Full Papers
the silyl ether, probably because of the instability of the silyl
ether formed from this silane. Finally, the dehydration of the al-
cohol to the alkene occurs.
Several experiments were performed to support this mecha-
nism. The reduction of deoxyanisoin was performed using deu-
terated dimethylphenylsilane (PhMe Si-D) catalyzed by Conclusions
2
MoO Cl (H O) (10 mol%) in THF under reflux [Eq. (1)]. After
2
2
2
2
purification, the deuterated alkene 2 was isolated in 37% yield
We have developed a new and efficient method for the direct
and selective deoxygenation of aryl ketones to the correspond-
ing aryl alkenes with the PhSiH /MoO Cl (H O) (5–10 mol%)
1
[
Eq. (1)]. The H NMR spectrum of this compound shows a sin-
glet at d=6.93 ppm, which integrates only one proton (Sup-
3
2
2
2
2
porting Information) and confirms the incorporation of deuteri-
system. This is the first methodology for the deoxygenation of
carbonyl compounds catalyzed by oxomolybdenum com-
plexes, and the results obtained extend the scope of the use
of high-valence oxomolybdenum complexes as efficient cata-
lysts for the deoxygenation of ketones.
1
um in 2. In contrast, the singlet at d=6.93 ppm in the H NMR
spectrum of alkene 3 obtained in the reaction with PhMe SiH
2
integrates two protons (Supporting Information).
If we compare this methodology with our proce-
dure for the deoxygenation of aryl ketones with the
[
14]
silane/oxorhenium complex system,
this new
method is more selective for the deoxygenation of
ketones to alkenes in excellent yields. In contrast, our
previous method produced an inseparable mixture of
alkane and alkene with a low yield of alkene in the reaction of
several substrates. The use of oxomolybdenum complexes for
the deoxygenation of ketones instead of oxorhenium com-
plexes makes the deoxygenation of ketones more economic
and less toxic.
We also investigated the deoxygenation of the alcohol 4 in
the presence of MoO Cl (H O) in THF under reflux without the
2
2
2
2
addition of the reducing agent to obtain the alkene 3 in 79%
yield [Eq. (2)]. This result shows that the oxomolybdenum com-
plex promotes the dehydration of the alcohol to the corre-
sponding alkene [Eq. (2)].
This new method has the following advantages: (1) excellent
catalytic activity; (2) easy and inexpensive preparation of the
catalyst; (3) the use of an environmentally friendly catalyst;
(
4) MoO Cl (H O) is air stable in ether solution; (5) applicability
2 2 2 2
to a large variety of ketones; (6) the reactions can be per-
formed under an air atmosphere without using dry solvents;
(
7) the catalyst can be used in more than one catalytic cycle.
The formation of the methylene group in the reduction of
Then, we explored the catalytic activity of MoO Cl (H O) in
the flavone (Table 4, entry 12) suggests that the PhSiH3/
MoO Cl (H O) system can also be applied to the direct reduc-
2
2
2
2
the dehydration of the alcohols 5 and 7 without the addition
of any reducing agent, which afforded the corresponding sty-
rene in 88% yield and 4-bromostyrene in 73% yield, deter-
2
2
2
2
tive deoxygenation of ketones to the corresponding alkanes.
The dehydration of alcohols 4, 5, and 7 to the alkenes cata-
lyzed by MoO Cl (H O) in good yields without a reducing
1
mined by H NMR spectroscopy using 1,2-dimethoxyethane as
2
2
2
2
the internal standard [Eqs. (3) and (4)]. In contrast, no reaction
was observed in the deoxygenation of the primary alcohol 2-
phenylethanol. The results obtained demonstrated that
MoO Cl (H O) catalyzes the dehydration of secondary alcohols.
agent suggests that this catalyst can be an excellent, inexpen-
sive, easily available, and environmentally friendly alternative
[
19]
to the oxorhenium complexes reported recently for the de-
oxygenation of alcohols and polyols, which is a very useful re-
action to deoxygenate biomass resources.
2
2
2
2
We believe that the deoxygenation of alcohols catalyzed by
MoO Cl (H O) involves the coordination of the alcohol to Mo
Further mechanistic studies, which include computational
studies, and new applications of this method with other oxo
complexes, reducing agents, and substrates are under investi-
gation in our group.
2
2
2
2
with the release of a ligand and the transfer of two hydrogen
atoms, which results in the formation of the alkene and a mole-
cule of water. Gebbink and co-workers have also reported the
deoxygenation of alcohols to the corresponding alkenes using
several MoO (acac’) complexes as catalysts without the addi-
2
2
tion of a reducing agent, but the yields of the alkenes ob-
Experimental Section
[18]
tained are, in general, low.
Deoxygenation of aryl ketones to the corresponding alkenes
with the PhSiH /MoO Cl (H O) system
3
2
2
2
2
To a solution of MoO Cl (H O) (5–10 mol%) and ketone (1.0 mmol)
2
2
2
2
in THF (3 mL) was added PhSiH (2.0–4.0 mmol). The reaction mix-
3
ture was stirred under reflux under an air atmosphere in a closed
ChemCatChem 2015, 7, 3503 – 3507
3506
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