Highly Diastereoselective Hydrosilane-Assisted Rhodium-Catalyzed Spiro-Type Cycloisomerization of Succinimide and Pyrazolone-Based Functional 1,6-Dienes

Article abstract of DOI:10.1002/asia.202100372

Organosilicon compounds are important reagents and synthetic intermediates that play a key role in the construction of new materials and complex products. Here we show a highly diastereoselective rhodium-catalyzed cycloisomerization of 1,6-dienes, in which the use of (EtO)₃SiH accelerates the intramolecular cyclization reaction to afford a novel spiro-fused succinimide and pyrazolone derivatives in moderate to excellent yields as a single diastereoisomer. The proposed mechanism involves an active Rh?H species from the hydrosilane that is the H-donor in this spiro-type cycloisomerization reaction.

Full text of DOI:10.1002/asia.202100372

DOI: 10.1002/asia.202100372
Communication
Highly Diastereoselective Hydrosilane-Assisted Rhodium-Catalyzed
Spiro-Type Cycloisomerization of Succinimide and Pyrazolone-
Based Functional 1,6-Dienes
Hui-Lin Li+,[a] Wei-Sheng Huang+,[a] Fang-Ying Ling,[a] Li Li,*[a] Jun-Hao Yan,[a] Hao Xu,[a] and
Li-Wen Xu*[a, b]
Dedicated to Prof. Andy Hor for his 65th birthday
ether dienes in 1971,[9] there are a few reports on the Rh-
Abstract: Organosilicon compounds are important reagents
and synthetic intermediates that play a key role in the
construction of new materials and complex products. Here
we show a highly diastereoselective rhodium-catalyzed
cycloisomerization of 1,6-dienes, in which the use of
(EtO)3SiH accelerates the intramolecular cyclization reaction
to afford a novel spiro-fused succinimide and pyrazolone
derivatives in moderate to excellent yields as a single
diastereoisomer. The proposed mechanism involves an
active RhÀ H species from the hydrosilane that is the H-
donor in this spiro-type cycloisomerization reaction.
catalyzed cycloisomerization of 1,6-dienes, and compatibility of
functional group is generally problematic for this reaction.[10] On
the other hand, the transition-metal-catalyzed cyclization of
α,ω-dienes generally underwent cycloisomerization with low
chemoselectivity to deliver
a mixture of isomers, which
demonstrates the difficulty in controlling chemo- and regio-
selectivity because of structurally diverse isomers.[11] Notably,
although recent efforts have been performed on the develop-
ment of new synthetic methods for constructing complex
carbocycles, the scope of the chemistry of the Rh-catalyzed
cycloisomerization reactions had not expanded to diastereose-
lective reductive cyclization assisted by hydrosilane, while some
reports described 1,6-dienes could be transformed into different
carbocycles in the presence of other transition-metal
catalysts[2–4] (Scheme 1A). Therefore, it is worth to explore and
develop a Rh-catalyzed reaction of 1,6-dienes with high level of
chemoselectivity among all the possibilities of reaction path-
ways.
Transition-metal-catalyzed cycloisomerization of α,ω-dienes
have been one of the most important and atom-economic
reaction processes in the construction of functionalized cyclic
products that are fundamental constituents of pharmaceutical
compounds and nature products, also in the field of functional
materials.[1] In this regard, extensive studies have been carried
out on the cycloisomerization-type cyclization of α,ω-dienes,
promoted by various transition-metal catalysts, including
nickel,[2] palladium,[3] ruthenium,[4] titanium,[5] Lewis super
acids,[6] and other metal catalysts.[7] Among these catalytic
methods, the synthetic potential of rhodium in cycloisomeriza-
tion of α,ω-dienes has become an attractive strategy in the
synthesis of carbocycles or heterocycles.[8] Although Malone
and co-workers reported the first example of the RhCl3-
catalyzed cycloisomerization of structurally simple α,ω-diallyl
[a] H.-L. Li,+ W.-S. Huang,+ F.-Y. Ling, Dr. L. Li, J.-H. Yan, H. Xu, Prof. Dr. L.-W. Xu
Key Laboratory of Organosilicon Chemistry and Material Technology of
Ministry of Education and Laboratory of Organosilicon Material Technology
of Zhejiang Province
College of Material, Chemistry and Chemical Engineering
Hangzhou Normal University, Hangzhou 311121 (P. R. China)
E-mail: 20060054@hznu.edu.cn
liwenxu@hznu.edu.cn
[b] Prof. Dr. L.-W. Xu
State Key Laboratory for Oxo Synthesis and Selective Oxidation
Suzhou Research Institute (SRI), Lanzhou Institute of Chemical Physics (LICP)
University of the Chinese Academy of Sciences (UCAS), Lanzhou 730000 (P.
R. China)
[+] The authors contributed equally.
Scheme 1. Competitive reaction pathways in the hydrosilane -assisted
transition-metal-catalyzed cycloisomerization of 1,6-dienes: from the trans-
formations of simple 1,6-dienes to heterocycle-functionalized 1,6-dienes
(this work).
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/asia.202100372This manuscript is part of a special
collection on Metals in Functional Materials and Catalysis.
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In addition, there is no report on the catalytic construction
of spiro-fused succinimide-derived functional N-heterocycles
with stereoselective version of Rh-catalyzed cycloisomerization
or reductive cyclization of functionalized 1,6-dienes to date. In
this context, succinimide and its derivatives are one of versatile
building blocks for synthetic chemistry and functional
materials,[12] allowing for subsequent transformations of skeletal
variation and investigation as drug candidates. One of the
challenges is the cyclization reaction could either compete with
reduction or hydrosilylation on the C=C doubles of 1,6-dienes
and the amide group due to the nature of SiÀ H bond activation
of hydrosilane with the aid of rhodium catalyst.[13] On the basis
of our previous studies on silicon-mediated organic synthesis,[14]
herein we want to present a novel Rh- catalyzed cycloisomeriza-
tion of succinimide or pyrazolone -derived 1,6-dienes in the
presence of hydrosilane to finish a highly chemoselective
synthesis of spiro-fused heterocycles with high yield and
diastereoselectivity.
87% isolated yield as a single diastereoisomer (entry 1 of
Table 1), whereas the corresponding product was obtained in
lower yield when Cy3P (tricyclohexylphosphine) was used as the
À
ligand in this process (entry 2). Surprisingly, t-Bu3PH+BF4
mainly led to reductive compounds (3a and 4a), Xantphos also
in favor of the reductive compounds (3a and 4a) and a small
amount of adduct 5a. It turned out that PPh3 and TPTP (tri(p-
tolyl)phosphine) were not suitable ligand to construct 2a
(entry 3–6). Further investigation showed that, when the
°
reaction was performed at a higher temperature (60 C), it
would increase the ratio of reductive product (3a and 4a).
Moreover, the replacement of toluene by other solvents in
the rhodium-catalyzed cycloisomerization of succinimide de-
rived 1,6-diene 1a normally gave a mixture of the spro-product
2a with reductive compounds (3a and 4a) (entry 8). In addition,
[Rh(cod)Cl]2 provided the corresponding product 2a in a slight
lower yield mixed with 12% reductive mixture (3a and 4a)
(entry 9), while other rhodium catalysts were tested and did not
provide any improvement in this cycloisomerization process.
Note that the cyclization did not occur in the presence of
Pd2(dba)3 (entry 10).
At the outset of the project, we explored the rhodium-
catalyzed reaction using succinimide derived 1,6-diene 1a with
(EtO)3SiH (triethoxysilane) as model substrates, the experimental
results are summarized in Table 1 (for the details of optimiza-
tions, see the ESI). The reaction was carried out smoothly in the
Furthermore, the hydrosilane was found to be crucial in this
rhodium-catalyzed cycloisomerization process, as the absent of
(EtO)3SiH only gave trace amount of 2a, while submitting 0.5
equivalent (EtO)3SiH to the reaction, 47% yield of 2a was
observed. Finally, other silanes were examined, such as Et3SiH
(triethyl silane), MePhSiH2 (methylphenyl silane) and PhSiH3
(phenyl silane), in which only Et3SiH provide 2a with a very
good chemical control and excellent diastereoselectivity similar
to (EtO)3SiH, but a slight decrease on the conversion of 1a.
MePhSiH2 gave a poor chemical selectivity (76:24). A lower
conversion of 1a was observed in the case of PhSiH3. On the
basis of these experimental results, we suggested that the
activation of rhodium catalyst by various hydrosilanes is
obviously different because of its differentiated ability in the
interaction of a hydrosilane with rhodium complex.[15] And the
steric hindrance of aryl or alkyl substituents on hydrosilane
would affect the formation of RhÀ H species (Si-MÀ H species),
which also made influence on the subsequent addition of metal
hydride to alkenes.
With those optimized conditions in hand, we examined the
scope of the reaction (Table 2). First, the reaction was carried
out with β-styryl derivative bearing a halogen substituent at the
meta-position. Pleasingly, meta-bromo and meta-chrolo deriva-
tives (1b, 1c) were converted into the spiro-products 2b and
2c in moderate yields with excellent diastereoselectivity. A
similar reactivity was observed for the meta-methoxy aryl
derivatives (1d). Bromo and methoxy substituents at ortho-
position were also tolerated, giving the corresponding products
in good yields (2e, 2f).
À
presence of [Rh(nbd)Cl]2 with Cy3PH+BF4 (tricyclohexylphos-
phine tetrafluoroborate) as a ligand source in toluene at room
temperature to afford a methylenecyclopentane derivate 2a in
Table 1. Optimization of reaction conditions.[a]
Entry Derivation from standard conditions 2a/(3a+4a)/5a[Yield%][b,c]
1
2
3
4
5
6
none
97/3/0[d]
71/2/0
0/68/4
0/86/14
2/0/22
0/0/69
À
Cy3P instead of Cy3PH+BF4
t-Bu3PH+BF4À instead of Cy3PH+BF4
À
Xantphos instead of Cy3PHÀ+BF4
À
PPh3 instead of Cy3PH+BF4
TPTP instead of Cy3PH+BF4
À
°
7
8
9
10
11
12
13
14
15
at 60 C instead of rt
DCM instead of toluene
[Rh(cod)]Cl2 instead of [Rh(nbd)Cl]2
Pd2(dba)3 instead of [Rh(nbd)Cl]2
Without (EtO)3SiH
93/7/0/0
89/9/0/0
88/12/0/0
NR
7/0/0
47/0/0
92/2/0/0
76/24/0/0
70/4/0/0
0.5 eqiuv of (EtO)3SiH was used
Et3SiH instead of (EtO)3SiH
MePhSiH2 instead of (EtO)3SiH
PhSiH3 instead of (EtO)3SiH
Then we examined the effect of para-substitution on the
cyclization transformation. Para-chloro and para-bromo sub-
stituents reacted smoothly leading to cyclized products (2g,
2h) in moderate yields. Similarly, para-methyl derivate 2i was
obtained in 71% yield. Electron donating substituent (methoxy),
iodo- and electron with-drawing substituent (trifluoromethyl)
on para-position were efficiently converted into the desired
[a] Unless otherwise noted, the reaction conditions were as follows: 1a
(0.2 mmol), silane (0.2 mmol), and in toluene (2.0 mL). [b] Determined by
GC-MS. [c] Unless otherwise noted, the dr of 2a is generally >20:1, which
1
was determined by H NMR on the crude reaction mixture. [d] 78%
isolated yield.
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molecule (Table 2). Moreover, a gram scale reaction of 1a
provided the desired product 2a without the erosion of isolated
yield and diastereoselectivity.
Table 2. Scope of the Rh-catalyzed cyclization of succinimide derived 1,6-
dienes 1.[a,b,c]
In this stage, a kinetic study for the formation of 2a was
also performed and determined by GC analysis (see the ESI).
According to the curve, 1a could be converted into 2a within
14 hours.
In addition, pyrazolones and related heterocycles are both
common structural motifs in pharmaceutical agents, natural
products and synthetic building blocks.[16] we consider using
pyrazolone derived 1, 6-dienes as a backbone in the reaction to
provide corresponding pyrazolone based spiro-derivatives, to
demonstrate the versatility of the methodology that we
developed: A highly diastereoselective rhodium catalyzed cyclo-
isomerization of 1,6-dienes assisted by triethoxysilane.
Under the standard conditions as optimized above, several
pyrazolone based 1,6-dienes, which modified with halogen
atoms, methyl and methoxy on the aromatic ring were tested
on the reaction system. To our delight, the cycloisomerization
of pyrazolone based 1,6-dienes were proved suitable materials
in this catalytic system, and to be a highly diastereoselective
process. As shown in Scheme 3, substrates 6a–g were reacted
smoothly, afforded the excepted pyrazolone based spiro-
products 7a–g in 64–83% isolated yield as a single diaster-
eoisomer.
Entry
[Ar1]
[Ar2]
2
Yield%
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
2p
2q
2r
87
60
62
61
73
87
66
60
71
93
80
85
76
92
82
57
86
90
79
3-BrC6H4
3-ClC6H4
3-MeOC6H4
2-BrC6H4
2-MeOC6H4
4-ClC6H4
4-BrC6H4
4-MeC6H4
4-MeOC6H4
4-IC6H4
4-CF3C6H4
2-naphthyl
2-thiophenyl
Ph
9
10
11
12
13
14
15
16
17
18
19
Then the attention was turned to the asymmetric synthesis
of chiral spiro-fused heterocycles. Under the modified reaction
conditions, several commercially available chiral phosphine
ligands were examined. On the basis of a large amount of
exploring experiments based on the screening of chiral P-
ligands and reaction parameters, most of chiral P-ligands turned
4-EtOC6H4
4-NO2C6H4
4-t-BuC6H4
3,5-diMeC6H3
3,5-bisCF3C6H3
Ph
Ph
Ph
Ph
2s
[a] Unless otherwise noted, the reaction conditions were as follows: 1
(0.2 mmol), silane (0.2 mmol), and in toluene (2.0 mL). [b] Isolated yield. [c]
Dr value of 2 Determined by .1H NMR (unless otherwise noted, the dr of 2
is generally >20:1).
products in high isolated yields (2j with 93%, 2k with 80%, and
2l with 85% respectively). Next, substitute bearing a 2-
naphythenyl group reacted smoothly, providing the expected
compound 2m in 76% yield. Thienyl-substituted 1,6-diene was
tested as well, the desired product 2n was obtained in 92%
yield.
To further extend the scope of application of our method-
ology, we next examined several N-aryl-succinimide based 1,6
dienes on the rhodium catalyzed cycloisomerization process.
Para-position on the aromatic ring bearing an electron donating
group, such as ethoxy and tert-butyl substituents reacted
smoothly, giving the desired products (2o and 2p) in good
yields, whereas electron with-drawing group, para-nitro sub-
stituent gave the excepted compound 2q in moderate yield.
Finally, 3,5-dimethy and 3.5-bis(trifluoromethyl) substituents
allowed the formation of the corresponding product (2r and
2s) in 90% and 79% isolated yield, respectively.
It is worth to mention that all the spiro-products were
observed with a complete diastereoselectivity. The X-Ray
crystallographic analysis of 2e led us to unambiguously
determine the relative and absolute configuration of the
Scheme 2. Rh-catalyzed cyclization of pyrazolone derived 1,6-dienes 6. [a]
Unless otherwise noted, the reaction conditions were as follows: 6
(0.2 mmol), silane (0.2 mmol), in toluene (2.0 mL). [b] Isolated yield.
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Scheme 3. Enantioselective Rh-catalyzed cyclization of 1. [a] Unless other-
wise noted, the reaction conditions were as follows: 1a (0.2 mmol), silane
(0.2 mmol), in toluene (2.0 mL). [b] Isolated yield. [c] ee determined by HPLC
on a chiral stationary phase.
Scheme 4. Plausible mechanism for the hydrosilane-assisted Rh-catalyzed
cycloisomerization.
out to be unsuitable partners in the construction of 2a, and no
data of enantioselectivity was observed in this reaction. Such
asymmetric rhodium-catalyzed cycloisomerization of succini-
mide or pyrazolone-derived 1,6-dienes is proved to be a highly
challenging reaction because of no previous reports on the
development of chiral P-ligand for this type of cycloisomeriza-
tion reaction. Inspired by our previous investigations on the
copper-catalyzed Huisgen cycloaddition reaction of succini-
mide-derived or pyrazolone-derived bisalkynes with azides in
the presence of chiral phosphine ligand Tao-Phos,[17] we
hypothesis that Tao-Phos and its analogues may show an ideal
control of enantioselectivity on this rhodium-catalyzed cyclo-
isomerization process.[17d] Unexpectedly, the asymmetric cyclo-
isomerization of 1a was obviously suppressed in the presence
of Tao-Phos. The expected product 2a was isolated as a single
diastereoisomer albeit with low yields and promising enantiose-
lectivities (Scheme 3). Among Tao-Phos and its analogues, (R,S)-
L1 provided the best enantioselectivity (69:31 er) and 37:63 er
was observed in the presence of (S,R)-L2 (see Scheme 3). It
should be noted that, this result was still the best record in this
reaction on the basis of our screening experiment of chiral P-
ligands (see the Scheme S2 and Table S11 of Supporting
Information).
of 1a, followed by a re-insertion to another C=C bond of the
intermediate 9 gives birth to the ring-formed intermediate 10.
Finally, the desired product was generated through β-H
elimination. Notably, we believed that oxygen of imide moiety
could coordinate with Rh center as a directing group, which
would be beneficial to the excellent diastereoselectivity in the
rhodium-catalyzed 1,6-cycloisomerization.
In summary, we developed an efficient catalytic diastereose-
lective synthesis of spiro-fused succinimide and pyrazolone
derivatives. This methodology, using (EtO)3SiH assisted rhodium
catalyst as the catalytic system allowed the formation of the
spiro-fused succinimide and pyrazolone derivatives in moderate
to high yields (57–93% yield) and excellent diastereoselectivity
(>20:1). The broad range scope of the reaction offers a new
protocol to synthesize spiro-fused succinimide and pyrazolone
derivatives. The extension to asymmetric transformation of
succinimide based 1,6-diene was also explored in the presence
of Tao-Phos, which developed by our research group. Although
the chiral control in this 1,6-diene cycloisomerization trans-
formation is not perfect, but Tao-Phos is the only effective chiral
phosphine ligand that can provide promising enantioselectivity
so far. We believe this work will possibly inspire researchers to
explore more effective ligands in this filed. Further effort to the
development of an asymmetric version for the transition-metal-
catalyzed cycloisomerization of 1,6-dienes are underway in our
laboratory.
Although the mechanism of rhodium-catalyzed cycloisome-
rization of 1,6-dienes remains unclear, we suggest that this
hydrosilane assisted Rh-catalyzed cyclization process initiated to
form a RhÀ H species, supported by the observation of a new
signal on 35P NMR data (see the ESI) and literature precedence.
As shown in Scheme 4, a plausible mechanism was proposed. Acknowledgements
First, the RhÀ H species would accommodate one of the two
C=C units with oxygen atom of imide moiety on 1a to form a
hydride-diene complex 8. In the next stage of oxidative
addition, the RhÀ H species could insert into one alkene moiety
This work was supported by the grants of National Natural
Science Foundation of China (21773051, 22072035, 21801056 and
21901056), Zhejiang Provincial Natural Science Foundation of
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K.Z. Jiang and X.Q. Xiao for their assistance on the MS and X-ray
analysis.
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Conflict of Interest
The authors declare no conflict of interest.
Keywords: rhodium
· hydrosilane · diastereoselectivity ·
cycloisomerization reaction · 1,6-dienes
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Version of record online: ■■■, ■■■■
Chem Asian J. 2021, 16, 1–6
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Communication
COMMUNICATION
A highly diastereoselective
rhodium-catalyzed cycloisomeriza-
tion of 1,6-dienes that is promoted
by hydrosilane has been established
to afford spiro-fused succinimide and
pyrazolone derivatives in moderate
to excellent yields as a single diaster-
eoisomer.
H.-L. Li, W.-S. Huang, F.-Y. Ling, Dr. L.
Li*, J.-H. Yan, H. Xu, Prof. Dr. L.-W. Xu*
1 – 6
Highly Diastereoselective Hydrosi-
lane-Assisted Rhodium-Catalyzed
Spiro-Type Cycloisomerization of
Succinimide and Pyrazolone-Based
Functional 1,6-Dienes