1
76
J. Robertson, R.S. Srivastava / Molecular Catalysis 443 (2017) 175–178
O
OH
MTO, Re
(Red.= PPh , H , P(OPh)
2 3
2 7
O
+
H O
R1
+
Red
2
R
R
R'
Ln
(VI)
R
3
R'
Mo
O
O
Scheme 1. Deoxygenation of epoxides with Re catalysts.
L
L
(
IV)
Mo
(VI)
Mo
Table 1
OH
H
◦
Deoxygenation of styrene oxide at 160 C for 24 h.
O
O
O
O
Entry
Catalysts
Styrene (%)
R1
1
2
3
4
5
6
7
Molybdenum metal powder
Sodium molybdate
MoO3
MoO2(acac)2
Ammonium molybdate
Mo(CO)6
Trace
Trace
Trace
2
R
O
5
Ln
24 (ethyl benzene trace amount)
75
(IV)
MoO2(dtc)2
Mo
O
R
O
L
(
IV)
O
Mo
Based on recent studies on the use of rhenium and ruthe-
nium complexes as valuable catalysts for organic reactions [26],
we initiated a project studying the deoxygenation of epoxides with
inexpensive, commercially or readily available molybdenum cata-
lysts and practical reductants. Molybdenum compounds are much
cheaper than rhenium and ruthenium compounds and the Mo-
compounds are mostly commercially available or can be easily
made. Herein, we now report the inexpensive molybdenum cat-
alyzed deoxygenation of epoxides to alkenes using a relatively
small amount of a sacrificial alcohol as a reductant under relatively
mild reaction conditions.
H O
2
R1
R
O
R1
Scheme 2. Proposed catalytic cycle for MoO2(dtc)2-catalyzed alcohol driven deoxy-
genation reaction. Possible reaction mechanism.
Deoxygenation of styrene oxide was carried out as a model sub-
◦
2
. Results and discussion
strate in anhydrous toluene at 160 C in the presence of MoO (dtc)
2
2
(10 mol% – Mo) with 2,4-dimethyl-3-pentanol as a reductant that
Our first efforts focused on the investigation of inexpensive
produced styrene in 75% yield for 24 h. The addition of 4 A˚ molecular
sieve does not change the product yield. Next we optimized a range
of temperatures and found that 160 C is the best temperature for
molybdenum-based catalysts for the deoxygenation of styrene
oxide to styrene. Subsequently we concentrated on the exam-
ination of various commercially available or easily synthesized
molybdenum compounds for the deoxygenation of epoxides
◦
the model substrate for optimum yield of alkene. Temperature for
the catalytic reaction involving less reactive epoxide was raised up
◦
(
Table 1). Molybdenum metal (Table 1, entry 1) showed little activ-
ity in the deoxygenation of styrene oxide, as did sodium molybdate
Table 1, entry 2). Likewise, molybdenum (VI) oxide, MoO (Table 1,
to 190 C, to get better yield of alkenes.
With the optimized reaction conditions in hand, a variety of
epoxides varying from terminal to cyclic epoxides were subjected
to the deoxygenation reaction (Table 2). The deoxygenation of all
the activated epoxides (bearing a phenyl group) such as styrene
oxide, ethyl-3-epoxyglacidate, 1,2-epoxy-3-phenoxypropane, and
␣,-epoxy carbonyl compounds produce better deoxygenated
products than inactivated linear and cyclic epoxides. Thus we found
that when styrene oxide subjected to deoxygenation in the pres-
(
3
entry 3), MoO (acac) (Table 1, entry 4), and ammonium molyb-
2
2
date (Table 1, entry 5) showed little or no activity in deoxygenation
reaction. However, Mo (CO)6 (Table 1, entry 6) and MoO (dtc)2
2
(
Table 1, entry 7) showed relatively good activity for deoxygenation
of styrene oxide. Since the MoO2 (dtc)2 was the best performing
catalysts tested in our deoxygenation of styrene oxide, we decided
to adapt the MoO (dtc) in order to optimize the catalytic behavior
ence MoO (dtc)2 catalyst, 2,4-dimethyl-3-pentanol as reductant
2
2
2
◦
of these complexes.
in the anhydrous toluene at 160 C produced styrene in 75% yield
After optimization of catalysts, we started to study the prelim-
inary optimization of reaction conditions for the deoxygenation
of epoxide with respect to solvent, reductant, and temperature.
(Table 2, entry 1). However, the terminal epoxide, 1,2-epoxyhexane
having an aliphatic group is very sluggish and produced only a
trace amount alkene (Table 2, entry 2). We also examined func-
tional group tolerance of the epoxides. Accordingly, we examined
the water-sensitive ether and ester bond tolerance under these
reaction conditions. Catalytic reaction of 1,2-epoxy-3-phenoxy
propane, a phenolic ether, is slow and produced a low yield of
alkene, 25% (Table 2, entry 3). The reaction with disubstituted
epoxides (ethyl-3-phenylglycidate) occurred smoothly with 53%
yields of alkene (Table 2, entry 4). The dicarbonyl epoxide such as
(2R,3S)-dimethyl oxirane-2,3-dicarboxylate bearing an ester link-
age was sluggish and produced the corresponding alkene in 17%
An initial reaction was originated with styrene oxide, MoO (dtc)2
2
(
3
10 mol%) in the presence and of sacrificial alcohol (2,3-dimethyl-
-pentanol), a good amount of styrene was produced. No product
were detected in the absence of 2,3-dimethyl-3-pentanol. The other
n
alcohols such as MeOH, EtOH, PrOH, 1-butenol, 1-octanol, Hexyl
alcohol, 2-methyl-2-butanol, iso-amyl-alcohol, and 3-methyl-1-
butanol, which acts as reductant as well as a solvent, gave alkenes
in poor yields. Likewise reaction in diols and triols such as ethy-
lene glycol, 1,2-propanediol, and glycerol produces large amounts
of uncharacterized byproducts in GC-MS. The attempted deoxy-
genation of styrene oxide in water showed no conversion and not
even traces of any of the expected products detected. Anhydrous
and hydrated toluene, THF, and dichloroethane were also exam-
ined for deoxygenation of styrene oxide and finally we found that
anhydrous toluene produces better yield than other solvents inves-
tigated.
◦
◦
for 24 h at 160 C and 33% for 48 h at 190 C (Table 2, entry 5).
The ␣,  -carbonyl (trans-1,3-diphenyl-2,3-epoxypropan-1-one),
a keto epoxide produces ␣, -unsaturated ketone in excellent
yields (Table 2, entry 6). Similarly the cyclic epoxides such as 1,2-
epoxycyclododecane and cyclohexeneoxide are not very active and
produce small amounts of the corresponding alkene in 15% and 12%
◦
respectively at 190 C for 24 h (Table 2, entry 7 and 8). Trans-stilbene