Direct Assembly of Polysubstituted Propiolamidinates via Palladium-Catalyzed Multicomponent Reaction of Isocyanides

Article abstract of DOI:10.1021/acs.orglett.9b03201

A straightforward approach for the assembly of different polysubstituted propiolamidnates via palladium-catalyzed multicomponent reaction of isocyanides, haloalkynes, and amines has been reported in which the C(sp)-C(sp²) and C(sp²)a

Full text of DOI:10.1021/acs.orglett.9b03201

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Direct Assembly of Polysubstituted Propiolamidinates via
Palladium-Catalyzed Multicomponent Reaction of Isocyanides
Meng Li,‡ Songjia Fang,‡ Jia Zheng,‡ Huanfeng Jiang,‡ and Wanqing Wu*,†,‡
†State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China
‡Key Laboratory of Functional Molecular Engineering of Guangdong Province, School of Chemistry and Chemical Engineering,
South China University of Technology, Guangzhou 510640, China
S
* Supporting Information
ABSTRACT: A straightforward approach for the assembly of different polysubstituted propiolamidnates via palladium-
catalyzed multicomponent reaction of isocyanides, haloalkynes, and amines has been reported in which the C(sp)−C(sp2) and
C(sp2)N bonds were constructed in one pot. This reaction featured in high efficiency, excellent chemoselectivity, and good
functional group compatibility. The synthetic utility of this method was also demonstrated.
midinates and their derivatives, as ubiquitous skeletons
occurring in agricultural chemicals, drug molecules, and
view of the employment of prefunctionalized precursors, the
direct assembly of propiolamidines from simple and available
starting materials is in great demand. Therefore, the develop-
ment of novel and convenient strategies for the efficient
construction of propiolamidines is highly desirable.
A
catalysts, have received considerable attention over the past
decades.1 Among them, propiolamidines are extraordinarily
attractive owing to their special chemical and structural
characteristics as well as their extensive applications in
synthetic chemistry.2 However, few methods have been
developed to provide propiolamidines, largely restricting their
further development.3 Traditionally, these compounds were
acquired through the addition reactions between terminal
alkynes and sterically bulky carbodiimides (Scheme 1a)4 or the
On the other hand, isocyanides, as versatile synthons in
transition-metal-catalyzed transformations, have been widely
investigated in recent years, affording numerous valuable
nitrogen-containing compounds.6 However, in contrast to a
large amount of transformations focusing on the insertion of
isocyanides into the aryl-7 or alkenylpalladium8 intermediates,
few reports are related to the combination of isocyanides with
alkynylpalladium species.9 This might be ascribed to the
limited approaches to alkynylpalladium species as well as the
competitive homocoupling reactions of alkyne complexes.10
Thus, the exploration of novel cross-coupling reactions
between isocyanides and the alkynylpalladium complex
remains interesting and meaningful. Based on our continuing
interest in palladium-catalyzed isocyanide transformations,11
herein we disclose a concise and convenient synthesis of
various polysubstituted propiolamidinates via a palladium-
catalyzed multicomponent reaction of isocyanides, haloalkynes,
and amines under mild conditions (Scheme 1c). In this
chemistry, the C(sp)−C(sp2) and C(sp2)N bonds were
constructed with high stereoselectivity and step economy in
one step. The key process of this transformation was the
migration insertion of isocyanides into the newly formed
alkynylpalladium intermediates.
Scheme 1. Synthesis of Polysubstituted Propiolimidines
1,4-metathesis reaction of nitrosoarenes with 3-en-1-ynamides
bearing different sulfonamides (Scheme 1b).5 Despite these
significant advances, the synthesis of propiolamidines contains
several challenging issues: (i) due to the present limitation of
substrate scope, the diversity of products and compatibility
with various functional groups needs to be improved; (ii) in
Received: September 9, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03201
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Organic Letters
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To evaluate the optimal conditions, we initially selected
aniline (1a), (bromoethynyl)triisopropylsilane (2a), and tert-
butyl isocyanide (3a) as the model substrates (Table 1).
Scheme 2. Substrate Scope of Primary Amines and
Isocyanides
a b
,
a b
,
Table 1. Optimization of Reaction Conditions
b
entry
catalyst
base
additive
solvent
yield (%)
1
2
3
4
5
6
7
8
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
Pd(OAc)2
PdCl2
PdCl2
K2CO3
K2CO3
K2CO3
K2CO3
DBU
DMF
15
20
82
50
49
69
60
10
65
92 (85)
74
90
78
DMSO
MeCN
Toluene
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
Et3N
CH3ONa
t-BuOK
Cs2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
9
10
11
12
13
14
15
AgTFA
Ag2CO3
AgOAc
AgTFA
AgTFA
AgTFA
c
76
69
d
a
Reaction conditions: 1a (0.1 mmol), 2a (0.15 mmol), 3a (0.125
mmol), catalyst (10 mol %), base (2.0 equiv), additive (20 mol %),
b
solvent (1.0 mL), at 100 °C for 4 h in air. nd = not detected. Yield of
4a was determined by 1H NMR using CH2Br2 as an internal standard.
c
The isolated yield is shown in parentheses. Under N2 atmosphere.
80 °C.
a
d
Reaction conditions: 1a−1t (0.10 mmol), 2a (0.15 mmol), 3 (0.125
mmol), PdCl2 (10 mol %), AgTFA (20 mol %), K2CO3 (2.0 equiv),
MeCN (1.0 mL), 100 °C, 4 h. Yield is isolated yield. Reaction
b
Delightfully, the desired product (E)-N-(tert-butyl)-N′-phenyl-
3(triisopropylsilyl)propiolimidamide (4a) was detected in 15%
NMR yield in the presence of PdCl2 (10 mol %) and K2CO3
(2.0 equiv) in DMF (1.0 mL) at 100 °C for 4 h (Table 1, entry
1). Subsequent screening of various solvents showed that
MeCN was the best choice, which might be ascribed to its
good coordinative ability to stabilize the reaction intermediates
(Table 1, entries 2−4).12 Next, different bases, such as DBU,
Et3N, CH3ONa, t-BuOK, and Cs2CO3, were tested. However,
no preferred results were observed (Table 1, entries 5−9). To
further promote this reaction, different kinds of silver salts
were examined (Table 1, entries 10−12). Pleasingly, AgTFA
was found to be effective and increased the yield of 4a from
82% to 92%. A lower yield was obtained when PdCl2 was
replaced with Pd(OAc)2 (Table 1, entry 13). Furthermore, a
slightly decreased yield was observed when the reaction
proceeded under an N2 atmosphere or at a lower temperature
(Table 1, entries 14 and 15). Meanwhile, ligands were
demonstrated to be not necessary in this reaction (see the
Supporting Information for details).
With the optimized reaction conditions established, the
generality and limitations of this strategy were explored
(Scheme 2). A variety of anilines and their derivatives were
found to be suitable for this transformation, converting to the
corresponding products in moderate to good yields. All of the
tested monosubstituted anilines with either electron-donating
or electron-withdrawing substituents at the para- or ortho-
positions of the benzene rings proceeded smoothly in this
reaction to afford the desired propiolamidinates 4a−4h in 74−
85% yields. Despite relatively low reactivity, polysubstituted
anilines also exhibited good compatibility for this chemical
conditions: 1u−1z (0.3 mmol), 2a (0.10 mmol), 3a (0.125 mmol),
PdCl2 (10 mol %), AgTFA (20 mol %), K2CO3 (2.0 equiv), MeCN
(1.0 mL), 100 °C, 6 h. Yield is isolated yield.
process (4i−4m). It should be noted that benzo[d]thiazol-2-
amine was well adopted in this protocol and transformed to
the desired product 4n in 68% yield. Furthermore, for aromatic
amines with an indolyl or naphthyl group at the ortho-position
of the aryl ring, the corresponding products 4o and 4p were
obtained in 78% and 86% yields, respectively. The structure of
4p was verified unambiguously by X-ray diffraction (CCDC
1916418). Additionally, when substrates with a styrene moiety
were applied in this transformation, the target products 4q and
4r were formed in moderate yields, providing potentials for
further elaborations. The examination of different isocyanides
revealed that sterically hindered 1-admantyl isocyanide and
1,1,3,3-tetramethylbutyl isocyanide were compatible with this
catalytic system, while other aliphatic isocyanides or aromatic
isocyanides only gave unfruitful results. Pleasingly, aliphatic
primary amines bearing long or bulky alkyl substituents such as
n-decyl, tert-butyl, and cyclopentyl groups worked well in this
reaction, converting to the expected products 4u−4x in
moderate to excellent yields.13 Similarly, homoallylic amines
were tolerated in this system, and the desired propiolamidines
4y and 4z could be obtained in 78% and 71% yields.
To further expand the substrate scope, various secondary
amines were then examined (Scheme 3). Under the standard
conditions, the transformations of symmetrical and unsym-
metrical linear amines proceeded well to give the correspond-
ing products 4aa−4ad in good to excellent yields. Moreover,
different saturated and unsaturated cyclic amines, including
B
DOI: 10.1021/acs.orglett.9b03201
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a
a
Scheme 3. Substrate Scope of Secondary Amines
Table 2. Effects of Various Haloalkynes
entry
substrate 2
2aa, X = Cl
2a, X = Br
2ac, X = I
2ao, R = TMS
2ap, R = TES
2aq, R = SiMe2 Bu
2ar, R = SiPh3
2as, R = Ph
product 4
yield (%)
1
2
3
4
5
6
7
8
4a
4a
4a
4ao
4ap
4aq
4ar
4as
78
85
53
nd
50
55
nd
trace
t
a
Reaction conditions: 1a (0.10 mmol), 2 (0.15 mmol), 3a (0.125
mmol), PdCl2 (10 mol %), AgTFA (20 mol %), K2CO3 (2.0 equiv),
MeCN (1.0 mL), 100 °C, 4 h. Yield is isolated yield. nd = not
detected.
a
Reaction conditions: 1 (0.30 mmol), 2a (0.10 mmol), 3a (0.125
mmol), PdCl2 (10 mol %), AgTFA (20 mol %), K2CO3 (2.0 equiv),
MeCN (1.0 mL), 100 °C, 6 h. Yield is isolated yield.
group (2ap) and a (tert-butyl)dimethylsilyl group (2aq) were
found to be suitable substrates, converting to the target
products 4ap and 4aq in moderate yields. However, neither
(bromoethynyl)trimethylsilane (2ao) nor (bromoethynyl)-
triphenylsilane (2ar) could be applied to this system,
indicating that this transformation might be more sensitive
to the steric hindrance of the haloalkyne substrates. In
addition, only a trace amount of the desired product was
detected by GC−MS when (bromoethynyl)benzene (2as) was
tested.
2,5-dihydro-1H-pyrrole, 1-methylpiperazine, thiomorpholine,
and indoline, were successfully converted to the propiolami-
dine products 4ae−4aj in 48−80% yields. The configuration of
the product 4aj was determined by X-ray crystallography
analysis (CCDC 1916428). Additionally, the transformations
of secondary amines with a cyano or benzyl group provided the
target products 4ak and 4al in 68% and 75% yields,
respectively.
To further evaluate the practicality and synthetic utility of
this reaction, a gram-scale experiment was carried out.
Gratifyingly, the desired propiolamidine was isolated in 70%
yield (Scheme 5a). Then a series of conversions of 4a were
performed, and different valuable structures could be
constructed. For example, the diphenylpropiolimidamide 6
was easily obtained via hydrolysis and Sonogashira coupling
Besides serving as versatile catalysts and ligands in organic
synthesis, it is well-known that amines are common and
important starting materials for many industrial products.
Thus, the late-stage functionalization of amines is of great
significance (Scheme 4). α-Methylbenzylamine (1am) was
Scheme 4. Late-Stage Functionalization of Multifunctional
Amines
a
Scheme 5. Derivatizations of Propiolamidine 4a
initially tested in this reaction, and the desired product 4am
was isolated in 65% yield. Notably, when the dehydroabietic
amine 1an, a multifunctional amine widely used in the rubber
industry, food manufacturing, and exploration of chiral
reagents,14 was subjected to this transformation, the function-
alization product 4an could be obtained in 84% yield,
suggesting the potential applications in modification of
complex molecular skeletons.
The effect of various haloalkynes on this transformation was
next investigated. Initially, when the halogen atom was
changed from bromine to chlorine or iodine, the propiolami-
dine product 4a was isolated in 78% and 53% yields,
respectively (Table 2, entries 1−3). Subsequent studies of
bromoalkynes with different silyl groups afforded quite
interesting results. The bromoalkynes bearing a triethylsilyl
a
Reaction conditions: (a) PdCl2 (5 mol %), AgTFA (10 mol %),
K2CO3 (1.5 equiv), MeCN (20 mL), 100 °C, 6 h; (b) (i) TBAF (1 M
in THF, 1.0 equiv), THF (2.0 mL), rt, 5 min, (ii) PdCl2(PPh3)2 (4
mol %), CuI (15 mol %), PhI (0.25 mmol), Et3N (2.0 mL), rt, 24 h;
(c) (i) TBAF (1 M in THF, 1.0 equiv), THF (1.0 mL), rt, 5 min, (ii)
CuI (10 mol %), benzylazide (1.2 equiv), MeCN (1.5 mL), 90 °C, 3
h; (d) AgOTf (5 mol %), HOTf (10 mol %), toluene (1.0 mL), 110
°C, 10 min; (e) NIS (1.5 equiv), CuI (10 mol %), MeCN (1.0 mL),
80 °C, 15 min; (f) NH2OH·HCl (1.5 equiv), EtOH (1.0 mL), 100
°C, 30 min.
C
DOI: 10.1021/acs.orglett.9b03201
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Letter
reaction of propiolamidine with iodobenzene (Scheme 5b).
Additionally, the click reaction of 4a gave a functionalized
triazole skeleton 7a in 62% yield (Scheme 5c). Furthermore,
the cyclization reaction of 6 catalyzed by AgOTf afforded 2-
aminoquinoline product 8 in good yield (Scheme 5d). More
interestingly, another important class of 2-aminoquinoline
derivative, 3-iodo-2-aminoquinoline 9, was produced in 75%
yield via electrophilic cyclization of 4a using N-iodosuccini-
mides as electrophiles (Scheme 5e). Finally, N,3-diphenylisox-
azol-5-amine (10) was synthesized in acceptable yield in the
presence of a slight excess of NH2OH·HCl (Scheme 5f).
In order to better clarify the reaction mechanism, several
control experiments were conducted. First, a 25% yield of
phenylmethanediimine (1a′) was isoalated when aniline 1a
and isocyanide 3a were employed as substrates. Considering
that the carbodiimide might be the intermediate, 1a′ was
prepared and reacted under the optimal conditions. However,
the reaction afforded no desired product 4a, which could
exclude that the carbodiimide was the intermediate for this
transformation (Scheme 6a,b). Moreover, a deuterium-labeling
neously, 1,3-hydrogen migration of 4a′ results in the formation
of the desired product 4a. Alternatively, although the oxidative
addition of a simple Pd(II) species to an alkynyl bromide has
rarely been investigated, the generation of an alkynylpalladium-
(IV) complex with haloalkyne derivatives as oxidative reagents
has been well documented, generally in the presence of a
stoichiometric amount of strong oxidant and a stable
cyclopalladium intermediate.17 Thus, another mechanism
involving a Pd(II)/Pd(IV) catalytic cycle could not be fully
excluded (see the Supporting Information for details).
In conclusion, we have developed a convenient approach for
different propiolamidines via palladium-catalyzed multicompo-
nent reaction of isocyanides, haloalkynes, and amines. This
transformation featured in the concomitant construction of
C(sp)−C(sp2) and C(sp2)N bonds with broad substrate
scope. More importantly, the practicability of this method was
demonstrated by the synthesis of diverse useful skeletons, such
as aminoquinolines, isoxazoles, and functionalized triazoles.
Further exploration of the reaction mechanism and synthetic
applications is ongoing in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
Scheme 6. Control Experiments
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b03201.
Experimental procedures, condition screening table,
characterization data, and copies of NMR spectra for
all products (PDF)
Accession Codes
CCDC 1916418, 1916428, and 1920494 contain 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 Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
experiment illustrated that the free hydrogen atom in the
propiolamidine product originated from aniline rather than
H2O in this system (Scheme 6c).
On the basis of these experiments and previous studies,7,8,15
a possible catalytic cycle for this reaction was proposed in
Scheme 7. Initially, oxidative addition of bromoalkyne 2a to
the Pd(0) species affords the acetylpalladium species I.16
Subsequent migration insertion of tert-butyl isocyanide (3a)
generates the imidoyl Pd(II) intermediate II, which would be
trapped by aniline (1a) to produce the key palladium complex
III. Finally, reductive elimination provides the unstable
product 4a′ and releases the active Pd(0) species. Simulta-
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: cewuwq@scut.edu.cn.
ORCID
Huanfeng Jiang: 0000-0002-4355-0294
Wanqing Wu: 0000-0001-5151-7788
Notes
Scheme 7. Possible Mechanism
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the National Key Research and Develop-
ment Program of China (2016YFA0602900), the National
Natural Science Foundation of China (21672072 and
21472051), and the Guangdong Natural Science Foundation
(2018B030308007) for financial support.
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DOI: 10.1021/acs.orglett.9b03201
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