F. Xie, X. Chen, X. Zhang et al.
Journal of Catalysis 398 (2021) 192–197
merits of regulable oxidative performance, intrinsic stability and
easy recyclability. In recent years, OMS-2 has gained considerable
attention [43–44], and the OMS-2 materials have exhibited attrac-
tive applications in selective oxidation due to the highly porous
structure, controllable valence state and mobility of oxygen ions
[45–46]. Moreover, cobalt-based nanocatalysts have greatly facili-
tated the advances of alcohol oxidation [47–48]. However, it is
important to note that these examples are only confined to the
well-known organic reactions, and their potentials in developing
innovative organic transformations have been scarcely demon-
strated. Herein, by developing an OMS-2 nanorod-supported cobalt
catalyst, we wish to report, for the first time, its utility in efficient
construction of imidazolinones via aerobic dehydrocyclization of
vicinal diols with amidines.
Scheme 1. Examples on the transformations of vicinal Diols to N-heterocycles.
2. Experimental
2.1. Synthesis of Co/OMS-2–800
Typically, the cobalt heterogeneous catalyst was prepared by
pyrolysis of the cobalt hydroxide immobilized on the pre-
synthesized OMS-2 materials [49–50]. Initially, the OMS-2 frame-
work was obtained from the reaction of KMnO4 and MnSO4 with
assistance of concentrated HNO3. After that, the in situ generated
Co hydroxide was embedded and deposited into the cage of
OMS-2 via impregnation method. Further, this supported hydrox-
ide catalyst was pyrolyzed under argon flow at 800 °C for 4 h,
which produced cobalt-doped octahedral molecular sieve (denoted
as Co/OMS-2–800, for more details see the supporting information
(SI)).
The crystal phase of the Co/OMS-2–800 has been analyzed by
XRD (Figure S1 in the SI). The peaks at 12.7°, 18°, 28.9°, 37.2°,
42°, 50°, 56.1°, 60.1°, 69.4°are assignable to lattice planes of
OMS-2, which is typical cryptomelane phase (JCPDS-00–020-
0908) [46]. This result indicates that the crystal form of OMS-2 is
retained even under high-temperature pyrolysis. Diffraction corre-
sponding to CoO at 2h = 36.4°, 42.4°, 61.5° were recognized with
JCPDS (43–1004) [45]. Moreover, Peaks related to Co3O4 (JCPDS-
43–1003) were observed for a reflection at 2h = 31.3°, 36.8°,
38.4°, 44.8°, 59.2°, 65.2° [45]. A typical IV isotherm with hysteresis
loop arose in the N2 adsorption–desorption isotherms of Co/OMS-
2–800, which suggests a highly porous structure (SI, Figure S2).
Notably, there is a dominant distribution of pore size centered at
2–4 nm, which makes for substrate diffusion.
The morphology and structural characterization of the prepared
Co/OMS-2–800 material was investigated by means of TEM, STEM
and EDS analyses. The TEM images (Fig. 1a-1b and Figure S3 (SI))
clearly demonstrate that the catalyst exists in the form of nanor-
ods. The average diameter of the nanorods was in the range of
85–90 nm. As shown in HAADF-STEM image (Fig. 1c) and corre-
sponding elemental mapping results (Fig. 1d-1f and Figure S4
(SI)), the impregnated Co was well dispersed over OMS-2 nanor-
ods. Furthermore, the signals of Co, Mn, K, C, O are highly over-
lapped and interconnected with each other, which are
surrounded by the carbon matrix. These results confirm that the
Co species present uniform dispersion on the surface of OMS-2.
To identify the surface chemistry of the developed Co/OMS-2–
800, the X-ray photoelectron spectroscopy (XPS) was then con-
ducted. The element contents are as follows: Co (2.44 wt%), C
(22.76 wt%), Mn (41.87 wt%), and O (32.93 wt%). Furthermore,
the content of Co loading was determined by ICP-OES (2.1 wt%),
which is very clsosely to the value detected by XPS (2.44 wt%).
These results indicate the Co species supported on OMS-2 are higly
uniform. The Co 2p spectrum (Figure S5a) shows characteristic
peak of Co2+ species with a binding energy of 780.6 ev [51–52],
Scheme 2. Envisaged new protocol.
of different substituted imidazolinones [33–35], these transforma-
tions generally suffer from one or more limitations such as the use
of less environmentally benign agents, the need for prefunctional-
ization steps to access specific agents, and difficult catalyst
reusability. In this context, the development of new strategies
enabling efficient access to various imidazolinones, preferably with
biomass-derived vicinal diols and reusable catalysts, would be
highly desirable.
As a continuation of our efforts toward the construction/functio
nalization of N-heterocycles [36–39], we envisioned a new strategy
for general synthesis of imidazolinone 3, that is, the combination of
aerobic dehydrocyclization of vicinal diols 1 and amidines 2 with
group migration. As illustrated in Scheme 2, the first catalytic
dehydrogenation (1st CDH) of 1 under the assitance of nanocobalt
and air (oxidant) forms hydroxyketone 1–1. Then, intermediate B
is generated via the condensation of amindine 2 with 1–1 followed
by the second catalytic dehydrogenation of A (2nd CDH) and base-
mediated amino addition to the carbonyl group (path a). Alterna-
tively, successive full catalytic dehydrogenation of diol 1, the cap-
ture of in situ formed dione 1–2 by amidine 2, and intramolecular
cyclization also rationalize the formation of B (pah b). Finally, the
subsequent group 1,2-migration [33] and protonation of C give rise
to the desired product 3. However, it is important to note that,
under aerobic dehydrogenative and basic conditions, vicinal diols
can easily undergo C-C bond cleavage to generate aldehydes D
[40]. Moreover, the generated aldehydes D unavoidably react with
two molecules of amidine 2 to form triazine by-products F (path c)
[41–42]. Hence, to achieve a chemoselective synthesis of product 3,
it is essential to find a compatible catalyst system to ensure that
the dehydrogenated intermediate 1–1 or 1–2 is timely traped by
amidine 2, thus supressing the decomposition of diols 1 to unde-
sired aldehydes.
With the above information, we believe that the development
of a suitable heterogeneous nanocatalyst would offer a solution
to achieve the desired synthetic purpose (Scheme 2). In compar-
ison with homogeneous catalysis, such type of catalyst has the
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