Organic Letters
Letter
mL), 0 °C with vigorous stirring. The results are listed in
Scheme 3. Gratifyingly, excellent product yields can be
a
Scheme 3. Scope of Alcohol Oxidation Using SAZADO
Figure 1. Recyclability of the SAZADO catalyst.
reused for the next cycle without further reactivation in 10
subsequent runs. The recovered catalyst was used repetitively
for 5 cycles with no loss of catalytic activities, proving the good
durability and stability of the SAZADO catalyst, which is a
sufficiently recyclable heterogeneous catalyst, and demonstrat-
ing the further advantage of this type of supported 1-Me-
AZADO. To further explore the cycle stability of the catalyst,
the control experiments with lower conversion were
performed. The results revealed that the SAZADO catalyst
conversion rate after six cycles (Figure S1). Following the
seventh run, the catalytic activity of SAZADO started to
display an obvious decrease. In the last cycle, the catalyst of
analyzed by elemental analysis (Table S2). The result
showcased that the content of nitrogen decreased obviously
in comparison to fresh SAZADO, probably because of the
instability of SAZADO under alkaline conditions, in which the
gradual hydrolysis of ester occurred. Obviously, the stability of
SAZADO is ascribed to the strong covalent bonding of the
nitroxyl radical unit inside the mesopores of SBA-15, instead of
simple physical adsorption with the surface of the mesoporous
silica, which can significantly reduce the leaching of the active
units from the solid into the solution phase during the catalysis
and recycling stages.
a
Conditions: 12 (0.2 mmol), KBr (2.4 mg, 0.02 mmol), SAZADO
(
20 mg), DCM (0.5 mL), aq. NaHCO (0.71 mL), NaClO (1.7
3
equiv), 10 min, 0 °C. Isolated yields and GC conversion rates (the
results in parentheses) are shown. NaClO (3.4 equiv). TBAB (0.75
b
c
d
e
equiv). Time: 20 min. A 1 mmol-scale synthesis: 114 mg of product
1
3g delivered.
obtained in the oxidation of alcohols with 27 examples; almost
no limitations were identified, and more importantly, no
overoxidized carboxylic acids were also observed during the
reactions. For further details, various functional groups on the
benzene rings of benzyl alcohol, such as bromo, chloro, nitro,
cyano, methyl, and methoxy, could be efficiently oxidized to
the corresponding aldehydes or ketones with 85−98% yields
within only 10 to 20 min. At the same time, we have also found
that the electronic properties of the aryl ring had an obvious
influence on the oxidation reaction. Compared to electron-rich
groups (i.e., −Me, −OMe), benzyl alcohol bearing electron-
In conclusion, the SBA-15 supported 1-Me-AZADO nitroxyl
radical, as a highly efficient heterogeneous catalyst in the
selective oxidation of alcohol, has been designed and
synthesized, which is the first example of the immobilization
of the AZADO derivative. Novel and useful features of this
approach included (1) the successful synthesis and the
immobilization of 1-Me-AZADO on mesoporous silica SBA-
poor groups (i.e., −Cl, −Br, −CN, and −NO ) afforded the
2
carbonyl compounds in relatively high yields. When (4-
methoxyphenyl)methanol was employed, the reaction afforded
corresponding aldehyde with 90% yield by increasing the
employed amount of NaClO to 3.4 equiv and prolonging the
reaction time to 20 min. Excellent yields were also obtained
with other activated heteroaromatic alcohols, including
pyridine and furan with an aldehyde group The system was
also applicable to sensitive groups such as double bonds and
triple bonds (terminal alkynes), affording the corresponding
carbonyl compounds with 95% and 90% yield, respectively.
Additionally, the catalytic system proved to be very effective
for oxidation of aliphatic primary and secondary alcohols, and
even sterically hindered alcohols. Among them, the alcohols
1
5, (2) extremely high catalytic efficiency for selective
oxidation of various primary, secondary, and even sterically
hindered alcohols for short reaction times (less than 20 min),
(
3) convenient separation and reutilization for at least six times
through a simple centrifugation manipulation, and (4) low-cost
and easily available support. Such strategy should be highly
useful for the synthesis of carbonyl compounds with various
functional groups. Importantly, the development and sustain-
able processes for the oxidation of various alcohols will enrich
the repertoire of valuable reagents and methodologies in
organic chemistry.
1
1
2x and 12z were respectively oxidized into target products
3x and 13z with 90% and 97% yields by addition of 0.75
equiv of tetrabutylammonium bromide (TBAB).
ASSOCIATED CONTENT
sı Supporting Information
■
Subsequently, the recyclability of our catalyst was examined
by employing benzyl alcohol as the substrate under Anelli’s
conditions (Figure 1). After the oxidation was completed, the
SAZADO was easily recovered by centrifugation and then
*
3
930
Org. Lett. 2021, 23, 3928−3932