Iron-catalyzed oppenauer-type oxidation of alcohols

Article abstract of DOI:10.1002/adsc.200900896

A (hydroxycyclopentadienyl)iron dicarbonyl hydride catalyzes the Oppenauer-type oxidation of alcohols with acetone as the hydrogen acceptor. Many functional groups are tolerant to the oxidation conditions. The same complex also catalyzes the dehydrogenation of diols to lactones. A mechanism involving the formation of iron-alcohol complexes and their rapid ligand exchange with free alcohols is proposed. The trimethylsilyl groups on the cyclopentadienyl ligand of the catalyst play a critical role in stabilizing the iron hydride and increasing the catalyst lifetime.

Full text of DOI:10.1002/adsc.200900896

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DOI: 10.1002/adsc.200900896
Iron-Catalyzed Oppenauer-Type Oxidation of Alcohols
Michael G. Coleman,a,b Alec N. Brown,a,c Brett A. Bolton,a and Hairong Guana,*
a
Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, OH 45221-0172, USA
Fax : (+1)-513-556-9239; phone: (+1)-513-556-6377; e-mail: hairong.guan@uc.edu
Current address: Department of Chemistry, Rochester Institute of Technology, Rochester, NY 14623-5603, USA
Current address: Department of Chemistry, University of Oregon, Eugene, OR, 97403-1253, USA
b
c
Received: December 23, 2009; Published online: April 7, 2010
Dedicated to Professor Charles P. Casey for his decades of services to the chemistry community.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900896.
Abstract: A (hydroxycyclopentadienyl)iron dicar-
bonyl hydride catalyzes the Oppenauer-type oxida-
tion of alcohols with acetone as the hydrogen ac-
ceptor. Many functional groups are tolerant to the
oxidation conditions. The same complex also cata-
lyzes the dehydrogenation of diols to lactones. A
mechanism involving the formation of iron-alcohol
complexes and their rapid ligand exchange with
free alcohols is proposed. The trimethylsilyl groups
on the cyclopentadienyl ligand of the catalyst play a
critical role in stabilizing the iron hydride and in-
creasing the catalyst lifetime.
These catalytic reactions may be easy to operate, but
the structures of the actual catalysts are ill-defined,
providing little mechanistic information for further
catalyst improvement.
Recently, the Casey group has reported the first
well-defined iron system that catalyzes the efficient
and chemoselective hydrogenation of aldehydes and
ketones.[8] Detailed studies on the key step, the hydro-
gen transfer from a (hydroxycyclopentadienyl)iron di-
carbonyl hydride (1) to aldehydes, are consistent with
a mechanism involving a concerted transfer of proton
and hydride to the substrates outside the coordination
sphere of Fe (Scheme 1).[9] The resulting alcohol com-
plexes have shown low kinetic stabilities, as evidenced
by the rapid exchanges between the coordinated alco-
hols and free alcohols at room temperature. In addi-
tion, isotope-labeling experiments have suggested that
the hydrogen transfer step is reversible, implying that
the oxidation of alcohols catalyzed by 1 is feasible if
Keywords: alcohols; dehydrogenation; homogene-
ous catalysis; iron; metal hydrides; oxidation
The oxidation of alcohols to the corresponding car- an efficient hydrogen acceptor is employed.
bonyl products is one of the most fundamental and After a series of experiments aimed to identify the
important reactions in organic synthesis. Typical alco- conditions for the oxidation of alcohols catalyzed by
hol oxidation methods involve stoichiometric amounts 1, we found that the reactions were best carried out at
of chromium- or manganese-based oxidants that are 608C with 3 mol% of 1 using acetone as both a hydro-
hazardous materials requiring special disposal proce- gen acceptor and a solvent (Table 1).[10] We also dis-
dures. To use environmentally more friendly oxidants,
chemists have developed transition metal-catalyzed
oxidations of alcohols in the presence of acetone,[1,2]
[4]
H2O2,[3] or O2 as the oxidant. However, most of
these catalytic systems rely on precious metals such as
Ru,[1] Ir,[2] and Pd.[4] The development of new cata-
lysts containing abundant and inexpensive metals, par-
ticularly those derived from iron, for the oxidation of
alcohols is highly desirable. It has been reported that
aerobic alcohol oxidation is catalyzed by FeACHTNUGTRNEG(UN NO3)3/
[5]
FeBr3 or FeCl3·6H2O/NaNO2/TEMPO,[6] and the
catalytic oxidation of secondary alcohols with t-
Scheme 1. Reversible hydrogen transfer from an iron hy-
BuO2H is affected by FeACTHNUTRGNEUNG
(NO3)3·9H2O/picolinic acid.[7] dride to an aldehyde.
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967

Michael G. Coleman et al.
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Table 1. Iron-catalyzed Oppenauer-type oxidation of alco-
hols.[a]
Scheme 3. Unfavourable equilibria for the oxidation of
simple primary alcohols.
Shvo catalyst,[12] where the C=C bonds migrate to
make the more stable a,b-enones.[1c]
Simple primary alcohols did not fully convert to the
aldehyde products under our oxidation conditions.
For instance, only 20% conversion was observed
when the oxidation of benzyl alcohol (0.5M in ace-
tone) was carried out at 608C for 24 h in the presence
of 3 mol% of 1. Presumably, the potential equilibria
for the oxidation of primary alcohols favour
RCH2OH/acetone rather than RCHO/2-propanol
(Scheme 3). Similarly, the observed partial oxidations
of 4-penten-1-ol, 2,2,2-trifluoro-1-phenylethanol, and
benzoin (Figure 1) under the same catalytic condi-
tions are due to the large negative reduction enthal-
pies for their oxidation products.[13] On the other
hand, the resonance energy gained from the oxidation
of cinnamyl alcohol (Table 1, entry 9) shifted the
equilibrium towards the a,b-unsaturated aldehyde, re-
sulting in a complete reaction. Furthermore, the dehy-
drogenation of diols catalyzed by the same iron hy-
dride was successful as the overall oxidation reactions
were driven by the favourable formation of lactones
from hemiacetal intermediates, which could be gener-
ated from the oxidation of one alcohol functionality
followed by cyclization (Scheme 4).[14]
[a]
Reaction conditions: 45 mmol of 1 and 1.5 mmol of the al-
cohol substrate in 3 mL of acetone at 608C under an
argon atmosphere.
Isolated yield.
A possible mechanism for the iron-catalyzed oxida-
tion of alcohols is proposed in Scheme 5. Previous
studies on the hydrogen transfer from 1 to acetone
[b]
covered that a variety of functional groups, including have supported the formation of an isopropyl alcohol
halogens, OMe, CF3, and C=C bonds, were unaffected complex A,[8] which can exchange the ligand with the
by the oxidation conditions. While iron hydride com- alcohol substrate via a coordinatively unsaturated in-
plexes are known to catalyze alkene isomerization re-
actions,[11] interestingly, no C=C bond migration was
observed during the iron-catalyzed oxidation of cho-
lesterol (Scheme 2), suggesting that our catalytic
system is highly chemoselective. This result is in con-
trast to Bꢁckvallꢂs study on oxidation of 3b-hydroxy-
ACHTUNGTRENNUNGsteroids catalyzed by an analogous ruthenium-based Figure 1. Substrates which are not fully oxidized.
Scheme 2. Iron-catalyzed oxidation of cholesterol.
Scheme 4. Iron-catalyzed dehydrogenation of diols.
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Iron-Catalyzed Oppenauer-Type Oxidation of Alcohols
diiron bridging hydride complex (d=À22.70 and d=
À22.36 for the reactions involving 3 and 4, respective-
ly).[17] This is reminiscent of the ruthenium chemistry
well established by Shvo and co-workers; however,
the diruthenium bridging hydride complex is the only
ruthenium species generated in these reduction reac-
tions (Scheme 6).[12] The effect of TMS groups on the
stability of 1 is not fully understood at the moment. It
is possible that the bulky TMS groups may prohibit
the intermolecular interaction between the hydride
and the OH moieties,[18] and thus prevent the loss of
H2 and the eventual decomposition of the iron com-
plex (Scheme 7). When the OH group is adjacent to
two TMS groups, the formation of a diiron bridging
hydride complex should also be unfavourable. Indeed,
we have yet to observe such a species in any of the in-
vestigated reactions involving the complex 1.
In summary, we have described a well-defined iron
catalyst for the oxidation of alcohols. The reactions
are easy to operate and selective for the alcohol func-
tionality. In particular, no side reactions such as C=C
bond isomerization and ring opening of cyclopropyl
Scheme 5. Proposed catalytic cycle for iron-catalyzed oxida-
tion of alcohols.
termediate B. The new alcohol complex A’ undergoes rings have occurred in our catalytic system. We have
dehydrogenation to release the oxidation product and
regenerate the iron hydride 1.
The trimethylsilyl (TMS) substituents on the cyclo-
pentadienyl ring have a profound influence on the
catalytic activity of 1. The Williams group has recently
reported that the oxidation of 1-phenylethanol cata-
lyzed by an iron tricarbonyl complex bearing a tetra-
phenyl-substituted cyclopentadienone ligand gives
acetophenone with
a
much lower efficiency
(4.4 turnovers after 4 d, compared to 33 turnovers
after 18 h in our system).[15] This group has hypothe-
sized that an iron hydride species 2 is likely the key
intermediate in the catalytic cycle. To further under-
stand the substituent effect, we set out to prepare re-
lated iron hydride complexes 3 and 4 (Figure 2) ac-
cording to the similar procedures that were used for
the synthesis of 1.[16] Unfortunately, rapid decomposi-
tion reactions precluded the purification and full char-
Scheme 6. The bridging hydride formation from the reduc-
acterization of the desired hydrides. Interestingly, the tion of carbonyl functionalities by a ruthenium hydride.
reaction between the crude hydride (3 or 4) and ace-
tone in toluene-d8 did not generate the expected iso-
propyl alcohol complex. Instead, free isopropyl alco-
hol was observed along with a number of iron species,
1
the H NMR spectrum of which was indicative of a
Figure 2. Iron dicarbonyl hydrides with aryl-substituted hy-
droxycyclopentadienyl rings.
Scheme 7. Potential decomposition pathway for 1.
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Michael G. Coleman et al.
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the best of our knowledge, this is the most efficient
iron system reported to date for the Oppenauer-type
oxidation reactions. We have also investigated the sta-
bility of related iron hydride complexes and have ad-
dressed the critical role of the substituents on the cy-
clopentadienyl rings of the iron catalysts. This finding
has provided useful guidelines for the further devel-
opment of iron-catalyzed oxidation reactions. Future
studies to elucidate the mechanistic details and devel-
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General Information
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All air-sensitive compounds were prepared and handled
under an argon atmosphere using standard Schlenk and
inert-atmosphere box techniques. Acetone was degassed by
bubbling argon for 15 min prior to use. Toluene-d8 was dis-
tilled from Na and benzophenone under an argon atmos-
phere. Hydride 1 was prepared as described in the litera-
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well as the NMR spectra of the oxidation products are pro-
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in acetone resulted in a slightly higher conversion;
however, it took a longer time for the reaction to reach
the equilibrium.
General Procedures for the Catalytic Oxidation of
Alcohols (Table 1, Scheme 2 and Scheme 4).
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1
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Acknowledgements
We thank the NSF-REU program (CHE-0754114) and the
University of Cincinnati for the generous support of this re-
search.
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