Sn(IV) phosphonates as catalysts in solvent-free Baeyer-Villiger oxidations using H2O2

Article abstract of DOI:10.1039/b807938b

We have designed a new family of layered Sn(iv)phosphonate (SnPP) materials which are very efficient catalysts in the BV oxidation of aromatic aldehydes without any solvent and using aqueous H₂O₂ (30%) as the oxidant. The Royal Society of Chemistry.

Full text of DOI:10.1039/b807938b

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Sn(IV) phosphonates as catalysts in solvent-free Baeyer–Villiger
oxidations using H2O2w
a
Sharath Kirumakki,a Sandani Samarajeewa,z Robert Harwell,wa
Atashi Mukherjee,wa Rolfe H. Herberb and Abraham Clearfield*a
Received (in Austin, TX, USA) 9th May 2008, Accepted 4th August 2008
First published as an Advance Article on the web 8th October 2008
DOI: 10.1039/b807938b
We have designed a new family of layered Sn(IV)phosphonate
(SnPP) materials which are very efficient catalysts in the BV
oxidation of aromatic aldehydes without any solvent and using
aqueous H2O2 (30%) as the oxidant.
solvent (Scheme 1). The electron donating substituents on the
aromatic ring are known to favor the formation of the ester
which forms the corresponding phenol on hydrolysis.10 The BV
reaction over 4-methoxybenzaldehyde (anisaldehyde) carried out
in the presence of different solvents using Sn(IV)phenylphospho-
nate (SnPP) as the catalyst showed that as the polarity of the
solvent decreased the activity of the catalyst increased (Fig. 1).
No benzoic acid was detected in the reaction mixture.
The Baeyer–Villiger (BV) oxidation is the oxidative cleavage of a
carbon–carbon bond adjacent to a carbonyl, which converts
ketones to esters and aldehydes to phenols. The conventional
BV oxidation involves the use of an organic peracid. This is one
of the main drawbacks of this procedure as the organic peracid is
expensive, hazardous and ecologically unattractive as it produces
carboxylic acid salts as the byproduct.1 The use of alternative
pathways such as the in situ generation of the organic peracids by
the action of oxygen with sacrificial aldehydes, or the use of H2O2
with a catalyst, has been widely researched. The use of hydrogen
peroxide is a ‘‘green’’ process as water is the byproduct of this
oxidation. The main challenges to this procedure are the compat-
ibility of the solvent with aqueous H2O2 and finding the right
catalyst which would activate the peroxide in the presence of
water. There are a number of reports on the oxidation of cyclic
ketones2–5 using hydrogen peroxide but only a few on aromatic
aldehydes mainly because of the poor reactivity of the aroma-
tic aldehydes. Homogenous catalysts such as SeO2 and arylsele-
ninic acids have been found to be efficient in the oxidation of
aromatic aldehydes using aqueous hydrogen peroxide.6 Corma
et al.7–10 have designed Sn-beta zeolite and Sn-MCM-41 hetero-
geneous catalysts for the reaction of aromatic aldehydes with
hydrogen peroxide in non halogenated solvents such as dimethyl-
formamide and dioxane. They suggest that the catalyst activates
the carbonyl compound and not the hydrogen peroxide and this
makes these catalysts more selective. We have developed a family
of porous Sn(IV)phenylphosphonates that are layered materials,
possess high surface area and with pore diameters in the 10–20 A
range.11,12 We have used these materials as catalysts in the BV
reaction and found them to be extremely active for the oxidation
of aromatic aldehydes using 30% aqueous H2O2 solution. The
most fascinating part of these catalysts is that the reaction
proceeds more efficiently in the absence of any organic solvents.
During the BV oxidation of an aromatic aldehyde various
products can be formed depending on the catalyst and the
This trend is opposite to what was observed over Sn-beta
zeolites.10 Amongst the different solvents used the conversion
was the best for the least polar solvent toluene, which gave the
aldehyde conversion of 42% and a selectivity to the phenol of
35%. We observed that in the absence of a solvent the conver-
sion went up to 54% and the selectivity to the phenol increased
to 68%. The hydrolysis of the ester to phenol is facile in the
absence of a solvent. This result is extremely important in the
BV oxidation as it eliminates the need to find a compatible
solvent and also advances the cause of the environment as it
eliminates the need for a solvent. We postulate that the SnPP
materials being inorganic–organic hybrids are good hosts to
both the organic reagents and H2O2, thus making them very
active catalysts in this reaction. The effect of the catalyst
characteristics on the reaction was studied by using a series of
catalysts synthesized using various phosphonic acids and
solvents. Sn(C6H5PO3)2 was synthesized using various solvent
systems and it was found that the catalyst formed in H2O-
DMSO(SnPP-D) or pure H2O (SnPP-F) was much more
reactive than the one synthesized in a H2O–EtOH (SnPP-E)
mixture. 4-methoxybenzaldehyde conversions of 68–70% were
observed when the catalyst prepared in H2O or H2O-DMSO
was used in the BV reaction whereas a conversion of 48% was
observed when the catalyst prepared in H2O–EtOH was used
(Table 1). The selectivity for the phenol was around 70% in all
a Chemistry Department, Texas A&M University, College Station,
77842 Texas, USA. E-mail: clearfield@mail.chem.tamu.edu
b Condensed Matter Physics Group, Racah Institute of Physics, The
Hebrew University, 91904 Jerusalem, Israel
w Electronic supplementary information (ESI) available: Experimental
procedure, X-ray patterns, Mossbauer spectra, BET isotherms. See
DOI: 10.1039/b807938b
z Undergraduate student.
Scheme 1 Baeyer–Villiger oxidation of an aromatic aldehyde.
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Fig. 2 SEM images of Sn(C6H5PO3)2: Scale for (A) 200 nm; (B)
1.50 mm.
Fig. 1 Effect of solvent on the BV reaction of anisaldehyde over SnPP
catalyst; Reaction conditions: 60 1C, 180 min, 3.7 mmol of aldehyde
and 4.5 mmol of 30% H2O2; 0.025 g catalyst (SnPP-F); 3 ml solvent.
cases, the selectivity is not a function of the catalyst but that of
the solvent. The surface area of catalyst prepared from H2O–
EtOH mixture was 233 m2 gꢁ1 whereas the ones made from
H2O or H2O-DMSO was far lower than this (B50 m2 gꢁ1).
The X-ray patterns of these samples show that they have the
same interlayer spacing of B15 A, however the crystallinity of
the sample prepared with EtOH is considerably lower than the
other two. The SEM images of these samples reveal that in the
presence of EtOH the Sn phosphonates tend to form nano-
particles which aggregate to form porous globules, whereas in
the presence of only H2O or DMSO the particles are much
bigger and better defined (Fig. 2A, 4B).
Fig. 3 Mossbauer spectra of a Sn(IV) phosphonate compound.
119Sn Mossbauer spectroscopic studies on these samples
¨
show that all of them give essentially the same isomeric shift
(IS) at 294 K: ꢁ0.318 + 0.009 mm sꢁ1 (Fig. 3).
The quadrupole splitting (QS) is very small and nearly
temperature independent. This suggests that the phenyl phos-
phonate ligands do not significantly influence the electronic
environment around the metal atom. The line widths (fwhm)
are on the order of B1.01 + 0.02 mm sꢁ1, indicating a small
range of geometries of the tin atom nearest neighbor ligands.
Temperature dependent Mossbauer studies on these samples
indicate that the tin atom in the SnPP-E lattice is slightly more
tightly bound, giving rise to a slightly higher lattice temperature
Fig. 4 SEM images of crosslinked Sn phosphonates.
than in SnPP-F. The temperature dependence of the isomer shift
is essentially identical for SnPP-E and SnPP-F. This might
hinder the ability of this catalyst to activate the aldehyde leading
Table 1 BV oxidation of 4-methoxybenzaldehyde over various SnPP
catalystsa
to a decreased conversion. The Mossbauer lattice temperatures,
¨
Inter-
layer
YM,13 are B128 K for SnPP-F and B143 K for SnPP-E.
We have prepared a series of catalysts in H2O–DMSO with
various phosphonic acids in between the layers. The interlayer
spacing, surface area and pore sizes of these materials are a
function of these phosphonic acids (Table 1). The Mossbauer
studies on these samples suggest that the Sn is in a nearly
octahedral environment in all cases and that the environment
around Sn(IV) is nearly the same. The 119Sn NMR spectra do
not change with the phosphonic acids and therefore the only
effect the phosponic acid has is to alter pore architecture and the
surface area of the Sn phosphonate. For pristine SnO2 sample,
an isotropic peak around ꢁ604 ppm flanked by sidebands is
observed9 which is characteristic of the octahedral coordination
of Sn4+. The NMR spectra of Sn phosphonates show a peak at
ꢁ821.5 ppm which suggests that the environment around the
Sn(IV) is considerably different from that of SnO2.12 SnO2 is not
an active catalyst for this reaction. The SEM images of these
Surface Con- Selectivity
version/phenol
Sample
No.
spacing Sn area/
wt%m2 gꢁ1
24.5 385
Catalyst
(d)/A
%
(%)
SnPP-A Sn(IV)(O3PC6H4- 13.4
C6H4PO3);
H2O–DMSO
SnPP-B Sn(IV) (O3PC6H5- 9.4
PO3);H2O–DMSO
SnPP-C Sn(IV)(C6H5PO3) 15.4
(HPO3);
H2O–DMSO
SnPP-D Sn(IV)(C6H5PO3)2; 15.4
H2O
SnPP-E Sn(IV)(C6H5PO3)2; 15.4
H2O–EtOH
88
100
30 399
28 285
32
83
70
75
27
49
70
42
68
70
69
67
26.5 233
25.8 48
SnPP-F Sn(IV)(C6H5PO3)2; 15.4
H2O–DMSO
a
Reaction conditions: 0.025 g catalyst, 60 1C, 180 min, 3.7 mmol of
aldehyde and 4.5 mmol of 30% H2O2.
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Table 2 BV oxidation of different 4-susbstituted benzaldehydes
Selectivity
Table 3 Comparison of catalytic activity of different Sn catalysts
with 4-methoxybenzaldehyde
Aldehyde
Conversion/%
phenol (%)
Sn
wt%
Conv/ Sel-phenol/
%
Catalyst
%
Comments
p-Ethylbenzaldehyde
p-Tolualdehyde
p-Anisaldehyde
46
52
58
55
89
100
100
100
Beta Zeolite
SnO2
B2
56
46
Acetonitrile 50%
H2O2 (ref. 10)
p-Ethoxybenzaldehyde
Sn Beta Zeolite B2
58
86
Toluene 35% H2O2
(ref. 9)
No solvent 30% H2O2
Reaction conditions: 0.025 g catalyst (SnPP-A) 80 1C, 60 min,
3.7 mmol of aldehyde and 4.5 mmol of 30% H2O2.
SnPP-A*
24.5 98
100
Reaction conditions: 0.05 g catalyst (* 0.025 g catalyst), 80 1C, 7 h,
3.7 mmol of aldehyde and 4.5 mmol of H2O2.
SnPP catalysts reveal that when the layers are cross-linked the
morphology is different from the ones which are not cross-
linked (Fig. 2 and 4). The cross-linked materials tend to form a
larger continuous particle by the stacking of several layers and
the uncross-linked ones tend to form smaller particles, which
often aggregated into spheres.11,12 The formation of pores and
consequently high surface areas in the cross-linked materials
can be explained analogous to the cross-linked Zr materials
where the pores develop by a coming together of layers of
unequal size during particle growth.14
The BV oxidation of cyclohexanone using 0.025 g of Sn
phosphonate (Sn-PPA) was carried out at 60 1C for 6 h. When
the cyclohexanone to 30% H2O2 ratio was 1 : 1.25 the conversion
of cyclohexanone was 68% with a caprolactone selectivity of
95%. When the same reaction was carried out at 90 1C for 20 h
and with a cyclohexanone to 30% H2O2 ratio of 1 : 3 the
conversion of cyclohexanone was 94% and the major product
was adipic acid with a selectivity of 85%. These results show that
the Sn phosphonates can be applied in the BV oxidation of a wide
range of substrates from aromatic aldehydes to cyclic ketones.
In conclusion, we have developed a new catalytic system
whose surface area and pore size can be tailored and tuned
over a wide range and these materials can catalyze the BV
reaction under facile solvent-less condition to give conversion
and selectivity levels higher than those reported in the litera-
ture so far. We also believe that chiral centres can be intro-
duced in the phosphonic acid moiety which would make this
catalyst an attractive chiral catalyst for the synthesis of asym-
metric lactones from racemic ketones.
We have used a series of SnPP catalysts to carry out BV
oxidation on 4-methoxybenzaldehyde (Table 1). It was found that
the conversion of the aldehyde was greatly influenced by the
catalyst employed. When Sn(IV)(O3PC6H4PO3) (SnPP-B) was
used as the catalyst the conversion was only 32%. The conversion
increased to 68% when Sn(IV)(C6H5PO3)2 (SnPP-D) was used
and a maximum conversion of 88% was observed over
Sn(IV)(O3PC6H4–C6H4PO3) (SnPP-A). There are two main
factors determining the efficiency of the catalyst; first is the
compatibility of the H2O2 with the catalyst and second the
accessibilty of the active centres to the aldehydes. These catalysts
being inorganic–organic hybrid materials are not very hydro-
phobic and therefore compatible with H2O2. The degree of
accessibilty of the aldehyde to the active centers therefore decides
the conversion levels in the reaction. When SnPP-F is used, the
conversion of anisaldehyde is around 68% and when we introduce
spacers in between the benzene ring in the form of phosphorous
acid groups, the surface area goes up from 48 to 285 m2 gꢁ1 and
the conversion also increases to 83%. SnPP-B has a more closely
packed structure than the other catalysts leading to inaccessible
active sites. We have used SnPP-A in BV oxidation reactions over
various substituted benzaldehyde substrates (Table 2).
This study was supported by the National Science Founda-
tion (NSF) through Grant DMR-0332453 and the Robert A.
Welch Foundation Grant 0673A, for which grateful acknow-
ledgment is made. Use of the TAMU Microscopy & Imaging
Center (MIC) facility is acknowledged.
Notes and references
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The conversion level follows the electron donating ability of
the p-substituents; and the order of reactivity is methoxy 4
ethoxy 4 methyl 4 ethyl. The only product formed in all these
cases was the phenol except when 4-ethylbenzaldehyde was used.
In this case a small amount (B10%) of the ester was also formed.
The catalyst SnPP-A was reused in the BV oxidation and it
was found that there was no perceivable change in the activity
of the catalyst. The X-ray pattern and Mossbauer spectra of
this material are virtually unchanged indicating that there is no
physico-chemical change to the material and there can be no
leaching of the Sn(IV) because of the nature of the octahedral
bonds holding the Sn within the layer. These catalysts show
much higher catalytic activity compared to other hetero-
geneous catalysts reported in the literature (Table 3), although
we must note that the amount of Sn contained in our system is
much higher than in the other Sn systems.
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5558 | Chem. Commun., 2008, 5556–5558