Air-stable, nitrile-ligated (cyclopentadienone)iron dicarbonyl compounds as transfer reduction and oxidation catalysts

Article abstract of DOI:10.1002/adsc.201100896

A series of air-stable, nitrile-ligated (cyclopentadienone)iron dicarbonyl compounds was synthesized and their activities as catalysts in the transfer reduction of acetophenone were explored. While all were active catalysts, the acetonitrile adduct was chosen for further study and was found to be active in the transfer reduction of aldehydes and ketones and in the Oppenauer-type oxidation of secondary alcohols. The acetonitrile catalyst exhibited activities similar to those of an analogous air-sensitive iron hydride, but unlike the iron hydride it was unreactive in carbonyl reductions using hydrogen gas. Copyright

Full text of DOI:10.1002/adsc.201100896

COMMUNICATIONS
DOI: 10.1002/adsc.201100896
Air-Stable, Nitrile-Ligated (Cyclopentadienone)iron Dicarbonyl
Compounds as Transfer Reduction and Oxidation Catalysts
a
a,b
a
a,
Taylor N. Plank, Jessica L. Drake, Daniel K. Kim, and Timothy W. Funk *
a
Department of Chemistry, Gettysburg College, 300 N. Washington St., Gettysburg, PA 17325, USA
Fax : (+1)-717-337-6270; phone: (+1)-717-337-6348; e-mail: tfunk@gettysburg.edu
Current address: Department of Chemistry, Merkert Chemistry Center, Boston College, 2609 Beacon St., Chestnut Hill,
b
MA 02467-3860, USA
Received: November 17, 2011; Published online: February 23, 2012
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100896.
Abstract: A series of air-stable, nitrile-ligated (cy-
clopentadienone)iron dicarbonyl compounds was
synthesized and their activities as catalysts in the
transfer reduction of acetophenone were explored.
While all were active catalysts, the acetonitrile
adduct was chosen for further study and was found
to be active in the transfer reduction of aldehydes
and ketones and in the Oppenauer-type oxidation
of secondary alcohols. The acetonitrile catalyst ex-
hibited activities similar to those of an analogous
air-sensitive iron hydride, but unlike the iron hy-
dride it was unreactive in carbonyl reductions using
hydrogen gas.
highly active and stable in the air but are based on
precious metals. Replacing platinum group metal cat-
alysts with iron-based catalysts is desirable because of
[
4]
ironꢀs abundance and low cost.
A few examples of iron compounds catalyzing
[
5,6]
transfer reductions of ketones are known,
and we
became interested in iron hydride 1 because of its
structural similarity to the versatile diruthenium
bridging hydride
2
known as Shvoꢀs catalyst
(Figure 1). Compound 1 is an active catalyst for
[8]
[
7]
[
6]
both transfer reductions and transfer oxidations,
and detailed mechanistic work by Casey and Guan
showed that its reduction mechanism is similar to that
[
6]
of 2. Unfortunately its major practical limitation is
its air sensitivity. While 1 is stable for a brief time in
the air, it must be stored at À308C under an inert at-
Keywords: alcohols; aldehydes; homogeneous catal-
ysis; iron; ketones; oxidation; reduction
[
6b,9]
mosphere.
Because of its potential as a catalyst for reductive
and oxidative transformations, attempts to access
user-friendly, air-stable derivatives of 1 have been
Carbonyl reductions and alcohol oxidations are fun- made by us and others. In all cases an air-stable (cy-
damental organic transformations, and a wide range clopentadienone)iron tricarbonyl compound was used
[10]
of stoichiometric reductants and oxidants is known to as a pre-catalyst and was treated with either water,
[
11]
[12]
mediate these processes. Efforts to minimize the trimethylamine N-oxide, or UV-Vis radiation to
amount of waste produced have led to the develop- remove a CO ligand and generate an active species in
ment of metal-catalyzed reactions, although stoichio- solution (Scheme 1). Catalyst turnover numbers
metric reductants or oxidants are still necessary to (TONs) were typically low in most cases (less than
induce catalyst turnover. Carbonyl transfer reduction 10 TONs) with the highest being 18. The low TONs
reactions (Meerwein–Ponndorf–Verley-type reduc- may be due to a combination of incomplete pre-cata-
tions) and alcohol transfer oxidations (Oppenauer-
type oxidations) are desirable from a practical stand-
point: the solvent also acts as a mild reductant or oxi-
[
1]
dant, respectively. Historically, complexes based on
main group metals, such as aluminum and boron, or
[
1]
on lanthanides have catalyzed these reactions, but
[2]
[3]
ruthenium and iridium catalysts are also known.
Many of the main group and lanthanide catalysts are
Lewis acidic and moisture sensitive and must be han- Figure 1. Iron hydride 1 and the structurally similar Shvoꢀs
dled with care. The transition metal catalysts are catalyst 2.
Adv. Synth. Catal. 2012, 354, 597 – 601
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597

Taylor N. Plank et al.
COMMUNICATIONS
[a]
Table 1. Synthesis of iron nitriles.
[b]
Catalyst
Nitrile
Yield [%]
4
5
73
72
6
7
8
65
75
72
[
a]
Reaction conditions: 3 (1 equiv.), nitrile (2 equiv.), and
trimethylamine N-oxide (1.2 equiv.) in acetone (0.025M
with respect to 3) at reflux for 20 h.
[b]
Scheme 1. Simplified reaction mechanism and access to cata-
lytic cycle from (cyclopentadienone)iron tricarbonyls.
Isolated yield.
Table 2. Transfer reduction of acetophenone with the iron
[a]
nitriles.
lyst activation and decomposition of the 16-electron
active species (A) in solution.
In an attempt to avoid these issues and access more
active air-stable catalysts, we decided to replace one
of the carbon monoxide ligands of the (cyclopentadi-
[b]
Entry
Catalyst
Conversion [%]
Ae none)iron tricarbonyl compounds with a labile
C
H
T
U
N
G
T
R
E
N
N
U
N
G
ligand that could dissociate in solution. The air-stable,
1
2
3
4
5
4
5
6
7
8
91
91
85
96
87
[
6b]
benzonitrile-ligated compound 4 is known and was
an appealing starting point because a series of iron
complexes containing aliphatic nitriles or substituted
benzonitriles bearing electron-donating or electron-
withdrawing groups could be synthesized. Using the
established procedure, a series of five nitrile-ligated
iron compounds was synthesized from 3, which is air-
stable and accessible in two steps from commercial
[a]
Reaction conditions: acetophenone (0.21 mmol, 1 equiv.)
and catalyst (0.021 mmol, 0.1 equiv.) in 2-propanol
0.4 mL) at 808C for 18 h.
Determined by H NMR spectra of crude reaction mix-
tures.
(
[b]
1
[
9,13]
chemicals (Table 1).
All of the iron nitriles in
Table 1 could be purified by flash chromatography
and all were air-stable, orange solids.
The activity of compounds 4–8 as catalysts in the covered that high yields in aldehyde transfer reduc-
transfer reduction of acetophenone in 2-propanol was tions occurred with 2 mol% of 7 (Table 3). Benzalde-
explored (Table 2). Catalyst loadings of 10 mol% hydes substituted with aryl halides were tolerated
were common when (cyclopentadienone)iron tricar- (Table 3, entries 2 and 4) as well as those bearing
[
10–12]
bonyl compounds were used,
so our initial explo- ortho substituents (Table 3, entries 5 and 6). 4-Cyano-
rations used the same amount. At 808C all five com- benzaldehyde was reduced in low yield (Table 3,
pounds were active catalysts with acetonitrile-derived entry 7), which we attributed to catalyst inhibition
7
affording 1-phenylethanol in the highest yield caused by nitrile coordination. No alkene reduction
(
Table 2, entry 4). At lower temperatures the conver- was observed in the reactions with a,b-unsaturated al-
[
14]
sions were reduced, presumably because nitrile disso- dehydes (Table 3, entries 9, 10, and 12).
Aliphatic
ciation to form the active 16-electron species (A in aldehydes are also reduced cleanly (Table 3, entries 11
Scheme 1) is favored at elevated temperatures. and 13). Imine reductions are also possible using
Based on the results in Table 2, we explored the ac- these conditions (Table 3, entry 14), although some
tivity of acetonitrile-ligated 7 in more detail. We dis- unreacted imine remains even after three days.
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Adv. Synth. Catal. 2012, 354, 597 – 601

Air-Stable, Nitrile-Ligated (Cyclopentadienone)iron Dicarbonyl Compounds
[a]
[a]
Table 3. Transfer reductions of aldehydes catalyzed by 7.
Table 4. Transfer reductions of ketones catalyzed by 7.
[b]
[b]
Entry
Substrate
Product
Yield [%]
Entry Substrate
1
Product
Yield [%]
92
1
2
85
96
2
93
3
4
98
76
3
4
87
98
5
88
5
63
6
7
87
8
1 (3.3:1 cis/
6
7
(18)
trans)
9
cis)
7 (1.4:1 trans/
8
9
83
8
35 (4:1 cis/trans)
98
97
97
[
[
a]
b]
1
1
0
1
Reaction conditions: ketone (1 mmol, 1 equiv.) and 7
0.05 mmol, 5 mol%) in 2 mL 2-propanol at 808C for
8 h.
Isolated yield.
(
1
12
85
ties. For example, the cis isomer was the major prod-
uct from the reduction of 2-methylcyclohexanone
1
1
3
4
83
(
Table 4, entry 6), indicating that the hydride was de-
[
c]
(67)
livered to the carbonyl from the side opposite relative
to the methyl group. The major isomer from the re-
duction of 4-tert-butylcyclohexanone is the trans com-
pound (Table 4, entry 7). (R)-Carvone is reduced in
low yield with a majority of the remaining mass being
[
a]
Reaction conditions: aldehyde (1 mmol, 1 equiv.) and 7
0.02 mmol, 2 mol%) in 2 mL 2-propanol at 808C for
18 h.
b]
(
[
[
Isolated yield; values in parentheses are NMR conver- unreacted starting material (Table 4, entry 8). Unlike
sions.
the reactions with a,b-unsaturated aldehydes, a mix-
ture of products was obtained when 4-phenyl-3-buten-
c]
Three days reaction time.
2-one (9) was reduced (Scheme 2). Some variation in
product distribution was observed over multiple runs.
Ketones were also reduced using 7 in 2-propanol The minor products could have formed from alkene
Table 4). A catalyst loading of 5 mol% was found to reduction of 9 or from alkene isomerization of major
(
generally give secondary alcohols in high yields. Both product 10 followed by tautomerization. This process
aromatic and aliphatic ketones reacted under these would yield 12, which could be reduced to 11.
conditions (Table 4, entries 1–4). The reduction of 2-
While aldehydes and ketones were reduced under
acetylpyridine did not go to completion (Table 4, transfer reduction conditions with 7, esters, alkynes,
entry 5), which again may be due to catalyst inhibition and aliphatic epoxides did not react (Figure 2). Inter-
by substrate coordination. Substituted cyclohexanones estingly, styrene oxide underwent a selective, reduc-
were reduced with low to modest diastereoselectivi- tive ring-opening to afford 2-phenylethanol in the
Adv. Synth. Catal. 2012, 354, 597 – 601
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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599

Taylor N. Plank et al.
COMMUNICATIONS
[a]
Table 5. Oppenauer-type oxidations catalyzed by 7.
[b]
Entry
1
Substrate
Product
Yield [%]
93
2
3
99
92
Scheme 2. Transfer reduction of ketone 9 with 7.
4
5
6
99
94
91
Figure 2. Substrates not reduced by 7.
presence of 7 and 2-propanol (Scheme 3). An analysis
1
of the H NMR spectrum of the crude reaction mix-
ture showed approximately 2% of the product from
7
69
8
9
(41)
98
Scheme 3. Reductive ring-opening of styrene oxide.
[a]
Reaction conditions: alcohol (1 mmol, 1 equiv.) and 7
0.05 mmol, 5 mol%) in 2 mL acetone at 908C in
(
nucleophilic ring-opening by 2-propanol at the more
sterically hindered epoxide carbon. This result is in
contrast to the reaction of styrene oxide with 1 in the
presence of dihydrogen. When iron hydride 1 was
a sealed vessel for 18 h.
Isolated yield; values in parentheses are NMR conver-
sions.
[b]
treated with styrene oxide and H in toluene, no ring-
2
[
6a]
opening was observed.
In addition to being active in transfer reductions,
nitrile-ligated compound 7 also catalyzed Oppenauer-
The success of compound 7 as an active transfer re-
type oxidation reactions (Table 5). Conversions were duction catalyst prompted us to test its activity in re-
low in acetone at reflux, so the reactions were carried ductions with H . Iron hydride 1 catalyzed carbonyl
2
out in sealed vessels at 908C. Secondary benzylic, al- reductions using three atmospheres of hydrogen at
[
6]
lylic, and aliphatic alcohols were all oxidized to ke- room temperature. Because nitrile dissociation from
tones (Table 5, entries 1–6). When an allylic alcohol 7 occurred at elevated temperatures, we used condi-
bearing a terminal alkene was treated with 7 and ace- tions identical to those with hydride 1 but at 808C
tone, the major product was due to alkene isomeriza- (Scheme 4). No reduction of acetophenone was ob-
tion (Table 5, entry 7). Due to a disfavored equilibri- served with 7. Substrates bearing nitriles completely
um, primary alcohols are typically not oxidized in
[
1]
high yields in Oppenauer oxidations, and that trend
can be seen here in the oxidation of benzyl alcohol
(
Table 5, entry 8). A diol underwent an oxidative cy-
ACHTUNGTRENNUNGc lization to form a lactone (Table 5, entry 9). The
yields of these reactions are equal to or higher than
those using 10 mol% of iron tricarbonyl 3 and trime-
thylamine N-oxide.
[
11a]
Scheme 4. Inactivity of 7 in reductions with H2.
600
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Adv. Synth. Catal. 2012, 354, 597 – 601

Air-Stable, Nitrile-Ligated (Cyclopentadienone)iron Dicarbonyl Compounds
[
6a]
inhibited the reactivity of 1 at room temperature,
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In summary, a series of air-stable, nitrile-ligated (cy-
clopentadienone)iron dicarbonyl compounds was syn-
thesized and their reactivities in transfer reductions
was explored. While they were all active catalysts, the
acetonitrile-ligated 7 was chosen for further examina-
tion. Compound 7 was found to catalyze transfer re-
ductions of aldehydes and ketones and Oppenauer-
type oxidations of secondary alcohols, but it was un-
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General Information
1
993, 12, 3753–3761.
All reactions were done under an atmosphere of nitrogen.
Reagent grade acetone and 2-propanol were degassed by
bubbling nitrogen through them for at least 15 min prior to
use, but no attempts were made to remove residual water.
The synthetic procedures and spectroscopic data of all iron
compounds along with the experimental details and spectro-
scopic data for all isolated compounds are provided in the
Supporting Information.
[
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[
[
General Procedure for Transfer Reductions
A solution of aldehyde or ketone (1 mmol) and 7 (0.02–
0.05 mmol, 2–5 mol%) in 2 mL of 2-propanol was stirred at
808C for 18 h. The volatiles were removed by evaporation
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under reduced pressure, and the residue was purified by
flash chromatography on silica gel.
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General Procedure for Oppenauer-Type Oxidations
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A solution of alcohol (1 mmol) and 7 (0.05 mmol, 5 mol%)
in 2 mL of acetone in a sealed vessel was stirred at 908C for
[
18 h. The volatiles were removed by evaporation under re-
duced pressure, and the residue was purified by flash chro-
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(
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Acknowledgements
40 h, ~3% of the saturated alcohol 2-methyl-3-phenyl-
1
1
-propanol was observed in the H NMR spectrum of
We thank Research Corporation for Science Advancement
and Gettysburg College for generously supporting this re-
search.
the crude reaction. None of this compound was ob-
served after 18 h of reaction time. See the Supporting
Information.
Adv. Synth. Catal. 2012, 354, 597 – 601
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