An oxidation of alcohols by oxygen with the enzyme laccase and mediation by TEMPO

Article abstract of DOI:10.1016/S0040-4039(01)01463-0

A simple and efficient oxidation of alcohols to carbonyl compounds by oxygen at room temperature is described; it requires the laccase/TEMPO mediator system as the catalyst. A possible mechanistic explanation is provided.

Full text of DOI:10.1016/S0040-4039(01)01463-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 7551–7553
An oxidation of alcohols by oxygen with the enzyme laccase and
mediation by TEMPO
Maura Fabbrini, Carlo Galli,* Patrizia Gentili and Daniele Macchitella
Dipartimento di Chimica and Centro CNR Meccanismi di Reazione, P.le A. Moro 5, I-00185 Rome, Italy
Received 6 July 2001; accepted 6 August 2001
Abstract—A simple and efficient oxidation of alcohols to carbonyl compounds by oxygen at room temperature is described; it
requires the laccase/TEMPO mediator system as the catalyst. A possible mechanistic explanation is provided. © 2001 Elsevier
Science Ltd. All rights reserved.
Laccases are multi-copper oxidases expressed under
ligninolytic conditions by white-rot fungi.1 In view of the
low redox potential, native laccases can oxidise only
phenolic fragments of lignin,2 with the concomitant
reduction of O2. However, the oxidation of non-phenolic
substrates can also take place on mediation by appropri-
ate substances.3 We have recently found that one such
No overoxidation products were in general observed,
while no conversion to oxidised products was obtained
in the absence of laccase, nor in the absence of TEMPO.
Inspection of Table 1 shows that the primary benzyl and
allyl alcohols (entries 1, 2, 3, 7 and 8) were converted into
the corresponding aldehydes in high yields. Secondary
benzyl alcohols (1 and 3, entries 4 and 5) were also
efficiently oxidised to ketones (2 and 4), while the tertiary
benzyl alcohol (5) was recovered unchanged, thus under-
lying the need for a CꢀH bond a to the alcohol group
for a successful oxidation. Primary and secondary alkyl
alcohols were oxidised in lower yields (entries 9, 10,
12–14), but doubling the reaction time increased the yield
significantly (entry 11). In these cases, substantial
amounts of the precursor were also recovered. Benzyl
ethers were converted to aldehydes, albeit in low yields
(entries 15 and 16). A quantitative conversion into
benzaldehyde was finally obtained with primary and
tertiary benzylamines (entries 17 and 18), but surprisingly
no oxidation to benzaldehyde took place with benzyl
mercaptan (entry 19), in spite of its structural analogy
with benzyl alcohol (entry 1), nor any disulfide product
was detected from it.
mediator
is
2,2%,6,6%-tetramethylpiperidine-N-oxyl
(TEMPO);4 we report here on a mild and environmental
friendly oxidation of alcohols, as well as of a few other
compounds (Eq. (1)), obtained with laccase from a strain
of Trametes villosa under mediation by TEMPO.
(1)
TEMPO is already known to catalyse efficiently the
oxidation of alcohols to carbonyl products by a number
of oxidants,5 including copper salts.6 In our particular
case, the catalytic oxidant of TEMPO would be the
copper-enzyme laccase.
The oxidation reactions were run at room temperature
in a 3 mL water solution, buffered at pH 5 (0.1 M citrate)
and preliminarily purged for 30 min with O2; a 24 h
reaction time was adopted. The initial concentrations
were: [subst.] 20 mM, [TEMPO] 6 mM, with 9 Units of
laccase.4 The results are reported in Table 1, and the
yields were determined by GC by the internal standard
method (with respect to 4-methoxyacetophenone).
These results support our previous suggestion about an
oxidation mechanism that proceeds through the oxo-
ammonium ion of TEMPO,4 as it is proposed for the
oxidation of alcohols by other oxidants and TEMPO.5,6
Laccase, having a redox potential in the range of 0.7–0.9
V,2 can easily oxidise the stable oxyl-radical form of
TEMPO to the oxoammonium ion (E° 0.2 V)7 (Eq. (2)).
(2)
Keywords: laccase; TEMPO; oxidation; alcohols; mediation.
* Corresponding author. Tel.: (39)0649913386; fax: (39)06490421;
e-mail: carlo.galli@uniroma1.it
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01463-0

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M. Fabbrini et al. / Tetrahedron Letters 42 (2001) 7551–7553
Table 1. Oxidations with oxygen by the laccase/TEMPO systema
a
Typical conditions: 24 h reaction time, rt; [subst.] 20 mM, [TEMPO] 6 mM, 3 U/mL of laccase, in 3 mL water solution at pH 4.5 (0.1 M citrate).
Oxygen had been initially purged for 30 min in the solvent.
Determined by GC analysis.
Longer reaction time (48 h).
Benzyl benzoate (3%) was additionally detected.
Shorter reaction time (7 h).
Without purging with O2.
With a lower [TEMPO], i.e. 2 mM.
With [TEMPO] 2 mM and only 1 U/mL of laccase.
b
c
d
e
f
g
h
i
As in h, but without purging with O2.
The last experiments of Table 1 show that a shorter
reaction time can be adopted with the more reactive
substrates, or a higher substrate/mediator ratio or even
a higher mediator/laccase ratio can be used, without a
major compromise in oxidation efficiency. Finally, the
need for initial purging with O2 becomes relevant only
under the more demanding conditions, where a high
substrate/laccase ratio is used.
This ion is the actual oxidant (Scheme 1), while laccase
would regenerate TEMPO from the generated hydroxyl-
amine. Then, either acid-induced disproportionation of
TEMPO, or further oxidation of it by laccase, would
form the oxoammonium ion once again. Since laccase is
in turn oxidised by oxygen, this synthetic procedure
represents an oxidation of alcohols by air, catalysed by
the laccase/TEMPO system.

M. Fabbrini et al. / Tetrahedron Letters 42 (2001) 7551–7553
7553
Scheme 1. Possible mechanism of oxidation by the laccase/TEMPO system.
Acknowledgements
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3. (a) Bourbonnais, R.; Paice, M. G.; Freiermuth, B.; Bodie,
E.; Borneman, S. Appl. Environ. Microb. 1997, 63, 4627–
4632; (b) Crestini, C.; Argyropoulos, D. S. Bioorg. Med.
Chem. 1998, 6, 2161–2169.
We thank Professor F. Minisci (Politecnico, Milano)
for disclosing us some preliminary results on his work
about
a
Mn–Cu–TEMPO catalysed oxidation of
4. Fabbrini, M.; Galli, C.; Gentili, P. J. Mol. Catal. B:
alcohols.6b Thanks are due to Novo Nordisk Biotech
(Denmark) for a generous gift of laccase.
Enzyme, submitted.
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Synthesis 1996, 1153–1174; (b) De Mico, A.; Margarita,
R.; Parlanti, L.; Vescovi, A.; Piancatelli, G. J. Org. Chem.
1997, 62, 6974–6977.
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