Direct Reductive Amination of Aldehydes via Environmentally Benign Bentonite-Gold Nanohybrid Catalysis

Article abstract of DOI:10.1002/adsc.201601042

An efficient, green and reliable method for the direct reductive amination of aldehydes was developed using an environmentally benign bentonite-gold nanohybrid catalyst. Use of this heterogeneous catalyst affords a variety of secondary amines in excellent yield under ambient reaction conditions in the presence of phenyldimethylsilane as mild hydride donor. The catalyst is recyclable, selective and is well applicable for the gram-scale preparation of secondary amines. (Figure presented.).

Full text of DOI:10.1002/adsc.201601042

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DOI: 10.1002/adsc.201601042
Direct Reductive Amination of Aldehydes via Environmentally
Benign Bentonite-Gold Nanohybrid Catalysis
R. J. Maya,a,b Susanna Poulose,b Jubi John,b and R. Luxmi Varmaa,b,*
a
Academy of Scientific and Innovative Research (AcSIR), CSIR-NIIST, Thiruvananthapuram – 695019, India
Organic Chemistry Section, CSIR-National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram –
b
695019, India
Fax : (+91)-471-249-1712; e-mail: lux_varma@rediffmail.com
Received: September 19, 2016; Revised: January 24, 2017; Published online: && &&, 0000
Supporting information for this article is available under http://dx.doi.org/10.1002/adsc.201601042.
Abstract: An efficient, green and reliable method for donor. The catalyst is recyclable, selective and is well
the direct reductive amination of aldehydes was de- applicable for the gram-scale preparation of secon-
veloped using an environmentally benign bentonite- dary amines.
gold nanohybrid catalyst. Use of this heterogeneous
catalyst affords a variety of secondary amines in ex- Keywords: aldehydes; bentonite-gold nanohybrid;
cellent yield under ambient reaction conditions in environmentally benign nature; reductive amination;
the presence of phenyldimethylsilane as mild hydride selective catalyst
Introduction
catalytic systems were also developed. In the devel-
oped methods for reductive amination, molecular hy-
Secondary amines are constructive intermediates with drogen, silanes, formates, isopropyl alcohol, and
incredible potential in organic synthesis, biological Hantzsch esters are used as hydrogen donors.[10,13–16]
systems, materials science, agrochemicals and pharma- Among these, silanes act as mild hydride sources
ceuticals.[1] The simplest method for the synthesis of which can reduce imines in combination with Lewis
secondary amines involves the reductive amination of or Brønsted acids. However, the homogeneous cata-
carbonyl compounds via an intermediate imine, which lytic systems are plagued with the problems of cata-
is a widely utilized method in pharmaceutical indus- lyst-product separation and recyclability, and hence
try.[2] Direct reductive amination constitutes a more industrial applications of these catalysts; especially in
proficient and undemanding route[3] contrary to indi- the pharmaceutical industry remain as a challenge.
rect methods, which involve the isolation of the unsta- Consequently developing a suitable heterogeneous
ble imine intermediate.[4] However, overalkylation of catalyst for the reductive amination reaction becomes
amines is a major problem in the direct reductive significantly desirable. In recent times, some palladi-
amination of aldehydes.[5] Traditional methods includ- um based heterogeneous catalysts were developed for
ing various tin[6] and boron[7] complexes for secondary the one-pot reductive amination of aldehydes.[17]
amine formation often suffer from harsh reaction con-
Supported gold nanoparticles exhibit unique reac-
ditions, low selectivity, poor yields and toxic by-prod- tivity and they have successfully catalyzed various re-
ucts. The reaction of alcohols and amines through the actions like carbon-carbon[18] and carbon-heteroatom
borrowing hydrogen strategy[8] is one of the most bond formation,[19] oxidations,[20] reductions,[21] etc.
powerful methods to prepare secondary amines. How- Even though AuNPs exhibited high efficiency in hy-
ever, improved methods capable of generating secon- dride transfer processes, the exploitation of AuNPs
dary amines at room temperature are needed for for amine synthesis through a reductive amination
green organic synthesis and in this milieu, direct re- strategy is rare.[22] Although the reported catalysts are
ductive amination remains as a potent tool in the dynamic for the reductive amination reaction, the de-
field of synthetic organic chemistry.
velopment of more efficient, green, selective, reusable
In recent times, precious metal (Rh, Ir, and Ru) and cost effective catalytic systems still remains as
based catalysts and H2 gas have been reported to pro- a challenge in both industrial and academic research
mote direct reductive amination.[9,10] In addition, bio- fields. We have recently reported a bentonite-gold
relevant metals such as iron[11] and copper[12] based nanohybrid catalyst (Au-MPBen) for selective oxida-
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tion of silanes to silanols.[23] Compared to other sup- of 4 nm on the support. The concentration of gold on
ports like metal oxides, zeolites, active carbon, carbon Au-MPBen catalyst was measured by ICP-MS analy-
nanotubes, etc., bentonite has several advantages such sis and it was found that 1 mg of catalyst was loaded
as cost effectiveness, ubiquitousness and environmen- with 5.24ꢁ10À7 moles of gold.
tally benign nature. Furthermore, it can be easily
Our studies commenced with a one-pot condensa-
modified chemically through the hydroxy groups pres- tion of benzaldehyde 1 (1.0 equiv.) and aniline 2
ent on its broken edges.[24] Herein, we report the (1.0 equiv.) in the presence of phenyldimethylsilane
direct reductive amination of aldehydes using our Au- (1.5 equiv.) as the hydride source and a catalytic
MPBen catalyst with dimethylphenylsilane as a mild amount of Au-MPBen (420 mmol%) in acetonitrile at
reducing agent at room temperature to afford a varie- room temperature under an argon atmosphere. The
ty of secondary amines in excellent yields. Dimethyl- reaction was found to be complete in an hour and
phenylsilane was utilized in the present reaction due after removal of the catalyst by centrifugation and
to its low toxicity, mild reducing ability and in addi- column chromatography the product obtained in 95%
tion, due to the mild environmental impact of the by- yield was characterized to be the secondary amine 3.
product silanol.
This promising result prompted us to investigate the
effect of solvents on the reaction and the results are
given in Table 1 (entries 1–5). It was found that under
similar reaction conditions, an optimum yield of 95%
of N-benzylaniline was produced when acetonitrile
Results and Discussion
The Au-MPBen catalyst was synthesized and charac- was used as the solvent.
terized following our literature precedent.[23] In short,
The superiority of Au-MPBen catalyst in the direct
the assembly of the nanohybrid catalyst commenced reductive amination reaction was established by per-
with the acid activation of bentonite with 4N HCl forming the reaction using acid activated bentonite
and the resultant product was then treated with (3- (Ben-4h), organofunctionalized bentonite (Ben-MP),
mercaptopropyl)trimethoxysilane
and gold nanoparticle-impregnated bentonite (Au-Ben)
phenyltriethoxysilane at room temperature to provide with no additional stabilizing agents and the benton-
ACHTUNGTRENNUNG
organofunctionalized bentonite. Subsequently, Au- ite-gold nanohybrid (Au-MPBen). When the reduc-
MPBen was synthesized by impregnating the organo- tive amination of benzaldehyde was performed with
functionalized bentonite with HAuCl4 followed by re- Ben-4h/Ben-MP as catalysts (Table 2, entries 1 and 2),
duction with NaBH4. The catalyst was characterized N-benzylaniline was not formed even after 24 h.
using XRD, FT-IR, ICP-MS, XPS and TEM. Figure 1 When Au-Ben was used as the catalyst (Table 2,
shows the existence of monodispersed, spherically- entry 3), though the transformation of benzaldehyde
shaped gold nanoparticles with an average diameter to N-benzylaniline occurred, AuNPs were found to
Figure 1. TEM images of fresh Au-MPBen (scale bar=50 nm and 100 nm).
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Table 1. Optimization of solvents for the Au-MPBen-cata-
the corresponding secondary amines (3b and 3d) in
near quantitative yields (93–94%) within 2 h. In the
cases of aldehydes containing electron-releasing
groups (entries 3 and 5), the reaction proceeded very
efficiently within 2 h resulting in the products 3c and
3e in 92 and 91% yields, respectively. Besides, 2-bro-
moaniline and 2-iodo-4-bromoaniline (entries 9 and
10) reacted with benzaldehyde, furnishing the corre-
sponding products 3i and 3j in quantitative yield (93
and 91%). Bulkier aldehydes such as 1-pyrenecarbox-
aldehyde and 2-naphthaldehyde (entries 6 and 7)
were readily converted to the desired secondary
amines (3f and 3g) in 85 and 89% yields within 4 h
and 2 h, respectively.
lyzed reductive amination.[a]
This methodology was also found to be applicable
to heterocyclic aldehydes and amines. When 8-amino-
quinoline (entry 8) was employed as the amino com-
ponent, the corresponding secondary amine 3h was
obtained in 91% yield within 3 h. The reaction of 2-
aminothiazole with benzaldehyde (entry 11) required
more time (5 h) for completion and afforded the cor-
responding secondary amine 3k in 71% yield. In addi-
tion, thiophen-2-aldehyde and 2-furfuraldehyde (en-
tries 12 and 13) were converted to corresponding
amines (3l and 3m) in almost quantitative yields (94
and 92%) in a short time.
[a]
Reaction conditions: benzaldehyde (0.5 mmol), aniline
(0.5 mmol), phenyldimethylsilane (0.75 mmol), room
temperature, argon atmosphere, 1 h, solvent (1 mL), Au-
MPBen (420 mmol%).
Furthermore, the selectivity of the Au-MPBen cata-
lyst towards reductive amination reaction was unam-
biguously established by the reactions of cinnamalde-
hyde and perillaldehyde (entries 14 and 15) which
provided the corresponding secondary amines 3n and
3o very effectively with 92 and 91% yields and both
the reactions were completed within 2 h.
Table 2. Comparison of various catalysts in the reductive
amination reaction.[a]
In order to extend the substrate scope of the cata-
lyst, aliphatic aldehydes and amines were introduced
in the reaction. Priopionaldehyde and butyraldehyde
(entries 16 and 17) delivered the corresponding prod-
ucts 3p and 3q in good yields (80% and 82%) within
12 h but aliphatic amines exhibited very low reactivity
towards the reductive amination reaction. Even after
24 h, the reactions of isopropylamine and benzyla-
mine with benzaldehyde did not furnish the products
(3r and 3s) in isolable yields.
[a]
Reaction conditions: benzaldehyde (0.5 mmol), aniline
(0.5 mmol), phenyldimethylsilane (0.75 mmol), room
temperature, argon atmosphere, 1 h, ACN (1 mL), cata-
lyst (420 mmol%).
The recyclability of Au-MPBen for the direct re-
ductive amination reaction was investigated by carry-
ing out the reaction using benzaldehyde and aniline.
Au-MPBen catalyst was recovered by centrifugation
leach out from the support, lessening its reliability as or filtration and reused in five consecutive runs with-
a feasible catalyst. Only the Au-MPBen (Table 2, out the loss of efficiency (Table 4). After fifth run,
entry 4) gave the efficient conversion of benzaldehyde TEM images (see the Supporting Information,
to N-benzylaniline.
SFig. 1) showed no significant change in the morphol-
In order to explore the generality of our Au- ogy of the catalyst. These observations confirmed the
MPBen catalyst in the direct reductive amination re- high efficacy and robust nature of this Au-MPBen
action, under optimized reaction conditions, a variety nanohybrid catalyst.
of aldehydes and amines were reacted and the results
Moreover, this catalytic system was also found to
are detailed in Table 3. Both para- and ortho-halogen- be applicable for the gram-scale synthesis of secon-
ated aromatic aldehydes (entries 2 and 4) provided dary amines (Scheme 1). When 1 g of benzaldehyde
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Table 3. Substrate scope of reductive amination reaction.[a]
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Table 3. (Continued)
[a]
Reaction conditions: aldehyde (0.5 mmol), amine (0.5 mmol), phenyldimethylsilane (0.75 mmol),
room temperature, argon atmosphere, ACN (1 mL), Au-MPBen (420 mmol%).
Table 4. Recycling experiments with the Au-MPBen cata-
ACHTUNGTRENNUNG
lyst.[a]
ly. But based on the percentage of surface gold atoms
(not the actual gold loading) that are exposed to the
substrate, TON and TOF values reached to 3004 and
502 hÀ1, respectively.
The presumed mechanism for the direct reductive
amination of aldehydes with phenyldimethylsilane as
the hydride donor may be similar to the pathways
suggested for similar noble metal hybrid catalysts
(Scheme 2).[25] The reaction starts with the formation
of imine A by the condensation of aldehyde and
amine. Phenyldimethylsilane undergoes oxidative in-
À
sertion of the Si H bond on the surface of the gold
nanoparticles, which produces an activated silane B
that is capable of reducing the imine. This activated
silane species then reacts with the imine, hydrosilyla-
tion occurs and the N-silylated amine is generated.
Water produced as a result of condensation reaction
is used for the hydrolysis of the N-silylated amine to
[a]
Reaction conditions: benzaldehyde (0.5 mmol), aniline
(0.5 mmol), phenyldimethylsilane (0.75 mmol), room
temperature, argon atmosphere, 1 h, ACN (1 mL), Au-
MPBen (420 mmol%).
Scheme 1. Gram-scale synthesis of N-benzylaniline using
Au-MPBen.
was treated with aniline in the presence of Au-
MPBen (1.05 mmol%) and phenyldimethylsilane as
hydride source, N-benzylaniline 3 was isolated in 75%
yield (1.3 g) within 6 h. Under these conditions, the
turnover number (TON) and turnover frequency
Scheme 2. Proposed mechanism for the Au-MPBen-cata-
(TOF) were calculated as 676 and 113 hÀ1, respective- lyzed reductive amination reaction.
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produce corresponding secondary amine and silanol is Gram-Scale Preparation of N-Benzylamine and TON
produced as a by-product. Silylation of the nitrogen and TOF Calculations
atom and reduction are concurrent reactions that
ensue during the direct reductive amination reaction.
in dry ACN (7 mL), 1.92 g of phenyldimethylsilane and the
To a mixture of 1 g of benzaldehyde and 0.878 g of aniline
catalyst, Au-MPBen, (1.05 mmol%) were added and the
mixture was allowed to stir at room temperature under an
argon atmosphere. The progress of the reaction was moni-
Conclusions
tored by TLC. After completion of the reaction, the catalyst
was removed by centrifugation and washed three times with
ACN. The filtrate was concentrated under vacuum and the
crude product was purified by column chromatography. N-
Benzylaniline was obtained successfully within 6 h; yield:
1.3 g (75%).
We have reported an efficient and green method for
the direct reductive amination of aldehydes using
a bentonite-gold nanohybrid, Au-MPBen as a hetero-
geneous catalyst with the mild reducing agent phenyl-
dimethylsilane. The catalytic system has a number of
advantages such as being easily separable, reusable,
economical, selective, environmentally benign, and
opertaing under mild reaction conditions to furnish
the amines in excellent yield. Au-MPBen afforded
secondary amines with a wide variety of aldehydes
and amines very effectively. This catalyst was also ap-
plicable for the gram-scale preparation of secondary
amines.
TON and TOF values based on the total amount of gold
were calculated as follows:
TON=total amount of product (mol)/total amount of gold
(mol)=0.0071/0.0000105=676.
TOF=TON/time (h)=676/6=113 hÀ1.
The percentage of gold atoms exposed to the surface of
AuNP was calculated based on the work of Boudart and
Djega-Mariadassou,[26] which is approximately 0.9/d, where
d is the spherical metal particle diameter in nm. Thus gold
nanoparticles with a diameter of 4 nm have about 22.5%
(0.9/4 nm) of their atoms lying on the surface of the AuNP.
As a result, adjusted TON and TOF values based on surface
atoms were obtained as 3004 and 502 hÀ1, respectively.
Experimental Section
General Procedure for the Direct Reductive
Amination of Aldehydes
Acknowledgements
To a mixture of aldehyde 1 (0.50 mmol) and amine 2
(0.50 mmol) in dry ACN (1 mL), phenyldimethylsilane
(0.75 mmol) and the catalyst, Au-MPBen, (420 mmol%)
were added and the mixture was allowed to stir at room
temperature under an argon atmosphere. The progress of
the reaction was monitored by TLC. After completion of
the reaction, the catalyst was removed by centrifugation and
washed three times with ACN. The filtrate was concentrated
under vacuum and the crude product was purified by
column chromatography to afford the secondary amine (3).
The authors thank CSIR Network project CSC-0135 for fi-
nancial assistance. RJM thanks UGC, New Delhi for a re-
search fellowship. We thank Mrs. Saumini Mathew, Mrs. S.
Viji, Mr. Saran P. Raveendran, Mr. Rakesh Gokul and Ms.
Athira S. of CSIR-NIIST, Trivandrum, for recording NMR
and mass spectra. We also thank Mr. Kiran Mohan of CSIR-
NIIST, Trivandrum, for transmission electron microscopic
analysis.
Recycling Experiment
References
To a mixture of benzaldehyde 1 (0.50 mmol) and aniline 2
(0.50 mmol) in dry ACN (1 mL), phenyldimethylsilane
(0.75 mmol) and the catalyst, Au-MPBen, (420 mmol%)
were added and the mixture was allowed to stir at room
temperature under an argon atmosphere. The progress of
the reaction was monitored by TLC. After completion of
the reaction, the catalyst was removed by centrifugation and
washed three times with ACN. The filtrate was concentrated
under vacuum and the crude product was purified by
column chromatography to afford N-benzylaniline (3). The
catalyst was reused for the second reaction after washing
with ACN. The reductive amination reaction was repeated
for four more cycles by reusing the recycled catalyst from
the previous reaction. After the fifth run, TEM analysis of
the catalyst was carried out to check for any changes in the
morphology.
[1] a) M. S. Gibson, in: The Chemistry of the Amino
Group, (Ed.: S. Patai), Wiley-Interscience, New York,
1968, pp 61; b) A. W. Czarnik, Acc. Chem. Res. 1996,
29, 112–113; c) J. P. Wolfe, S. Wagaw, J.-F. Marcoux,
S. L. Buchwald, Acc. Chem. Res. 1998, 31, 805–818;
d) S. A. Lawrence, in Amines: Synthesis Properties and
Applications, Cambridge University Press, Cambridge,
2004.
[2] S. Warren, P. Wyatt, Organic Synthesis: the disconnec-
tion approach, 2nd edn., Wiley-Blackwell, Oxford, 2008,
p 54.
[3] a) B.-M. Park, S. Mun, J. Yun, Adv. Synth. Catal. 2006,
348, 1029–1032; b) G. Shang, Q. Yang, X. Zhang,
Angew. Chem. 2006, 118, 6508–6510; Angew. Chem.
Int. Ed. 2006, 45, 6360–6362; c) Q. Yang, G. Shang, W.
Gao, J. Deng, X. Zhang, Angew. Chem. 2006, 118,
3916–3919; Angew. Chem. Int. Ed. 2006, 45, 3832–3835;
d) T. Imamoto, N. Iwadate, K. Yoshida, Org. Lett. 2006,
Adv. Synth. Catal. 0000, 000, 0 – 0
6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÞÞ
These are not the final page numbers!

FULL PAPERS
asc.wiley-vch.de
8, 2289–2292; e) M. T. Reetz, O. Bondarev, Angew.
Chem. 2007, 119, 4607–4610; Angew. Chem. Int. Ed.
2007, 46, 4523–4526; f) Y.-Q. Wang, S.-M. Lu, Y.-G.
Zhou, J. Org. Chem. 2007, 72, 3729–3734; g) C. Li, C.
Wang, B. Villa-Marcos, M. J. Xiao, J. Am. Chem. Soc.
2008, 130, 14450–14451; h) Z. Han, Z. Wang, X. Zhang,
K. Ding, Angew. Chem. 2009, 121, 5449–5453; Angew.
Chem. Int. Ed. 2009, 48, 5345–5349; i) W. Li, G. Hou,
M. Chang, X. Zhang, Adv. Synth. Catal. 2009, 351,
3123–3127; j) A. Baeza, A. Pfaltz, Chem. Eur. J. 2010,
16, 4003–4009.
V. Tararov, A. Bçrner, J. Org. Chem. 2003, 68, 4067–
4070; f) G. D. Williams, R. A. Pike, C. E. Wade, M.
Wills, Org. Lett. 2003, 5, 4227–4230; g) D. Imao, S. Fuji-
hara, T. Yamamoto, T. Ohta, Y. Ito, Tetrahedron 2005,
61, 6988–6992; h) T. Bunlaksananusorn, F. Rampf, Syn-
lett 2005, 17, 2682–2684; i) A. Robichaud, A. N. Ajjou,
Tetrahedron Lett. 2006, 47, 3633–3636; j) L. Rubio-
Pꢂrez, F. J. Pꢂrez-Flores, P. Sharma, L. Velasco, A. Cab-
rera, Org. Lett. 2009, 11, 265–268; k) D. Steinhuebel,
Y. K. Sun, K. Matsumura, N. Sayo, T. Saito, J. Am.
Chem. Soc. 2009, 131, 11316–11317; l) C. Li, B. Villa-
Marcos, J. Xiao, J. Am. Chem. Soc. 2009, 131, 6967–
6969; m) B. Sreedhar, P. S. Reddy, D. K. Devi, J. Org.
Chem. 2009, 74, 8806–8809.
[4] a) M. J. Palmer, M. Wills, Tetrahedron: Asymmetry
1999, 10, 2045–2061; b) F. Spindler, H.-U. Blaser, C.
Malan, B. Pugin, F. Spinder, H. Steiner, M. Studer,
Adv. Synth. Catal. 2003, 345, 103–151; c) W. Tang, X.
Zhang, Chem. Rev. 2003, 103, 3029–3069; d) T. C.
Nugent, M. EI-Shazly, Adv. Synth. Catal. 2010, 352,
753–819.
[11] S. Fleischer, S. Zhou, K. Junge, M. Beller, Chem. Asian
J. 2011, 6, 2240–2245.
[12] S. Werkmeister, K. Junge, M. Beller, Green Chem.
2012, 14, 2371–2374.
[5] a) M. B. Smith, J. March, Marchꢀs Advanced Organic
Chemistry, Wiley, New York, 2001, p 1187; b) A. F.
Abdel-Majid, K. G. Carson, B. D. Harris, C. A. Maryan-
off, R. Shath, J. Org. Chem. 1996, 61, 3849–3862;
c) R. F. Borch, H. D. Durst, J. Am. Chem. Soc. 1969,
91, 3996–3997.
[6] a) E. R. Burkhardt, B. M. Coleridge, Tetrahedron Lett.
2008, 49, 5152–5155; b) H. Alinezhad, M. Tajbakhsh, F.
Salehian, F. K. Fazli, Tetrahedron Lett. 2009, 50, 659–
661; c) W. Y. Liao, Y. F. Chen, Y. X. Liu, H. G. Duan,
J. L. Petersen, X. D. Shi, Chem. Commun. 2009, 6436–
6438; d) M. Tokizane, K. Sato, Sakami, Y. Imori, C.
Matsuo, T. Ohta, Y. Ito, Synthesis 2010, 1, 36–42;
e) P. V. Ramachandran, P. D. Gagare, K. Sakavuyi, P.
Clark, Tetrahedron Lett. 2010, 51, 3167–3169.
[13] a) D. Dubꢂ, A. A. Scholte, Tetrahedron Lett. 1999, 40,
2295–2298; b) R. Apodaca, W. Xiao, Org. Lett. 2001, 3,
1745–1748; c) B.-C. Chen, J. E. Sundeen, P. Guo, M. S.
Bednarz, R. Zhao, Tetrahedron Lett. 2001, 42, 1245–
1246; d) T. Mizuta, S. Sakaguchi, Y. Ishii, J. Org. Chem.
2005, 70, 2195–2199; e) O.-Y. Lee, K.-L. Law, C.-Y. Ho,
D. Yang, J. Org. Chem. 2008, 73, 8829–8837; f) S. En-
thaler, Catal. Lett. 2011, 141, 55–61.
[14] a) M. Kitamura, D. Lee, S. Hayashi, S. Tanaka, M.
Yoshimura, J. Org. Chem. 2002, 67, 8685–8687; b) R.
Kadyrov, T. H. Riermeier, Angew. Chem. 2003, 115,
5630–5632; Angew. Chem. Int. Ed. 2003, 42, 5472–5474;
c) S. Ogo, K. Uehara, T. Abura, S. Fukuzumi, J. Am.
Chem. Soc. 2004, 126, 3020–3021; d) E. Byun, B. Hong,
K. A. De Castro, M. Lim, H. Rhee, J. Org. Chem. 2007,
72, 9815–9817; e) C. Wang, A. Pettman, J. Basca, J.
Xiao, Angew. Chem. 2010, 122, 7710–7714; Angew.
Chem. Int. Ed. 2010, 49, 7548–7552; f) D. OꢃConnor, A.
Lauria, S. P. Bondi, S. Sabaet, Tetrahedron Lett. 2011,
52, 129–132.
[7] a) H. Kato, I. Shibata, Y. Yasaka, S. Tsunoi, M.
Yasuda, A. Baba, Chem. Commun. 2006, 40, 4189–
4191; b) P. D. Pham, P. Bertus, S. Legoupy, Chem.
Commun. 2009, 41, 6207–6209.
[8] a) M. H. S. A. Hamid, P. A. Slatford, J. M. J. Williams,
Adv. Synth. Catal. 2007, 349,1555–1575; b) T. D. Nixon,
M. K. Whittlesey, J. M. J. Williams, Dalton Trans. 2009,
753–762; c) R. Yamaguchi, K. Fujita, M. Zhu, Hetero-
cycles 2010, 81, 1093–1140; d) G. E. Dobereiner, R. H.
Crabtree, Chem. Rev. 2010, 110, 681–703; e) G. Guille-
na, D. J. Ramon, M. Yus, Chem. Rev. 2010, 110, 1611–
1641; f) L. He, X.-B. Lou, J. Ni, Y.-M. Liu, Y. Cao, H.-
Y. He, K.-N. Fan, Chem. Eur. J. 2010, 16, 13965–13969;
g) C.-H. Tang, L. He, Y.-M. Liu, Y. Cao, H.-Y. He, K.-
N. Fan, Chem. Eur. J. 2011, 17, 7172–7177; h) C. Guna-
nathan, D. Milstein, Science 2013, 341, 249–257; i) L.
Tang, Y. Yang, L. Wen, S. Zhang, Z. Zha, Z. Wang,
Org. Chem. Front. 2015, 2, 114–118.
[15] D. Gnanamgari, A. Moores, E. Rajaseelan, R. H. Crab-
tree, Organometallics 2007, 26, 1226–1230.
[16] a) S. Hoffmann, M. Nicoletti, B. List, J. Am. Chem.
Soc. 2006, 128, 13074–13075; b) R. I. Storer, D. E. Car-
rera, Y. Ni, D. W. C. MacMillan, J. Am. Chem. Soc.
2006, 128, 84–86; c) A. Kumar, S. Sharma, R. A.
Maurya, Adv. Synth. Catal. 2010, 352, 2227–2232.
[17] a) E. Byun, B. Hong, K. A. De Castro, M. Lim, H.
Rhee, J. Org. Chem. 2007, 72, 9815–9817; b) B. Sreed-
har, P. S. Reddy, D. K. Devi, J. Org. Chem. 2009, 74,
8806–8809; c) S. Wei, Z. Dong, Z. Ma, J. Sun, J. Ma,
Catal. Commun. 2013, 30, 40–44; d) M. Nasrollahzadeh,
New J. Chem. 2014, 38, 5544–5550.
[9] a) V. I. Tararov, A. Bçrner, Synlett 2005, 203–211;
b) T. C. Nugenta, M. El-Shazlya, Adv. Synth. Catal.
2010, 352, 753–819; c) D. Talwar, N. P. Salguero, C. M.
Robertson, J. Xiao, Chem. Eur. J. 2014, 20, 245–252.
[10] a) H. U. Blaser, H. P. Buser, H. P. Jalett, B. Pugin, F.
Spindler, Synlett 1999, 867–868; b) V. I. Tararov, R. Ka-
dyrov, T. H. Riermeier, A. Bçrner, Chem. Commun.
2000, 1867–1868; c) T. Gross, A. Seayad, A. Moballigh,
M. Beller, Org. Lett. 2002, 4, 2055–2058; d) Y. Chi,
Y. G. Zhou, X. Zhang, J. Org. Chem. 2003, 68, 4120–
4122; e) R. Kadyrov, T. H. Riermeier, U. Dingerdissen,
[18] a) A. S. K. Hashmi, L. Schwarz, J.-H. Choi, T. M. Frost,
Angew. Chem. 2000, 112, 2382–2385; Angew. Chem.
Int. Ed. 2000, 39, 2285–2288; b) S. Wang, L. Zhang, J.
Am. Chem. Soc. 2006, 128, 14274–14275; c) J. Xie, H.
Li, J. Zhou, Y. Cheng, C. Zhu, Angew. Chem. 2012,
124, 1278–1281; Angew. Chem. Int. Ed. 2012, 51, 1252–
1255; d) J. P. Brand, J. Waser, Angew. Chem. 2010, 122,
7462–7465; Angew. Chem. Int. Ed. 2010, 49, 7304–7307;
e) Z. Shi, C. He, J. Org. Chem. 2004, 69, 3669–3671;
f) X.-Q. Yao, C.-J. Li, J. Am. Chem. Soc. 2004, 126,
6884–6885; g) J.-J. Kennedy-Smith, S.-T. Staben, F. D.
Adv. Synth. Catal. 0000, 000, 0 – 0
7
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÞÞ
These are not the final page numbers!

FULL PAPERS
asc.wiley-vch.de
Toste, J. Am. Chem. Soc. 2004, 126, 4526–4527; h) T.
de Haro, C. Nevado, J. Am. Chem. Soc. 2010, 132,
1512–1513; i) C. Wei, C.-J. Li, J. Am. Chem. Soc. 2003,
125, 9584–9585.
assero, Angew. Chem. 2005, 117, 7052–7055; Angew.
Chem. Int. Ed. 2005, 44, 6892–6895.
[21] a) B. S. Takale, S. M. Tao, X. Q. Yu, X. J. Feng, T. Jin,
M. Bao, Y. Yamamoto, Org. Lett. 2014, 16, 2558–2525;
b) M. Zhang, H. Yang, Y. Zhang, C. Zhu, W. Li, Y.
Cheng, H. Hu, Chem. Commun. 2011, 47, 6605–6666;
c) D. Astruc, F. Lu, J. R. Aranzaes, Angew. Chem. 2005,
117, 8062–8083; Angew. Chem. Int. Ed. 2005, 44, 7852–
7878; d) A. Corma, P. Serna, H. Garcia, J. Am. Chem.
Soc. 2007, 129, 6358–6363.
[22] a) R. Kumar, E. Gravel, A. Hagꢄge, H. Li, D. Verma,
I. N. N. Namboothiri, E. Doris, ChemCatChem 2013, 5,
3571–3535; b) S. Liang, P. Monsen, G. B. Hammond, B.
Xu, Org. Chem. Front. 2016, 3, 505–505; c) W. Yang, L.
Wei, F. Yia, M. Cai, Catal. Sci. Technol. 2016, 6, 4554–
4545; d) z. Ke, X. Cui, F. Shi, ACS Sustainable Chem.
Eng. 2016, 4, 3921–3939.
[19] a) C.-G. Yang, C. He, J. Am. Chem. Soc. 2005, 127,
6966–6967; b) F. Mo, J. M. Yan, D. Qiu, F. Li, Y. Zhang,
J. Wang, Angew. Chem. 2010, 122, 2072–2076; Angew.
Chem. Int. Ed. 2010, 49, 2028–2032; c) J. Zhang, C.-G.
Yang, C. He, J. Am. Chem. Soc. 2006, 128, 1798–1799;
d) K. D. Hesp, M. Stradiotto, J. Am. Chem. Soc. 2010,
132, 18026–18027; e) L. Ye, L. Cui, G. Zhang, L.
Zhang, J. Am. Chem. Soc. 2010, 132, 3258–3259; f) C.
Brouwer, C. He, Angew. Chem. 2006, 118, 1776–1779;
Angew. Chem. Int. Ed. 2006, 45, 1744–1747; g) A.
Leyva, A. Corma, J. Org. Chem. 2009, 74, 2067–2074;
h) S. Biswas, J. S. M. Samec, Chem. Commun. 2012, 48,
6586–6588.
[20] a) B. Guan, D. Xing, G. Cai, X. Wan, N. Yu, Z. Fang,
L. Yang, Z. Shi, J. Am. Chem. Soc. 2005, 127, 18004–
18005; b) A. S. K. Hashmi, C. Lothschutz, M. Acker-
mann, R. Doepp, S. Anantharaman, B. Marchetti, H.
Bertagnolli, F. Rominger, Chem. Eur. J. 2010, 16, 8012–
8019; c) C.-F. Xu, M. Xu, Y.-X. Jia, C.-Y. Li, Org. Lett.
2011, 13, 1556–1559; d) Y. Liu, F. Song, S. Guo, J. Am.
Chem. Soc. 2006, 128, 11332–11333; e) M. A. Cinellu,
G. Minghetti, F. Cocco, S. Stoccoro, A. Zucca, M. Man-
[23] R. J. Maya, J. John, R. L. Varma, Chem.Commun. 2016,
52, 10625-10628 .
[24] a) A. Vaccari, Catal. Today 1998, 41, 53; b) H. Shi, T.
Lan, T. J. Pinnavaia, Chem. Mater. 1996, 8, 1584–1515.
[25] M. Stratakis, H. Garcia, Chem. Rev. 2012, 112, 4469–
4444.
[26] M. Boudart, G. Djꢂga-Mariadassou, in: Kinetics of Het-
erogeneous Catalytic Reactions, Princeton University
Press, Princeton, N.J., 1984, p 26.
Adv. Synth. Catal. 0000, 000, 0 – 0
8
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Direct Reductive Amination of Aldehydes via
Environmentally Benign Bentonite-Gold Nanohybrid
Catalysis
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Adv. Synth. Catal. 2017, 359, 1 – 9
R. J. Maya, Susanna Poulose, Jubi John, R. Luxmi Varma*
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