Rh(III)-catalyzed diastereodivergent spiroannulation of cyclic imines with activated alkenes

Article abstract of DOI:10.1021/acs.orglett.7b02678

Rh(III)-catalyzed [3 + 2] annulation of cyclic N-sulfonyl or N-acyl ketimines with activated alkenes has been realized, leading to the synthesis of spirocycles with three continuous stereogenic centers. This atom-economic reaction proceeded efficiently under mild and redox-neutral conditions via a C-H activation pathway, and the coupling is diastereodivergent, with the diastereoselectivity being controlled by silver additives.

Full text of DOI:10.1021/acs.orglett.7b02678

Letter
pubs.acs.org/OrgLett
Rh(III)-Catalyzed Diastereodivergent Spiroannulation of Cyclic Imines
with Activated Alkenes
Bingxian Liu,† Panjie Hu,† Ying Zhang,† Yunyun Li,‡ Dachang Bai,† and Xingwei Li*,†,‡
†Henan Key Laboratory of Organic Functional Molecules and Drug Innovation, School of Chemistry and Chemical Engineering,
Henan Normal University, Xinxiang 453007, China
‡Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
S
* Supporting Information
ABSTRACT: Rh(III)-catalyzed [3 + 2] annulation of cyclic N-sulfonyl or N-acyl ketimines with activated alkenes has been
realized, leading to the synthesis of spirocycles with three continuous stereogenic centers. This atom-economic reaction
proceeded efficiently under mild and redox-neutral conditions via a C−H activation pathway, and the coupling is
diastereodivergent, with the diastereoselectivity being controlled by silver additives.
with alkynes. Wei reported the oxidative coupling of
benzosultam with heterocycles.11 Nishimura applied Ir(I)
catalysis to [3 + 2] annulation of benzosultam and N-acyl
ketimines with 1,3-dienes via C−H activation (Scheme 2).8a−c
etal-catalyzed C−H activation and functionalization has
Mprovided important approaches for step-economic
synthesis of various drug-related chemicals and organics.1
Among these, tandem [3 + 2] annulation reactions between
readily available arenes and unsaturated bonds have been
widely investigated.2−4 This type of reaction involves a C−H
bond activation process followed by in situ transformations of
the directing group, which offers a straightforward route to
access cyclic compounds5 and is especially useful for the
construction of spirocycles that are otherwise difficult to
synthesize.6−13
Scheme 2. Construction of Spirocycles by Annulation with
Alkenes
Despite this progress, discovery of novel synthetic methods
that enable rapid assembly of spirocycles in a highly concise
manner remains a formidable challenge.14 N-containing
spirocycles of Type A are important motifs in numerous
bioactive products and pesticides (Scheme 1).15 Previously, a
number of metal-catalyzed [3 + 2] spiroannulation reactions for
the formation of aminocyclopentene derivatives have been
disclosed. For example, the groups of Dong,7 Nishimura,8d,e Li,9
Wang,10 and Cramer12 reported the annulation of aromatics
Scheme 1. Examples of Important N-Containing Spirocycles
Recently, Kim reported annulative couplings between N-acyl
ketimines and activated olefins under harsh conditions (Scheme
2),13 where the olefins were mostly limited to maleimides,
fumarates, and maleates to give a single diastereomer. We now
report mild and efficient [3 + 2] spiroannulation between
arenes and CF3-substituted enones with simultaneous con-
Received: August 28, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02678
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b
,
struction of three continuous stereogenic centers, and the
diastereoselectivity could be tuned by the silver additive.
Our initial studies revealed that, in the presence of
[Cp*RhCl2]2/AgSbF6 (4 mol %/16 mol %) and Cu(OAc)2
(50 mol %), the coupling of N-sulfonyl ketimine 1a with CF3-
substituted enone16 2a in DCE afforded two spirocycles (3aa
and 3aa′) in overall quantitative yield with a dr of 1.3:1 (Table
1, entry 1). Both products have been characterized by X-ray
Scheme 3. Scope of Ketimine Substrates
ab
,
Table 1. Optimization Studies
entry
solvent
additive
yield (%)
dr
temp (°C)
1
2
3
4
DCE
TFE
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
−
>99
>99
trace
>99
nr
1.3:1
1:1
60
60
60
40
40
40
40
25
40
40
40
40
MeOH
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
−
1.6:1
c
5
−
1:4
6
75
7
AgOAc
95
3.4:1
4.4:1
1:4.6
1:5
8
AgOAc
94
9
AgTFA
84
a
10
11
AgSbF6
AgOTf
72
The reaction was performed under Conditions A, B, or C in DCE
b
(2.0 mL) under Ar for 12 h. Isolated yield. The dr was determined by
86
1:7.8
1:4.7
c
1H NMR spectroscopy. The reaction was performed at a scale of 2.5
d
12
AgOTf
83
a
mmol of 1a under Conditions A.
Reaction conditions: 1a (0.2 mmol), 2a (0.24 mmol), [Cp*RhCl2]2/
AgSbF6 (4 mol %/16 mol %), and an additive (50 mol %) in a solvent
b
(2.0 mL) for 12 h under Ar. Isolated yield. The dr of 3aa:3aa′ was
additive, the products were all obtained in good to excellent
yields and moderate dr’s (46−98% total yield, 1.7−5.5:1 dr). It
has been reported that N-acyl ketimine generated in situ from 3-
hydroxy-3-aryllisoindolin-1-ones by dehydration can undergo
[3 + 2] annulation with alkynes.8c,e Thus, 4a was chosen to
react with 2a under the standard conditions A, furnishing the
desired products 5aa and 5aa′ in totally 64% yield and 1:10.4
dr. By simple optimization using the AgTFA additive
(Conditions B; see Supporting Information (SI)), the products
were obtained in excellent yield (97%) and moderate dr
(1:5.2). Several substituted N-acyl ketimines (4b−4d) were
then examined, and they all gave moderate to high yield (42%−
86%, 1:4−8 dr). To demonstrate the synthetic utility of the
catalytic reaction, a 2.5-mmol-scale synthesis of 3aa and 3aa′
was performed and a high yield was realized (Conditions A,
80%, 1:3.5 dr).
The scope of alkenes was also established (Scheme 4), and
the reaction system allowed the annulations of various CF3-
substituted enones and aryl vinyl ketones. CF3-substituted
enones bearing halogen groups (2c−e), methoxyl (2f), CF3
(2g), and NO2 (2h) at the para-position of the phenyl group all
reacted smoothly with 1a under Conditions A to give the
corresponding products in moderate to excellent yield and dr
(59−99% yield, 1:2.3−14 dr). The reactions were efficient with
introduction of EWGs such as CF3 (2g) and NO2 (2h).
Substrates bearing an ortho- or meta-substituent (2i−2k) were
also examined, and they exhibited good reactivity. Notably, only
product 3ak′ was detected when 2-methyl-substituted alkene
(2k) was used as a coupling partner (64%). Coupling of alkenes
with disubstituted aryl (2l−2o), furyl (2p), and naphthyl (2q)
all afforded the desired products with good efficiency and
selectivity. Switching to the AgOAc additive consistently
c
determined by 1H NMR spectroscopy. No AgSbF6 was used.
d
AgNTf2 was used instead of AgSbF6 (16 mol %).
crystallography. Changing the solvents turned out to be futile
(entries 2,3), and decreasing the reaction temperature only
slightly increased the diastereoselectivity (entry 4). Screening of
additives revealed that AgOAc was effective to increase the
diastereoselectivity up to 4.4:1 (entry 8), but using AgTFA or
AgOTf gave the inverse diastereoselectivity (entries 9 and 11).
Control experiments were also carried out. In the absence of
AgSbF6, no product was detected. However, the products could
be generated without any additive (entry 6). When AgSbF6 was
used as a halide scavenger as well as a silver additive (entry 10)
or when AgNTf2 was used as a halide scavenger (entry 12), the
reaction proceeded with a lower yield and diastereoselectivity.
Finally, we chose the conditions in entries 8 (Conditions C)
and 11 (Conditions A) as the diastereodivergent standard
conditions.
Substituted N-sulfonyl ketimines (1a−f) were allowed to
undergo annulation with 2a under Conditions A to explore the
scope of the arene substrate (Scheme 3). It was observed that
an electron-donating group such as methoxyl (1d) benefited
the reaction while electron-withdrawing groups such as para-F
(1c) decreased the reaction efficiency. The position of the
substituent also affected the reaction. In contrast to ketimine
1d, ketimine 1e with an ortho-OMe group coupled with 2a to
give a single product in good yield (3ea′, 62%). This is likely
due to the steric effect that leads to a more crowded transition
state during insertion of the imine group. The ketimine 1f with
a thienyl group only reacted with a low yield. When the
reactions were carried out under Conditions C with the AgOAc
B
DOI: 10.1021/acs.orglett.7b02678
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b
,
Scheme 4. Scope of Alkenes
Scheme 5. H/D Exchange and KIE Experiments
additive-dependent migratory insertion into the imine moiety.
In the presence of a more cationic AgOTf salt, the silver cation
likely facilitates the migratory insertion by activation of the
carbonyl to place the carbonyl and the nitrogen in a cis
orientation through a N,O-chelation to the Rh or Ag for
stabilization.
In summary, we have realized Rh(III)-catalyzed efficient [3 +
2] annulation for the synthesis of diverse spirocycles with three
stereogenic centers under mild conditions. The reactions are
highly efficient and atom-economical. The diastereoselectivity
can be tuned by the silver additive. Two types of cyclic ketimine
directing groups proved to be viable, and the scope of alkenes
was investigated and found to be quite satisfactory. Given the
rapid assembly of spirocycles, mild and oxidant-free conditions,
broad substrate scope, and diastereodivergence, this method
may find applications in the synthesis of related complex
structures.
a
The reaction was performed under Conditions A, C, or D in DCE
b
(2.0 mL) under Ar for 12 h. Isolated yield. The dr was determined by
1H NMR spectroscopy.
changed the diastereoselectivity with a good yield and moderate
selectivity. In most cases, the two diastereomers are isolable.
Aryl vinyl ketones (2r−2t) and nitroalkene (2u) were also
compatible. With a simple change of the silver additive to
AgOMs (see SI), the coupling afforded the annulated products
in excellent yield with moderate dr (Conditions D). In contrast,
some common alkenes failed to show any reactivity.
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.7b02678.
To gain some insight into the mechanism of this reaction, H/
D exchange experiments were carried out. H/D exchange with
AcOH-d4 was detected at the ortho-positions of the phenyl
group of 1a in the presence or absence of olefin 2a under
Conditions A (Scheme 5a). H/D exchange was also observed at
the ortho′ position of product 3aa (3aa′ was not analyzed,
Scheme 5b). These results indicated reversibility of the C−H
cleavage. KIE experiments were then conducted by both
intermolecular competition and independent reactions using 1a
and 1a-d5. Consistently large KIEs were obtained, indicating
that cleavage of the C−H bond is probably involved in the
turnover-limiting step (Scheme 5c and 5d). The large
magnitude might also suggest a tunneling effect in C−H
activation.16 A possible mechanism is given in the SI on the
basis of our experimental results and literature precedents.
Cyclometalation of 1a affords a five-membered rhodacyclic
intermediate. Subsequent key processes include the regio- and
diastereoselective insertion of the alkene to give a Rh(III)-
enolate, in which the stereochemistry of the olefin is retained
(trans orientation). The Rh−C(enolate) then undergoes silver
Crystallographic data (CIF, CIF, CIF)
Detailed experimental procedures, characterization of
new compounds, and copies of NMR spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: xwli@dicp.ac.cn.
ORCID
Dachang Bai: 0000-0001-7342-2966
Xingwei Li: 0000-0002-1153-1558
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The NSFC (Nos. 21525208 and 21472186), research fund
from Henan Normal University (5101034011009), and start-up
■
C
DOI: 10.1021/acs.orglett.7b02678
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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Org. Lett. XXXX, XXX, XXX−XXX