Relay Rh(II)/Pd(0) dual catalysis: Selective construction of cyclic all-quaternary carbon centers

Article abstract of DOI:10.1021/acs.orglett.6b02958

A novel relay Rh(II)/Pd(0) dual catalysis strategy that promotes the divergent reaction of α-diazo-carbonyl compounds with allylic carboxylates for the selective construction of cyclic all-quaternary carbon centers has been developed. This binary catalyst

Full text of DOI:10.1021/acs.orglett.6b02958

Letter
pubs.acs.org/OrgLett
Relay Rh(II)/Pd(0) Dual Catalysis: Selective Construction of Cyclic
All-Quaternary Carbon Centers
Zi-Sheng Chen, Xiao-Yan Huang, Jin-Ming Gao, and Kegong Ji*
Shaanxi Key Laboratory of Natural Products & Chemical Biology, College of Science, Northwest A&F University, Yangling 712100,
PR China
*
S Supporting Information
ABSTRACT: A novel relay Rh(II)/Pd(0) dual catalysis strategy that promotes the divergent
reaction of α-diazo-carbonyl compounds with allylic carboxylates for the selective construction
of cyclic all-quaternary carbon centers has been developed. This binary catalyst system rendered
domino C−H insertion/allylic alkylation process under mild conditions. Remarkably, the
domino catalytic reaction shows good selectivity and excellent tolerance to various func-
tionalities and is operationally simple.
significant challenge in organic synthesis concerns the
A
development of efficient catalytic reactions that furnish
1
all-quaternary carbon centers. All-quaternary carbon centers,
especially cyclic all-quaternary carbon centers, are prevalent
throughout most classes of naturally occurring biologically active
2
compounds and pharmaceutical agents. Although significant
efforts have been devoted to the effective construction of all-
1
quaternary centers in recent years. New methodologies that
could be advantageous in terms of functional-group tolerance,
mild reaction conditions, and the use of readily available and
stable starting materials are still highly desired.
Dual catalysis is an ideal strategy to synthesize heterocyclic
3
scaffolds, as multiple bonds can be formed in one-pot. Beyond
the rapid construction of molecular complexity from stable and
readily available starting materials, time and cost efficiencies
resulting from the lack of purification of intermediates make such
transformations attractive. Indeed, the combination of transition
3b−f
metal catalyst with organo-catalysts,
such as amino-catalysts,
phosphoric acids, and N-heterocyclic carbene catalysts, has
emerged as a new and powerful strategy. However, dual catalysis
with the right combination of transition metal catalysts is far
4
,5
less explored, which is in part due to the difficulty in ensuring
redox-compatibility between the catalysts, avoiding catalysts
4
i
deactivation. Moreover, the mechanistic complexity of these
dual catalytic systems has resulted in limited understanding and
thus restricted development of effective dual transition-metal
catalytic reactions. Recently, Liang demonstrated the palla-
dium(0)-catalyzed coupling of allyl carboxylates with α-diazo
carbonyl compounds to construct acyclic quaternary carbon
Figure 1. Developments in relay Pd/Rh dual catalysis.
7
6
well established. However, Pd-catalyzed cross-couplings are
centers. Herein we report our discovery of a highly compati-
nowadays recognized as one of the most powerful and reliable
ble Rh(II)/Pd(0) dual catalysis that promotes the reaction of
α-diazo-β-ketoesters 1 with allyl carboxylates 2 for the selective
construction of cyclic all-quaternary carbon centers (dr > 20:1)
8
tools for the C−C bond formations. Although rapid develop-
ments in these two important areas have been achieved, it is
highly desirable to develop novel methodologies through com-
bining these two areas together. In recent years, the com-
bination of mechanistically distinct carbene transformation and
(
Figure 1d). This binary catalyst system rendered domino
Rh(II)-carbene-induced C−H insertion/Pd-catalyzed allylic
alkylation process.
Diazo compounds have found wide applications in organic
synthesis. In particular, the Rh-catalyzed diverse transforma-
7
Received: September 30, 2016
tions of diazo compounds involving the Rh-carbene species are
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02958
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
cross-coupling in a single catalytic cycle has been introduced
Scheme 1. Relay Rh(II)/Pd(0)-Catalyzed Reaction of
α-Diazo-β-ketoesters 1 and Allyl Carboxylates 2a
a
as a new strategy for the formation of C−C bonds, which
9
involved a single catalyst. However, some transformations
would be difficult or unattainable with monocatalysis. Thus, it
may provide access to entirely new reactivity patterns by
combining Pd and Rh catalysts in a single flask. The daunting
challenge for this transformation resides in the compatibility
between Rh and Pd catalysts. Jeong and coauthor demonstrated
highly compatible Rh(I)/Pd(0) system for the allylic alkylation/
4a,5b
Pauson Khand reaction (Figure 1a).
Lautens also developed
Rh(I)/Pd(0) system for formal hydroarylation/C−N coupl-
5
f−k
ing (Figure 1b).
Recently, the cross-compatible Pd(II)/
4n
5
l
Rh(III) system and Pd(0)/Rh(II) system were also reported.
Our reaction design involves that the β-lactone 5 was generated
first from the Rh(II)-carbene-induced intramolecular C−H
10c,f
insertion of α-diazo-β-ketoesters 1.
Subsequently, Pd(0)-
11
catalyzed allylic alkylation with allyl carboxylates 2 would
access β-lactones 3 bearing an α-quaternary carbon center, which
are often found in biologically active compounds (Figure 1d).
12
To put the above hypothesis to the test, our investiga-
tion began with α-diazo-β-ketoester 1a and allyl benzoate 2a at
t
room temperature in the presence of 1.0 mol % Rh ( BuCO ) ,
2
2 4
2
.0 mol % [PdCl(allyl)] , and 2.2 mol % Xantphos. Under these
2
conditions, the desired β-lactone 3aa was obtained in 71% yield
and with excellent diastereoselectivity (dr > 20:1) (entry 1,
Table S1). The X-ray crystallography and 2D NMR analysis of
the β-lactones 3 indicated that the methyl and allyl groups
were in a trans position (see SI). Meanwhile, the product 7 was
also detected, which was attributed to the Rh-catalyzed O−H
insertion of benzoic acid with excessive α-diazo-β-ketoester 1a.
Encouraged by this initial result, we proceeded to optimize the
reaction. After screening different Pd-catalysts (entries 2−5,
Table S1), it was found that Pd(PPh ) did not deliver isolable
a
Reaction conditions: 1 (0.36 mmol, 1.8 equiv), 2a (0.20 mmol), in
toluene (2.0 mL) at room temperature. Yields are isolated yield.
b
c
Compound 1k (0.28 mmol, 1.4 equiv) was used. Reaction carried
out at 50 °C.
3
4
coupling products (entry 5, Table S1). The efficiency of reaction
was dramatically decreased by changing the Xantphos ligand to
other phosphine ligands (entries 7−10, Table S1) or the Rh(II)
catalysts (entries 11−12, Table S1). In addition, the yield could
be further improved by changing the substrate ratio of 1a/2a
from 2.0:1.0 to 1.8:1.0 (entry 15, Table S1). Moreover, control
experiments indicated that both palladium and rhodium are
required for the reactivity. For example, when under the only
Rh(II) catalyst, only the C−H inserted β-lactone 5a was detected
We next investigated the reactivity of α-diazo-β-ketoester 1a
and allyl carboxylate 2b having 3-phenyl group. However, the
corresponding product 3ab could be obtained in low yield under
13
the optimized reaction conditions. To our delight, it was found
that the addition of 2.0 equiv of K CO , which may help the Pd-
2
3
catalyzed allylic alkylation, could significantly improve the yield
affording 3ab in 65% yield, while using only 1.4 equiv of α-diazo-
β-ketoester 1a. To further demonstrate the versatility of the
reaction, a series of allyl carboxylates 2 were reacted with α-diazo-
β-ketoesters 1a (Scheme 2). Not surprisingly, the 3-phenyl
substituted allyl carboxylates 2 having electron-donating methyl
(2c) group and the electron-withdrawing fluoro (2e), chloro (2f),
trifluoromethyl (2j), and nitro (2k) substituents at para-position
of phenyl ring were well tolerated. The allyl carboxylates 2l having
naphthyl substituent could also effectively react to afford the
desired product 3al in 77% yield. Interestingly, only (E)-geometry
3ad was also obtained from the reaction with sterically demanded
ortho-methoxyphenyl substituted allyl carboxylate 2d (E/Z =
5.9:1). In addition, it has been found that reaction with the E/Z-
mixture of allyl carboxylates having bromide groups at the o-, m-,
and p-positions (2g−2i) could give the corresponding (E)-geo-
metry β-lactone products (3ag−3ai) without (Z)-geometry in
good yields. These results clearly indicated that the geometry of
(
entry 16, Table S1). However, when the reaction was carried out
in the presence of only [ClPd(allyl)] /Xantphos, the Pd(0)-
2
catalyzed cross-coupling of allylic carboxylates with α-diazo6
carbonyl compounds did not proceed at all (entry 17, Table S1).
Other allyl carboxylates such as allyl acetate did not improve the
13
yield and resulted in a 62% yield of 3aa.
After having optimized the conditions, reaction of various
α-diazo-β-ketoesters 1 and allyl benzoate 2a afforded the cor-
responding β-lactones 3aa−3ma in moderate to high yields
(Scheme 1). The reaction was not significantly affected by
electronic effect of the substituents on the aromatic ring of
α-diazo-β-ketoesters 1. Both electron-rich (1b and 1c) and
electron-poor (1d−1k) α-diazo-β-ketoesters can be readily
employed. In particular, fluoro (1d), bromide (1g and 1h),
trifluoromethyl (1i), nitro (1j), and nitrile (1k) groups were well
tolerated. Furthermore, by elevating the reaction temperature to
1
3
olefin of allyl moiety is isomerized via η -η π-allyl Pd complexes,
5
0 °C for 6 h, the furan-containing α-diazo-β-ketoesters 1m
and the Pd-catalyzed allylic alkylation occurred at the sterically
14
could also be successfully incorporated into the product 3ma in
moderate yield. Unfortunately, the reaction did not give the
corresponding product 3na from alkyl substituted α-diazo-β-
less demanding terminal carbon. The methallyl benzoate 2m
1
2
(R = H, R = Me) also afforded the corresponding 3am.
When allyl carboxylates 2a were treated with α-diazo-β-
ketoesters 1o under the above conditions, the corresponding
1
0c
ketoester 1n.
B
DOI: 10.1021/acs.orglett.6b02958
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Relay Rh(II)/Pd(0)-Catalyzed Reaction of
α-Diazo-β-ketoesters 1a and Allyl Carboxylates 2
Scheme 3. Relay Rh(II)/Pd(0)-Catalyzed Reaction of
α-Diazo-β-ketoesters 1o and Allyl Carboxylates 2
a
a
a
Reaction conditions: 1a (0.28 mmol, 1.4 equiv), 2 (0.20 mmol), in
a
Reaction conditions: 1o (0.24 mmol, 1.2 equiv), 2 (0.20 mmol), in
toluene (2.0 mL) at room temperature. Yields are isolated yield.
toluene (2.0 mL) at room temperature. Yields are isolated yield.
b
c
Compound 2d (E/Z = 5.9:1) was used. Compound 2g (E/Z = 3:1)
b
d
e
Compound 1o (1.2 mmol, 1.2 equiv) and 2b (1.0 mmol) were used
was used. Compound 2h (E/Z = 50:1) was used. Compound 2i
E/Z = 33:1) was used.
c
d
in toluene (10.0 mL). Compound 2d (E/Z = 5.9:1) was used. Com-
(
e
pound 2g (E/Z = 3:1) was used. Compound 2h (E/Z = 50:1) was
used. Compound 2i (E/Z = 33:1) was used.
f
β-lactones 3oa could not be obtained. Surprisingly, a 2,3-dihydro-
H-inden-1-one 4a was obtained in 77% yield, a result that we
1
attribute to Rh-catalyzed 1,5-C−H insertion instead of 1,4-C−H
insertion. Next, to demonstrate the generality of this reac-
tion, various allyl carboxylates 2 were reacted with α-diazo-β-
ketoesters 1o. As shown in Scheme 3, this reaction afforded the
corresponding products 4 in high to excellent yields. Allyl car-
boxylates 2 bearing electron-donating (2c and 2d) and electron-
withdrawing groups (2e−2k) can be used. Functional groups
including fluoro, trifluoromethyl, nitro, and 2-naphthyl groups
were well tolerated in this reaction. Notably, the halogen sub-
stituents on the aromatic ring, especially bromide (2g−2i),
remained inert in the reaction, allowing further transformations
through cross-coupling reactions. Also, the reaction was not
noticeably affected by the position of substituents on the aro-
matic ring of allyl carboxylates, although the reaction rate
decreased in the case of the substituents on ortho-positions of the
aromatic ring of allyl carboxylates (2d and 2g). Finally, the
molecular structures of 3ha and 4k were unambiguously con-
In summary, we have developed a novel method for the
formation of two different C−C bonds on the same carbon atom
in one-pot by relay Rh(II)/Pd(0) dual catalysis. This binary
catalyst system rendered domino C−H insertion/allylic
alkylation process under mild conditions. Notably, this reac-
tion provides a new strategy for the construction of cyclic all-
quaternary carbon centers. The corresponding β-lactones 3 and
15
firmed through X-ray crystallography.
In order to shed light on the mechanism, several experiments
were performed. First, α-diazo-β-ketoesters 1a and 1o were
subjected to the intramolecular C−H insertion using only Rh(II)
carboxylates as the catalyst, respectively. The desired β-lactone
2
,3-dihydro-1H-inden-1-one 4 can also be selectively obtained,
depending on the choice of α-diazo-carbonyl compounds. Fur-
thermore, the relay catalytic process shows good selectivity and
excellent tolerance to various functionalities, which is operation-
ally simple.
1
0a−d,f
5a
was obtained in 61% yield by Rh-catalyzed 1, 4−C-H
insertion of the 1a (eq 1). However, reaction with the α-diazo-β-
ketoesters 1o afforded the 1,5-C−H insertion product 6a in 70%
yield, while the 1,4-C−H insertion product was not detected at
all. This result clearly suggested that 1,5-C−H insertion is
preferable to 1,4-C−H insertion (eq 2). We next focused our
ASSOCIATED CONTENT
Supporting Information
■
*
S
11
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b02958.
attention on Pd-catalyzed allylic alkylation of 5a and 6a with
allyl benzoate 2a, which were readily achieved by employing
[
PdCl(allyl)] /Xantphos in toluene. These results clearly imply
Detailed experimental procedures and characterization
data for 3 and 4 (PDF)
X-ray crystallographic data for 4k (CIF)
X-ray crystallographic data for 3ha (CIF)
2
that this catalytic system was a relay Rh(II)/Pd(0) dual catalysis,
and the relay catalytic system could provide superior efficiency
compared to its stepwise counterpart.
C
DOI: 10.1021/acs.orglett.6b02958
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
2
014, 79, 12159−12176. (l) Zhang, S.-S.; Wu, J.-Q.; Liu, X. G.; Wang,
AUTHOR INFORMATION
Corresponding Author
■
*
̃
́ ́
H. G. ACS Catal. 2015, 5, 210−214. (m) Galvan, A.; Fananas, F. J.;
Rodríguez, F. Eur. J. Inorg. Chem. 2016, 2016, 1306−1313.
(n) Yamamoto, K.; Qureshi, Z.; Tsoung, J.; Pisella, G.; Lautens, M.
Org. Lett. 2016, 18, 4954−4957.
jikegong@nwsuaf.edu.cn
Notes
(
6) Chen, Z.-S.; Duan, X.-H.; Zhou, P.-X.; Ali, S.; Luo, J.-Y.; Liang, Y.-
The authors declare no competing financial interest.
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■
Methods for Organic Synthesis with Diazo Compounds; Wiley: New York,
This work was supported by the National Natural Science Foun-
dation of China (No. 21402154 and 21502150), the Chinese
Universities Scientific Fund (No. 2452015437 and 2452016093),
and the Youth Training Programme of Northwest A&F University
No. Z111021508 and Z111021404). We thank Dr. Hong-li
Zhang for NMR analysis in Northwest A&F University.
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DOI: 10.1021/acs.orglett.6b02958
Org. Lett. XXXX, XXX, XXX−XXX