Transition-metal-free insertion reactions of alkynes into the C-N σ-bonds of imides: Synthesis of substituted enamides or chromones

Article abstract of DOI:10.1039/c8cc03059f

A transition-metal-free tandem process for the synthesis of substituted enamides and chromones is presented. The insertion of isolated alkynes into the C-N σ-bonds of imides is involved in this tandem process. In the case of alkynones bearing an ortho-bro

Full text of DOI:10.1039/c8cc03059f

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
=
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Transition-Metal-Free Insertion Reactions of Alkynes into C-N σ-
Bond of Imides: Synthesis of Substituted Enamides or Chromones
Received 00th January 20xx,
Accepted 00th January 20xx
Zhong Zheng, Ye Wang, Murong Xu, Lingkai Kong, Mengdan Wang and Yanzhong Li*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
transition-metal-free tandem process for the synthesis of active compounds.12 Since the lone-pair electrons of the
substituted enamides and chromones is presented. An insertion of nitrogen atom are delocalized onto the carbonyl group,
isolated alkynes into C-N σ-bond of imides is involved in this rendering the C-N bonds of imides stronger and a challenging
tandem process. In the cases of alkynones bearing an ortho- goal for
a
direct insertion reaction.13 Nevertheless,
bromo-substituted aryl ring, chromones were selectively formed considerable efforts have been devoted to develop direct
via O-cyclization pathway. A variety of substituted enamides and alkyne or alkene insertion into C-N bonds, particularly in an
chromones were prepared in good to high yields.
Enamides are an interesting class of compounds as both
occurrence in natural products1 and versatile building blocks in
O
organic synthesis,2 particularly in the synthesis of chiral amines
Matsubara & Kurahashi et al.: Nickel-Catalyzed Decarbonylative Addition of Phthalimides to
Alkynes
by enantioselective hydrogenation.3 On the other hand, the
O
O
10 mol% Ni(cod)2
40 mol% PMe3
Ar
Pr
N
chromone ring is a common crucial structure in numerous of
naturally occurring products.4 It also plays important role in
medicinal chemistry and drug research.5 Despite of their wide
utility in organic chemistry, efficient approaches to the
synthesis of highly functionalized enamides or chromones6 are
still scarce. Traditional procedures for the synthesis of
chromone derivatives usually require multi-step preparation of
substrates7 or transition-metal catalysis,8 and the formation of
side-products or severe decomposition of starting materials
might lead to unsatisfactory yields.5a, 9 Hence, it is an intriguing
goal to synthesize highly functionalized enamides and
chromones in a high atom economy manner with readily
available substrates.
+
N
Ar
Pr
Pr
(b)
(c)
PhMe, 110 °C
O
Pr
Stoltz et al.: Synthesis of Aryl Ketoamides via Aryne Insertion into Imides
O
O
O
TMS
2.0 eq. TBAT
PhMe, 60 °C
+
N
NH
OTf
H
O
This work: transition-metal-free C-N sigma-bond insertion/cyclization with ynones
O
O
R3
R2 NH
R3
O
O
O
R1
K2CO3
(d)
+
R3
N
H
R3
R1
R2
O
1a
2a
enamide 3, up to 85 % yield
Br
O
O
O
O
O
N-cyclization
R3
R3
K2CO3
R1
+
2a
R1
R1
HN
R2
R2
N
R2
Br
R3
O
R3
O
3a
(e)
The direct insertion of unsaturated bonds into C-N σ-bonds
is a useful and efficient transformation in organic synthesis,
since it allows the construction of C-C and C-N bonds at the
same time without any byproducts.10 However, due to the
inert nature of C-N σ-bonds, it is challenging to achieve direct
insertion under mild conditions. Imides are bench available
chemicals, and versatile building blocks in a variety of chemical
transformations,11 especially in the preparation of biologically
not detected
R3
O
O
O
R2
O
HO
R2
N
O-cyclization
O
N
R3
R1
R3
R3
Br
5
chromone , up to 95 % yield
Scheme 1. Insertion of carbon-carbon triple-bonds into C-N σ-bonds
intramolecular manner or others.14a-e Murakami et al. reported a
Pd-catalyzed intramolecular insertion of alkenes into the C−N bond
of β‑lactams (Scheme 1a).13 A Ru-catalyzed intramolecular C-N
bond insertion with formyl translocation was achieved by Li
group.14e Pd-catalyzed synthesis of functionalized indoles via
intramolecular alkyne insertion into C-N bonds was reported by Liu
et al.15b The intermolecular alkyne insertion into C-N bonds was
disclosed by Matsubara and Kurahashi et al., which involved a Ni-
catalyzed decarbonylation of phthalimide carbonyl (Scheme 1b).16a
Transition metal free insertion reactions of alkynes into C-N σ-
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bonds have also been described,17 but they are mainly relied on
using reactive arynes (Scheme 1c)18 or heterocyclic arynes,19 and
the scope of substrates were limited by the cyclic aryne precursors.
We have disclosed that insertion of isolated alkynes into C-C σ-
bonds of dicarbonyl compounds could occur under transition-
metal-free conditions.20 As the continuation of our interest in
activation of inert C-X (X = C, N etc) chemical bonds, we envisaged
that a base-promoted insertion of isolated alkyne into C-N σ-bond
could occur with appropriate substrates such as imide. With two
carbonyls bounded to the nitrogen atom, the acidity of imide N-H is
increased and more susceptible to form anion under basic
even after 12 hours. Increasing the amount of K2CO3 (entry 15)
slightly reduced the yield to 75%. The formation of 3a was
totally inhibited in the absence of a base (entry 16). These
results indicated that a basic condition is mandated for this
process. Other polar solvents such as DMF and DMAc afforded
lower yields of 3a. Structure of 3a was confirmed by X-ray
crystallography.21
Table 2. Synthesis of tetrasubstituted enamides a, b
conditions. Herein we wish to report
a transition-metal-free
protocol which involves insertion of readily available ynones into C-
N σ-bonds of imides, resulting in tetra-substituted enamides.
Moreover, by using ortho-bromo-substituted ynones, multi-
substituted chromones were obtained via a selective O-cyclization
pathway. To the best of our knowledge, there have been no reports
on the insertion of isolated alkynes into C-N σ-bonds under
transitional-metal-free conditions.
Table1. Optimization of reaction conditions for the formation of enamide 3a a,b
a Reactions were conducted on a 0.2 mmol scale with the ratio for 1:2 = 1.0:2.0 under
N2. b Isolated yields.
With the optimal reaction condition in hand (Table 1, entry
3), the scope of this reaction was subsequently investigated
(Table 2). Firstly, substituent effects on the aryl ring of the
imides were studied (Table 2, 3a-3c). It was cleared that imides
with electron-neutral (R3 = Ph), electron-donating (R3 = 4-
MePh) and electron-withdrawing groups (R3 = 4-ClPh) on the
aryl ring were suitable for this reaction, offering the desired
enamides 3a-3c with moderate to high yields. However, a
strong electron-donating group (R3=4-OMePh) afforded no
desired product. It should be noted that enamides 3b and 3c
were obtained as 1:1 isomers, possibly due to the imine-
enamine tautomerization of the enamide. Substituent effects
of R1 on the aryl ring were subsequently explored (Table 2, 3d-
3i). Substrates with electron-donating groups (4-Me, 4-OMe
and 3,4,5-(OMe)3) were suitable for this reaction, giving the
a All reactions were carried out under N2 atmosphere with a 0.2 mmol scale.
b Isolated yields.
We started the investigation by using acyclic imide 2a as
model substrate. When the reaction of ynone 1a with imide 2a
was carried out using K2CO3 (2.0 eq.) in DMSO at room
temperature, no reaction was observed even after 24 h (Table
1, entry 1). With an elevated temperature of 80 oC, to our
delight, the desired enamide could be obtained in 68% yield
(entry 2). Further increasing the reaction temperature to 130
oC led to an increased yield of 80% (entry 3). Changing the
amount of imide 2a did not give better results (entries 4 and
5). When the ratios of ynone and imide were changed to 2:1
and 1.5:1, 3a were obtained with 74% and 60% yield,
respectively (entries 6 and 7). By employing Cs2CO3 as the base
gave the desired product with 78% yield in less than one hour
(entry 8). Other inorganic bases, such as K3PO4 (entry 9), tBuOK
(entry 10) and NaOH (entry 11) were less effective, provided
3a with 38-60% yields. KOAc (entry 12) and organic bases such
as Et3N and 1,4-diazabicyclo[2.2.2]octane (DABCO) (entries 13
and 14) were ineffective, resulting in trace amounts of product
corresponding products 3d-3f with yields ranging between
58%-67%. Electron-withdrawing R1 groups (4-Cl, 4-Br) gave
slightly better yields than electron-donating groups, affording
3g and 3h in 63% and 76% yields, respectively. A furan-
substituted ynone was also compatible, offering 3i in 73%
yield. Napthtyl substituted substrate could give the
corresponding product 3j in 66% yield as well. When ynones
bearing the same substituents as that of imide (R3=R1) were
employed, the desired products 3k and 3l were obtained in
62% and 80% yields respectively as a single isomer. Next, R2
substituents of the aryl ring on the triple bond were screened.
The electron-donating aryl group (4-PhMe, 4-PhOMe) gave the
2 | J. Name., 2012, 00, 1-3
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corresponding products 3m and 3n in 69% and 41% yields,
To clarify the reaction mechanism, control experiments were
An enamide
respectively. Electron-withdrawing aryl group (4-ClPh) also performed using ynone 4a and imide 2a
.
afforded desired 3o with 58% yield. However, alkyl intermediate 3p was obtained in 78% yields under 50 oC with a
substituents such as methyl and n-butyl failed to give the longer reaction time of 7.5 h (Scheme 2). When 3p was
desired products.
Table 3. Synthesis of chromones a,b
exposed to the optimized reaction condition, the desired 5a
was obtained with 75% yields in 2.5 h. In the absence of K2CO3,
the formation of 5a was not observed even after 12 h, and
85% intermediate 3p was recovered. This indicates that a basic
condition is mandated for both the C-N bond cleavage step
and the cyclization step.
O
O
R2
R3
O
O
O
O
2.0 eq. K2CO3
DMSO
130 o
N
R3
+
R1
R1
R3
N
R3
R2
H
Br
C
chromone 5
ynone 4
imide 2
O
O
O
O
O
O
O
O
F
N
R3
N
Ph
N
Ph
Ph
R3
F
Ph
O
R3=Ph, 5a, 2 h, 75%
5d, 1 h, 67%
5e, 2.5 h, 75%
R3=4-MePh, 5b, 2 h, 66%
R3=4-ClPh, 5c, 1 h, 94%
O
O
R2
O
O
O
O
O
N
Ph
Cl
MeO
MeO
N
Ph
N
Ph
Ph
R2=4-ClPh, 5h, 1 h, 73%
O
Ph
O
Ph
R2=1-Naphthyl, 5i, 3 h, 33%
R2=4-MePh, 5j, 1 h, 80%c
R2=4-MeOPh, 5k, 3 h, 86%c
R2=3,4,5-(MeO)3Ph, 5l, 2 h, 94%c
R2=Me, 1 h, --
5g, 3.5 h, 56%c
Scheme 2. Control experiments
5f, 1 h, 62%
OMe
Based on our experimental results and previously reported
works,20 a plausible reaction mechanism has been proposed
MeO
O
OMe
O
(Scheme 3). Firstly, under
undergoes a Michael-type addition with ynone to give anion
intermediate which presumably undergoes an
a basic condition, imide 2a
N
F
O
A
,
5m, 3.5 h, 78%c
X-ray of 5e
intramolecular nucleophilic addition/ring-opening sequence
resulting in a alkyne insertion into C-N σ-bond to afford
a Reactions were conducted on a 0.2 mmol scale with the ratio for 4:2 = 1.0:2.0 under
N2. b Isolated yields. c Ratio for 4:2 changed to 2:1.
intermediate
enamide 3a (X=H). With ortho-bromo-substituted ynone (e.g.:
X=Br), imine-enamine tautomerization of intermediate
followed by nucleophilic aromatic substitution (SNAr)22 gives
chromone 5a
C. Hydrolysis of C leads to the formation of
With a series of enamides being prepared, we postulated
that ynones bearing ortho-bromide-substituted phenyl ring on
the carbonyl would undergo an intramolecular nucleophilic
aromatic substitution (SNAr)22 with oxygen or nitrogen atom to
give heterocyclic products. To our delight, when the reaction
of 1-(2-bromophenyl)-3-phenylprop-2-yn-1-one 4a with 2a was
performed under the optimal reaction conditions, a desired
chromone 5a was selectively generated in 75% yield, and no N-
cyclization product was detected. Next, we engaged in
expanding the substrate scope to synthesize substituted
chromones (Table 3). Firstly, the effect of the substituent R3 of
the imide aryl ring was explored. Good to excellent yields of
chromones were acquired irrespective of the presence of
electron-donating (R3=4-PhMe) or electron-withdrawing
C
.
groups (R3=4-PhCl) (Table 3, 5a 5c). Next, the R1 substituents
-
on the aryl ring was explored. Both electron-donating and
electron-withdrawing groups on the aryl substituent R1 were
tolerable to the reaction conditions, affording the
Scheme 3. Proposed mechanism
In summary, we have developed an efficient base-
promoted procedure for the synthesis of substituted enamides
and chromone derivatives starting from ynones and imides.
This procedure is featured with the insertion reaction of
ynones into C-N σ-bond as key process, which represents the
first example of base promoted insertion reaction of isolated
internal alkynes into C-N σ-bonds. It provides an atom-
economic and highly efficient way for the preparation of
densely-functionalized enamides and chromones. Further
efforts to explore the insertion reaction for the synthesis of
various types of carbo- or heterocyclic compounds are in
progress in our lab.
corresponding products 5d-5g in good to high yields. The
scope of R2 substituents on the triple bond termini was also
investigated. Various aryl groups were applicable to this
reaction, including both electron-deficient and electron-rich
aryl rings (5h-5l). Naphthyl substrate afforded the desired 5i
with a lower yield, possibly due to the steric hindrance.
Electron-rich aryl ring led to better results, in which 3,4,5-
trimethoxyphenyl substrate gave product 5l with 94% yield.
Substrate with 3,4,5-trimethoxyphenyl R2 and electron-
withdrawing R1 group (2-Br-4-F) could also afford product 5m
in 78% yield.
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Acknowledgement
We thank the National Natural Science Foundation of China
(grant no. 21272074) and the Program for Changjiang Scholars
and Innovative Research Team in University (PCSIRT) for
financial support.
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Conflicts of interest
There are no conflicts to declare.
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DOI: 10.1039/C8CC03059F
Transitionalꢀmetalꢀfree insertion of ynones into CꢀN σꢀbonds of imides for the
chemoꢀselective synthesis of chromones or enamides.