Intermolecular iodofunctionalization of allenamides with indoles, pyrroles, and furans: Synthesis of iodine-substituted: Z -enamides

Article abstract of DOI:10.1039/c6cc05046h

A new method was developed to synthesize iodine-substituted Z-enamides through N-iodosuccinimide-mediated intermolecular iodofunctionalization of allenamides with indoles, pyrroles, and furans. These reactions proceed rapidly and tolerate a broad scope of substrates. The conjugated sulfimide ion species probably acts as the key intermediate.

Full text of DOI:10.1039/c6cc05046h

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Intermolecular Iodofunctionalization of Allenamides with indoles,
pyrrole, and furan: Synthesis of iodine-substituted Z-enamides
Honghe Li, Xiaoxiao Li*, Zhigang Zhao*, Ting Ma, Chenyang Sun and Bowen Yang
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A new method was developed to synthesize iodine-substituted Z-
enamides through N-iodosuccinimide-mediated intermolecular
iodofunctionalization of allenamides with indoles, pyrrole, and
furan. These reactions proceed rapidly and tolerate a broad scope
of substrates. The conjugated sulfimide ion species probably acts
as the key intermediate.
R
EWG
N
previous work
LM
H
α
LMX
X
Nu
A
B
a
γ
NuH
R
R
.
Nu
N
N
R
or
N
b
EWG
EWG
R
EWG
EWG
N
BA*-H
α
Iodine
reagent
BA*
c
γ
R
N
R
NuH
1,4-addition
R
N
The regio- and stereo-controlled functionalization of carbon–
carbon double bonds has enormous potential in organic
synthesis.1 Among them, allenamides have recently proven to
be important synthetic intermediates, undergoing diverse
interesting transformations.2 In particular, this class of
“electron-rich” π systems is receiving growing attention in
organic synthesis through metal-assisted electrophilic
activation.3 The coordination of transition-metal complexes to
Nu
N
I
I
EWG
EWG
I
EWG
This work
Nu = indoles, pyrrole, furan
Fig. 1 Conventional (metal) and
free) electrophilic activation of allenamides.
“unconventional” (metal-
elegant examples of intramolecular cyclizations involving
allenamides have been reported to date.9
Enamides10 are an interesting class of substrates because of
their use in the construction of heterocycles and chiral amines
as well as their occurrence in several natural product
frameworks.11 Our research group has focused on the
intermolecular cycloaddition12 or nucleophilic addition13 of
allenamides via gold catalysis to afford the corresponding
enamides. To the best of our knowledge, the iodine reagent
mediated intermolecular nucleophilic addition of allenamides
has not been reported. In this paper, we report the N-
the allenyl framework
intermediates that can smoothly undergo condensation with
a variety of nucleophilic agents both at the α or γ position
(Figure 1, pathway ). However, few examples of the metal-
“C=C=C” affords positively charged
A
a
free electrophilic activation of allenamides have been reported
so far.4 Recently, Bandini and co-workers reported a chiral
Brønsted acid (BA)-assisted dearomatization of indoles (NuH)
through the electrophilic activation of allenamides (Figure 1,
pathway b
).5
In the recent years, iodine reagents have received
considerable attention as inexpensive, nontoxic, and readily
available electrophiles for interactions with double bonds to
create new stereogenic centers in an efficiently controlled
manner (reagent-controlled stereoselective reactions),6 or with
alkynes to create vinyl iodides,7 thus allowing diverse
functionalization using metal-catalyzed coupling chemistry.8
Iodine electrophile activates allenamides, however, still poses
challenges with respect to low regioselectivity. Only a few
iodosuccinimide-mediated
intermolecular
iodofunctionalization of allenamides with indoles, pyrrole, and
furan, providing iodine-substituted enamides. The reaction
generates a new C–C bond at the γ-position of allenamides and
affords iodine-substituted enamides with Z-configuration
(Figure 1, pathway c).
Based on our previous results with respect to the catalyst-free
addition of indoles to allenamides,14 we began our studies by
exploring the reaction between phenyl allenamide (1a) with 1-
methylindole (2a). The optimization results are summarized in
Table 1. The reaction afforded the desired product 4a in 9%
yield under the promotion with elemental Iodine electrophiles
in DCM, along with significant amounts of degradation product
4-methyl-N-phenylbenzenesulfonamide (Table 1, entry 1). The
use of NIS as the promoter afforded 4a in 66% yield, along
College of Chemistry and Environmental Protection Engineering, Southwest
University for Nationalities, Chengdu 610041, PR China
Email: lixiaoxiao.2005@163.com; zzg63129@163.com
Electronic Supplementary Information (ESI) available: Experimental details,
compound characterization, and NMR spectra. CCDC 1448325 for 7i and CCDC
1479962 for 7l. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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a
Table 1 Screening of optimal conditionsa
Table 2 Substrate scope of allenamides (
1)
R
R
3b (1.05 equiv.)
.
N
N
+
N
CH3CN, rt
PG
PG
I
N
1a-p
2a
4a-p
Allenamide
Product
4
(%)b
Entry
R
PG
5
(%)c
1
1
1a
Ph
Ts
Ts
Ts
Ts
Ts
Ts
Ts
Ts
4a (86)
4b (84)
4c (89)
4d (79)
4e (79)
4f (80)
4g (78)
4h (83)
4i (84)
4j (84)
4k (77)
4l (81)
4m (81)
4n (86)
4o (82)
4p (71)
8
9
2
1b
1c
1d
1e
1f
4-MeC6H4
2-MeC6H4
4-MeOC6H4
3-FC6H4
3
8
1a/2a
Iodine reagent
(equiv.)/sol
Yield of
4a (%)c
Yield of 5a
Entry
(equiv.)
(%)d
4
8
5
9
1
2
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:2
1:1.2
1:0
3a (3)/DCM
3b (3)/acetone
3b (2)/acetone
3b (1.05)/acetone
3b (1.05)/DCM
3b (1.05)/CHCl3
3b (1.05)/CCl4
9
ND
6
4-FC6H4
8
66
67
69
35
40
22
63
86
26
55
70
86
78
0
18
7
1g
1h
1i
4-BrC6H4
8
3
15
8
2,4-(Me)2C6H4
8
4
10
9
3,5-(MeO)2C6H4 Ts
8
5
18
10
1j
Bn
Ts
Ts
Ts
Ts
Ts
<3
<3
<3
<3
<3
<3
<3
6
7b
18
11
1k
1l
4-MeOC6H4CH2
4-FC6H4CH2
PhCH2CH2
n-Bu
33
12
8
3b (1.05)/DCE
31
13
1m
1n
1o
1p
9
3b (1.05)/CH3CN
3b (1.05)/toluene
3a (1.05)/CH3CN
3c (1.05)/CH3CN
3b (1.05)/CH3CN
3b (1.05)/CH3CN
3b (1.05)/CH3CN
8
14
10
11
12
13
14
15e
29
15
3,5-(MeO)2C6H4 Ac
2-oxazolidinone
7
16
a Conditions:
14
1 (0.1 mmol), 2a (0.2 mmol), CH3CN (3 mL), rt,
b
8
c
within 1 min. Yield of isolated product. The yield of
determined from the 1H NMR spectral data.
5 was
9
31%/6%(6a)
a
Under the optimal conditions, the substrate scope of
allenamides was investigated in reactions with 1-
methylindole (2a) (Table 2). Substrates containing an aryl
Unless noted, all the reactions were carried out at 0.1 mmol
b
scale in 3 mL solvent at rt within 1 min. 1a was consumed
(1)
c
within 5 min. Yield of isolated product. Yield of 5a was
determined by the 1H NMR spectroscopy of the reaction
d
1
group substituted with an electron-donating or electron-
withdrawing group, such as methyl, methoxy, fluoro, and
bromo, efficiently reacted with 2a to afford the desired
products 4b–g in 78–89% yields, exhibiting no obvious
substituent effect (Table 2, entries 2–7). Both 2,4-dimethyl-
and 3,5-dimethoxy-substituted phenyl allenamides efficiently
reacted with 2a to afford 4h (83%) and 4i (84%), respectively
(Table 2, entries 8–9). Although benzyl allenamide 1j and 4-
fluoro-substituted benzyl allenamide 1l were suitable for
preparing 4j and 4l in 84% and 81% yields, 4-methoxy
substituted benzyl allenamide 1k gave the corresponding
product 4k in 77% yield (Table 2, entries 10–12). Furthermore,
the reactivity of different aliphatic substituted allenamides was
also investigated. Both phenethyl and n-butyl substituted
allenamides produced products 4m and 4n in 81% and 86%
yields, respectively (Table 2, entries 13 and 14). The reaction is
also efficient with allenamides when acyl was used in the place
of tosyl as the amino-protecting group. Thus, the reaction of
3,5-dimethoxy-substituted phenyl allenamide 1o with an acyl
substituent furnished 4o in 82% yield, 2-oxazolidinone
allenamide 1p also provided the corresponding adduct 4p in
71% yield (Table 2, entries 15 and 16). Notably, under the
e
mixture.
Yield
of
4-methyl-N-
phenylbenzenesulfonamide/5a
/
6a = 36%/31%/6%.
with 5a as a by-product in 18% yield as determined from the
1H NMR spectral data (Table 1, entry 2). A decrease in the
iodine reagent loading to 2.0 equiv. led to a slightly increased
reaction yield of 67% (Table 1, entry 3). Moreover, the yield of
4a increased to 69% when the iodine reagent loading was
reduced to 1.05 equiv. (Table 1, entry 4). Subsequent
screening of the solvents led to an increase in the yield to 86%
when CH3CN was used as the solvent, whereas DCM, CHCl3,
CCl4, DCE, and toluene were not as effective (Table 1, entries
5–10). Notably, 55% yield of 4a could also be obtained when
1.05 equiv. of electrophile 3a was used in CH3CN (Table 1,
entry 11). The process was also promoted with 3c (Table 1,
entry 12). Similar results were obtained when the amount of
2a was reduced to 2.0 equiv. (Table 1, entry 13). The yield of
4a decreased to 78% when the loading of 2a was only 1.2
equiv. (Table 1, entry 14). In the control experiment, 4-methyl-
N-phenylbenzenesulfonamide, 5a, and 6a were obtained in
36%, 31%, and 6% yields, respectively without 2a as the
nucleophile (Table 1, entry 15).
optimal conditions, the yields of the by-product 5 were <9%
2 | J. Name., 2012, 00, 1-3
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a
Table 3 Substrate scope of indole substrates (2)
3b
3b
(1.05 equiv.)
CH3CN, rt
+
+
.
N
N
H
N
(1)
(2)
HN
I
Ts
1a
Ts
2j
7j (62%),5j (16%)
(1.05 equiv.)
CH3CN, rt
N
.
.
N
O
O
I
Ts
Ts
1a
2k
7k (67%), 5k (7%)
Indole
2
Product
(%)
Entry
R1
R2
R3
5
(%)c
N
7
N
3b (1.05 equiv.)
N
N
+
(3)
1
2
3c
4d
5
2b
H
5-MeO
5-F
H
H
H
H
7b (57)
7c (77)
7d (43)
7e (50)
7f (80)
7g (66)
7h (66)
7i (70)
18
13
11
25
10
19
15
14
N
N
CH3CN, rt
H
Ts
I
Ts
2c
2d
2e
2f
1a
2l
7l (96%)
H
H
Scheme 1 Iodofunctionalization of other substrates.
5-Br
4-Me
7-Me
H
H
H
H
H
obtained in 62% and 67% yields, whereas imidazole 2l afforded
the 1,2-addition product 7l in 96% yield, providing the
evidence of the reaction mechanism. Moreover, 7l was a
crystalline solid, and single-crystal X-ray analysis confirmed its
structure (Fig. 2, right).15
Based on our experimental results and precedents in the
literature,9b,16 we propose a mechanism for the reaction as
shown in Scheme 2. The first step of the reaction involves
electrophilic halogenation via interaction between iodine
electrophile and the π system of allenamide 1a to form
6
2g
2h
2i
H
H
7
Me
H
H
8
H
Bn
a Conditions: 1a (0.1 mmol),
2 (0.2 mmol), CH3CN (3 mL), rt,
within 1 min.
b Yield of isolated product.
c The yield of was determined from the 1H NMR spectral data.
5
d Isolated yield after 24 h.
iodiranium intermediate I. This affords the key intermediate
conjugated sulfimide ion species II, that undergoes a
when 1-methylindole was used as the nucleophile.
decyclization reaction through the delocalization of the
nitrogen lone pair towards the alkene. Subsequently, II
undergoes 1,4-addition with nucleophile 2a, affording iodine-
substituted enamide 4a. The formation of 1,2-addition product
7l indicates that the key intermediate is the conjugated
Next, indole substrates
2 were investigated by reacting with
phenyl allenamide (1a) (Table 3). Indole 2b and 5-MeO-
substituted indole derivative 2c were successfully converted
into 7b and 7c in 57% and 77% yields, respectively (Table 3,
entries 1 and 2). The indole substrates with halo substituents,
however, showed low reactivities. For example, 5-fluoroindole
2d and 5-bromoindole 2e afforded products 7d and 7e in only
43% and 50% yields, respectively, even after 24 h (Table 3,
entries 3 and 4). Methyl substituents were also placed at
sulfimide ion species II
.
N
N
N
Ts
7l
I
different positions of the indolyl core (2f–2h), resulting in
satisfactory yields. 4-Methylindole provided the C3-addition
product 7f in 80% yield (Table 3, entry 5), whereas 2-
methylindole and 7-methylindole provided the C3-addition
products 7g and 7h both in 66% yield (Table 3, entries 6 and 7).
Indole bearing an N-benzyl group was also able to furnish 7i in
70% yield, and the absolute configuration of the
intermolecular nucleophilic addition products was further
determined to be (Z) by the single-crystal diffraction study of
7i (Fig. 2, left).15 The yields of the by-products were in the 10–
20% range according to their 1H NMR spectral data.
2m 1,2-addition
"I+ source"
.
N
N
N
I
Ts
Ts
Ts
I
Iodiranium
intermediate I
1a
sulfimide ion II
2a 1,4-addition
N
In an attempt to extend this method, we explored the
iodofunctionalization of other heterocyclic substrates, such as
pyrrole 2j, 2-methylfuran 2k, and imidazole 2l. As shown in
scheme 1, C2-addition products 7j and 7k were successfully
I
Ts
N
4a
Scheme 2 Proposed reaction mechanism.
To prove the practicality of this iodine-promoted reaction, a
gram-scale synthesis of the iodine-substituted enamide 4a was
performed. When 1.09 g of allenamide 1a (4 mmol) was used,
1.63 g of the desired product 4a was obtained in 85% yield
within 1 min, indicating that this transformation is easy to
scale up to gram scale without a loss in efficiency (Scheme 3).
To further demonstrate the potential application of this
protocol, 4a was reacted with phenylboronic acid under Suzuki
Fig. 2 X-ray structure of compound 7i (left) and 7l (right).
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6
C. Romano, M. Jia, M. Monari, E. Manoni and M. Bandini,
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Scheme 3 Gram-scale synthesis and synthetic application.
cross-coupling conditions17, the corresponding coupling
7
8
For reviews of double bonds activation via iodine reagents,
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2011, 7. 847-859; (b) Y. Yamamoto, I. D. Gridnev, N. T. Patil
product
9
was isolated in good yield.
In summary,
a
facile synthetic protocol for iodine-
and T. Jin, Chem. Commun., 2009, 5075-5087.
substituted Z-enamides was established through NIS-mediated
intermolecular iodofunctionalization of allenamides with
indoles, pyrrole, and furan, affording the desired 1,4-addition
products in good yields under mild conditions. Moreover,
when imidazole was used as the nucleophile, the
corresponding 1,2-addition product was obtained in a good
yield, indicating that the key intermediate was the conjugated
sulfimide ion species. Importantly, the products of this
reaction, iodine-substituted enamides, should facilitate diverse
functionalization by metal-catalyzed cross-coupling chemistry.
The potential utilization and extension of this interesting
synthetic methodology are currently underway.
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This work was financially supported by the Fundamental
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