CoCl2 catalysed decarboxylation-oxidation of mandelic acids by molecular oxygen

Article abstract of DOI:10.1039/b307186c

A series of mandelic acid derivatives was oxidised by molecular oxygen using cobalt(II) chloride as the catalyst. Benzaldehyde and/or benzoic acid derivatives were obtained in high selectivities, depending on the aromatic ring substitution. Different oxidation mechanisms are operating, depending on the mandelic acid substitution.

Full text of DOI:10.1039/b307186c

View Article Online / Journal Homepage / Table of Contents for this issue
P A P E R
CoCl catalysed decarboxylation-oxidation of mandelic acids by
2
molecular oxygen
a
ab
c
d
Isabelle Favier, Elisabet Du n˜ ach,* Dominique H e´ brault and Jean-Roger Desmurs
a
Laboratoire Ar oˆ mes, Synth e` ses et Interactions, Universit e´ de Nice-Sophia Antipolis,
Parc Valrose, 06108, Nice cedex 2, France
b
c
d
Laboratoire Chimie Bioorganique (CNRS UMR 6001), Universit e´ de Nice-Sophia
Antipolis, Parc Valrose, 06108, Nice cedex 2, France
Rhodia Organique Fine, Centre de Recherches, BP 62, 85, Av. Fr e` res Perret, 69192,
Saint Fons cedex France
Rhodia Organique Fine, 190, Av. Thiers, 69457, Lyon cedex 06, France
Received (in Montpellier, France) 24th June 2003, Accepted 16th September 2003
First published as an Advance Article on the web 29th October 2003
A series of mandelic acid derivatives was oxidised by molecular oxygen using cobalt(II) chloride as the catalyst.
Benzaldehyde and/or benzoic acid derivatives were obtained in high selectivities, depending on the aromatic
ring substitution. Different oxidation mechanisms are operating, depending on the mandelic acid substitution.
Introduction
Cobalt complexes, and in particular cobalt(II) chloride, consti-
tute versatile catalysts in organic synthesis and have been
1
2
used, for example, in the monohydroxylation of b-keto esters,
3
for the synthesis of b-amino alcohols from oxiranes and for
the acylation of anisole.
4
In combination with molecular oxygen, the oxidation of
alkenes to epoxides with cobalt complexes has been described
5
–8
with Schiff base ligands, cryptates or porphyrins. Under O2 ,
the formation of cobalt(III) superoxo species, responsible for
the oxidation of the alkenes, has been proposed. The oxidative
2
cleavage of alkenes versus their epoxidation by a Co(II)/O sys-
tem has been studied for the cleavage of olefins into carbonyl
9
derivatives and for the oxidative cleavage of isoeugenol to
Scheme 1 Oxidation reaction of mandelic acid derivatives showing
the numbering scheme
1
0
vanillin. The oxidative cleavage of a-diols has also been
reported. The same system has also been applied for the oxi-
dation of alcohols to the corresponding carbonyl compounds,
1
1
7
ꢀ
obtained in DMSO at 125 C with 1 mol % catalyst under
1
for the oxidation of alkylbenzenes to carboxylic acids and the
2
2
atmospheric O pressure. It was observed that the oxidation
1
3
reaction could be optimised by operating under basic condi-
tions; the addition of 1.5 equiv. (versus substrate) of a 50%
aqueous solution of sodium hydroxide afforded the best yields,
as compared to the reaction in DMSO alone or with AcOH as
the additive. Both DMSO and NaOH seem to play an impor-
tant role in accelerating the oxidation. The results of the oxida-
oxidation of cyclic ethers to lactones.
The oxidation of mandelic acids has been examined under a
1
4
variety of systems. However, the Co-catalysed oxidative de-
carboxylation of mandelic acid derivatives has not yet been
reported. We present here our results on the use of the
2
CoCl /O
2
system for the oxidation of such substrates. More-
for the
oxidation of a-ketols, and epoxides, as well as for the oxida-
2
tion of several mandelic acid derivatives (1) under O (1 atm) in
over, we recently examined the use of Bi(0)/O
2
1
5
a cobalt(II) chloride catalysed reaction are presented in Table 1.
Unsubstituted mandelic acid (1a) reacted smoothly, with a
conversion of 42% after 24 h. No important evolution of the
system was observed with longer reaction times. A 89:11 mix-
ture of 2a and 3a was formed in 62% yield. The remaining
product was phenylglyoxylic acid (4a), obtained in 38% yield.
Interestingly, better conversions were obtained with substi-
tuted mandelic acids, either with electron-donating or with
electron-withdrawing substituents. Thus, the reactivity of
p-chloro-, p-fluoro- and p-trifluoromethylmandelic acids (1b–
1d) possessing electron-withdrawing substituents was exam-
ined; these substrates afforded conversions of 75–100% after
24 h. A clean reaction with a quantitative yield of the corres-
ponding benzoic acids (2b–2d) and benzaldehydes (3b–3d)
was obtained. However, the selectivity 2:3 was very dependent
on the nature of the substituents. Thus, for p-fluoro- and
1
6,17
tion of mandelic acids.
catalytic systems are presented.
Some comparisons between both
Results and discussion
Oxidation of mandelic acid derivatives catalysed by CoCl /O
2
2
The oxidation of several mandelic acid derivatives (1) was
examined under different reaction conditions, using CoCl as
2
the catalyst. As illustrated in Scheme 1, the oxidative cleavage
of mandelic acids mainly affords benzoic acid derivatives (2)
and/or benzaldehyde derivatives (3).
ꢀ
In a water medium at 80 C, the conversion of 1a was less
2 2
than 5% after 24 h and in H O–NaOH or H O–AcOH media
the conversion of 1a was less than 18%. The best results were
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
6
2
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 6 2 – 6 6

View Article Online
Table 1 Oxidation of mandelic acid derivatives by molecular oxygen,
ꢀ
catalysed by cobalt(II) chloride (1 mol %) in DMSO–NaOH at 125 C
a
Reaction % Conversion % Yield
of 2 þ 3
Substrate time/h
of 1
% Selectivity 2:3
1
1
1
1
1
1
1
a
b
c
d
e
f
24
24
24
24
42
75
62
100
100
100
57
89:11
29:71
82:18
83:17
1:99
98
Scheme 2 Mechanistic pathways for the oxidation of mandelic acids
100
97
0.5
8
2
62
100
88
85
16:84
1:99
2
catalysed by CoCl . These reactions were carried out for
g
unsubstituted 1a, for the mandelic acid derivative 1f with an
electron-rich aryl ring and for 1d, with an electron-poor aryl
moiety.
a
Yield of products calculated on converted substrate.
p-trifluoromethylmandelic acids (1c and 1d) the corresponding
benzoic acid derivatives 2c and 2d were formed as the
major compounds in 82–83% selectivities. In contrast, for
p-chloromandelic acid (1b) the main product was
p-chlorobenzaldehyde (3b) with a selectivity of 71%.
(a) Oxidation of mandelic acid, benzaldehyde and phenyl-
glyoxylic acid. With 1 mol % of CoCl
in DMSO–NaOH at
2
ꢀ
125 C, unsubstituted mandelic acid (1a) was 35% converted
after 2 h to yield benzoic acid (2a) and phenylglyoxylic acid
(4a) with a selectivity of 57:43. After 24 h, the conversion of
1a was of 42% and some benzaldehyde (3a) was present, with
a 2a:3a:4a selectivity of 55:7:38 (Scheme 3).
Both benzaldehyde (3a) and ketoacid (4a), possible inter-
mediates in the formation of benzoic acid (2a), were subjected
to oxidation under the same conditions. The oxidation of 3a
afforded a complete conversion after 2 h, with the formation
of 30% of 2a and 70% of a sulfoxide derivative (5a) issued from
the reaction of 3a and DMSO. However, compound 5a was
not isolated during the oxidation of 1a.
The reactivity was enhanced with electron-rich substituents
1e–1g). p-Hydroxymandelic acid (1e) was converted in 0.5 h
(
and vanillic acid (1g) in 2 h. Yields of 2 þ 3 of 57–88% was
accounted for by the presence of some polymeric material in
the cases of 1e and 1g and of an additional ketoacid derivative
(
4f, 12%), in the case of 1f. The selectivities 2:3 were here in
favour of the formation of the corresponding aldehydes 3e–
g, with 2:3 ratios up to 1:99 in the cases of 4-hydroxymandelic
3
acid (1e) and vanillic acid (1g). In the case of the p-methoxy
substituent in 1f, the selectivity towards the aldehyde 3f was
8
4%.
The oxidation of ketoacid 4a occurred at a slow rate and
gave a conversion of only 16% after 2 h, with the exclusive
formation of 2a. The conversion of 4a reached 37% after
The kinetics of the consumption of different mandelic acid
derivatives is presented in Fig. 1
2
4 h. The slow oxidation of 4a is in agreement with the fact
it was isolated in 38–43% selectivity from the oxidation of 1a.
Taking into consideration the data of Scheme 3 and the fact
that the sulfoxide derivative 5a was not formed during the oxi-
dation of 1a, the mechanism of formation of 2a from 1a by the
Mechanistic considerations
The reactivity and selectivity differences observed for the oxi-
dation of mandelic acid derivatives are probably due to differ-
ent mechanisms operating in the oxidative Co catalysed
system. The oxidation of mandelic acids to the corresponding
benzoic acids and/or benzaldehyde derivatives involves an oxi-
dative decarboxylation reaction, which can occur via paths
CoCl system can occur through the two different pathways
2
‘‘a’’ and ‘‘b’’ of Scheme 2. The main pathway ’’b’’ involves
a first oxidation of 1a to aldehyde 3a, followed by its further
and rapid oxidation into 2a. Surprisingly, in this oxidation
process from 1a compound 5a is not observed. Possibly the
‘
‘a’’ or ‘‘b’’ shown in Scheme 2. Thus, derivatives 1 can be
oxidised to the corresponding a-ketoacids 4 via path ‘‘a’’,
with further decarboxylation to aldehydes 3 or oxidative
decarboxylation to benzoic acids 2.
Alternatively, following path ‘‘b’’, the oxidative decarboxy-
lation of 1 can directly afford aldehydes 3, which may be
further oxidised to acids 2. In order to determine the main
reaction pathways for the differently substituted mandelic
acid derivatives, mechanistic studies were carried out on the
oxidation of substrates 1, as well as on the oxidation of the
corresponding benzaldehydes 3 and ketoacid derivatives 4,
Fig. 1 Kinetics of the oxidation of mandelic acid derivatives by the
ꢀ
Scheme 3 Oxidation of mandelic acid (1a), benzaldehyde (3a) and
phenylglyoxylic acid (4a) catalysed by CoCl
catalytic system CoCl
2
/O
2
in DMSO–NaOH at 125
C
2
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 6 2 – 6 6
63

View Article Online
rapid oxidation of the intermediate aldehyde species prevents
the formation of 5a.
In competitive pathway ‘‘a’’, phenylglyoxylic acid (4a) is
formed and accumulated and is only slowly oxidised into 2a.
This pathway ‘‘a’’ is to be considered as a minor contribution,
as illustrated in Scheme 4.
(b) Oxidation of 4-methoxymandelic acid, 4-anisaldehyde
and 4-methoxyphenylglyoxylic acid. The oxidation of 4-meth-
oxy substituted mandelic acid (1f) mainly afforded the corre-
sponding aldehyde derivative 3f with a conversion of 62%
after 8 h and a selectivity 2f:3f:4f of 14:74:12. As shown
in Fig. 2, the oxidation of 4-anisaldehyde (3f) was much faster
than that of 4-methoxymandelic acid (1f). Interestingly, the
oxidation of 4-methoxyphenylglyoxylic acid (4f) was very
slow.
Fig. 2 Oxidation of 4-methoxymandelic acid (1f), 4-anisaldehyde (3f)
and 4-methoxyphenylglyoxylic acid (4f) catalysed by CoCl
2
Table 2 presents the results obtained for the oxidations of 1f,
f and 4f catalysed by CoCl . As already observed for the oxi-
2
dation of 3a, the oxidation of 4-anisaldehyde (3f), which was
completely converted in 2 h, afforded mainly the sulfoxide deri-
vative 5f (analogous to 5a in Scheme 3) in 90% yield while
Table 2 Oxidation of 4-methoxymandelic acid (1f), 4-anisaldehyde
3f) and 4-methoxyphenylglyoxylic acid (4f) catalysed by CoCl
3
(
2
Substrate Time/h % Conversion Products % Product yield
4
-methoxybenzoic acid (2f) was only formed in 10% yield.
3
f
2
100
2f
10
90
The sulfoxide derivative 5f was not observed during the
oxidation of 1f.
5
f
4f
1f
24
8
20
62
2f
2f
3f
4f
100
14
2
Our data of Fig. 2 and Table 2 indicate that the CoCl cat-
alysed oxidation of 4-methoxymandelic acid (1f) should mainly
proceed through mechanistic pathway ‘‘b’’ in Scheme 2, invol-
ving the direct formation of 4-anisaldehyde (3f). The car-
boxylic acid 2f should be formed from the oxidation of 3f.
The ketoacid 4f is not a plausible intermediate in the oxidation
of 1f to 3f or to 2f.
74
12
Comparative studies between the oxidation of mandelic acid
and
derivatives under molecular oxygen catalysed by CoCl
by Bi(0)
2
The oxidative decarboxylation of substrates 1 has been
recently examined in a Bi(0) catalysed reaction in the presence
(
c) Oxidation of 4-trifluoromethylmandelic acid, 4-trifluoro-
methylbenzaldehyde and 4-trifluoromethylphenylglyoxylic acid.
In order to examine the relative reaction rates of a mandelic
acid derivative with electron-withdrawing substituents, the oxi-
dation of 4-trifluoromethylmandelic acid (1d) was compared to
that of 4-trifluoromethylbenzaldehyde (3d) and 4-trifluoro-
methylphenylglyoxylic acid (4d). As shown in Fig. 3, both
the aldehyde 3d and the ketoacid 4d reacted faster than 1d.
The results of the oxidation of 1d, 3d and 4d are summarised
in Table 3.
The oxidation of 4-trifluoromethylbenzaldehyde (3d) gave
0% of 4-trifluoromethylbenzoic acid (2d) after complete
conversion in 2 h. The oxidation of 4-trifluoromethylphenyl-
glyoxylic acid (4d) afforded exclusively the carboxylic acid 2d
with a conversion of 75% after 7 h. In the case of the oxidation
1
6
of molecular oxygen in DMSO–AcOH. As in the Co
catalysed oxidation, compounds 2 and 3 were obtained with
selectivities depending on the substitution on the aryl ring.
However, on comparing both catalytic systems, we observed
that the reactions do not always afford the same main product
for the same substituent. In order to better compare the CoCl
2
and the Bi(0) systems for the oxidation of 1, Table 4 sum-
marises the results obtained in the Bi(0) catalysed reactions,
including conversions, yields of 2 and 3 and selectivities.
The comparison of the data of Tables 1 and 4 indicates that
from the point of view of the reaction rate and conversion,
6
metallic bismuth is a slightly more efficient catalyst than CoCl
for the oxidation of 1a, 1f and 1g. Similar reactivities were
2
observed for the oxidation of 4-trifluoromethylmandelic acid
1d). For the remaining cases, CoCl showed a better catalytic
2
activity than Bi(0).
2
of 1d to 2d by CoCl occurring with 83% selectivity, both
mechanisms, either via path ‘‘a’’ or path ‘‘b’’ in Scheme 2
are compatible with the available data.
(
We can conclude that the Co-catalysed oxidative decarboxy-
lation of derivatives 1 gives rise to the reaction products 2 and
3
in very different ratios, ranging from 1:99 to 89:11, depending
on the substrate substitution. The reaction follows different
pathways depending on the nature of the aryl group. Thus, for
the methoxy-substituted 1f, the aldehyde 3f was selectively
obtained following mainly path ‘‘b’’. For unsubstituted 1a
or for CF
the main reaction products, respectively, and both pathways
‘a’’ and ‘‘b’’ are operative.
3
-substituted 1d the carboxylic acids 2a and 2d were
‘
Fig. 3 Oxidation of 4-trifluoromethylmandelic acid (1d), 4-trifluoro-
methylbenzaldehyde (3d) and 4-trifluoromethylphenylglyoxylic acid
(4d) catalysed by CoCl
2
Scheme 4 Mechanistic pathways for the oxidation of mandelic acid
1a) catalysed by CoCl
(
2
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
6
4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 6 2 – 6 6

View Article Online
Table 3 Oxidation of 4-trifluoromethylmandelic acid (1d), 4-trifluoro-
methylbenzaldehyde (3d) and 4-trifluoromethylphenylglyoxylic acid
(
4d) catalysed by CoCl
2
Substrate Time/h % Conversion Products % Product yield
3d
4d
1d
2
7
100
75
2d
2d
2d
60
100
83
24
100
3
d
17
Scheme 6 Selectivity in the oxidation of 4-methoxymandelic acid (1f)
with two different catalytic systems
16
Table 4 Oxidation of mandelic acid derivatives 1 catalysed by Bi(0)
Conclusions
a
%
Substrate Time/h of 1
Conversion % Yield
of 2 þ 3
2 2
The CoCl /O system in DMSO–NaOH constitutes a novel
% Selectivity 2:3
and efficient catalytic system for the oxidative decarboxylation
of mandelic acid derivatives. Benzoic acids or benzaldehyde
derivatives are obtained selectively, depending on the substitu-
ents present on the aromatic ring. In the reactivity examined
for a series of para-substituted mandelic acids, it was observed
that for OH, OMe and Cl substituents, the oxidation led to
selective formation of the corresponding aldehydes 3. In con-
1
1
1
1
1
1
1
a
b
c
d
e
f
24
24
24
24
6
54
56
87
98
97
99
98
98
98
93:7
98:2
72
97
97:3
99:1
100
77
23:77
62:38
13:87
6
0.6
trast, for H, F and CF
carboxylic acids 2 were formed in selectivities higher than
0%. The cobalt(II) chloride catalysed oxidation of mandelic
3
substituents, the corresponding
g
97
a
8
Yield of products calculated on converted substrate.
acid derivatives is proposed to follow different mechanistic
pathways, depending on the substitution on the aryl ring.
Interestingly, comparison of the oxidation of the same
substrates by a Bi(0)/O2 system presents some similarities,
although in the cases of 4-chloromandelic acid and 4-methoxy-
mandelic acid, the main products obtained were different and
the selectivities 2:3 were completely reversed.
However, important changes in the selectivities of both Bi(0)
and Co(II) catalysed reactions were observed. When benzalde-
hyde derivatives 3 are the main compounds formed, as in the
case of 1e and 1g, a better selectivity is obtained with CoCl
2
as compared to Bi(0). On the contrary, when benzoic acid deri-
vatives 2 are the main products as in the oxidation of 1a, 1c
and 1d, better selectivities towards 2 are obtained with Bi(0).
In two cases, the selectivity of the two catalytic systems is
completely reversed. Thus, for the oxidation of 4-chloro- and
Experimental
Commercially available products were used without further
purification. HPLC analysis was effectuated with a Waters
Millipore apparatus, with a m-Bondapack C18 Waters 9 mm,
4
-methoxymandelic acid (1b and 1f, respectively), the CoCl
2
catalysed oxidation affords mainly the corresponding benzal-
dehyde derivatives 3b and 3f, whereas in the Bi(0) catalysed
oxidation, the corresponding benzoic acid derivatives 2b and
f are selectively obtained. With the Co(II) system, 4-chloro-
benzaldehyde (3b) and 4-anisadehyde (3f) are formed with
1% and 84% selectivities, respectively. With the Bi catalytic
3
H
0 cm ꢁ 3.9 mm column. The eluent was a 80:20 mixture of
2
O–MeOH with H
3
PO
4
(0.5%). The elution was carried out
in isocratic mode and the products were
2
ꢂ1
at 1 mL min
detected by UV at l ¼ 256 nm.
7
system, 4-chloro- and 4-methoxybenzoic acids (2b and 2f) are
formed in 98% and 62% yields, respectively.
General oxidation procedure
Scheme 5 illustrates the fact that the oxidation of 1b can be
oriented towards the formation of either 2b or 3b, according to
the nature of the catalytic system. Thus, 2b or 3b can be selec-
tively obtained by using the Bi(0) or the Co(II) systems, respec-
tively. A similar Scheme 6 can be proposed for the oxidation
of 1f, indicating that either the aldehyde 3f or the carboxylic
The reactions were carried out under 1 atm molecular oxygen.
Mandelic acid or one of its derivatives (2 mmol) was dissolved
in DMSO (5 mL) in the presence of anhydrous CoCl (0.02
2
mmol) and NaOH (3 mmol of a 50% aqueous solution). The
ꢀ
mixture was stirred at 125 C and the consumption of the start-
ing material was followed by HPLC and/or by H NMR. The
1
2
acid 2f can be selectively obtained with CoCl or Bi(0) as the
crude reaction mixture was hydrolysed with 5 mL aqueous 1 M
HCl solution saturated with NaCl and extracted with ethyl
acetate (5 ꢁ 10 mL). Organic layers were collected and washed
twice with an aqueous 0.1 M HCl solution saturated with
catalyst, respectively, though with a lesser selectivity.
For the Bi(0) catalysed oxidations, path ‘‘a’’ via the ketoacid
4
(Scheme 2) was shown to be the main pathway followed for
the oxidation of 1a and 1d to the corresponding carboxylic
acids 2a and 2d.
NaCl, dried over MgSO and filtered off. The products were
4
1
analysed and quantified by HPLC and by H NMR and their
spectral data compared to those of authentic samples.
References
1
2
J. Iqbal, M. Mukhopadhyay and A. K. Mandal, Synlett, 1997,
76.
X. Baucherel, E. Levoirier, J. Uziel and S. Jug e´ , Tetrahedron Lett.,
000, 41, 1385.
8
2
3
4
J. Iqbal and A. Pandey, Tetrahedron Lett., 1990, 31, 575.
J. Iqbal, M. A. Khan and N. K. Nayyar, Tetrahedron Lett., 1991,
32, 5179.
Scheme 5 Selectivity in the oxidation of 4-chloromandelic acid (1b)
with two different catalytic systems
5
D. K. Chand and P. K. Bharadwaj, Inorg. Chem., 1997, 36, 5658.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 6 2 – 6 6
65

View Article Online
6
7
A. K. Mandal and J. Iqbal, Tetrahedron, 1997, 53, 7641.
T. Punniyamurthy, B. Bhatia, M. M. Reddy, G. C. Maikap and
J. Iqbal, Tetrahedron, 1997, 53, 7649.
B. C. Das and J. Iqbal, Tetrahedron Lett., 1997, 38, 1235.
(a) Y. H. Lin, I. D. Williams and P. Li, Appl. Catal., A, 1997, 150,
12 Y. Yoshino, Y. Hayashi, T. Iwahama, S. Sakaguchi and Y. Ishii,
J. Org. Chem., 1997, 62, 6810.
13 P. Li and H. Alper, J. Mol. Catal. A, 1992, 72, 143.
14 See, for example: (a) C. Walling and K. Amaranath, J. Am. Chem.
Soc., 1982, 104, 1185; (b) D. Ip and J. Rocek, J. Am. Chem. Soc.,
1979, 101, 6311.
8
9
2
21; (b) X. Baucherel, J. Uziel and S. Jug e´ , J. Org. Chem., 2001,
6
6, 4504.
15 C. Coin, V. Le Boisselier, I. Favier, M. Postel and E. Du n˜ ach,
Eur. J. Org. Chem., 2001, 735.
16 I. Favier, F. Giulieri, E. Du n˜ ach, D. H e´ brault and J. R. Desmurs,
Eur. J. Org. Chem., 2002, 1984.
17 I. Favier and E. Du n˜ ach, Tetrahedron, 2003, 59, 1823.
1
0
1
R. S. Drago, B. B. Corden and C. W. Barnes, J. Am. Chem. Soc.,
986, 108, 2453.
P. Mastrorilli, G. P. Suranna, C. F. Nobile, G. Farinola and L.
1
1
Lopez, J. Mol. Catal. A: Chem., 2000, 156, 279.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
6
6
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 6 2 – 6 6