Aerobic oxidation of amines to imines by cesium-promoted mesoporous manganese oxide

Article abstract of DOI:10.1021/acscatal.5b00325

Selective oxidation of amines to imines is one of the most studied reactions in the field of heterogeneous catalysis. Cs ion-promoted mesoporous manganese oxide (meso Cs/MnO_x) was synthesized using an inverse surfactant micelle as a soft template. The chemical and structural properties of the mesoporous manganese oxide material were characterized by powder X-ray diffraction, nitrogen sorption, electron microscopy, and X-ray photoelectron spectra. The meso-Cs/MnO_x material presented an aggregated nanocrystalline nature with monomodal mesoporous size distributions. The catalyst was found to be effective in the oxidation of amines to imines under aerobic conditions. The meso Cs/MnO_x exhibited oxidation of primary, secondary, cyclic, aromatic, and aliphatic amines to imines, and the conversions reached as high as >99%. The catalyst was also effective in oxidative cross-condensation of two different amines to produce asymmetrically substituted imines. Surface-active Mn³⁺ species along with labile lattice oxygen were found to play an important role in the catalytic activity. Mild reaction conditions (air atmosphere and absence of any oxidative or basic promoters), ease of product separation by simple filtration, and significant reusability make this mesoporous manganese oxide material an economical and ecofriendly catalyst for the syntheses of versatile imine derivatives.

Full text of DOI:10.1021/acscatal.5b00325

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Article
Efficient Aerobic Oxidation of Amines to Imines by
Cesium Promoted Mesoporous Manganese Oxide
Sourav Biswas, Biswanath Dutta, Kankana Mullick, Chung-Hao Kuo, Altug S Poyraz, and Steven L. Suib
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b00325 • Publication Date (Web): 09 Jun 2015
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Efficient Aerobic Oxidation of Amines to Imines by
Cesium Promoted Mesoporous Manganese Oxide
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†
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Sourav Biswas , Biswanath Dutta , Kankana Mullick , Chung-Hao Kuo , Altug S. Poyraz and
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Steven L. Suib *
†
Department of Chemistry, University of Connecticut, Uꢀ3060, 55 North Eagleville Rd., Storrs,
Connecticut 06269 (USA), Fax: (+1) (860)ꢀ486ꢀ2981, Eꢀmail: steven.suib@uconn.edu
‡
Institute of Materials Science, University of Connecticut, Uꢀ3060, 55 North Eagleville Rd.,
Storrs, Connecticut 06269 (USA).
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ABSTRACT: Selective oxidation of amines to imines is one of the most studied reactions in the
field of heterogeneous catalysis. Cs ion promoted mesoporous manganese oxide (meso
Cs/MnO ) was synthesized using inverse surfactant micelle as a soft template. The chemical and
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structural properties of the mesoporous manganese oxide material were characterized by powder
Xꢀray diffraction (PXRD), nitrogen sorption, electron microscopy and Xꢀray photoelectron
spectra (XPS). The mesoꢀCs/MnO material presented aggregated nanocrystalline nature with
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monomodal mesoporous size distributions. The catalyst was found to be effective in oxidation of
amines to imines under aerobic conditions. The meso Cs/MnO exhibited oxidation of primary,
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secondary, cyclic, aromatic, and aliphatic amines to imines, where the conversions reached as
high as >99%. The catalyst was also effective in oxidative cross condensation of two different
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amines to produce asymmetrically substituted imines. Surface active Mn species along with
labile lattice oxygen were found to play an important role in the catalytic activity. Mild reaction
conditions (air atmosphere and absence of any oxidative or basic promoters), ease of product
separation by simple filtration and significant reusability make this mesoporous manganese
oxide material an economical and ecofriendly catalyst for the syntheses of versatile imine
derivatives.
KEYWORDS: Amine oxidation, Mesoporous Material, Manganese Oxide, Ion Promoted,
Heterogeneous Catalysis.
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1. Introduction
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Imines are a class of nitrogen based compounds with high reactivity due to the presence of
unsaturated C=N double bonds. Derivatives of imines have versatile uses in the production of
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heterocyclic chemicals and pharmaceutical compounds . The traditional approach of imine
synthesis involves condensation reaction between amines and carbonyl compounds. The process
requires Lewis acid catalysts, dehydrating agents, activated aldehydes and prolonged reaction
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time, therefore restrain the use of this process practically and environmentally . As an
alternative approach, the direct oxidation of amines has attracted considerable attention for
synthesis of imines. Stoichiometric oxidants such as chromate, permanganate, and 2ꢀ
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iodoxybenzoic acid have been used for the oxidation of amines to imines. However, the use of
stoichiometric oxidants has several drawbacks, which are the production of undesirable toxic
waste and difficulties in product separation.
Improvement of catalytic oxidation reaction is a captivating and demanding technology in the
field of heterogeneous catalysis. Due to considerable increasing environmental concerns,
chemists are constantly looking for cleaner synthetic strategies with high atom economy. In
comparison with different kinds of oxidizing agents, air is the most economical and green
oxidant. Catalytic oxidation under ambient aerobic conditions is recognized as the ultimate
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reaction in past decades in the production of fine chemicals . Some established catalytic
systems for the oxidation of amines to imines include precious metal based catalysts such as
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palladium, platinum, gold , other systems like ruthenium
, graphene oxide , metal organic
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frameworks , αꢀMnO , copper
and photocatalysts like TiO2
, BiVO4 and Nb O . In
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spite of good yields and selectivity, all of the catalysts either have cumbersome preparation
methods or require the use of harsh reaction conditions such as high pressure, light irradiation,
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and use of oxidative promoters
. Moreover, limited activity of aliphatic amines with undesired
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side reactions is another flaw for the reported systems
. Despite good activity in the syntheses
of symmetrical imines, syntheses of asymmetrically substituted imines by cross condensation of
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two amines have barely been discussed
. Therefore, design of an effective heterogeneous
catalyst system for the oxidation of amines is desirable, which ideally involves mild aerobic
atmospheric conditions, high TOF, avoidance of any additives, high reusability, and activity
towards syntheses of symmetrical and asymmetrical substituted imines.
Mesoporous metal oxides are capable of acting as superior heterogeneous catalysts due to their
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intrinsic features of high surface area and tunable pore sizes
. High surface area provides a
higher concentration of active sites per unit mass of substrate, whereas tunable pore size
expedites adsorption and mass transfer. Among the various metal oxides, manganese oxides
acquired a great deal of interest in catalytic oxidation reactions. Catalytic properties of
manganese oxide are strongly related to the exchangeable redox properties of different oxidation
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states of Mn . Concepts of structural defects, lattice oxygen, and acidꢀbase properties are
fundamental for understanding the origin of catalytic performance of manganese oxide. We have
previously demonstrated that surface residing promoter ions can enhance the catalytic activity of
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manganese oxides
. A new series of mesoporous materials [University of Connecticut (UCT)
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mesoporous materials] has been discovered recently by our group . Unlike conventional
mesoporous material synthesis, an inverse surfactant micelle was used as soft template. Precise
control over the pore size, surface area, and crystallinity by simple heat treatment cycles make
these types of materials superior over their nonporous counterparts in different catalytic
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applications such as oxidation of alcohols, low temperature CO and CH oxidation, and water
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splitting reactions
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With the UCT material synthesis approach, we have recently reported mesoporous Cs
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promoted manganese oxide as an efficient oxidation catalyst . Incorporation of a trace amount
of Cs in the manganese oxide resulted in superior activity enhancement in the aerobic oxidation
reactions. For instance, the catalyst was found to oxidize structurally different alcohols to
aldehydes selectively (100% selectivity) with very high conversion (as high as 100%). The
oxidation ability was further confirmed by the activation of relatively inert CꢀH bonds under
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solvent free conditions. The presence of surface active Mn species along with accessible lattice
oxygen and bifunctional character (oxidative and basic) of the catalyst were found to be the
factors responsible for the catalytic performance. Herein, we report the aerobic oxidation of
amines to imines and oxidative cross condensation of two different amines (benzylamine and
other aromatic and aliphatic amines) with mesoporous Cs promoted manganese oxide material
(
meso Cs/MnO ). Use of air as the sole oxidant, ambient reaction conditions, high yields of
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imines, and absence of additives make this process superior over other heterogeneous catalytic
systems. The mechanistic pathways and kinetic analysis were also studied in detail. This is the
first example of additive free and nonꢀphotochemical heterogeneous catalytic system for
synthesizing symmetrical and asymmetrical imines from diverse amines under aerobic
atmospheric conditions.
2. Experimental Section
2.1 Synthesis of mesoporous Cs/MnOx:
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In a typical synthesis 0.02 mol of manganese nitrate tetrahydrate (Mn (NO ) ·4H O) and 0.134
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mol of 1ꢀbutanol were added into a 120 mL beaker. To this solution 0.00034 mol of
poly(ethylene glycol)ꢀblockꢀpoly(propylene glycol)ꢀblockꢀpoly(ethylene glycol) (Pluoronic
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P123, PEO PPO PEO , molar mass 5750 g mol ) and 0.032 mol of concentrated nitric acid
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(HNO ) were added and stirred at room temperature until the solution became clear (light pink).
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To this clear solution 200 µL of 1.0 M cesium nitrate (CsNO ) aqueous solution was added
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maintaining the Mn/Cs molar ratio of 100/1. The resulting clear solution was then kept in an
oven at 120°C for 3 h under air. The black product was collected and washed with excess
ethanol, centrifuged, and dried in a vacuum oven overnight. At the end, the dried black powders
were subjected to a heating cycle. First they were heated at 150°C for 12 h and cooled down to
room temperature under ambient conditions followed by a second heating step of 250°C for 3 h.
Mesoporous MnO was synthesized by the same procedure without adding any CsNO solution.
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2.2 Catalyst Characterization
Powder XꢀRay diffraction (PXRD) measurements were performed on a Rigaku Ultima IV
diffractometer (Cu Kα radiation, λ=1.5406 Å) with an operating voltage of 40 kV and a current
of 44 mA. The lowꢀangle PXRD pattern was collected over a 2 theta range of 0.5–8° with a
ꢀ1
continuous scan rate of 0.5° min , where the wideꢀangle PXRD pattern was collected over a 2
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theta range of 5–75° with a continuous scan rate of 2.0° min . Nitrogen sorption experiments
were performed by a Quantachrome Autosorbꢀ1ꢀ1C automated adsorption system. The material
was degassed at 150°C for 5 h (ramp rate 10°C / min) under helium prior to measurement. The
surface area was calculated using the Brunauer–Emmett–Teller (BET) method and the Barrett–
Joyner–Halenda (BJH) method was utilized to calculate the pore sizes and pore volume from the
desorption branch of the isotherm. The surface morphology was determined with a Zeiss DSM
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82 Gemini field emission scanning electron microscope (FEꢀSEM) with a Schottky emitter at
an accelerating voltage of 2.0 kV having a beam current of 1.0 mA. Highꢀresolution transmission
electron microscopy (HRꢀTEM) experiments were carried out on a JEOL 2010 FasTEM
microscope at an operating voltage of 200 kV. The samples were prepared by casting the
suspension of material on a carbon coated copper grid. Xꢀray photoelectron spectroscopy (XPS)
was done on a PHI model 590 spectrometer with multiprobes (ΦPhysical Electronics Industries
Inc.), using AlꢀK radiation (λ= 1486.6 eV) as the radiation source and was fitted using CasaXPS
software (version 2.3.12). The powder samples were pressed on carbon tape mounted on
adhesive copper tape stuck to a sample stage placed in the analysis chamber. For correction of
surface charging, the C 1s photoelectron line at 284.6 eV was taken as a reference. A mixture of
Gaussian (70%) and Lorentzian (30%) functions was used for the leastꢀsquares curve fitting
procedure. The temperature programmed desorption (TPD) experiments were performed with a
Thermolyne 79300 model tube furnace equipped with an MKS gas analyzer coupled with a
quadrupole mass selective detector. The samples were treated with Ar for 2 h at 250°C before the
experiment. Ar was used as a carrier gas in the experiments at a constant flow rate in the
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temperature range 50°C to 600°C at a ramp of 10°C min . The amount of Cs in the material was
determined with a Perkin Elmer/DRCꢀe inductively coupled plasma mass spectrometer (ICPꢀ
MS) CETAC laser ablation unit, whereas Perkin Elmer Optima 7300DV inductively coupled
plasma optical emission spectrometer (ICPꢀOES) was used to determine the Mn concentration.
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.3 Catalytic Activity Measurements
.3.1 Oxidation of amines to imines: In a typical amine oxidation reaction, a mixture of
amine (0.5 mmol), catalyst (25 mg) and toluene (5 mL) was added in a 25 mL round bottom
flask equipped with a condenser. The reaction mixture was heated to reflux under vigorous
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stirring (700 rpm) for the required time under an air balloon. After reaction, the mixture was
cooled and the catalyst was removed by filtration. The product analysis was done using GCꢀMS
(gas chromatographyꢀmass spectrometry). The conversion was determined based on the
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concentration of amines. The selectivity was calculated on the basis of imines, aldehydes, and
cyanides as the only products. Most reactions were repeated twice and the average values were
used.
Pure imines were obtained by washing and evaporation of the solvent. The reaction was cooled
to room temperature after completion (having negligible amount of aldehydes and cyanides
along with imines). The catalyst was removed by centrifugation and washed several times with
toluene. The filtrate was collected and kept for evaporation of solvent and finally dried under
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vacuum. The imine products were identified by the H NMR and C NMR spectra.
2.3.2 Cross-coupling of amines: In a typical crossꢀcoupling reaction, a mixture (1/3 molar
ratio) of benzylamine and other aliphatic and aromatic amines, catalyst (25 mg) and toluene (5
mL) were put in a 25 mL round bottom flask equipped with a condenser. The reaction mixture
was heated to reflux with vigorous stirring (700 rpm) for the required time under an air balloon.
After reaction, the mixture was cooled, the catalyst was removed by filtration, and GCꢀMS was
used to analyze the filtrate. The conversion was determined based on concentration of
benzylamine (limiting reactant). The selectivity was calculated based on the self and cross
products of benzylamine.
2.4 Analysis of reaction products
The GCꢀMS analyses were performed by a 7820A GC system connected with a mass detector
of 5975 series MSD from Agilent Technologies and a nonpolar crossꢀlinked methyl siloxane
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column with dimensions of 12 in × 0.200 mm × 0.33 µm was used. The H and C NMR spectra
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were recorded on a Bruker AVANCE IIIꢀ 400 MHz spectrometer. H NMR spectra were
collected at 400 MHz with chemical shift referenced to the residual peak in CDCl (δ: H 7.26
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. Results
3.1 Structural characterization of Meso Cs/MnOx
The physicochemical and morphological features of the material were characterized
extensively by powder Xꢀray diffraction (PXRD), N sorption, scanning electron microscopy
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SEM), transmission electron microscopy (TEM), Xꢀray photoelectron spectroscopy (XPS),
temperature programmed desorption (TPD) and inductive coupled plasma (ICP) techniques. The
PXRD pattern (Figure 1A) at low angle region (0.5° to 8°) displayed a single peak, which
represented the irregular porous nature of the materials. No distinct diffraction pattern was
observed in the wide angle (5° ꢀ 75°) range (Figure 1B), which indicated the poorly crystalline
nature of the material. The mesoporosity of the material was confirmed by N sorption
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experiments (Figure 1C), where a Type IV adsorption isotherm followed by a Type I hysteresis
loop was observed. Pore size of meso Cs/MnOx was calculated as 3.4 nm by BJH method
(Figure 1D) from desorption branch of the isotherm, whereas the surface area was (calculated by
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BET method) 79 m g . SEM images (Figure 2) showed that aggregated nanoparticles formed
quasiꢀspherical micron sized morphology. TEM image (Figure 3A) also revealed the random
packing of nanoꢀoxide particles. Unlike PXRD, the high resolution TEM (Figure 3B) indicated
the crystalline nature of the material. The lattice fringes were calculated as 0.27 nm, which can
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be correlated to the (222) plane of Mn O (bixbyite) phase, which was in agreement with the
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literature findings . Poorly crystalline nature of the material can be the reason for absence of any
diffraction peaks under PXRD.
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The Mn 2p XPS spectra in Figure S1B consist two peaks, which were assigned as Mn 2p3/2 and
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Mn 2p1/2 core levels. The binding energy of 641.5 eV and 653.5 eV can be attributed to Mn
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oxidation state
. The O 1S spectra (Figure S1A) showed a broad peak of binding energy in the
range of ~ 527 to 536 eV, indicating the presence of multiple oxygen species. Deconvolution of
O 1S spectra (Figure S1A) specified the existence of three different binding oxygen species,
namely, structural or lattice oxygen (O ), surface adsorbed oxygen (O ) and adsorbed water or
s
ads
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hydroxyl group (O ) . The relative amount of structural oxygen (O ) was higher than the other
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type of oxygen species (Figure S1, Table S1). O ꢀTPD data under a stream of Ar gas are shown
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in Figure S2. In general, desorptions of surface adsorbed oxygen species and labile lattice
oxygen species with different MnꢀO bond strengths cause the appearance of peaks in O ꢀTPD
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profile. A major loss of O from the meso Cs/MnO material was observed at 570°C, which can
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be ascribed as the evolution of lattice oxygen from the manganese oxide (Figure S2). ICPꢀMS
was employed to verify the amount of Cs in the material. A very low concentration of Cs (Mn/Cs
molar ratio was 604/1), quite different than the nominal ratio (100/1) was detected. Excess ions
were probably removed by washing.
3.2 Optimization of reaction condition
Oxidative coupling of amines to imines over the metal oxides was discussed extensively in the
1
5,17
literature
and a generally accepted reaction mechanism is described in Scheme S1. The
reaction is initiated by the oxidative dehydrogenation of amines to form the RCH=NH
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intermediate followed by reacting with the water molecules to form aldehyde with exclusion of
ammonia. The intermediate aldehyde was then condensed with the amines to form the target
imine molecules. In the beginning of our studies, we have selected the oxidation of 4ꢀ
methoxybenzylamine as a model reaction for developing the optimal reaction conditions. First,
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the reactions were performed by meso Cs/MnO in different polarity of solvents at their
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respective boiling point temperatures (Entry 1ꢀ4, Table 1). Toluene (the one with highest boiling
point, 110°C) gave the highest conversion (>99%) (Entry 4, Table 1) and selectivity of 80%
towards the imine, among the tested solvents and was selected for the purpose. Further
optimization was employed to increase the selectivity of imine formation over cyanide and
aldehyde intermediates. Since the formations of other side products are favored at high
temperatures, the reaction was performed at relatively lower temperature (80°C). High selectivity
of imine (>99%) can be achieved at 80°C, but conversion decreased significantly to 45% (Entry
5, Table 1). Furthermore, the variation of selectivity for imine with catalyst loading was
established for meso Cs/MnO (Entry 5ꢀ7, Table 1). While no significant changes in conversion
x
and selectivity were observed with the catalyst loading of 100 mg (Entry 6, Table 1), decreasing
the catalyst amount to 25 mg was found to be sufficient to drive the reaction towards a
quantitative conversion (>99%) and high selectivity (96%) (Entry 7, Table 1). The reaction was
completed in 5 h in aerobic atmospheric conditions without use of any additives. When pure
oxygen was used instead of air as oxidant (Entry 8, Table 1), the reaction was completed even at
shorter times (as short as 3 h). A significant decrease of conversion (53%) was observed under
nitrogen atmosphere in 8h (Entry 9, Table 1).
3.3 Comparison with different catalysts
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Comparison with the wellꢀestablished redox catalysts is essential for newly developed highly
active catalysts. Very low conversion (10%) was achieved in the absence of any catalyst in 8 h
(Entry 7, Table S2). No improvement of conversion was observed using commercial nonporous
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Mn O indicating it to be totally inactive in the present condition (Entry 6, Table S2). Using
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potassium containing manganese oxide octahedral molecular sieves (KꢀOMSꢀ2 ) selectivity for
imines decreased drastically to 42% with formation of high concentration of cyanide (due to
dehydrogenation of amine) and aldehyde products. Other manganese oxide based catalysts like
amorphous manganese oxide and birnessite were also tested under the present reaction
conditions (Entry 4ꢀ5, Table S2). Meso MnO without Cs promoter (synthesized by the same
x
UCT method) resulted in similar (>99%) conversion (Entry 2, Table S2) but much lower
selectivity (78%) compare to meso Cs/MnO (96% selectivity).
x
In order to determine the role of Cs in the present amine oxidation catalytic system, a series of
experiments were performed by changing the Cs loading in the material. The aerobic oxidation
of relatively inert secondary amine (1,2,3,4ꢀtetrahydroisoquinoline) was selected as the model
reaction . A remarkable increase of conversion (Table S3) was observed when Cs was
incorporated in the meso MnO material. The nonpromoted meso MnO material exhibited only
x
x
12 % conversion (Entry 1, Table S3) after 1 h, whereas 87 ꢀ95% conversions (Entry 2ꢀ4, Table
S3) were achieved by Cs incorporation. The meso Cs/MnO material with the highest Cs loading
x
(0.16%) gave maximum conversion (94%) under identical reaction conditions (Entry 4, Table
S3).
All of these factors (high activity for both primary and secondary amines, air as oxidant and
atmospheric condition) allowed meso Cs/MnO preferable to be superior than the other reported
x
catalytic systems where high pressure oxygen set up (graphene oxide), tertꢀbutyl hydroperoxide
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(
TBHP) additives (αꢀMnO ), light irradiation (photoꢀcatalysts) or precious metal based systems
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were inevitable for achieving the catalytic efficiency.
3.4 Oxidation of amines to imines
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Having analyzed the optimum reaction conditions for 4ꢀmethoxybenzylamine oxidation, the
substrate scope and limitations were explored for different type of amines. The results of
catalytic oxidation of various amines including aromatic, aliphatic, primary and secondary amine
derivatives are listed in Table 2. The isolated yield of the some of the imines are given in Table 2
1
13
(see values in parenthesis) along with their H NMR and C NMR data (see supporting
information). Meso Cs/MnO was able to oxidize aromatic (Entry 1 ꢀ 9, Table 2), aliphatic (Entry
x
10, Table 2), primary (Entry 1 ꢀ 7, Table 2), secondary (Entry 8ꢀ9, Table 2) and cyclic (Entry 9,
Table 2) amines to corresponding imines with excellent conversion and selectivity. No
dehalogenated product was detected in case of amines with halogen substitution (Entry 4 ꢀ 5,
Table 2). Amine containing S as heteroatom was converted to the imine effectively (Entry 6,
Table 2). The catalyst was also useful for oxidation of relatively bulky biphenyl (1ꢀ
napthylmethylamine) system (Entry 7, Table 2). In case of oxidation of secondary amine (Entry
8, Table 2), conversion was less and much longer reaction time (20 h) and as well as higher
amount of catalyst (50 mg) were required. Oxidation of 1,2,3,4ꢀtetrahydroisoquinoline (Entry 9,
Table 2) produced a cyclic imine with notable conversion (95%) and selectivity (95%).
Remarkable catalytic activity of meso Cs/MnO was attributed to the successful oxidation of a
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long chain inactive aliphatic amine (1ꢀdodecylamine) towards corresponding imines (Entry 10,
Table 2), though longer reaction time (24 h) was required and selectivity was quite smaller
(50%). The other byproduct was dodecanenitrile, which was formed by the dehydrogenation of
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the reactive RCH=NH intermediate. On the other hand, oxidation of aniline (lacking of αꢀH
atoms) produced azo benzene as the only oxidation product (Entry 11, Table 2).
The oxidation of benzylamine derivatives bearing electron donating (Entry 2 ꢀ 3, Table 2) as
well as electron withdrawing groups (Entry 4 ꢀ 5, Table 2) were successfully converted to the
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imines. Benzylamine with electron withdrawing groups (CF and Cl) displayed >99% conversion
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ꢀ1
with TOF 1.56 h , whereas presence of electron donating groups (OCH and CH ) resulted in
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ꢀ1
ꢀ1
>
99% conversion but lower TOF (0.78 h for 4ꢀmethylbenzylamine and 0.68 h for 4ꢀ
methoxybenzylamine). Hammett equation can be used to interpret the electronic or steric
46
influence of the substituents on the reaction rate . The relative rates of oxidation of para
substituted benzylamines (pꢀCl, pꢀH, pꢀMe, pꢀOMe) were investigated. A linear relationship was
+
found between ln (k /k ) and Brown–Okamato constant (σ ) (Figure 4). The slope of the plot
x
H
p
resulted a reaction constant (ρ) value of 1.264, which illustrated the development of a negative
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charge at the reaction center in the transition state of the rateꢀlimiting step .
3.5 Cross coupling of amines
Excellent activities in the Cs/MnO mediated oxidation of amines to imines encouraged us to
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check the feasibility of this atmospheric aerobic protocol in the synthesis of unsymmetrical
imines. Reactions between benzylamine and 4ꢀmethoxybenzylamine were performed in different
molar ratios (Entry 1ꢀ3, Table 3). The reaction having 1/3 molar ratio of benzylamine/4ꢀ
methoxybenzylamine exhibited the highest selectivity towards the unsymmetrical imine (77%
selectivity) with minimum formation of symmetrical imine (23% selectivity). Then, a variety of
structurally different amines and benzylamine were tested with the best optimized reaction
condition (1/3 molar ratio). Using amines with electron releasing and electro withdrawing groups
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produced unsymmetrical imines with high (73 – 89%) selectivity (Entry 3 ꢀ 5, Table 3). An
unsymmetrical imine having a heteroatom S was synthesized (58% selectivity) by the reaction
between benzylamine and 2ꢀthiopheneamine using the established method (Entry 6, Table 3).
The method was further utilized to perform a reaction between an aromatic amine (benzylamine)
and aliphatic amines (butylamine and dodecylamine) (Entry 7 ꢀ 8, Table 3). The selectivity
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(>99%) towards the unsymmetrical imines was excellent compared to the coupling of two
aromatic amines. A very low selectivity (30% selectivity) of unsymmetrical imine was observed
by a reaction between benzylamine and aniline.
3.6 Reusability and heterogeneity
Steady reusability and negligible leaching of active species are the two important aspects of the
heterogeneous catalysts. The reusability test was conducted for the oxidation of 4ꢀ
methoxybenzylamine. At the end of the reaction, the catalyst was removed by filtration, washed
with ethanol and reactivated at 250°C for 45 min prior to reuse. Reactivation was required to
remove the adsorbed species from the catalyst surface. The catalyst retained the activity and
selectivity even after the fourth cycle (Figure S3A). PXRD analyses of assynthesized and the
recovered catalysts indicated that the amorphous nature of the catalyst was preserved (Figure
S3B). The hot filtration test was also carried out to verify any possible leaching of active species
from the catalyst surface. No further consumption of 4ꢀmethoxybenzylamine took place after
filtering off the catalyst at 47% conversion (Figure S4). Moreover, a very minute change of Cs
amount (0.04%) was observed in the meso Cs/MnO after the reaction by ICPꢀMS. All of these
x
results signified truly heterogeneous nature of meso Cs/MnO , which was not only active but
x
also had sustainability and recyclability.
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3
.7 Kinetic and mechanistic study
The high activity and selectivity of meso Cs/MnO as an amine oxidation catalyst system
x
motivated us to gain an insight into the mechanistic details of the reaction. In order to determine
if any radical intermediates were involved in the reaction, we used a radical scavenger (2,6ꢀdiꢀ
tertꢀbutylꢀ4ꢀmethylphenol). The scavenger had no effect on the reaction rate of 4ꢀ
methoxybenzylamine oxidation, which ruled out the formation of any radical intermediates.
Time dependent experiments on oxidation of 4ꢀmethoxybenzylamine were conducted using meso
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Cs/MnO to study reaction kinetics (Figure S5). Continuous sampling was undertaken in the
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course of 1h and conversion and selectivity were determined by GCꢀMS. Formation of
benzaldehyde in minutes was observed during the reaction pathway along with the desired imine
as the major product. Kinetic experiments depicted a first order rate equation with respect to
ꢀ1
amine (Figure S6) having a rate constant of 0.42 min . The apparent activation energy was
ꢀ1
calculated as 11.5 KJ mol from the Arrhenius plot (Figure S7). Kinetically relevant elementary
steps in the reaction pathways were measured by changing the benzylic H to D in benzylamine.
From the first order reaction plot between ln[benzylamine] and ln[benzylamineꢀα,αꢀd ], the
2
obtained kinetic isotope effect (KIE) value was 2.04 (Figure S8). This result signified abstraction
of the α CꢀH proton of amine as the rate determining step in the reaction.
4
. Discussions
The material was synthesized following the synthesis procedure of University of Connecticut
38
(
UCT) mesoporous materials . Transition metal and ion precursors loaded inverse micelles grew
packed in a random fashion to build the mesoporous structure. The reaction solvent was acidic
(
HNO ) 1ꢀbutanol solution. Evaporation of the solvent (by heat treatment) along with the NOx
3
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formation (by thermal decomposition of nitrates) had driven the reaction. The nitrate ions had
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dual roles. First, the hydrotropic nature of the nitrate ions hydrated the core of inverse
surfactant micelles by penetrating through it, which pulled the positively charged manganese
oxoꢀclusters to the core of the inverse micelles by increasing the solubility. Second, the nitrate
ions decomposed to form the NOx species, which controlled the pH and the sol gel chemistry of
Mn sols. Solvent extraction by ethanol was performed to remove surfactants to from the
mesoporous materials. Chemisorbed species (nitrates and carboxylates) were removed by a heat
treatment at 150°C for 12 h and were followed by a second heat treatment at 250°C for 3h.
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Catalytic aerobic oxidation of amines to imines were performed by meso Cs/MnO . Diverse
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amine derivatives were converted to the corresponding imines efficiently. Catalytic reaction of
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halobenzene derivatives without dehalogenation is a challenge . We tested oxidation of 4ꢀ
chlorobenzylamine to verify any dehalogenation. >99% selectivity towards imine with no
dehalogenated products were observed (Entry 4 ꢀ 5, Table 2), indicating the effectiveness of
meso Cs/MnO in oxidation of halogen containing substrates. Substrates bearing hetero atoms
x
like S or N are known to poison transition metal oxide based catalysts due to strong coordination
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of the heteroatom to the transition metal centers . However, in the present study, 2ꢀthiophene
benzylamine was converted efficiently to the corresponding imines with no significant change of
TOF and selectivity compared to the benzylamines (Entry 6, Table 2). Oxidation ability of meso
Cs/MnO was further tested for selective oxidation of relatively inert secondary amines (Entry 8
x
ꢀ
9, Table 2). As observed, the secondary amines can also be transformed to the corresponding
imines with very high conversions (45 – 95%) and excellent selectivity (95 – 99%), but a much
longer reaction time (20 h) and a higher amount of catalysts (50 mg) were required. This may be
due to the steric effect as it was relatively difficult to abstract the H atoms next to the NꢀH group.
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The reaction with dibenzylamine produced benzaldehyde as the side product, which can be
attributed to the oxidative cleavage of C=N bond of imine. Aliphatic amines are relatively
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difficult to oxidize as mentioned in the previous studies . However, in case of oxidation of long
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chain inactive 1ꢀdodecylamine by meso Cs/MnO , an imine was formed having twenty four
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carbon atoms. Such kind of reaction with inactive aliphatic amines has not been observed in
previous studies. The bifunctional nature (oxidative and basic) of the meso Cs/MnO is believed
x
to be the reason for the superior activity of the long chain 1ꢀdodecylamine, as the
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dehydrogenation of amine can be promoted by the basic nature of the catalyst . The easy
diffusion and transportation of 1ꢀdodecylamine in the mesoporous network may be the other
reason behind the activity. The lower selectivity (50%) of product imine compared to the
aromatic systems (mostly >90%) can be attributed to the lower stabilization due to the absence of
conjugated aromatic rings.
Along with symmetrical imine formation, the catalyst was also effective for the formation of
asymmetrical imines. Depending on the molar ratio and nucleophilic properties of the partner
amines, asymmetrical imines can be synthesized efficiently. The reaction between an aromatic
amine (benzylamine) and aliphatic amines (1ꢀbutylamine and 1ꢀdodecylamine) produced
asymmetrical imine with >99% selectivity. This can be attributed to the higher nucleophilic
nature of aliphatic amines than that of aromatic amines. Once the benzaldehyde was formed, the
more nucleophilic aliphatic amines competed with the benzylamine to minimize the selfꢀ
coupling and yielded the unsymmetrical amines with a very high selectivity (>99%). The
difference in nucleophilic properties also explained the relatively less selective unsymmetrical
imine production from benzylamine and aniline (Entry 9, Table 3). The lower nucleophilic
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property of anilines than benzylamine drove the reaction towards selfꢀoxidative coupling of
benzylamines (70% selectivity).
5
3
In general a MarsꢀVanꢀKrevelen mechanism , is anticipated to occur for oxidation types of
reactions over manganese oxides. According to the mechanism, structural defects and labile
lattice oxygen are the key properties of manganese oxide catalysts. In this study, the poorly
crystalline nature of the meso Cs/MnOx promoted facile supply of lattice oxygen, as observed by
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XPS and O ꢀTPD studies. Basicities of the metal oxides are known to be increased due to the
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presence of electropositive alkali metal ions. In this study, incorporation of trace amounts of
electropositive Cs ions induced basicity in the meso MnO , which can be considered as surface
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defects of the material
. As shown in CO chemisorption experiments (Figure S9), an increase
2
of CO uptake was observed for the meso Cs/MnO material at three different temperatures,
2
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which indicated an increase of basicity of the meso MnO after incorporation of Cs. The
x
presence of basic sites in the meso Cs/MnO material can enhance the oxidative dehydrogenation
x
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of amine by aiding the deprotonation of α CꢀH bond . Secondary amines are considered as
24
tough substrates for oxidation due to inactiveness of α CꢀH bonds . However, in our study a
87% increase of conversion (Entry 1ꢀ4, Table S3) of oxidation of an inert secondary amine
(
1,2,3,4ꢀtetrahydroisoquinoline) was observed by incorporation of Cs in meso MnO . All of these
x
experiments signified the poorly crystalline and bifunctional nature of meso Cs/MnO towards
x
the augmented catalytic activity in oxidation of amines.
Based on experiments and the literature, we propose a mechanism for oxidation of amines by
meso Cs/MnOx (Scheme S2). Adsorbed amine molecules transferred an electron to the Mn
center, followed by an elimination of an α CꢀH proton and an NꢀH proton to form the RCH=NH
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intermediate (Scheme S3). The surface active Mn centers simultaneously involved a one electron
reduction , which can lead to facile release of lattice oxygen and thereby promoting the oxidation
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ability . The labile lattice oxygen reoxidized the Mn center back with production of H O ,
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which could easily be decomposed over manganese oxide and form water. The abstraction of an
α CꢀH proton produced the negatively charged intermediate (Scheme S3), which was the rate
determining step as indicated by the linearity of the Hammett plot [positive value (1.264) of
reaction constant (ρ)] and isotope labelling study. The trapping of negatively charged
intermediate (by abstraction of α CꢀH proton) by addition of radical inhibitor was unsuccessful,
may be due the formation of the intermediate inside the mesoporous network of the catalyst,
where transportation of radical inhibitor was not proper. The supply of oxygen from the air is
crucial for the catalytic activity, as loss of lattice oxygen should be replenished by the oxygen
from the air. This also was supported by the observation that the elimination of air by nitrogen
diminished the reaction rate (Entry 9, Table 1). The reactive RCH=NH intermediate was rapidly
hydrolyzed by the water and formed the aldehyde, which was condensed with another amine
molecule to generate the corresponding imine.
The role of adsorbed water in the catalyst surface (as observed by XPS) is also believed to be
significant in the reaction system. A faster reaction rate was observed by the deliberate
introduction of water in the reaction, which further confirms our process to follow the above
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ꢀ1
mentioned reaction pathways . The activation energy (11.5 KJmol ) was significantly lower
than the previous study using αꢀMnO catalyst and TBHP oxidant conducted at room
2
temperature. The activation energy data suggested a lower energy pathways for meso Cs/MnOx
system at present reaction conditions. The high surface area and the monomodal uniform
mesoporous size distribution are the other important factors, related to the catalytic activity. This
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ꢀ1
can be attributed as enhanced catalytic activity of meso Cs/MnO (surface area 78 m g )
x
2
ꢀ1
compare to commercial nonporous Mn O (surface area 11 m g ) The high surface area of meso
2
3
Cs/MnO exposed the lattice oxygen more to the surface, whereas the mesoporous network
x
10
11
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promoted the adsorption and molecular transportation of the substrates.
5. Conclusion
In summary, we documented mesoporous Cs promoted manganese oxide as an efficient,
selective and reusable, and therefore environmentally benign catalyst for the catalytic oxidation
of amines to imines. The material exhibited uniform monomodal mesoporous size distribution
2
ꢀ1
(
pore diameter 3.4 nm) with high surface area (78 m g ). The catalyst was able to oxidize
diverse amine derivatives to produce imines with very high percent conversions (as high as
99%) and high selectivity (50 – 100%). The reactivity of the material towards inactive amines
>
were quite significant. The material was found to be oxidized long chain aliphatic amines (i.e. 1ꢀ
dodecylamine) with quantitative conversion (>99%) and high selectivity (50% selectivity). The
material was also capable to synthesize asymmetrical imines by coupling of benzylamine and
other aromatic and aliphatic amines with excellent selectivity (as high as >99%). Our
3
+
mechanistic investigation revealed an involvement of surface active Mn species, labile lattice
oxygen, and the bifunctional (oxidative and basic) nature of the catalyst towards the augmented
catalytic activity. High surface area, low crystalline nanoparticle nature and presence of
mesoporous structure are the other crucial factors. Further investigations of the active sties of the
catalyst are underway.
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(B
) 150
(
A
) 2500
2000
1
00
50
0
1
500
000
1
500
0
2
4
6
8
10
20
30
40
50
60
70
2
Theta (degree)
2
Theta (degree)
(
C
)
(D) 1.0
60
0.8
0.6
40
0.4
0
.2
.0
20
0
2
4
6
8
10
0.0
0.2
0.4
0.6
0.8
1.0
Pore diameter (nm)
Relative Pressure (P/P0)
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Figure 1. Structural characterization of meso Cs/MnO . PXRD patterns (A) Low angle (0.5° ꢀ
x
8
°), (B) Wide angle (5°ꢀ75°), (C) N sorption isotherm and (D) BJH desorption pore size
2
distribution.
10
11
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Figure 2. SEM images of meso Cs/MnO at (A) low and (B) higher magnifications.
x
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5
5
5
5
5
5
5
5
6
Figure 3. TEM images of meso Cs/MnO at (A) low and (B) higher magnification. The
x
measured lattice distances of 0.27 nm can be indexed to bixbyite Mn O (222) planes.
2
3
0
1
2
3
4
5
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9
0
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9
0
0.4
0.2
0.0
p-Cl
Slope = 1.264
p-H
2
R = 0.998
-0.2
-0.4
-0.6
-0.8
-1.0
p-CH
3
p-OCH
3
-0.8
-0.6
-0.4
-0.2
0.0
0.2
+
σ
p
Figure 4. Hammett plot of competitive oxidation of para substituted benzylamine at 15 min of
reaction time. Reaction condition: amine (0.5 mmol), catalyst (25 mg), solvent (5 mL), 110°C,
+
air balloon, 15 min. A linear relationship between ln(k /k ) and Brown–Okamoto constant (σ )
x
H
p
for para substituted benzylamines with slope (ρ) of 1.264 were obtained, which indicated the
formation of negatively charged intermediate.
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a
Table 1. Optimization of oxidative coupling of 4ꢀmethoxy benzyl amine
b
c
d
Entry
Solvent
Temp. Time Oxidant Conversion Selectivity
TON
(h)
(%)
(%)
(°C)
1
2
3
4
5
Methanol
Dioxane
Hexane
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
60
8
8
8
8
8
8
5
3
8
Air
Air
Air
Air
Air
Air
Air
O2
50
55
94
100
100
80
0.84
0.92
0.73
1.67
0.75
0.83
3.33
3.00
1.76
100
66
44
110
80
100
45
100
85
e
6
110
110
110
110
100
100
90
f
7
96
f
8
100
100
f
9
N2
53
a
Reaction conditions: 4ꢀmethoxy benzyl amine (0.5 mmol), meso Cs/MnO (50 mg), solvent (5
mL), air balloon, 3ꢀ5 h. Conversions were determined by GCꢀMS based on concentration of
amines. The other products were aldehyde and cyanide. TON = moles of amine converted per
mole of catalyst. nd = not determined by GCꢀMS. 100 mg catalyst. 25 mg catalyst
x
b
c
d
e
f
.
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5
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0
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5
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9
0
1
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3
4
5
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7
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9
0
1
2
3
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
a
Table 2. Aerobic oxidation of amines to imines by meso Cs/MnOx
b
b
c
Entry
Substrate
Time (h)
Conversion
%)
Selectivity
TON
(
(%)
93
1
2
3
3
4
5
>99(82)
3.33
3.33
3.33
NH2
>99
75
96
NH2
>99(80)
NH2
O
4
5
2
2
>99
98
96
3.33
3.33
NH2
NH2
Cl
>99(96)
F3C
6
7
4
4
>99
>99
92
80
3.33
3.33
NH2
S
NH2
d
8
20
3
45
99
95
1.50
3.33
N
H
d
9
>99
NH
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>99
15
50
nd
3.33
nd
NH2
1
0
e
11
NH
2
10
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a
Reaction conditions: amines (0.5 mmol), meso Cs/MnO (25 mg), toluene (5 mL), 110°C, air
x
b
balloon. Conversion and selectivity was determined by GCꢀMS based on the concentration of
amines. Numbers in parenthesis refer to yields of isolated products. TON = moles of amines
converted / moles of catalyst. Meso Cs/MnO (50 mg). Azobenzene was the only product. nd
=
c
d
e
x
not detected.
a
Table 3. Aerobic oxidative cross coupling of benzylamine with different amines
R2
meso Cs/MnOx
NH2
N
N
+
R NH
2
2
+
Toluene, air, 110 °C
1a
2a
c
d
Entry
Substrate
Molar
ratio
Conversion
%)
Selectivity
b
(
(%)
1
a
2a
25
1
2
3
2/1
1/2
1/3
>99
>99
>99
62
NH2
O
30
23
70
77
NH2
O
O
NH2
4
5
1/3
1/3
>99
>99
10
27
89
73
NH2
NH2
F3C
6
7
1/3
1/3
>99
>99
42
nd
58
NH2
S
>99
NH2
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2
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2
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5
5
5
5
5
5
5
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1/3
1/3
>99
15
nd
70
>99
30
NH2
10
NH
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
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9
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0
1
2
3
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5
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9
0
1
2
3
4
5
6
7
8
9
0
a
Reaction conditions: benzylamine and other aromatic and aliphatic amines (0.5 mmol), meso
Cs/MnO (25 mg), toluene (5 mL), 110°C, 3 h, air balloon. Molar ratio of benzylamine and
different amines. Conversion was determined by GCꢀMS based on the concentration of
benzylamine. Selectivity was calculated based on selfꢀcoupled (1a) and crossꢀcoupled (2a)
b
x
c
d
products of benzylamine. nd = not detected.
ASSOCIATED CONTENT
Supporting Information: List of chemicals, XPS, O ꢀTPD, kinetic study, reusability study,
2
1
13
isotopic study, calculation of activation energy, and H NMR and C NMR of typical products
are given in the supporting information.This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*
Steven L. Suib, Email: steven.suib@uconn.edu, Fax: (+1) (860)ꢀ486ꢀ2981 (USA).
ACKNOWLEDGMENT
SLS thanks support of the US Department of Energy, Office of Basic Energy Sciences,
Division of Chemical, Biological and Geological Sciences under grant DEꢀFG02ꢀ
8
6ER13622.A000. SB thanks Sniega Stapcinskaite of Center of Environmental Science and
Engineering, University of Connecticut, Storrs for assistance in ICPꢀMS and Dr. Alfredo M.
AngelesꢀBoza of Department of Chemistry, University of Connecticut, Storrs for helpful
discussions.
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Nicolaou, K.; Mathison, C. J.; Montagnon, T. Angew. Chem. 2003, 115, 4211ꢀ
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Zhang, P.; Gong, Y.; Li, H.; Chen, Z.; Wang, Y. Nat. Commun. 2013, 4, 1593.
Parmeggiani, C.; Cardona, F. Green Chem. 2012, 14, 547ꢀ564.
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Wang, J.ꢀR.; Fu, Y.; Zhang, B.ꢀB.; Cui, X.; Liu, L.; Guo, Q.ꢀX. Tetrahedron Lett.
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