Cooperative Catalysis with Coupled Chiral Induction in 1,3-Dipolar Cycloadditions of Azomethine Ylides

Article abstract of DOI:10.1002/chem.201801433

1,3-Dipolar cycloadditions (1,3-DC) between imino esters (as precursors of N-metallated azomethine ylides) and π-deficient alkenes are promoted by cooperative asymmetric Lewis acid/Br?nsted base catalysis. The components of these catalytic pairs are silver salts derived from enantiopure commercially available BINOL-based phosphoric acids and Cinchona alkaloids. Chiral phosphoric silver(I) salts promote HOMO raising of in situ formed 1,3-dipoles, whereas protonated cinchona alkaloids generate a LUMO lowering for the dipolarophiles resulting in a global acceleration of the 1,3-DC. The best results were obtained with BINOL-derived silver phosphate and hydrocinchonine. Matching between both cooperative metallo- and organocatalyst results in an enhanced enantiomeric excess, superior to that reached by both separate components. NOESY experiments and DFT calculations are compatible with a non-covalent interaction (hydrogen bond) between both catalysts, which results in close contacts and mutually coupled chiral environments.

Full text of DOI:10.1002/chem.201801433

A Journal of
Accepted Article
Title: Cooperative Catalysis with Coupled Chiral Induction in 1,3-
Dipolar Cycloadditions of Azomethine Ylides
Authors: Jose Miguel Sansano, Alberto Cayuelas, Olatz Larrañaga,
Verónica Selva, Carmen Nájera, Takahiko Akiyama, Abel de
Cózar, José I. Miranda, and Fernando P. Cossío
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To be cited as: Chem. Eur. J. 10.1002/chem.201801433
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Cooperative Catalysis with Coupled Chiral Induction in 1,3-
Dipolar Cycloadditions of Azomethine Ylides
Alberto Cayuelas ,[a][b] Olatz Larrañaga,[b][d] Verónica Selva, [a][b][c] Carmen Nájera,* [a][b] Takahiko
Akiyama,[e] José M. Sansano,*[a][b][c] Abel de Cózar,[b][d][f] José I. Miranda,[g] and Fernando P. Cossío¥[b][d]
Dedicated to Prof. Miguel Yus on the occasion of his retirement.
Abstract: 1,3-Dipolar cycloadditions (1,3-DC) between imino esters
(as precursors of N-metallated azomethine ylides) and deficient
alkenes are promoted by cooperative asymmetric Lewis acid-
Brønsted base catalysis. The components of these catalytic pairs are
silver salts derived from enantiopure commercially available BINOL-
based phosphoric acids and Cinchona alkaloids. Chiral phosphoric
silver(I) salts promote HOMO raising of in situ formed 1,3-dipoles,
whereas protonated cinchona alkaloids generate a LUMO lowering of
the dipolarophiles resulting a global acceleration of the 1,3-DC. The
best results were obtained with BINOL-derived silver phosphate and
hydrocinchonine. Matching between both cooperative metallo- and
organocatalyst results in an enhanced enantiomeric excess, superior
to that reached by both separate components. NOESY experiments
and DFT calculations are compatible with a non-covalent interaction
(hydrogen bond) between both catalysts, which results in close
contacts and mutually coupled chiral environments.
of Figure 1, the selection of different combinations of catalysts
provided access to all possible stereoisomers of a target
compound in high enantiomeric excess. Therefore, this strategy
is based on the independent chiral induction generated by both
catalysts (Figure 1a). This model contrasts with the clear double
chiral activation of a single substrate one (Figure 1b).
Scientists learn many key synthetic concepts from nature. The
simple and sophisticated strategies employed in biocatalysis have
been implemented in modern asymmetric synthesis,
1 , 2
synergistic/cooperative catalysis
being one of the most
Figure 1. Possible cooperative catalytic models.
fascinating areas (Figure 1).3 Due to the opposite nature of chiral
catalysts involved in these types of processes, they are free to
interact to each other and give inactive species. To overcome this
drawback it is necessary to select them in such a way that a
reversible binding can occur allowing the corresponding substrate
activation separately. Concerning the interaction of two chiral
entities,4 successful metal catalyst-metal catalyst, metal catalyst-
organocatalyst, and organocatalyst-organocatalyst have been
reported.3
This necessary reversible binding, associated with the high
stereocontrol exhibited, for example, by cycloadditions in their
corresponding transition states allows to expand the scope and
synthetic utility of them.6,7,8 Particularly, catalytic enantioselective
9
, 10
1,3-dipolar cycloaddition (1,3-DC)
involving stabilized
azomethine ylides and electrophilic alkenes have been promoted
using different strategies: a) monofunctional chiral metal
complexes;10a,c b) bifunctional chiral complexes;10a,c,11 c) double
chiral activation of metal catalysts;12 d) monofunctional chiral
organocatalysts;10a,c and e) bi- or polyfunctional chiral
organocatalysts.13 However, no cooperative catalysis generated
by two chiral entities, bound by non-covalent interactions,
activating both the dipole and the dipolarophile has been reported
to date (Figure 1c).14
In this work, the effects of introducing chirality in the Lewis
acid catalyst, together with the chirality of the Brønsted base, in
the diastereo- and enantioselective 1,3-DCs between imino
esters 1 and electrophilic alkenes 2 to give enantiomerically
enriched prolines 3, will be surveyed (Scheme 1). For this purpose,
several Cinchona alkaloids 4-9 as chiral Brønsted bases and a
series of chiral BINOL-derived silver phosphates,1516 generated
by mixing the corresponding chiral phosphoric acids 10-16 with
silver carbonate for 1 h, at room temperature, will be evaluated.
Initially, the solution containing the chiral silver phosphate
was allowed to react with imino ester 1a and N-methylmaleimide
(NMM, 2a), in the presence (or absence) of the chiral base
(reagents added in this order), in toluene as solvent, at rt avoiding
Within a wider context, Carreira et al.5 firstly introduced the
concept of stereodivergent dual catalysis for the enantioselective
-allylation of aldehydes. In that case, related to the (a) strategy
[a] Dr. A. Cayuelas, V. Selva, Prof. Dr. C. Nájera, Prof. Dr. J. M. Sansano
Departamento de Química Orgánica. Universidad de Alicante.
Ctra. Alicante-San Vicente s/n, 03080-Alicante, Spain.
[b] Centro de Innovación en Química Avanzada (ORFEO-CINQA).
[c] Institute of Organic Synthesis. Universidad de Alicante.
[d] Dr. O. Larrañaga, Dr. A. de Cózar, Prof. Dr. F. Cossío
Departamento de Química Orgánica I, Facultad de Química,
Universidad del País Vasco/Euskal Herriko Unibertsitatea UPV/EHU., P.
K. 1072, E-20018 San Sebastián, Spain.
[e] Prof. Dr. Takahiko Akiyama
Department of Chemistry, Faculty of Science. Gakushuin University, 1-
5-1 Mejiro, Toshima-ku, Tokyo, 171-8588 Japan.
[f] IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain.
[g] Dr. José I. Miranda
SGIker NMR Facility. Universidad del País Vasco.
*Corresponding Author for experimental: cnajera@ua.es, jmsansano@ua.es.
¥Corresponding Author for calculations: fp.cossio@ehu.es.
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(Table 1, entry 12). Identical result of 3a was achieved employing
the chiral phosphate (R)-13/Ag2CO3 and 4a (Table 1, entry 14),
but phosphoric acids 11, 12, and 14-16 furnished poor
enantioselections of 3a in low ee. The most important control in
the enantioselection was provided by the chiral base (Table 1,
entry 13) meanwhile the chiral silver phosphate cooperated to get
the maximum level of chiral induction (Table 1, entry 12). The
opposite configuration was easily generated using cinchonidine 7
together with (R)-10/Ag2CO3 giving ent-endo-3a in good
conversion and 93% ee (data not included in Table 1). The
absolute configuration of endo-3a was assigned by comparison of
HPLC data and the specific optical rotations of purified samples
with the reported ones in previous articles. 18 So, the best
conditions described in entry 12, employing commercially
available chiral sources, were used for both mechanistic and
scope studies.
Table 1. Optimization of the 1,3-DC between imino ester 1a and NMM 2a.
Entry
1
Chiral acid
Base
Conversion (%)[a]
100
ee (%)[b]
__
Scheme 1. Cinchona alkaloids 4-9 and chiral BINOL-derived phosphoric acids
10-16 employed in 1,3-DC between imino ester 1a and N-methylmaleimide 2a.
(R)-10
60 ent-endo-3a
__
__
__
2
(R)-11
(R)-12
(R)-13
(R)-10
(R)-10
(R)-10
(R)-10
(R)-10
(R)-10
(S)-10
96
34 ent-endo-3a
light exposure (Scheme 1 and Table 1).17 Preliminary results,
obtained in the absence of a base, revealed that the best chiral
silver phosphate was formed by silver carbonate and acid (R)-10
giving ent-endo-3a in excellent both conversion and
diastereoselectivity (>98:2 determined by 1H NMR) although with
moderate enantioselectivity (Table 1, entry 1). The mixture (S)-
10/Ag2CO3 afforded the same results yielding cycloadduct endo-
3a instead. The employment of phosphoric acids 11-13 gave
lower enantioselectivity (Table 1, entries 2-4) whilst reactions
performed with hindered silver phosphates, derived from acids
14-16, furnished almost racemic endo-3a (data not included in
Table 1). The bifunctional character of these chiral silver
phosphates in this 1,3-DC was disrupted by the introduction of an
external base, such as trimethylamine, lowering the
enantioselectivity of the process (Table 1, entry 5). In the
presence of Cinchona alkaloids as chiral bases (4a, 6-9), the
results were satisfactory, especially when cinchonine 6,
cinchonidine 7 (in this example ent-endo-3a was obtained), and
hydrocinchonine 4a were used (Table 1, entries 6-10).
Hydrocinchonine 4a was selected for the next experiments
because a higher purity of the crude endo-3a product was
observed by 1H NMR.Under these conditions, other solvents such
as THF, CH2Cl2, tert-butyl methyl ether, acetonitrile and methanol
were tested and the ee was not improved (data not included in
Table 1). The lowering of the temperature to 0 ºC was not efficient
in terms of conversion and enantioselectivity. The matched
combination between phosphoric acid (R)-10/Ag2CO3/4a was
demonstrated after analysis of the enantioselectivity obtained in
the reaction mediated by the catalytic system formed by
mismatched (S)-10/Ag2CO3/4a triad (Table 1, entry 11).
According to this set of results, it was considered that the order of
the addition of reagents (Cinchona alkaloid was added at the end)
facilitated the reaction of the (R)-10/Ag2CO3 by itself with low
enantioselectivity such as it was demonstrated in entry 1 of Table
1. So, a different addition order was attempted (Figure 2) where a
better interaction chiral base-imino ester was ensured. This new
procedure allowed to obtain cycloadduct endo-3a in 99% ee
3
100
100
100
100
91
20 ent-endo-3a
4
26 ent-endo-3a
5
Et3N
4a
6
52 ent-endo-3a
6
83
7
83
8
7
97
83 ent-endo-3a
9
8
97
63 ent-endo-3a
10
11
12[c]
13[c]
14[c]
9
100
97
65
20
99
83
99
4a
4a
4a
4a
(R)-10
100
100
100
__
(R)-13
[a] Determined by analysis of crude 1H-NMR spectra. [b] Determined by standard or SCF
HPLC using columns with chiral stationary phases. [c] Reaction performed using the
addition protocol described in Figure 2.
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Figure 2. Optimized addition sequence of cooperative catalysts 1+23.
points shown in Figure 4, TSa is the one of lowest energy because
of optimal interaction between the phosphoric and alkaloid units.
The solvent accessible surface (SAS) shows close contacts
between both chirality sources, in agreement with the NOESY
experiments. Consequently, a quite rigid structure is formed,
which generates and enhances chiral induction with respect to the
separate catalysts.
In order to better understand our experimental results, we
analyzed in detail the parent 1,3-DC between imino ester 1a and
NMM 2a in the presence of Ag2CO3 and catalysts (R)-10 and
hydrocinchonine 4a. On the basis of previous studies19 on the
mechanism of asymmetric catalyzed 1,3-DC between azomethine
ylides and π-deficient alkenes, the catalytic cycle gathered in
Figure 3 was considered. In addition, we optimized several
stationary points associated with this mechanism by means of
DFT calculations, which were carried out at the
M06(PCM,solvent=PhMe)6-31G(d) & LANL2DZ//B3LYP/6-31G
(d)&LANL2DZ level of theory.20
According to this catalytic cycle, the protocol described in
Figure 2 ensures that when the substrates 1a and 2a are added,
the Ag+ cation formed a chiral salt with the corresponding
phosphoric acid. In the presence of both catalysts, imino ester 1a
generated an N-metalated azomethine ylide INT1 that can react
with dipolarophile 2a or with its activated complex INT2. However,
when the structure of INT1 was optimized, and its reaction with
2a was analyzed, we found that the chiral information of the
phosphoric acid is not optimal to reach an effective blockage of
one prochiral face of the dipole and low theoretical
enantioselectivity towards ent-endo-3a was obtained (eecalc= -16,
see the SI for further details), in qualitative agreement with the
experimental results (Table 1, entries 1-5). Similarly, DFT analysis
of the reaction of INT2 with the N-metallated azomethine ylide of
1a in the absence of (R)-10 resulted in a modest ee of 77%, also
in qualitative agreement with the experimental results (see also
the SI for additional details).
In parallel to activation of 1a, phosphoric acid 10 or its silver
salt 10-Ag+ (Figure 3) can interact with Brønsted base 4a to form
non-covalent complex INT3 or, after forming the azomethine ylide
moiety, INT4. In both intermediates, hydrogen bonding between
the hydroxy group of 4a and the phosphate group was shown by
our DFT calculations. In addition, when the O-allyl derivative 4b,
was tested as chiral base in the 1,3-DC between 1a and 2a in the
presence of Ag2CO3 and (R)-10, the formation of racemic endo-
3a was observed. Besides, 1H-NMR experiments with the 4a/(R)-
10 pair showed that the quinuclidine moiety of 4a was protonated
and the surrounding proton resonances changed accordingly.
Finally, NOESY correlations between 4a-H+ and the aromatic
protons of (R)-10 were observed, thus confirming the presence of
complexes INT3 and INT4 gathered in Figure 3. In contrast, no
NOESY contacts were observed in solution experiments with the
4b/(R)-10 pair. From intermediate INT4, cycloadducts INT5 were
formed via saddle points TS. This step of the catalytic cycle leads
to endo-3a with concomitant release of the catalysts as the 10-
Ag+/4a pair or INT3 intermediate.
Figure 3. Proposed catalytic cycle for the successful cooperative non-covalent
catalysis involving catalysts 10 and 4a via double FMO activation.
This induction results in a preferential [a(Si)···d(Si)/b(Re)···c(Re)]
supra-supra interaction (Figure 4) that ultimately leads to
cycloadduct endo-3a, in nice agreement with the experimental
results. The other transition structures TSc-d exhibit less than
optimal interactions. In particular, the N+H···O=P hydrogen
bonding between both catalytic units were calculated to be less
efficient that the OH···O=P interactions, a result compatible with
the lack of enantioselectivity observed in the presence of O-allyl
derivative 4b (vide supra). All these calculations result in a
computed enantiomeric excess of eecalc=92%, in good agreement
with our experimental result of 99% (see the SI for further
information).
We also calculated the possible transition structures leading to
the INT5 precursors connected with endo-3a and ent-endo-3a,
denoted as TSa-d in Figure 4. We considered both OH···O=P and
N+H···O=P contacts between 10--Ag+ and 4a-H+. Our calculations
showed concerted although asynchronous [4s+2s] topologies,
in which the C···C bonds being formed are more advanced at
the distal position with respect to the 4a+H···O=C(NMM)
interaction. These saddle points also show double FMO activation
between both reactants. The phosphate moiety promotes a
HOMO raising of the azomethine ylide and the protonated alkaloid
unit generates a LUMO lowering via the above-mentioned
hydrogen bond between 4a-H+ and 2a. Among the four saddle
Figure 4. Transition structures TSa-d associated with 1,3-DCs between 1a and
2a to give endo-3a and ent-endo-3a in the presence of catalysts Ag2CO3, 4a
and (R)-10. All structures were fully optimized at the M06(PCM,solvent=Ph Me)
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/6-31G(d)&LANL2DZ//B3LYP/&-31G(d)&LANL2DZ. Bond distances are given
in Å. Numbers in brackets are the relative free energies at 298 K, in kcal/mol.
Generalitat Valenciana (PROMETEOII/2014/017), the Gobierno Vasco/
Eusko Jaurlaritza (GV/EJ, Grant IT673-13) and the University of Alicante.
This work was partially supported by a Grant-in-Aid for Scientific Research
from JSPS (17H03060).
With this information, the survey of the versatility of this
cooperative enantioselective 1,3-DC was carried out using
BINOL-derived silver phosphate and hydrocinchonine (Figure 5).
NMM dipolarophile 2a was a suitable dipolarophile for these
transformations and reacted satisfactorily with methyl, isopropyl
and tert-butyl benzylideneglycinates in good yields and very high
to excellent diastereo (>98:2 dr) and enantioselectivities of endo-
3a-c, methyl ester being the most appropriate starter. Alanine
derivative 1 was also allowed to react with the catalytic system
and 2a affording endo-3d in 93% ee. Prolinate endo-3e was
diastereoselectively obtained in 61% yield and 95% ee (Figure 5).
More hindered substrates and different dipolarophiles as ortho-
substituted arenes in the imino group of 1 and tert-butyl acrylate
were tested together in the same reaction affording endo-3f in
good yield (78%) and excellent ee (>99%).
Keywords: asymmetric catalysis • cooperative catalysis • cycloadditons •
DFT calculations • enantioselective synthesis
β-Nitrostyrene is an excellent dipolarophile that, generally,
promotes an exo-approach towards the dipole, but in this reaction,
unexpected endo-cycloadduct 3g was the major diastereoismer
isolated with a 95% ee. This result also supports the proposed
interaction dipolarophile (nitro group)-protonated quinuclidine
described in INT4 (Figure 3). Chalcone and 4-methoxychalcone
were also essayed with methyl iminoglycinate 1a giving excellent
enantioselectivity and high yields of endo-3h-j.
Figure 5. Scope of the cooperative non-covalent 1,3-DC catalysis in the
presence of catalysts 4a and (R)-10.
In conclusion, in this work an efficient endo-diastereo and
enantioselective 1,3-DC of azomethine ylides based on a
cooperative asymmetric Lewis acid-Brønsted base catalysis has
been presented. Silver(I) salts of chiral phosphoric acids are poor
asymmetric catalysts for this reaction. Cinchona alkaloids, in turn,
are efficient in terms of chiral induction. In contrast, matched non-
covalent coupling between both cooperative catalysts results in a
significant improvement of the observed ee’s. We think that the
concepts presented in this communication can be applied to other
reactions involving doubly FMO-activated nucleophilic and
electrophilic substrates.
Acknowledgements
We gratefully acknowledge financial support from the Spanish Ministerio
de Economía y Competitividad (MINECO) (projects CTQ2013-43446-P
and CTQ2014-51912-REDC) the Spanish Ministerio de Economía,
Industria y Competitividad, Agencia Estatal de Investigación (AEI) and
Fondo Europeo de Desarrollo Regional (FEDER, EU) (projects CTQ2016-
76782-P, CTQ2016-80375-P and CTQ2016-81797-REDC), the
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Entry for the Table of Contents
COMMUNICATION
Better together. Enhanced non-
additive enantiocontrol is observed in
asymmetric 1,3-DC between imino
esters and alkenes in the presence of
commercially available Lewis acid-
Brønsted base catalytic pairs. This
cooperative catalysis is based on
FMO-mediated activation of both
reactants. The reinforced combined
chiral induction relies on contacts
between both catalysts mediated by
a non-covalent interaction under mild
conditions.
A. Cayuelas, O. Larrañaga, V. Selva, C.
Nájera,* T. Akiyama, J. M. Sansano, A.
de Cózar, J. I. Miranda, and F. P.
Cossío¥
Page No. – Page No.
Cooperative Catalysis with Coupled
Chiral Induction in [3+2]
Cycloadditions
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[1]
When two or more substances that produces an effect greater than the sum of their individual effects can be considered as synergism. In addition, two or
more entities working or acting together willingly for a common purpose or benefit involving mutual assistance in working towards a common goal is
considered cooperativity. According to these similar definitions we establish that a synergy can be produced by two catalysts in a double activation catalysis
or in a single activation catalysis. It is convenient to point that a dual o bifunctional catalysis: a) S. Matsunaga, M. Shibasaki, Chem. Commun. 2014, 50,
1044-1057; (or even a cascade catalysis) are not considered as cooperative or synergistic processes: b) A. E. Allen, D. W. C. MacMillan, Chem. Sci. 2012,
3, 633-658.
[2]
[3]
In nature, for example, reductases or oxidases are combining with NADP+/NADPH cofactor allowing hyghly efficient specific cooperative asymmetric
transformations. a) Multiheme Cytochromes (Eds.: C. A. Salgueiro, J. M. Dantas), Springer, Berlin, 2016; b) Monooxygenase, Peroxidase and Peroxygenase
Properties and Mechanisms of Cytochrome P450 (Advances in Experimental Medicine and Biology), (Eds.: E. G. Hrycay, S. M. Bandiera), Springer, Berlin,
2016.
For recent reviews and books, see: a) Homo- and Heterobimetallic Complexes in Catalysis: Cooperative Catalysis, (Ed.: P. Kalck), Springer, Berlin, 2016; b)
M.-H. Wang, K. A. Scheidt, Angew. Chem. 2016, 128,15134–15145; Angew. Chem. Int. Ed. 2016, 55, 14912-14922; c) S. Afewerki, A. Córdova, Chem. Rev.
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[4]
[5]
The employment of a chiral catalyst and a racemic catalyst has been frequently used in this cooperative mode.
a) S. Krautwald, D. Sarlah, M. A. Schafroth, E. M. Carreira, Science, 2013, 340, 1065-1068. b) S. Krautwald, M. A. Schafroth, D. Sarlah, E. M. Carreira, J.
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pp 1681–1688; b) H. Yao, Y. Zhou, X. Chen, P. Zhang, J. Xu, H. Liu, J. Org. Chem. 2016, 81, 8888-8899; c) D. B. Ramachary, P. S. Reddy, K. S. Shruthi,
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J. L. Vicario, Chem. Eur. J. 2017, 23, 2764-2768; b) L. Naesborg, F. Tur, M. Meazza, J. Blom, K. S. Halskov, K. A. Jørgensen, Chem Eur. J. 2017, 23, 268-
272.
[6]
[7]
[8]
[9]
For a recent example of asymmetric cooperative [2+2+2] cycloaddition, see: M. Auge, A. Feraldi-Xypolia, M. Barbazanges, C. Aubert, L. Fensterbank, V.
Gandon, E. Kolodziej, C. Ollivier, Org. Lett. 2015, 17, 3754-3757.
For reviews dealing with general 1,3-DC, see: a) M. S. Singh, S. Chowdhury, S. Koley, Tetrahedron 2016, 72, 1603-1644; b) G. Pandey, D. Dey, S. K. Tiwari,
Tetrahedron Lett. 2017, 58, 699-705; c) H. A. Dondas, M. G. Retamosa, J. M. Sansano, Synthesis 2017, 49, 2819-2851.
[10] For reviews concerning catalytic enantioselective 1,3-DCs, see: a) T. Hashimoto, K. Maruoka, Chem. Rev. 2015, 115, 5366-5412; b) B. Bdiri, B.-J. Zhao, Z.-
M. Zhou, Tetrahedron: Asymmetry 2017, 28, 876-899.
[11] See, for example: a) H. Y. Kim, H. J. Shih, W. E. Knabe, K. Oh, Angew. Chem. Int. Ed. 2009, 48, 7420-7423; b) W. Zeng, G.-Y. Chen, Y.-G. Zhou, Y.-X. Li,
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Li, H. Kim, K. Oh, Org. Lett. 2015, 17, 1288-1291; e) Hybrid cinchona alkaloid-amino acid derived amidophosphanes (CA-AA-amidphos) acted, in combination
with silver salts, as mutifuntional catalysts. H. Wang, Q. Deng, Z. Zhou, S. Hu, Z. Liu, L.-Y. Zhou, Org. Lett. 2016, 18, 404-407.
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