Hierarchically assembled helicates as reaction platform – from stoichiometric Diels–Alder reactions to enamine catalysis

Article abstract of DOI:10.3762/BJOC.16.195

The stereoselectivity of a Diels–Alder reaction within the periphery of hierarchically assembled titanium(IV) helicates formed from mixtures of achiral, reactive and chiral, unreactive ligands was investigated in detail. Following the pathway of the chiral induction, the chiral ligands, solvents as well as substituents at the dienophile were carefully varied. Based on the results of the stoichiometric reaction, a secondary amine-catalyzed nitro-Michael reaction is performed as well which afforded reasonable diastereoselectivities.

Full text of DOI:10.3762/BJOC.16.195

Hierarchically assembled helicates as reaction platform – from
stoichiometric Diels–Alder reactions to enamine catalysis
David Van Craen, Jenny Begall, Johannes Großkurth, Leonard Himmel,
Oliver Linnenberg, Elisabeth Isaak and Markus Albrecht*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2020, 16, 2338–2345.
Institute of Organic Chemistry, RWTH Aachen University, Landoltweg
https://doi.org/10.3762/bjoc.16.195
1, 52074 Aachen, Germany
Received: 17 July 2020
Email:
Accepted: 10 September 2020
Published: 24 September 2020
Markus Albrecht* - markus.albrecht@oc.rwth-aachen.de
*
Corresponding author
This article is part of the thematic issue "Molecular recognition" and is
dedicated to the memory of Carsten Schmuck.
Keywords:
Diels–Alder reaction; enamine catalysis; hierarchical helicates;
remote-control; stereoselectivity
Guest Editor: J. Niemeyer
©
2020 Van Craen et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The stereoselectivity of a Diels–Alder reaction within the periphery of hierarchically assembled titanium(IV) helicates formed from
mixtures of achiral, reactive and chiral, unreactive ligands was investigated in detail. Following the pathway of the chiral induction,
the chiral ligands, solvents as well as substituents at the dienophile were carefully varied. Based on the results of the stoichiometric
reaction, a secondary amine-catalyzed nitro-Michael reaction is performed as well which afforded reasonable diastereoselectivities.
Introduction
Carbon–carbon (C–C) bond-forming reactions play a key role in et al. [12] are important examples in this context. Recently, we
organic chemistry. Hereby the stereoselectivity of the reaction described the use of hierarchically assembled helicates as tem-
is highly important due to the different behavior of stereoiso- plates for stereoselective Diels–Alder reactions via a post-func-
mers in human metabolism [1,2]. Stereocontrol was achieved tionalization process [13]. Catechol ligands L-H2 with an ester
either via an auxiliary [3-7] or a catalyst [8], both providing the functionality in the 3-position were prepared via conversion of
stereoinformation necessary for induction during the C–C bond the acid chloride of 2,3-dihydroxybenzoic acid to the corre-
formation. Catalytic approaches for C–C bond-forming reac- sponding esters. These ligands underwent a complexation with
tions even found their way into the relatively young field of titanoyl(IV) bisacetylacetonate and lithium carbonate initially
supramolecular chemistry, e.g., regioselective Diels–Alder reac- forming a mononuclear “Werner-type” triscatecholate
tions within supramolecular hosts as described by Fujita et al. titanium(IV) complex. Two of these monomers dimerized in a
[
9-11] or stereoselective nucleophilic substitutions by Raymond consecutive step to obtain a non-covalently linked helicate
2338

Beilstein J. Org. Chem. 2020, 16, 2338–2345.
(
Scheme 1). The dimerization took place via the coordination of dynamic and fast diastereomerizing/epimerizing monomer was
three lithium cations acting as bridges between two monomeric the major species in DMF switching off the stereoselectivity
complex units [13-20]. [13].
Enantioselectivities up to 25% ee at elevated temperature Herein we investigated the induction pathway and significantly
32% ee at 0 °C) depending on the substrate were achieved in a optimized the stereoselectivity of the reaction. Furthermore, a
(
Diels–Alder reaction by introducing two different substituted catalytic approach was introduced which paves the way to the
catechol ester ligands during the complex formation: (1) A final goal of supramolecular stereoselective catalysis with hier-
diene-substituted ligand 1-H2 for the Diels–Alder reaction archical helicates as homogeneous catalysts.
[
21,22] and (2) a chiral ligand 2-H2 for the stereocontrol [13].
Cleaving the complex under acidic conditions resulted in the
desired enantiomerically enriched product 9 and enabled the
recovery of the chiral ligand 2-H2 (Scheme 2) [13].
Results and Discussion
Stereoselective Diels–Alder reactions in the
periphery of hierarchically assembled
The solvent choice allowed on/off-switching of the stereoselec- helicates
tivity of the Diels–Alder reaction. In THF the stereochemically Elucidating the induction pathway of the Diels–Alder reaction
locked dimer of the hierarchical helicate was present. Here is vital for the optimization of the system described above and
stereoselectivity was turned on. On the other hand, the highly for the development of future processes based on the principle
Scheme 1: Formation of hierarchically assembled lithium-bridged titanium(IV) helicates as well as the ligands used for the stereoselective Diels–Alder
reaction.
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Beilstein J. Org. Chem. 2020, 16, 2338–2345.
information of, e.g., a chiral auxiliary, Lewis acid or catalyst. In
the previously reported system two different induction path-
ways were conceivable: (1) A chiral ligand is located close to
the diene and controls the stereochemistry of the cycloaddition.
(
2) The chiral ligand controls the helicity of the helicate (ΔΔ or
ΛΛ) and the helix induces the stereoselectivity of the
Diels–Alder reaction.
To find out which of the induction pathways takes over the
control of the Diels–Alder reaction in the periphery of the heli-
cates, a specific helicity was induced at an achiral diene bear-
ing helicate. It has been described before that an addition of
chiral ammonium salts leads to the preference of a specific twist
at the helicate [32]. As inductor, (R)-1-phenylethylammonium
chloride was added to the racemic hexadiene-substituted heli-
cate [Li3(1)6Ti2]−. The chiral salt influences the helicity of the
monomeric complexes and which dimerize to the right-handed
(
ΔΔ) helicate [32]. As the process is slow, the mixture of the
ammonium salt and complex was stirred for two weeks at room
temperature in methanol. Thereafter, the solvent was removed
and the Diels–Alder reaction with N-benzylmaleimide was per-
formed at elevated temperature in THF. The reaction yielded
the racemic product after purification. Scheme 3 is showing that
Scheme 2: Previously reported on/off switch for “remote-controlled”
23-31] stereoselectivity of a Diels–Alder reaction by use of different
solvents. The heteroleptic complexes are mixtures with an average
ligand distribution as shown [13].
[
to use self-assembled coordination platforms (or as in the the induction of stereochemistry of the Diels–Alder reaction
present case mixtures thereof) to control stereoselective C–C depends on the chirality at the chiral ligand and not at the helix.
bond-forming reactions. Stereoinduction usually relies on This allows improvement of the stereoselectivity by using more
spatial proximity of the prochiral carbon atoms and a chiral appropriate sterically hindered or rigid chiral ligands. In addi-
Scheme 3: Elucidating the pathway of the stereoinduction of the Diels–Alder reaction. Ten equivalents of chiral ammonium salt are added to the hier-
archical helicate in methanol and stirred for two weeks. Afterwards methanol is removed and the residue is dissolved in THF to perform a Diels–Alder
reaction at the side chain.
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Beilstein J. Org. Chem. 2020, 16, 2338–2345.
tion, a solvent screening was performed in which solvents were Diels–Alder reaction due to their poor ability to stabilize lithi-
used which favor the dimer. This is imminent for good enantio- um cations. Chloroform showed the best induction with 32% ee
selectivities because the presence of a high amount of stereola- followed by dichloromethane with 25% ee, both with 50% yield
bile monomer switches off the selectivity [13].
(Table 1).
Solvent dependence
Ligand screening
Initially the solvent dependence of the stereochemical induc- In a second optimization step, the chiral ligands have been
tion of the Diels–Alder reaction by the phenylethyl-derived varied. An increase of stereoselectivity was achieved by using
ligand 2 was studied using N-benzylmaleimide (8e) as dieno- the helicates with a statistical ligand distribution
phile (Table 1). The solvents dioxane (17% ee) and acetone Li[Li3(1)3(L*)3Ti2] (L* = 3–7-H2). The given formula only de-
(
14% ee) showed a slight decrease of the enantioselectivity scribes the ratio of the ligands but in fact a statistical mixture of
compared to THF (21% ee). The yields of the reactions were complexes Li[Li3 (L*)6 Ti2 ], Li[Li3 (1)(L*)5 Ti2 ],
rather moderate. On the other hand, the use of acetonitrile had Li[Li3(1)2(L*)4Ti2], Li[Li3(1)3(L*)3Ti2], Li[Li3(1)4(L*)2Ti2],
no significant influence on the yield compared to acetone while Li[Li3(1)5(L*)1Ti2], and Li[Li3(1)6Ti2] is present. Expanding
the enantioselectivity dramatically dropped to 8% ee. In this the aromatic unit to a naphthyl group in 3-H2 resulted in an
case the lower selectivity correlated with the increasing lithium increase of the enantioselectivity to 44% ee. Even better selec-
solvating capability of the solvent resulting in a higher propor- tivities were obtained with 4-H2 bearing an indanyl [33,34] sub-
tion of the monomer and thus in lower stereoselectivities. In stituent which combines a stereogenic center implemented in a
contrast to this, less polar solvents such as dichloromethane and ring system providing rigidity as well as an aromatic residue.
chloroform resulted in increasing stereoselectivities in the The enantioselectivity increased to 58% ee (Table 1).
Table 1: Optimization of the stereoselectivity achieved of the Diels-Alder reaction at hierarchical helicates with solvent and chiral ligand screening.
Entry
L*
9
R
solvent
T
yield
[%]
ee
[%]
[°C]
1
2
3
4
5
6
7
8
9
2
2
2
2
2
2
3
4
5
6
7
4
4
4
4
e
e
e
e
e
e
e
e
e
e
e
a
b
c
d
Bz
Bz
Bz
Bz
Bz
Bz
Bz
Bz
Bz
Bz
Bz
Me
Et
THF
70
105
60
86
44
65
65
65
65
65
65
65
65
65
65
77 [13]
53
21 [13]
17
14
8
dioxane
acetone
MeCN
DCM
50
44
50
25
32
44
58
46
16
−8
43
39
18
49
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
50
71
64
61
10
11
12
13
14
15
64
11
76
79
t-Bu
Cy
82
80
aReactions performed in closed tubes.
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Beilstein J. Org. Chem. 2020, 16, 2338–2345.
Besides the aromatic ligands, terpenyl-substituted ligands were vestigated as catalysts. The ligands with potential catalytic ac-
investigated, too. The largest ligand 7-H2 with a cholesteryl tivity were synthesized in a three-step approach (Scheme 4).
moiety favored the opposite enantiomer, however, only with Initially the amino alcohols 10a–d were protected with a Boc
−
8% ee in only 11% yield. The low yield may be attributed to group [42,43]. Esterification of the protected alcohols 11a–d
the poor solubility of the helicate. The other terpene [35,36] [33,44] with 2,3-dioxosulfinylbenzoyl chloride obtained from
derived systems Li[Li3(1)3(5)3Ti2] and Li[Li3(1)3(6)3Ti2] 2,3-dihydroxybenzoic acid and thionyl chloride afforded the
showed a different behavior. The (1S,2S,3S,5R)-3-pinanyl- N-Boc-substituted catechol ligands 12a–d [33,36]. They were
substituted Li[Li3(1)3(5)3Ti2] yielded 46% ee, while the com- deprotected under acidic conditions with hydrochloric acid
plex bearing a ʟ-(−)-borneyl residue Li[Li3(1)3(6)3Ti2] showed yielding ligands 13a–d-H2 [33,36] as ammonium chloride salts.
only 16% ee. The yields were higher than 60%. A possible
reason for the significant drop in enantioselectivity by switching
from ligand 5 to 6 was due to the different dimerization behav-
ior. The homoleptic helicate Li[Li3(6)6Ti2] shows a lower
dimerization tendency compared to Li[Li3(5)6Ti2] [35,36].
Thus, the higher amount of undesired monomer in solution of
Li[Li3(1)3(6)3Ti2] resulted in a partial switch-off of the stereo-
selectivity.
Screening of the dienophile
The variation of the dienophile was studied in chloroform using
the helicate Li4[(1)3(4)3Ti2]. N-Maleimides 8a and 8b with a
methyl and an ethyl residue showed higher yields and a lower
induction in comparison to the benzyl derivative 8e with
4
3% ee and 39% ee (Table 1). The poorest result was obtained
by using dienophile 8c with a tert-butyl substituent (82% yield,
8% ee). This maleimide gave the lowest induction in our
1
previous work, too [13]. Thus, no improvement was made in
comparison to the 15% ee [13] achieved with chiral ligand 2-H2
in THF as solvent. The cyclohexyl-substituted dienophile 8d
showed a higher induction (49% ee and 80% yield) than 8a and
8
b, but could not reach the results of 8e. The described optimi-
zation of the reaction conditions based on solvent, chiral ligand,
and substituent at the dienophile resulted in a nearly threefold
increase of the enantioselectivity compared to the earlier de-
scribed results [13].
Scheme 4: Synthesis of the ligands with secondary amine-containing
substituents.
The obtained ligands 13a–d-H2 were used together with the
The screening showed the opportunity to use hierarchically chiral ligands 2,4,5-H2 for the formation of hierarchical heli-
formed helicates with mixtures of ligands as platforms to cates with a statistical ligand ratio which were formed from
control the stereochemistry of C–C bond-forming reactions. 1 equivalent of 13-H2 and 5 equivalents of 2-H2, 4-H2, and
However, it would be of great advantage to transfer the find- 5-H2.
ings to catalytic C–C bond-forming reactions which are cata-
lyzed by hierarchical helicates containing chiral ligands for The catalytic activity of the amine ligands was tested first by
stereocontrol and achiral catalytically active ligands.
using the uncoordinated ligand 13a-H2 substituted with a
N-methylethylamine moiety. The reaction was performed in
DMSO-d6 due to solubility limitations of the ligand. Fast and
Enamine-catalyzed nitro-Michael reactions
The nitro-Michael reaction [37-40] seemed to be suitable to be easy measurement of the yield and the diastereoselectivity was
performed at hierarchically assembled helicates due to the reac- possible by NMR spectroscopy. The nitro-Michael reaction of
tion’s “benchmark character” [41]. Therefore, ligands bearing 3 equivalents propanal (14) and β-nitrostyrene (15) with
secondary amine residues were introduced instead of the diene 25 mol % of 13a-H2 after 2 days at room temperature resulted
ligands. Again helicates with a statistical distribution of chiral in 45% yield of product 16 and a nearly 1:1 diastereomeric ratio
ligands and of the new amine ligands in the complex were in- (Table 2).
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Beilstein J. Org. Chem. 2020, 16, 2338–2345.
Table 2: Enamine-catalyzed nitro-Michael reaction with hierarchically assembled helicates.a
Entry
catalyst
mol %
T
t
[d]
yield
[%]
dr
[°C]
1
2
3
4
5
6
7
8
9
b
13a-H2
25
15
15
15
7.5
15
15
15
15
rt
rt
rt
0
2
3
3
7
1
3
3
3
3
45c
0
52:48c
–
Li4[(13a)1(2)5Ti2]
Li4[(13b)1(2)5Ti2]
Li4[(13b)1(2)5Ti2]
Li4[(13b)1(2)5Ti2]
Li4[(13c)1(2)5Ti2]
Li4[(13d)1(2)5Ti2]
Li4[(13b)1(4)5Ti2]
Li4[(13b)1(5)5Ti2]
88
71
48
0
83:17
87:13
65:35
–
70
rt
rt
rt
rt
0
–
13
27
80:20
80:20
aNo enantioselectivity was achieved. bReaction was performed in DMSO-d6 (0.26 M) due to solubility limitations of the free ligand with 3 equiv of
propanal. cValues determined by integration of the crude NMR spectrum of the reaction.
Catalysis at the “statistical” helicates was carried out with tivity. A reasonable diastereoselectivity of 60% de was ob-
5
equivalents of propanal (14) in order to gain a higher conver- served for both catalysts. The limited solubility of these com-
sion. Beside a significant control over the diastereomeric ratio plexes caused a significant reduction of the yield at room tem-
no enantioselectivity was achieved with helicates as catalysts. perature and due to this the reaction was not performed at lower
Catalysts at concentrations of 15 mol % were used in CDCl3 at temperatures.
room temperature and 0 °C with three or seven days of reaction
time. The conversion was controlled by NMR spectroscopy and Conclusion
TLC. The helicate Li4[(13a)1(2)5Ti2] did not lead to any An optimization of the Diels–Alder reaction taking place in the
conversion at room temperature (Table 2). The catalyst periphery of hierarchically assembled helicates was carried out.
Li4[(13b)1(2)5Ti2] with an ethyl-substituted amine worked well It was based on the elucidated induction pathway showing that
resulting in 88% yield and 66% de at room temperature. The the stereoselectivity was due to the proximity of the chiral units
diastereomeric excess increased slightly to a maximum of 74% of ligand 2 to the diene unit. The helicity of the helicate did not
de (dr 87:13) at 0 °C. A dramatic decrease to 30% de was ob- have a significant influence. After optimization of solvent,
served by lowering the catalyst loading to 7.5 mol % while in- chiral ligand, and substituent at the dienophile stereoselectivity
creasing the temperature to 70 °C. No enantioselectivity was was nearly tripled. Up to 58% ee was achieved in the
observed using the helicate Li4[(13b)1(2)5Ti2] as catalyst. The Diels–Alder reaction in chloroform with the indanyl-substi-
helicates Li4[(13c)1(2)5Ti2] and Li4[(13d)1(2)5Ti2] with an iso- tuted chiral ligand 4-H2 and N-benzylmaleimide (8e) as the
propyl-substituted ethylamine and a cyclic secondary amine dienophile.
ligand as catalytically active unit showed no conversion in the
nitro-Michael reaction. Solubility problems were the supposed In addition, the transition from the stoichiometric Diels–Alder
reason for this observation. Thus, the amine ligand 13b-H2 reaction to a catalytic nitro-Michael reaction was described
seemed to be an appropriate component to make helicates from utilizing secondary amine ligands as catalysts. Only amine
ligand mixtures which possess catalytic activity.
ligand 13b-H2 seemed suitable in the catalysis with the corre-
sponding statistical helicates. With other complexes solubility
Exchange of the chiral ligand 2 by other chiral ones resulted in problems arose. Li4[(13b)1(2)5Ti2] was the most efficient cata-
the corresponding complexes Li4[(13b)1(4)5Ti2] and lyst discussed in this study and provided good yields of up to
Li4[(13b)1(5)5Ti2], but did not lead to a control of enantioselec- 88% at room temperature. Suitable diastereoselectivities were
2343

Beilstein J. Org. Chem. 2020, 16, 2338–2345.
5
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Supporting Information
1
1
1
Supporting Information File 1
Synthetic procedures, characterization data, SFC and HPLC
conditions and copies of 1H and 13C NMR spectra of new
compounds.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-16-195-S1.pdf]
1
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1
Acknowledgements
We sincerely thank Prof. Magnus Rüping, Cornelia Vermeeren,
Prof. Carsten Bolm and Marcus Frings for the determination of
the enantio- and diastereoselectivities via SFC and HPLC. Parts
of this publication have been published in the Ph.D. thesis [33]
of Dr. David Van Craen.
1
7.Baumert, M.; Albrecht, M.; Winkler, H. D. F.; Schalley, C. A. Synthesis
2010, 953–958. doi:10.1055/s-0029-1218684
1
8.Albrecht, M.; Isaak, E.; Baumert, M.; Gossen, V.; Raabe, G.;
Fröhlich, R. Angew. Chem. 2011, 123, 2903–2906.
doi:10.1002/ange.201006448
Angew. Chem., Int. Ed. 2011, 50, 2850–2853.
doi:10.1002/anie.201006448
Funding
1
9.Van Craen, D.; de Groot, M.; Albrecht, M.; Pan, F.; Rissanen, K.
Z. Anorg. Allg. Chem. 2015, 641, 2222–2227.
doi:10.1002/zaac.201500543
Support by the international graduate school SeleCa – Selec-
tivity in Chemo- and Biocatalysis of the DFG (Deutsche
Forschungsgemeinschaft) is gratefully acknowledged.
2
0.Van Craen, D.; Rath, W. H.; Huth, M.; Kemp, L.; Räuber, C.;
Wollschläger, J. M.; Schalley, C. A.; Valkonen, A.; Rissanen, K.;
Albrecht, M. J. Am. Chem. Soc. 2017, 139, 16959–16966.
doi:10.1021/jacs.7b10098
ORCID® iDs
David Van Craen - https://orcid.org/0000-0002-0501-2233
Johannes Großkurth - https://orcid.org/0000-0001-6525-2872
Markus Albrecht - https://orcid.org/0000-0002-3501-1477
2
1.Chakrabarty, R.; Stang, P. J. J. Am. Chem. Soc. 2012, 134,
14738–14741. doi:10.1021/ja3070073
2
2.Wang, M.; Lan, W.-J.; Zheng, Y.-R.; Cook, T. R.; White, H. S.;
Stang, P. J. J. Am. Chem. Soc. 2011, 133, 10752–10755.
doi:10.1021/ja204155r
Preprint
2
2
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3.van Leeuwen, P. W. N. M.; Rivillo, D.; Raynal, M.; Freixa, Z.
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4.Ohmatsu, K.; Ito, M.; Kunieda, T.; Ooi, T. Nat. Chem. 2012, 4,
A non-peer-reviewed version of this article has been previously published
as a preprint: https://doi.org/10.3762/bxiv.2020.82.v1
473–477. doi:10.1038/nchem.1311
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