Well-defined iron catalysts for the acceptorless reversible dehydrogenation-hydrogenation of alcohols and ketones

Article abstract of DOI:10.1021/cs5009656

Acceptorless dehydrogenation of alcohols, an important organic transformation, was accomplished with well-defined and inexpensive iron-based catalysts supported by a cooperating PNP pincer ligand. Benzylic and aliphatic secondary alcohols were dehydrogenated to the corresponding ketones in good isolated yields upon release of dihydrogen. Primary alcohols were dehydrogenated to esters and lactones, respectively. Mixed primary/secondary diols were oxidized at the secondary alcohol moiety with good chemoselectivity. The mechanism of the reaction was investigated using both experiment and DFT calculations, and the crucial role of metal-ligand cooperativity in the reaction was elucidated. The iron complexes are also excellent catalysts for the hydrogenation of challenging ketone substrates at ambient temperature under mild H₂ pressure, the reverse of secondary alcohol dehydrogenation.

Full text of DOI:10.1021/cs5009656

Research Article
pubs.acs.org/acscatalysis
Well-Defined Iron Catalysts for the Acceptorless Reversible
Dehydrogenation-Hydrogenation of Alcohols and Ketones
Sumit Chakraborty,†,⊥ Paraskevi O. Lagaditis,‡,⊥ Moritz Forster,§ Elizabeth A. Bielinski,∥ Nilay Hazari,∥
̈
Max C. Holthausen,§ William D. Jones,*,† and Sven Schneider*,‡
†Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
‡Institut fur Anorganische Chemie, Georg-August-Universitat, Tammannstraße 4, 37077 Gottingen, Germany
̈
̈
̈
§Insitut fur Anorganische und Analytische Chemie, Goethe-Universitat, Max-von-Laue-Strasse 7, 60438 Frankfurt am Main, Germany
̈
̈
∥Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, United States
S
* Supporting Information
ABSTRACT: Acceptorless dehydrogenation of alcohols, an
important organic transformation, was accomplished with well-
defined and inexpensive iron-based catalysts supported by a
cooperating PNP pincer ligand. Benzylic and aliphatic
secondary alcohols were dehydrogenated to the corresponding
ketones in good isolated yields upon release of dihydrogen.
Primary alcohols were dehydrogenated to esters and lactones,
respectively. Mixed primary/secondary diols were oxidized at
the secondary alcohol moiety with good chemoselectivity. The
mechanism of the reaction was investigated using both
experiment and DFT calculations, and the crucial role of
metal−ligand cooperativity in the reaction was elucidated. The iron complexes are also excellent catalysts for the hydrogenation
of challenging ketone substrates at ambient temperature under mild H2 pressure, the reverse of secondary alcohol
dehydrogenation.
KEYWORDS: iron, catalysis, acceptorless dehydrogenation, hydrogenation, metal−ligand cooperativity
Scheme 1. Cobalt-Catalyzed Acceptorless Dehydrogenation
of Alcohols and Dehydrogenative Coupling of Primary
Alcohols and Amines
INTRODUCTION
■
Catalytic acceptorless alcohol dehydrogenation (AAD) is a
convenient, atom-economical approach for alcohol oxidation
without the need for an oxidant.1 The reaction is also highly
relevant to the field of organic-hydrogen-storage-materials as it
provides a unique opportunity to release H2 from sustainable
sources, such as biomass-derived alcohols and carbohydrates
under mild conditions.2 Furthermore, reactive carbonyl com-
pounds generated from AAD can be transformed into other
useful organic materials such as imines and amides.3 From a
thermodynamic point of view, alcohol dehydrogenation is
generally an uphill process (i.e., endothermic) at room
temperature;4 however, the release of H2 gas has a favorable
positive entropic contribution and the de/hydrogenation
equilibrium can be driven by removal of H2.5
Despite the significance of this reaction, homogeneous
catalysts for AAD protocols mostly employ precious and
heavy metals such as Ru,6 Rh,7 Ir,6o,8 and Os.9 In comparison,
the same reaction with catalysts that utilize nonprecious, earth-
abundant metals is much less developed. Hanson and co-
workers reported a cobalt catalyst (1) for AAD which is sta-
bilized by a bis(phosphino)amine (PNP) ligand (Scheme 1).10
Several secondary aromatic and aliphatic alcohols were
dehydrogenated under oxidant-free conditions. Labeling studies
indicated an initial reversible alcohol dehydrogenation step
involving a cobalt hydride intermediate. Moreover, a catalytic
cycle consisting of CoI/III intermediates was proposed, and the
importance of metal−ligand cooperativity was highlighted.11 In
the case of a primary alcohol, 1 was found to be much less
effective. Nevertheless, dehydrogenation of primary alcohols
was achieved in the presence of primary amines, and the
corresponding imines were isolated from the Schiff-base
reaction.10 Noticeably, for the dehydrogenative coupling of
primary alcohols and amines, comparable catalytic activities to
previously reported Ru-based catalysts were reported (1 mol %
catalyst 1), whereas AAD required much higher catalyst
loadings (5 mol %).
Received: July 7, 2014
Revised: September 24, 2014
Published: September 25, 2014
© 2014 American Chemical Society
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Most recently, several groups independently reported the
synthesis of iron complexes (2, 3, 4, and related species)
supported by chelating PNP ligands and their utilization in de/
hydrogenation catalysis (Scheme 2).12−17 Initially, the Beller
Table 1. Acceptorless Dehydrogenation of 1-Phenylethanol
Using Iron Pincer Complexes (2 and 3) as the Precatalysts
catalyst
Scheme 2. Catalytic Applications of PNP Supported Iron
Complexes
entry
(loading)
solvent
additive time (h) NMR conv. (%)
1
2
3
2 (3 mol %)
2 (3 mol %)
2 (1 mol %)
3 (3 mol %)
3 (3 mol %)
THF
24
24
24
24
24
24
87
100
100
79
toluene
toluene
THF
a
4
KOtBu
5
6
THF
<5
0
toluene
a
10 mol % of KOtBu was used with respect to catalyst 3.
examined by introducing the gas through a thick cannula into a
separate reaction vessel containing cyclooctene, 4 mol %
RhCl(PPh3)3, and 3 mL of THF at 60 °C.19 Analysis of these
reaction products by GC-MS revealed clean formation of
cyclooctane, confirming the release of H2 upon AAD with
precatalyst 2. Further confirmation of H2 formation is provided
by GC headspace gas analysis (see the Supporting Informa-
tion). Complex 3 was also found to be an AAD precatalyst in
the presence of KOtBu, although lower conversions were
obtained after the same time (entry 4). Almost no conversion
was observed with complex 3 in the absence of base (entry 5),
and no reaction occurred in the absence of any iron catalyst
(entry 6).
The substrate scope of AAD with precatalyst 2 was
investigated (Scheme 3). Secondary benzylic alcohols (A−I,
K, L) are dehydrogenated to the corresponding acetophenone
derivatives in good isolated yields (65−92%). The reaction is
tolerant to a variety of functional groups such as -OMe, -Me,
and -NO2 as well as halides (F, Cl, Br, and I). ortho-Methylated
substrate D exhibits slower reaction rates than para and meta
substituted substrates C and E. Importantly, for alcohols with
electron-withdrawing substituents (F−I), the catalyst loading
can be further lowered to 0.1 mol % (TON = 103), albeit with
longer reaction times. In addition to aromatic substrates, the
aliphatic secondary alcohol cyclohexanol (J) was successfully
dehydrogenated to give cyclohexanone. A substrate with an
ester functional group (K) was also tolerated under these
conditions.
Both homogeneous and heterogeneous catalysis was recently
proposed for (transfer-)hydrogenation with related iron
precatalysts, and distinguishing mechanisms can be challeng-
ing.20 All (de)hydrogenation reactions reported here re-
mained as transparent colored solutions throughout the
reaction. Beller previously reported that MeOH reforming
with 2/KOH was unaffected by substoichiometric amounts of
PMe3 and hence indicative of homogeneous catalysis. Likewise,
in our case the Hg-poisoning test did not affect the reaction;
however, mercury does not always inhibit iron nanoparticle
catalysts.20e,f Therefore, kinetic indications are generally more
significant. Dehydrogenation of F with 4 was followed over
time (see the Supporting Information), and no induction
period was observed as, e.g., for Morris’ heterogeneous iron
transfer catalyst.20e,f Furthermore, in the dehydrogenation of A
(catalyst 2) the addition of a second batch after full conversion
resulted in 53% conversion within the same reaction time. This
result suggests that decomposition of 2 does not form a
catalytically more active species (see below).
group used complex 2 for catalytic H2 production from
methanol in the presence of KOH.12 The reaction proceeds at a
remarkably low catalyst loading (ppm level), and the high
catalyst thermal stability (>90 °C) is noteworthy. Guan15 and
Beller16 subsequently utilized precatalysts 2 and 3 for ester
hydrogenation, and Jones17 reported the reversible de/
hydrogenation of N-heterocycles with these complexes. The
hydride amido species 4, which had been previously proposed
by Beller as a crucial catalytically active intermediate, was also
isolated and could be directly used as catalyst for this reaction.17
At the same time, Hazari and Schneider established that 4 can
be used as a catalyst for formic acid dehydrogenation and
described the equilibrium of 4 with the cis- and trans-dihydrides
[Fe(H)2CO{HN(CH2CH2PiPr2)2}] (5a/b) upon H2 addition/
elimination.14 As a joined, ongoing effort to develop base metal
catalysts for de/hydrogenation, we here report new protocols
for AAD of secondary and primary alcohols with the well-
defined iron catalysts 2−4, as had been predicted in a
theoretical study.18 Furthermore, we show that these species
are also active for the reverse ketone hydrogenation of
challenging substrates and describe initial experimental and
computational mechanistic studies.
RESULTS AND DISCUSSION
■
We first studied AAD of 1-phenylethanol with precatalysts 2
and 3 under a variety of catalytic conditions (Table 1). The best
results were obtained in refluxing toluene under a slow, steady
N2 flow (entry 3). Under these conditions with 1 mol %
catalyst (2) loading 1-phenylethanol was quantitatively
1
converted to acetophenone within 24 h as determined by H
NMR spectroscopy. The identity of the liberated gas was
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Scheme 3. Iron-Catalyzed Acceptorless Dehydrogenation of Alcohols using Precatalyst 2
Besides secondary alcohols, primary alcohols and diols were
also examined. AAD of benzyl alcohol with precatalyst 2 gives
full conversion after 8 h in refluxing toluene and benzyl ben-
zoate was isolated as the sole product from this reaction (eq 1).
(4-hydroxy-2-butanone). Precedence for such high secondary
over primary alcohol chemoselectivity in homogeneous alcohol
oxidation has been observed for only a few precious metal-
based systems.6n,25
A plausible homogeneous mechanism for the iron-catalyzed
alcohol dehydrogenation is outlined in Scheme 4. Based on our
Scheme 4. Proposed Catalytic Cycle for the
Dehydrogenation of Alcohols
Product formation of the formal dehydrogenative Tishchenko
reaction presumably results from addition of substrate to
intermediate benzaldehyde and subsequent dehydrogenation of
the hemiacetal (see below). Accordingly, intramolecular,
dehydrogenative condensation of primary diols, such as 1,2-
benzenedimethanol (M) and 1,5-pentanediol (N), using
precatalyst 2, readily produces the corresponding lactones,
phthalide, and δ-valerolactone. Several groups have previously
reported homogeneous catalysts for the base-free transfer-
dehydrogenation of diols to lactones in the presence of a
hydrogen acceptor such as acetone.21 However, reports related
to base-free acceptorless conversion are extremely rare,22 and
the use of a first-row transition metal-based homogeneous
catalyst was only recently described for the first time.23,24
The chemoselectivity of AAD was explored using two
substrates with both primary and secondary alcohol functional
groups. Complex 2 selectively dehydrogenates the secondary
alcohol moiety in 1-phenyl-1,2-ethanediol (O), leaving the
primary alcohol unaffected. While the methine C−H bond
in O is weakened in the presence of the adjacent phenyl group
replacement of the phenyl group with a methyl substituent
(P) also results in exclusive secondary alcohol oxidation
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current understanding of the dehydrogenation of N-hetero-
cycles17 and formic acid14 we propose that complex 4 is directly
on the catalytic cycle. Previously, it was established that 4
reversibly adds H2 to give mainly trans-dihydride complex 5a
and smaller amounts of cis-dihydride 5b, which is in equilibrium
with 5a according to EXSY NMR experiments.14 Minor
quantities of free N(CH2CH2PiPr2)2 and iron(0) complex
[Fe(CO)2{HN(CH2CH2PiPr2)2}] (6)13 are also observed. The
relevance of 4 within the catalytic cycle of AAD is supported by
a stoichiometric control reaction of 4 with 2 equiv of 1-butanol
at room temperature. Slow, selective substrate conversion to
n-butyl-butanoate is accompanied by formation of the same
iron products (5a, 5b, 6) and free ligand as determined by 31P
NMR spectroscopy, without detection of other intermediates.
Hydrogen transfer from the substrate to 4 is conceivable either
by a concerted pathway or stepwise through an alkoxide
intermediate (Scheme 4), which remains at this point
unresolved on experimental grounds. However, our computa-
tional results indicate low barriers for a concerted mechanism
(see below). Comparison of the stoichiometric reactions of
4 with H2 and 1-butanol, respectively, indicate considerably
faster catalyst degradation to iron(0) and free ligand with
alcohol as hydrogen source. This observation suggests that
formation of inactive 6 might be initiated by H2 reductive
elimination from 5b at low H2 concentrations. In contrast, H2
elimination from 5a to amide 4 was shown under vacuum14,17
and closes the cycle in Scheme 4.
the case of 1-heptanol, small amounts of 1-heptanal were also
seen, supporting a mechanism with initial aldehyde formation
(see above).
The reaction mechanism underlying the AAD mediated by
bifunctional iron catalyst 4 was further examined using DFT
calculations (Scheme 5). We used MeOH and MeOCH2OH as
model substrate and intermediate, respectively, and PMe2
truncated 4. Relative free energies reported below have been
obtained at the RI-B3PW91-D3BJ/def2-QZVPP//B3LYP/
def2-SVP level of density functional theory and refer to the
temperature regime employed in the experiments (120 °C). At
these conditions, the overall formation of methyl formate is
calculated to be slightly endergonic (endothermic) with ΔRG =
+4.0 kcal mol−1 (ΔRH = +11.6 kcal mol−1), which is in excel-
lent agreement with experimental data (see the Supporting
Information). Starting from the five-coordinate amido complex
and methanol (A1, Scheme 5), the initial formation of an
encounter complex (A2) via N···H···O hydrogen bonding is
slightly exergonic. From here, concerted O−H/C−H hydrogen
transfer from the substrate to the amido species exhibits a
moderate barrier (TS_A2). It leads to the formation of an
encounter complex between formaldehyde and trans-dihydride
A3, which is, however, unbound at ΔG393 so that formaldehyde
is liberated without barrier, in an exergonic step (A3 → A4).
Subsequently, the reaction of formaldehyde with a second
equivalent of the substrate to an intermediate hemiacetal (A5)
occurs in an iron-catalyzed reaction sequence with a small
overall reaction barrier (see discussion and Scheme 6 below).
Regeneration of the amido complex from the trans-dihydride
A5 is a multistep process catalyzed by the substrate (A6-A9,
Scheme 5, bottom): The trans-dihydride forms an encounter com-
plex with methanol, featuring simultaneous Fe−H···H−O dihy-
drogen bonding and N−H···O hydrogen bonding (A6), which
assists an overall rate-limiting, synchronous proton transfer via
TS_A6 to form dihydrogen complex A7. For this step, indirect
amine to hydride proton transfer via substrate mediated proton
shuttling (Scheme 5) was computed to be slightly favored by
ΔΔG393 = 3.0 kcal mol−1 over direct proton transfer without
substrate involvement (see the Supporting Information). At this
point we note that rate-limiting H2 elimination is in line with
the observation of 5a in the stoichiometric reaction of 4 with
1-butanol (see above). Further, such Brønsted-acid catalyzed NH
to hydride proton transfer was previously demonstrated experi-
mentally for a related Ru(PNP) hydride complex.26 The resulting
nonclassical dihydrogen complex (A7) is stabilized by a hydrogen
bond between the basic amide group and MeOH. Elimination of
H2 via TS_A7 and decoordination of MeOH (A8) regenerates
the amido complex (A1), accompanied by a minute barrier.
Our results discussed so far are in reasonably good
agreement with a recent DFT study by Yang focusing on the
dehydrogenation of ethanol to acetaldehyde catalyzed by
complex 4.18 As a consequence of the higher heat of
hydrogenation of formaldehyde with respect to acetaldehyde,
H2 transfer from ethanol to 4 was computed to be less
endergonic (0.6 kcal mol−1) in this study than we found for our
methanol model. Also, Yang reported a stepwise ionic pathway
for the alcohol dehydrogenation step, whereas we found a
concerted pathway for the dehydrogenation of MeOH.
Importantly, however, the barrier for the alcohol-assisted
proton shuttling within 5a represents the highest overall barrier
in both studies. Hence, the computed reaction profiles for this
branch of overall AAD to ester are relatively robust with respect
This mechanistic proposal suggests that complex 4 should be
an active catalyst for AAD under base free conditions.
Accordingly, 1-phenylethanol is selectively converted to
acetophenone in boiling toluene with 1 mol % 4 as the catalyst
(eq 2). Likewise, selective, dehydrogenation of several primary
alcohols to the respective esters is catalyzed by 4 with catalyst
loadings as low as 0.1 mol % and conversions between 62 and
90% within 20 h (Table 2, entries 1−4). Under the same
Table 2. Iron-Catalyzed Dehydrogenation of Primary
Alcohols Using Catalyst 4
a
entry
R
catalyst loading
NMR conv. (%)
1
2
3
4
n-C3H7
0.1 mol %
0.1 mol %
0.1 mol %
0.1 mol %
0.1 mol %
0.4 mol %
90 (75)
80 (72)
62 (60)
n-C4H9
n-C5H11
n-C6H13
n-C3H6OH
CH2-Cy
b
67 (62)
e
5
99 (85)
76
6
a
b
Isolated yield in parentheses. 2% of 1-heptanal were also detected.
e
Product: γ-butyrolactone.
conditions, conversion of 1,4-butanediol to γ-butyrolactone is
quantitative (entry 5). High conversion of the branched alcohol
1-cyclohexylmethanol (76%) requires slightly higher catalyst
loading (0.4 mol %, entry 6). Importantly, no products other
than the esters were observed for these reactions; although in
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Scheme 5. Computed Lowest Free-Energy Pathways for Methanol AAD at Catalytic Conditions (120°C) with Model Catalyst
a
[FeH(CO){N(CH2CH2PMe2)2}]
a
Blue: AAD from H3COH to H3COCH2OH and H2 (see Scheme 6 for the iron-catalyzed hemiacetal formation through reaction of MeOH
with formaldehyde). Red: AAD from H3COCH2OH to H3COC(O)H and H2. Free energies ΔG and enthalpies ΔH in kcal mol−1 computed at the
RI-B3PW91-D3BJ/def2-QZVPP//B3LYP/def2-SVP level of DFT.
to the computational methods and to catalyst and substrate
truncation, which lends further credibility to these results.
In the second branch of the overall reaction (Scheme 5, red),
the hemiacetal formed in the first part of the reaction sequence
is further dehydrogenated by the parent amide (B1) yet in a
slightly different fashion: In contrast to the concerted hydrogen
transfer onto the amido catalyst found in the case of MeOH,
we did not find a single transition state for a concerted de-
hydrogenation of the hemiacetal but a stepwise reaction
sequence: After formation of the N···H···O hydrogen bridged
complex B2 in a slightly exergonic step, protonation of the
amido ligand occurs with a low barrier of 6.1 kcal mol−1
(TS_B2). Yet, although the resulting complex B3 is a clearly
characterized stationary point identified by intrinsic-reaction-
coordinate following calculations running downhill from the
preceding transition state based on total energies, it does not
represent a stable species on the free energy surface. Hydrogen
transfer to the metal center (TS_B3) proceeds with a low
barrier to give complex B4. The latter subsequently
decoordinates methyl formate yielding the trans-dihydrido
intermediate B5 in an exergonic step, akin to the situation
found for the methanol dehydrogenation. The trans-dihydrido
intermediate then undergoes the same methanol catalyzed H2
elimination as reported above to close the catalytic cycle
(Scheme 5, bottom). Hence, irrespective of some technical
differences, both the methanol dehydrogenation (A2 → A4)
and the hemiacetal dehydrogenation (B2 → B5) represent
single elementary steps in the free energy regime, the former
endergonic and the latter exergonic, with effective free-energy
barriers of ΔG393 = 19.0 kcal mol−1 and 7.2 kcal mol−1,
respectively, and without occurrence of intermediates.
For the hemiacetal formation step from formaldehyde and
methanol sketched in Scheme 5 we investigated three different
routes. In line with recent theoretical work of Azofra et al. we
compute large activation barriers for the noncatalyzed as well as
the methanol-assisted reaction steps (Schemes 6a and 6b).30
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Scheme 6. Computed Free-Energy Pathways for Hemiacetal Formation from Methanol and Formaldehyde: (a) Direct Coupling
a
Step; (b) Methanol Assisted Step; and (c) Iron Catalyzed Path
a
Free energies ΔG and enthalpies ΔH in kcal mol−1 computed at the RI-B3PW91-D3BJ/def2-QZVPP//B3LYP/def2-SVP level of DFT.
Table 3. Iron-Catalyzed Hydrogenation of Acetophenones
entry
R
catalyst (loading)
p (bar)
solvent
time (h)
T
NMR conv. (%)
1
2
3
4
5
6
7
OMe
OMe
OMe
H
2 (1 mol %)
5.5
5.5
5.5
1
toluene
THF
8
8
8
2
4
2
4
r.t.
r.t.
100
100
100
100
100
100
100
a
3 (1 mol %)
4 (1 mol %)
toluene
THF
r.t.
4 (0.2 mol %)
4 (0.1 mol %)
4 (0.1 mol %)
4 (0.05 mol %)
r.t.
H
5
THF
r.t.
H
5
THF
50 °C
50 °C
H
5
THF
a
10 mol % of KOtBu with respect to catalyst 3 was used as activator.
We found, however, a low-barrier reaction sequence cata-
lyzed by iron complex A1 commencing with a barrierless
addition of methanol across the Fe−N bond to form A11 in a
moderately exergonic step. Subsequent formation of a loose
encounter complex with formaldehyde (A12) initiates the
C−O coupling step via TS_A12, and liberation of the hemi-
acetal regenerates the catalyst A15. Hence, with an overall
free-energy barrier of 8.9 kcal mol−1, the hemiacetal forma-
tion step is efficiently catalyzed by the iron amido com-
plex (complete presentation of path (c) in the Supporting
Information).
Overall, the calculations fully support our initial mechanistic
speculations and emphasize the role of metal−ligand co-
operativity in the reaction course. Since the AAD reaction of
primary alcohols to esters represents the reverse process of
ester hydrogenation, as described by Beller and Guan,15,16 our
computational results are also relevant for this reaction.
Ketone hydrogenation, on the other hand, represents the
reverse reaction of secondary alcohol dehydrogenation (see
above) but has not been previously examined with these
Fe(PNP) catalysts. The hydrogenation of carbonyl compounds
with bifunctional iron catalysts has been examined in recent
years by the groups of Casey, Morris and Milstein.27 However,
some substrates still remain challenging with respect to
conversion and/or chemoselectivity.
The model substrate 4′-methoxyacetophenone was hydro-
genated (5.5 bar H2) in toluene in the presence of pre-
catalyst 2 (1 mol %) at room temperature (Table 3, entry 1).
Quantitative conversion to 1-(4′-methoxyphenyl)ethanol was
achieved within 8 h as determined by NMR spectroscopy.
Complex 3 in the presence of KOtBu (10 mol %) also serves as
an equally effective precatalyst (entry 2). Moreover, when the
five-coordinate complex 4 was used as the catalyst, similar
catalytic activity was observed (entry 3). Further possible
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reduction of the catalyst loading or pressure is demonstrated
for catalyst 4: Acetophenone is selectively hydrogenated
to 1-phenylethanol at room temperature within 2 h using
impressively low catalyst loading and H2 pressure (0.2 mol % 4,
1 bar H2, entry 4). The catalyst loading can be further reduced
to 0.1 mol % at slightly higher pressure and reaction times
(5 bar H2, r.t., 4 h, entry 5) or temperature (5 bar H2, 50 °C,
2 h, entry 6) and even down to 0.05 mol % still at mild and
practical conditions for full completion (5 bar H2, 50 °C, 4 h,
entry 7), i.e. TON > 2000. Hence, with respect to activity this
system is comparable with the best known homogeneous iron
catalysts for ketone hydrogenation.27e,k A PMe3 poisoning
experiment (10 equiv with respect to 4) was also carried out for
acetophenone hydrogenation (0.1 mol % 4, 5 bar H2, 25 °C).
Substrate conversion (68%) was slightly reduced, but catalysis
was not inhibited as typically observed for other homogeneous
iron catalysts.27j
EXPERIMENTAL SECTION
■
General Experimental Information. Unless otherwise
noted, all the organometallic compounds were prepared and
handled under a nitrogen atmosphere using standard Schlenk
and glovebox techniques. Dry and oxygen-free solvents such as
THF and pentane were collected from an Innovative Tech-
nology PS-MD-6 solvent purification system28 and used through-
out the experiments. Toluene was vacuum distilled from a purple
solution of Na and benzophenone and stored over 4 Å molecular
sieves. CDCl3 was used without further purification. 1H and 13
C
NMR were recorded on a Bruker Avance-400 or Bruker Avance-
300 spectrometer. Chemical shift values in H and 13C NMR
1
spectra were referenced internally to the residual solvent
resonances. GC-MS spectra were recorded on a Shimadzu
QP2010 instrument. Complexes 2−4 have been previously
reported in the literature.12−17 All the products isolated from
the dehydrogenation and hydrogenation reactions are known
compounds.29
However, besides high activity, the chemoselectivity of
catalyst 4 is also remarkable. For example, 2-acetylpyridine
(eq 3) is converted in quantitative yield to the respective
Catalytic Dehydrogenation of 1-Phenylethanol. In a
glovebox, an iron complex (25 μmol), KOtBu (if required),
and 1-phenylethanol (833 μmol or 2.5 mmol) were mixed with
5 mL of solvent in a 50 mL flame-dried Schlenk flask. A
condenser was attached to the flask, and the whole setup was
brought outside the glovebox. The Schlenk flask containing the
homogeneous mixture was then stirred under reflux for 24 h.
During the reflux, the solution was continuously bubbled with
N2 (∼1 atm), and the liberated H2 gas was allowed to escape
through an outlet port (a needle). After the reaction, the
solution was allowed to cool to room temperature, filtered
through a short silica gel column, and eluted with THF (10−
15 mL). The resulting filtrate was evaporated under vacuum
to afford an oily residue. Product identification and conver-
alcohol with 0.1 mol % 4 at 5 bar H2 (50 °C, 4 h). In
comparison, Milstein’s iron catalyst required 15 h (87% yield)
under the same conditions,27e and Morris’ catalyst gave only
20% yield.27k As also reported for Morris’ catalyst (95%
yield),27k 4 catalyzes the chemoselective hydrogenation of
5-hexen-2-one to the respective unsaturated alcohol (eq 4),
1
sions were obtained from a H NMR spectrum in CDCl3
(Supporting Information).
Dehydrogenation of 1-phenylethanol (1 mol % catalyst 2) in
the presence of elemental mercury (1.5 g, 7.5 mmol) left the
catalytic activity unaffected.
which provides strong evidence for an outer sphere (bifunc-
tional) mechanism. In addition, 4 exhibits high chemoselectivity
with an α,β-unsaturated ketone (eq 5), and only minor
For gas analysis, dehydrogenation of 1-phenylethanol (1 mol %
catalyst 2) was carried out for 10 h, and the gas in the
headspace was collected by a gastight syringe. H2 was detected
by GC-analysis as compared to an authentic sample of H2 (see
the Supporting Information). Alternatively, dehydrogenation
of 1-phenylethanol (302 μL, 2.5 mmol) with catalyst 2 (10 mg,
25 μmol) in toluene (5 mL) was carried out after connecting
the head space to a flask charged with RhCl(PPh3)3 (23 mg,
25 μmol) and cyclooctene (81 μL, 625 μmol) in THF (5 mL)
at 60 °C. After 24 h, 84% of the cyclooctene was converted to
cyclooctane, according to GC-MS.
formation of the respective saturated ketone (12%) and alcohol
(2%) is observed. In contrast, Milstein’s iron catalyst is not
selective with respect to these three products, and Morris’
catalyst exhibited no conversion.
For successive addition experiment, in a glovebox a flame-
dried 50 mL Schlenk flask was charged with catalyst 2 (10 mg,
25 μmol), 1-phenylethanol (302 μL, 2.5 mmol), and toluene
(5 mL). The solution was stirred under reflux for 24 h under a
nitrogen atmosphere, and H2 was allowed to escape through
an outlet port. After 24 h, a second batch of 1-phenylethanol
(302 μL, 2.5 mmol) was introduced into the system, and the
dehydrogenation reaction was carried out for an additional
24 h. After this time, the solution was allowed to cool to room
temperature, filtered through a silica gel column, and eluted
with more toluene (10 mL). The resulting filtrate was evap-
orated under vacuum, and the percentage conversion for the
In summary, inexpensive homogeneous catalysts using
earth-abundant iron have been utilized for the acceptorless
dehydrogenation of alcohols and diols and for the reverse
hydrogenation of challenging ketone substrates. The yields
and chemoselectivities under mild, base free conditions
for this system are remarkable compared with previous iron
catalysts. Initial mechanistic examinations are in agreement
with homogeneous catalysis for de/hydrogenation. The five-
coordinate iron amido hydride species 4 is proposed to
be the active catalyst in the dehydrogenation reaction, as
supported by DFT calculations. Currently, our efforts are
focused toward more detailed mechanistic studies of the
dehydrogenation and hydrogenation reactions, which will
be reported in the future.
1
second catalytic run was calculated from the relative H NMR
integrations.
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ACS Catalysis
Research Article
General Procedure for the Iron-Catalyzed Dehydro-
genation of Alcohols (A−P) with Catalyst 2. In a glovebox,
a 50 mL flame-dried Schlenk flask was charged with catalyst 2
(10 mg, 25 μmol), an alcohol substrate (2.5 or 25 mmol), and
5 mL of toluene (substrates A−L) or 5 mL of THF (substrates
M−P). After attaching a condenser to the Schlenk flask,
the solution was stirred at 120 °C for a specific time under a
constant N2 flow. A needle was placed through the septum on
top of the condenser to remove the liberated H2 gas. After the
reaction, the solution was allowed to cool to room temperature,
filtered through a short silica gel column, and eluted with THF.
The resulting filtrate was evaporated under vacuum to afford
the pure product. 1H and 13C{1H} NMR spectra of the
products were recorded in CDCl3 and matched with the
chemical shifts reported in the literature. Percentage yields were
also calculated for these isolated products.
of trans-4-phenyl-3-buten-2-one, the gas pressure was adjusted
to 10 bar once solutions were injected. Small aliquots of the
reaction mixture were sampled from a stainless-steel sampling
dip tube attached to the Parr reactor. All samples for gas
chromatography (GC) analyses were diluted to a total volume
of approximately 1 mL using oxygenated methanol. The
conversion of hydrogenated ketones was analyzed by an Agilent
Technologies 7890A gas chromatograph equipped with a
DB-5MS (30 m × 0.25 mm; film thickness 0.25 μm) with an
autoinjector (Agilent 7683B). Hydrogen was used as a mobile
phase at a column pressure of 5 psi with a split flow rate of
1 mL/min. The injector temperature was 300 °C, and the FID
temperature was 300 °C. One method was used to analyze the
substrates; the oven temperature began at 40 °C for 1 min, then
15 °C per minute to 200 °C, and finally 200 °C for 1 min. The
retention times for substrates and hydrogenated products are
provided in the Supporting Information.
Catalytic Dehydrogenation of Benzyl Alcohol with
Complex 2. In a glovebox, a flame-dried 50 mL Schlenk flask
was charged with catalyst 2 (10 mg, 25 μmol), benzyl alcohol
(259 μL, 2.5 mmol), and toluene (5 mL). The solution inside
the flask was then stirred at 120 °C for 8 h under a constant N2
flow. H2 was allowed to escape through an outlet port. After the
reaction, the solution was allowed to reach room temperature,
filtered through a short silica gel column, and eluted with THF.
The resulting filtrate was evaporated under vacuum to afford an
PMe3 Poisoning Test in the Hydrogenation of
Acetophenone with Catalyst 4. In a glovebox, a vial was
charged with 4 (5 mg, 0.013 mmol) and 10 mL of THF, a second
vial was charged with acetophenone (1553 mg, 12.821 mmol) and
10 mL of THF, and a third vial was charged with PMe3 (10 mg,
0.119 mmol) and 5 mL of THF. Each solution was then
transferred to a syringe equipped with a 12 in. needle and
stoppered with a rubber septum. The syringes loaded with
catalyst and substrate solutions were taken out of the glovebox
and injected to the prepared Parr reactor against a flow of
hydrogen gas (25 °C, 5 bar H2). After 20 min, the syringe with
the PMe3 solution was injected into the Parr reactor under
1
oily material. H NMR spectrum recorded for this residue in
CDCl3 showed quantitative formation of benzyl benzoate. No
1
trace of benzaldehyde was found in the H NMR spectrum.
General Procedure for the Catalytic Dehydrogenative
Coupling of Alcohols or Diols with Complex 4. A Schlenk
flask was charged with 4 (0.005 mmol), alcohol, and toluene
(2 mL) in a drybox. The flask was then connected to a Schlenk
vacuum line, equipped with a condenser and an oil bubbler.
The solution was heated (120 °C) with stirring in an open
system under a flow of argon for 20 h. After exposing the flasks
to air and cooling, naphthalene was added as an internal
1
a flow of H2, and conversion was monitored over time by H
NMR and GC-FID analysis.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, product characterization data, and
details of DFT calculations. This material is available free of
charge via the Internet at http://pubs.acs.org.
1
standard and analyzed by H NMR spectroscopy in CDCl3.
General Procedure for the Iron-Catalyzed Hydro-
genation of 4′-Methoxyacetophenone. In a glovebox, a
25 mL stainless steel Parr pressure reactor was loaded with an
iron complex (25 μmol), KOtBu (if required), 4′-methoxy-
acetophenone (375 mg, 2.5 mmol), and 5 mL of toluene (or
THF). The reactor was sealed, flushed with H2 three times, and
finally placed under 80 psig of H2 pressure. The solution was
then stirred at room temperature for 8 h. After the reaction, the
solution was filtered through a short silica gel column and
eluted with THF. The resulting filtrate was evaporated to
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: jones@chem.rochester.edu.
*E-mail: sven.schneider@chemie.uni-goettingen.de.
Author Contributions
⊥These authors (S.C. and P.O.L.) contributed equally.
Notes
1
The authors declare no competing financial interest.
dryness to afford the pure hydrogenation product. H and
13C{1H} NMR spectra of the product were recorded in CDCl3
and matched with the reported spectra in the literature.
ACKNOWLEDGMENTS
■
General Procedure for Hydrogenation Studies with
Catalyst 4. All of the hydrogenation reactions were performed
at constant pressures using a stainless steel 50 mL Parr
hydrogenation reactor. The temperature was maintained at 50
or 70 °C using an oil bath. The reactor was flushed several
times with hydrogen gas at 5 atm prior to the addition of
catalyst and substrate. In a glovebox, a vial was charged with 4
(2 mg, 0.005 mmol) and 5 mL of THF, and a separate vial was
charged with substrate (5.128 or 2.564 mmol) and 5 mL of
THF. Each solution was then transferred to a syringe equipped
with a 12 in. needle and stoppered with a rubber septum. Both
syringes were taken out of the glovebox and injected to the
prepared Parr reactor against a flow of hydrogen gas. In the case
Parts of this work were funded by the Center for Electro-
catalysis, Transport Phenomena, and Materials (CETM) for
Innovative Energy Storage, an Energy Frontier Research Center
funded by the U.S. Department of Energy, and the ESD
NYSTAR program (W.D.J.). S.S. and M.C.H. acknowledge
support through the COST Action 1205 (CARISMA). P.O.L. is
grateful to the Alexander von Humboldt Foundation for a
postdoctoral scholarship. N.H. and E.A.B. acknowledge support
from the National Science Foundation through the Center for
the Capture and Conversion of CO2 (Grant No. CHE-
1240020). Calculations have been performed at the Center for
Scientific Computing (CSC) Frankfurt on the FUCHS and
LOEWE-CSC high-performance compute clusters.
4001
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ACS Catalysis
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