color, as
was the case for biphenyl imidazole derivatives 10. Evidently,
the staining was due to the formation and accumulation of
stable phenoxyl-nitroxyl radicals (TLC control). To prove the
dimeric structure of the obtained product, we used NMR spec-
troscopy along with elemental analysis data.
Obtained biphenyl bis(imidazole) 13 was poorly soluble in
Thus, signals typical for a symmetrical structure were
observed in the spectra of bis(imidazole) 13 in a triuoroacetic
acid solution. For example, singlets of gem-methyl and t-butyl
groups shied downeld to 2.06 and 1.57 ppm, respectively, in
the proton spectrum of compound 13. Protons of the biphenyl
0
0
part represent the AA BB system with doublets at 8.05 and
1
3
8
.51 ppm. The C NMR spectrum of 13 is even more remark-
able in this respect (downeld shiing of several signals). Thus,
low-eld signals at 166.0 and 194.3 ppm were attributed to
Fig. 5 Experimental meff (T) dependence for radical 11a.
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Scheme 3 The Suzuki–Miyaura homocoupling reaction involving 4H-imidazole N-oxide 1 and preparation of symmetrical hybrid diradical 15.
carbon atoms respectively associated with nitrogen atoms of bis(phenol) 13 and formation of hybrid diradical 15, which was
nitrone and imino groups of the heterocycle (which corre- isolated in a high yield by gentle evaporation of the reaction
sponds to the downeld shiing of these signals by 18–19 ppm). solution in a solid form as a dark brown powder. We found that
Apparently, the covalent interaction of bis(imidazole) 13 with compound 15 could be puried by ash chromatography on
triuoroacetic acid contributes to the downeld shiing of silica gel without any noticeable decomposition. The diradical
1
13
signals of the H and C nuclei. CF
3
COOH is known to be is highly soluble in chlorinated solvents (methylene chloride,
) but is virtually insoluble in nonpolar or
capable of protonating a number of organic compounds, for chloroform, and CCl
4
instance, ketones, various imino derivatives, and cyclic nitro- protic/aprotic solvents (hexane, ether, ethanol, and acetone).
nes, by shiing an appropriate carbon signal of the double C]X The use of other oxidizing systems to transform bis(phenol) 13
1
3
bond in the C NMR spectrum into a low eld from 1.5 to (for example, DDQ or an aqueous alkaline solution of potassium
24
27.5 ppm.
ferricyanide) also led to the preferential formation of compound
The oxidation of biphenyl bis(imidazole) 13 with a large 15, but there was signicant accumulation of impurities in this
excess of lead dioxide (20–30 equiv.) in a dilute chloroform case. Isolated from chloroform, the diradical is a crystal solvate
solution for 15–30 min resulted in complete conversion of containing one molecule of the solvent. A peak at m/z 782.4 with
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conjugated diradicals, corresponding to A–D–A triads in which
the ter(quater)phenyl or ter(quater)thiophene core will be the
donor linker. Obviously, the 1,3,2-dioxoborolane derivative of
4H-imidazole 3-oxide synthesized here can be applied to obtain
asymmetric hetero diradicals bearing different types of a para-
27
magnetic nucleus, which are appealing as building blocks for
functional materials, particularly, for creation of components of
a quantum computer.
Experimental
General
Analytical-grade reagents and solvents were used, and reactions
Fig. 6 Cyclic voltammograms for monoradical 8 and diradical 15 in were monitored by TLC. Column chromatography and TLC were
3
a CH CN solution.
performed using Acros silica gel 60A (0.035–0.070 mm) and
Sorbl PTLC-AF-UV 254 (Russia), respectively. Mass spectra
1
were registered on a DFS high-resolution mass spectrometer. H
1
3
the intensity of ꢄ4% corresponding to the [M + 2] ion (Fig. S5 in NMR and C NMR spectra were recorded on Bruker AV-300, AV-
ESI†) is observed in the high-resolution mass spectrum of dir- 400, DRX-500 spectrometers at 300/400/500 and 75/100/125
adical 15. It is worth noting that all the previously obtained MHz, respectively, for 1–5% solutions of compounds in
hybrid monoradicals in their high-resolution mass spectrum CDCl , DMSO-d , or CF COOH (for bis(imidazole) 13). Residual
3
6
3
are characterized by an [M + 1] ion. Other characteristic signals proton signals from the deuterated solvents served as references
1
13
in the mass spectrum of compound 15 are ions at m/z 766 (12%) [DMSO-d (2.50 ppm), CDCl (7.24 ppm), for H spectra].
C
6
3
and 750 (18%), corresponding to a sequential loss of one and NMR chemical shis are reported in reference to an undeu-
two oxygen atoms by the diradical molecule.
terated residual solvent [CDCl (76.9 ppm) or DMSO-d (39.4
3
6
1
3
It should be noted that this is the rst example of formation ppm)]. The assignment of signals of carbon atoms in C NMR
of a stable dimeric phenoxyl-nitroxide that can apparently exist spectra of 4H-imidazole derivatives 1 and 10a–d was carried out
in various hybrid forms (15-NN, 15-NP, and 15-PP), including on the basis of an earlier spectral study on the spectra of cyclic
28
exotic biradical species with a zwitterionic structure whose nitrones. Fourier transform infrared (FT-IR) spectra were
charged parts are separated by the diphenoquinone linker (15- acquired in KBr pellets on a Bruker Vector-22. The UV-Vis
PPQZ and 15-NPQZ) as well as diamagnetic zwitterionic struc- spectra were obtained for EtOH solutions using a Hewlett-
ture 15-DZ (Scheme 3).
Packard HP 8453 spectrophotometer. Raman spectrum of dir-
Comparative cyclic voltammograms of radicals 8 and 15 are adical 15 was recorded with a Bruker Optics SENTERRA
presented in Fig. 6. Iodine-substituted radical 8 yields two microscope spectrometer equipped with a 785 nm direct diode
ꢀ
1
irreversible one-electron oxidation peaks at ꢀ0.48 and 0.76 V, laser with 1 cm resolution and 25 mW laser power. The
ꢀ1
which correspond to formation of the oxoammonium cation Raman shi range considered was between 170 and 3000 cm
.
and radical dicationic species. Nonetheless, diradical 15 Elemental analyses were performed on an automatic CNS
undergoes reversible two-electron oxidation at E1/2 ¼ ꢀ0.63 V analyzer Euro EA 3000. The melting points were determined on
forming a dioxammonium feature (Scheme S2 in ESI†). Further an FP 81 HT instrument, Mettler Toledo.
oxidation of dioxammonium dication proceeds quasi-reversibly
at 0.80 V.
Cyclic voltammetry measurements were conducted in
CH CN solution on computer-controlled P-8nano
a
3
a
The EPR spectral and structural characteristics of this potentiostat/galvanostat (Elins, Russia) in combination with
unusual diradical and its magnetic analysis will be the subject a three-electrode cell (Gamry); 0.1 M n-Bu NPF was applied as
4
6
of our future study and deserve a separate publication.
a supporting electrolyte. Pt, a Pt wire, and Ag/AgCl served as
a working, counter, and reference electrode, respectively. The
reference electrode was calibrated by measurement of the redox
potentials of ferrocene.
Conclusions
Thus, we showed that 5-(para-iodophenyl)-4,4-dimethyl-2-(3,5-
2 2
Arylboronic acids 9a–d, B (pin) (bis(pinacolato)diboron) 12,
di-tert-butyl-4-hydroxyphenyl)-4H-imidazole 3-oxide possessing and palladium catalysts were obtained from commercial sour-
high reactivity in Pd-catalyzed processes can readily react with ces (Acros Organics); 3,5-di-t-butyl-4-hydroxy-benzaldehyde 6
various arylboronic acids as well as bis(pinacolato)diboron in was synthesized according to a procedure described in the
29
one-pot tandem synthesis with formation of the corresponding literature. All the other reagents were purchased from Acros
biphenyl derivatives in high yields. The products can be easily Organics or Alfa-Aesar.
oxidized to stable hybrid mono- and diradicals of the phenoxyl-
Synthesis of key 2-(3,5-di-t-butyl-4-hydroxyphenyl)-5-(4-iodo-
nitroxyl series. We are planning to extend this approach to other phenyl)-4,4-dimethyl-4H-imidazole 3-oxide 1, its subsequent
25
types of cross-coupling (e.g., the Stille reaction and direct cross-coupling reactions, and oxidation of products are
26
arylation of donor heterocycles ) to synthesize new dimeric described in ESI.†
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X-Ray structural analysis
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The unit cell parameters and experimental intensities of radical
2
3
11a were measured at 296(2) K with a Bruker Kappa Apex II CCD
˚
diffractometer with Mo Ka radiation (l ¼ 0.71073 A) and
a graphite monochromator. The structure was solved by direct
methods and rened in an anisotropic (isotropic for H)
approximation using the SHELX-97 soware suite. Positions
of the hydrogen atoms were calculated and rened by means of
the riding model. Analysis of the geometry and intermolecular
30
4 G. Hu, S. P. Kitney, S. M. Kelly, W. Harrison and M. O'Neill,
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A. Krishna, A. V. Lunchev and A. C. Grimsdale, in Synthetic
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M. Leclerc and J.-F. Morin, Wiley-VCH Verlag GmbH & Co.
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D. G. Hall, Boronic Acids, Wiley-VCH Verlag GmbH & Co.
KGaA, Weinheim, 2005.
3
1
interactions was carried out in the PLATON soware.
Crystallographic data of 11a:‡ C31
H
35
N
2
O
2
, FW ¼ 467.61,
˚
monoclinic, P2
1
/c a 11.5429(7), b 14.3317(8), c 16.995(1) A,
ꢁ
˚
3
ꢀ3
b 109.552(2) , V 2649.3(3) A , Dcalc 1.172 g cm , m(Mo-Ka) 0.073
ꢀ
1
ꢁ
mm , F(000) 1004.0, 50 398 measured reections qfull 31.19 ,
completeness 99.7% (qmax), 7602 independent (Rint 0.0558), 324
parameters, R 0.0954 [for 5116 observed I > 2s(I)], wR 0.3367
7
1
2
(
ꢀ
all data), GooF ¼ 1.308, largest diff. peak and hole 0.414 and
ꢀ
3
0.728 e A .
8
(a) P. G. Gildner and T. J. Colacot, Organometallics, 2015, 34,
5
497; (b) G. A. Molander and B. Canturk, Angew. Chem., Int.
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Ed., 2009, 48(49), 9240; (c) R. Martin and S. L. Buchwald,
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A. Lomanovich, I. Grigor'ev and D. Mazhukin, Book of
Abstracts of 14-th International Conference on Molecule-based
Magnets, Saint Petersburg, Russia, 2014.
Continuous-wave EPR spectra were acquired on a Bruker EMX
ꢀ
5
spectrometer at room temperature in dilute (ca. 5 ꢂ 10 M)
toluene solutions degassed by means of repeated freeze–pump–
thaw cycles. At modulation 0.01 mT@100 KHz and MW power 2
mW, the spectra were recorded as single slow scans (ca. 3 h).
Isotropic g values were determined using solid DPPH (2,2- 10 (a) M. Souto, C. Rovira, I. Ratera and J. Veciana,
diphenyl-1-picrylhydrazyl) as a standard and were found to be
typical for cyclic nitroxides. The accuracy of determining
hyperne coupling constants and g values was estimated at
CrystEngComm, 2017, 19, 197; (b) M. Souto, V. Lloveras,
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Chem. Lett., 2016, 7, 2234; (c) S. Vela, M. Souto, I. Ratera,
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V. K. Cherkasov, G. A. Abakumov, A. G. Starikov and
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Magnetic measurements
Magnetic susceptibility of a polycrystalline sample of 11a was
measured with a Quantum Design MPMSXL SQUID magne-
tometer in the temperature range 2 to 300 K in a magnetic eld
of up to 5 kOe.
Conflicts of interest
There are no conicts of interest to declare.
Acknowledgements
The authors would like to thank the Russian Foundation for
Basic Research (grants no. 15-03-02741 and 17-53-50043) and
the Multi-Access Chemical Service Center SB RAS for spectral
and analytical measurements.
1
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