G. Zhu et al.
Molecular Catalysis 505 (2021) 111516
2. Experimental
catalyst activity of BPS-Cu@SiO2 and BPS-Ir@SiO2. The results disclosed
that BPS-Ir@SiO2 showed much higher catalytic activity (Table 1). After
a series of explorations, it was revealed that base was important for this
reaction and sodium hydroxide was best base. Next, the effect of solvents
was also explored and it was found that dioxane could produce 84 %
yield of desired product. However, CH2Cl2 and CH3OH couldn’t promote
this reaction. Further screening revealed toluene was the most effective
solvent. After a series of condition screenings, we were pleased to find
that the best result (95 % yield) was gained with BPS-Ir@SiO2, while
only ligand could not catalyze this transformation (entry 13). It should
be noted that the reaction of benzyl alcohol with tert-butanesulfinamide
couldn’t happen in water (Table 1, entry 18). Finally, we tested tem-
perature and reaction time under the condition of BPS-Ir@SiO2 and
optimal solvent. It was disclosed that the combination of 120 ◦C with
24 h was suitable conditions.
2.1. The synthesis of BPS-Cu@SiO2
The unsymmetrical benzotriazole-pyridinyl-silane (BPS) ligand (L)
was synthesized in three steps and purified with moderate yield.
(Scheme 2) [18]. With BPS in hand, the corresponding iridium and
copper catalysts were prepared as follows: To a mixture of ligand L
(504 mg, 1.1 mmol) and CuI (57 mg, 0.30 mmol) in a dried Schlenk tube
with a stirring bar, acetonitrile (15 mL) was added under N2 atmo-
sphere, the mixture was stirred at room temperature for 12 h until a
yellow sediment (BPS-Cu) appeared. After removing the solvent, the
sediment was washed with dichloromethane, water and ethanol for
three times. Subsequently, SiO2 (600 mg) was added to the Schlenk tube
with BPS-Cu (270 mg), DMSO as a solvent, the resulting mixture was
stirred vigorously at 140 ◦C for 48 h, BPS-Cu@SiO2 was obtained after
filtrating and washing with water and ethanol for four times. Mean-
while, BPS-Ir@SiO2 was also obtained according to the upper steps (see
supporting information for details).
Subsequently, the substrate expansion of this reaction of various
benzyl alcohols and tert-butanesulfinamides was next examined
(Table 2). It was revealed that the substrates with -F, -Br, -Cl on the aryl
group moieties could produce the corresponding N-benzyl-2-methyl-
propane-2-sulfinamides in high yields. Benzyl alcohols with ꢀ OCH3 or
-X substituents were converted into the desired products in good yields.
Interestingly, piperonyl alcohol and 2-naphthalenemethanol could also
react with 2-methylpropane-2-sulfinamide smoothly and generate the
products in moderate yields (4 l, 4 m).
3. Results and discussion
After the synthesis, BPS-Cu@SiO2 and BPS-Ir@SiO2 were both
carefully characterized through energy dispersive spectrometer (EDS),
X-ray photoelectron spectrometry (XPS) and transmission electron mi-
croscopy (TEM).
The selective synthesis of substituted ketones and unsaturated
carbonyl compounds from benzyl alcohols is a challenging issue, which
are both atom-efficient processes, only water (or water with hydrogen
gas) produced as byproducts. Considering the success of BPS-Ir@SiO2
catalyzed reaction of benzyl alcohol and tert-butanesulfinamide, we next
focused on the reaction of ketones and benzyl alcohols.
TEM images of Fig. 1b, c showed that copper composite was suc-
cessful supported on the carrier, and the Fig. 1d demonstrated that
iridium was successful supported on the SiO2 [19]. The structural
characteristics and morphologies of the catalysts (BPS-Cu@SiO2 and
BPS-Ir@SiO2) were observed clearly through the transmission electron
microscopy.
After a series of tests, we found BPS-Cu@SiO2 could generate 6a in
high yields with high selectivity, while BPS-Ir@SiO2 only gave a mixture
of phenylpropiophenone and chalcone. Therefore, BPS-Cu@SiO2 was
used to explore the reaction of benzyl alcohols and substituted ketones.
As shown in Table 3, a wide range of alcohol substrates with functional
groups were all tolerated, including -Cl, -F, -Br, ꢀ OCH3 and ꢀ CH3, the
corresponding unsaturated carbonyl compounds were achieved in good
to excellent yields. The substrates bearing ꢀ CH2CH3 and -Ph gained the
corresponding products in 80 %, 86 % yields, respectively. 2-Naphthale-
nemethanol was also suitable to this transformation, yielding 6z in 94 %
yield.
In addition, EDS studies were conducted to further examine these
composites, and the EDS pattern showed peaks at 2.3, 8.0 and 9.1 keV
were characteristic peaks of Cu, and in next EDS image, the peaks at 2.8,
9.3 and 11.3 keV belonged to Ir (Fig. 2a, b). The contents of copper and
iridium in the catalysts were obtained 8.56 % and 2.35 % mass fraction
from EDS. The chemical composition and surface chemical states of the
catalyst composite were shown via X-ray photoelectron spectroscopy
(XPS). The wide XPS spectra indicated the presence of C, N and Cu el-
ements (Fig. 2c) and it was clear that C, N and Ir elements were detected
from Fig. 2d, and the peaks at binding energies of 935 eV and 953 eV
belonged to Cu, and the peaks at 60 eV and 63.6 eV came from iridium.
All the characterizations provided a direct proof for the successful
preparation of BPS-Cu@SiO2 and BPS-Ir@SiO2 composites.
3.1. Mechanism exploration
The excellent catalytic activity of BPS-Cu@SiO2 and BPS-Ir@SiO2
system in these transformations stimulated us to investigate the possible
reason of these two BPS-based copper and iridium catalytic systems. We
noticed that BPS-metal was coupled with SiO2 through silane partner of
ligand, which was an important factor for this high catalytic activity. It
Compared to classical borrowing hydrogen reaction of amines with
alcohols, the transformation of tert-butanesulfinamide with alcohols is
much more difficult to realize. Therefore, the reaction of benzyl alcohol
and tert-butanesulfinamide was selected as a model reaction to test the
Scheme 1. The benzotriazole-pyridinyl-silane based copper and iridium catalysts.
2