Rhodium-Catalyzed Enantioselective Alkenylation of Cyclic Ketimines: Synthesis of Multifunctional Chiral α,α-Disubstituted Allylic Amine Derivatives

Article abstract of DOI:10.1021/acs.orglett.8b00651

By employing a simple open-chain chiral phosphite-olefin ligand, a highly enantioselective rhodium-catalyzed alkenylation of 1,2,5-thiadiazole 1,1-dioxide-type cyclic ketimines with diverse vinylboronic acids was achieved under mild conditions at room tem

Full text of DOI:10.1021/acs.orglett.8b00651

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Rhodium-Catalyzed Enantioselective Alkenylation of Cyclic
Ketimines: Synthesis of Multifunctional Chiral α,α-Disubstituted
Allylic Amine Derivatives
Yi Li,§,†,‡ Bo Liu,§,†,‡ and Ming-Hua Xu*,†
†State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, 555 Zuchongzhi Road,
Shanghai, 201203, China
‡University of Chinese Academy of Sciences, Beijing, 100049, China
S
* Supporting Information
ABSTRACT: By employing a simple open-chain chiral phosphite-olefin ligand, a highly enantioselective rhodium-catalyzed
alkenylation of 1,2,5-thiadiazole 1,1-dioxide-type cyclic ketimines with diverse vinylboronic acids was achieved under mild
conditions at room temperature. This protocol provides an efficient and practical access to multifunctional α,α-disubstituted
chiral allylic amines bearing quaternary stereocenters in high yields (up to 99%) with good to excellent ee’s (up to 99%).
Scheme 1. Enantioselective Construction of α,α-
hiral amines bearing an α-allyl functionality are of
Disubstituted Allylic Amines
C
particular interest because they are exceptionally valuable
building blocks in organic synthesis and have found wide
application in medicinal chemistry and pharmaceutical
industry.1 Over the past decades, considerable efforts have
been devoted to their asymmetric synthesis.2−8 Accordingly,
three main strategies have been developed to access chiral
allylic amines (Scheme 1a), including transition-metal-catalyzed
asymmetric allylic amination,3,4 asymmetric rearrangement of
allylic imidates,5 and asymmetric addition of alkenyl nucleo-
philes to imines,6,7 which have proven to be powerful.
However, despite these remarkable advances, convenient and
efficient methods for the preparation of α,α-disubstituted allylic
amines bearing a tetrasubstituted stereogenic center at the
allylic position remain largely underdeveloped due to the
significant synthetic challenges associated with the enantiose-
lective installation of a quaternary carbon stereocenter.
To date, there have been very few reports on the successful
catalytic asymmetric construction of α,α-disubstituted chiral
allylic amines. For example, Nguyen and Kleij have made great
progress in asymmetric allylic amination using tertiary allylic
imidates and substituted vinyl cyclic carbonates as rhodium/
palladium allyl precursors.4a,b Peters demonstrated that 3,3-
disubstituted allylic trifluoroacetimidates were capable of a
highly enantioselective aza-Claisen rearrangement under the
catalysis of planar-chiral pentaphenylferrocenyl palladacy-
cles.5b,c Comparatively, transition-metal-catalyzed asymmetric
addition of alkenyl nucleophiles to ketimines appears to be a
simple and attractive strategy for straightforward access to
chiral α,α-disubstituted allylic amines in terms of substrate
availability. However, achieving high yield and enantioselectiv-
ity through the direct control of chiral catalysts still remains a
very challenging task, and the problem is mainly attributed to
Received: February 23, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00651
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
the intrinsic lower reactivity of ketimines and the inefficient
enantiofacial differentiation. To our knowledge, few studies
have been attempted so far, and only three single examples
involving achieving high enantioselectivity through Rh-
catalyzed asymmetric addition of alkenylboron reagents to
keimines were reported by Hayashi,6e Lam,6f Kong and
McLaughlin6g (Scheme 1b). Notably, an alternative route to
chiral allylic amines bearing quaternary stereocenters was
recently developed by Jia through nickel-catalyzed direct
addition of styrenes to highly activated ketimines.7 Thus,
there is an unmet need for highly efficient asymmetric
approaches that could enable easy access to versatile α,α-
disubstituted chiral allylic amine derivatives.
Table 1. Optimization of Reaction Conditions for
Alkenylation of 1a with 2a
a
b
c
In recent years, our group has focused on the development of
effective chiral olefin ligands for asymmetric catalysis.9 In 2014,
we reported the design of a new class of chiral phosphite-based
open-chain olefin ligands10 and found they showed higher
activity and enantioselectivity than sulfur-olefins in an
asymmetric 1,2-addition of (E)-styrylboronic acids to six-
membered cyclic N-sulfonyl aldimines, allowing for the
construction of α-monosubstituted allylic amine frameworks
in an enantioselective manner.10a Motivated by this success, and
in conjunction with our interest in this field, we became
intrigued by the feasibility of their application in the more
challenging enantioselective alkenylation of ketimines to access
α,α-disubstituted allylic amines bearing quaternary stereogenic
centers. Herein, we disclose our development of an exception-
ally mild and convenient method that allows catalytic
asymmetric addition of vinylboronic acids to 3-substituted-4-
aryl-1,2,5-thiadiazole 1,1-dioxides for synthesis of various highly
enantioenriched thiadiazoline products. Such molecules con-
taining a unique α,α-disubstituted allylic amine framework and
sulfonamide functionality are of particular interest to organic
chemists and pharmaceutical scientists,11 but there are no
reports of their asymmetric synthesis via direct enantioselective
catalysis.
We initiated the work by evaluating simple open-chain chiral
phosphite-olefin L1 as ligand for the reaction of 3-ethoxy-4-
phenyl-1,2,5-thiadiazole 1,1-dioxide (1a) and (E)-styrylboronic
acid (2a) under the conditions of [Rh(COE)2Cl]2 (1.5 mol %)
and aqueous K3PO4 (1 M) in toluene at room temperature. To
our delight, the reaction proceeded smoothly and gave the
expected alkenylation product 3aa in excellent yield (99%) with
a promising enantioselectivity (83% ee) (Table 1, entry 1). This
result suggested that simple chiral phosphite-olefin ligands
could promote the reaction efficiently. Accordingly, to attain
higher enantioselectivity, a variety of (R)-binol-derived
phosphite-olefin L2−L9 that contained different aromatic
moieties attached to the double bond were evaluated. It was
found that the enantioselectivity was slightly improved with the
introduction of a bulkier substituent onto the para-position of
phenyl ring (L3−L4, entries 3−4). Further investigation
revealed that L5−L6 bearing methoxy groups at the 3,5- or
3,4,5-position of the phenyl ring gave an obvious improvement
in enantioselectivity, while maintaining the same excellent
reactivity (entries 5−6). Changing the methoxy group in L5 to
the sterically more bulky tert-butyl group exhibited the greatest
preference, in terms of both catalytic activity and enantiocon-
trol (L7, 99% yield and 92% ee) (entry 7). In addition, ligands
that contain large naphthyl groups at the terminal (L8, L9) or
two methyl/phenyl substituents at the 3- and 3′-positions of
the binaphthyl unit (L10, L11) were also prepared.
Unfortunately, no superior results were obtained (entries 8−
entry
L
additive
solvent
yield (%)
ee (%)
1
L1
L2
L3
L4
L5
L6
L7
L8
L9
L10
L11
L7
L7
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K2HPO4
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
DCE
99
96
99
99
99
99
99
99
99
91
99
99
99
83
83
86
85
90
91
92
85
88
55
43
93
94
2
3
4
5
6
7
8
9
10
11
12
13
DCE
a
Reactions were performed with 1a (0.1 mmol), 2a (0.2 mmol),
[Rh(COE)2Cl]2 (1.5 mol %), L (3 mol %), and 1.0 equiv of additive
(1.0 M) in 1 mL of dry solvent at room temperature. Isolated yield.
Determined by chiral HPLC.
b
c
11). Following up on the encouraging results obtained with L7,
effects related to the reaction solvent and additive were
carefully examined (see Supporting Information (SI)). Finally,
the combination of DCE and K2HPO4 was selected to be the
optimal choice, giving the best result with 94% ee and 99%
yield (entry 13).
With the optimal conditions in hand, we sought to evaluate
the scope and generality of the reaction. As summarized in
Scheme 2, a variety of 3-substituted 4-aryl-1,2,5-thiadiazole 1,1-
dioxides (1) were all successfully reacted with (E)-styrylboronic
acid (2a), providing the corresponding chiral thiadiazoline
products (3) having an α,α-disubstituted allylic amine frame-
work mostly in both high yields and enantioselectivities (up to
99% yield and 99% ee). In general, 4-phenyl-1,2,5-thiadiazole
1,1-dioxide species bearing different R substituents, such as
ethoxyl, methoxyl, isopropoxy, and n-butylamino, at the 3-
position of cyclic ketimines were proven to be suitable
substrates, as they all exhibited high reactivity and led to the
formation of highly enantiomerically enriched products 3aa−
3da (92−96% ee). Furthermore, substrates with various meta-
and para-substituted functional groups in the Ar rings were well
tolerated, affording the corresponding products 3ea−3ja with
generally high enantioselectivities in excellent yields. In
addition, the 2-naphthyl-substituted substrate also afforded
the desired product (3ka) in very high yield (97%) and ee
(97%). The electronic properties of the aromatic ring did not
appear to affect the reaction yields and enantiomeric excess
values. However, a large decrease in enantioselectivity was
observed in the cases of cyclic ketimines bearing 1-naphthyl
(3la) and 2-fluorophenyl (3ma), probably due to steric
disadvantage. Notably, when ketimines containing a hetero-
B
DOI: 10.1021/acs.orglett.8b00651
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Organic Letters
Letter
line products (3eb, 3kb, 3ob, 3ec, 3jc, 3kc, 3nc, 3ed, and 3kd)
in up to 99% yield with outstanding enantioselectivities (92−
97% ee). Remarkably, the reaction also tolerates vinylboronic
acids with varied alkyl substituents, allowing access to various
desired alkenylation products 3ee, 3ef, 3eg, 3oe, 3of, and 3og
in good yields with the same high level of enantioselectivity
(91−96% ee). These results suggest that significant variation of
the olefin substituent has little effect on the reaction
enantioselectivity. Thus, a broad range of sterically and
electronically different vinylboronic acids could be employed
with ease in this direct asymmetric alkenylation, indicating that
this protocol is a promising method for constructing diverse
chiral allylic amine frameworks bearing a tetrasubstituted
stereogenic center. The absolute configuration of the
alkenylation products was unambiguously established to be S
by single-crystal X-ray analysis of 3ec (Figure 1).
Scheme 2. Rh/L7-Catalyzed Asymmetric Alkenylation of 1
with 2a
a b c
, ,
a
The reaction was performed with 1 (0.1 mmol), 2a (0.2 mmol),
[Rh(COE)2Cl]2 (1.5 mol %), L7 (3.0 mol %), and 1.0 equiv of
b
c
K2HPO4 (1 M) in 1 mL of DCE at rt. Isolated yield. Determined by
chiral HPLC.
aromatic functionality such as thienyl and furanyl groups were
used as the substrates, the alkenylation reactions proceeded
equally well and afforded the desired products 3na−3pa in high
yields with up to 99% ee.
Figure 1. X-ray crystal structure of (S)-3ec.
Subsequently, the scope with respect to the vinylboronic
acids was examined to demonstrate the generality of this
alkenylation (Scheme 3). Gratifyingly, reactions involving a
variety of (E)-styrylboronic acids bearing either electron-
donating or electron-withdrawing functional groups with a
series of 3-ethoxy-4-aryl-1,2,5-thiadiazole 1,1-dioxides were all
found to be successful, affording the expected chiral thiadiazo-
To demonstrate the practicality of this protocol, a gram-scale
reaction between ketimine 1a and vinylboron reagent 2a was
carried out (see SI). Under the standard conditions, the
reaction proceeded smoothly and gave 1.01 g of 3aa in equally
high yield (98%) without loss of the enantioselectivity (94%
ee).
To further showcase the synthetic utility of this method, the
transformations of alkenylation products 3aa and 3ba into a
valuable vicinal diamine and α-aryl-β,γ-alkenyl α-amino amides
were evaluated (Scheme 4). First, reduction of 3aa with NaBH4
Scheme 3. Rh/L7-Catalyzed Asymmetric Alkenylation of
Vinylboronic Acid 2
a b c
, ,
Scheme 4. Access to Enantioenriched Vicinal Diamine and
β,γ-Alkenyl-α-Amino Amides
in THF at room temperature afforded the chiral cyclic
sulfamide 4 in quantitative yield, and subsequent treatment of
4 with 1,3-diaminopropane led to the efficient formation of
vicinal diamine 5 in 95% yield with untarnished ee (eq 1). Very
gratifyingly, treatment of 3ba under melting conditions gave the
corresponding sulfahydantoin 6 without losing optical purity.
a
The reaction was performed with 1 (0.1 mmol), 2 (0.2 mmol),
[Rh(COE)2Cl]2 (1.5 mol %), L7 (3.0 mol %) and 1.0 equiv of
b
c
K2HPO4 (1 M) in 1 mL of DCE at rt. Isolated yield. Determined by
chiral HPLC. The reaction was performed at 60 °C.
d
C
DOI: 10.1021/acs.orglett.8b00651
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Organic Letters
Letter
Further removal of the sulfonyl with LiAlH4 or ring cleavage
with MeMgBr could lead to α-aryl-β,γ-alkenyl α-amino amides
7 and 8 in good yields, while maintaining the same level of
enantioselectivity (eq 2). It is noteworthy that this kind of β,γ-
unsaturated α-amino acid and their derivatives are important
structural motifs presented in many biologically active natural
products and pharmaceutical agents,12 and catalytic asymmetric
approaches to access such quaternary-containing β,γ-unsatu-
rated α-amino acid units remain considerably underdeveloped.
Finally, the availability of the current methodology can also
be featured by the intriguing synthetic transformations of the
alkenylation product 3aa (Scheme 5). Treatment of 3aa with
heterocycle sulfamide derivatives that possess great potential in
drug discovery. We believe that this method will find wide
application in organic synthesis and medicinal chemistry.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00651.
Experimental procedures, characterization data, and
copies of NMR and HPLC spectra (PDF)
Accession Codes
Scheme 5. Transformations of Alkenylation Product 3aa
CCDC 1814140−1814141 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: xumh@simm.ac.cn.
ORCID
Ming-Hua Xu: 0000-0002-1692-2718
Author Contributions
§Y.L. and B.L. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
allyl bromide in the presence of K2CO3 in DMF at room
temperature for 2 h, followed by the olefin metathesis using the
second-generation Grubbs catalyst, afforded the dihydropyrrole
derivative 9 in 90% yield for two steps. Similarly, treatment of
3aa with 3-bromopropyne, followed by cyclization under
typical Pauson−Khand reaction conditions, efficiently delivered
chiral quaternary carbon-containing fused cyclopenta[c]proline
derivatives 10a and 10b in 34% and 47% yields without erosion
of the ee. The stereochemistry of 10a and 10b were determined
by X-ray crystallographic analysis and NOE experimentation
(see SI), respectively. Furthermore, ammoniation of alkenyla-
tion product 3aa could also be conducted smoothly in the
presence of ammonia or benzylamine to provide the
corresponding products 11 and 12 in excellent yield without
any loss of enantioselectivity. Interestingly, the proton signal
indicated that CN of compound 11 is located outside the
ring, which is different from the case in compound 12.
In summary, we have developed an efficient protocol for the
challenging asymmetric addition of alkenyl nucleophiles to
ketimines. Under rhodium catalysis with a simple open-chain
chiral phosphite-olefin as a ligand, the reaction of diverse
vinylboronic acids with varied 3-substituted-4-aryl-1,2,5-thia-
diazole 1,1-dioxides could proceed smoothly at room temper-
ature, enabling reliable access to a broad range of functional
α,α-disubstituted chiral allylic amines bearing quaternary
stereocenters in high yields with good to excellent enantiose-
lectivities (up to 99% ee). The method is promising and
notable because it can be applied to access highly valuable
enantioenriched β,γ-unsaturated α-amino amides and vicinal
diamines with ease of removal of sulfonyl. Furthermore, the
products can be easily transformed into a series of interesting
■
We thank the National Natural Science Foundation of China
(21472205, 81521005, 21502207, 21325209) and the Shanghai
Municipal Committee of Science and Technology
(15YF1414600) for financial support.
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