I. Gumus et al.
Molecular Catalysis 501 (2021) 111363
Ir-Zr-MOF [82], etc. The Au/ZnAl
2
O
3
catalyst presented the high per-
for efficient one-pot imine formation using Au/MIL-101 can be
explained in Fig. 7. First, alcohol forms an alcoholate with the assistance
of the base, and it binds to unsaturated Au atoms bearing positive charge
density located at corners, edges and steps of the NPs. Then, Au-H and
formance among the reported Au catalysts at mild temperatures with
ꢀ 1
high TOF =39.1 h , due to the active surface oxygen species and the
acidic-basic property of support, but it suffer from limited substrate
scope (Table 4, entry 8) [56]. Similarly, Au/HAP as catalyst displayed
Au-alkoxide species form with the transfer of a
α
-H (ꢀ CH
2
-) from the
◦
high performance at 60 C and after 3 h the yield and selectivity of imine
alcoholate to Au atoms. This is generally regarded as the initial step
along alcohol oxidation over Au surfaces [2,34]. The role of oxygen in
the reaction medium in this stage is to remove the hydrogen from cor-
ners/edges/steps of the Au NPs, leading to the catalytic cycle. After that,
by removing a hydrogen from the Au-alkoxide, the desired benzalde-
hyde product is produced (Step 1).
was 99 %, but in this reaction substrat/catalyst molar ratio was 100. In
this base-free system, a delicate cooperation between metallic gold and
the acid/base sites of the hydroxyapatite (HAP) surface was play a key
role in determining the efficiency and compatibility of the Au/HAP
catalyst for not only the rate-determining alcohol oxidation but also for
the subsequent condensation step of the tandem reaction (Table 4, entry
After the benzaldehyde formed, the reaction can proceed in two
possible pathways: (i) the produced benzaldehyde with a water mole-
cule to form gem-diol, which can be oxidized into carboxylic acid in-
termediate or (ii) the produced benzaldehyde with amine molecule to
form instable carbinolamine intermediate. We predicted that the reac-
tion pathway proceeds through an unstable carbinolamine intermediate
because carboxylic acid was not observed during the reaction in our
system. MIL-101 has a high density of open chromium centres, which
can act as Lewis acid sites for catalysis and along possible pathway, the
9
) [60].
On the other hand, the catalytic activity of Au/TiO
2
in oxidative
coupling of amines and alcohols has been studied by several groups in
the absence and/or presence base (Table 4, entries 11–15) [67,77,78].
According to these studies, product selectivity (imine, amine and others)
depends on significantly to reaction conditions such as temperature,
base and oxygen (Table 4, entries 4–7). In the examples reported by Sun
et al. and Ishida e. al., when the reactions carried out without base at 60
◦
–
and 120 C under O
2
atmosphere, the yield of imine was 88 (after 36 h)
C
–
O group of the benzaldehyde coordinates with this Lewis acid sites,
–
–
and 45 % (after 22 h), respectively. Azobenzene was formed as
by-product due to the direct aerobic oxidation of aniline in the presence
of excess oxygen (Table 4, entries 4 and 7). Ishida et al. also reported
which increases the electrophilicity of the C O group [84]. Probably
this nucleophilic attack effectively prevents the formation of
over-oxidized by-products. Then, as a result of the nucleophilic addition
–
reaction to the C O group of the unreacted amine, an carbinolamine is
that when the reactions performed with a base under 0.1 MPa N
2
–
pressure, the imine selectivity was low (secondary amine, ester, amide
were formed as by-products). The results of He et al. were interesting for
formed. The formed unstable carbinolamine undergoes dehyration by
acid-catalyzed a pathway to form the corresponding imine (Step 2).
Consequently, Au/MIL-101 catalyst is appropriate material for the one
pot tandem synthesis of imines because Au NPs offer oxidation activity
while MIL-101 provides Lewis acid sites during the condensation step.
this tandem reaction. According to the report, in a base-free system
◦
under 5 atm N
2
at 120 C, the main product was secondary amine, while
imine was formed only as a byproduct. The adding of base to the reac-
tion lead to formation of the imine product (after 14 h, ca. 50 %) [78].
There is a very limited number of MOF-based catalysts for the one-
pot imine synthesis from alcohols and amines (Table 2, entries
4
. Conclusions
1
3
8–20). Among all the ever-reported MOF-based catalysts, the prepared
In conclusion, an efficient and stable bifunctional Au/MIL-101
ꢀ 1
.0 %-Au/MIL-101 catalyst exhibited the highest TOF (51.47 h ) for
catalyst was successfully developed by using the liquid phase impreg-
nation method which showed excellent catalytic performance when
applied in the one-pot tandem synthesis of various imines. The effects of
various factors on the catalytic activity such as reaction atmosphere,
solvent, temperature and base have been also investigated. We found
that amount of oxygen and base are necessary for the high selectivity to
benzaldehyde of benzyl alcohol which is the rate-determining step for
one-pot synthesis of N-benzylidenaniline from benzyl alcohol and
aniline.
3
.3. Mechanism study of the reactions
The above experimental findings suggest that the catalytic pathway
Fig. 7. Possible reaction pathway for the one-pot imine synthesis (yellow colored balls represent Au atoms) (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article).
9