The design of a spiro-pyrrolidine organocatalyst and its application to catalytic asymmetric Michael addition for the construction of all-carbon quaternary centers

Article abstract of DOI:10.1039/c5cc02765a

A novel chiral spiro-pyrrolidine silyl ether organocatalyst has been designed. Its catalytic asymmetric effect is demonstrated by the Michael addition reaction, which affords the desired products with an all-carbon quaternary center in up to 99% ee and 87

Full text of DOI:10.1039/c5cc02765a

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Received 00th January 20xx,
Accepted 00th January 20xx
A Spiro-Pyrrolidine Organocatalyst and Its Application
to Catalytic Asymmetric Michael Addition for the
Construction of All-Carbon Quaternary Centers
DOI: 10.1039/x0xx00000x
www.rsc.org/
Jin-Miao Tian,a Yong-Hai Yuan,a Yong-Qiang Tu,*,a, b Fu-Min Zhang,a Xiao-Bo Zhang,a Shi-Heng
Zhang,a Shao-Hua Wang,a and Xiao-Ming Zhanga
characteristics above was noticed. We hypothesized such a species
might be modified to a new chiral organocatalyst, which we
expected would present some unique asymmetric induction property.
Based on a further understanding of the aminocatalysis mechanism
and the structure feature of JørgensenꢀHayashi catalyst,2d we
designed and prepared the corresponding silyl etherꢀtype catalysts 1ꢀ
3 for the investigation of their catalytic efficiency.
A novel chiral spiro-pyrrolidine silyl ether organocatalyst has been
designed. Its catalytic asymmetric effect is demonstrated by the
Michael addition reaction, which affords the desired products
with an all-carbon quaternary center in up to 99 % ee and 87 %
yield. Furthermore, the reaction process has been investigated via
in situ NMR experiments.
Since there is no universal catalyst effective for all chemical
reactions, developing new catalyst (ligand) is always a focus in the
field of asymmetric catalysis.1 Among the catalysts developed
during the past decades, two types of catalysts have drawn our
attention. The first one is aminocatalysts with pyrrolidine framework,
such as proline and its derivatives as well as the MacMillan
imidazolidinones (Figure 1a). They have played a vital role in
organocatalysis due to their success in CꢀX (X = C, N, O etc.) bond
formation.2,3,4 The second one is the metalꢀcomplex catalysts with
C2ꢀaxial spiroꢀligands, such as SpirOP developed by Chen’s group,
SDP developed by Zhou’s group and SKP developed by Ding’s
group (Figure 1b), which also have presented powerful and special
stereocontrol ability induced by the spiroꢀbackbone.5 Recently, we
are focusing with great interest on the cooperative effect combining
above two structural features, i.e. pyrrolidine framework and
spirobicyclic rigidity. Driven by this idea and in connection with the
efficient stereoselective construction of spiroꢀpyrrolidines during the
course of our natural product synthesis,6 a 1ꢀazaꢀspiroꢀ[4.4]ꢀnonaneꢀ
5ꢀone intermediate which possesses the dual structural
O
MeN
OTMS
Ph
Ph
rgensen-Hayashi
catalyst
Ph
tBu
COOH
N
N
N
H
H
(a)
(b)
H
MacMillan
imidazolidinone
J
Ø
Proline
N
H
OR
PPh2
PPh2
1
R=TBS
2 R=TBDPS
3 R=Ac
our new catalysts
O
O
O
O
PPh2
Ph2P
PPh2 Ph2
P
SDP
SKP
SpirOP
Figure 1. Design of an Organocatalyst with Pyrrolidine and
Unsymmetrical Spirobicyclic Framework
Construction of allꢀcarbon quaternary units is an important and
challenging task in organic synthesis, especially for the acyclic
system.7,8 Among the reported methods approaching these units,
Michael addition represents one of the most practical methods,8,9 and
several asymmetric versions have also been realized under the
catalysis of metalꢀcomplexes such as CuꢀNHC, PdꢀPyOx and Rhꢀ
diene (Figures 2a).10,11 In contrast, the organocatalytic asymmetric
Michael addition is not so effective and underexplored.12 Aiming at
this subject, Kwiatkowski, Zhang and Hayashi’s group have done
some pioneering work recently (Figure 2a).13,14 In this
communication, we present a more efficient and convenient Michael
addition of acyclic unsaturated aldehydes with nitromethane under
the promotion of our newly developed catalyst 2.
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Table 1. Optimization of the asymmetric Michael Additiona
Catalyst
(equiv)
Entry
T(oC)
Additive T/con(%)
Y(%)b
Eec
Figure 2. Overview of Michael Addition for Constructing Allꢀ
Carbon Quaternary Units
1 (0.2)
2 (0.2)
3 (0.2)
4 (0.2)
5 (0.2)
6 (0.2)
7 (0.2)
2 (0.2)
2 (0.2)
2 (0.2)
2 (0.2)
2 (0.2)
2 (0.2)
2 (0.1)
2 (0.05)
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
-5
BA
BA
1d/100
1d/100
1d/100
2d/24
2d/50
2d/75
2d/42
2d/50
1d/80
1d/50
3d/ꢀꢀ
87
85
75
<5
<5
<5
7
75%
86%
55%
1%
1
2
3
BA
Initially, organocatalysts 1ꢀ3 were prepared easily in gramꢀscale
using chiral substrate induced hydroamination/ semipinacol
rearrangement reaction as the key step instead of catalytic
asymmetric means15 so as to achieve high optical purity and simplify
the synthesis process. As shown in Scheme 1 (for details, please see
Supporting Information), an 8ꢀstep synthesis was performed to give
catalysts 1ꢀ3 from known compound 8 in about 30 % total yield.
Their absolute configurations were confirmed by Xꢀray diffraction of
an amino tosylate 12 derived from intermediate 11.16 Note that
although we have tried to prepare similar type of catalyst with a
TMS group, it has been ruled out due to its instability.
4
BA
5
BA
14%
70%
60%
85%
84%
82%
ꢀꢀ
6
BA
7
BA
8d
ꢀꢀꢀ
40
33
16
ꢀꢀ
9
NaOAc
Na2CO3
PTS
BA
10
11
12
13
14
15
2.5d/100
7d/20
7d/95
7d/30
85
16
65
19
90%
89%
88%
88%
ꢀ10
ꢀ5
BA
Scheme 1. Approach for the our designed organocatalyst.
BA
ꢀ5
BA
a): According to the general procedure in Table 1 of section 3 in the
Supporting Information. b): Isolated yield. c): The enantiomeric
excess was determined by chiral HPLC analysis. d): without BA
Subsequently, the substrate scope of the reaction was investigated
with a variety of β,βꢀdisubstituted α,βꢀunsaturated aldehydes. The
results showed that all aldehydes screened could react well to
generate the desired products in moderate to good yields and with
good to high enantioselectivites (Table 2). Noticeably in the case of
substrate S1, variation of Eꢀ or Zꢀconfiguration of the double bond
still afforded nearly the same enantioselectivity and yield of the
product (entry 1). In addition, the steric hindrance caused by R group
at βꢀposition affected significantly the enantioslectivity of the
reaction. Accordingly, the bigger the steric hindrance was, the better
the enantioselectivity was (entries 1, 2 vs 7, entries 4, 5 vs 6).
Meanwhile, substrates with the βꢀphenyl ring bearing electronꢀ
donating group (MeO, S8) or electronꢀwithdrawing group (NO2, S10)
did not affect the enantioselectivity (entry 6). Furthermore, the
reaction could tolerate the substrates bearing heteroatoms such as S,
Br, N and Cl (entries 3, 5, 7 and 9). Moreover, substrates containing
exocyclic C=C bond also reacted well to give the good to excellent
enantioselectivities (entries 8 and 9). The absolute configurations of
all products were deduced from those of P10 and P14, whose
absolute configurations were determined by comparing their optical
rotations with the reported data.14a,14b,18 To further expand the scope
of nucleophile and thus identify the unique character of catalyst 2,
nitroethane was examined as nucleophile. As indicated in Table 2
(entries 10 and 11), although the diastereoselectivity was similar to
Hayashi’s results,14a the reaction also proceeded well to give
excellent enantioselectivites.
With sufficient catalysts 1ꢀ3 in hand, their catalytic properties
were surveyed toward the Michael addition of model substrate
geranial S1 with nitromethane as the nucleophile, owing to its easy
availability and diverse transformation to other nitrogenꢀcontaining
functionalities (Table 1).17 To our delight, in the presence of
catalysts 1, 2 or 3 and benzoic acid (BA), the expected product P1
was obtained with good yield and enantioselectivity with toluene as
the solvent (entries 1ꢀ3). In contrast, Lꢀproline and Jørgensenꢀ
Hayashi catalyst were not effective, and only poor yields and modest
enantioselectivities were observed (entries 4ꢀ7). Although the
introduction of TBDPS group was much easier with the newly
designed catalyst, the direct comparison of 2 with Jørgensenꢀ
Hayashi catalyst (R2 = TBDPS) was not successful.18 Moreover, the
presence of BA was found to be crucial to speed up the reaction and
improve the yield (entry 2 vs 8). While other additives like NaOAc
and Na2CO3 would decrease the reaction rate (entries 9 and 10), and
the use of pꢀToluenesulfonic acid (PTS) gave even worse result
(entry 11). Solvents such as DCM, Et2O, MeCN and MeOH could
not improve the enantioselectivity (for details, please see Supporting
Information). A further survey of the reaction temperature showed
that lower temperature indeed slightly increased enantioselectivity,
but seriously decreased the reaction rate (entries 12 and 13).
Furthermore, decreasing the catalyst loadings would slow down the
reaction, though the enantioselectivity was not affected significantly
(entries 14 and 15). At last, we chose entry 12 as the optimal
condition.
Table 2. Substrate Scope of the Asymmetric Michael Addition of
Unsaturated Aldehydes with Nitromethanea
2 | J. Name., 2012, 00, 1-3
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Figure 3. Proposed Reaction Mechanism
In conclusion, through combination of two structural
characteristics, pyrrolidine framework and spirobicyclic rigidity, a
novel kind of chiral spiroꢀpyrrolidine silyl ether catalysts 1ꢀ3 was
designed and prepared easily in gram scale for the first time, and
their absolute configurations were confirmed via the derivative 11 by
Xꢀray diffraction. As a complement to current organocatalysts, such
type of catalysts could provide additional choices for carrying out
new catalytic asymmetric transformations. Accordingly, their
catalytic efficiency was demonstrated by an enantioselective
Michael addition reaction using catalyst 2, which could afford the
desired products with an allꢀcarbon quaternary center with up to 99%
ee and up to 87% yield. Further investigation of new organocatalysts
based on the chiral spiroꢀpyrrolidine and their catalytic properties are
ongoing.
Notes and references
1
(a) K. Mikami and M. Lautens, New Frontiers in Asymmetric Catalysis,
WileyꢀInterscience: Hoboken, NJ, 2007; (b) H. Pellissier, Recent
Developments in Asymmetric Organocatalysis, Royal Society of
Chemistry, 2010.
a): According to the general procedure in section 3 of the Supporting
Information. b): Isolated yield. c): The enantiomeric excess was
determined by chiral HPLC analysis. d) Nitroethane was used as the
nucleophile instead of nitromethane.
2
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addition, some in situ NMR experiments were performed with S8
and CH3NO2 under the catalysis of 2 in Dꢀtoluene on a 600MHz
NMR spectrometer (for related NMR spectra see Supporting
Information). Firstly, when S8 and 2 were mixed in Dꢀtoluene, the
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h. That could rationalize that the Eꢀ, Zꢀ or mixted E/Zꢀ S1 could react
to give the sole product P1 with nearly the same ee and yield (Table
2, entry 1), as they afforded the same dienamine intermediate 8a
(Figure 3) before Michael addition. Secondly, when S8, catalyst 2,
BA and CH3NO2 were mixed in Dꢀtoluene according to the standard
operation, the product P8 was observed immediately, and completely
formed within 16 h. However, in any case above, no iminium
intermediate was observed, possibly because it reacted with CH3NO2
too fast to be detectable. Based on these facts and related literature,2d
a possible process of this reaction was proposed to involve an initial
dienamine formation, then fast Michael addition via an iminium
formation, and final hydrolysis to release catalyst 2 and generate
product P8 (Figure 3). Particularly for the key stereoꢀcontrolled
addition of iminium 8b, the neucleophilic CH3NO2 preferred to
approach from the Siꢀface to form the quaternary center with a (S)ꢀ
configuration in the product P8, instead of the Reꢀface because of
the bigger steric hindrance caused by OTBDPS.
3
4
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4 | J. Name., 2012, 00, 1-3
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