A Convenient Nitroxyl Radical Catalyst for the Selective Oxidation of Primary and Secondary Alcohols to Aldehydes and Ketones by O2 and H2O2 under Mild Conditions

Article abstract of DOI:10.1021/op034063o

A new macrocyclic tetrafunctional nitroxyl radical, I, developed by Ciba Specialty Chemicals, is a particularly effective catalyst in combination with Mn(II) and Co(II) or Cu(II) nitrates for the selective oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones by air or O ₂ under mild conditions (ambient temperature and pressure) or H ₂O₂. A distinctive feature of I is the possibility of easy recovery and recycles, due to its low solubility, particularly as ammonium salt, in most organic solvents, which makes it especially useful for practical applications.

Full text of DOI:10.1021/op034063o

Organic Process Research & Development 2003, 7, 794−798
Articles
A Convenient Nitroxyl Radical Catalyst for the Selective Oxidation of Primary
and Secondary Alcohols to Aldehydes and Ketones by O and H O under Mild
2
2 2
Conditions
,
†
†
†
‡
§
Francesco Minisci,* Francesco Recupero, Marianna Rodin o` , Massimiliano Sala, and Armin Schneider
Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”, Politecnico di Milano, Via Mancinelli 7, 20131
Milano, Italy, Ciba Specialty Chemicals S.p.A., Via Pila 6/3 40044 Sasso Marconi (BO) fraz. Pontecchio Marconi, Italy,
and Ciba Specialty Chemicals Inc., Building, R.1059.4, 4002 Basel, Switzerland
3
Abstract:
variety of procedures. A convenient method, involving a
A new macrocyclic tetrafunctional nitroxyl radical, I, developed
by Ciba Specialty Chemicals, is a particularly effective catalyst
in combination with Mn(II) and Co(II) or Cu(II) nitrates for
the selective oxidation of primary and secondary alcohols to
2 2 2
two-phase system (CH Cl /H O), sodium hypochlorite as
oxidant and bromide ion as cocatalyst, has been developed
4
by Montanari and co-workers; a modified “Montanari
process” has been recently utilised for the production of fine
5
the corresponding aldehydes and ketones by air or O
2
under
chemicals. Several metal salt complexes were also utilised
mild conditions (ambient temperature and pressure) or H
2
O
2
.
in combination with TEMPO for the aerobic oxidation of
alcohols: cupric salts were effective with benzylic alcohols,
A distinctive feature of I is the possibility of easy recovery and
recycles, due to its low solubility, particularly as ammonium
salt, in most organic solvents, which makes it especially useful
for practical applications.
while they proved to be less effective with nonbenzylic
6
alcohols; RuCl
2
(PPh
3
)
3
is an efficient catalyst at 100 °C,
7
2
and a more complex catalyst involving CuBr‚Me S, per-
fluoroalkylated bipyridine, and perfluorooctane-chloroben-
zene biphasic solvent system, was utilised at 90 °C.
Recently we have reported in a preliminary communica-
Introduction
The oxidation of primary and secondary alcohols to the
corresponding aldehydes and ketones is a basic transforma-
tion of great interest for general synthetic purposes and for
practical applications, and a large variety of oxidising
reagents has been utilised successfully. Environmentally
reliable catalytic systems in combination with cheap oxidants,
such as O or H O , are desirable for practical purposes.
2 2 2
Organic persistent nitroxyl radicals such as TEMPO (2,2,6,6-
tetramethylpiperidin-1-oxyl) or nonpersistent radicals such
as PINO (phthalimido-N-oxyl, generated in situ from N-
tion9a that a catalytic system, involving Mn(NO
3
)
2
in
combination with Co(NO ) or Cu(NO ) , is particularly
3 2 3 2
effective for the aerobic oxidation of carbonyl compounds
under very mild conditions (cyclohexanone is oxidised to
adipic acid by oxygen with high selectivity at ambient
temperature and pressure), but it is completely inert towards
alcohols; it becomes very effective for the aerobic oxidation
of primary and secondary alcohols in combination with
TEMPO, which however inhibits the further oxidation of
aldehydes and ketones. This procedure appears particularly
convenient for the synthesis of aldehydes and ketones from
the corresponding alcohols, due to the simple and mild
1
2
hydroxyphthalimide) have been shown to be particularly
effective catalysts for the selective aerobic oxidation of
primary and secondary alcohols to the corresponding alde-
hydes and ketones.
(
(
(
2) Minisci, F.; Punta, C.; Recupero, F.; Fontana, F.; Pedulli, G. F. Chem.
Commun. 2002, 688.
3) de Nooy, A. E. J.; Besemer, A. C.; van Bekkum, H. Synthesis 1996, 1153,
a review.
4) Anelli, P. L.; Biffi, C.; Montanari, F.; Quici, S. J. Org. Chem. 1987, 52,
2559; J. Org. Chem. 1989, 54, 2970; Anelli, P. L.; Biffi, C.; Montanari, F.;
Quici, S. Org. Synth. 1990, 69, 212.
(
(
(
5) Bjørsvik, H.-R.; Liguori, L.; Costantino, F.; Minisci, F. Org. Process Res.
DeV. 2002, 6, 197.
6) Semmelhack, M. F.; Schmid, C. R.; Cortes, D. A.; Chou, S. J. Am. Chem.
Soc. 1984, 106, 3374.
7) Dijksman, A.; Arends, I. W. C. E.; Sheldon R. A. Chem. Commun. 1999,
The use of persistent nitroxyl radicals, such as TEMPO,
for the oxidation of alcohols has been largely explored by a
1591; Dijksman, A.; Arends, I. W. C. E.; Sheldon R. A. Platinum Met.
ReV. 2001, 45, 15; Dijksman, A.; Marino-Gonzalez, A.; Mairata i Payeras,
A.; Arends, I. W. C. E.; Sheldon, R. A. J. Am. Chem. Soc. 2001, 123, 6826.
†
Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”,
(8) Betzemeier, B.; Cavazzini, M.; Quici, S.; Knochel, P. Tetrahedron Lett.
2000, 41, 4343.
(9) (a) Minisci, F.; Recupero, F.; Fontana. F.; Bjørsvik, H.-R.; Liguori, L. Synlett
2002, 610. (b) Minisci, F.; Fumagalli, C.; Pirola, R. ( Lonza S.p.A.). WO
0158845, WO 0187815, 2001. (c) Minisci, F.; Recupero, F.; Pedulli, G. F.;
Lucarini, M. J. Mol. Catal. A 2003, in press (a review).
Politecnico di Milano.
‡
Ciba Specialty Chemicals S.p.A.
Ciba Specialty Chemicals Inc.
§
(1) Cecchetto, A.; Fontana, F.; Minisci, F.; Recupero, F. Tetrahedron Lett. 2001,
4
2, 6651.
7
94
•
Vol. 7, No. 6, 2003 / Organic Process Research & Development
10.1021/op034063o CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/26/2003

conditions (the oxidation takes place at room temperature
considering that the catalysis is effective only in acidic
medium, as it will be discussed later on.
and atmospheric pressure of air or O
2
), the high selectivity,
and the cheap metal salt catalyst.1
Results and Discussion
TEMPO has two important functions: it generates the
oxoammonium salt, which is responsible for the alcohol
oxidation and inhibits the further oxidation of aldehydes and
ketones, which occurs by free-radical chain processes under
the same conditions in the absence of TEMPO; this last,
being persistent, quickly traps a variety of free radicals (eq
The radical of structure I, as reported in the Ciba patent,
has been investigated as catalyst for the aerobic oxidation
of alcohols.
1).
The advantage of using TEMPO, compared to the aerobic
2
oxidation involving the PINO radical, is related to the
general character for the oxidation of benzylic and non-
benzylic primary alcohols to the corresponding aldehydes,
while the use of N-hydroxyphthalimide, which generates in
situ the PINO radical, is limited to benzylic alcohols (with
nonbenzylic derivatives the oxidation of primary alcohols
leads to carboxylic acids, even at low conversions).
I has been prepared by oxidation of the amine precursor
N-H f N-O ), which is a commercial product, CHIMAS-
SORB 966, sold by Ciba Specialty Chemicals. The oxidation
of the amine precursor has been carried out according to the
procedures described for the other tetramethylpiperidine
derivatives, involving the use of hydrogen peroxides or
peracetic acid as oxidants.
•
(
This different behaviour of the two nitroxyl radicals,
10
TEMPO and PINO, was explained by the bond dissociation
enthalpies of the O-H bond of the corresponding N-
hydroxyderivatives (69.9 kcal/mol for the N-hydroxy-2,2,6,6-
tetramethylpiperidine and 88.1 kcal/mol for the N-hydroxy-
phthalimide). Consequently PINO can induce free-radical
chain processes, the selectivity of which is determined by
polar, enthalpic, and captodative effects in the hydrogen
abstraction step, while TEMPO inhibits such processes.
The advantage of using N-hydroxyphthalimide concerns
its low solubility in several solvents, which facilitates the
recovery and the recycle of the catalyst that are more difficult
with TEMPO. PINO, however, is much less stable than
TEMPO, and it can decompose in competition with the
regeneration of N-hydroxyphthalimide, particularly with less
The tetranitroxyl radical I has a low solubility in most
organic solvents, and its use as heterogeneous catalyst for
the oxidation of alcohols is effective with sodium hypochlo-
1
1
rite, but it gave poor results in combination with Mn(II)
and Co(II) or Cu(II) nitrates for the aerobic oxidation because
of poor solubility. I, however, is soluble in acidic medium,
due to the presence of amino groups and is particularly
soluble in acetic acid, which is a suitable solvent according
to the procedure for the aerobic oxidation catalysed by
TEMPO and metal salts. Thus, the nitroxyl radical I in
combination with Mn(II) and Co(II) or Cu(II) nitrates
catalyses the aerobic oxidation of primary and secondary
alcohols to the corresponding aldehydes and ketones in acetic
acid solution with high selectivity under mild conditions
-1
reactive substrates (a value of 0.1 s for the first-order self-
decay of PINO has been determined10 at 25 °C); a key step
in the catalysis by N-hydroxyphthalimide is, in fact, the
hydrogen abstraction by the PINO radical (eq 2).
(
room temperature and atmospheric pressure of oxygen or
air). The results with a variety of alcohols are reported in
Table 1. Blank experiments have shown that in the absence
of the radical I or the metal salts, no substantial oxidation
occurs.
The reaction mechanism involves the disproportionation
of the nitroxyl radical, catalysed by the acidic medium (eq
3
), with the formation of the oxoammonium salt, which is
known to be the actual oxidant of the alcohol (eq 4).
Recently a Ciba patent11 has reported a stable and
persistent polynitroxyl radical, characterised by low solubility
in many organic solvents and therefore suitable to be easily
recovered and recycled, making particularly convenient its
utilisation as catalyst for the aerobic oxidations of practical
interest. Moreover, the presence of amino groups makes the
recovery of the catalyst as the ammonium salt still easier,
The nitroxyl radical is regenerated by oxygen and the
metal salt catalytic system (eq 5).
(
10) Amorati, R.; Lucarini, M.; Mugnaini, V.; Pedulli, G. F.; Minisci, F.; Fontana,
F.; Recupero, F. Greci, L. J. Org. Chem. 2003, 68, 1747.
11) Zedda, A.; Sala, M.; Schneider, A. (Ciba Specialty Chemicals Holding Inc.).
WO 02/058844 A1, 2002.
(
Vol. 7, No. 6, 2003 / Organic Process Research & Development
•
795

Table 1. Aerobic oxidation of alcohols to aldehydes and
ketones, catalysed by the nitroxyl radical I in combination
with Mn(II) and Co(II) nitratesa
the activity. Our hypothesis is that the catalyst undergoes a
partial degradation during the aerobic oxidation; in particular,
the methylene groups in the R-position to the nitrogen are
very sensitive to oxidation. It is likely the Co(III) and the
Mn(III) salts intermediates of the oxidation contribute to
regenerate the nitroxyl radical from hydroxylamine derivative
I/
reaction
time/h
yield/
%
alcohol
OH
mol % procedure
T/°C
PhCH
PhCH
PhCH
PhCH
PhCH
PhCH
PhCH
PhCH
PhCH
PhCH
2
2
2
2
2
2
2
2
2
2
0.5
1.2
2.5
1.2
2.5
2.5
2.5
1.2
0.5
2.5
1.2
1.2
1.2
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
1.2
1.2
1.2
1.2
2.5
1.2
2.5
2.5
2.5
2.5
2.5
A
A
A
A
B
B
A
A
A
C
A
A
B
C
B
A
B
C
B
B
A
B
C
B
A
C
B
C
A
A
A
A
A
A
A
2
20
20
20
20
20
20
40
40
40
20
20
20
20
20
20
20
20
20
20
20
40
20
20
20
25
20
20
20
40
40
25
25
40
40
40
99
97
98
97
96
93
97
95
98
99
99
94
97
99
95
95
94
94
92
94
98
97
96
90
96
98
95
97
98
96
97
93
99
92
94
OH
1
0.6
1
2
2
0.3
0.5
1
1
0.8
1
4
1
3
3
3
3
5
5
2
5
5
4
1
1
3
1
1.5
2
1
1
1.5
2
(eq 7), formed in eqs 3 and 4. Unfortunately, they may also
OH
OHb
oxidise the amino groups by an electron-transfer process (eqs
8 and 9), leading to the oxidative degradation of the catalyst.
OH
OHb
OH
OH
OH
OHb
p-MeO-C
p-MeO-C
p-MeO-C
p-MeO-C
6
6
6
6
H
H
H
H
4
4
4
4
-CH
-CH
-CH
-CH
2
2
2
2
OH
OHb
OH
OHb
m-MeO-C
6
H
4
-CH
2
OH
p-Cl-C
p-Cl-C
p-Cl-C
6
6
6
4
H -CH
4
H -CH
4
H -CH
2
2
2
OH
OH
OHb
To overcome this possible oxidative degradation we
thought it right to add a strong acid to the reaction medium
to protonate more extensively the amino groups of the
catalyst; the protonated amino groups are quite inert towards
the electron-transfer oxidation (eq 8) and the subsequent
oxidative degradation (eq 9).
Indeed, the overall processes was considerably improved
by carrying out the oxidation in acetic acid in the presence
of a suitable amount of p-toluenesulphonic acid:
o-NO
m-NO
2
-C
-C
6
H
4
-CH
-CH
2
OH
2
6
H
4
2
OH
OH
OH
OHb
p-NO
p-NO
p-NO
2
-C
-C
-C
6
H
4
4
4
-CH
-CH
-CH
2
2
2
6
6
H
H
2
2
6
m-C H
4
-(CH
2
OH)
2
Ph-CHOH-CH
Ph-CHOH-CH
3
b
3
furfuryl alcohol
furfuryl alcoholb
2
-(hydroxymethyl)pyridine
cinnamyl alcohol
(i)The higher acidity favours the disproportionation of the
2
2
-methyl-pentanol
-nonanol
nitroxyl radical (eq 3), making the overall process faster.
However, an excess of acid must be avoided to prevent the
esterification of the alcohol substrate in esters of the acetic
acid, the products which are inert toward the aerobic
oxidation.
cyclohexanol
cyclooctanol
cyclododecanol
2.5
a
Procedure A with oxygen, Procedure B with air. b Cu(II) nitrate instead of
Co(II) nitrate.
(ii) The protonation of the amino groups of the catalyst
increases the electrophilic character of the oxoammonium
salt and therefore increases the rate of the addition of the
alcohol (eq 10), which is the rate-determining step of the
The overall stoichiometry of the reaction is given by eq
6.
1
2
oxidation.
On the other hand, the further aerobic free-radical
oxidation of aldehydes and ketones, which easily occurs
under the same conditions in the absence of I, is completely
inhibited by the presence of I, which quickly traps intermedi-
ate free radicals (eq 1). The oxidation rate is not only affected
by the temperature but also by the concentration of the
catalyst. This behaviour is useful because it allows regulating
the oxidation rate by the concentration of the nitroxyl, which
is more stable at lower reaction temperature. The amount of
the catalyst is relatively unimportant, as concerns its cost
contribution on the overall process, because it can be easily
recovered and recycled.
Attempts to recover and recycle the catalyst from acetic
acid solution gave only moderate results; the recovery of
the catalyst is relatively simple by evaporating acetic acid
and utilising a suitable solvent, such as methyl-tert-butyl
ether or ethyl acetate, which dissolves the reaction product
Recent results,12 concerning the oxidation of substituted
benzyl alcohols catalysed by nitroxyl radicals, show that
electron-donating groups activate and electron-withdrawing
groups deactivate the reaction in agreement with the forma-
tion of the complex between the oxoammonium salt and the
alcohol as the rate-determining step.
(iii) The most relevant effects of increasing the acidity,
however, are related to the fact that the oxidative degradation
of the catalyst is practically inhibited and the recovery and
recycle of the protonated catalyst with substantially un-
changed activity is much easier.
Several groups have addressed the problem of the
recycling nitroxyl radicals, which are rather expensive, by
(aldehydes and ketones), while the insoluble catalyst is easily
(12) Minisci, F.; Recupero, F.; Cecchetto, A.; Gambarotti, C.; Punta, C.; Faletti,
recovered. However, the recovered catalyst has partially lost
R.; Paganelli, R.; Pedulli, G. F. Eur. J. Org. Chem. Submitted.
796
•
Vol. 7, No. 6, 2003 / Organic Process Research & Development

designing heterogeneous variants of TEMPO.13-17 In all these
cases, however, the catalysts are active by using NaOCl as
oxidant, while they appear rather inactive for the aerobic
oxidation. Thus, the nitroxyl radical I appears to be, as far
as we know, the only catalyst suitable for the aerobic
oxidation of alcohols under mild conditions and for an easy
recycling.
In general, if the catalyst are soluble, it is not easy to
recover them from the final reaction mixture. Otherwise, in
the aerobic oxidation, if the catalyst is not in solution, the
reaction does not proceed as well as in homogeneous
conditions. The new nitroxyl radical I combines both the
solubility in the reaction solvent (acetic acid) and the
feasibility to recover the catalyst from the reaction mixture
after removing the solvent.
Table 2. Oxidation of alcohols to aldehydes and ketones by
, catalysed by the nitroxyl radical I in combination with
Mn(II) and Co(II) nitrates
2 2
H O
alcohol
I/mol % reaction time/h T/°C yield/%
PhCH
PhCH
PhCH
PhCH
PhCH
PhCH
2
2
2
2
2
2
OH
OH
OH
OHa
OH
OHb
7.5
5.0
2.5
1.2
2.5
2.5
1.2
1.2
1.2
2.5
2.5
2.5
2.5
8
12
16
2
8
8
25
25
25
50
25
25
50
50
50
50
50
50
50
97
98
96
93
-
-
p-MeO-C
p-Cl-C
p-Me-C
p-NO -C
6
H
4
-CH
-CH OH
-CH OH
-CH OH
2
OH
1.5
96
93
98
94
98
96
92
6
H
6
4
2
2
2
2
8
5
6
H
4
2
2
6
H
4
2
2
-nonanol
cyclohexanol
1-heptanol
The catalytic system can be utilised for the aerobic
oxidation of cycloalkanes to dicarboxylic acids, which are
compounds of particular industrial interest; in particular
adipic, suberic and 1,12-dodecandioc acids (from cyclohex-
ane, cyclooctane, and cyclododecane) are valuable starting
materials for the synthesis of polyamides and other polymeric
materials. A general industrial process for the production of
dicarboxylic acids involves the autoxidation of cycloalkanes
to mixtures of cycloalkanols and cycloalkanones at low
conversion to have high selectivity, followed by the oxidation
of those mixtures with nitric acid, which has, however, some
drawbacks, resulting mainly from the corrosivity of nitric
acid, and environmental problems, due to the formation of
nitrogen oxides.
a
By using the couple Cu(II)/ Mn(II) nitrates. b By using the couple Cu(II)/
Co(II) nitrates.
Compared to oxygen, the use of aqueous hydrogen
peroxide is especially beneficial, since its process requires
relatively simple reactor design of the production plants,
which can result in convenience for the production of
moderate amounts of fine chemicals. However a significant
drawback is represented by the high activation energy
required for the oxidation of many organic compounds by
hydrogen peroxide so that catalysis is often necessary for
the reaction to occur.
We have investigated the possibility of catalysing the
hydrogen peroxide oxidation of alcohols by I in combination
with the same metal salts (Mn(II) and Co(II) or Cu(II)
nitrates) utilised for the aerobic oxidation. The results (Table
An alternative approach is provided by the aerobic
oxidation of the mixtures of cycloalkanols and cyclo-
alkanones catalysed by the nitroxyl radical developed by Ciba
Specialty Chemicals (in combination with Mn(II) and Co-
2
) show that the Mn(II)/Co(II) nitrates couple is an effective
catalyst in combination with I for the oxidation of alcohols
by hydrogen peroxide (eq 11), while the couples Mn(II)/
Cu(II) or Co(II)/Cu(II) give poor results. It appears that the
copper salt catalyses the decomposition of H
involving the oxidation of the alcohol.
(II), or Cu(II) nitrates) according to the procedure reported
in this work, the separation of the nitroxyl catalyst I and the
final aerobic oxidation of the pure cycloalkanones to dicar-
boxylic acids by the same metal salt catalytic system (Mn-
2 2
O without
(
(
II) and Co(II) or Cu(II) nitrates) under very mild conditions
room temperature and atmospheric pressure of oxygen),
following the procedure recently reported by this research
9
group.
After oxygen, hydrogen peroxide is a particularly con-
venient oxidant of organic compounds for several reasons:
In the absence of the nitroxyl radical I or of the Mn(II)/
Co(II) nitrates couple no substantial oxidation occurs,
suggesting that also with hydrogen peroxide the actual
oxidant of the alcohol is the oxoammonium salt (eq 4), which
is continuously regenerated by the combination of hydrogen
peroxide and the metal salt catalysis. Also in this case the
oxidation rate is affected by the concentration of I. The
nitroxyl catalyst I can be recovered and recycled from acetic
acid, without loss of activity, as described for the aerobic
oxidation processes.
(i) high oxidation potential (E° ) 1.77V),
(ii) formation of water as reduction product, thus mini-
mising the environmental drawbacks which are often in-
volved with other oxidants, including a cheap oxidant such
as sodium hypochlorite,
(iii) the low cost,
(iv) the low molecular mass.
(
(
(
(
(
13) Fey , T.; Fischer, H.; Bachmann, S.; Albert, K.; Bolm, C. J. Org. Chem.
2
001, 66, 8154.
14) Brunel, D.; Lentz, P.; Sutra, P.; Deroide, B.; Fajula, F.; Nagy, J. B. Stud.
Surf. Sci. Catal. 1999, 125, 237.
15) Verhoef, M. J.; Peters, J. A.; van Bekkum, H. Stud. Surf. Sci. Catal. 1999,
Experimental Section
All the starting materials and catalysts were commercial
products and used without further treatment. The nitroxyl
radical I was prepared according to the Ciba patent.11
General Procedures for the Aerobic Oxidation of
Alcohols. Procedure A. A solution of 6 mmol of the alcohol
1
25, 465-472.
16) Ciriminna, R.; Blum, J.; Avnir, D.; Pagliaro, M. Chem. Commun. 2000,
441.
17) Dijksman, A.; Arends, I. W. C. E.; Sheldon, R. A. Chem. Commun. 2000,
71-272; Synlett 2001, 102-104.
1
2
Vol. 7, No. 6, 2003 / Organic Process Research & Development
•
797

in 35 mL of acetic acid, each specific amount of the nitroxyl
catalyst I reported in Table 1, and 0.25 mol % of Mn(II)
and Co(II) or Cu(II) nitrates, was stirred at the specified
temperature under oxygen atmosphere at ambient pressure
for the times reported in Table 1. The acetic acid was then
evaporated at ca. 50 Torr, and the reaction products were
extracted with methyl-tert-butyl ether (MTBE) or with ethyl
acetate (the nitroxyl radical I is insoluble in these solvents
and can be recovered and recycled). All the reaction products
are known, and GC and GC-MS analyses were performed.
The products were identified through a comparison of the
analytical data with those of authentic samples and the yields
were estimated by GC with the internal standard technique.
In few cases the reaction products were isolated in experi-
ments on larger scale (60 mmol of alcohol) simply by
evaporating the solvent and obtaining a purity>98%. The
recovered catalyst I was recycled with partial loss of activity.
Procedure B. The experiment was carried out as for the
procedure A, except that the stirred reaction solution was
maintained under air at atmospheric pressure.
at room temperature for 3 h under air at atmospheric pressure.
The analysis, the isolation of the reaction products, and the
recovery of the catalyst were carried out according to
procedure A. The nitroxyl catalyst has been recycled for four
times without significant loss of activity, as p-toluenesul-
phonic acid salt. No additional p-toluenesulphonic acid was
required for the recycles. After the catalyst I was recycled
four times, the yield of benzaldehyde with respect to the
benzyl alcohol was 95%.
General Procedure for the Oxidation of Alcohols by
Hydrogen Peroxide. A solution of the alcohol (6 mmol),
30% aqueous H O (15 mmol) in 35 mL of acetic acid, the
2 2
amount of the nitroxyl catalyst I reported in Table 2 and
0.25 mol % of Mn(II) and Co(II) nitrates was stirred at the
specified temperature for the time reported in Table 2. The
workup, analysis, and recovery of the catalyst I were carried
out according to procedure A. The catalyst I was recycled
five times in the oxidation of benzyl alcohol without
significant loss of activity: the oxidation yield of the benzyl
alcohol with respect to benzaldehyde was 96%
Procedure C. A solution of 92.6 mmol of the alcohol,
2
.25 mmol of the nitroxyl radical catalyst I, 4.5 mmol of
Received for review May 28, 2003.
OP034063O
p-toluenesulphonic acid, 1.8 mmol of Mn(II) nitrate and 1.8
mmol of Co(II) nitrate, in 100 mL of acetic acid was stirred
798
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Vol. 7, No. 6, 2003 / Organic Process Research & Development