Copper-Catalyzed Tandem Cross-Coupling and Alkynylogous Aldol Reaction: Access to Chiral Exocyclic α-Allenols

Article abstract of DOI:10.1021/acs.orglett.1c01712

An enantioselective copper-catalyzed tandem cross-coupling/alkynylogous aldol reaction has been developed. The tetrasubstituted allenoates containing both central and axial chirality have been obtained in moderate to good yields and excellent enantio-and

Full text of DOI:10.1021/acs.orglett.1c01712

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Letter
Copper-Catalyzed Tandem Cross-Coupling and Alkynylogous Aldol
Reaction: Access to Chiral Exocyclic α‑Allenols
Guangyang Xu, Zhen Wang, Ying Shao, and Jiangtao Sun*
Cite This: Org. Lett. 2021, 23, 5175−5179
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ABSTRACT: An enantioselective copper-catalyzed tandem cross-
coupling/alkynylogous aldol reaction has been developed. The
tetrasubstituted allenoates containing both central and axial
chirality have been obtained in moderate to good yields and
excellent enantio- and diastereoselectivity. Distinct from the
previous use of Cu(I) salts, this protocol features the use of
copper(II) salts as a catalytic precursor in this asymmetric cross-
coupling reaction.
opper-catalyzed cross-coupling reactions of terminal
electrophile to trap the in situ generated allenoate-Cu(I)
intermediate (Scheme 2b).7 As part of our research on carbene
transformations, we previously reported a series of tandem
reactions for synthesizing heterocycles and functionalized
molecules through the tandem reactions of terminal alkynes
and diazoacetates.8 Inspired by Wang’s seminal work,3 and by
introducing a Machael-acceptor into the alkyne skeleton, we
also reported a tandem coupling of alkynes with diazo
compounds and subsequent cyclization to synthesize tetrasub-
stituted allenoates.8b Also encouraged by Liu’s protocol,6 we
envisioned that, by introducing a strong electrophile such as
the aldehyde moiety into the alkyne molecule, the chiral
exocyclic allenols bearing axially chiral allenes would be
obtained through an effective catalytic system. As depicted in
Scheme 2c, the reaction might start with the formation of
copper species Int-2 via the preferential formation of copper
acetylide Int-11 or copper-carbene Int-1′,2,9 which would
undergo alkynyl migratory insertion to form propargyl copper
species Int-3. Then, the alkynylogous Aldol reaction10 would
occur to yield the allenoate copper species Int-4. Alternatively,
the 1,3-copper migration of Int-3 forms allenoate-copper
intermediate Int-3′, which can be trapped by the aldehyde
moiety to generate Int-4, too. Finally, protonation of Int-4
delivers the final chiral exocyclic α-allenol and regenerates the
copper catalyst. It should be noted that nominal catalysis by
Cu(II) complexes involves prior reduction to Cu(I) by either
diazo compounds11 or terminal alkynes.12
C
alkynes with diazo compounds represent one important
protocol to synthesize polysubstituted allenes and allenoates.1,2
A common accepted mechanism for the allene formation
involves the formation of copper species I which undergoes
migratory insertion to generate a propargyl copper species II
(Scheme 1). Subsequent protonation produces the final di- or
Scheme 1. Common Mechanism
trisubstituted allene (path a). Pioneered by the Wang group,
this copper species II can be trapped through the addition of a
strong electrophile-allyl halide and directly converted to
tetrasubstituted allene (path b).3
By using a chiral copper catalytic system, the axially chiral
allenes could be obtained via this protocol.4−6 In 2015, Liu and
co-workers reported the first highly enantioselective diazo-
alkyne coupling reaction for synthesizing chiral allenoates via a
copper(I)-chiral cationic guanidinium salt system (Scheme
2a).4 Later, Wang and co-workers developed a Cu(I)-chiral
bisoxazoline-catalyzed coupling of diazoalkanes with terminal
alkynes for the enantioselective synthesis of trisubstituted
allenes.5 In 2017, the Ley group established a Cu(I)-chiral
pyridinebis(imidazoline) system to prepare chiral disubstituted
allenes by the coupling of diazo compounds and propargylated
amines.6 Recently, Liu and co-workers discovered an
asymmetric three-component reaction to reach chiral tetrasub-
stituted allenoates, in which the isatin was used as an
Received: May 21, 2021
Published: June 17, 2021
https://doi.org/10.1021/acs.orglett.1c01712
© 2021 American Chemical Society
Org. Lett. 2021, 23, 5175−5179
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a
Scheme 2. Previous Reports and Our Design
Table 1. Optimization of the Reaction Conditions
b
c
yield
ee
d
entry
[Cu]
ligand solvent
(%)
(%)
dr
1
2
3
4
5
6
7
8
CuI
L1
L1
L1
L1
L1
L2
L3
L4
L5
L6
L7
L8
L9
L4
L4
L4
L4
L4
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCE
33
24
79
0
15
0
78
87
68
<5
41
62
44
0
59
31
79
56
57
94
95
>19:1
>19:1
>19:1
Cu(MeCN)4BF4
Cu(tfacac)2
Cu(OTf)2
CuBr2
50
>19:1
Cu(tfacac)2
Cu(tfacac)2
Cu(tfacac)2
Cu(tfacac)2
Cu(tfacac)2
Cu(tfacac)2
Cu(tfacac)2
Cu(tfacac)2
Cu(tfacac)2
Cu(tfacac)2
Cu(tfacac)2
Cu(tfacac)2
Cu(cod)(tfacac)
78
97
86
9:1
>19:1
>19:1
9
10
11
12
13
14
15
16
17
18
96
80
63
19:1
5:1
3:1
toluene
MeCN
DCM
DCM
76
19
93
92
>19:1
1:1
13:1
>19:1
e
a
Reaction conditions: A mixture of copper catalyst (0.005 mmol),
chiral ligand (0.0055 mmol), 4 Å MS (60 mg), 1a (0.1 mmol), and
solvent (4 mL) was stirred for 1 h at 25 °C under argon; then, 2 (0.15
b
c
mmol) was added and stirred at 40 °C. Isolated yield. ee was
d
determined by chiral HPLC analysis. dr was determined by crude 1H
NMR. Without 4 Å MS. Cu(tfacac)2 = copper(II) trifluoroacetyla-
We commenced our studies by examining the copper-
catalyzed cross-coupling reaction between alkyne 1a and α-
phenyl diazoacetate 2a (Table 1). As it was demonstrated that
chiral bisoxazoline (Box) was a privileged chiral ligand in the
copper-catalyzed cross-coupling of terminal alkynes with
diazoacetates, we tested Box L1 as the chiral ligand to
combine with a series of copper salts (5 mol %) in this
reaction. The initial experiment with CuI and L1 in
dichloromethane at 40 °C delivered the desired product 3aa
in 33% yield and 57% ee (entry 1), and the use of
Cu(MeCN)4BF4 gave 3aa in 94% ee, but only in 24% yield,
albeit in excellent diastereoselectivity (>19:1) (entry 2).
Considering the poor catalytic reactivity for Cu(I) salts, we
then attempted to screen Cu(II) salts in this reaction.
Gratifyingly, Cu(tfacac)2 exhibited a good catalytic reactivity
and selectivity, affording 3aa in 79% yield and 95% ee (entry
3). However, other Cu(II) salts gave poor results. For example,
Cu(OTf)2 was ineffective (the self-coupling reaction of diazo
was observed as the major reaction, entry 4), and CuBr2
showed both poor catalytic reactivity and selectivity (entry 5).
Next, using Cu(tfacac)2 as the copper catalyst, a series of Box
ligands were evaluated. It was found that decreasing the ring
size of Box ligands gave poor results. No catalytic reactivity was
observed for Box L2 (n = 4) (entry 6). The use of L3 (n = 5)
provided 3aa in 78% yield and 78% ee (entry 7). Typically,
using Liu’s Box ligand L4 (n = 7) delivered 3aa in 87% yield
e
cetonate.
and 97% ee (entry 8).13 These results indicated that the Box
ligands with a small bite-angle are beneficial to both reactivity
and enantioselectivity. Further ligand screening was then
performed. Box ligand L5 containing a gem-dimethyl linkage
showed moderate catalytic reactivity and decreased enantiose-
lectivity (entry 9); however, L6 bearing a gem-dibenzyl linkage
gave only a trace amount of 3aa (entry 10). The PyBox ligand
L7 exhibited moderate catalytic reactivity and good enantio-
selectivity (entry 11). Further modifications to the ligand
scaffold gave inferior results. For instance, L8 and L9 exhibited
moderate reactivity and enantioselectivity but poor diaster-
eoselectivity (entries 12 and 13). Subsequent solvent screening
failed to improve the reaction. No reaction occurred when 1,2-
dichloroethane (DCE) was used (entry 14). Toluene gave 3aa
in moderate yield and enantioselectivity (entry 15), and
acetonitrile gave both poor enantio- and diastereoselectivity
(entry 16). It should be noted that both the yield and
enantioselectivity of 3aa slightly dropped without the addition
of a molecular sieve (entry 17). Finally, to verify whether the
Cu(II) complexes might be reduced to Cu(I) in the reaction
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system, Cu(cod)(tfacac) was examined, and 3aa was obtained
in 56% yield with 92% ee (entry 18).
Subsequently, the scope of the reaction with respect to
alkynes 1 was investigated (Scheme 4). Various alkynes
Under the optimal reaction conditions, the scope of diazo
compounds was then evaluated (Scheme 3). A range of α-aryl
a b
,
Scheme 4. Scope of Alkyne
a b
,
Scheme 3. Scope of Diazo Compounds
a
Reaction conditions: 1 (0.1 mmol), 2a (0.15 mmol), Cu(tfacac)2
(0.005 mmol), L4 (0.0055 mmol), 4 Å MS (60 mg), DCM (4 mL),
b
c
40 °C. Yields of isolated products. ee was determined by chiral
d
e
HPLC analysis. Performed at 50 °C. Cu(tfacac)2 (0.01 mmol) and
f
L4 (0.011 mmol) were used. Performed at 60 °C.
bearing either electron-donating or electron-withdrawing
groups at different positions of the aromatic ring were
smoothly reacted with diazoacetate 2a, providing the
corresponding exocyclic α-allenols (3ba−3ia) in 61−91%
yield, 90−96% ee, and 9:1 to >19:1 dr. Replacing the aldehyde
moiety with ketone in the alkyne substrate, the product 3ja
containing a tetrasubstituted stereocenter was obtained in 63%
yield, 80% ee, and 8:1 dr. However, the reaction of 2-(but-3-
yn-2-yl)benzaldehyde with 2a led to poor diastereoselectivity
(1.3:1), of which the major isomer 3ka was isolated in 45%
yield and 87% ee. The minor isomer was isolated in 20% yield
and 97% ee. The comparatively lower yield for 3fa, 3ia, and 3la
is attributed to the self-coupling of diazoacetates. For 2-(but-3-
yn-1-yl)benzaldehyde, the desired product was obtained in
30% yield and 66% ee. With respect to 2-ethynylbenzaldehyde,
the desired product 3ma was not observed, but the diazo self-
coupling product was detected.
The synthetic utility of this protocol was then demonstrated
(Scheme 5). A gram-scale synthesis of 3ae was performed
(Scheme 5a). Under the optimal reaction conditions, the
reaction with 3 mmol of 1a and 4.5 mmol of 2e afforded 1.04 g
of 3ae (87% yield) with 95% ee. With the reduction of 3ae
with DIBAL-H followed by AgNO3-promoted cyclization,
chiral spiro compound 5 was isolated in 35% yield and 96%
ee.14 Moreover, iodolactonization of 3ae with I2 led to the
corresponding spiro-iodobutenolide 6 in 55% yield and 94%
ee.15 Finally, the reaction of 2-acetaldehyde phenylacetylene 7
with 2a gave the corresponding product 8 in 93% yield, 83%
ee, and 6:1 dr (Scheme 5b).
a
Reaction conditions: 1a (0.1 mmol), 2 (0.15 mmol), Cu(tfacac)2
(0.005 mmol), L4 (0.0055 mmol), 4 Å MS (60 mg), DCM (4 mL),
b
c
40 °C. Yields of isolated products. ee was determined by chiral
HPLC analysis. Cu(tfacac)2 (0.01 mmol) and L4 (0.011 mmol)
were used. Combined yield.
d
e
diazoacetates containing either electron-donating or electron-
withdrawing substituents on the phenyl ring reacted smoothly
with 1a to give the corresponding exocyclic α-allenols (3ab−
3al) in 55−92% yield, 87−96% ee, and an excellent dr value
(>19:1). Typically, the strong electron-withdrawing substitu-
ents, such as OCF3 (3af) and CF3 (3ag), were tolerated. The
absolute configuration of 3ad was assigned as (R, Sa) by X-ray
diffraction analysis. α-Heteroaryl diazoacetates, such as α-
thienyl diazoacetate and α-indole diazoacetate, reacted well
with 1a, providing the corresponding products 3am and 3an in
moderate yields and excellent enantioselectivities. The ester
moiety of phenyl diazoacetate was also examined. The methyl
ester gave the product 3ao in almost the same yield but with a
diminished enantioselectivity (83% ee) and relatively poor
diastereoselectivity (8:1). The tert-butyl ester afforded 3ap in a
slightly higher yield (90%) and 96% ee. However, the benzyl
ester led to both lower yield and enantioselectivity, and also a
poor dr value (3aq). Unfortunately, for α-alkyl diazoacetate,
the reaction was messy, and the corresponding product 3ar
was not obtained.
Control experiments were conducted to understand the
reaction mechanism (Scheme 6). Under the optimal reaction
conditions, the reaction of terminal alkyne 9 with 2a produced
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Scheme 5. Further Elaboration
ASSOCIATED CONTENT
* Supporting Information
■
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01712.
Experimental procedures along with characterizing data
and copies of NMR spectra (PDF)
Accession Codes
CCDC 2079107 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Scheme 6. Control Experiments
Jiangtao Sun − Jiangsu Key Laboratory of Advanced Catalytic
Materials & Technology, School of Petrochemical
Engineering, Changzhou University, Changzhou 213164, P.
R. China; orcid.org/0000-0003-2516-3466;
Email: jtsun@cczu.edu.cn
Authors
Guangyang Xu − Jiangsu Key Laboratory of Advanced
Catalytic Materials & Technology, School of Petrochemical
Engineering, Changzhou University, Changzhou 213164, P.
R. China
Zhen Wang − Jiangsu Key Laboratory of Advanced Catalytic
Materials & Technology, School of Petrochemical
Engineering, Changzhou University, Changzhou 213164, P.
R. China
Ying Shao − Jiangsu Key Laboratory of Advanced Catalytic
Materials & Technology, School of Petrochemical
Engineering, Changzhou University, Changzhou 213164, P.
R. China; orcid.org/0000-0002-7379-8266
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01712
allenoate 10 in 62% yield and 58% ee (Scheme 6a). The three-
component reaction of 9, 2a, and benzaldehyde gave a 58%
yield of 10, and the benzaldehyde was recovered (Scheme 6b).
These two experiments indicated the preferential reaction of
the alkyne moiety with diazo in alkyne substrate 1; subsequent
addition to the aldehyde moiety then gave the final product in
Scheme 1c. Moreover, Scheme 6b also indicates that this
copper catalytic system did not support an intermolecular
three-component reaction, because the protonation is
preferred over the subsequent addition to aldehyde. The
deuterium-labeling experiments were also conducted. The
reaction of D-1a with 2a generated D-3aa in 89% yield and
96% ee, with 10% deuterium labeling (Scheme 6c). The
addition of 3 equiv of D2O resulted in 14% deuterium labeling
of D-3aa (Scheme 6d). These two experiments indicated that
the deprotonation and protonation did occur in the whole
reaction process.
In summary, we have developed a copper-catalyzed
asymmetric construction of chiral exocyclic α-allenols from
the reaction of diazoacetates and terminal alkynes, featuring
the concurrent construction of both central and axial chirality.
Key to realize this high enantio- and diastereoselective reaction
is the use of Cu(tfacac)2 as the catalyst precursor, which is
distinct with the use of Cu(I)-salts in previous allene formation
through coupling of diazo compounds and terminal alkynes.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(21971026) and the Jiangsu Key Laboratory of Advanced
Catalytic Materials & Technology (BM2012110) for their
financial support.
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