Baeyer–Villiger oxidations with a difference: molecular sieve redox catalysts
for the low-temperature conversion of ketones to lactones
Robert Raja, John Meurig Thomas and Gopinathan Sankar
Davy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London, UK
W1X 4BS. E-mail: dawn@ri.ac.uk
Received (in Liverpool, UK) 4th January 1999, Accepted 5th February 1999
Redox molecular sieve catalysts MAlPO-36 (M = Mn or Co)
convert cyclopentanone, cyclohexanone, 2-methylcyclohex-
anone and adamantan-2-one to their corresponding lactones
with high efficiency (selectivities in excess of 90%, conver-
sions in the range 50 to 85%), in the presence of O2 and
PhCHO as sacrificial oxidant.
MnALPO-18. The structure12 of aluminium phosphate No. 36
(IZA structure code ATS) has well-defined, oval-shaped
channels (6.5 3 7.5 Å) (Fig. 1), which, by appropriate
preparative means,7,12 may be lined with a substantial number
of either cobalt or manganese ions framework-substituted in
place of AlIII ions. Similarly, two other types of materials,
AlPO-5 (AFI) and AlPO-18 (AEI) with pore dimensions of 7.3
and 3.8 Å, respectively, were synthesised with specific metal
ions having identical concentrations.11 By calcining the as-
prepared MAlPOs (M = Co or Mn) samples in oxygen at 550
°C, the substituted ions may be converted12 to their +3 oxidation
states, to a degree that is dependent on the structure; the trend in
the concentration of the +3 state in the three structures is AlPO-
18 > AlPO-36 > AlPO-5.11 The results of Bayer–Villiger
oxidation of a number of ketones to lactones, performed with
these catalysts, are given in Table 1, and a typical kinetics plot
for the MnAlPO-36 catalyst is shown in Fig. 2.†
Although the mechanistic details of the aerobic oxidation of
ketones to lactones reported here still requires elucidation,
preliminary studies (cf. ref. 6) point to the fact that the active
centres in the redox catalysts are the CoIII (or MnIII) ions that
line the micropores. We know that the higher conversions are
effected by the framework-substituted higher valence ions in the
molecular sieve catalysts for several reasons. Firstly, divalent
ion substituted AlPO-36 analogues such as Mg2+ (or Zn2+) that
are not raised to higher oxidation states upon calcination in O2
yield conversions not significantly different from those of the
Mukaiyama sacrificial oxidations with benzaldehyde alone (see
Table 1, entries 5 and 7). Secondly, in a parallel experiment
(Table 1, entry 6) using the smaller pore CoALPO-18 (or
MnALPO-18), where all the transition-metal ions are in the +3
oxidation state,7,11 and the diameter of micropores is 3.8 Å, no
catalytic conversion of cyclohexanone to e-caprolactone occurs
when benzaldehyde is used as a sacrificial oxidant because the
In 1991 Mukaiyama and co-workers reported1 that various
aldehydes could be smoothly oxidised to their corresponding
carboxylic acids by molecular oxygen in the presence of
NiII(dmp)2 catalysts [Hdmp = 1,3-bis(p-methoxyphenyl)pro-
pane-1,3-dione]. They also reported2 an efficient method for
epoxidising olefins using the same catalyst, O2 and sacrificial
aldehydes (Scheme 1).
These conditions are of considerable interest in that they offer
attractive alternatives to the use of environmentally less
acceptable oxidants (many of which function stoichiometrically
rather than catalytically) such as CrO3, KMnO4, Pb(OAc)4,
RuO4 Ag2O and Co(acac)3.3 H2O2 in the presence of an
appropriate catalyst (e.g. by methyltrioxorhenium4) is also a
good oxidant for the conversion of olefins to epoxides and
cyclic ketones to lactones5 (Scheme 2).
In pursuit of our goal to design and synthesise microporous
solid catalysts for the selective aerobic oxidation of hydro-
carbons, we have found that transition metal ion (framework)
substituted aluminophosphates (MAlPO, where M = transition
metal ions) can effectively convert alkanes6–8 and aldehydes to
their corresponding carboxylic acids. This prompted us to
investigate the potential of these microporous solids for
oxidation of ketones, in the presence of a sacrificial aldehyde,
using air as an oxidant. Sacrificial aldehydes for the conversion
of ketones to lactones (an example of Baeyer–Villiger oxida-
tion) have been shown to be effective using various homoge-
neous catalysts.
Several heterogeneous catalytic systems based on hydro-
talcites9 or heteropolyoxometalates10 have been used to effect
Baeyer–Villiger oxidations. However, to the best of our
knowledge, there are no reports of the use of microporous redox
molecular sieves for the shape-selective conversion of ketones
to lactones. Taking account of the redox properties11 and pore
dimensions of a range of metal ion substituted AlPOs, we have
identified three different structures of potential catalysts that
contain small amounts (up to 4 atom%) of either manganese or
cobalt redox cations. The catalysts we have used are CoALPO-
36, MnALPO-36, CoAlPO-5, MnAlPO-5, CoALPO-18 and
Scheme 1
Scheme 2
Fig. 1 Representation of the AlPO-36 catalyst, where one of the aluminium
sites is occupied by manganese. The reaction scheme of the formation of the
perbenzoic acid intermediate from benzaldehyde effected by MnIII ions and
molecular O2 is also shown.
Chem. Commun., 1999, 525–526
525