Catalytic Enantioselective Reaction of Allenylnitriles with Imines Using Chiral Bis(imidazoline)s Palladium(II) Pincer Complexes

Article abstract of DOI:10.1002/anie.201702429

The first highly enantioselective reaction of allenylnitriles with imines has been developed. Excellent yields and enantioselectivities were observed for the reaction with various imines using chiral Phebim-Pd^II complexes. This process offers a simple and efficient synthetic route for various functionalized α-vinylidene-β-aminonitriles and their derivatives.

Full text of DOI:10.1002/anie.201702429

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201702429
German Edition: DOI: 10.1002/ange.201702429
Asymmetric Synthesis
Catalytic Enantioselective Reaction of Allenylnitriles with Imines
Using Chiral Bis(imidazoline)s Palladium(II) Pincer Complexes
Masaru Kondo, Masashi Omori, Tsubasa Hatanaka, Yasuhiro Funahashi, and
Shuichi Nakamura*
Abstract: The first highly enantioselective reaction of allenyl-
nitriles with imines has been developed. Excellent yields and
enantioselectivities were observed for the reaction with various
imines using chiral Phebim-PdII complexes. This process offers
a simple and efficient synthetic route for various functionalized
a-vinylidene-b-aminonitriles and their derivatives.
Allene compounds and their derivatives have been proven to
be useful building blocks for the preparation of pharmaceut-
ical targets,[1] and their unique reactivity renders them
versatile intermediates in organic syntheses.[2] Therefore,
their synthetic importance has prompted large interest to
develop the asymmetric synthesis of chiral compounds having
an allenyl group.[3] One of the most efficient methods for the
preparation of chiral compounds having an allenyl group is
the stereoselective addition of allenyl compounds carrying
electron-withdrawing groups with electrophiles. There are
several reports for the enantioselective addition reaction of
allenylesters with imines,[4,5] carbonyl compounds,[6] and
activated olefins,[7] however, to the best of our knowledge,
there are no reports on the catalytic enantioselective reaction
of allenylnitriles with imines (Figure 1).[8] On the other hand,
we recently developed the activation of nitrile compounds
using chiral bis(imidazoline) palladium(II) pincer com-
plexes.[9,10] In these catalyst systems, palladium catalysts
enhanced the acidity of the a-proton in alkylnitriles to give
palladium ketenimides, which react with imines to give
products. We expected that the bis(imidazoline) palladium
system could be applied to the reaction of allenylnitriles with
imines, because allenylnitriles have reasonable acidity (pKa =
21.0 in DMSO), similar to allylnitrile (pKa = 20.7 in
DMSO).[11] Therefore, chiral palladium catalysts activate the
acidity of allenylnitriles, then the carbanion of allenylnitriles
Figure 1. Palladium pincer complexes activate nitrile compounds.
attacks imines to give chiral a-vinylidene-b-aminonitriles.
Herein, we report the first highly enantioselective reaction of
allenylnitriles with imines using palladium(II) pincer com-
plexes with chiral bis(imidazoline)s as a chiral Lewis acid
catalyst.
The enantioselective reaction of allenylnitrile 2a
(1.5 equiv) with various imines 1 (1.0 equiv) was carried out
by using palladium catalysts 4a–d (2 mol%) and silver
acetylacetonate (2 mol%) in THF (Table 1). Although the
reaction of N-diphenylphosphinoyl imine 1a (R = DPP) and
N-Boc imine 1b with allenylnitrile 2a using catalyst 4a and
Ag(acac) did not afford any products (entries 1 and 2), the
reaction of N-tosyl imine gave product 3c in 73% yield with
54% ee (entry 3). Furthermore, the reaction with N-trime-
thylsilylethanesulfonyl imine 1d (R = SES) gave product 3d
in moderate yield with 71% ee (entry 4). In order to improve
enantioselectivity, we optimized the structure of bis(imidazo-
line) catalysts 4b–d (entries 5–7). As a result, the reaction
using catalyst 4d having an acetyl group (R1) and 2,4,6-
trimethylphenyl group (Ar) emerged as the most suitable
catalyst for this reaction giving product 3d with high
enantioselectivity but low yield due to the generation of
a by-product by the over-reaction of product with imines
(entry 7). To our delight, the reaction at a lower temperature
suppressed the generation of by-product to give product 3d in
good yield with high enantioselectivity (entry 8). Finally, the
addition of 20 mol% of trimethylsilylalcohol (TMSOH)
improved the reactivity and yield of the product (entry 9).
Next, we examined the reaction of 2a with various imines
1d–p using 4d, Ag(acac), and TMSOH (Table 2). The
reaction of various imines 1e–j having an electron-withdraw-
ing group, such as a fluoro, chloro, bromo, and trifluoromethyl
group, in the meta or para position gave products 3e–j in high
yield with high enantioselectivity (63–88%, 99% ee,
entries 1–7). The reaction with imines 1k bearing an elec-
[*] M. Kondo, M. Omori, Prof. S. Nakamura
Department of Life Science and Applied Chemistry, Graduate School
of Engineering, Nagoya Institute of Technology
Gokiso, Showa-ku, Nagoya 466-8555 (Japan)
E-mail: snakamur@nitech.ac.jp
M. Kondo, Prof. S. Nakamura
Frontier Research Institute for Material Science
Nagoya Institute of Technology
Gokiso, Showa-ku, Nagoya 466-8555 (Japan)
Prof. T. Hatanaka, Prof. Y. Funahashi
Department of Chemistry, Graduate School of Science
Osaka University
1-1 Machikaneyama, Toyonaka, Osaka 560-0043 (Japan)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201702429.
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Table 1: Optimization of the reaction of allenylnitrile 2a with various
imines 1a–d using 4–Ag(acac).
tolerated in this reaction condition and gave product 3p with
good stereoselectivity (entry 13). To test the scalability,
a gram-scale reaction was performed using 2 mol% of the
catalyst 4d (entry 14). The reaction of 1d (1.0 g) with 2a
proceeded to give the product 3d in 89% yield with 99% ee.
Furthermore, the reaction of 3.0 equiv of racemic terminal
ethyl-substituted allenylnitrile 2b with imine 1d gave a dia-
stereomer mixture of product 3q in good yield with high
enantioselectivity (Scheme 1).[12] To the best of our knowl-
edge, these results are first examples of the enantioselective
reaction of allenylnitriles with imines.
Entry
1
Catalyst
t [h]
Yield [%]
ee [%]
1
2
3
4
5
6
7
1a
1b
1c
1d
1d
1d
1d
1d
1d
4a
4a
4a
4a
4b
4c
4d
4d
4d
72
72
48
72
72
72
72
72
24
–
–
–
–
Scheme 1. Reaction of racemic terminal ethyl-substituted allenylnitrile
2b with imine 1d using 4d–Ag(acac) catalyst.
73
49
14
trace
47
82
88
54
71
65
–
91
99
99
We next examined the transformation of a-vinylidene-b-
aminonitrile 3d. The reaction of 3d using silver(I) nitrate and
calcium carbonate in acetone/water gave the intramolecular
cyclization product 5 in high yield without the loss of
enantiopurity (Scheme 2a).[13] Palladium acetate catalyzed
8[a]
9[a,b]
[a] The reaction was carried out at À608C. [b] TMSOH (20 mol%) was
added.
Table 2: The reaction of allenylnitrile 2a with various imines 1d–p using
4d–Ag(acac).
Entry
1
R
T [8C]
t [h]
Yield [%]
ee [%]
1
2
3
4
5
6
7
1d
1e
1 f
1g
1h
1i
1j
1k
1l
1m
1n
1o
1p
1d
Ph
3-FC6H4
4-FC6H4
À60
À60
À60
À60
À60
À60
À60
À30
À60
À60
À60
À60
10
24
24
24
96
48
24
24
168
72
144
168
168
168
24
88
88
88
74
63
84
80
62
85
71
63
70
64
89
99
99
99
99
99
99
99
98
99
91
96
99
86
99
3-ClC6H4
4-ClC6H4
3-BrC6H4
3-CF3C6H4
3-MeOC6H4
3-PhC6H4
1-Naphthyl
2-Naphthyl
2-Furyl
8[a]
9[a]
10
11
12
13[a]
14[b]
=
4-ClC6H4CH CH
Ph
À60
[a] The reaction was carried out using 5 mol% of 4d and Ag (acac).
[b] 1.0 g of 1d was used.
tron-donating methoxy group showed high enantioselectivity
but moderate yield (entry 8). The reaction with biphenyl
imine 1l also gave product 3l in 85% yield with 99% ee
(entry 9). Naphthyl and a heteroatom containing imines 1m–
o also gave products 3m–o with high enantioselectivity
(entries 10–12). The reaction of conjugated imine 1p was
Scheme 2. Transformation of a-vinylidene-b-aminonitriles to chiral 2,5-
dihydro-1H-pyrroles 5, 6 and oxazolidinone 10.
2
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the coupling-cyclization of 3d with allyl bromide, and
afforded product 6 in moderate yield without racemization
(Scheme 2b).[14] Ozonolysis of 3d gave acyl cyanide,[15] which
was directly converted into amino acid methyl ester 7 using
MeOH and Et3N.[16] The reduction of 7 with LiBH4 afforded
amino alcohol 8,[17] which was cyclized to oxazolidinone 9.[18]
Cleavage of the SES group from 9 using TBAF gave 10
without any loss of enantiopurity (Scheme 2c). The absolute
configuration of 10 was determined as (S), based on the value
of the specific rotation from a previous paper.[19]
The assumed catalytic cycle for the reaction of allenylni-
trile with imines is shown in Figure 2. The addition of
Ag(acac) to palladium complex 4 evokes the exchange
reaction of bromide in 4 to acetylacetonate (complex A).
Coordination of the palladium in complex A to the cyano
Scheme 3. Reaction of allenylester 2c with imine 1d using 4d–Ag.
We examined the optimization of the structure of complex
4d and allenylnitrile 2a using the Gaussian09[21] B3LYP/
LANL2DZ method (Figure 3). The optimization results
Figure 3. The optimized structure of the complex and the proposed
transition state for the reaction of allenylnitrile with an imine using 4d.
showed that the cumulene-type intermediate and nitrogen
in allenylnitrile coordinate the palladium cation. Based on the
calculation result and the absolute configuration of products,
the assumed transition state for the reaction of palladium
cumulene-type intermediate with imines using 4d is depicted
in Figure 3.
Avoiding steric repulsion between the substituent on
imine and the mesityl group in 4d, the imine approaches the
cumulene-type intermediate of allenylnitrile to give the (S)-
isomer.
In conclusion, we developed the first highly enantioselec-
tive reaction of allenylnitrile with imines using chiral phebim-
PdII catalyst. Various imines can be applicable in the process.
This process offers a simple and efficient synthetic route for
a-vinylidene-b-aminonitriles and their derivatives. Further
studies are in progress to determine the potential of these
catalytic systems to other processes.
Figure 2. Proposed reaction cycle of the reaction of allenylnitrile with
imines using 4–AgI.
group in allenylnitrile affords cationic complex B, which was
observed by ESI-Mass analysis of the mixture of 4d, Ag(acac)
and allenylnitrile in a 1:1:1 ratio (cation mode, calcd for
C56H60N5O2Pd: 940.4, found: 940.3, see the Supporting
Information). Complex C then reacts with N-SES imines to
give complex D, which subsequently undergoes protonation
and decomplexation, resulting in products 3 and regenerated
complex A.[20]
Acknowledgements
This work was partly supported by a Grant-in-Aid for
Scientific Research from the MEXT (Japan) and Tokyo
Ohka Foundation.
In order to clarify the activation of allenylnitrile by
palladium catalyst, the reaction of allenylester 2c with 1d
using the 4d was examined (Scheme 3). However, the
reaction of allenylester 2c with 1d using 4d–Ag(acac) catalyst
did not give any products. This reactivity is in sharp contrast
to the reaction of 2a with 1d using 4d–Ag(acac). This result
implied that the palladium pincer complex selectively coor-
dinates and activates the cyano group in 2a.
Conflict of interest
The authors declare no conflict of interest.
Keywords: allenyl compounds · asymmetric synthesis ·
Lewis acids · pincer complex
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Manuscript received: March 7, 2017
Final Article published: && &&, &&&&
4
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Communications
Asymmetric Synthesis
M. Kondo, M. Omori, T. Hatanaka,
Y. Funahashi,
S. Nakamura*
&&&& — &&&&
Catalytic Enantioselective Reaction of
Allenylnitriles with Imines Using Chiral
Bis(imidazoline)s Palladium(II) Pincer
Complexes
Getting a grip on allenyles: The first
highly enantioselective reaction of alle-
nylnitriles with imines has been devel-
oped (see scheme; SES=silylethanesul-
fonyl). Excellent yields and enantioselec-
tivities were observed for the reaction
with various imines using chiral Phebim–
PdII complexes. This process offers
a simple and efficient synthetic route for
various functionalized a-vinylidene-b-
aminonitriles and their derivatives.
Angew. Chem. Int. Ed. 2017, 56, 1 – 5
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!