708 J. CHEM. RESEARCH (S), 1998
J. Chem. Research (S),
1998, 708±709$
Catalytic Behavior of Niobium(V)±
Tetraphenylporphyrin in the Oxidation of
Cyclohexene with Hydrogen Peroxide Evaluated
1
by H NMR Spectroscopy$
Carlos Roland Kaiser,* Mona A. Abdel-Rehim,
Mauro Cesar S. Machado, Eduardo Lauande T. Souza and
Elizabeth Roditi Lachter
Instituto de Quõmica±DQO, Universidade Federal do Rio de Janeiro, Fundao, CT, Bloco A,
21949-900, Rio de Janeiro±RJ, Brazil
The catalytic efficiency of the niobium(V)±tetraphenylporphyrin complex in the oxidation reaction of cyclohexene with
1
aqueous hydrogen peroxide was evaluated using H NMR spectroscopy.
Oxidations promoted by synthetic porphyrin catalysts are of
relevance to previous studies of the chemistry of cytochrome
P-450 in living organisms.1 Due to their importance, these
biomimetic processes have received much attention in recent
years.2 In general the oxidant used is iodosylbenzene while
the use of other oxidants such as alkyl hydroperoxides,
Scheme 1
molecular oxygen and hydrogen peroxide has also been
investigated. The metal of the porphyrin ring can be iron,
also the substrate have at least one distinct signal between
d 3.0 and d 7.2 due to the more deshielded protons: for
example, cyclohexene (d 5.76), cyclohexene oxide (d 3.08),
cyclohex-2-en-1-ol (d 4.20), cyclohexene-1,2-diol (d 3.78) and
cyclohex-2-en-1-one (d 7.16). The strong solvent signal at
d 7.90 does not interfere and therefore the lock was made
by way of D2O sealed in a capillary. Thus, deuterium
solvent was not necessary. The conversion and selectivity
were measured by signal integration based on the substrate.
Calibration curves were not necessary although an internal
standard can be used. The results show that due to the low
oxidizing power of the hydrogen peroxide the reactions with
equimolar quantities and with or without catalyst are unpro-
ductive (entries 2 and 1). Better conversions are obtained
with an excess of the oxidant in the presence of the catalyst
(compare entries 3 and 4). Small amounts of organic acids
or bases and higher temperatures (entries 5 to 14) also led
to better conversions. Acetic acid gave the best results
although it has a major in¯uence on the transformation of
the epoxide 2 to the corresponding diol 3 (entries 9±12).
We had also noted a slow and progressive decomposition
manganese, chromium or osmium. Previous studies have
given the best results with iodosylbenzene or alkyl hydro-
peroxides and iron or manganese porphyrins.2 Hydrogen
peroxide gives good selectivities and conversions in the
epoxidation of several alkenes in the presence of molyb-
denum or tungsten tetraryl- and octaethyl-porphyrins.3
However, using molecular oxygen, visible light and tri-m-
oxo-niobium(V)±tetra-p-tolylporphyrin as catalyst the con-
versions are very low.4 Using tri-m-oxo-niobium(V)±tetraphe-
nylporphyrin [(NbTPP)2O3] for the oxidation of cyclohexene
by tert-butyl hydroperoxide led to total conversion in 12 h
with 30% selectivity for the epoxide.5 The present work pre-
sents the catalytic behaviour of the (NbTPP)2O3 complex in
the oxidation reaction of cyclohexene 1 with aqueous hydro-
gen peroxide. 1H NMR spectroscopy was used in place of
the usual gas chromatography to examine this reaction.
All reactions were carried out in biphasic media formed
from chloroform and aqueous H2O2 and taking an aliquot
of the organic phase for the analysis at times and conditions
as described in Table 1. The interesting point of these 1H
NMR analyses in CHCl3 is that the possible products and
Table 1 Oxidation of cyclohexene by hydrogen peroxide in the presence of (NbTPP)2O3
Entry
Catalytic systema
T/8C
t/h
Epoxide 2d (%)
Diol 3d (%)
Turnoverb
c
1
2
3
4
5
6
7
8
1eq H2O2
1eq H2O20.1eq py
40
40
40
40
40
55
55
55
40
55
55
55
55
55
24
24
24
24
24
24
48
72
24
24
48
72
24
48
<0.5
1.3
1.1
4.5
9.3
11.0
23.4
36.1
13.5
15.1
25.5
38.1
5.3
Ð
<1.0
2.6
2.3
<0.5
<0.5
<0.5
1.0
3.5
6.7
c
10eq H2O2
10eq H2O2
7.1
10eq H2O20.1eq py
10eq H2O20.1eq py
10eq H2O20.1eq py
10eq H2O20.1eq py
10eq H2O20.1eq AcOH
10eq H2O20.1eq AcOH
10eq H2O20.1eq AcOH
10eq H2O20.1eq AcOH
10eq H2O20.1eq EtOH
10eq H2O20.1eq EtOH
14.7
20.7
43.0
88.0
27.0
37.1
67.9
114.7
8.3
25.5
5.4
9
10
11
12
13
14
10.9
22.0
42.2
<0.5
2.9
12.2
21.6
apy pyridine, AcOH acetic acid, EtOH ethyl alcohol. bTurnover mmol products/mmol catalyst. cWithout catalyst. dScheme 1.
*To receive any correspondence.
of the catalyst in the presence of this acid (colour change
from dark red to pale yellow and the characteristic bands
disappear in the UV-VIS analysis).5 In neither of the
$This is a Short Paper as de®ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1998, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).