Dehydrogenation and oxidative coupling of alcohol and amines catalyzed by organosilicon-supported TiO2@PMHSIPN

Article abstract of DOI:10.1039/c4ra06685e

The catalytic dehydrogenation and tandem transformation of aromatic alcohols, including oxidative coupling of alcohols and amines, were achieved successfully using a catalytic amount of organosilicon-supported titania (TiO₂@PMHSIPN), which enables the efficient synthesis of aromatic aldehydes, imines, and benzimidazoles in good to excellent yields. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra06685e

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Dehydrogenation and oxidative coupling of alcohol
and amines catalyzed by organosilicon-supported
TiO2@PMHSIPN†
Cite this: RSC Adv., 2014, 4, 34681
a
a
a
a
Hu Wang,‡ Jin Zhang,‡ Yu-Ming Cui, Ke-Fang Yang, Zhan-Jiang Zhenga
Received 5th July 2014
Accepted 31st July 2014
*
ab
*
and Li-Wen Xu
DOI: 10.1039/c4ra06685e
www.rsc.org/advances
The catalytic dehydrogenation and tandem transformation of number of heterogeneous catalysts including supported gold
aromatic alcohols, including oxidative coupling of alcohols and (Au), platinum (Pt), rhenium (Re), silver (Ag), and copper (Cu)
amines, were achieved successfully using a catalytic amount of nanoparticles have been developed for the dehydrogenation of
organosilicon-supported titania (TiO2@PMHSIPN), which enables the alcohols.7 And most of these metal nanoparticles were sup-
efficient synthesis of aromatic aldehydes, imines, and benzimidazoles ported on various inorganic materials, such as TiO2, Al2O3, SiO2,
in good to excellent yields.
CeO2, or hydrotalcites, in which the use of inorganic materials
played crucial role in achieving good results. For example,
Kaneda et al. found the use of other heterogeneous catalyst such
as Ag/SiO2 or Ag/TiO2 instead of Ag/HT (hydrotalcite) led to trace
amounts of acetophenone in the dehydrogenation of 1-phe-
nylethanol.7f Wang and coworkers have also demonstrated that
Au nanoparticles (with uniform mean size of 3 nm) loaded onto
various supports exhibited different catalytic activities in the
dehydrogenation of alcohols to aldehydes, and found the use of
hydrotalcite (HT) with both strong acidity and strong basicity
provided the best catalytic performance.8 And interestingly, in
comparison to that of aerobic oxidation of alcohols, few reports
have mentioned the dehydrogenation of alcohols with good
efficiency.7–9 Notably, from the viewpoint of environmental
benign and atom efficiency of the reaction, a catalytic dehy-
drogenation of alcohols to carbonyl compounds is ideal and
safety. Although heterogeneous precious metals-based catalysts
were reported to be effective in this reaction, the development of
new and cheap heterogeneous catalyst for this reaction is highly
desirable.
The selective oxidation of alcohols to carbonyl compounds or
other derivatives is one of the most important and fundamental
reactions in organic chemistry and catalysis.1 Conventionally,
the oxidation of alcohols is carried out by metal-based oxidants
in stoichiometric amounts. And on the other hand, precious
metal-based homogenous or heterogeneous systems, including
gold, silver, and palladium, have been used as efficient catalysts
with molecular oxygen or organic oxidants (such as TEMPO and
PINO) as the sole oxidants to promote the selective oxidation of
alcohols.2 However, besides the strict demand for selective
oxidation promoted by these metal reagents or catalysts, the
reactions generate enormous amounts of toxic salts waste as
well as the recovery problem of expensive metal catalysts. To
meet environmental and economical acceptability with highly
activity, much efforts have been developed to heterogeneous
catalytic systems for aerobic or dehydrogenative oxidation of
alcohols.3–6 Especially, the dehydrogenation of alcohols is of
paramount importance in organic synthesis and catalysis owing
to wide applications of carbonyl compounds as important and
irreplaceable intermediate in chemical industries, medicinal
chemistry, and various research elds.5,6 In the past decade, a
Very recently, we have developed a new approach to
organosilicon-supported TiO2@PMHSIPN and mono-dispersed
hybrid Ag@TiO2 nanocomposites that obtained through
titanium-promoted
cross-linking
reduction
of
poly-
methylhydrosiloxane (PMHS)-based semi-interpenetrating
networks (PMHSIPN) and subsequent supporting of nano-
silver.10 In our synthetic strategy, the TiO2 was designed to be
highly dispersed on the functional organosilicon materials,
which is benecial to the formation of Ag@TiO2 nanoparticles
in uniform size and shape under mild conditions. Interestingly,
the fabricated Ag@TiO2 located on PMHS-based semi-
interpenetrating networks (nanoAg@TiO2@PMHSIPN) exhibi-
ted excellent catalytic activities in the alkynylation of
aKey Laboratory of Organosilicon Chemistry and Material Technology of Ministry of
Education, Hangzhou Normal University, Hangzhou 310012, P. R. China. E-mail:
liwenxu@hznu.edu.cn; Fax: +86 2886 5135; Tel: +86 2886 5135
bState Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of
Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, P. R. China. E-mail:
licpxulw@yahoo.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra06685e
‡ Mr H. Wang and Mr J. Zhang contributed equally to this work.
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triuoromethyl ketones and a,b-unsaturated triuoromethyl supported the generation of molecular hydrogen because of
ketones with terminal alkynes. In these reactions, the hybrid reduction of eneones 4a–c to ketones 5a–c with promising
Ag@TiO2@PMHSIPN catalyst showed a high level of catalytic selectivity (90/10 ꢀ 10 : 90). Encouraged by this promising
activity for the selective 1,2-alkynylation of various tri- result and Zhao's ndings on photocatalytic oxidation of alco-
uoromethyl ketones and a,b-unsaturated triuoromethyl hols,11 we then carried out the investigation on silver-free
ketones to a wide broad of uorinated alcohols in water. TiO2@PMHSIPN-catalyzed dehydrogenation of 1-phenyl-
However, similarly to Kaneda's nding on nanosilver catalysis,7f ethanol (1a) with propiophenone (3c). Interestingly, the
we also found that Ag@TiO2@PMHSIPN catalyst did not substituted enone 4c was obtained in 40% yield under the
promote the dehydrogenation of 1-phenylethanol (Scheme 1, solvent-free conditions (160 ꢁC, and 15 h). Therefore, we
eqn 1). In previous work,10 the catalytic activity of TiO2@ continued to study the scope of various substituted 1-indanone
PMHSIPN in organic reactions was unclear. Inspired by the (3d–h) in the TiO2@PMHSIPN-catalyzed dehydrogenation of 1-
work of oxygen atom transfer (OAT)-based photocatalytic phenylethanol (1a) at 160 ꢁC (Scheme 2). Unexpectedly, the yield
oxidation of alcohols by TiO2 in the presence of molecular of the desired products including the aldehyde and its deriva-
oxygen that reported by Zhao and coworkers,11 we hypothesized tives were dramatically arise from the substituent on phenyl
the hybrid TiO2@PMHSIPN could be used as a catalyst in the ring of 1-indanone. As shown in Scheme 2, except 5-chloride-1-
dehydrogenation of alcohols. To the best of our knowledge, indanone (3f), other substituted indanones were inactive in this
there was no report on the additive-free TiO2-catalyzed dehy- dehydrogenation-aldol condensation reaction because of poor
drogenation of alcohols for the synthesis of aldehydes and its conversion. Despite the catalytic activity of silver-free TiO2@
derivatives. Herein, we report the TiO2@PMHSIPN-catalyzed PMHSIPN catalyst, this dehydrogenation-aldol condensation
oxidation of alcohols to aldehydes and imines under different reaction was inferior to that of Ag@TiO2@PMHSIPN. We were
reaction conditions, in which the TiO2@PMHSIPN was found to pleased to nd the dehydrogenation of 1-phenylethanol gave
be a good catalyst in the oxidative synthesis of benzimidazoles high yields of benzaldehyde (88% yield). This nding promoted
from alcohols and benzene-1,2-diamine, which further sup- us to investigate the effect of solvent on the 5-chloride-1-
ported the efficient formation of aromatic aldehyde by the indanone (3f)-assisted TiO2@PMHSIPN-catalyzed dehydroge-
titanium-catalyzed dehydrogenation.
nation of 1-phenylethanol.
Initially, we evaluated the catalytic activity of Ag@TiO2@
Table 1 showed almost no reaction in toluene and other
PMHSIPN in the dehydrogenation of 1-phenylethanol in the solvents below 130 ꢁC (Entries 2–5), while excellent yield of
presence of various ketones at high temperature under solvent- benzaldehyde was obtained in nitrobenzene at 160 ꢁC (Entry 6).
free conditions (no addition of solvent except substrates). We With the optimized conditions in hand (2 mol% TiO2@
found that the addition of inorganic bases (such as K2CO3) was PMHSIPN, nitrobenzene as solvent, 160 ꢁC), we then studied the
crucial to this conversion. Mixtures of enones 4 and substituted scope of various benzyl alcohols and aromatic allylic alcohols.
ketones 5 were obtained in low to moderate selectivity (Scheme As shown in Scheme 3, most of benzyl alcohols (primary alco-
1, eqn 2, 2 mol% of catalyst was referred to the amount of hols) and aromatic allylic alcohols 1j and 1k (3-phenylprop-2-en-
ketone). The results clearly indicated that the silver nano- 1-ol and 4-phenylbut-3-en-2-ol respectively) afforded the desired
particles exhibited promising activity in the dehydrogenation
and subsequent aldol condensation of alcohols in the presence
of ketones. In addition, the formation of reductive ketone 5a–c
Scheme
1 Preliminary results of Ag@TiO2@PMHSIPN-promoted Scheme 2 The evaluation of TiO2@PMHSIPN-catalyzed coupling of
dehydrogenation of 1-phenylethanol.
benzyl alcohol with substituted indanones.
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Table 1 The effect of solvent on the 5-chloride-1-indanone (3f)-
aromatic ketones or enones was also unsuccessful. This result
indicates that steric effect was disadvantageous to the
dehydrogenation of alcohol in this reaction. Although the
results shown in Scheme 3 were not perfect in the synthesis of
aromatic aldehydes, the TiO2@PMHSIPN was rstly proven to
be an effective heterogeneous catalyst in the dehydrogenation of
benzyl alcohols.
assisted
TiO2@PMHSIPN-catalyzed
dehydrogenation
of
1-
phenylethanola
To obtain more information about the TiO2@PMHSIPN-
catalyzed dehydrogenation of benzyl alcohols, we then studied
the oxidative coupling of benzyl alcohols with amines.12
Notably, imines are useful backbones for organic synthesis,
pharmaceuticals, and necessary intermediates in many elds.
Since nitrobenzene was not easily removed under reduced
pressure, a variety of substituted anilines and benzyl alcohols
were carried out in this reaction under solvent-free conditions.12
To our delight, anilines containing electron-withdrawing or
electron-donating substituents reacted smoothly with benzyl
alcohol to give the desired imines in excellent conversions
(Table 2, entries 1–15). And a similar effect was observed with
Entry
Solvent
Temperature (ꢁC)
Yieldb (%)
c
1
2
3
4
5
6
—
160
120
130
120
120
160
88
<5
<5
<5
<5
99
Toluene
Xylene
DMSO
DMF
Nitrobenzene
a
Reaction conditions: 1-phenylethanol (1 mmol), TiO2@PMHSIPN
(calculated 2 mol% Ti based on the amount of alcohol), 5-chloride-1-
indanone (3f) was used as additive (20 mol%), in solvent (2 mL),
b
under argon. Aer the reaction, the products were analyzed by GC
using 1,3,5-trimethylbenzene as an internal standard and conrmed
c
by GC-MS aer separation of the TiO2@PMHSIPN catalyst. No
addition of solvent except substrates (solvent-free conditions).
Table 2 Results of the oxidative coupling of amines with alcohols
leading to the formation of iminesa
Entry
R1
R2
Yieldb (%)
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
4-Br
2-Br
2,4-Cl
2,6-Cl
4-Cl
4-Me
2-OMe
3-OMe
4-OMe
2-F
2-Me
H
94
99
99
99
99
99
88
99
99
92
94
92
99
92
99
93
96
99
59
84
99
99
99
98
99
99
3-Me
4-Me
2-OMe
2-OEt
4-Br
4-NO2
2-Cl
4-OEt
2,5-Cl
2,4-OMe
3-Cl
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
2-Me
4-Cl
3-Br
4-Me
4-Me
4-Me
4-Me
4-Me
4-Me
4-Me
4-Me
4-Me
4-Me
4-Me
Scheme 3 Dehydrogenation of benzyl alcohols to aromatic aldehydes
or ketones promoted by TiO2@PMHSIPN.
aromatic aldehydes in moderate to good yields (up to 99%) with
excellent selectivity (>99%). However, except bromide or
uoride-substituted benzyl alcohols, using ortho-substituted
benzyl alcohols as substrates led to low yields (16–39% yields).
Moreover, the dehydrogenation of secondary alcohols to
a
Reaction conditions: benzyl alcohol (1 mmol), TiO2@PMHSIPN
(calculated 2 mol% Ti based on the amount of alcohol), aromatic
amine (0.2 mmol), under argon.
b
Upon completion of the
dehydrogenation-initiated reaction and cooling, the mixture was
dissolved in DCM for GC and GC-MS analyses.5a
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various substituted benzyl alcohols with electron-withdrawing for the acid- and dehydrogenation/condensation of benzyl
or electron-donating groups. As shown in Table 2, except 2,6- alcohols with benzene-1,2-diamine to the catalytic synthesis of
dichloride substituted benzyl alcohol (Entry 19), almost all the benzimidazoles. Under optimized conditions similar to the
benzyl alcohols smoothly reacted with 4-methylaniline to give above reaction, we studied the oxidative coupling of benzyl
the corresponding imines in excellent conversion under the alcohols with benzene-1,2-diamine catalyzed by TiO2@
same catalytic conditions. These results supported that the PMHSIPN. Interestingly, as shown in Scheme 5, the reaction
addition of aromatic amines allowed the efficient the trans- could be carried out smoothly regardless of the electron-
formation of alcohols and amines into imines in the presence of withdrawing or the electron-donating group on the aromatic
TiO2@PMHSIPN under reaction conditions.
rings of alcohols. Good to excellent yields of the desired benz-
Encouraged by these promising results showed in Table 2, imidazoles were obtained in most cases. However, in some
we looked at extending the catalytic methods described above to cases, steric effects affected this reaction to some extent. For
other substrates. We then investigated the oxidative coupling of example, benzyl alcohol with ortho-substituted chloride or
benzyl alcohols with 4-methylbenzenesulfonamide (TsNH2) in bromide group resulted in moderate yields (53% and 69% yield
the presence of TiO2@PMHSIPN. As shown in Scheme 4, the respectively). From these results, the TiO2@PMHSIPN catalyst
catalytic conditions were also applicable to the conversion of may work to favor the dehydrogenation of alcohol to aldehyde,
benzyl alcohols to aldehyde intermediates in the presence of while the activity of this catalyst in subsequent amination and
TsNH2, whereby the desired Ts-imines were formed in excellent oxidative cyclization was not good enough.
yields and selectivity. In comparison to previous reports on the
Based on above results, a plausible mechanism for the
oxidative coupling of benzyl alcohols with amines, the advan- TiO2@PMHSIPN-catalyzed dehydrogenation of alcohols and
tages of this method are its general applicability in giving good tandem transformation of corresponding aldehydes in the
yields of the products under solvent-free conditions.
presence of amines/diamines is given in Fig. 1. In this catalytic
Similarly to imines, benzimidazole and its derivatives also process, titanium worked both as acid and base, in which all the
occupy pivotal roles in the synthesis of natural products and shiing of H to the surface of TiO2@PMHSIPN occurs as a
other materials in therapeutic applications such as antiulcer, reversible process (hydrogen-transfer catalysis).16 The reaction
analgesic, anti-inammatory, and anticancer.13 More recently, begins with the dehydrogenation of benzyl alcohol to the cor-
transition metal-catalyzed synthesis of benzimidazoles from responding aldehyde aer abstraction of hydrogen on benzylic
alcohols has been reported with one-pot tandem approach.14 carbon, which undergoes selective transfer hydrogenation of
Especially in 2010, Shiraishi and coworker15 reported a new the hydride species. A condensation then occurs between the
method for the acid- and synthesis of benzimidazoles by one- aldehyde and aromatic amine to give imine. For the synthesis of
pot multiple catalytic transformation of alcohols and aryldi- benzimidazoles, the intramolecular addition of amine to imine
amines at room temperature, in which the platinum nano- leads to the formation of dihydrobenzimidazole by cyclization.
particles supported on a TiO2 semiconductor enabled efficient In the last step (step 3), dehydrogenation of dihydro-
benzimidazole production under photoirradiation (l > 300 nm). benzimidazole affords the desired product by
It is proposed that a platinum-assisted photocatalytic oxidation hydrogen-transfer process.
on TiO2 and a catalytic dehydrogenation on the surface of the
particles contributed greatly to this highly efficient synthesis of
benzimidazoles. In this work, Shiraishi and coworker also
found that low yield of corresponding product was obtained
using pure TiO2 as catalyst under photoirradiation (24 h, up to
57% yield).15
a second
Thus to further validate the activity of TiO2@PMHSIPN in
the dehydrogenation of alcohols, we investigated the possibility
Scheme 4 Oxidative coupling of benzyl alcohol and TsNH2 catalyzed Scheme
5
Oxidative coupling of benzyl alcohol and benzene-
by TiO2@PMHSIPN.
1,2-diamine catalyzed by TiO2@PMHSIPN.
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51303043, 51203037 and 21173064), Zhejiang Provincial Natural
Science Foundation of China (LR14B030001), and Hangzhou
Science and Technology Program (20130533B15).
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Fig. 1 Possible reaction mechanism for the TiO2@PMHSIPN-cata-
lyzed dehydrogenation of alcohols and tandem transformation of
corresponding aldehydes.
In summary, this work is the rst to achieve the selective
catalytic dehydrogenation and tandem transformation of
aromatic alcohols using transition metal-free and
organosilicon-supported titanium material under solvent-free
reaction conditions. The organosilicon-supported titanium
(TiO2@PMHSIPN) enables efficient construction of imines and
benzimidazoles, which involve bifunctional titanium-assisted
oxidation of alcohol and subsequent coupling of amine or
oxidative coupling diamine. Moreover, this process has signif-
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synthesis of imines and benzimidazoles: (1) the titanium cata-
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Acknowledgements
The authors gratefully thank the nancial support of the
National Natural Science Foundation of China (NSFC, no.
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