DOI: 10.1002/open.201900104
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Self-Suspended Nanoparticles for N-Alkylation Reactions: A
New Concept for Catalysis
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The catalytic activity of snowman-like and core-shell Fe3O4/Au
nanoparticles (NPs), obtained through “wet chemistry”
approach which directly restitutes nanocatalysts stable and
highly active in the reaction medium, was tested towards N-
alkylation reactions. The nanocatalysts were tested for the
synthesis of secondary amines. The core-shell NPs, thanks to the
surface properties, homogeneous dispersion and intimate
connection with reagents in the catalyst medium, exhibited an
excellent catalytic activity (e.g. >99% yield and conversion of
aniline in very short time and mild conditions). Owing to the
magnetic part, the nanoparticles can be easily separated and
reused, showing an almost stable activity after 10 cycles.
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a
1. Introduction
ment of an easy recoverable and/or recyclable nanocatalysts
can reduce environmental impact and costs.
Different agrochemicals, pharmaceuticals, bioactive and natural
identical molecules are produced from intermediates such as N
containing molecules, e.g. amines.[1] For the syntheses various
approaches have been proposed and developed.[2–7] The so
called borrowing hydrogen method, consisting in an alcohol
based alkylation, is a very promising approach for the synthesis
Surface modifications[60–62] can provide an hydrophobic
nature to stable nanoparticles in reaction media, avoiding
agglomeration and aggregation. Although, it is so promising,
there are few examples of catalysis promoted by self-suspended
nanoparticles and none, at the best of our knowledge, to
promote N-alkylation reactions. n recent years, magnetically
recyclable nano-catalysts have attracted much attention in
organic synthesis. Apart from an ease of separability, they also
have other advantages such as low toxicity simple preparation
technique and are low-cost. [1] The magnetic nano-catalyst can
broadly be divided into two types: (i) supported magnetic
nano-catalyst, the active metal is deposited on freshly prepared
Fe3O4 nano-material, and (ii) a mixed ferrite nano-catalyst with
general formula MFe2O4 (where M is a divalent cation other
than iron), all starting materials are incorporated into one pot.
[2] Gold catalysts, that had been for long time considered inert,
have obtained considerable attention because of the catalytic
activity and selectivity showed at the small sizes.[43] This activity
strongly depends on: the particles size; crystallographic
nature;[63] etc… In particular, gold has been found to exhibit
unique catalytic properties for a number of liquid and vapor
phase reaction.[64] Moreover, magnetite is a soft Lewis acid in
organic synthesis[58,64,65] and can catalyze N-alkylation reaction,
e.g. through hydrogen autotransfer.[58,59]
of
secondary
amines[8–17]
attracting
recently
great
attention.[11,12,18–25] Indeed, it is an atom-economic and environ-
mental friendly approach, as alcohols are abundant and water is
the only by-product. Homogeneous catalysts have been
extensively studied[26–40] and catalysts based on Ru,[21] Ir,[41] Pd,[42]
Au,[43] Pt[44] have been successfully used. On the other hand,
homogeneous catalysts suffer from some typical drawbacks
such as recovery, handling of metal complexes, etc. Homoge-
neous catalysts recovery is a key topic in industrial chemistry.
Also, heterogeneous catalysis has been explored.[21–25,45À 59]
Heterogeneous catalysts containing metals such as Pd,[21–25]
Ru,[45] Au,[46] Ag[47–48] and Fe[58,59] have been used for N-alkylation
of amines with alcohols. However, they require multistep and
complex procedures and often result in reduced activity and
selectivity. Indeed, good activity can be achieved at temper-
[26,32–39,43,46,50À 59]
°
atures higher than 140 C.
This is mainly due to
diffusion limitations and heterogeneity of the active sites.
Nanoparticles (NPs) because of size and surface area are ideal
candidates to support catalysts, suppling large load and
improved activities. On the other hand, the design and develop-
Here we report, for the first time, the catalytic activity
towards N-alkylation reactions of Fe3O4/Au snowman-like
(Fe3O4/AuSM) and Fe3O4/Au core-shell (Fe3O4/AuCS) NPs. They
were obtained through a “wet chemistry” approach, which is a
scalable and reliable synthesis method. It directly restitutes
nanoparticles stable in solution, without requiring further
modification,[62,66–68] thanks to the surfactant molecules covering
the NPs during synthesis and allowing a behaviour between
homogeneous and heterogeneous catalysis. In particular, the
NPs can be dispersed and stabilized in solution because of the
small size and the ligand coating, whereas magnetic part allows
easily separation.
[a] Prof. M. Sarno, Dr. C. Cirillo, Dr. M. Iuliano
Department of Industrial Engineering and Centre NANO_MATES
University of Salerno
Via Giovanni Paolo II, 132-84084 Fisciano (SA), Italy
E-mail: msarno@unisa.it
Supporting information for this article is available on the WWW under
©2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons
Attribution Non-Commercial NoDerivs License, which permits use and dis-
tribution in any medium, provided the original work is properly cited, the
use is non-commercial and no modifications or adaptations are made.
The two different nanocatalysts were prepared to explore:
ligand covered nanometric size gold surfaces; and heterojunc-
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© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA