Cooperative Pd/Cu Catalysis: Multicomponent Synthesis of Tetrasubstituted Imidazolones from Methyl α-Isocyanoacetates, Primary Amines, and Aryl(vinyl) Iodides

Article abstract of DOI:10.1021/acs.orglett.7b03479

Three-component reaction of methyl α,α-disubstituted α-isocyanoacetates, primary amines, and aryl(vinyl) halides in the presence of Pd(OAc)₂ (0.05 equiv) and Cu₂O (1.0 equiv) provided 2,3,5,5-tetrasubstituted imidazolones via the for

Full text of DOI:10.1021/acs.orglett.7b03479

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Cooperative Pd/Cu Catalysis: Multicomponent Synthesis of
Tetrasubstituted Imidazolones from Methyl α‑Isocyanoacetates,
Primary Amines, and Aryl(vinyl) Iodides
Antonin Clemenceau, Qian Wang, and Jieping Zhu*
́ ́
Laboratory of Synthesis and Natural Products, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Federale de
Lausanne, EPFL-SB-ISIC-LSPN, BCH 5304, 1015 Lausanne, Switzerland
S
* Supporting Information
ABSTRACT: Three-component reaction of methyl α,α-disubstituted α-
isocyanoacetates, primary amines, and aryl(vinyl) halides in the presence of
Pd(OAc)2 (0.05 equiv) and Cu2O (1.0 equiv) provided 2,3,5,5-tetrasubstituted
imidazolones via the formation of three chemical bonds. A copper-mediated
migratory insertion of the isocyano group into the N−H bond of the amine
followed by lactamization and Pd-catalyzed cross-coupling of the in situ
generated amidinyl copper species with aryl(vinyl) halides accounted for the reaction outcome.
Scheme 1. Synthesis of Tetrasubstituted Imidazolones
midazolone, a nonaromatic heterocycle, is found in bioactive
1
Inatural products and is a key structural element responsible
for the luminescent properties of green fluorescent proteins
(GFP).2 It is a core structure of a number of pharmaceuticals
displaying potent inhibitory activities against fatty acid
synthases3 and angiotensin II receptor antagonists.4 For example,
irbesartan, a marketed drug for the treatment of hypertension,
contains this heterocycle.5 Different synthetic methods toward
the substituted imidazolones6 have been developed including,
mainly: (a) the transformation of the Erlenmeyer azlactones;7
(b) the cyclization of the amino acid derived formamidines,8 α-
amidoamides,9 and α-isocyanoacetamides;10 and (c) the
functionalization of the simple imidazolones.11 Recently,
Hoarau, Bischoff, and co-workers reported a Pd-catalyzed direct
C−H functionalization of imidazolones 1 for the synthesis of the
C-2 functionalized counterpart 2 (Scheme 1a).12 Pirali et al.
developed a synthesis of the C-2 arylated imidazolones by
reaction of the α-isocyanoacetamides with benzynes (Scheme
1b).13
reaction mixture (entry 1, Table 1). Screening other cocatalysts
(entries 2−11) indicated that Cu2O formed the most productive
couple with Pd leading to the desired three-component adduct
2a in 72% isolated yield (entry 7). The impact of base on the
reaction outcome was also examined. Among the two best
copper copromoters, addition of a base increased the product
yield in the case of CuI (entries 4 vs 5), but exerted a deleterious
effect on the reaction mediated by Cu2O (entries 7 vs 12−14).
The reaction temperature of 130 °C seemed optimum as the
reaction carried out at other temperatures led to the product with
diminished yields (entries 7 vs 15, 16). Performing the reaction
in DMSO (entry 17) or with reduced loading of Cu2O (entries
18, 19) resulted in low yields of 2a. However, reaction proceeded
smoothly in the absence of phosphine ligand to provide 2a, albeit
with reduced yield (entries 21).20 Finally, performing the
reaction with Pd or Cu alone afforded 2a in much reduced
Metal-catalyzed domino processes initiated by isocyanide
insertion reaction has recently been developed into a powerful
tool for the rapid construction of heterocycles.14−17 In
connection with our research program aimed at exploiting the
diverse reactivities of the isocyano group,18 we reported a silver
nitrate-catalyzed synthesis of the 3,5,5-trisubstituted imidazo-
lones 1 from methyl α-isocyanoacetates 3 and primary amines
4.19 As a continuation of this research topic, we describe herein a
three-component synthesis of 2,3,5,5-tetrasubstituted imidazo-
lones 2 by a Pd/Cu-catalyzed reaction of methyl α,α-
disubstituted α-isocyanoacetates 3 with primary amines 4 and
aryl(vinyl) halides 5 (Scheme 1c).
Methyl α,α-dibenzyl α-isocyanoacetates (3a, R1 = R2 = Bn),
benzylamine (4a), and 4-iodotoluene (5a) were chosen as test
substrates to evaluate the feasibility of our approach. Combining
AgNO3, the optimum metal salt for catalyzing the reaction
between 3a and 4a, with Pd catalyst afforded only an intractable
Received: November 9, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03479
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Organic Letters
Letter
a
Table 1. Synthesis of 2,3,5,5-Tetrasubstituted Imidazolones 2a: Survey of Reaction Conditions
b
entry
Pd/ligand
Lewis acid
AgNO3
base
yield (%)
1
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(dba)2
DBU
DBU
DBU
DBU
degradation
2
AgOTf
Yb(OTf)3
CuI
33
23
68
57
55
3
4
5
CuI
6
CuCl
h
7
Cu2O
75(72)
8
CuOTf
CuOAc
[(MeCN)4Cu]BF4
Cu(OTf)2
Cu2O
58
21
50
6
9
10
11
12
13
14
DBU
28
57
27
66
66
41
34
54
66
62
Cu2O
Et3N
Cu2O
Cs2CO3
c
15
Cu2O
d
16
Cu2O
e
17
Cu2O
f
18
Cu2O
g
19
Cu2O
20
21
22
23
Cu2O
Pd(OAc)2
Cu2O
i
Cu2O
43
Pd(OAc)2/PPh3
21
a
Standard conditions: 3a (0.1 mmol), 4a (0.15 mmol), 5a (0.2 mmol), Lewis acid (1.0 equiv), Pd catalyst (5 mol %), ligand (10 mol %), base (1.0
b
c
d
e
f
equiv), DMF (1.0 mL), 130 °C. NMR yield using CH2Br2 as an internal standard. 110 °C. 150 °C. Reaction was performed in DMSO. 10 mol
% of Cu2O was used. 25 mol % of Cu2O was used. Yield of isolated product in parentheses. Yield referred to C2-unsubstituted imidazolone
resulting from the Cu-promoted amination/lactamization sequence (cf. ref 19).
g
h
i
yield indicating a strong synergistic effect of these two metals in
catalyzing/mediating this transformation (entries 22, 23).
Overall, the optimum conditions found consisted of performing
the reaction in DMF at 130 °C in the presence of Pd(OAc)2 (5
mol %), PPh3 (10 mol %), and Cu2O (1.0 equiv). Under these
conditions, imidazolone 2a was isolated in 72% yield. The
reaction was chemoselective since direct N-arylation of primary
amine 4a by aryl iodide was not observed under these conditions.
This novel three-component reaction displayed a broad
application scope (Scheme 2). Aryl iodides bearing electron-
donating (Me, OMe) and electron-withdrawing groups (F, CF3,
NO2) at different positions were appropriate substrates for this
reaction. Various alkyl amines, linear, branched, and function-
alized, such as tryptamine (2m), participated well in this reaction.
However, aniline failed to participate in this reaction. Reactions
involving chiral amines (2e, 2f) and α-amino ester (2u) afforded
the three-component adducts in good yields without racemiza-
tion. The α,α-disubstituted α-isocyanoacetates bearing different
substituents reacted with amines and aryl iodide to afford the
adducts, including spiroimidazolones (2o, 2p), without event.
The protocol can also be applied to complex natural products.
Thus, reaction of 3α-amino-5-cholestene21,22 with 4-iodotoluene
(5a) and methyl α,α-diisobutyl isocyanoacetate afforded 2v in
52% yield. The α-nonsubstituted and α-monosubstituted α-
isocyanoacetates23 and methyl 2-isocyanobenzoate failed to
undergo the three-component coupling reaction. Finally,
performing the reaction of 3a, 4a, and 5a at 1.0 mmol scale
afforded the three-component adduct 2a in 90% isolated yield.
Vinyl bromides, including the 1,2-disubstituted, 1,1-disub-
stituted, and tetrasubstituted ones, reacted smoothly with 3 and 4
to provide the 2-vinyl substituted imidazolones (2w−2y) in good
yields (Scheme 3a). Based on this novel three-component
reaction, more complex domino process can be designed for the
one-pot synthesis of the polyheterocyclic compounds. For
example, reaction of ortho-bromobenzylamine (6) with methyl
isocyanoacetates 3 furnished the tricyclic compounds 7a and 7b,
respectively (Scheme 3b).
To gain insight on the reaction mechanism, the following
control experiments were carried out (Scheme 4a). The reaction
of the presynthesized 3,5,5-trisubstituted imidazolone 1a with
iodide 5a under our optimized conditions led only to the
recovery of 1a.24 In accordance with Hoarau and Bischoff’s
observation on the importance of the base on the C2−H
arylation of imidazolones, the same reaction performed in the
presence of benzylamine (1.5 equiv) afforded indeed 2a, albeit in
only 26% yield. The low yield of 2a indicated nevertheless that
the formation of transient imidazolone 1a followed by C−H
arylation, although feasible, was not the main reaction pathway
under our reaction conditions. Taking into consideration the
above results, a possible reaction sequence leading to
imidazolone 2 is depicted in Scheme 4b. Coordination of the
isocyano and the amino groups to the copper affording complex
A. Migratory insertion from A would afford B. Intramolecular
B
DOI: 10.1021/acs.orglett.7b03479
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Three-Component Synthesis of 2-Aryl Substituted
Imidazolones 2
Scheme 3. Three-Component Synthesis of 2-Vinyl
Substituted Imidazolones and Tricyclic Compounds
Scheme 4. Possible Reaction Pathway and Control
Experiments
a
Conditions: 3 (0.1 mmol), 4 (0.15 mmol), 5 (0.2 mmol), Cu2O (1.0
equiv), Pd(OAc)2 (5 mol %), PPh3 (10 mol %), DMF (1.0 mL), 130
°C.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b03479.
amidation followed by transmetalation of the organocopper
species with R4Pd(II)X C (R4 = aryl, vinyl), generated in situ by
oxidative addition of 5 to Pd(0), would produce complex D.25
Reductive elimination would then furnish imidazolone 2 with
concurrent regeneration of the palladium(0) catalyst.
In summary, we have developed a palladium-catalyzed, Cu2O-
mediated three-component reaction of methyl α,α-disubstituted
α-isocyanoacetates 3 with primary amines 4 and aryl(vinyl)
iodides 5. Three chemical bonds were created in this process
leading to diversely substituted 2,3,5,5-tetrasubstituted imidazo-
lones 2 in good yields. This multicomponent reaction, applicable
to a wide range of substrates, constitutes novel access to this
medicinally important nonaromatic heterocycle.
Experimental procedures, product characterization data,
and 1H and 13C NMR spectra for new compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jieping.zhu@epfl.ch.
ORCID
Jieping Zhu: 0000-0002-8390-6689
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.orglett.7b03479
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ACKNOWLEDGMENTS
■
We thank EPFL (Switzerland), Swiss National Science
Foundation (SNSF) for financial support.
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