Copper-catalyzed asymmetric alkynylation of cyclic: N -sulfonyl ketimines

Article abstract of DOI:10.1039/c7cc02159c

A Cu-catalyzed asymmetric alkynylation of cyclic N-sulfonyl ketimines was developed, providing the corresponding chiral α-tertiary amines with up to 98% ee. The method tolerates some variations in cyclic N-sulfonyl ketimine and alkyne scope. These products could be used in several transformations, in particular, the products of 6-membered cyclic N-sulfonyl ketimines could be easily converted to linear chiral α-tertiary amines. This asymmetric alkynylation provides an efficient, gram-scale, low-cost transition-metal catalyzed synthesis of chiral α-tetrasubstituted propargylamines.

Full text of DOI:10.1039/c7cc02159c

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Copper-Catalyzed Asymmetric Alkynylation of Cyclic N-sulfonyl
Ketimines
Received 00th January 20xx,
Accepted 00th January 20xx
Zheng Ling,a Sonia Singh,a,b Fang Xie,*a Liang Wu,a and Wanbin Zhang*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
Cu-catalyzed asymmetric alkynylation of cyclic N-sulfonyl In general, for asymmetric addition reactions, a cyclic
ketimines was developed, providing the corresponding chiral α- framework is preferred for the stereoselective synthesis of α-
tertiary amines with up to 98% ee. The method tolerates some tetrasubstituted propargylamines. However, propargylamine
variations in cyclic N-sulfonyl ketimine and alkyne scope. These products bearing a cyclic scaffold have limited use as synthetic
products could be used in several transformations, in particular, building blocks or intermediates. We hypothesized that the
the products of 6-membered cyclic N-sulfonyl ketimines could be asymmetric addition of ketimines possessing a cyclic scaffold,
easily converted to linear chiral α-tertiary amines. This followed by a simple ring opening of the cyclic products, could
asymmetric alkynylation provides an efficient, gram-scale, low- be used to generate linear α-tertiary amines. These amines
cost transition-metal catalyzed synthesis of chiral α- could then be implemented as synthetic building blocks for a
tetrasubstituted propargylamines.
wide range of natural products and bioactive compounds
containing chiral α-tertiary amines.8 Cyclic N-sulfonyl ketimines,
which have been used in addition reactions with various
nucleophiles, aza-Diels-Alder reactions and allylic substitution
reactions,2k,9 have been chosen as the preferred substrates
because the SO2 group can be easily removed (Scheme 1, (c)).
It is also worth noting that chiral cyclic sulfonamides have
found extensive use in a number of medicines10 such as 5-HT2
receptor antagonists, HIV-1 inhibitors, HCV (Hepatitis C virus)
NS5b inhibitors, and selective CRTh2 antagonists.11
Additionally, to ensure good reactivity and better
functionalization, we chose cyclic N-sulfonyl α-ketiminoesters
The construction of chiral α-tertiary amines is still a subject
of intensive research in organic chemistry.1,2 Therein, chiral α-
tetrasubstituted propargylamines are useful intermediates in
organic synthesis and are versatile scaffolds for the
preparation of natural products and bioactive compounds.3
The metal-catalyzed asymmetric alkynylation of ketimines is
one of the most direct, efficient and popular methods.4
However, construction of α-tetrasubstituted propargylamines
is still challenging due to the low reactivity of ketimines and
poor stereocontrol. To date, there are only several reports
concerning the catalytic enantioselective alkynylation of
ketimines catalyzed by metals such as Cu,5 Zn,6 and Rh.7
(a) Maruoka's and Watson's works:
Cu(I)
R3
H
N
N
The Cu-catalyzed alkynylation of ketimines is favored
because of its low cost, operational simplicity and good
stability of the copper reagent. In recent years, the Maruoka
and Watson groups have reported asymmetric alkynylations
for the preparation of isoquinolines with α-diaryl
tetrasubstituted stereocenters, with yields in excess of 76%
and 98% ee, respectively (Scheme 1, (a)).5a,5d On the other
R2
R3
R2
R1
R1
O
O
N
N
N
up to 76% yield
up to 98% ee
Ph
Ph
(b) Shibasaki's work:
S
S
Ph
Ph
P
mesitylcopper
Ph
Ph
Ph
HN
R1
P
R3
H
N
Ph
R1
R2
R2
R3
P
P
up to 79% yield
up to 80% ee
Ph
Ph
hand, the Shibasaki group reported
a direct catalytic
(c) This work:
asymmetric addition of terminal alkynes to ketimines to give
linear α-tetrasubstituted propargylamines with good yields (up
to 79%) and moderate enantioselectivity (up to 80% ee)
(Scheme 1, (b)).5b,5c
X
X
XH
NH2
SO2
SO2
NH
R2
Ar
[Cu] / chiral ligand
H
R2
R2
open ring
∗
N
∗
COOR1
COOR1
COOR1
X = none or O atom
Ar
Ar
Scheme 1 Cu-catalyzed asymmetric alkynylation of ketimines.
with relatively high activity to generate the alkynylation
products.12 Herein we describe an efficient protocol for the
preparation of amines containing
a
tetrasubstituted
stereogenic centre using a direct Cu-catalyzed asymmetric
addition of terminal alkynes to N-sulfonyl α-ketiminoesters.13
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At the beginning of this study, the Cu-catalyzed addition of conditions, giving the corresponding products with good to
terminal alkyne 2a to cyclic N-sulfonyl α-ketiminoester 1a was excellent enantioselectivities (92-98% ee) and good yields (70-
carried out in the absence of ligand (Scheme 2). Unsurprisingly, 94%)
(3ab, 3ad and 3af-3al). Electron-withdrawing and
no reaction occurred. Pyridine-bis(oxazolines) (PyBox) ligand electron-donating substituents (e.g. Cl and OMe) at the meta
L1 was then tested in this catalytic system. To our delight, the position of the phenyl ring resulted in a relative decrease in
reaction proceeded well, giving the desired product in 94% yields (65-80%) and enantioselectivities (85-91% ee) (3ac, 3ae).
isolated yield and 17% ee. Subsequent screenings using PyBox Changing the phenyl group to a β-naphthyl group gave the
ligands with different substituents on the oxazoline rings (L2, expected product (3am) with excellent enantioselectivity (91%
L3) were carried out (Scheme 2), however, no acceptable ee) and moderate yield (59%). Furthermore, even in the case
results were obtained. Using bisoxazoline (Box) ligand L4 of 2-ethynylthiophene, the expected product
greatly improved the enantioselectivity to 68%. Changing the afforded with excellent enantioselectivity (93% ee) and good
substituents on the oxazoline rings of the Box ligands (L5-L8 yield (81%). Hex-1-yne, ethynylcyclopropane and
(3an) was
)
showed that phenyl-substituted groups (L4) afforded better ethynyltrimethylsilane were also applied to this reaction
results. Subsequently, the effect of substituents at the carbon system but no reaction occurred.
atom connecting the two oxazoline rings was investigated. Low
to moderate enantioselectivities were provided when
substituents were cyclic groups (L9-L11). The substituent (R3)
was then changed to H, ethyl, allyl, n-pentyl and benzyl groups
for screening (L12-L16). As a result, L15 exhibited the best
catalytic efficiency, providing the desired product with 85% ee
O2
S
O2
S
O2
S
O2
S
and in 53% isolated yield.
NH
NH
COOEt
Cl
NH
NH
COOEt
COOEt
COOEt
Cl
Cl
3aa, 88%, 96% ee 3ab, 82%, 92% ee 3ac, 80%, 85% ee 3ad, 85%, 98% ee
O2
S
NH
O2
S
NH
O2
S
NH
O2
S
NH
COOEt
COOEt
COOEt
COOEt
MeO
Br
F
F3C
3ae, 65%, 91% ee
3ag, 70%, 93% ee
3af, 78%, 98% ee
O2
3ah, 91%, 98% ee
O2
S
NH
O2
S
NH
O2
S
NH
S
NH
COOEt
COOEt
COOEt
COOEt
MeOOC
NC
O2N
3ak, 90%, 96% ee
Ph
3al, 75%, 96% ee
a
3aj, 94%, 98% ee
Reaction conditions: 1a (0.10 mmol), 2a (0.15 mmol),
3ai, 93%, 93% ee
.
O2
S
O2
S
NH
Cu(OAc)2 H2O (0.01 mmol), L* (0.012 mmol), LiOAc (0.10 mmol) and
5Å MS (60 mg) at 80 C in toluene (1 mL) for 20 h. b Isolated yield. c
o
NH
d
The ee was determined by HPLC analysis.
The absolute
COOEt
COOEt
configuration of 3aa was determined as R according to ref. 7b.
Scheme 2 Screen of chiral ligandsa,b,c,d
S
.
3am, 59%, 91% ee
3an, 81%, 93% ee
Next, optimization of the reaction conditions was carried
out using L15 as a ligand. After screening of various inorganic
bases, solvents, copper salts, temperature and adjustment of
the ligand and copper ratio, the desired product was obtained
in 88% yield and 96% ee under the optimal reaction conditions
a
Reaction conditions: 1a (0.10 mmol), 2 (0.15 mmol), Cu(OAc)2
(0.01 mmol), L15 (0.015 mmol), LiOAc (0.10 mmol) and 5Å MS (60
o
b
c
mg) at 90 C in toluene (1 mL) for 2 days. Isolated yield. The ee
d
was determined by HPLC analysis. The absolute configuration of
3aa was determined as R according to ref. 7b.
Scheme 3 Scope of alkyne substratesa,b,c,d
(
1a (0.10 mmol), 2 (0.15 mmol), Cu(OAc)2 (0.01 mmol), L15
o
.
(0.015 mmol), LiOAc (0.10 mmol) and 5Å MS (60 mg) at 90 C
in toluene (1 mL) for 2 days.).14
The scope of substrate
1 was then explored (Scheme 4).
Good to excellent enantioselectivities (89-95% ee) were
obtained for substrates bearing a variety of ester groups
(COOMe, COOiPr, and COOnBu) (3ba-3da). In addition, a series
of fused 5-membered cyclic N-sulfonyl α-ketiminoesters
possessing electron-donating or electron-withdrawing
With the optimal reaction conditions in hand, a range of
terminal alkynes were then investigated with this catalytic
system (Scheme 3). Electron-withdrawing groups at the ortho
or para-position of the phenyl group, such as, Cl, Br, CF3,
COOMe, CN, NO2 and Ph, were all amenable to the reaction
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substituents at the 5- or 7-position of the phenyl groups
proceeded smoothly to afford the desired products with
3
excellent enantioselectivities (90-98% ee) and good to
excellent yields (56-97%) (3ea-3oa). A substrate bearing 4,6-
dimethyl-substituted phenyl group provided its corresponding
product with only 40% ee and 62% yield (3pa). Notably,
naphtha-fused N-sulfonyl α-ketiminoester also gave the
desired product with 91% ee and in 87% yield (3qa).
Scheme 5 Possible mechanism.
a
Reaction conditions: 4 (0.10 mmol), 2a (0.15 mmol), Cu(OAc)2
(0.01 mmol), L15 (0.015 mmol), LiOAc (0.10 mmol) and 5Å MS (60
o
b
c
mg) at 105 C in toluene (1 mL) for 24 hours. Isolated yield. The
ee was determined by HPLC analysis.
Scheme 6 Asymmetric alkynylation of 6-membered cyclic N-
sulfonyl α-ketiminoestersa,b,c and its ring opening reactions.
Considering the importance of the structural skeleton of
the alkynylation products, we explored the practicality of our
methodology. A gram-scale reaction using substrate 1a and
alkyne 2a was carried out (Scheme 7). The product 3aa was
obtained with results comparable to those shown in Scheme 3.
In addition, 3aa could be converted to alkyl and alkenyl
a Reaction conditions, please see Scheme 3.
Scheme 4 Scope of N-sulfonyl α-ketiminoestersa.
compounds (7, 8) via hydrogenation (Scheme 8).
A possible mechanism for the current transformation is
illustrated in Scheme 5. Initially, the Cu(II) salt can be reduced
to (phenylethynyl)copper(I) using phenyl acetylene and
LiOAc.15
coordination with (phenylethynyl)copper (
addition intermediate III via complex II 16
intermediate III furnishes the target product 3aa and
N-Sulfonyl
α-ketiminoesters
1a
) to generate the
Protonation of
undergoes
I
.
Scheme 7 Gram-scale reaction.
regenerates (phenylethynyl)copper (
next catalytic cycle.
I) which can take part in
Additionally, to further explore the scope of the reaction,
two fused 6-membered cyclic N-sulfonyl α-ketiminoesters 4a
and 4b were also examined in this catalytic system (Scheme 6).
A
substrate bearing a methyl ester group 4a gave its
Scheme 8 Transformations of 3aa.
corresponding product 5a with 91% ee and 74% yield. The ring
opening reaction of 5a went smoothly to afford linear chiral α-
In conclusion, we have demonstrated a direct Cu-catalyzed
asymmetric alkynylation of 5-membered cyclic N-sulfonyl α-
ketiminoesters with good to excellent yields and
enantioselectivities. A relatively wide substrate scope of
tertiary amino alcohol 6 with 94% yield and 90% ee.
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also be applied to 6-membered cyclic N-sulfonyl α-
ketiminoesters, whose products can be converted easily to
linear chiral α-tertiary amines via simple ring opening reactions.
The National Natural Science Foundation of China (Nos.
21572129, 21232004 and 21620102003) and Science and
Technology Commission of Shanghai Municipality (Nos.
14XD1402300 and 15JC1402200) are acknowledged for
financial support. We also thank Shanghai Jiao Tong University
(SJTU) and the Instrumental Analysis Center of SJTU.
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