Deprotonation of Benzoxazolium Salt: Trapping of a Radical-Cation Intermediate

Article abstract of DOI:10.1021/acs.orglett.8b03928

The deprotonation of N-2,6-diisopropylphenyl-substituted benzoxazolium tetrafluoroborate 1 with NaH results in the formation of electron-rich diaminodioxaethylene 2. The radical cation salt 2^·+·BF₄^- is found to be an inter

Full text of DOI:10.1021/acs.orglett.8b03928

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Deprotonation of Benzoxazolium Salt: Trapping of a Radical-Cation
Intermediate
Svetlana V. Klementyeva,*,† Pavel A. Abramov,‡,§ Nikolay V. Somov,∥ Yulia B. Dudkina,⊥
Yulia H. Budnikova,⊥ and Andrey I. Poddel’sky#
†Kazan Federal University, A.M. Butlerov Institute of Chemistry, 420008, Kremlevskaya str. 29/1, Kazan, Russian Federation
‡Nikolaev Institute of Inorganic Chemistry SB RAS, Prosp. Lavrentieva 3, 630090 Novosibirsk, Russian Federation
§Novosibirsk State University, Pirogova str. 2, 630090 Novosibirsk, Russian Federation
∥Lobachevsky State University of Nizhny Novgorod, Prosp. Gagarina 23, 603950 Nizhny Novgorod, Russian Federation
⊥Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientific Center of RAS, Arbuzov Str. 8, 420088 Kazan, Russian
Federation
#G.A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences, Tropinina str. 49, 603137 Nizhny Novgorod,
Russian Federation
S
* Supporting Information
ABSTRACT: The deprotonation of N-2,6-diisopropylphen-
yl-substituted benzoxazolium tetrafluoroborate 1 with NaH
results in the formation of electron-rich diaminodioxaethylene
2. The radical cation salt 2·+·BF4 is found to be an
−
intermediate product in the redox reaction leading from 1
to 2.
ince the late 1960s, an epoch of coordination compounds
with N-heterocyclic carbenes (NHCs) as ligands initiated
Although stable sterically demanding acyclic aminooxycar-
benes are known,16 neither free NOHC nor its dimer has been
isolated so far, in spite of the fact that the existence of NOHCs
was theoretically predicted.17 Taking into account a vast
variety of stable carbenes and considering as a challenge the
task to synthesize noncoordinated oxazol-2-ylidene derivatives,
we have prepared benzoxazolium salt 1 and attempted its
deprotonation. As a result, we have isolated the corresponding
olefinic dimer of dioxadiazafulvalene 2 and surprisingly faced a
S
1
2
̈
by the pioneering works of Wanzlick and Ofele and carried
on with the subsequent studies of Lappert3 has been
continuing. The surge of interest in this field was caused by
the first synthesis and X-ray-determined structural character-
ization of stable NHCs by Arduengo in the early 1990s.4 Since
then, use of manifold NHCs in coordination and organo-
metallic chemistry has afforded numerous metal complexes
whose applications have spread from homogeneous catalysis5
to functional materials.6 Among other different types of singlet
carbenes are carbenes in which one or both heteroatoms are
different from nitrogen,7−10 monoheteroatom-substituted
carbenes,8−11 and even carbocyclic stable carbenes.8−12 The
replacement of nitrogen in NHCs by either sulfur or oxygen is
recognized to be one of the most efficient tools for tuning of
electronic properties and bulkiness of such compounds. A
stable thiazol-2-ylidene as well as dimer dithiadiazafulvalene
have been isolated and structurally characterized by means of
single-crystal X-ray diffraction.13 Meanwhile, N,O-heterocyclic
carbenes (NOHCs) remain transient species, and even metal
complexes with oxazol-2-ylidene ligands are quite rarer being
mostly prepared by indirect methods via isocyanide deriva-
tives.14 Recently, a convenient straightforward synthetic route
to such complexes based on the reaction of oxazolium salt with
a metal precursor and an external base has been developed.15
nonpresumable paramagnetic species 2•+·BF4 .
−
For the synthesis of 1 we have chosen a high-yielding and
easy to perform synthetic pathway15a,18 based on the treatment
of 4,6-di-tert-butyl-N-(2,6-diisopropylphenyl)-o-aminophenol
with triethyl orthoformate under acidic conditions19 (Scheme
1). Benzoxazolium salt 1 was obtained as a white solid in 89%
yield. It is readily soluble in polar solvents such as CH3CN,
THF, and CHCl3, moderately soluble in diethyl ether, and
1
nearly insoluble in hydrocarbons. The H and 13C{1H} NMR
spectra of 1 revealed the characteristic oxazolium proton
(OCHN) resonance at 10.54 ppm and the corresponding
carbon resonance at 193.2 ppm (see the SI).
Received: December 9, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03928
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
107.5° and 108.0°, respectively. The C1−C3 and C1′−C3′
bonds in 2 slightly lengthen in comparison to 1.
Scheme 1. Synthesis and Deprotonation of Benzoxazolium
Salt 1
But what was a mysterious fuchsia-pink color which arose
instantlyupon contact of benzoxazolium salt 1 with deproto-
nation agent? It should be noted that when the basic agent
(NaH) was used, we were able to observe a fuchsia-pink color
on a time scale of weeks. Unfortunately, all of our attempts to
isolate this bright colored substance failed. The evaporation of
solvents in vacuo as well as replacement of the polar medium
by a less polar (diethyl ether) or nonpolar one (aromatic or
saturated hydrocarbons) led to oily products and a change in
color to tints of brown or even bleaching. At last, we succeeded
in obtaining a radical cation salt 2•+·BF4 (Scheme 2) by
−
means of heterogeneous reaction.22
Scheme 2. Possible Routes for Formation of 2
The molecular structure of 1 in the crystal was determined
by single-crystal X-ray diffraction analysis. All structural
parameters are very similar to the N-phenyl-substituted
analogue reported previously15a (Table S2). The oxazole ring
in 1 is essentially planar with the largest deviation from the
mean plane of 0.004 Å. The arene ring of the 2,6-
diisopropylphenyl substituent is almost perpendicularly ori-
ented to this plane with a dihedral angle of 88.0°. The nitrogen
atom N1 is also in a nearly planar environment and lies only
0.006 Å above the plane formed by carbon atoms C2, C3, and
C4.
The deprotonation of benzoxazolium salt 1 was performed
with NaH using a conventional synthetic protocol (Scheme 1).
The reaction mixture turned bright fuchsia-pink immediately
by mixing of oil-free sodium hydride with a solution of 1 in
THF. An intensive evolution of hydrogen and simultaneous
precipitation of sodium tetrafluoroborate was observed. The
solution was losing color gradually while stirring and eventually
became pale yellow-greenish. Instead of a stable oxazol-2-
ylidene, we have succeeded in the isolation of dimer
dioxadiazafulvalene 2 as pale yellow crystals in 67% yield20
(Scheme 1).
According to the single-crystal X-ray diffraction analysis, the
observed bond lengths and valence angles in compound 2
(Table S2) are very similar to the corresponding structural
parameters of analogues carbene dimer with benzothiazole
rings.13 The central double C2C2′ bond of 1.312(6) Å
corresponds to a normal olefin.21 The solid-state geometry of
dimer 2 is close to planar, and there is a 3.0° twist about the
central double bond. One oxazole ring is essentially planar
within 0.004 Å. In addition, the arene ring of 2,6-
diisopropylphenyl substituent and the oxazole mean plane
form an approximately right angle (89.4°). The nitrogen atom
N1 in the planar oxazole ring is 0.010 Å outside the plane of its
attached carbon atoms C2, C3, and C4. The largest deviation
from the mean plane of the more distorted oxazole ring is
equal to 0.033 Å. The 2,6-diisopropylphenyl substituent is
twisted by 81.6° with respect to the corresponding oxazole
mean plane. The nitrogen atom N1′ in the distorted oxazol
ring is 0.152 Å above the plane of its attached carbon atoms
C2′, C3′, and C4′. The steric repulsion of two bulky 2,6-
diisopropylphenyl groups leads to the expected elongation of
N1−C2 and O1−C2 (as well as N1′−C2′ and O1′−C2′)
bonds by about 0.1 Å relative to benzoxazolium salt 1 and
decrease of N1−C2−O1 and N1′−C2′−O1′’ angles up to
−
Fuchsia-pink single crystals of 2·+·BF4 suitable for X-ray
diffraction were obtained by deprotonation of a benzoxazolium
salt 1 in toluene. A slow heterogeneous deprotonation of
poorly soluble salt 1 probably afforded crystals of ionic product
2•+·BF4 , which also possesses a scanty solubility in nonpolar
−
aromatic hydrocarbons. Radical cation 2•+ corresponds to a
centrosymmetrical dimer with a central carbon−carbon bond
of 1.371(4) Å in length. This bond is elongated in comparison
with the double carbon−carbon bond in the neutral dimer 2
and normal double CC bond in organic substances21 (1.33
Å). On the other hand, it is substantially shorter than the
normal single C−C bond21 (1.54 Å). Noteworthy, the
observed value is typical, for example, for a tetrathiafulvalene
radical-cation23 and dithiadiazafulvalene radical-cation24 as
well as for boron-linked tetraaminoethylene radical cations25
and thus confirms the positive charge +1 on the dioxadiaza-
fulvalene core of the species 2•+. Both benzoxazole moieties are
twisted by 5.1° about such a sesquilateral central carbon−
carbon bond. The increase of the distance between two
oxazole parts of radical cation in 2•+ with respect to the neutral
dimer 2 leads to a decrease of steric repulsion and results in the
intermediate geometric parameters of the corresponding
oxazole rings. The N1−C2 and O1−C2 bonds as well as
valence angle N1−C2−O1 in 2•+ reveal intermediate values
B
DOI: 10.1021/acs.orglett.8b03928
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
between cation in 1 and neutral dimer 2 (Table S2). The
oxazole rings in 2•+ are planar within 0.011 Å. The arene rings
of the 2,6-diisopropylphenyl substituents are twisted by 85.1°
with respect to their oxazole mean planes. The nitrogen atoms
deviate from the planes of their attached carbon atoms by
0.132 Å.
tion of 1,3-thiazolium salts in the presence of base is one of the
most employed pathways toward dithiadiazafulvalene and gives
rise to a wide range of various substituted electron-rich
ethylenes,13,28 while only two cyclic thiazol-2-ylidenes have
been described to date.13
Concerning NHCs electronic stabilization rather than steric
demand was found to play a crucial role in the carbenes
dimerization.29 As for N,S-heterocyclic carbenes (NSHCs), the
only N-2,6-diisopropylphenyl-substituted one is a stable
crystalline solid at ambient temperature, while even the N-
mesityl-substituted analogue is persistent in solution up to 0
°C.13 Thus, the bulkiness of the substituents becomes the most
important factor for kinetic stabilization of NSHCs and
especially NOHCs. Indeed, we could not observe a free
carbene 3 (Scheme 2), while we succeeded in the isolation of
It is noteworthy that the structural parameters of
compounds 2 and 2•+ are very close to the quite rare examples
of NHC−cyclic (alkyl)(amino) carbene (CAAC) heterodimers
and their very recently reported radical cations.26
A radical-cation 2•+ demonstrates an isotropic EPR
spectrum at gi = 2.0036 with a complicated hyperfine structure
(Figure 1) which was reliably simulated using the program
its dimer 2 and trapped radical cation salt 2•+·BF4 , though the
−
formation and transformation of the latter is opened to
question. Noteworthy, the energetic preference of the
dioxadiazafulvalene 2 in comparison with a free carbene 3
was estimated by DFT UB3LYP/6-31g(d,p) calculations and
amounts to 33.0 kcal/mol (see the SI for details). Generally,
both for NHCs and NSHCs two mechanisms of dimerization
and formation of the electron-rich olefins are discussed: either
a direct reaction of two carbenes (“Wanzlick equilibrium”) or
an indirect reaction where the carbene reacts with the
corresponding azolium salt followed by a deprotonation of
the intermediate to yield the ethylenic dimer.28,30 The indirect
ionic mechanism is more preferable and has been supported
experimentally by 13C NMR investigations for some deriva-
tives.30a,31 Being not able to rule out the direct carbene
coupling, we suppose, that dimerization of 3 most likely
proceeds by the nucleophilic addition of the carbene 3 to its
benzoxazolium precursor 1 leading to the protonated dimer
intermediate 4 and followed by an acid−base reaction
intramolecular proton transferto yield dioxadiazafulvalene 2
(Scheme 2). Thus, the radical cation 2•+ probably arises from
some redox reaction of intermediate 4 resulting in the
elimination of atomic hydrogen rather than proton. The
Figure 1. Spin densities as predicted by the Mulliken population
analysis (a) and representation of SOMO for 2•+ (b) according to the
DFT UB3LYP/6-31g(d,p) calculations. Experimental and simulated
isotropic EPR spectrum of radical cation 2•+ in THF (c).
WinEPR Simfonia 1.25. The observed hyperfine structure of
the spectrum is caused by hyperfine splitting of signal from the
unpaired electron on two equivalent nuclei of nitrogen (14N, I
= 1, 99.63%) with HFC constant ai(14N) = 4.44 G, two pairs of
protons in the third and fifth positions of both six-membered
aromatic rings with constants ai(2 1H5) = 1.25 G and ai(2 1H3)
= 0.77 G, and four equivalent protons of methine fragment of
following reduction of 2•+·BF4 with NaH gives dimer 2 as
−
1
isopropyl groups with ai(4 HCH of iPr) = 0.29 G (Scheme 2).
final product. Unfortunately, the conditions providing redox
reactions in this system are unsure at this time. As a
confirmation of the proposed mechanism, we have carried
out the reaction of 2 with HBF4 and observed the formation of
The HFS of the spectrum points out the π-character of the
radical cation 2•+ with the delocalization of spin density
throughout the π-systems of both benzoxazole moieties
confirmed by the DFT UB3LYP/6-31g(d,p) calculations (for
details, see the SI). The spin density distribution is in good
agreement with the shape of SOMO radical cation 2•+. The
Mulliken population analysis confirms the ratio of the found
HFCs with a larger one on the nitrogen atoms (Figure 1). The
observance of HFC with methine protons that are located
above and below the π-system can be rationalized by their
conjunction with a π-molecular orbital of an unpaired electron.
Dioxadiazafulvalene 2 shows absorption only in the UV
region (330−400 nm), whereas the THF solution of
−
radical cation salt 2•+·BF4 with the simultaneous hydrogen
evolution (for details see the SI).
The cyclic voltammogram of 2 is well comparable with
NHC−CAAC heterodimers previously reported26 and shows
two reversible one electron waves at E11/2 = −0.48 and E2
=
1/2
+0.22 V vs Fc+/Fc. These redox processes correspond to the
successive formation of radical cation 2•+ and dication 22+
(Scheme 3), which were also generated in situ by both
electrolysis and chemical oxidation using the treatment of 2
with 1 and 2 equiv of AgBF4 (for details see the SI). In
addition, the stoichiometric reaction between neutral dioxa-
diazafulvalene 2 and its dication 22+ gave the radical cation 2•+,
indicating reversible electron transfer.
−
compound 2•+·BF4 features two broad absorbance bands in
the range of 360−410 and 450−600 nm in the UV−vis
spectrum. The second one sets conditions for fuchsia-pink
coloration (for details, see the SI). The TD-DFT UB3LYP/6-
31g(d,p) calculations show that the greatest contribution to
the long-wave absorption comes from the SOMO-LUMO
transition in 2•+ (97%).
In conclusions, a novel NOHC precursor 1 has been
synthesized using triethyl orthoformate for cyclization of 4,6-
di-tert-butyl-N-(2,6-diisopropylphenyl)-o-aminophenol under
acidic conditions. Deprotonation of this bulky benzoxazolium
salt 1 afforded dimer diaminodioxaethylene 2 as the final
product, and the formation of the stable carbene instead. Along
It is well-known that the deprotonation of imidazolium salts
in basic medium afforded either stable NHCs4 or tetramino-
ethylenes.27 Noteworthy, the synthetic strategy of deprotona-
with dioxadiazafulvalene 2, radical-cation salt 2•+·BF4 has
−
C
DOI: 10.1021/acs.orglett.8b03928
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
the Spectral-Analytical Center of FRC Kazan Scientific Center
of RAS for technical assistance with our research.
Scheme 3. Oxidation of 2
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This work was supported by the Russian Foundation for Basic
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