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doi.org/10.1002/chem.202005138
Chemistry—A European Journal
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Heterogeneous Catalysis |Hot Paper|
Visible-Light-Mediated Oxidative Amidation of Aldehydes by
Using Magnetic CdS Quantum Dots as a Photocatalyst
Dedicated to the 100th anniversary of chemistry at Nankai University
Abstract: A magnetic CdS quantum dot (Fe3O4/polydopa-
mine (PDA)/CdS) was synthesized through a facile and con-
venient method from inexpensive starting materials. Charac-
terization of the prepared catalyst was performed by means
of FTIR spectroscopy, XRD, SEM, TEM, energy-dispersive X-
ray spectroscopy, and vibrating-sample magnetometer tech-
niques. Fe3O4/PDA/CdS was found to be a highly active pho-
tocatalyst for the amidation of aromatic aldehydes by using
air as a clean oxidant under mild conditions. The photocata-
lyst can be recovered by magnetic separation and success-
fully reused for five cycles without considerable loss of its
catalytic activity.
modified Staudinger reaction.[20] From the perspective of green
chemistry, these processes generate a large amount of byprod-
ucts and chemical waste, so they need to be improved or re-
placed. Thus, much effort is being devoted to making the for-
mation of amide bonds more “green.” Among reported amide-
formation reactions, the direct cross-dehydrogenative coupling
(CDC) of aldehydes with amines is one of the most economical
methods to prepare amides, because the starting materials are
readily available and only hydrogen is produced as a byprod-
uct.
Introduction
The continuously growing demand for the development of
green and valuable transformations in organic synthesis has in-
spired chemists to address environmental and energy issues
through an operative and scalable technology.[1] In this con-
text, visible-light-mediated photoredox catalysis has become a
versatile and powerful strategy for reducing thermochemical
or electrochemical activation energy by using safe, abundant,
clean, and renewable light sources.[2–5] However, the main con-
tributions come from homogeneous photoredox catalysts,
such as transition-metal complexes based on ruthenium or iri-
dium, as well as various organic dyes.[6–8] Photoredox catalysis
with heterogeneous photocatalysts has the inherent advantag-
es of easy catalyst separation and recycling and is of great in-
terest from industrial and academic points of view.[9–13]
Various methods have been reported to form amide bonds
with aldehydes and amines as starting materials (Scheme 1).
Aldehydes are activated by forming acyl bromides,[21] acyl cya-
nides,[22] nitriles,[23] or active esters[24] as intermediates and are
then coupled with amines (Scheme 1a). Amines can also be
converted into N-chloramine[25] or anionic amido complexes[26]
as reactive intermediates to react with aldehydes (Scheme 1b).
In addition, catalytic methods with copper,[27] copper/silver,[28]
iron,[29] transition metals (Pd, Rh, and Ru),[30] and lanthanide[31]
have been reported (Scheme 1c). However, some of these
strategies have inherent shortcomings, such as the employ-
ment of expensive transition-metal catalysts, elevated tempera-
ture, or superfluous use of strong oxidants. In recent photo-
chemical studies, visible-light photoredox catalysis has provid-
ed optional approaches for the oxidative amidation of alde-
hydes by using a phenazinium salt,[32] rose Bengal,[33] boron-di-
pyrromethene (BODIPY),[34] silyl-substituted quinolizinium
compounds,[35] hemicyanine derivatives (CAT),[36] Ag/graphitic
carbon nitride (g-C3N4),[37] or anthraquinone (AQN)-based orga-
nophotocatalysts[38] (Scheme 1d). Nonetheless, most existing
methods involve the use of expensive catalysts and require ad-
ditives, and the separation of homogeneous photocatalysts is
still problematic. All of the deficiencies mentioned above
prompted us to continuously explore a less costly, greener,
heterogeneous, and simpler catalyst system under base-/addi-
The synthesis of amides is of great significance in organic
synthesis, because an amide is a key functional group in mate-
rials, natural products, and medicines.[14] Recent estimates from
the U.S. patent literature suggest that amidation reactions rep-
resent approximately 15% of all transformations[15] To date, nu-
merous methods for constructing amide bonds have been re-
ported.[16] The most well-established routes for the formation
of amides involve the reaction of amines with activated car-
boxylic acids or their corresponding derivatives,[17] the Schmidt
reaction,[18] the Beckmann rearrangement reaction,[19] and
[a] L. Xu, S.-Z. Zhang, W. Li, Prof. Z.-H. Zhang
Hebei Key Laboratory of Organic Functional Molecules
National Demonstration Center for Experimental Chemistry Education
College of Chemistry and Material Science, Hebei Normal University
Shijiazhuang 050024 (P.R. China)
Supporting information and the ORCID identification numbers for the
authors of this article can be found under:
Chem. Eur. J. 2021, 27, 5483 –5491
5483
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