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Journal of the Iranian Chemical Society
The form of the silica (often a gel or diatomaceous earth),
amount of OPA, and preparation method vary to meet the
different acid strength requirements of the individual reac-
tions, but an extensive period of heat treatment, generally
in a calcination kiln at temperatures of 120 °C and above,
tends to be a common step within each process. Condensa-
tion occurs at high temperature during calcination to give
surface bridging phosphate (Si–O–P=O), diphosphorus pen-
taoxide (P2O5), and pyrophosphate groups accompanied by
SiO2 is dried between 100 and 200 °C, a large portion of
OPA is uncombined, i.e., no phosphate bond is created. A
few reports deal with the preparation and characterization
of non-calcined OPA catalysts. As a part of our program
toward the development of environmentally benign proto-
cols, herein, we report such a silica-supported OPA catalyst
for pinacol-pinacolone rearrangement. We have successfully
characterized this silica-supported catalyst with the help of
31P NMR, TGA, DSC, FT-IR, XRD, and titration.
resulting homogenized, white free flowing powder is stored
tightly capped, studied, and used as obtained without any
more drying or calcination.
General procedure for pinacolic rearrangement
In a typical procedure, pinacol 1a (1 mmol) is simply
adsorbed on 2 eq (600 mg) of catalyst and heated without
stirring in an oil bath. The crude product is then extracted by
dichloromethane (2*10 mL) and evaporated under vacuum.
The crude mixture is then purified by flash chromatography
by heptane/AcOEt if needed. The obtained products are then
analysed and identified by mass spectrometry, 1H, and 13
NMR (see ESI).
C
Results and discussion
Structure of non‑calcined OPA/SiO2 catalyst
Different analytical techniques have been used to character-
ize the structures of surface species of OPA catalyst. The
surface properties of amorphous silica, which is considered
ence of silanol functionalities [38]. The OH groups act as
the centers of molecular adsorption during their specific
interaction with adsorbates capable of forming a hydrogen
bond with the OH groups, or, more generally, of undergoing
donor–acceptor interaction [38].
Experimental
Materials and instruments
All commercially available products and solvents were used
without further purification. Reactions were monitored by
TLC (Kieselgel 60F254 aluminum sheet) with detection by
UV light or potassium permanganate acidic solution. Col-
umn chromatography was performed on silica gel 40–60 µm.
Flash column chromatography was performed on an auto-
Surface OH groups are subdivided as follows (Fig. 1):
(1) isolated free (single silanols), SiOH; (2) geminal free
(geminal silanols or silanediols), –Si(OH)2; (3) vicinal, or
bridged, or OH groups bond through the hydrogen bond
(H-bonded single silanols, H-bonded geminals, and their
H-bonded combinations). On the SiO2 surface there also
exist surface siloxane groups or Si–O–Si bridges with oxy-
gen atoms. Moreover, there is structurally bond water inside
the silica skeleton leading to internal silanol groups. When
OPA is adsorbed on silica, different bonds can be observed
such as hydrogen bonding with surface or core silanols (IV)
or creation of bridged structures such as I, II, or III (Fig. 1).
Figure 2 illustrates XRD spectra of OPA/SiO2. The struc-
ture of the catalyst is greatly affected by OPA loading. A
broad peak attributed to SiO2 is seen around 2θ=22°. The
ill-defined shape of the peak proves that silica is amorphous.
The pattern for the OPA/SiO2 shows diffraction peaks that
indicate the presence of some crystalline phases, although
the presence of an amorphous phase is also evident. The
XRD spectra of OPA/SiO2 are different from those reported
by Xie et al. for SiO2/P2O5 but shares similarities with those
reported by Takezawa et al. for calcined OPA/SiO2 [39, 40].
The XRD spectra of the catalyst after three runs of reaction
did not show any significant variation.
matic apparatus, using silica gel cartridges. 1H, 31P, and 13
C
NMR spectra were recorded on a 400 MHz/54 mm ultra-
long hold. Chemical shifts (δ) are quoted in parts per million
(ppm) and are referenced to TMS as an internal standard.
Coupling constants (J) are quoted in hertz. FT-IR spec-
tra of the silica support, free OPA, and catalyst have been
recorded using a Varian 600-IR Series spectrometer. The
samples were scanned within a range of 400–4000 cm−1.
The thermal stability of OPA/SiO2 was investigated by ther-
mogravimetric analysis (TGA) using a PerkinElmer TGA
4000 apparatus and differential scanning calorimetry (DSC)
using a PerkinElmer DSC 6000 apparatus coupled with an
intracooler.
Preparation of the catalyst OPA/SiO2 by wet
impregnation
33 wt.% OPA/SiO2 was prepared by adding liquid 75% OPA
on a suspension of silica gel (70–200 mesh) in Et2O. The
suspension is stirred at room temperature during 1 h, and
then, the solvent is removed under moderate vacuum. The
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