CHEMCATCHEM
COMMUNICATIONS
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
ꢂ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 749 – 752 751