Synthesis and Catalytic Activity of (3,4-Diphenylcyclopentadienone)Iron Tricarbonyl Compounds in Transfer Hydrogenations and Dehydrogenations

Article abstract of DOI:10.1021/acs.organomet.8b00037

Four (3,4-diphenylcyclopentadienone)iron tricarbonyl compounds were synthesized, and their activities in transfer hydrogenations of carbonyl compounds and transfer dehydrogenations of alcohols were explored and compared to those of the well-established [2,5-(SiMe₃)₂-3,4-(CH₂)₄(η⁴-C₄C=O)]Fe(CO)₃ (3). A new compound, [2,5-bis(3,5-dimethylphenyl)-3,4-diphenylcyclopentadienone]iron tricarbonyl (7), was the most active catalyst in both transfer hydrogenations and dehydrogenations, and compound 3 was the least active catalyst in transfer hydrogenations. Evidence was found for product inhibition of both 3 and 7 in a transfer dehydrogenation reaction, with the activity of 3 being more heavily affected. A monomeric iron hydride derived from 7 was spectroscopically observed during a transfer hydrogenation, and no diiron bridging hydrides were found under reductive or oxidative conditions. Initial results in the transfer hydrogenation of N-benzylideneaniline showed that 3 was a significantly less active catalyst in comparison to the (3,4-diphenylcyclopentadienone)iron tricarbonyl compounds.

Full text of DOI:10.1021/acs.organomet.8b00037

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis and Catalytic Activity of (3,4-
Diphenylcyclopentadienone)Iron Tricarbonyl Compounds in Transfer
Hydrogenations and Dehydrogenations
Timothy W. Funk,* Andrew R. Mahoney,† Rebecca A. Sponenburg,‡ Kathryn P. Zimmerman,
Daniel K. Kim,§ and Emily E. Harrison
Department of Chemistry, Gettysburg College, Gettysburg, Pennsylvania 17325, United States
S
* Supporting Information
ABSTRACT: Four (3,4-diphenylcyclopentadienone)iron tri-
carbonyl compounds were synthesized, and their activities in
transfer hydrogenations of carbonyl compounds and transfer
dehydrogenations of alcohols were explored and compared to
those of the well-established [2,5-(SiMe3)2-3,4-(CH2)4(η4-
C4CO)]Fe(CO)3 (3). A new compound, [2,5-bis(3,5-
dimethylphenyl)-3,4-diphenylcyclopentadienone]iron tricarbonyl (7), was the most active catalyst in both transfer
hydrogenations and dehydrogenations, and compound 3 was the least active catalyst in transfer hydrogenations. Evidence
was found for product inhibition of both 3 and 7 in a transfer dehydrogenation reaction, with the activity of 3 being more heavily
affected. A monomeric iron hydride derived from 7 was spectroscopically observed during a transfer hydrogenation, and no
diiron bridging hydrides were found under reductive or oxidative conditions. Initial results in the transfer hydrogenation of N-
benzylideneaniline showed that 3 was a significantly less active catalyst in comparison to the (3,4-diphenylcyclopentadienone)
iron tricarbonyl compounds.
INTRODUCTION
■
While (cyclopentadienone)iron tricarbonyl compounds have
been known since the 1950s,1 it was not until 2007 that a
report on their catalytic activity in carbonyl reductions was
published.2 Initial experimental3 and computational4 mecha-
nistic studies of the iron hydroxycyclopentadienyl hydride 15
supported bifunctional reactivity, where both the iron and the
oxygen of the cyclopentadienone carbonyl were involved in a
concerted hydrogen transfer (Scheme 1). This reactivity is
similar to that observed for Shvo’s catalyst (2, Figure 1).6
Figure 1. Catalytically important iron and ruthenium carbonyl
Compound 1 is sensitive to oxygen, but the catalytic cycle can
compounds.
Scheme 1. Proposed Mechanism Using 1: Clockwise for
Carbonyl Reduction and Counterclockwise for Alcohol
Oxidation
also be entered by removing a carbonyl ligand from air-stable
(cyclopentadienone)iron tricarbonyl 37 using basic water,8
light,9 or trimethylamine N-oxide,10 which generates unsatu-
rated species A in solution.
Iron compounds 1 and 3 and derivatives of 3 have been
shown to react in a variety of oxidative and reductive
transformations,11 and recent studies have taken advantage of
their ability to catalyze both oxidations and reductions using the
borrowing hydrogen approach.12 In general, compounds 1 and
3 have been used the most often, but when other catalysts are
used, a majority of them contain a bicyclic cyclopentadieno-
ne.8b,10,12d,i,13 Relatively few catalysts without rings fused to the
cyclopentadienone have been explored, and of those that have,
Received: January 19, 2018
© XXXX American Chemical Society
A
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Article
tetraphenylcyclopentadienone compound 41 is the most
common.8a,13b,c,14 A close look at these studies suggests that
4 and its derivatives are less efficient catalysts in comparison to
those bearing fused rings in the 3- and 4-positions of the
cyclopentadienone in reductions with H2, but they are
competitive with or superior to bicyclic catalysts in transfer
oxidations and reductions. To gain more insight into this
potential trend, 3−7 were synthesized and their activities in the
transfer hydrogenation of carbonyl compounds and the transfer
dehydrogenation of alcohols were explored. Compounds 4−7
were chosen because they offered varying degrees of steric
hindrance in the 2- and 5-positions of the cyclopentadienone
ring.
positions of the cyclopentadienone13e have been reported. In
these solid-state structures, it varies as to whether or not the
phenyl groups in the 2- and 5-positions are coplanar with the
cyclopentadienone. Interestingly, the 1H and 13C NMR spectra
of 7 at room temperature show that the methyl groups on the
DMPh rings are equivalent, indicating that in solution there is
free rotation around the C−C bond connecting the cyclo-
pentadienone to these substituents.
Catalyst Activities. The reduction of acetophenone using
isopropyl alcohol as the hydrogen source was chosen as a
representative transfer hydrogenation, and the reaction progress
was monitored over time to gain insight into the catalytic
activities of compounds 3−7 (Figure 2). Interestingly, the TMS
RESULTS AND DISCUSSION
■
Catalyst Synthesis. The general synthetic route for
(cyclopentadienone)iron tricarbonyl compounds is shown in
Scheme 2. Substituted cyclopentadienones can be purchased or
Scheme 2. General Synthesis of Iron Compounds 4−6
synthesized from the corresponding symmetrical ketone and
benzil,15 and they react with iron pentacarbonyl at elevated
temperature to afford the desired (cyclopentadienone)iron
tricarbonyl compounds as air-stable solids. Compounds 416 and
51c,17 are known, and their corresponding cyclopentadienones
are commercially available, with 8b being a Diels−Alder dimer
that undergoes a retro-Diels−Alder process under the reaction
conditions.15b The diethyl cyclopentadienone 8c has also been
synthesized previously,15b but its iron tricarbonyl compound
(6) has not been reported.
Figure 2. Acetophenone reduction using 3−7. Conversions were
determined by GC relative to biphenyl. The consumption of
acetophenone (conversion) tracked with the formation of 1-phenyl-
ethanol (yield).
catalyst 3 significantly underperformed relative to all four of the
(3,4-diphenylcyclopentadienone)iron tricarbonyl compounds
under these conditions. Compound 7 was the most active
catalyst, resulting in >90% conversion within 6 h, and 4 and 5
had about the same activities and led to 90% conversion over a
longer period of time. No clear activity trend based on the size
of the substituent in the 2- and 5-positions of the cyclo-
pentadienone ring emerged.
Access to compound 7 required the synthesis of cyclo-
pentadienone 13,18 and our approach is shown in Scheme 3.
Scheme 3. Synthesis of Iron Compound 7
To see if the trend found in acetophenone reduction was
general, the activities of catalysts 3 and 7 were explored in the
transfer hydrogenation of other carbonyl compounds (Table
1). Overall, ketones were reduced in higher conversions using 7
in comparison to 3 (Table 1, entries 1−7). Substrate electronics
affected the reactions, with electron-poor ketones being
reduced more readily with both 3 and 7 (Table 1, entries 2
and 3). Aliphatic ketones were also reduced more efficiently by
7 (Table 1, entries 5−7). When an α,β-unsaturated ketone (4-
phenyl-3-buten-2-one) and aldehyde (cinnamaldehyde) were
used, mixtures of products from carbonyl reduction, alkene
reduction, or both were observed with both catalysts (Table 1,
entries 8 and 9, respectively), and 3 was more selective at
reducing only the carbonyl group relative to 7. Unhindered
aldehydes were reduced in high conversions with both catalysts
(Table 1, entries 9, 10, and 13), but 7 outperformed 3 when a
hindered aldehyde was used (Table 1, entry 11). Interestingly,
There are multiple reports of the synthesis of ketone 12,19 and
in our hands the two-step TosMIC approach20 afforded the
highest yield and purest product. Ketone 12 reacted with benzil
to afford the desired cyclopentadienone 13 in 84% yield, and
the procedure outlined in Scheme 2 was used to access 7.
Iron compounds 4−7 were characterized by NMR and IR
spectroscopy and mass spectrometry. The atom connectivity of
this class of compounds is well established, and X-ray crystal
structures of 416a and other (cyclopentadienone)iron tricar-
bonyl compounds bearing phenyl groups in the 2- and 5-
B
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Article
a
unconjugated alkene was observed (Table 1, entries 4, 7, 10,
12, and 13, respectively).
Table 1. Transfer Hydrogenations with 7 and 3
The same catalysts were used in the oxidation of 4-phenyl-2-
butanol (Figure 3). During the first 6 h, catalysts 3, 5, and 7 had
Figure 3. 4-Phenyl-2-butanol oxidation using 3−7. Conversions were
determined by GC relative to biphenyl. The consumption of 4-phenyl-
2-butanol (conversion) tracked with the formation of 4-phenyl-2-
butanone (yield).
similar activities, but ultimately 7 led to higher conversions
more quickly than the others. Compound 4 has already been
shown to be an active transfer dehydrogenation catalyst, but
both 5 and 7 outperformed it under these conditions.13b As in
the carbonyl reductions, no clear activity trend based on steric
hindrance in the 2- and 5-positions of the cyclopentadienone
arose. In a recent study of a reaction using 3 as part of a
cocatalyst system in a borrowing hydrogen process, an
induction period was observed and was proposed to be due
to the time needed for catalyst activation by trimethylamine N-
oxide, but we have found no evidence for an induction period
with any of these catalysts, including 3.22
Catalysts 3 and 7 were tested in the transfer dehydrogenation
of other alcohols to see if the same activity pattern emerged
(Table 2). In alcohol oxidations 7 was as or more active than 3,
although the differences in conversions between the two were
not as large as in the carbonyl reductions. Secondary benzylic
alcohols were oxidized by both catalysts (Table 2, entries 1−4),
but 7 was more active than 3 when an electron-poor aromatic
ketone formed (Table 2, entry 2). Catalyst 7 was also more
active in the dehydrogenation of acyclic and cyclic secondary
aliphatic alcohols (Table 2, entries 5 and 6, respectively).
Interestingly, both catalysts reacted preferentially with the cis
isomer of 4-tert-butylcyclohexanol, but 3 was more selective
than 7 (Table 2, entry 6). Due to a disfavored equilibrium,
transfer dehydrogenations of primary alcohols to aldehydes are
notoriously challenging,23 and our attempt with 3-phenyl-1-
butanol afforded low conversions with both 7 and 3. In addition
to the aldehyde, an esterfrom an oxidative couplingformed
with both catalysts;24 it was the major product with 7 but the
minor product using 3.
a
Reaction conditions for transfer hydrogenations: ketone or aldehyde
(2.5 mmol), iron compound (0.05 mmol), anhydrous trimethylamine
N-oxide (0.05 mmol), and biphenyl (0.63 mmol) in 5 mL of degassed
b
isopropyl alcohol at 80 °C. Determined by GC relative to biphenyl
and matched yield values calculated using 1H NMR spectroscopy.
c
1
Determined by relative integration values in the H NMR spectra.
when the same aldehyde reacted with an acetonitrile derivative
of 3 under very similar reaction conditions, a high yield of 2,4,6-
trimethylbenzyl alcohol was obtained.21 Substrates bearing
nitriles are known to deactivate this class of catalysts,2,21 and
although 7 was more active than 3, low conversions were still
observed (Table 1, entry 12). While substrates containing a
variety of reducible groups were explored, no evidence for
reduction of the aryl chloride, ester, nitro, nitrile, or
Product Inhibition. Figures 2 and 3 show that catalyst 3
does not turn over much beyond 24−30 h, and when some of
the reactions with 3 in Tables 1 and 2 were allowed to run for
C
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a
Table 2. Transfer Dehydrogenations with 7 and 3
Figure 4. Acetophenone reduction using 3 or 7 with and without 1
equiv of product added at t = 0. Conversions were determined by GC
relative to biphenyl. The consumption of acetophenone (conversion)
tracked with the formation of 1-phenylethanol (yield).
a
Reaction conditions for transfer dehydrogenations: alcohol (2.5
mmol), iron compound (0.063 mmol), anhydrous trimethylamine N-
oxide (0.063 mmol), and biphenyl (0.63 mmol) in 5 mL of degassed
b
acetone at reflux. Determined by GC relative to biphenyl and
matched yield values calculated using 1H NMR spectroscopy.
c
d
Reaction time was 72 h. Determined by a combination of GC and
1
relative integration values in the H NMR spectra; the mixture of 4-
tert-butylcyclohexanol used had a 2.9/1 trans/cis ratio. Determined by
relative integration values in the H NMR spectra.
e
1
longer times, negligible increases in conversion were observed.
Additionally, the rate of acetophenone reduction with 3 was
lower than that for all the other catalysts explored. Two
possible explanations for this plateauing behavior are catalyst
decomposition and product inhibition. To explore if product
inhibition was occurring, 1 equiv of 1-phenylethanol was added
to acetophenone reduction reactions using 3 or 7, and
conversion over time was monitored (Figure 4). In the case
of 7, the initial rate and final conversion were slightly lower
when product was added, but no decrease in rate or final
conversion was observed in the reaction using 3. In fact, the
final conversion for the reaction with 3 was about 10% higher
when the product was added at the beginning of the reaction.
These data suggest that product inhibition is not causing the
decreased activity of 3 in the reduction of acetophenone and
therefore some other factorpossibly catalyst decomposi-
tionis contributing to its lack of activity after approximately
24 h.13f
Figure 5. 4-Phenyl-2-butanol oxidation using 3 or 7 with and without
1 equiv of product added at t = 0. Conversions were determined by
GC relative to biphenyl. The consumption of 4-phenyl-2-butanol
(conversion) tracked with the formation of 4-phenyl-2-butanone
(yield).
overall conversion was 15−20% lower when product was
added. A much more dramatic decrease was observed in the
reaction with 3, where the product-spiked reaction struggled to
reach 10% conversion over 72 h. While some form of product
inhibition is occurring in both of these reactions, it is much
more dramatic with catalyst 3. The coordination of alcohols to
unsaturated species A has been observed in both the solution
and solid phases,3a but no evidence of an analogous ketone-
bound intermediate has been provided. Taken together, these
data suggest the observed product inhibition in alcohol
oxidations is not due to ketone coordinating to a 16-electron
species analogous to A but may be due to an increased rate in
The same product-inhibition experiments were performed
for 4-phenyl-2-butanol oxidations using 3 or 7: 1 equiv of 4-
phenyl-2-butanone product was added at the beginning of the
reactions (Figure 5). With 7 the initial rate decreased and the
D
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the reverse reaction (reduction of ketone to alcohol) and a shift
in the equilibrium of the reaction, both due to increased
concentrations of ketone.
Scheme 5. Possible Resting State Structures
Detection of Iron Hydrides. Next we chose to gain some
insight into the active catalyst species present during transfer
hydrogenation reactions using 7. When 7 was treated with 1
equiv of trimethylamine N-oxide at 80 °C in 1:1 toluene-d8/
isopropyl alcohol, an iron hydride signal at −10.72 ppm was
1
observed in the H NMR spectrum. The chemical shift was
state structure that forms must not trap it irreversibly. While we
did not observe any diiron bridging hydride species using 7,
Guan and co-workers attributed peaks at approximately −22
ppm (in toluene-d8) to them when they treated crude (2,5-
diphenylhydroxycyclopentadienyl)iron dicarbonyl hydrides
with acetone.27
similar to that of 1 (−11.62 ppm in C6D6)5 and the analogous
ruthenium compound (2,5-diphenyl-3,4-bis(p-tolyl)-
hydroxylcyclopentadienyl)ruthenium dicarbonyl hydride
(−9.76 ppm in THF-d8),25 and it suggests that monomeric
iron hydride 14 formed in solution (Scheme 4). Diruthenium
Preliminary Imine Reduction Study. The difference in
activity between 3 and the other catalysts encouraged us to
perform an initial experiment comparing their activities in an
imine reduction. (Cyclopentadienone)iron tricarbonyl com-
pounds are known to catalyze imine reductions and reductive
aminations,13c−e,h,29 but 4 is the only catalyst bearing a 3,4-
diphenylcyclopentadienone that has been used, and it afforded
low conversions when it was used in a reductive amination with
molecular hydrogen.13c In our study the transfer hydrogenation
of N-benzylideneaniline reached completion within 4 h using
catalysts 5 and 6 (Figure 6). The two catalysts with aromatic
Scheme 4. In Situ Formation of Proposed Iron Hydride 14
bridging hydride 2 and its derivatives are formed under similar
conditionsby heating (cyclopentadienone)ruthenium tricar-
bonyl compounds in alcoholsbut we found no evidence for a
diiron bridging hydride compound similar to 2, whose hydride
is significantly more shielded at −17.75 ppm in C6D6.26 Excess
acetophenone was added to our iron hydride solution, and the
1
reaction was monitored at 70 °C by H NMR spectroscopy.
Catalyst turnover was occurring under these conditions, and
the signal at −10.72 ppm remained with no other hydrides
appearing. These data are consistent with monomeric iron
hydride 14, not a diiron bridging hydride, being the resting
state of the catalyst during the transfer hydrogenation reaction.
Increased catalytic activities of (hydroxycyclopentadienyl)iron
and ruthenium hydrides bearing sterically bulky TMS groups
have been attributed to their inability to form catalytically
inactive bridging hydrides, and the sterically bulky 3,5-
dimethylphenyl groups may also be inhibiting their forma-
tion.3b,27
To examine the active catalyst in solution during a transfer
dehydrogenation reaction, a solution of 7 in acetone-d6 was
treated with trimethylamine N-oxide and excess 4-phenyl-2-
butanol at 50−60 °C. Catalyst turnover occurred under these
conditions4-phenyl-2-butanone and isopropyl alcohol-d6
were observedand 1H NMR spectroscopy showed very
small peaks at −4.87 ppm and −10.82 ppm. No peaks around
−20 ppm were present; thus, while the resting state of the
catalyst is unclear, no bridging hydrides analogous to 2 were
present. If we assume that an unsaturated species analogous to
A forms in solution, it could dimerize as a way to stabilize itself.
Some possible resting state structures based on other dimeric
species that have been proposed to form in reactions using
structurally similar catalysts are shown in Scheme 5. Compound
15 is analogous to known ruthenium dimers that have a low-
energy barrier of dissociation and readily react with a variety of
Lewis bases, including alcohols.28 Alternatively, the formation
of iron dimers with bridging carbonyl ligands (16) has been
proposed as a possible explanation for why certain (cyclo-
pentadienone)iron tricarbonyl compounds bearing phenyl
groups in the 2- and 5-positions show poor catalytic activity
in reductive aminations.13c Catalyst 7 remained active
throughout the course of the reaction; therefore, any resting
Figure 6. N-Benzylideneaniline reduction using 3−7. Conversions
were determined by GC relative to biphenyl. The consumption of N-
benzylideneaniline (conversion) tracked with the formation of N-
benzylaniline (yield).
substituents in the 2- and 5-positions of the cyclopentadienone
(4 and 7) afforded complete conversions over a longer period
of time, and after 2 days the reaction using 3 had not reached
40% conversion. A recent report by Facchini et al. also showed
that 3 was an ineffective catalyst for the transfer hydrogenation
of imines with isopropyl alcohol, and their [bis-
(hexamethylene)cyclopentadienone]iron tricarbonyl com-
pound was significantly more active.13h More thorough,
detailed studies are needed, but these initial results suggest
that catalysts with sterically small groups in the 2- and 5-
positions of the cyclopentadienone are more active than those
bearing larger substituents.
E
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Complex Prepared by Reppe and Vetter. J. Am. Chem. Soc. 1956, 78,
SUMMARY AND CONCLUSIONS
■
3621−3624. (c) Hubel, W.; Braye, E. H.; Clauss, A.; Weiss, E.;
̈
I n s u m m a r y , t h e a c t i v i t i e s o f f o u r ( 3 , 4 -
diphenylcyclopentadienone)iron tricarbonyl compounds in
transfer hydrogenations of carbonyl compounds and transfer
dehydrogenations of alcohols were explored and compared to
those of commonly used 3. Compound 3 led to the lowest
overall conversions in the reactions that were monitored over
time, and compound 7 afforded the highest conversions the
most quickly in the same reactions. Catalyst 7 was as active as
or more active than 3 in all of the carbonyl reductions and
alcohol oxidations tested. Monitoring conversion over time
allowed us to see that the activity of 3 decreased dramatically
after approximately 24−30 h and that there appeared to be
some form of product inhibition in alcohol oxidations.
Spectroscopic evidence for the presence of a monomeric iron
hydride in transfer hydrogenations using 7 was found, and there
was no indication that a diiron bridging hydride analogous to 2
was present in reduction or oxidation reactions with 7. Finally,
initial evidence for dramatic differences in catalyst activity in the
transfer reduction of an imine was provided. Overall, these
studies illustrate that catalysts with phenyl rings in the 3- and 4-
positions of the cyclopentadienone are active in transfer
hydrogenations and dehydrogenations and deserve a more
comprehensive exploration. Further studies will be focused on
understanding why and how cyclopentadienone substitution
affects catalytic activity.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00037.
■
S
(6) For reviews, see: (a) Conley, B. L.; Pennington-Boggio, M. K.;
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Experimental and synthetic details and NMR spectra
(PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail for T.W.F.: tfunk@gettysburg.edu.
ORCID
Timothy W. Funk: 0000-0002-8828-1446
Present Addresses
(7) (a) Knolker, H.-J.; Heber, J. Transition Metal-Diene Complexes
̈
■
in Organic Synthesis, Part 18. Iron-Mediated [2+2+1] Cycloadditions
of Diynes and Carbon Monoxide: Selective Demetalation Reactions.
Synlett 1993, 1993, 924−926. (b) Knolker, H.-J.; Heber, J.; Mahler, C.
̈
H. Transition Metal-Diene Complexes in Organic Synthesis, Part 14.
Regioselective Iron-Mediated [2+2+1] Cycloadditions of Alkynes and
Carbon Monoxide: Synthesis of Substituted Cyclopentadienones.
Synlett 1992, 1992, 1002−1004. (c) Pearson, A. J.; Shively, R. J., Jr.;
Dubbert, R. A. Iron Carbonyl Promoted Conversion of α,ω-Diynes to
(Cyclopentadienone)iron Complexes. Organometallics 1992, 11,
4096−4104.
†A.R.M.: Food, Pharma, and Medical Solutions, The Dow
Chemical Company, Collegeville, PA 19426, United States.
‡R.A.S.: Quantitative Bioelement Imaging Center, Northwest-
ern University, Evanston, IL 60208, United States.
§D.K.K.: Department of Chemistry, Princeton University,
Princeton, NJ 08544, United States.
(8) (a) Thorson, M. K.; Klinkel, K. L.; Wang, J.; Williams, T. J.
Mechanism of Hydride Abstraction by Cyclopentadienone-Ligated
Carbonylmetal Complexes (M = Ru, Fe). Eur. J. Inorg. Chem. 2009,
2009, 295−302. (b) Fleischer, S.; Zhou, S.; Junge, K.; Beller, M.
General and Highly Efficient Iron-Catalyzed Hydrogenation of
Aldehydes, Ketones, and α,β-Unsaturated Aldehydes. Angew. Chem.,
Int. Ed. 2013, 52, 5120−5124. (c) Tlili, A.; Schranck, J.; Neumann, H.;
Beller, M. Discrete Iron Complexes for the Selective Catalytic
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a Single Investigator Cottrell
College Science Award from Research Corporation for Science
Advancement (19750) and by a UR American Chemical
Society Petroleum Research Fund grant (52162-UR1).
(9) Berkessel, A.; Reichau, S.; von der Hoh, A.; Leconte, N.;
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Neudorfl, J.-M. Light-Induced Enantioselective Hydrogenation Using
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