Manganese-catalyzed selective oxidation of aliphatic C-H groups and secondary alcohols to ketones with hydrogen peroxide

Article abstract of DOI:10.1002/cssc.201300378

An efficient and simple method for selective oxidation of secondary alcohols and oxidation of alkanes to ketones is reported. An in situ prepared catalyst is employed based on manganese(II) salts, pyridine-2-carboxylic acid, and butanedione, which provides good-to-excellent conversions and yields with high turnover numbers (up to 10 000) with H₂O₂ as oxidant at ambient temperatures. In substrates bearing multiple alcohol groups, secondary alcohols are converted to ketones selectively and, in general, benzyl C-H oxidation proceeds in preference to aliphatic C-H oxidation. Manganeasy! Mn^II, picolinic acid, and butanedione make a smart mix for oxidation of secondary alcohols and alkanes and especially benzylic groups to ketones at room temperature with near stoichiometric amounts of hydrogen peroxide, low catalyst loadings, and high turn-over numbers. Copyright

Full text of DOI:10.1002/cssc.201300378

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DOI: 10.1002/cssc.201300378
Manganese-Catalyzed Selective Oxidation of Aliphatic CÀH
groups and Secondary Alcohols to Ketones with Hydrogen
Peroxide
Jia Jia Dong,[a] Duenpen Unjaroen,[a] Francesco Mecozzi,[a] Emma C. Harvey,[a]
Pattama Saisaha,[a] Dirk Pijper,[a] Johannes W. de Boer,[b] Paul Alsters,[c] Ben L. Feringa,[a] and
Wesley R. Browne*[a]
An efficient and simple method for selective oxidation of sec-
ondary alcohols and oxidation of alkanes to ketones is report-
ed. An in situ prepared catalyst is employed based on manga-
nese(II) salts, pyridine-2-carboxylic acid, and butanedione,
which provides good-to-excellent conversions and yields with
high turnover numbers (up to 10000) with H2O2 as oxidant at
ambient temperatures. In substrates bearing multiple alcohol
groups, secondary alcohols are converted to ketones selective-
ly and, in general, benzyl CÀH oxidation proceeds in prefer-
ence to aliphatic CÀH oxidation.
Introduction
Selective oxidation of alcohols to ketones together with direct
conversion of CÀH bonds to alcohols and ketones are crucial,
yet also highly challenging, processes in synthetic organic
chemistry, pharmaceuticals, and fine and bulk chemical synthe-
sis (Scheme 1).[1] Traditional methods for achieving such trans-
Several highly effective catalytic methods for the oxidation
of alcohols with H2O2 have been reported, not least the system
of Noyori and co-workers, based on tungsten oxide/PTC
(where PTC is phase transfer catalyst),[5] and the systems of
Beller and co-workers, employing iron catalysts[6] and catalysts
based on rhenium, molybdenum, and tungsten oxides.[7]
However, the use of high temperatures and/or catalyst load-
ings and potentially toxic PTCs drives the search for alternative
methods. Manganese-based catalysts are attractive in CÀH and
alcohol oxidations because of their generally low toxicity and
the often high reaction rates and turnover numbers that can
be achieved even at room temperature. Manganese catalysts,
based on salen ligands[8] have been employed in the oxidation
of alcohols with iodosobenzene as oxidant[8a] and catalyzed
enantioselective kinetic resolution of secondary alcohols.[8b]
Manganese catalysts based on polypyridyl and triazacyclono-
nane-based ligands,[9–11] as well as porphyrins,[12] have also
been applied.[13]
Scheme 1. Oxidation of alkanes and alcohols to ketones.
formations, although highly effective in general, are becoming
increasingly undesirable because of rising demands to reduce
environmental impact and increase the mass and energy effi-
ciency of processes. Hence the drive to develop benign meth-
ods compels us to explore the use of methods using 1st row
transition-metals and clean (mass-efficient) oxidants.[2] Hydro-
gen peroxide is a highly favorable oxidant in this regard,
second only to oxygen,[3] with water as the sole by-product.[4]
Recently, our group reported an efficient method for the ep-
oxidation and cis-dihydroxylation of alkenes catalyzed by an
in situ prepared manganese(II) catalyst and which is near-stoi-
chiometric in H2O2.[14–16] This catalytic system consists of a MnII
salt, pyridine-2-carboxylic acid, and sub-stoichiometric amounts
of ketone, and showed good-to-excellent selectivity, high turn-
over numbers (up to 300000) and high turnover frequencies
(up to 30 sÀ1) at room temperature, with a wide solvent
scope.[16]
[a] J. J. Dong, D. Unjaroen, F. Mecozzi, Dr. E. C. Harvey, Dr. P. Saisaha,
Dr. D. Pijper, Prof. Dr. B. L. Feringa, Prof. Dr. W. R. Browne
Stratingh Institute for Chemistry, University of Groningen
Nijenborgh 4, 9747 AG, Groningen (The Netherlands)
Fax: (+31)50-363-4296
E-mail: w.r.browne@rug.nl
Here, we report the application of this catalytic system to
the oxidation of alcohols and alkanes under ambient condi-
tions and low catalyst loadings (Scheme 2). For substrates that
do not bear alkene moieties, we show that high yields and se-
lectivity can be achieved in the oxidation of secondary alcohols
with good-to-excellent selectivity of secondary over primary al-
cohol oxidation. Furthermore, at higher catalyst loadings
[b] Dr. J. W. de Boer
Catexel BV BioPartner Center Leiden
Galileiweg 8, 2333 BD Leiden (The Netherlands)
[c] Dr. P. Alsters
DSM Innovative Synthesis
PO Box 18, 6160 MD Geleen (The Netherlands)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201300378.
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hexanone was obtained. Importantly, the reactions proceeded
without formation of significant amounts of side products
(e.g., Baeyer–Villiger oxidation products and double oxidation).
These conditions were applied to a series of secondary alco-
hols (Table 1). Cyclic and acyclic aliphatic alcohols were con-
verted cleanly to their corresponding ketone products (Table 1,
entries 2 and 3). For 5-nonanol, the conversion achieved was,
however, lower, which was ascribed to its lower solubility in
acetonitrile compared with the other alcohols examined. The
relatively wide solvent scope of the present catalyst system,
which was previously demonstrated,[16] allows for the low solu-
bility of 5-nonanol to be overcome by using acetone in place
of acetonitrile and provided 5-nonanone in high yield (Table 1,
entry 4).
Scheme 2. Oxidation of cyclohexanol to cyclohexanone.
(0.1 mol%) selective CÀH oxidation at benzylic positions can
be achieved as well as CÀH oxidation of alkanes.
Results and Discussion
In the present study, the oxidation of alcohols was initially in-
vestigated using conditions optimized earlier for alkene oxida-
tion (Scheme 2),[16] that is, using substrate (0.5m), manganese
perchlorate (0.01 mol%), sodium acetate (1.0 mol%), pyridine-
2-carboxylic acid as catalyst (0.5 mol%), and butanedione
(0.5 equiv. in acetonitrile) with H2O2 (1.5 equiv.). Under these
conditions, cyclohexanol was oxidized to cyclohexanone in
73% yield, albeit with incomplete conversion (Table 1, entry 1).
With 3.0 equiv of H2O2, full conversion and 91% yield of cyclo-
The oxidation of sterically encumbered alcohols was also in-
vestigated. 2,4-dimethyl-pentan-3-ol was converted (71%) with
68% yield to 2,4-dimethyl-pentan-3-one (Table 1, entry 5). Simi-
larly the natural product isoborneol was converted to camphor
in excellent yield (Table 1, entry 6). The reactions in general
were found to be scalable, without significant difference in
conversion or yield (see the Supporting Information for de-
tails). Notably, at larger scale, complete conversion was ach-
ieved by extraction of the product and unreacted starting ma-
terial from the reaction mixture and subjecting the mixture to
the same reaction conditions a second time. In the case of iso-
borneol at a 4 g scale, full conversion and an isolated yield of
87% was achieved.
Table 1. Oxidation of secondary alcohols.[a]
Entry[a] Substrate
Conv.[b] [%] Product
Isolated Yield [%][c]
91[b]
full
(73%)
A series of secondary benzylic alcohols were oxidized to
their corresponding ketones under these reaction conditions,
in good yields (Table 1). Even bromo-phenyl or oxidatively sen-
sitive methoxy-phenyl-bearing substrates proceeded in moder-
ate-to-good conversion. In general, lower conversion can be
overcome by decreasing the initial concentration of the sub-
strate as with,[16] for example, 1-(4’-methoxyphenyl)ethanol
(Table 1, entry 11).
1
(73[d]
)
2[e]
full
78
3[e]
4[f]
97
80
88
72
5
6
71
68[b]
95
The oxidation of hemiacetals was explored through tetrahy-
dro-2H-pyran-2-ol, which can be viewed as a cyclic alcohol
with an ether functional group. Full conversion and moderate
yield (44%) of valerolactone was achieved. The moderate yield
of the desired product was primarily a result of the ring open-
ing to generate the corresponding carboxylic acid in situ
(Scheme 3).
full
7
97
92
90
88
90
77
80
75
8
9
The selective oxidation of secondary aliphatic and aromatic
alcohols in the presence of primary alcohols was explored
through competition experiments (Schemes 4 and 5). Oxida-
tion of an equimolar mixture of 1- and 2-phenyl-ethanol pro-
vided 75% conversion of 1-phenyl-ethanol to acetophenone
with only trace conversion of 2-phenyl-ethanol. For substrates
10
11[g]
12
70
78
64
76
[a] Reaction conditions: 0.5m substrate (1 mmol), 50 mm Mn(ClO4)2·6H2O,
2.5 mm pyridine-2-carboxylic acid, 5.0 mm NaOAc, 0.25m butanedione,
and 1.5m H2O2 in acetonitrile. [b] Conversion and yield, based on sub-
strate, determined by 1H NMR spectroscopy. [c] Isolated yield, unless
stated otherwise. [d] 0.75m H2O2 (1.5 equiv. with regard to substrate) was
used. [e] 0.4m substrate (0.8 mmol). [f] In acetone. [g] 0.25m substrate
(0.5 mmol).
Scheme 3. Oxidation of tetrahydro-2H-pyran-2-ol to valero-lactone (yield de-
termined by H NMR spectroscopy).
1
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trast, oxidation of toluene proceeded with lower con-
version (35–40%) and low yield of a mixture of
benzyl alcohol (8–10%), benzaldehyde (8–10%) and
benzoic acid and with a low mass balance as a result
of the formation of an insoluble polymeric material.
1-Ethyl-4-methyl-benzene was found to undergo
good conversion (70%) with moderate yield of 4-
methyl-acetophenone as the main product, albeit
with significant amounts of
Scheme 4. Competition experiment between 1-phenyl- and 2-phenyl-ethanol (for condi-
tions, see Table 1).
a
white polymeric by-product
(Scheme 7). In general, for cyclic
systems, full conversion was ach-
ieved with moderate-to-excellent
isolated yields of the corre-
sponding ketone (Table 2, en-
tries 5–9). Surprisingly, in many
cases, substantial amounts of
the intermediate alcohol product
were obtained, which would
suggest that the rate of CÀH oxi-
dation, although slower, is nev-
Scheme 5. Chemoselective oxidation of secondary over primary alcohol moieties (for conditions, see Table 1).
bearing both secondary and primary aliphatic alcohol moieties,
oxidation of the secondary alcohol also proceeded preferential-
ly (Scheme 5).
ertheless comparable with the rate of alcohol oxidation. The
doubly oxidized product, anthracene-9,10-dione, was obtained
from 9,10-dihydroanthracene in excellent yield, (Table 2,
entry 7). In contrast, the mono-oxidized alcohol and ketone
products were obtained as the primary product in the oxida-
tion of 9,10-dihydrophenanthrene (Table 2, entry 9).
In addition to the oxidation of secondary alcohols to ke-
tones, the direct oxidation of methylene units to ketones with
the present catalytic system was explored.
The direct oxidation of aliphatic compounds such as cyclo-
hexane and cyclooctane proceeded with good conversion and
selectivity to the corresponding mono-ketone product.
(Table 2, entries 1 and 2). In contrast to alcohol oxidation, how-
ever, higher catalyst loadings (0.1 mol%) and up to 4.0 equiv.
of H2O2 were required (Scheme 6). In addition to the ketone
The selective oxidation of 1,2,3,4-tetrahydro-1-naphthol to
the corresponding ketone, without further oxidation at the
other benzylic position, confirms that benzylic positions are
marginally less susceptible to oxidation than secondary aryl al-
cohols (Table 1, entry 10). Nevertheless, if there is sufficient oxi-
dant available, further oxidation to the diketone products is
observed.
p-Methoxy- and o-nitro-phenylethanes could be converted
to the corresponding aryl-methyl-ketones as well (Table 2,
entry 10 and 11). The lower yield of 1-(4-methoxy)phenyl-etha-
none reflects the lower conversion also observed for the corre-
sponding alcohol 1-(4-methoxyphenyl)ethanol (Table 1,
entry 11). Similarly, lower conversion was observed for o-nitro-
phenylethane (Table 2, entry 11).
Scheme 6. Oxidation of cyclohexane to cyclohexanone under optimized re-
action conditions.
Surprisingly, oxidation of 2-benzyl-pyridine to phenyl(pyri-
din-2-yl)methanone proceeded with good conversion and se-
lectivity without formation of the N-oxide product (Table 2,
entry 12).
product, the corresponding alcohol, together with a mixture of
diketones, was observed in amounts that were dependent on
the exact reaction conditions.
At
catalyst
loadings
of
0.1 mol%, the oxidation at the
benzylic positions for a wide
range of alkylated aromatics
could be achieved at room tem-
perature (Table 2, entries 3–12).
For both ethylbenzene and pro-
pylbenzene, the corresponding
aromatic ketone product was
obtained with good selectivity
Scheme 7. Oxidation of 1-ethyl-4-methyl-benzene to 4-methyl-acetophenone under optimized reaction
(Table 2, entries 3 and 4). In con- conditions.
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siderably more oxygen; however, this can also be dis-
counted because of the relatively low level of cata-
lase type activity observed. Hence, the presence of
both alcohol and ketone products can be ascribed to
sequential oxidation rather than, for example, a Rus-
sell’s mechanism[17] between alkyl radicals and
oxygen.
Table 2. Oxidation at alkyl and benzylic positions.[a]
Entry Substrate
Conv.
[%]
Product/s (isolated yield [%])[b]
1
2
63
60
(34[c])
(41[c])
3
4
80
50
(50[c])
(34)
(13[c])
(3[c])
Conclusions
Herein, we demonstrate that selective oxidation of
secondary alcohols can be achieved at room temper-
ature with an in situ prepared manganese catalyst
with high turnover numbers (up to 10000) and with
near stoichiometric amounts of H2O2. The reaction is
scalable from 100 mg to 4 g and, in many examples,
highly selective. Although we have demonstrated
previously that this catalyst system is tolerant of sev-
eral common protecting groups,[16] the selectivity of
the catalyst towards secondary alcohols over primary
alcohols is also demonstrated, which reduces the
need for the introduction prior to oxidation and sub-
sequent removal of protecting groups and is compli-
mentary to catalytic methods for selective primary al-
cohol oxidation based on copper and TEMPO-based
(TEMPO=2,2,6,6-tetramethyl-1-piperidinyloxyl) cata-
lysts.[3c,6a] For benzylic alcohol oxidation and in partic-
ular for cyclic systems, selective oxidation to the
monoketone product can be achieved under mild
conditions and with good efficiency in terms of the
oxidant H2O2.
5
6
full
full
(60)
(66)
(12[c])
(9[c])
(4[c])
(25)
7
full
(99)
8[d]
full
90
75
25
(76)
(37[c])
(28)
9
(50[c])
(17[c])
10
11
(17)
12
13
80
83
(73)
(34)
Experimental Section
[a] Reaction conditions: 0.5m substrate (1 mmol), 5 mm pyridine-2-carboxylic acid,
0.5 mm Mn(ClO4)2·6H2O, 10 mm NaOAc, 0.75m butanedione, and 2.0m H2O2 in aceto-
nitrile. [b] Isolated yield, based on substrate, unless stated otherwise. [c] 1H NMR yield,
based on substrate, see the Supporting Information for details. [d] 0.25m substrate
(0.5 mmol).
Caution: The drying or concentration of solutions that
potentially contain H2O2 should be avoided. Prior to
drying or concentrating, it is necessary to test for the
presence of H2O2 by using peroxide test strips followed
by neutralization on solid NaHSO3 or another suitable re-
ducing agent. When working with H2O2, suitable protec-
tive safeguards should be in place at all times.
The selectivity towards methylene over methyl CÀH groups
Caution: Exposure to butanedione vapors has been
linked with lung disease. It should be handled in a properly venti-
lated fumehood and exposure to vapors should be avoided.
was investigated using 1-methylpyrrolidin-2-one. Good conver-
sion (83%) to and moderate isolated yield (34%) of 1-methyl-
pyrrolidine-2,5-dione was achieved (Table 2, entry 13); however,
the selectivity was poor with several side products also ob-
served.
General procedure for oxidation of secondary alcohols
With regard to the mechanism for conversion of aliphatic
CÀH groups to alcohols and subsequently to ketones, it is
likely that an active species similar to that previously reported
in the oxidation of alkenes[15,16] is responsible. The involvement
of active oxygen species such as oxygen and hydroxyl radicals
should also be considered, however. The direct involvement of
atmospheric oxygen can be excluded based on mass balance;
at the conversions observed, it would not be possible to
obtain the oxygen required from the dissolved oxygen present.
The disproportionation of the H2O2 present could provide con-
The alcohol (1 mmol) was added to a stock solution containing
Mn(ClO4)2·6H2O (0.01 mol%, 0.0361 mg) and pyridine-2-carboxylic
acid (0.5 mol%, 0.123 mg) in acetonitrile to generate a final sub-
strate concentration of 0.5m. NaOAc (aq. 0.6m, 1 mol%, 16.7 mL)
and butanedione (0.5 equiv., 43.5 mL) were added to generate
a final volume of 2 mL. The solution was stirred with cooling in an
ice/water bath before addition of H2O2 (50 wt%, 3.0 equiv., 170 mL).
After 12–16 h stirring at room temperature, brine (10 mL) was
added and the reaction was extracted with dichloromethane. The
combined organic layers were reduced in vacuo. The products
were isolated by flash column chromatography on silica gel of
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spectroscopy (see the Supporting Information).
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General procedure for oxidation at alkyl and benzylic moiet-
ies CÀH
The alkane (1 mmol) was added to
a stock solution of
Mn(ClO4)2·6H2O (0.1 mol%, 0.361 mg) and pyridine-2-carboxylic
acid (1.0 mol%, 0.246 mg) in acetonitrile to generate a final sub-
strate concentration of 0.5m. NaOAc (aq. 0.6m, 2 mol%, 33.4 mL)
and butanedione (1.5 equiv., 130.5 mL) were added to generate
a final volume of 2 mL. The solution was stirred with cooling in an
ice/water bath before addition of H2O2 (50 wt%, 4.0 equiv., 227 mL).
After 12–16 h, brine (10 mL) was added and the reaction was ex-
tracted with dichloromethane (thrice 10 mL). The combined organ-
ic layers were reduced in vacuo. The products were isolated by
flash column chromatography on silica gel of mesh particle size
230–400. Products were characterized by NMR spectroscopy (see
the Supporting Information). In certain cases a solid material was
also obtained, which, on the basis of FTIR, Raman, and NMR analy-
sis, appeared to be a polymer.
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Acknowledgements
This research has been performed with the framework of the
CatchBio program. The authors thank the European Research
Council (Starting Investigator Grant 279549, FM, DU, WRB), the
University of Groningen (Ubbo Emmius studentship, PS), the
Foundation for Technology and Science (STW Grant No. 11059,
JWdB, WRB) and Smart Mix program of the Netherlands Ministry
of Economic Affairs and the Education, Culture and Science,
DSM, Organon and Avantium (J.J.D., E.C.H., B.L.F, W.R.B.) for fi-
nancial support. COST action CM1003 is acknowledged for dis-
cussion.
[14] D. Pijper, P. Saisaha, J. W. de Boer, R. Hoen, C. Smit, A. Meetsma, R.
Hage, R. P. van Summeren, P. L. Alsters, B. L. Feringa, W. R. Browne,
Dalton Trans. 2010, 39, 10375–10381.
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sters, R. Hage, B. L. Feringa, W. R. Browne, Org. Biomol. Chem. 2010, 8,
4444–4450.
Keywords: alcohol oxidation · alkane oxidation · catalysis ·
manganese
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W. R. Browne, ACS Catal. 2012, 2, 1087–1096.
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Received: April 22, 2013
Revised: June 23, 2013
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These are not the final page numbers!

FULL PAPERS
J. J. Dong, D. Unjaroen, F. Mecozzi,
E. C. Harvey, P. Saisaha, D. Pijper,
J. W. de Boer, P. Alsters, B. L. Feringa,
W. R. Browne*
Manganeasy! MnII, picolinic acid, and
butanedione make a smart mix for oxi-
dation of secondary alcohols and alka-
nes and especially benzylic groups to
ketones at room temperature with near
stoichiometric amounts of hydrogen
peroxide, low catalyst loadings, and
high turn-over numbers.
&& – &&
Manganese-Catalyzed Selective
Oxidation of Aliphatic CÀH groups
and Secondary Alcohols to Ketones
with Hydrogen Peroxide
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 6
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These are not the final page numbers!