OMS-2 for aerobic, catalytic, one-pot alcohol oxidation-wittig reactions: Efficient access to α,β-unsaturated esters

Article abstract of DOI:10.1002/cctc.201300942

Manganese oxide octahedral molecular sieve (OMS) materials with well-defined pores have been extensively studied over two decades due to their intriguing chemical and physical properties. OMS-2, the synthetic cryptomelane form of manganese oxide, was synthesized by a modified reflux method and was found to be highly active for obtaining α,β-unsaturated esters (up to 95 % yield and with high diastereoselectivities) from a variety of benzyl, heteroaryl, allyl and alkyl alcohols via one-pot alcohol oxidation-Wittig reaction. The transformation utilizes air as the stoichiometric oxidant, and the inexpensive catalyst can be recovered and reused. Filter and use again! Porous manganese oxide molecular sieve based catalysts were found to efficiently promote the oxidation of a variety of alcohols to the aldehydes, which reacted insitu with stabilized Wittig reagent, providing almost exclusively E-α,β-unsaturated esters in good to excellent yields. The heterogeneous catalyst used was made from inexpensive starting materials, and the recovered catalyst was found to be reusable with a modest loss in activity.

Full text of DOI:10.1002/cctc.201300942

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DOI: 10.1002/cctc.201300942
OMS-2 for Aerobic, Catalytic, One-pot Alcohol Oxidation-
Wittig Reactions: Efficient Access to a,b-Unsaturated
Esters
Jagadeswara R. Kona,[a] Cecil K. King’ondu,[a, c] Amy R. Howell,*[a] and Steven L. Suib*[a, b]
Manganese oxide octahedral molecular sieve (OMS) materials
with well-defined pores have been extensively studied over
two decades due to their intriguing chemical and physical
properties. OMS-2, the synthetic cryptomelane form of manga-
nese oxide, was synthesized by a modified reflux method and
was found to be highly active for obtaining a,b-unsaturated
esters (up to 95% yield and with high diastereoselectivities)
from a variety of benzyl, heteroaryl, allyl and alkyl alcohols via
one-pot alcohol oxidation-Wittig reaction. The transformation
utilizes air as the stoichiometric oxidant, and the inexpensive
catalyst can be recovered and reused.
The first report of this one-pot, two reaction sequence came
from Ireland and Norbeck[1] using a Swern oxidation, followed
by a Wittig reaction. Later, Dess–Martin periodinane,[2] IBX,[3]
PCC,[4] SO3·Py,[5] TEMPO-BAIB,[6] which are homogeneous, and
BaMnO4,[7] MnO2,[4b,8] NBS or NIS (NBS: N-bromosuccinimide;
NIS: N-iodosuccinimide) in combination with MnO2 (ꢀ10–
40 equivalents),[9] which are heterogeneous, have been used as
oxidants. Catalytically, Ru-complexes,[10] Ni nanoparticles,[11] and
Ru/AlO(OH),[12] are active in these reactions. The aforemen-
tioned reactions, whether stoichiometric or catalytic, are per-
formed in halogenated solvents (CH2Cl2 and CHCl3) for long re-
action times. In some cases bases as additives are used. In ad-
dition, toxic and stoichiometric reagents are often used. The
catalytic procedures involve mostly noble metal based cata-
lysts, which are very expensive. Herein, we report an alterna-
tive benign approach that is aerobic, catalytic, and highly effi-
cient for the synthesis of valuable a,b-unsaturated esters in
high yields. Moreover, the catalyst is easily prepared from inex-
pensive starting materials.
From an economical and environmental point of view, one-pot
multi-step chemical transformations are attractive because of
the reduction in the number of work-ups, purifications, and
subsequent cost for waste disposal. Moreover, if the transfor-
mation is performed under air with heterogeneous, recovera-
ble, and reusable catalysts made from inexpensive and non-
toxic starting materials, the economic and environmental ad-
vantages are further multiplied. One-pot, tandem oxidation-
Wittig reactions are powerful tools for the efficient generation
of alkenes through the oxidation of primary alcohols to alde-
hydes, followed by “in situ” reaction of the carbonyl with phos-
phorus ylides [Eq. (1)].
Octahedral molecular sieve (OMS) materials of manganese
oxide are crystalline, porous, mixed valent, and semi-conduc-
tive and have shown diverse applications, including adsorp-
tion, battery materials, ion-exchange, and catalysis. OMS-2
(Figure 1), the synthetic cryptomelane form of manganese
oxide with the chemical composition KMn4+7Mn3+O16·nH2O,
has a 2ꢀ2 tunnel architecture, bearing edge and
corner shared MnO6 octahedral units with K+ and
H2O moieties as the tunnel occupants.[13] The tunnel
dimensions of OMS-2 materials were found to be
[a] J. R. Kona, Dr. C. K. King’ondu, Prof. A. R. Howell, Prof. S. L. Suib
Department of Chemistry
University of Connecticut
55 N Eagleville Road, Storrs, CT 06269 (USA)
Fax: (+1)860-486-2981
E-mail: steven.suib@uconn.edu
amy.howell@uconn.edu
[b] Prof. S. L. Suib
Institute of Materials Science
University of Connecticut
97 N Eagleville Road, Storrs, CT 06269 (USA)
[c] Dr. C. K. King’ondu
Department of Sustainable Energy Science and Engineering
Nelson Mandela African Institution of Science and Technology
P. O. Box 447, Arusha (Tanzania)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300942.
Figure 1. Structure of OMS-2 (CrystalMakerꢁ representation).
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4.6ꢀ4.6ꢀ6.5 ꢃ with an average oxidation state of 3.8 for Mn,
due to Mn4+ major and Mn3+ minor oxidation states. Since its
initial preparation in 1994,[14] OMS-2 has captured broad atten-
tion because of its outstanding catalytic activity. OMS-2 has
been used in the oxidation of alcohols,[15] dehydrogenations,[16]
the epoxidation of alkenes,[17] epoxide ring opening,[18] the syn-
thesis of imines from alcohols,[19] the synthesis of 2-aminodi-
phenylamine,[20] quinoxaline synthesis,[21] the oxidation of cy-
clohexane,[22] dye degradation,[23] the oxidation of 9H-fluo-
rene,[24] water oxidation,[25] and the synthesis of amides from al-
cohols.[26]
Table 1. Optimization of benzyl alcohol oxidation-Wittig reaction.[a]
Entry
OMS-2
[mg]
Gas
t
[h]
Conversion[b]
[%]
1
2
3[c]
4[d]
5
15
15
15
15
15
10
N2
Ar
air
air
air
O2
4
4
4
4
4
4
30
32
2
39
>99
>99
6
The one-pot oxidation-Wittig reaction of benzyl alcohol was
studied (Table 1) using different solvents and gases with OMS-
2 to optimize the reaction conditions. Under an atmosphere of
the inert gases, nitrogen or argon (Table 1, Entries 1–2), a maxi-
mum conversion of 32% to the corresponding a,b-unsaturated
ester was observed. In the presence of air (Table 1, Entry 5), the
reaction proceeded smoothly under toluene to give the ester,
with very high conversion (>99%). Similarly, high conversion
to a,b-unsaturated ester was ob-
[a] Reaction conditions: benzyl alcohol (1.0 mmol), Ph3P=CH-COOEt
(1.0 eq), OMS-2, toluene (5.0 mL), and reflux at 1108C in gas (balloon).
[b] Based on GC-MS. [c] 1,4-dioxane, reflux at 1018C. [d] acetonitrile,
reflux at 858C.
tained with oxygen. However,
when oxygen was used, the re-
action proceeded to complete
conversion with lower catalyst
loading (Entry 6, Table 1). Since
air is greener and more econom-
ical than oxygen and because of
safety concerns associated with
solvents at elevated tempera-
tures under O2, air was chosen
as the co-oxidant for most of
the oxidation-Wittig reactions.
The substrate scope of the re-
action was examined with
Table 2. OMS-2 catalyzed one-pot alcohol oxidation-Wittig reactions.[a]
Entry
Reactant
Product
Yield[b]
[%]
Selectivity[c]
E:Z
TON[d]
1
2
89
90
96:4
97:3
49
29
3
4
5
6
7
8
91
80
87
95
90
91
97:3
98:2
92:8
94:6
94:6
95:5
15
29
21
61
25
37
a
wide range of alcohols
(Table 2). The catalyst loading
was adjusted for each substrate
to provide complete conversion
in 4 h. In all cases 100% selectiv-
ity for the formation of a,b-unsa-
turated esters was observed. The
isolated yields were excellent
with the lowest yields coming
from products that were some-
what volatile (Entries 13, 14, 17,
and 18) and the reactants that
were difficult to oxidize (En-
tries 19 and 20). The selectivity
for the (E)-isomer was generally
very high (>9:1, except En-
tries 16 and 20).
9[e]
10
11
85
92
91
99:1
97:3
96:4
8
29
18
The turnover numbers (TON)
represent the efficiency of the
OMS-2 in oxidizing a particular
substrate. All of the reactions
went to completion, except
those involving 1-octanol and 5-
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formation. However, the Wittig
reagent must accelerate the re-
moval of the aldehyde and,
therefore, the rate of oxidation.
There are several things
about the catalyst that are
worth noting. First, the catalyst
preparation is very straightfor-
ward and is slightly modified
from our earlier descriptions.[14]
Mn(OAc)2, rather than MnSO4, is
used as the MnII source, and
concentrations of reagents are
varied (refer to supporting infor-
mation for full description). The
OMS-2 surface area is greater
(115 m2 gÀ1) than our earlier
(98 m2 gÀ1) description.[15f] To
the best of our knowledge, this
is the highest reported surface
area for the regular OMS-2 ma-
terials prepared under reflux
conditions without co-solvents.
This is likely a result of the al-
tered morphology, which con-
sists of discrete bundles of
nanorods, rather than long
fiber-like structures. As with the
previously described OMS-2, an
important feature is that the
catalysis is heterogeneous. After
Table 2. (Continued)
Entry
Reactant
Product
Yield[b]
[%]
Selectivity[c]
E:Z
TON[d]
12
83
98:2
18
13
14
72
77
92:8
94:6
12
12
15
16
82
79
90:10
77:23
12
15
17
68
70
74
65
91:9
97:3
94:6
82:18
12
12
8
18
19[f]
20[f]
8
[a] Reaction conditions: alcohol (1.0 mmol), Ph3P=CH-COOEt (1.0 eq), OMS-2, toluene (5.0 mL), and reflux at
110 8C for 4 h in air (balloon). [b] Isolated yield [c] Selectivity determined from GC-MS. [d] Turn over number
(TON)=(moles of alcohol converted)(moles of catalyst)À1
. Formula weight of OMS-2 (KMn8O16.nH2O) is
734.6 gmolÀ1 (excluding the water). [e] The reaction was performed in toluene (10.0 mL) for 9 h. [f] alcohol
(2.0 mmol), Ph3P=CH-COOEt (2.2 eq), toluene (5.0 mL), and 808C for 24 h in O2 (balloon).
methyl-1-hexanol. For the reactions that went to completion,
higher catalyst loading was needed for some substrates to
effect the complete conversion (>99%) in 4 h time. The reac-
tions of unactivated alkyl alcohols (Entries 19 and 20) that do
not go to complete completion require higher catalyst loading,
long reaction time (24 h) and an O2 atmosphere to effect the
maximum conversion. In general, with the exception of the
sterically congested 2,4,6-trimethyl substituted benzyl alcohol
(Entry 9), benzyl alcohols were more readily oxidized than het-
eroaromatic (Entries 12–14), allylic (entries 15 and 17), propar-
gylic (Entry 16), and alkyl (Entries 18–20) alcohols. The efficien-
cies do not appear to be related to the electronic nature of
the aromatic rings, as simple electron rich (Entries 2 and 4) and
electron deficient (Entries 7 and 8) benzylic alcohols have simi-
lar TONs. Electron deficient (Entry 12) and electron rich (En-
tries 13 and 14) heteroaromatic compounds also do not differ
greatly in TON. The TONs suggest that shape/size play an im-
portant role in interactions with the catalyst. Overall, OMS-2
was found to be highly active for a wide variety of alcohols to
produce the a,b-unsaturated esters.
the oxidation-Wittig reaction was performed on benzyl alcohol,
OMS-2 was filtered and the reaction mixture was extracted
with 1% HNO3. The extract was analyzed with inductively cou-
pled plasma-atomic emission spectroscopy (ICP-AES), the con-
centration of potassium and manganese were at the ppb level,
indicating there was negligible metal leaching. The analysis
rules out catalysis due to leached metal species and confirms
that the oxidation was performed in a heterogeneous fashion.
Also, significantly, X-ray diffraction, scanning electron microsco-
py, and transmission electron microscopy data show that after
catalytic runs the composition does not change and that the
structure and morphology are largely unchanged.
Reusability of the catalyst, OMS-2, was studied (Table 3) on
piperonyl alcohol oxidation-Wittig reaction. After each cycle,
OMS-2 was separated by filtration and washed with toluene,
ethanol, and diethyl ether (ꢀ3.0 mL each) and dried at 2008C
for 2 h. The recovery of OMS-2 was above 90%. The filtrate
that contained crude reaction mixture was analyzed. There was
no significant loss in the catalytic activity even after the fourth
cycle.
If the Wittig reagent is not present, the rate of oxidation of
the alcohol is much slower. For example, when benzyl alcohol
was treated with OMS-2 under the conditions in Table 2, only
27% conversion resulted in 4 h. This suggests that the alde-
hyde remains in the vicinity of the catalyst subsequent to its
An aerobic, highly efficient, one-pot alcohol oxidation/Wittig
reaction for the synthesis of a,b-unsaturated esters has been
developed. The initial oxidation is effected with catalytic
amounts of OMS-2, synthesized by a modified precipitation
method. The method, which uses a non-halogenated solvent,
worked successfully for a wide variety of alcohols to produce
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cycle 4
>99
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Acknowledgements
This work was supported by the grant (DE-FG02-
86ER13622A0000) from the Department of Energy, Office of Basic
Energy, Division of Chemical Sciences, Bioscience and Geoscien-
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Mohammad for helpful discussions.
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Keywords: alcohols · heterogeneous catalysis · manganese
oxide · ylides · unsaturated esters
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