M. Al-Hashimi et al. / Tetrahedron Letters 46 (2005) 4365–4368
4367
silsesquioxane–silica matrix, we refer to them as
EPBPSS. In this work we have used ceriumEPBPSS3
materials (where 3 refers to the Q/Ta + Tb ratio).
formed rapidly and selectively in substrates having a
primary alcohol or ester group. We previously showed
that primary alcohols are oxidised at a slow rate if the
reaction mixture is stirred at the reflux temperature.13
Tetrahydrothiophene gave excellent selectivity for the
sulfoxide with NaBrO3 as reoxidant but gave only sul-
fone with TBHP. Confirmation that the Ce(IV)/Ce(III)
couple is important in these reactions was obtained by
demonstrating that the oxidation worked equally well
starting with Ce(III)EPBPSS3. For all substrates tested
with the Ce(IV)EPBPSS3–NaBrO3 system, a good
degree of selectivity for the sulfoxide could be obtained
after some optimisation of conditions. The heteroge-
neous system competed well with homogeneous CAN–
NaBrO3 for reactive sulfides like thioanisole. The less
reactive diphenyl sulfide however was converted to sulf-
oxide after 144 h, using the heterogeneous system at
80 °C, compared to 3 h using the homogeneous CAN–
NaBrO3 system at room temperature. Overall, the selec-
tivities observed for heterogeneous oxidation of sulfides
using the Ce(IV)EPBPS–NaBrO3 system compare extre-
mely well with reported selectivities for other heteroge-
neous catalysts and indicate that this system is more
selective than the corresponding Ce(IV)EPBPSS3–
TBHB system.
O
O
O
O
O
O
Si
O
O
Si
P
Si
P
Si
O
O
O
O
EPS
HO
HO
HO
HO
EPBPSS
P
HO
O
O
O
HO
We have already reported on a range of metalEPS
and metalEPBPSS derivatives, including cobalt(II),
cerium(IV) and vanadyl derivatives and on their catalytic
oxidation chemistry of allylic groups, primary and sec-
ondary alcohols and allylic alcohols.11–15 In this letter
we report our initial work on the catalytic oxidation of
a variety of sulfides utilising CeEPBPSS3 as catalyst
and sodium bromate as oxidant. Similar reactions utili-
sing TBHP as oxidant were carried out in a few cases for
comparison.
2. Results
In a future paper we will report on further oxidation
chemistry using other active redox metalEPBPSS3 cata-
lysts and a variety of co-oxidants.
We reported earlier on the oxidation of alcohols with
the related CeEPS–sodium bromate system where effec-
tive heterogeneous oxidation of alcohols was achieved.
We now report on the oxidation of a range of sulfide
substrates using the related system CeEPBPSS3–
NaBrO3. The results are shown in Table 1. As far as
we are aware no cerium(IV) heterogeneous reagents
have been reported for the oxidation of sulfides
although the homogeneous system, CAN–NaBrO4
(where CAN is cerium ammonium nitrate) has been re-
ported as a reagent for the oxidation of alcohols.16 CeE-
PBPSS3, was prepared by treating an aqueous
suspension of the disodium phosphonate material,
Na2EPBPSS3,11 with an aqueous solution of cerium
ammonium nitrate. The yellow solid obtained was fil-
tered, washed with copious amounts of water and then
with ether and finally dried at 100 °C at 0.01 Torr.
References and notes
1. Sato, K.; Hyodo, M.; Aoki, M.; Zheng, X. Q.; Noyori, R.
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Chem. 1967, 32, 3865–3868; Trahanovsky, W. S.; Young,
B. L. J. Org. Chem. 1967, 32, 2349–2350.
3. Discussion
In all cases except one, the reactions resulted in 100%
conversion of sulfide. Typical procedures are given.18,19
For thioanisole and the Ce(IV)EPBPSS3–NaBrO3
system, increasing the reaction temperature from 30 to
50 °C using 0.8 mmol of the reoxidant,20 resulted in a
quicker reaction and a 2% improvement in selectivity
for sulfoxide. Increasing the oxidant from 0.8 to
1.5mmol had no effect at 30 °C and a 3% reduction in
selectivity at 50 °C. Selectivity for the sulfoxide would
appear to be slightly higher for reactions employing
0.8 mmol of the reoxidant NaBrO3 compared to those
employing 1.5mmol of the reoxidant. Oxidation of thio-
anisole with Ce(IV)EPBPSS3 and TBHP as reoxidant
resulted in good selectivity but considerably slower reac-
tions than those utilising NaBrO3. The sulfoxide is