Nickel(0)-catalyzed enantioselective annulations of alkynes and arylenoates enabled by a chiral NHC ligand: Efficient access to cyclopentenones

Article abstract of DOI:10.1002/anie.201408364

Cyclopentenones are versatile structural motifs of natural products as well as reactive synthetic intermediates. The nickel-catalyzed reductive [3+2] cycloaddition of α,β-unsaturated aromatic esters and alkynes constitutes an efficient method for their synthesis. Here, nickel(0) catalysts comprising a chiral bulky C₁-symmetric N-heterocyclic carbene ligand were shown to enable an efficient asymmetric synthesis of cyclopentenones from mesityl enoates and internal alkynes under mild conditions. The bulky NHC ligand provided the cyclopentenone products in very high enantioselectivity and led to a regioselective incorporation of unsymmetrically substituted alkynes.

Full text of DOI:10.1002/anie.201408364

Angewandte
Chemie
DOI: 10.1002/anie.201408364
Asymmetric Catalysis
Nickel(0)-Catalyzed Enantioselective Annulations of Alkynes and
Arylenoates Enabled by a Chiral NHC Ligand: Efficient Access to
Cyclopentenones**
Joachim S. E. Ahlin, Pavel A. Donets, and Nicolai Cramer*
Abstract: Cyclopentenones are versatile structural motifs of
natural products as well as reactive synthetic intermediates. The
nickel-catalyzed reductive [3+2] cycloaddition of a,b-unsatu-
rated aromatic esters and alkynes constitutes an efficient
method for their synthesis. Here, nickel(0) catalysts comprising
a chiral bulky C1-symmetric N-heterocyclic carbene ligand
were shown to enable an efficient asymmetric synthesis of
cyclopentenones from mesityl enoates and internal alkynes
under mild conditions. The bulky NHC ligand provided the
cyclopentenone products in very high enantioselectivity and led
to a regioselective incorporation of unsymmetrically substi-
tuted alkynes.
C
yclopentenones are important structural motifs in many
natural products and bioactive compounds,[1] and are used as
key synthetic intermediates of substituted cyclopentanones.[2]
These characteristics promoted the development of a broad
variety of synthetic methods to access cyclopentenones.[3]
Among them, the Pauson–Khand reaction is a particularly
useful three-component process.[4] Asymmetric variants have
been devised,[5] however intermolecular reactions with
unbiased olefins remain problematic. Although catalytic
reactions with cobalt and other transition metals are
reported,[6] stoichiometric metal–carbonyl complexes and/or
the use of toxic carbon monoxide often make the develop-
ment of alternative processes attractive.[7,8] In this respect,
reductive Ni-catalyzed[9] [3+2] annulations of phenyl enoates
and alkynes to give cyclopentenones were recently reported
independently by Ogoshi et al.[10] and Montgomery et al.[11]
Both substrates, the enoate and the alkyne, are common,
stable, and readily accessible building blocks. Along the same
the lines, the enoate that serves as three-carbon component
omits the use of CO (Scheme 1). Given the relevance of the
cyclopentenone scaffold, an efficient catalytic asymmetric
variant of this method enabled by a suitable chiral phosphine
or carbene ligand is a desirable and synthetically valuable
goal. However, despite intense research efforts and the
Scheme 1. Synthesis of cyclopentenones. a) [2+2+1] cycloaddition
(classical Pauson–Khand reaction), b) reductive Ni0-catalyzed [3+2]
cycloaddition, c) asymmetric reductive Ni0-catalyzed [3+2] cycloaddi-
tion.
resulting growing number of chiral N-heterocyclic carbenes,
only very scarce examples of asymmetric NHC–Ni catalysis
have been reported so far.[12] This discrepancy is somewhat
surprising, as simple achiral workhorse NHCs, such as IPr and
IMes, belong to the most powerful and versatile ligands in Ni
catalysis. It might be rationalized by the differences in the
steric and electronic characteristics of many chiral N-hetero-
cyclic carbenes with respect to IPr or IMes, often rendering
them not competent for the envisaged transformation. These
shortcomings represent an important gap in asymmetric
catalysis.
Herein we report a Ni-catalyzed annulation of a wide
range of acrylates and alkynes enabled by a chiral N-
heterocyclic carbene for the asymmetric synthesis of cyclo-
pentenones. In addition to the challenge of finding an
appropriate chiral (NHC or phosphine) ligand for this
transformation, the nature of the reduction system to close
the catalytic cycle is critical for the reaction outcome
[Eq. (1)]. While Ogoshiꢀs system consisting of zinc powder
and iPrOH as solvent required very high reaction temper-
atures of 1308C,[10] thus impeding efficient asymmetric
[*] J. S. E. Ahlin, Dr. P. A. Donets, Prof. Dr. N. Cramer
Laboratory of Asymmetric Catalysis and Synthesis
EPFL SB ISIC LCSA
BCH 4305, CH-1015 Lausanne (Switzerland)
E-mail: Nicolai.cramer@epfl.ch
Homepage: http://isic.epfl.ch/lcsa
[**] This work was supported by the Swiss National Science Foundation
(no. 137666). We thank Dr. R. Scopelliti for X-ray crystallographic
analysis of compounds 3la and 6. NHC=N-heterocyclic carbene.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201408364.
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reactions, Montgomeryꢀs triethylborane/protic solvent proto-
col[11] allowed homogenous reaction mixtures at lower
temperatures. However, in our case, the advantage of using
chiral carbene ligands came along with two issues: 1) the
competing transesterification of the reactive phenol ester with
the added methanol gave unreactive methyl cinnamate 4, and
2) the direct 1,4 reduction gave saturated esters 5.
In order to get a first overview over the potential of the
ligand, we evaluated a set of chiral N-heterocyclic carbenes
using phenyl cinnamate and 3-hexyne as model substrates
(Table 1, entries 1–4; for the full optimization study, see the
trimethyl phenyl group (entry 8) provided the optimal
balance, giving 3aa in 64% yield and 97.5:2.5 e.r. Further
increase in the steric bulk with an even larger 2,6-diisopropyl
group shut down the formation of 3aa completely and led
almost exclusively to the formation of the 1,4-reduced
product (entry 9). Restriction of the amounts of MeOH and
BEt3 had a positive influence and the optimal ratio consisted
of five equivalents of alcohol and two equivalents of borane.
Switching to CPME (cyclopentyl methyl ether) as the solvent
allowed to reduce the reaction temperature, and the bulkier
tBuOH as proton source almost completely suppressed
transesterification and reduction pathways, thus increasing
the yield of cyclopentenone to 92%, but slightly reducing the
enantioselectivity (entry 11). Given the favorable character-
istics of the bulky IPr ligand for the cyclopentenone
formation, we explored L5, a ligand with a mixed design
between L2 and IPr. Notably, L5 gave rise to a substantially
superior reactivity and selectivity, resulting in 3aa in 92%
yield and 3:97 e.r. (entry 12).[14] Moreover, under these
conditions, the catalyst loading could be reduced to
1.25 mol% without affecting the reaction outcome signifi-
cantly (entry 13). Such loadings are still rather rare for
complex Ni0-catalyzed transformations. L5 was previously
used only once in a single trial in asymmetric catalysis,
resulting in a very poor selectivity for Pd-catalyzed oxindole
formation.[15] This outcome is in stark contrast to the excellent
results of the C2-symmetric carbene L2 for the same trans-
formation as reported by Kꢁndig. This striking performance
discrepancy for two different reactions is clearly another
testimony that one cannot conclude on the power of a chiral
ligand for a specific transformation from a few established
benchmark reactions.
Table 1: Optimization of the cyclopentenone formation.[a]
Entry L*
R
Solvent R’OH
Yield e.r.[c]
[%][b]
1[d]
2
L1 Ph
L2 Ph
L3 Ph
L4 Ph
L2 4-MeO-C6H4
L2 4-CF3-C6H4
L2 3,5-Me2-C6H3
L2 2,4,6-Me3-C6H2 THF
L2 2,6-iPr2-C6H3
L2 CH(Ph)2
THF
THF
THF
THF
THF
THF
THF
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
32
52
1
76:24
92:8
74:26
–
3[d]
4[d]
5
0
53
27
82
64
1
92:8
6
7
8
9
10
11[e]
12[e]
13[f]
14
81:19
91.5:8.5
97.5:2.5
97:3
–
94:6
3:97
2.5:97.5
2:98
THF
THF
0
L2 2,4,6-Me3-C6H2 CPME
L5 2,4,6-Me3-C6H2 CPME
L5 2,4,6-Me3-C6H2 CPME
tBuOH 92
tBuOH 92
tBuOH 71
tBuOH 60
L5 3,5-Me-C6H3
CPME
To explore the generality of the optimized process, we
evaluated different substitution pattern on the acrylic ester. A
wide range of cinnamic esters were tolerated (Scheme 2). The
aromatic portion accommodated the most common electron-
donating and electron-withdrawing groups. Irrespective of the
position of the substituents (ortho, meta, or para), the yields
and selectivities were consistently high. Notably, substrates
with condensed arenes, heterocyclic substituents such as 3-
furyl, 3-thienyl reliably provide cyclopentenones 3ba–3qa.
However, a 2-furyl group reduced both yield and selectivity,
presumably because of a chelation of the nickel center with
the oxygen atom of the furan. The absolute configuration of
the cyclopentenones was unambiguously established by X-ray
crystallographic analysis of the ferrocenyl-containing deriva-
tive 3la.[16] Alkyl-substituted acrylates, such as 1p and 1q,
undergo the annulation reaction, however with lower selec-
tivity. Next, different internal alkynes were evaluated. A
variety of dialkyl alkynes with functional groups were well
tolerated and provided the cyclopentenones with excellent
enantioselectivities. However, diaryl alkynes did not provide
the desired cyclopentenones under these conditions.
[a] Reaction conditions: enoate (0.10 mmol), 3-hexyne (0.15 mmol),
[Ni(cod)2] (10.0 mmol), L* (11.0 mmol), MeOH (0.8 mmol), BEt3
(0.5 mmol), at 708C for 17 h. [b] Yields of isolated products. [c] Deter-
mined by HPLC on a chiral stationary phase. [d] At 508C. [e] With
[Ni(cod)2] (5.0 mmol), L* (7.0 mmol), tBuOH (0.5 mmol), BEt3
(0.2 mmol), at 508C. [f] 1.25 mol% catalyst, [Ni(cod)2] (1.25 mmol), L5
(1.30 mmol), tBuOH (0.5 mmol), BEt3 (0.2 mmol), at 508C for 42 h,
79% conversion. CPME=cyclopentyl methyl ether, Np=naphthyl.
Supporting Information). The ratio 3aa/4/5 varies signifi-
cantly, depending on the chiral ligand NHC*. From the tested
carbene scaffolds, only the Kꢁndig-type ligands[13] L1 and L2
gave the desired enone product 3aa in reasonable yields and
selectivity, while many other carbenes or phosphine ligands
failed. With this initial hit, the role of the aromatic ester was
investigated next. Electron-poor aryl groups (entry 6) led to
a reduced enantioselectivity and a largely increased propen-
sity for transesterification with the methanol additive. Elec-
tron-rich aryl groups (entry 5) reacted more sluggishly and
alkyl esters (entry 10) were completely unreactive. We found
that the steric bulk of the phenol group is highly important for
the reactivity and selectivity. In this respect, a larger 2,4,6-
We next evaluated nonsymmetrical internal alkynes.
Especially for these challenging substrates, the advantage of
the “mixed-design” carbene L5 over the C2-symmetrical
carbene L2 was apparent (Scheme 3). L5 resulted in dramat-
ically improved regioselectivities, while concomitantly
increasing the enantioselectivity. Although aryl alkyl alkynes
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Chemie
Figure 1. X-ray crystal structure of a NiII complex 6.
active in the reaction (presumably because of the difficulty to
access the corresponding Ni0 species), it showed that the 2,6-
diisopropyl phenyl group provided an increased shielding
compared to the chiral side chains. The importance of this
increased bulk for a better orientation of the alkyne also
manifested itself in the achiral transformations to give 3ag.
IMes with IPr gave 19:1 rs (73%) and IMes only gave 3:1 rs
(84%). The high enantioselectivity also suggested that only
a single chiral side chain of the carbene is involved in the
enantioselection, presumably during the facial-selective coor-
dination and incorporation of the enoate (7!8, Scheme 5).
To demonstrate the practicality of the process, we carried
out a gram-scale reaction using substrate 1h and a catalyst
loading of 2.5 mol% nickel (Scheme 4). The desired product
3ha was isolated in a comparable yield of 89% and
a selectivity of 98.5:1.5 e.r.
Scheme 2. Scope of enantioselective synthesis of cyclopentenones 3.
Reaction conditions: 1 (0.10 mmol), 2 (0.15 mmol), [Ni(cod)2]
(5.0 mmol), L5 (7.0 mmol), tBuOH (0.5 mmol), BEt3 (0.2 mmol), at
508C for 17 h. [a] With L2. [b] MeOH used instead of tBuOH at 708C.
cod=cycloocta-l,5-diene, Mes=mesityl, Phth=phthaloyl.
The following mechanistic scenario for the cycloaddition
is plausible (Scheme 5).[10–11,18] First, both the enoate and
alkyne substrates coordinate to the Ni0 catalyst, which bears
the chiral NHC ligand (7). Next, the enantioselectivity-
determining step of the process is the oxidative cyclization,
Scheme 4. Gram-scale synthesis of cyclopentenone 3ha.
Scheme 3. Performance of L5 versus L2 with challenging unsymmet-
rical alkynes. Reaction conditions: 1 (0.10 mmol), 2 (0.15 mmol),
[Ni(cod)2] (5.0 mmol), L* (7.0 mmol), tBuOH (0.5 mmol), BEt3
(0.2 mmol), at 508C. rs=regioselectivity.
turned out to be less reactive, they provided the correspond-
ing enone. Noteworthy, in this case the regiochemistry was
reversed with L5.[17]
The performance of L5 prompted us to evaluate its
structure more closely, and the corresponding NiII complex 6
is depicted in Figure 1.[16] Although 6 turned out to be not
Scheme 5. Plausible mechanism of the Ni-catalyzed asymmetric cyclo-
addition.
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
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Received: August 19, 2014
Published online: && &&, &&&&
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Keywords: annulation · asymmetric catalysis · carbenes ·
cyclopentenones · nickel
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3452; j) W. Li, J. Montgomery, Chem. Commun. 2012, 48, 1114 –
1116.
[19] BEt3 has been shown to undergo partial methanolysis to
B(OMe)Et2 (see K.-M. Chen, K. G. Gunderson, G. E. Hardt-
mann, K. Prasad, O. Repic, M. J. Shapiro, Chem. Lett. 1987,
1923 – 1926). Replacing BEt3 by B(OMe)Et2 resulted in an
inactive catalytic system and no conversion was observed.
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Asymmetric Catalysis
J. S. E. Ahlin, P. A. Donets,
N. Cramer*
&&&& — &&&&
Nickel(0)-Catalyzed Enantioselective
Annulations of Alkynes and Arylenoates
Enabled by a Chiral NHC Ligand: Efficient
Access to Cyclopentenones
Cyclization: Nickel(0) catalysts with
a chiral bulky C1-symmetric N-heterocy-
clic carbene ligand enabled the efficient
asymmetric reductive [3+2] cycloaddition
of enoates and alkynes, providing sub-
stituted cyclopentenones under mild
conditions. The system provided the
products in very high enantioselectivity
and led to a regioselective incorporation
of unsymmetrically substituted alkynes.
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!