Rhodium(I)-Catalyzed Coupling-Cyclization of C=O Bonds with α-Diazoketones

Article abstract of DOI:10.1021/acs.orglett.8b01541

An unprecedented intermolecular nucleophilic attack of C=X bonds (X = O and S) on the rhodium(I)-carbenes has been developed. This transformation allows for the coupling-cyclization of aroylamides with α-diazoketones and provides concise access to 2,4,5-trisubstituted 1,3-oxazoles and 1,3-thiazoles with a broad tolerance of functional groups.

Full text of DOI:10.1021/acs.orglett.8b01541

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Rhodium(I)-Catalyzed Coupling−Cyclization of CO Bonds with
α‑Diazoketones
Ziyang Chen, Xinwei Hu, Junmin Huang, and Wei Zeng*
Key Laboratory of Functional Molecular Engineering of Guangdong Province, Guangdong Engineering Research Center for Green
Fine Chemicals, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641,
China
S
* Supporting Information
ABSTRACT: An unprecedented intermolecular nucleophilic
attack of CX bonds (X = O and S) on the rhodium(I)-
carbenes has been developed. This transformation allows for
the coupling−cyclization of aroylamides with α-diazoketones
and provides concise access to 2,4,5-trisubstituted 1,3-
oxazoles and 1,3-thiazoles with a broad tolerance of functional groups.
ransition-metal-catalyzed cross-coupling between two
fragments provides a powerful platform for chemistry
although this type of transformation could provide an
alternative approach for the assembly of structurally diverse
molecules.
T
transformations.1 Among the different coupling partners,
diazo-compound-derived metal carbenes have played impor-
tant roles in modern organic chemistry because these active
intermediates could couple with arenes,2 alkanes,3 olefins,4
alcohols,5 alkynes,6 carboxylic acids,7 amines8 and others9 to
construct different C−X bonds (X = C, N, O, S, Si, etc.). In
comparison, the coupling reactions involving metal carbenes
and carbonyl (CO) group are rarely explored. In this regard,
Ibata and Padwa developed a Rh(II)-catalyzed intramolecular
coupling reaction of carbonyl group (CO) with diazo
compounds, in which carbonyl oxygen attacks Rh(II) carbenes
to generate carbonyl ylides (Scheme 1a).10 For the
It is well-known that amides belong to readily available
carbonyl synthons, and these compounds have been widely
utilized as “nitrogen” and carbonyl sources to make hetero-
cycles, in which amidonitrogen generally acts as a “nucleo-
philic” monomer to trigger a reaction.12 Although Che and Hu
developed cross-couplings of amides with Ru(II) and Rh(II)-
carbenes to furnish α-imino esters and prolines (Scheme 1b),13
other transition-metal-carbene-involved coupling reactions
with amides are extremely rare. However, considering that
various metal carbenes, especially for the Rh(I)-carbene,
possess particular and interesting reactivity in a few catalytic
C−C bond-forming processes,14 exploring a novel coupling
model between Rh(I)-carbenes and amides is desirable and
also will possibly provide a promising approach to construct
nitrogen-containing heterocycles. Herein, we disclose a Rh(I)-
catalyzed coupling−cyclization of amides with diazo com-
pounds, in which an unusual nucleophilic attack of carbonyl
oxygen on Rh(I)-carbenes led to the formation of 2,4,5-
trisubstituted 1,3-oxazoles derivatives. These 1,3-oxazole
skeletons are commonly encountered in bioactive natural
products and pharmaceuticals.15
Scheme 1. Distinct Coupling Pathways between Diazo
Compounds and Amides
Optimization conditions for the coupling−cyclization of
carbonyl CO bonds with diazo compounds are summarized
in Table 1. Initial efforts focused on screening various Mn(I),
Co(III), Pd(II), Rh(III), Rh(II), and Rh(I) catalysts, which
could possibly enable the cross-coupling of benzoylamide (1a)
with α-diazo-β-ketoester (2a) in the presence of AgOAc (5
mol %) in EtOAc (2.0 mL) at 100 °C under an Ar atmosphere
for 10 h (Table 1, entries 1−6). Gratifyingly, we did find that
different types of metal carbenes showed different chemical
selectivities, leading to the formation of 1,3-oxazole (3-1a) and
intermolecular version, only Yoshikai recently employed
ketimines to act as carbonyl group equivalents and couple
with diazo compounds under a copper catalytic system,
affording multisubstituted pyrroles through nucleophilic attack
of ketimine nitrogen on copper carbenes.11 To date, the
intermolecular coupling reaction between carbonyl CO
bonds and diazo compounds has not yet been explored,
Received: May 16, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01541
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a b
,
Table 1. Optimization of the Reaction Parameters
Scheme 2. Scope of Aroylamides
b
entry
catalyst
additive
solvent
yield (%) (3-1a/4a)
1
2
3
4
5
6
7
8
Mn(CO)5Br
Cp*Co(CO)I2
Pd(OAc)2
Rh2(OAc)4
RhCl3
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgClO4
AgNTf2
AgBF4
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
−
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
toluene
CH3CN
THF
0/0
trace/0
26/trace
0/32
22/18
[Cp*RhCl2]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
<5/trace
32/trace
12/21
61/trace
66/trace
9
10
11
12
13
14
15
16
17
18
c
90/trace
64/11
40/trace
80/trace
<5/trace
87/trace
53/0
DMF
DCE
EtOAc
EtOAc
d
AgSbF6
56/0
a
All the reactions were performed using amide 1 (0.4 mmol) and
diazo compound 2a (0.40 mmol) with [Rh(COD)Cl]2 catalysts (3
mol %) in the presence of AgSbF6 (5 mol %) in EtOAc (2.0 mL) at
100 °C for 8 h under Ar in a sealed reaction tube. Followed by flash
chromatography on SiO2. Isolated yield. 59% yield on 1.0 mmol
scale.
a
Unless otherwise noted, all the reactions were performed using
amide 1a (0.40 mmol) and diazo compound 2a (0.40 mmol) in the
presence of catalysts (3 mol %) with an additive (5 mol %) in solvent
(2.0 mL) at 100 °C for 8 h under Ar in a sealed reaction tube.
b
c
b
Followed by flash chromatography on SiO2. Isolated yield.
c
Employing 2 mol % of [Rh(COD)Cl]2 produced a 68% yield of 3-
d
1a. The reaction was carried out at 80 °C.
the corresponding 1,3-oxazoles (3-1b and 3-1d) in good to
excellent yields (82% and 63%).16 In comparison, electron-
withdrawing group (4-F, 4-Cl, 4-Br, 4-NO3, and 4-CF3)-
substituted benzamides led to poorer substrate conversion,
affording moderate to good yields of the products 3-1e−3-1j
(47% − 69%).17 Meanwhile, the steric hindrance of the meta-
or ortho-substituent on benzamides inhibited the reaction
conversion to some degree (49% for 3-1c and 45% for 3-1g).
Moreover, the 3,4- or 3,5-disubstituted benzamides also
tolerate the assembly of the cyclized products, and different
types of substituents and substitution at various positions of
phenyl ring did not significantly affect the reactivity (3-1k−3-
1n, 49−64% yields). It should be noted that 1-naphthoyla-
mide, 2-furoylamide, and isobutyramide18 are still amenable
under our reaction conditions with moderate yields (50% for
3-1o, 62% for 3-1p, and 43% for 3-1q), but pyridine-2-
carboxylic acid amide did not produce the target products 3-
1q.19
Subsequently, we evaluated the scope of α-diazo-β-
ketoesters (Scheme 3) and found electron-rich phenyl-
substituted diazo compounds could be efficiently transferred
into 1,3-oxazoles in 81−94% yields (3-2a, 3-2b, and 3-2d).20
On the contrary, electron-deficient phenyl-substituted diazo
compounds resulted in lower yields of the products 3-2e−3-2h
(57−77% yields). Besides the α-diazo-β-phenyl-substituted
ketoesters, the α-diazo-β-(2-naphthyl)-substituted ketoester
and α-diazo-β-(2-furyl)-substituted ketoester also gave good
yields of 1,3-oxazoles 3-2i (68%) and 3-2j (65%). Moreover,
further evaluation revealed that α-diazo-β-alkyl-substituted
ketoesters and α-diazo-β-diketone could proceed smoothly to
access 4-alkyl-5-acyl-substituted 1,3-oxazoles in moderate
yields (3-2k−3-2m, 48−53% yields). It is gratifying that the
α-amido-β-ketoester (4a), respectively. Among them, Mn-
(CO)5Br, Cp*Co(CO)I2, and [Cp*RhCl2]2 catalysts did not
efficiently enhance the coupling reaction at all (entries 1, 2 and
6), and Rh(II)-carbene was easily attacked by the amidoni-
trogen of 1a to afford a 32% yield of α-amido-β-ketoester 4a
(entry 4). On the contrary, Pd(OAc)2 and [Rh(COD)Cl]2
could successfully allow the coupling−cyclization of amide 1a
with diazo compound 2a to produce 1,3-oxazole 3-1a through
the nucleophilic attack of the CO group of 1a on the metal
carbenes, and [Rh(COD)Cl]2 was proven to be the most
efficient catalyst (compare entry 3 with 7). However, RhCl3
did not show good chemical selectivity (compare entries 3 and
7 with 5), affording 1,3-oxazole (3-1a) and 4a in a ratio of
almost 1:1 (entry 5). Subsequently, various silver salts
including AgClO4, AgNTf2, AgBF4, and AgSbF6 were further
evaluated in the presence of [Rh(COD)Cl]2 catalysts in order
to increase the reaction conversion, and AgSbF6 could
signficantly improve the yield of 3-1a from 32% to 90%
(compare entries 7−10 with 11). Finally, the additional solvent
screening revealed that toluene, 1,2-dichloroethane (DCE),
DMF, etc. gave inferior results (compare entries 12−16 with
11). It should still be noted that the reaction yield would be
decreased to 53% in the absence of AgSbF6 (compare entry 17
with 11), and also running the reaction at lower temperature
(80 °C) led to poorer reaction conversion (compare entry 11
with 18).
With the optimized reaction conditions known, we then
investigated the scope of aroylamides using 2-diazo-3-oxo-3-
phenyl-propionic acid ethyl ester (2a) as the coupling reagent.
As shown in Scheme 2, the electron-rich amides, such as 4-
methyl-benzamide and 4-methoxy-benzamide, could produce
B
DOI: 10.1021/acs.orglett.8b01541
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b
,
Scheme 3. Scope of α-Diazo-β-Ketoesters
Scheme 5. Possible Reaction Mechanism
diazo-β-ketoester (2a) with Rh(I) catalysts affords rhodium(I)
carbene complex A with the extrusion of a N2 molecule.
Subsequently, nucleophilic attack of carbonyl oxygen of amide
2a on Rh(I) carbene A generates intermadiate B, which further
produces β-ketoester C via metal protonation with concom-
itant regeneration of Rh(I) catalysts. Finally, C undergoes an
intramolecular cyclization/dehydration cascade to yield 2,4,5-
trisubstituted 1,3-oxazole 3a.
In conclusion, we have developed a novel Rh(I)-catalyzed
coupling−cyclization of carbonyl CO bonds with α-diazo-β-
ketoesters. This transformation proceeds through a rare
intermolecular nucleophilic attack of carbonyl oxygen on
Rh(I)-carbenes and provides efficient access to versatile 2,4,5-
trisubstituted 1,3-oxazole skeletons. Wide functional group
tolerance from aroylamides and diazo compounds is observed.
a
All the reactions were performed using benzamide 1a (0.4 mmol)
and diazo compound 2 (0.40 mmol) with [Rh(COD)Cl]2 catalysts (3
mol %) in the presence of AgSbF6 (5 mol %) in EtOAc (2.0 mL) at
100 °C for 8 h under Ar in a sealed reaction tube. Followed by flash
b
chromatography on SiO2. Isolated yield.
reaction system could be further extended to thiobenzamides,
furnishing 2,4,5-substituted 1,3-thiazoles (3-2n and 3-2o) in
acceptable yields using 1a as coupling partners.
ASSOCIATED CONTENT
* Supporting Information
■
Several control experiments were performed to elucidate the
plausible mechanism. First, the coupling−cyclization of
benzamide (1a) with α-diazo ester (2a) in the presence of
Rh2(OAc)4 catalysts produced α-amido-β-ketoester 4a (32%
yield) through a nucleophilic attack of carboxamido nitrogen
on the Rh(II)-carbenes, but 4a could not further produce 1,3-
oxazole (3-1a) under our standard conditions, implying that α-
amido-β-ketoester (4a) was not the possible intermediate of
this transformation (Scheme 4a). Then, the Rh(I)-catalyzed
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01541.
Detailed experimental procedures, characterization data,
1
13
copies of H NMR and
C NMR spectra for all
isolated compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zengwei@scut.edu.cn.
ORCID
Scheme 4. Preliminary Mechanistic Studies
Wei Zeng: 0000-0002-6113-2459
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the National Key Research and Develop-
ment Program of China (No. 2016YFA0602900), NSFC (No.
21372085), and Guangdong Province Science Foundation
(No. 2017B090903003) for financial support.
coupling reaction of N-methyl-benzamide (1q) with diazo
compound 2a afforded the α-acyoxy-β-ketoester 6, which is
derived from the hydrolysis of ketoimine 5. This result
demonstrated that a nucleophilic attack of carbonyl oxygen on
the Rh(I)-carbenes was involved in this reaction (Scheme 4b).
Moreover, the competive coupling−cyclization between diazo
compound 2a and aroylamides (1b and 1c) differing in
electronic effects implied that nucleophilic attack on Rh(I)-
carbenes possibly belongs to the rate-determining step in the
cyclization process (Scheme 4c).
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C
DOI: 10.1021/acs.orglett.8b01541
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
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DOI: 10.1021/acs.orglett.8b01541
Org. Lett. XXXX, XXX, XXX−XXX