Relay Rh(ii)/Pd(0) dual catalysis: Synthesis of α-quaternary β-keto-esters: Via a [1,2]-sigmatropic rearrangement/allylic alkylation cascade of α-diazo tertiary alcohols

Article abstract of DOI:10.1039/c9cc08559a

A relay Rh(ii)/Pd(0) dual catalysis that enables domino [1,2]-sigmatropic rearrangement/allylic alkylation of α-diazo tertiary alcohols is described. This transformation represents a highly efficient method for the one-pot synthesis of α-quaternary β-keto-esters under mild conditions, in which two separate C-C σ-bonds at the carbenic center were formed in a straightforward manner.

Full text of DOI:10.1039/c9cc08559a

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Relay Rh(II)/Pd(0) dual catalysis: synthesis of
a-quaternary b-keto-esters via a [1,2]-sigmatropic
rearrangement/allylic alkylation cascade of
a-diazo tertiary alcohols†
Cite this:DOI: 10.1039/c9cc08559a
Received 2nd November 2019,
Accepted 6th December 2019
Xian-Xu Wang, Xiao-Yan Huang, Sen-Hao Lei, Fang Yang, Jin-Ming Gao,
DOI: 10.1039/c9cc08559a
Kegong Ji
and Zi-Sheng Chen
*
rsc.li/chemcomm
A
relay Rh(II)/Pd(0) dual catalysis that enables domino [1,2]-
sigmatropic rearrangement/allylic alkylation of a-diazo tertiary
alcohols is described. This transformation represents a highly
efficient method for the one-pot synthesis of a-quaternary b-keto-
esters under mild conditions, in which two separate C–C r-bonds at
the carbenic center were formed in a straightforward manner.
The efficient construction of quaternary carbon centers remains a
critical challenge in organic synthesis.1 Among various methodo-
logies, the gem-difunctionalization of diazo compounds2,3,9 via
metal-carbenoids is a well-documented process for accessing
quaternary carbon centers. This directly introduces two different
functional groups at a carbenic center (Scheme 1). Recent mile-
stones in transition metal catalyzed cross-couplings involving metal-
carbenoid migratory insertions have mostly been successful3–8
(Scheme 1, strategy A). Pioneered by Van Vranken4a and
Barluenga,3a,4b,c the research groups of Wang,3c,d,5 Liang,6a,b
Yu,7 and Rovis8a have further developed the migratory insertion
and coupling cascade reaction of in situ generated palladium,
rhodium, or copper carbenoids, which realizes the efficient
construction of quaternary carbon centers. Remarkably, Hu9,10
and others11 also established the method of electrophilic trapping
of metal-carbenoid induced ylides or zwitterionic intermediates
(Scheme 1, strategy B), which provides an alternative route to
quaternary carbon centers. Despite the apparent advances, these
two strategies are mostly limited to donor/acceptor (D/A) diazo
compounds. Therefore, developing new and efficient catalytic
systems to address other types of diazo compounds under mild
conditions is highly desired.
Scheme 1 Metal-carbenoids for the construction of quaternary carbon
centers.
for the selective construction of cyclic quaternary carbon
centers (Scheme 2a).12a,c,d In this cooperative dual catalysis
process, the catalytically generated Rh(II)-carbenoids (A) and
p-allyl Pd(II) intermediates (B) are involved. The sigmatropic
rearrangement is recognized as one of the most attractive
fashions for the formation of quaternary carbon centers.13–16
In particular, a series of practical tandem semipinacol rearran-
gements developed by Tu and co-workers14,15 have been
successfully applied in the total synthesis of natural products.
Maruoka16a,b and Feng16c have also explored Lewis acid catalyzed
homologation of aldehydes or ketones with diazo compounds.
This results in a carbon-chain extension or ring expansion at a
carbonyl group, involving a single catalyst. However, some
chemical transformations would be inefficient or unattainable
with mono-catalysis. Recently, Gong and co-workers established
a highly compatible rhodium/bifunctional urea dual-catalysis
system17 that rendered a [1,2]-sigmatropic rearrangement/Michael
addition cascade of a-diazo alcohols with nitroalkenes.
Previously, we demonstrated for the first time the cross-
compatible Rh(II)/Pd(0) dual-catalysis system.12 This promotes
the divergent reaction of b-keto-a-diazo-carbonyl compounds
(acceptor/acceptor diazo compounds) with allylic carboxylates
Inspired by the above studies, we envisioned that the
Rh(II)-catalyst induced [1,2]-sigmatropic rearrangement18 of
a-diazo tertiary alcohols (1) would efficiently generate the enol
ester intermediates (E). Subsequently, Pd(0)-catalyzed allylic
alkylation with allyl tert-butyl carbonate (2) would access acyclic
Shaanxi Key Laboratory of Natural Products & Chemical Biology,
College of Chemistry and Pharmacy, Northwest A&F University, Yangling 712100,
Shaanxi, P. R. China. E-mail: chenzsh@nwsuaf.edu.cn
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9cc08559a
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significantly improved the yield affording 3aa in 94% yield,
which may facilitate the Pd(0) catalyzed allylic alkylation (entry 2).
Further optimization showed that the reaction efficiency was
largely affected by changing the Pd catalysts (entries 3–5). For
example, Pd(PPh3)4 and PdCl2 were unable to promote the
reaction, probably because of the deactivation effect within this
catalytic system.19 Various phosphine ligands were also tested
(entries 6–8). However, the utilization of mono-dentate phosphine
P(2-furyl)3 (entry 6) had a deleterious effect. The other bidentate
phosphine ligands (entries 7 and 8) did not give better results
than those observed with Xantphos. In addition, when other Rh
catalysts such as RhCl(PPh3)3 and Rh2(OAc)4 were used, the
yields were diminished too (entries 9 and 10). After considerable
investigations, a protocol based on Rh2(tBuCO2)4, [Pd(allyl)Cl]2,
Xantphos, and Cs2CO3 in toluene at room temperature afforded
3aa in 98% yield (entry 11).
With the ideal conditions in hand, we then tested the scope
of a-diazo tertiary alcohols, which could easily be prepared
from the commercially available a-diazo ethyl acetate and
ketones (Table 2).20 Various a-diazo alcohols bearing different
substituents at the para positions of the phenyl rings
performed well, affording the corresponding products (3ba–3fa)
in good to excellent yields. Notably, the halogen substituents were
all tolerated in the presence of a palladium catalyst, thus providing
the opportunity for further transformations through coupling
reactions (3da–3fa). In addition to the carbon-chain extension, this
approach could also be effectively utilized for the ring expansion of
cyclic ketones (3ga–3ja). Interestingly, the reaction proceeded
well over substrates derived from dissymmetrical ketones, in
which two different migration products can simultaneously
be obtained. While using the chain dissymmetrical di-aryl
ketones, the electron-rich aryl group migration is dominant
in the selective formal C–C insertion process (3l vs. 3l0).
Scheme 2 Construction of quaternary carbon centers via Rh(II)/Pd(0)
dual-catalysis.
a-quaternary allylated b-keto-esters (3), in which two different
C–C bond formations on the carbenic center were formed in a
candid manner (Scheme 2b).
At the outset, the redox-compatible Rh(II)/Pd(0) dual catalytic
system12 established by our group was selected to investigate
the reaction of an a-diazo tertiary alcohol (1a) with an allyl tert-
butyl carbonate (2a) (Table 1).
As expected, when the reaction was carried out at room
temperature in the presence of Rh2(tBuCO2)4 (1.0 mol%),
[Pd(allyl)Cl]2 (2.0 mol%), and Xantphos (2.2 mol%), the desired
product (3aa) could be isolated in 10% yield (entry 1). We were
pleased to find that the addition of K2CO3 (2.0 equiv.)
Table 2 Scope of a-diazo tertiary alcoholsa
Table 1 Optimization of the reaction conditionsa
Entry Rh2L4
[Pd]
Ligand
Base
—
3aab (%)
1
2
3
4
5
6
7
8
Rh2(tBuCO2)4 [Pd(allyl)Cl]2 Xantphos
10
94
0
Rh2(tBuCO2)4 [Pd(allyl)Cl]2 Xantphos K2CO3
Rh2(tBuCO2)4 Pd(PPh3)4
Rh2(tBuCO2)4 PdCl2
—
K2CO3
Xantphos K2CO3
Xantphos K2CO3
o5
86
38
89
92
71
76
98
Rh2(tBuCO2)4 Pd(OAc)2
Rh2(tBuCO2)4 [Pd(allyl)Cl]2 P(2-furyl)3 K2CO3
Rh2(tBuCO2)4 [Pd(allyl)Cl]2 Segphos
K2CO3
K2CO3
Rh2(tBuCO2)4 [Pd(allyl)Cl]2 Dppf
9
10
11
RhCl(PPh3)3
Rh2(OAc)4
[Pd(allyl)Cl]2 Xantphos K2CO3
[Pd(allyl)Cl]2 Xantphos K2CO3
Rh2(tBuCO2)4 [Pd(allyl)Cl]2 Xantphos Cs2CO3
a
a
Reaction conditions: 1a (0.12 M, 1.2 equiv.), 2a (0.1 M, 1.0 equiv.),
Reaction conditions: 1 (0.12 M, 1.2 equiv.), 2a (0.10 M, 1.0 equiv.),
Rh2L4 (1.0 mol%), [Pd] (4.0 mol%), based on Pd, and the ligand Rh2(tBuCO2)4 (1.0 mol%), [Pd(allyl)Cl]2 (2.0 mol%), and Xantphos
(2.2 mol%) in toluene at room temperature. Isolated yields.
b
(2.2 mol%) in toluene at room temperature. Yields represent isolated yields.
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Table 3 Scope of functionalized allyl tert-butyl carbonatesa
Scheme 3 Asymmetric one-pot synthesis of chiral acyclic a-quaternary
allylated b-keto-esters.
conducted at room temperature by combining Rh2(tBuCO2)4
with Ito’s {[Pd(allyl)Cl]2-(R)-BINAP} catalyst,22a,b which was the
most enantioselective catalyst for the asymmetric allylic alkylation
of cyclic 1,3-diketones.22c The response of the reaction was only
the racemic product 3aa in 80% yield. After a series of efforts
(see Table S1, ESI†), chiral ligand L8 shows moderate reactivity
and enantio-selectivity, affording 3aa in 59% yield and 53% ee
(Scheme 3).
In summary, we have developed a highly efficient relay
Rh(II)/Pd(0) dual catalysis for the one-pot synthesis of acyclic
a-quaternary allylated b-ketoesters. Under mild conditions,
this binary catalyst system rendered domino [1,2]-sigmatropic
rearrangement/allylic alkylation of a-diazo tertiary alcohols,
in which two separate C–C s-bonds on the same carbon atom
were formed in an upfront manner. Further extension of the
Rh(II)/Pd(0) dual-catalyzed system with diazo compounds is
underway in our laboratory.
a
Reaction conditions: 1a (0.12 M, 1.2 equiv.), 2 (0.10 M, 1.0 equiv.),
Rh2(tBuCO2)4 (1.0 mol%), [Pd(allyl)Cl]2 (2.0 mol%), and Xantphos (2.2
mol%) in toluene at room temperature. Yields represent isolated yields.
b
Reaction carried out at 60 1C.
In contrast, the alkyl group migrates prior to the aryl group
(3m vs. 3m0), when the a-diazo alcohol (derived from benzo-
cyclohexanone) was employed. The migration process revealed
in this case is similar to that in previous reports.13b,c
Next, we turned our attention to study an array of functio-
nalized allyl tert-butyl carbonates. As shown in Table 3,
the preparative scope was in general observed regardless
of whether electron-donating (2c–f) or electron-withdrawing
(2g–k) groups were present or not. In particular, substrates
having fluoro (2g), chloro (2h–j), and bromo (2k) groups were
well-tolerated, giving the corresponding products (3ag–3ak)
in excellent yields. Similarly, the 1-naphthyl or 2-thiophenyl
substituted allyl carbonate (2l or 2m) was also successfully
incorporated into the product, 3al or 3am obtained in high
yield. Although the reaction rate decreased in the case of the
substituents on the ortho-positions of the aromatic ring of
g-substituted allyl carbonates (2d, 2h, and 2l), the efficiency of
this transformation was not noticeably affected by the position
of the substituents. Gratifyingly, this method could be extended
to 3-methyl-substituted allyl carbonate (2n) (R = CH3), in which
the palladium complex did not rapidly undergo b-hydride
elimination.
Asymmetric dual transition-metal catalysis is an enduring
challenging subject,21 in part due to the redox-compatibility of
the catalysts, catalyst deactivation, and the need for balanced
kinetics. This is required to suppress undesired catalytic path-
ways and to come up with a controlled reaction sequence.19
Next, we also attempted to explore an asymmetric relay Rh(II)/
Pd(0) dual catalysis for gaining access to chiral acyclic
a-quaternary allylated b-ketoesters. First, the reaction was
The current work was made possible by the support of the
National Natural Science Foundation of China (No. 21402154),
the Natural Science Foundation of Shaanxi Province (No. 2019JQ647
and 2018JQ2072), the Chinese Universities Scientific Fund
(No. 2452016093), and the Youth Training Project of Northwest
A&F University (No. 2452017034).
Conflicts of interest
There are no conflicts to declare.
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