LETTER
391
An Efficient Tandem Oxidation of Cyclohexanol to e-Caprolactone with
Peroxyacids and TEMPO Catalyst in Ionic Liquids as Solvents
Tandem
O
xidation
n
of Cyclohex
n
anol to
e
-Ca
a
prolactone Chrobok*
Silesian University of Technology, Department of Chemical Organic Technology and Petrochemistry,
ul. Krzywoustego 4, Gliwice 44-100, Poland
Fax +48(32)2371032; E-mail: anna.chrobok@polsl.pl
Received 31 October 2010
(2,2,6,6-tetramethylpiperidine-1-oxyl), have become the
catalysts of choice for this reaction, in which oxygen, so-
dium hypochlorite, potassium peroxomonosulfate, or
bis(acetoxy)iodobenzene can serve as oxidants.4
Abstract: The new one-pot tandem oxidation of cyclohexanol to e-
caprolactone with potassium peroxomonosulfate or m-chloroper-
oxybenzoic acid as oxidation agents and ionic liquids as solvents is
described. A 2,2,6,6-tetramethylpiperidine-1-oxyl radical with tet-
rabutylammonium bromide as the co-catalyst was used. A solution
of KHSO5 in the ionic liquid 1-butyl-3-methylimidazolium tetra-
fluoroborate [bmim]BF4 facilitates the tandem oxidation of alcohol
to lactone. In classic solvents, this reaction can only be carried out
to the ketone formation step. This is most probably due to the ability
of [bmim]BF4 to dissolve both alcohol and KHSO5. In the case of
using m-chloroperoxybenzoic acid, ionic liquids provide an effi-
cient medium for this reaction. This new method enables e-capro-
lactone formation with high yields (75–80%).
The tandem reaction concerning the one-pot oxidation of
secondary alcohols to lactones or esters would be benefi-
cial for organic chemists. Only limited information con-
cerning this problem can be found in the literature.
Sepulveda et al.5 described the oxidation of cyclohexanol
with 70% H2O2 in acetonitrile in the presence of the cata-
lysts: H3PW12O40 and Cs2.5H0.5PW12O40 at 90 °C. Al-
though the conversion is influenced by the pressure of
oxygen, the decomposition of H2O2 was observed. The se-
lectivity of e-caprolactone was achieved from 16–27%.
Other effective methods for the one-pot conversion of al-
cohol to lactone were based on the use of m-chloroper-
oxybenzoic acid (MCPBA) as an oxidant. Lipton et al.6
presented a method using catalytic amounts of cyclic
chromate ester, which facilitated the oxidation of cyclo-
hexanol to lactone in dichloromethane as a solvent at
room temperature with 93% yield. Cella et al.7 discussed
another method involving the utilisation of MCPBA and
2,2,6,6-tetramethylpiperidine hydrochloride as catalysts
for the effective oxidation of cyclohexanol to e-caprolac-
tone in dichloromethane at room temperature with high
yields. An interesting method describing a strategy for ox-
idation of cyclohexanol to e-caprolactone involves molec-
ular oxygen and N-hydroxyphthalimide as a catalyst.8
During the oxidation at 75 °C, one molecule of H2O2 is
created in situ and, combined with a Lewis acid, serves as
the reaction conditions for lactone formation.
Key words: ionic liquids, oxidation, alcohols, tandem reaction, lac-
tones
The oxidation reactions of alcohols to ketones and ketones
to lactones are some of the most important transforma-
tions in organic synthesis and are used in many different
applications, including the synthesis of antibiotics, ste-
roids, pheromones, and monomers for polymerisation.1
Among these reactions is the crucial synthesis of e-capro-
lactone. The major use of e-caprolactone is in the produc-
tion of polycaprolactone. This is a biodegradable
polyester that is used as an additive in resins or as an im-
plantable biomaterial. For this reason, the synthesis of e-
caprolactone has become a model reaction for this study.
The oxidation of ketones to lactones or esters in the Baey-
er–Villiger (BV) reaction uses organic peroxyacids or hy-
drogen peroxide as the most commonly used oxidants.
However, Baeyer and Villiger used Caro’s acid (potassi-
um peroxomonosulfate, KHSO5) in their pioneering
work; this oxidising agent was used for BV reactions only
occasionally.2 For the oxidations with KHSO5, an aque-
ous reaction medium must be employed, which can lead
to the hydrolysis of lactones or esters. In previous work,
an alternative method with ionic liquids (IL) as solvents
was presented.3
Herein, a new method for the one-pot tandem oxidation of
alcohol to lactone is presented. The method is based on
the use of MCPBA or Oxone® (commercial source of po-
tassium peroxomonosulfate) as oxidants with a TEMPO
radical and tetrabutylammonium bromide (TBAB) as the
oxidant system in IL solvents. To the best of the author’s
knowledge, this is the first method describing the one-pot
oxidation of alcohol to lactone in IL with MCPBA as an
oxidant. Additionally, there are no reports concerning the
use of KHSO5 for a tandem oxidation.
Although the BV reaction is relatively easy to carry out,
the oxidation of alcohols, especially these secondary to
ketones, is much more problematic. During the broad
range of research, nitroxyl radicals, e.g., TEMPO
In Table 1, the results of the oxidation of cyclohexanol to
e-caprolactone are presented. Among the IL, 1-butyl-3-
methylimidazolium tetrafluoroborate {[bmim]BF4} was
chosen as the solvent.
SYNLETT 2011, No. 3, pp 0391–0395
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Advanced online publication: 13.01.2011
DOI: 10.1055/s-0030-1259316; Art ID: G29810ST
© Georg Thieme Verlag Stuttgart · New York