Synthesis of Cyclopentadienes for Cyclopentadienyl Ligands via Cp*RhIII-Catalyzed Formal sp3C-H Activation/Spiroannulations

Article abstract of DOI:10.1021/acs.orglett.0c03982

An efficient Cp*RhIII-catalyzed formal C(sp3)-H activation/spiroannulation of alkylidene Meldrum's acids with alkynes has been developed using catalytical Cu(OAc)2 and air as the oxidant. This reaction demonstrates a new and straightforward approach to spirocyclopentadienes with Meldrum's acid moieties in good to excellent yields under mild reaction conditions with a broad substrate scope. Notably, this protocol provides a novel and straightforward approach to cyclopentadienes with various substitution patterns and the corresponding cyclopentadienyl-type ligands from simple substrates.

Full text of DOI:10.1021/acs.orglett.0c03982

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Letter
Synthesis of Cyclopentadienes for Cyclopentadienyl Ligands via
Cp*RhIII-Catalyzed Formal sp3 C−H Activation/Spiroannulations
Yongzhuang Wang, Xiaoli Huang, Qin Wang, Yuhai Tang, Silong Xu, and Yang Li*
Cite This: Org. Lett. 2021, 23, 757−761
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ABSTRACT: An efficient Cp*RhIII-catalyzed formal C(sp3)−H activation/spiroannulation of alkylidene Meldrum’s acids with
alkynes has been developed using catalytical Cu(OAc)2 and air as the oxidant. This reaction demonstrates a new and straightforward
approach to spirocyclopentadienes with Meldrum’s acid moieties in good to excellent yields under mild reaction conditions with a
broad substrate scope. Notably, this protocol provides a novel and straightforward approach to cyclopentadienes with various
substitution patterns and the corresponding cyclopentadienyl-type ligands from simple substrates.
Scheme 1. Transition-Metal-Catalyzed Phenol- or Enol-
Directed C−H Activation/Spiroannulations with Alkynes
ransition-metal-catalyzed C−H activation/annulations
Thave emerged as one of the most straightforward and
efficient synthetic approaches to construct cyclic compounds.1
However, these reactions have been rarely investigated for the
synthesis of carbocycles due to the highly required heteroatom-
directing strategy in C−H activation.2 Phenol- or enol-directed
C(sp2)−H activation/spiroannulations with alkynes have been
developed as a novel access to various spirocyclic carbocycles
through phenol dearomatization or keto−enol tautomerization
by many groups3 (Scheme 1, previous work). In 2017, Yao et
al.4 developed a pentamethylcyclopentadienyl rhodium
(Cp*RhIII)-catalyzed formal C(sp3)−H activation/spiroannu-
lation of α-arylidene pyrazolones with alkynes for the efficient
synthesis of spirocyclopentadienes via a similar enol-directed
group, and the asymmetric version was then realized by
Waldmann et al.5 (Scheme 1, III). Of note, cyclopentadienyl
(Cp) ligands are very popular and have been widely used in
transition-metal catalysts because of their unique structures
and binding properties.6,7 Moreover, Cp−metal catalysts have
also had a great development in C−H activation in the past
decade,1,8,9 of which the commercially available Cp*Rh is one
of the most frequently used catalysts.9 Recently, as alternatives
for Cp*, different Cp ligands were found to largely change and
even switch the catalytic activity and selectivity of Cp−metal
catalysts.10 However, most synthetic methods of substituted
Cp ligands were based on the cyclopentadiene precursors
obtained via derivatization of cyclopentadienes or cyclo-
pentenones.6,11 Although direct [3 + 2] cyclizations via the
previous C−H activation/spiroannulations have provided a
potential method for cyclopentadienes, which are difficult to
convert to Cp ligands due to the stable all-carbon quaternary
centers.3−5 Herein, in consideration of the rich reactivity12 and
potential enol-directing ability of Meldrum’s acid moiety
(Scheme 1a), we developed a useful Cp*RhIII-catalyzed formal
C(sp3)−H activation/spiroannulation of alkylidene Meldrum’s
Received: December 1, 2020
Published: January 13, 2021
https://dx.doi.org/10.1021/acs.orglett.0c03982
© 2021 American Chemical Society
Org. Lett. 2021, 23, 757−761
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acid with alkynes to afford spirocyclopentadienes, which can be
easily converted to precursor cyclopentadienes of various Cp
ligands, in good to excellent yields under mild conditions
(Scheme 1b).
Alkylidene Meldrum’s acids could easily isomerize into the
enol tautomer owing to the strong electron-withdrawing ability
of the cyclic ester moiety12,13 (Scheme 1a). So we began with
our research by applying 2,2-dimethyl-5-(1-phenylethylidene)-
1,3-dioxane-4,6-dione (1a) and diphenylethyne (2a) as model
substrates (Table 1). Initially, the metal catalysts were tested in
With the optimized reaction conditions in hand, we then
investigated the substrate scope of alkylidene Meldrum’s acids
and alkynes (Scheme 2), which showed that this reaction
a
Scheme 2. Scope of the Spiroannulation Reaction
a
Table 1. Optimization of the Reaction Conditions
b
entry
catalyst
solvent
x
yield (%)
1
2
3
[RhCp*Cl2]2
[IrCp*Cl2]2
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
THF
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
0.25
0.10
0.05
53
trace
trace
50
48
10
21
12
15
33
[RuCl2(p-cymene)]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
c
4
5
d
6
7
8
9
10
11
12
13
14
DCE
toluene
MeOH
DMF
dioxane
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
7
71
e
e
87
62
e
e
g
15
16
17
nr
nr
ef
g
,
0.10
2.0
a
ef
h
,
Isolated yields. Reaction conditions: [Cp*RhCl2]2 (2.5 mol %),
nd
Cu(OAc)2 (10 mol %), 1a (0.2 mmol, 1.0 equiv), 2a (0.3 mmol, 1.5
equiv) at 80 °C under N2 for 2 h. Displacement ellipsoids are drawn
at the 50% probability level. The ratio of regioisomers.
a
Unless specified, reaction conditions: catalyst (2.5 mol %), 1a (0.2
mmol, 1.0 equiv), 2a (0.3 mmol, 1.5 equiv), solvents (2.0 mL), at 80
b
c
b
c
d
°C under N2. Isolated yields. 100 °C. Room temperature for 16 h.
e
f
g
h
Under air. No 2a. No reaction. 1a decomposed.
demonstrates good generality. Above all, various alkylidene
Meldrum’s acids were examined in this reaction, affording the
corresponding products in good to excellent yields (3a−k, 70−
87%). Neither the electronic nature nor the steric hindrance of
the aryl group in alkylidene Meldrum’s acids had any obvious
influence upon the transformation (3a−i). Heteroaryl- or
alkyl-substituted alkylidene Meldrum’s acids were also
compatible, generating the corresponding spirocyclopenta-
dienes 3j and 3k in 70% yield. Subsequently, a range of
alkynes were tested under standard conditions, which showed
that symmetrical alkynes with various aryl substituents were
well-tolerated and provided products 3l−s in 70−82% yields.
Unsymmetrical alkynes bearing an aryl or ester group also
proceeded smoothly to give the desired spirocyclopentadienes
in good yields (60−70%), albeit with poor regioselectivities
(3t−v). Notably, 1-phenyl-1-propyne delivered product 3w in
a good yield as a single regioisomer, which indicated that the
alkyne insertion step preferentially occurred at the alkyne
carbon bearing the alkyl group. Finally, dialkyl acetylene was
successfully demonstrated, albeit with a lower yield of 42%
(3x). The absolute structures of 3a were confirmed by X-ray
crystallography (see Scheme 2 and Supporting Information).
CH3CN with Cu(OAc)2 as the stoichiometric oxidant at 80
°C, which manifested that [Cp*RhCl2]2 could nicely
accomplish the formal C(sp3)−H activation/spiroannulations
and afford the desired spirocyclopentadiene 3a in 53% yield
(entries 1−3). However, screening of the temperature and
solvents did not improve the yields (entries 4−11).
Considering the high reactivity of Meldrum’s acids toward
Lewis acids,14 we reduced the amount of Cu(OAc)2 while
using air as a green oxidant. To our delight, a dramatic
enhancement of the yield was obtained (entries 12 and 13),
and the highest yield of 87% was accomplished using 0.10
equiv of Cu(OAc)2 (entry 13). Further reduction of the
amount of Cu(OAc)2 decreased the yields significantly, which
indicated that a specific amount of Cu(OAc)2 was required to
ensure the rapid formation of catalytically active rhodium
catalyst and also its oxidative ability (entry 14, vide infra).
Moreover, control experiments indicated that Cu(OAc)2
played a very critical role in this reaction (entry 15). Finally,
the reactions were performed without the rhodium catalyst,
which proved that excess Cu(OAc)2 could decompose the
cyclic Meldrum’s acid moiety (entries 16 and 17).
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To further shed some light on the mechanism of the
reaction, the deuterium-incorporated alkylidene Meldrum’s
acid (1a-D, 60% D) and 1,2-bis(4-fluorophenyl)ethyne (2c)15
were applied in the deuterium-labeling study under otherwise
identical conditions (Scheme 3). The reaction afforded the
spiroannulations have the highly reactive Meldrum’s acids
moieties, which tend to occur C−C bond cleavage with the
loss of CO2 and acetone under various conditions.12 So we
tried to demonstrate a novel and straightforward approach to
various cyclopentadienes and the corresponding Cp ligands
from spirocyclopentadienes 3. At the outset of the study, a
scaled-up reaction was carried out at a 5 mmol scale by
employing the model substrates 1a and 2a, affording the
spirocyclopentadiene 3a (1.77 g) in 84% yield without the
obvious loss of yield (Scheme 5a). Gratifyingly, spirocyclo-
Scheme 3. Mechanistic Studies
Scheme 5. Scale Up and Synthesis of Multisubstituted
Cyclopentadienes and the Corresponding Rhodium
Catalysts
deuterium-labeling product 3n-D in 70% yield with a lower
deuteration of 56% compared to that of substrate 1a-D, which
implies that the C−H activation step might be reversible.16
Based on the preliminary results and relevant literature,3,4
a
reasonable mechanism is proposed, as shown in Scheme 4. To
Scheme 4. Proposed Mechanism
pentadienes 3 can be easily applied for the synthesis of a range
of cyclopentadienes 4 with different substitution in good yields
(62−89%) via simple reduction with NaBH4 (Scheme 5b; for
proposed mechanism, see the Supporting Information).
Cyclopentadienes 4 without quaternary carbon have always
served as the precursors of different Cp ligands.6 Subsequently,
the Cp-coordinated Rh catalyst 5a was obtained in 80% yield
using modified cyclopentadiene 4a7c (Scheme 5c). Finally, it
was found that CpRhIII catalyst 5a was also proven to be an
effectual catalyst for the formal sp3 C−H activation/
spiroannulation of substrates 1a and 2a, affording spirocyclo-
pentadiene 3a in 79% yield, albeit with a longer reaction time
(Scheme 5d).
begin with, the catalytically active Cp*Rh(OAc)2 catalyst is
formed by the ligand exchange of [Cp*RhCl2]2 with
Cu(OAc)2. 1,5-Hydride migration of 1a generates the enolized
1a′, which then coordinates to the activated rhodium catalyst
followed by the formation of a six-membered rhodacycle
intermediate A via the reversible C−H activation. Next, the
coordination and migratory insertion of alkyne 2a produces the
eight-membered rhodacycle intermediate B, which subse-
quently undergoes the favored Rh 1,3-migration to generate
the key six-membered spirorhodacycle intermediate C, while
relieving the steric interaction between the phenyl group and
Meldrum’s acid moiety. Finally, reductive elimination from
intermediate C delivers the product 3a and Rh(I) species,
which is oxidized by Cu(OAc)2 to finish the catalytic cycle.
Notably, Cu(OAc)2 is generated by the oxidation of air and
reused in the reaction.17
In conclusion, a useful Cp*RhIII-catalyzed formal C(sp3)−H
activation/spiroannulation of alkylidene Meldrum’s acids with
alkynes was developed using catalytic Cu(OAc)2 and air as a
green oxidant, affording spirocyclopentadienes with a Mel-
drum’s acid moiety in good to excellent yields under mild
conditions. This protocol also provides a novel and
straightforward approach to various cyclopentadienes without
quaternary carbons, and the corresponding cyclopentadienyl-
type ligands from simple substrates. Further studies to explore
the mechanism and new application of this methodology are
underway in this lab.
The planar Cp skeleton is a privileged scaffold8 in the field of
ligand design, so its synthesis has attracted a long-standing
interest from the synthetic community.18 Spirocyclopenta-
dienes 3 obtained by newly developed C−H activation/
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ASSOCIATED CONTENT
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03982.
Experimental procedures, compounds characterization
data, and copies of NMR spectra (PDF)
Accession Codes
CCDC 1996544 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,
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AUTHOR INFORMATION
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Corresponding Author
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Yang Li − Department of Material Chemistry, School of
Chemistry and Xi’an Key Laboratory of Sustainable Energy
Materials Chemistry, Xi’an Jiaotong University, Xi’an
710049, P.R. China; orcid.org/0000-0002-9311-3412;
Email: yanglee@mail.xjtu.edu.cn
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Yongzhuang Wang − Department of Material Chemistry,
School of Chemistry and Xi’an Key Laboratory of
Sustainable Energy Materials Chemistry, Xi’an Jiaotong
University, Xi’an 710049, P.R. China
Xiaoli Huang − Department of Material Chemistry, School of
Chemistry and Xi’an Key Laboratory of Sustainable Energy
Materials Chemistry, Xi’an Jiaotong University, Xi’an
710049, P.R. China
Qin Wang − Department of Material Chemistry, School of
Chemistry and Xi’an Key Laboratory of Sustainable Energy
Materials Chemistry, Xi’an Jiaotong University, Xi’an
710049, P.R. China
Yuhai Tang − Department of Material Chemistry, School of
Chemistry and Xi’an Key Laboratory of Sustainable Energy
Materials Chemistry, Xi’an Jiaotong University, Xi’an
710049, P.R. China
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Silong Xu − Department of Material Chemistry, School of
Chemistry and Xi’an Key Laboratory of Sustainable Energy
Materials Chemistry, Xi’an Jiaotong University, Xi’an
710049, P.R. China; orcid.org/0000-0003-3279-9331
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03982
Notes
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ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(Nos. 21602167, 21871218), the China Postdoctoral Science
Foundation (No. 2019M663660), and the Fundamental
Research Funds for the Central Universities. We thank Prof.
Yanzhen Zheng (Frontier Institute of Science and Technology,
Xi’an Jiaotong University) for his assistance with X-ray
crystallographic analysis. We thank Miss Ying Zhang and Lu
Bai at the Instrument Analysis Center of Xi’an Jiaotong
University for their assistance with HRMS analysis.
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1
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Org. Lett. 2021, 23, 757−761