On the Diastereoselectivity of the Complexation of Ketopinic Acid-Derived 2-Acyloxy-1,3-cyclohexadienes and the Configurational Stability of Dienol-Fe(CO)3Complexes. A Case Study

Article abstract of DOI:10.1021/acs.organomet.1c00354

In the course of an attempt to synthesize 2-acyloxycyclohexa-1,3-diene-Fe(CO)3 complexes in nonracemic form, we reinvestigated the "fully diastereoselective"Fe(CO)3 complexation of (S,S)-2-ketopinoyloxy-1,3-cyclohexadiene, which had been described by Yeh and co-workers (Organometallics 2001, 20, 289-295). However, after cleaving off the chiral auxiliary unit, we only obtained racemic complexes, also for a related substrate. For this reason, we performed control experiments to exclude possible racemization mechanisms and confirmed the configurational integrity of the dienol-Fe(CO)3 intermediates using stereochemically defined dihydrocarvone-derived complexes. We finally could show that the complexation described by Yeh actually proceeds without any detectable diastereoselectivity. At the end, the virtually inseparable diastereomers of the chiral complexation products could be distinguished by careful NMR and chiral HPLC analyses.

Full text of DOI:10.1021/acs.organomet.1c00354

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Article
On the Diastereoselectivity of the Complexation of Ketopinic Acid-
Derived 2‑Acyloxy-1,3-cyclohexadienes and the Configurational
Stability of Dienol-Fe(CO)3 Complexes. A Case Study
Lars Hemmersbach, Steffen Romanski, Svetlana Botov, Andreas Adler, Jörg-Martin Neudörfl,
and Hans-Gunther Schmalz*
̈
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ABSTRACT: In the course of an attempt to synthesize 2-
acyloxycyclohexa-1,3-diene-Fe(CO)3 complexes in nonracemic
form, we reinvestigated the “fully diastereoselective” Fe(CO)3
complexation of (S,S)-2-ketopinoyloxy-1,3-cyclohexadiene, which
had been described by Yeh and co-workers (Organometallics 2001,
20, 289−295). However, after cleaving off the chiral auxiliary unit, we
only obtained racemic complexes, also for a related substrate. For this
reason, we performed control experiments to exclude possible
racemization mechanisms and confirmed the configurational integrity
of the dienol-Fe(CO)3 intermediates using stereochemically defined
dihydrocarvone-derived complexes. We finally could show that the
complexation described by Yeh actually proceeds without any
detectable diastereoselectivity. At the end, the virtually inseparable
diastereomers of the chiral complexation products could be distinguished by careful NMR and chiral HPLC analyses.
INTRODUCTION
■
Since their first preparation by Reihlen and co-workers,1 diene-
Fe(CO)3 complexes have enjoyed much attention. In
particular, they have found important application as synthetic
building blocks2 and even as bioactive agents.3 Some years ago,
Figure 2. Planar chirality of substituted η4-cyclohexadiene-Fe(CO)3
complexes.
we introduced acyloxy-substituted cyclohexadiene-Fe(CO)3
complexes as a promising class of enzyme-triggered CO-
releasing molecules (ET-CORMs).4 As an example, the methyl
fumarate-derived FumET-CORMs rac-1-A and rac-1-B (Figure
1) were shown to exhibit outstanding anti-inflammatory
properties.5
nonracemic form. These are (1) resolution of racemates
(either by physical, chemical, or enzymatic means);6 (2)
enantioselective transformation of prochiral complexes;7 (3)
direct enantioselective complexation;8 and (4) diastereoselec-
tive complexation of (temporarily) chirally modified sub-
strates.9
Since previous studies in our group had shown that only low
selectivity factors are observed in the kinetic resolution of rac-2
type complexes using esterases such as PLE or LCR4b and the
reported enantioselective complexation protocols proved to be
inapplicable in the preparation of this type of compounds, we
intended to synthesize complexes of type 2 by diastereose-
Figure 1. Structures of racemic FumET-CORMs rac-1-A/B.5
A general feature of such substituted η4-diene-Fe(CO)3
complexes is their planar chirality (Figure 2). Accordingly,
we were interested in preparing the ET-CORMs in nonracemic
form to investigate whether the enantiomers might exhibit
different biological activity.
Received: June 14, 2021
In principle, there are four conceptually different approaches
to synthesize planar chiral η4-diene-Fe(CO)3 complexes in
© XXXX The Authors. Published by
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lective complexation of a chirally modified cyclohexadiene
ligand of type 4 and subsequent replacement of the chiral
auxiliary moiety by an achiral acyl residue (Scheme 1).
According to our plan (Scheme 1), the next task was to
cleave the chiral auxiliary unit from complex 6. Of the various
conditions tested, only the reaction of 6 with hydrazine in
THF proceeded smoothly, and the initially formed cyclo-
hexadienolate-Fe(CO)3 complex (7) was directly trapped with
TIPS-OTf to give the stable O-silylated product 8. Following
our previously developed protocol,4b this compound was then
converted to the 2-pivaloyloxy-cyclohexadiene-Fe(CO)3 com-
plex 9 by in situ TIPS deprotection (with TBAF) and re-
esterification with pivaloyl chloride (Scheme 3).
Scheme 1. Retrosynthetic Analysis
Scheme 3. Conversion (Transesterification) of 6 to rac-9
Among the various reported methods for the diastereose-
lective complexation of chiral or chirally modified dienes,9 we
considered the method of Yeh and co-workers10 to be the most
promising for our purposes, as these authors claimed complete
diastereoselectivity in the complexation of cyclohexadiene
derivatives derived from ketopinic acid (Scheme 2).
Scheme 2. Complexation of the Chiral Diene 5 According to
Yeh10
However, to our great surprise, the product resulting from
the transesterification sequence proved to be fully racemic
(rac-9), as shown by HPLC analysis on a chiral stationary
phase. A racemic reference sample of rac-9 was prepared
independently from rac-8, which was obtained by direct
complexation.4b While the enantiomers of the intermediate (8/
ent-8) could not be separated by chiral HPLC, an optical
rotation value of ≤0.5° indicated that a racemic mixture had
already been obtained (from 6) earlier at the stage of 8
(Scheme 3).
Initially, we had no explanation as to why a racemate was
formed, and we wondered whether racemization might have
occurred at the stage of the dienolate complex intermediate 7.
Since we were not aware of any report that investigated the
configurational stability of planar-chiral dienolate complexes,
we decided to probe their stability using complexes derived
from (R)-(−)-carvone (10). To this end, the side chain of 10
was first selectively hydrogenated11 and the resulting enone
was converted to the dienol acetate 11 with LDA and acetic
anhydride.4a Complexation of 11 gave the two diastereomers
13a and 13b, which were easily separated by column
chromatography. Using Fe2(CO)9 in ether, the products
were formed in a 2:1 ratio. Remarkably, the diastereoselectivity
could be increased to 6:1 d.r., again in favor of the trans-
diastereomer 13a, by using the Fe(CO)3 transfer reagent 12.12
The structure of the two diastereomers (13a and 13b) was
unambiguously determined by X-ray crystallography (Figure
4).
RESULTS AND DISCUSSION
■
Accordingly, we started our investigation with the preparation
and complexation of the chiral cyclohexadienylester 5
following the published procedure. And indeed, we were able
to reproduce the reported transformation, and the data of the
isolated product matched those reported by Yeh for the
diastereomerically pure complex 6 (Scheme 2).10
We also managed to obtain a crystal of 6, which was
analyzed by X-ray crystallography to confirm the proposed
structure, including the relative and absolute configuration
(Figure 3). Interestingly, two conformers were found in the
crystal, differing in the orientation of the bicyclic ketopinic acid
moiety (rotation along the bond connecting the bicyclo[2.2.1]-
heptane unit to the attached ester group).
Figure 3. Structures of both rotamers of complex 6 present in the
crystalline state.
Figure 4. Structures of the diastereomeric carvone-derived complexes
13a (left) and 13b (right) in the crystalline state.
B
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In close analogy to the transformation of 6 to rac-8 (Scheme
3), the separated diastereomerically pure complexes 13a and
13b were then treated with hydrazine in THF, and the formed
dienolate intermediates were directly trapped by addition of
TIPS-OTf to give the O-silylated products 14a and 14b,
respectively (Scheme 4). Careful NMR analysis revealed within
the limits of detection that, in both cases, the products (14a
and 14b) were obtained as essentially pure diastereomers
(Figure 5).
Scheme 5. Possible Racemization of Complexes of Type 15
through a Base-Mediated [1,5]-Hydrogen Shift?
Scheme 6. Synthesis and Complexation of the Chiral
Acyloxycyclohexadiene 17
Scheme 4. Synthesis of Diastereomerically Pure Complexes
13a and 13b Starting from (R)-(−)-Carvone (10) and Their
Conversion to 14a and 14b, Respectively
conditions as before (Fe2(CO)9 in Et2O) to give complex 18,
again as an apparently pure diastereomer (as in the case of 6).
The reaction of complex 18 under the established conditions
of ester cleavage (hydrazinolysis and O-silylation) gave the
complexed TIPS dienol ether 19, from which the pivaloate rac-
20 (corresponding to rac-9) was obtained as before (Scheme
7). Again, the product (rac-20) was shown to be fully racemic
Scheme 7. Transesterification of 18 to rac-20 via the TIPS-
Protected Dienol-Fe(CO)3 Complex 19
by means of chiral HPLC. Moreover, a negligible [α]D value
indicated the formation of a racemic mixture already at the
stage of the intermediate rac-19.
Thus, having demonstrated in two cases that the exchange of
the chiral auxiliary of the primary complexation products (6
and 18) for an achiral acyl residue yielded only racemic
products, we concluded that the Yeh complexation does not in
fact proceed diastereoselectively. This means that, contrary to
expectation, the putatively pure samples of 6 and 18,
respectively, were actually obtained only as 1:1 mixtures of
inseparable diastereoisomers. However, we were somewhat
puzzled because Yeh had reported in his publication10 that the
diastereomer of 6 shows a characteristic 1H NMR signal of the
internal olefinic H atom at 5.24 ppm, and such a signal was
definitely not present in our spectra of 6 or 18.
Therefore, as another control experiment (Scheme 8), we
synthesized a diastereomeric mixture (6/dia-6) by esterifica-
tion of the racemic dienolate complex rac-7 (obtained in situ
from rac-8) with nonracemic ketopinic acid chloride (22).
Initially, we actually observed significant amounts (up to 40%)
of a second component in the crude product mixture that
showed the above-mentioned signal in the 1H NMR. However,
this putative diastereomer proved to be surprisingly unstable
and quickly vanished from the CDCl3 solution under oxidation
(rust formation). In acetone-d6, it proved to be significantly
more stable.
1
Figure 5. Superimposed olefinic sections of the H NMR spectra of
13a (red) and 13b (blue) and the corresponding silylethers 14a and
14b derived thereof.
The fact that the transesterification of the enantiomerically
and diastereomerically pure complexes 13a and 13b occurred
without any notable loss of stereochemical integrity strongly
indicated the configurational stability of the cyclohexadieno-
late-Fe(CO)3 intermediates.
Nevertheless, we could not completely exclude some
obscure racemization (Scheme 3), e.g., a (possibly base-
mediated) [1,5]-hydrogen shift occurring in conjunction with a
haptotropic migration of the Fe(CO)3 unit in complexes of
type 15 (Scheme 5).
Even though such a racemization mechanism (Scheme 5)
seemed very unlikely to us, we decided to investigate a second
system that lacks the possibility of a [1,5]-H shift. To this end,
the dimedone-derived cyclohexanone 1613 was α-deprotonated
with LDA in THF and the resulting dienolate was O-acylated
with ketopinic acid chloride prepared in situ (Scheme 6). The
resulting dienolester 17 was then complexed under the same
C
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diastereomers in both cases was strengthened. This conclusion
was finally confirmed by a very detailed analysis of the NMR
spectra of 6/dia-6 and 18/dia-18, respectively. Indeed, a closer
look at the 13C spectra at 125 MHz revealed a tiny doubling of
most signals, which was more pronounced in DMSO-d6 than in
Scheme 8. Control Experiment: Synthesis of a
Diastereomeric Mixture of Ketopinoyl-Cyclohexadiene-
Fe(CO)3 Complex 6/dia-6 and the Dienol complex rac-23
1
CDCl3. Also, in the H NMR of 6/dia-6, the existence of two
diastereomers is reflected by a splitting of the internal olefinic
signal, although this is not very distinct in CDCl3 (and was
misinterpreted by Yeh as a 3J coupling of 4 Hz). In DMSO-d6,
however, the splitting is very pronounced and clearly proves
the presence of two diastereomers (Figure 7).
The high oxidation sensitivity indicated that the second
compound was not a diastereomer of 6 but rather the dienol
complex rac-23. Indeed, treatment of the TIPS complex rac-8
with TBAF, followed by aqueous workup, afforded rac-23 as a
highly sensitive compound, showing the characteristic 1H
NMR signal at 5.35 ppm (in acetone-d6) (Figure 6). As
1
Figure 7. Section of the H NMR spectrum (internal olefinic H) of
6/dia-6 at 35, 45, and 55 °C (500 MHz, in DMSO-d6).
In order to exclude the possibility that the doubling of the
signals might not result from an impeded interconversion of
the rotamers (Figure 3), spectra were also recorded at higher
temperatures. However, no coalescence was observed at
temperatures up to 55 °C. For both diastereomers of 6/dia-
6, the internal olefinic signals (at 5.77 and 5.82 ppm,
respectively) appeared as double-doublet with coupling
constants of 3J = 6.7 Hz and 4J = 2.1 Hz, which was consistent
with our expectations. Noteworthy, the signals of both
diastereomers shifted slightly with increasing temperature to
higher field (Figure 7).
Figure 6. Section of the 1H NMR (500 MHz in acetone-d6) of
mixtures containing 6/dia-6 and rac-23 prepared according to either
Scheme 8 (red) or Scheme 9 (green).
As additional evidence, we finally succeeded in identifying
conditions that allowed the analytical separation of the
diastereomers (6/dia-6) by means of chiral HPLC (see
Supporting Information). The HPLC examination of a crystal
of 6 (used for X-ray crystal structure analysis) revealed the
presence of (quite predominantly) only one diastereomer. This
demonstrates the selective crystallization tendency of only the
one diastereomer (6), which may have contributed to the
original misinterpretation of the complexation outcome.
additional evidence, reaction of the ketopinic acid-derived
complex 6 (prepared by complexation according to Scheme 2)
with LiHMDS also led to the formation of the intermediate
rac-7, from which either the dienol complex rac-23 (after
aqueous workup) or the O-silylated dienol complex rac-8 was
obtained, the latter upon reaction with TIPS triflate (Scheme
9; Figure 6).
Having thus refuted Yeh’s claim that the diastereomeric
complexes 6 and dia-6 are readily distinguishable by NMR, our
conviction that the putative “fully diastereoselective” complex-
ation of the chirally modified dienes 5 and 17 proceeded
without any stereoselectivity to give a 1:1 mixture of
CONCLUSION
■
We have carefully demonstrated that the Fe(CO)3 complex-
ation of 1,3-cyclohexadiene ligands bearing a chiral ketopi-
noyloxy substituent does not proceed diastereoselectively as
reported.10 Therefore, much to our disappointment, this
method did not open a suitable option for the synthesis of
planar chiral acyloxy-substituted 1,3-cyclohexadiene complexes
in nonracemic or even enantiomerically pure form, as was our
original aim. Using dihydrocarvone-derived complexes 13a and
13b, we could once again demonstrate the high configurational
integrity of the chiral diene-Fe(CO)3 substructure, including
that of dienol and dienolate complexes which, to the best of
our knowledge, had never been investigated in this respect
before.
Scheme 9. Formation of the Cyclohexadienol-Fe(CO)3
Complex rac-7 through Base-Induced Cleavage of 6 (as
Indirectly Proven by Its Conversion into rac-23 or rac-8,
Respectively)
As this case study reflects, it took a lot of effort to clean up
an erroneous result from the literature. However, considering
D
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(c) Iwata, C.; Takemoto, Y. [Fe(diene)(CO)3] complexes as a
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the poorer experimental capabilities 20 years ago, we have no
reason to blame Yeh and co-workers for the misinterpretation
of their results. Scientific progress depends on constant review
and revision of experimental facts.
The question remains as to how planar-chiral 2-acylox-
ycyclohexa-1,3-diene Fe(CO)3 complexes of type 2 can be
prepared in nonracemic form. Besides racemate resolution, the
development of reliable methods based on diastereoselective
complexation of (temporarily) chirally modified substrates
appears to be an interesting challenge that should be worth
solving.
(3) (a) Schlawe, D.; Majdalani, A.; Velcicky, J.; Heßler, E.; Wieder,
T.; Prokop, A.; Schmalz, H.-G. Iron-Containing Nucleoside
Analogues with Pronounced Apoptosis-Inducing Activity. Angew.
Chem., Int. Ed. 2004, 43, 1731−1734. (b) Hirschhäuser, C.; Velcicky,
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Wieder, T.; Schmalz, H.-G. Nucleoside Analogues with a 1,3-Diene-
Fe(CO)3 Substructure: Stereoselective Synthesis, Configurational
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M.; Amslinger, S.; Schmalz, H.-G. Acyloxybutadiene Iron Tricarbonyl
Complexes as Enzyme-Triggered CO-Releasing Molecules (ET-
CORMs). Angew. Chem., Int. Ed. 2011, 50, 2392−2396. (b) Romanski,
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00354.
■
sı
Experimental procedures, NMR spectra of all new
compounds, HPLC chromatograms, crystallographic
details (PDF)
Accession Codes
CCDC 2086570−2086572, and 2097519 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
S.; Kraus, B.; Guttentag, M.; Schlundt, W.; Rucker, H.; Adler, A.;
̈
AUTHOR INFORMATION
Corresponding Author
Neudörfl, J.-M.; Alberto, R.; Amslinger, S.; Schmalz, H.-G.
Acyloxybutadiene tricarbonyl iron complexes as enzyme-triggered
CO-releasing molecules (ET-CORMs): a structure−activity relation-
ship study. Dalton Trans. 2012, 41, 13862−13875. (c) Botov, S.;
Stamellou, E.; Romanski, S.; Guttentag, M.; Alberto, R.; Neudörfl, J.-
M.; Yard, B.; Schmalz, H.-G. Synthesis and Performance of Acyloxy-
diene-Fe(CO)3 Complexes with Variable Chain Lengths as Enzyme-
Triggered Carbon Monoxide-Releasing Molecules. Organometallics
2013, 32, 3587−3594. (d) Sitnikov, N. S.; Li, Y.; Zhang, D.; Yard, B.;
Schmalz, H.-G. Design, Synthesis, and Functional Evaluation of CO-
Releasing Molecules Triggered by Penicillin G Amidase as a Model
Protease. Angew. Chem., Int. Ed. 2015, 54, 12314−12318.
■
Hans-Gunther Schmalz − Department of Chemistry,
̈
University of Cologne, 50939 Köln, Germany; orcid.org/
0000-0003-0489-1827; Email: schmalz@uni-koeln.de
Authors
Lars Hemmersbach − Department of Chemistry, University of
Cologne, 50939 Köln, Germany
Steffen Romanski − Department of Chemistry, University of
Cologne, 50939 Köln, Germany
Svetlana Botov − Department of Chemistry, University of
Cologne, 50939 Köln, Germany
Andreas Adler − Department of Chemistry, University of
Cologne, 50939 Köln, Germany
(5) Bauer, B.; Göderz, A.-L.; Braumuller, H.; Neudörfl, J. M.;
̈
Röcken, M.; Wieder, T.; Schmalz, H.-G. Methyl Fumarate-Derived
Iron Carbonyl Complexes (FumET-CORMs) as Powerful Anti-
inflammatory Agents. ChemMedChem 2017, 12, 1927−1930.
Jörg-Martin Neudörfl − Department of Chemistry, University
of Cologne, 50939 Köln, Germany
(6) (a) Alcock, N. W.; Crout, D. H. G.; Henderson, C. M.; Thomas,
S. E. Enzymatic resolution of a chiral organometallic ester:
enantioselective hydrolysis of 2-ethoxycarbonylbuta-1,3-dienetricarbo-
nyliron by pig liver esterase. J. Chem. Soc., Chem. Commun. 1988,
746−747. (b) Kappes, D.; Gerlach, H.; Zbinden, P.; Dobler, M.;
König, W. A.; Krebber, R.; Wenz, G. Optically Active Isoprene-
(tricarbonyl)iron(0) and Methyltrimethylenemethane(tricarbonyl)-
iron(0). Angew. Chem., Int. Ed. Engl. 1989, 28, 1657−1659.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.1c00354
Notes
The authors declare no competing financial interest.
(7) (a) Roush, W. R.; Wada, C. K. Application of η4-Diene Iron
Tricarbonyl Complexes in Acyclic Stereocontrol: Asymmetric Syn-
thesis of the as-Indacene Unit of Ikarugamycin (A Formal Total
Synthesis). J. Am. Chem. Soc. 1994, 116, 2151−2152. (b) Takemoto,
Y.; Baba, Y.; Honda, A.; Nakao, S.; Noguchi, I.; Iwata, C.; Tanaka, T.;
Ibuka, T. Asymmetric synthesis of (diene)Fe(CO)3 complexes by a
catalytic enantioselective alkylation using dialkylzincs. Tetrahedron
1998, 54, 15567−15580. (c) Fukuda, A.; Kobayashi, Y.; Kimachi, T.;
Takemoto, Y. Synthetic studies on macrolactin A by using a
(diene)Fe(CO)3 complex. Tetrahedron 2003, 59, 9305−9313.
(8) (a) Birch, A. J.; Raverty, W. D.; Stephenson, G. R. Chirality
transfer in the coordination sphere of iron. Organometallics 1984, 3,
1075−1079. (b) Knolker, H.-J.; Hermann, H. Asymmetric Catalysis in
the Complexation of Prochiral Dienes by the Tricarbonyliron
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the German
Research Foundation (DFG; project SCHM 857-20-1) and the
Fonds der Chemischen Industrie (doctoral fellowship to S.R.).
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https://doi.org/10.1021/acs.organomet.1c00354
Organometallics XXXX, XXX, XXX−XXX