Palladium-catalysed ligand-free reductive Heck cycloisomerisation of 1,6-en-α-chloro-enamides

Article abstract of DOI:10.1039/c9cc00537d

The first example of an intramolecular hydroarylation of 1,6-en-α-chloro-enamides was achieved by a palladium-catalysed ligand-free reductive Heck cycloisomerisation with no competing Heck-cyclised by-product.

Full text of DOI:10.1039/c9cc00537d

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DOI: 10.1039/C9CC00537D
Journal Name
COMMUNICATION
Palladium-Catalysed Ligand-Free Reductive Heck
Cycloisomerisation of 1,6-En-α-Chloro-Enamides
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
b
c
a,c
Yangyang Hou,† Jing Ma,† Hongyi Yang, Edward A. Anderson, Andy Whiting and Na Wu*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The first example of an intramolecular hydroarylation of 1,6-en-α- elimination by-product was observed.
Sarpong's Heck coupling
chloro-enamides was achieved by a palladium-catalysed ligand-
free reductive Heck cycloisomerisation with no competing Heck-
cyclised by-product.
R3
Pd(OAc)2 (5 mol%)
NaTFA (1 equiv.)
+
R2
R2
R3
N
OHC
N
2
or K CO3 (2 equiv.)
Cl
R1 DMF or 1,4-dioxane
OHC
R1
1
2
0 oC, 1 h
3
8
1
6 examples, 63-93%
Reductive processes in metal-catalysed organic synthesis are often
well understood, involving common reductants such as dihydrogen,
Tang's reduction
H
Cl
Ph
H
H
N
1
Pd(PPh3)2Cl2 (5 mol%)
CuI (3 mol%)
formates, formic acid and activated alcohols. Similarly, palladium-
Ph
N
Ph
Ph
catalysed hydroarylation (reductive Heck reactions) between
arylhalides and alkenes, typically involve: (i) alkylamines, formates
Ms
Et N, DMF, 80 oC
Ms
3
4
5
92%, E:Z > 95:5
Ar2
Cl
This work
Ar2
2
and activated alcohols as the hydride source in the process; (ii)
R
Ar2
R
R
TMSCl
R1
Pd(II)
neutral or anionic aryl–Pd complexes, and electron-poor olefins and
N
Ts
N
R1
2
a,3 and
styrene (preferred olefin substrates for insertion);
(iii) key
N
Ts
6
7
Ts
R1
8
aryl-Pd species, coordinatively saturated by ligands (phosphines, N-
heterocyclic carbenes, halides and acetates) to inhibit β-H-Pd
Scheme 1. Heck coupling and reduction of Chloro-enamides.
4
elimination side-reactions.
Ynamides and enamides are versatile functional groups that are
finding use as fascinating building blocks for the synthesis of
nitrogen-containing compounds. Recently, Sarpong reported
Herein, we report a palladium-catalysed reductive Heck
cyclisation of 1,6-enynamides. In contrast to the common features
of reductive Heck reactions, we report here that: (i) hydroarylation
9
intermolecular Heck coupling reactions of bench-stable α-halo
5
of styrene occurred through an intramolecular hydride transfer
1
0
eneformamides in DMF or 1,4-dioxane and Tang reported a
reduction of the α-halo-enamide to the enamide using Et N as a
and an indolyl alkylpalladium(II)-pecies was reduced through an
6
3
intermolecular hydride transfer likely from i-PrOH (or 1,4-dioxane )
1
1
reductant (Scheme 1). In order to explore the balance of reactivity
and stability of α-halo-enamides, we prepared more electrophilic α-
chloro tosylmides 7a and employed Sarpong’s Heck conditions to
test the potential intramolecular cyclisation of 7a. However, our
approach was distinct from Sarpong’s Heck, in that a reductive Heck
cyclised 8a was obtained exclusively, rather than Heck cyclised 8a’
as H-donor, confirmed by D-isotope exchange studies; (ii) chloride
dissociation of an electrophilic α-chloro-enamide was realised in the
absence of alkylammonium salts as halide abstractors and a cationic
Pd(II)-enamide Heck coupling proceeded with both electron-neutral
7
and electron-rich styrenes; (iii) interestingly, the key enamide-Pd
8
species was free from ligands saturation; and (iv) no β-H-Pd
(
Table 1, entries 1-3). Alternatively, using activated alcohols as the
solvent (which was employed in alkenylpalladative reduction of
a. State Key Laboratory for the Chemistry and Molecular Engineering of Medicinal ynamides by Anderson ), 8a was also afforded in satisfying yields
12
Resources, School of Chemistry and Pharmaceutical Science, Guangxi Normal
University, Gulin, Guangxi, 541004, China. E-mail: wuna07@gxnu.edu.cn.
(entries 4-13). Surprisingly, when electron-rich palladium ligands
were employed, which were expected to prohibit β–H-Pd
elimination according to coordinatively saturation of Pd(II) and
Pd(0), the reductive Heck cyclisation was supressed (entries 14-18).
b. Chemistry Research Laboratory, Oxford University, 12 Mansfield Road, Oxford,
OX1 3TA, UK.
c.
In general, PdCl is more sustainable in 1,4-dioxane (entries 1 and 2)
Department of Chemistry, Durham University, South Road, Durham DH1 3LE, UK.
2
E-mail: na.wu@durham.ac.uk.
than that in i-PrOH, as the precipitation of palladium black was
immediately observed in i-PrOH.
†
These authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Based on the optimised results, entries 2 and 3 (Table 1) were
applied to explore the substrate scope using 2-styryl-α-chloro-
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J. Name., 2013, 00, 1-3 | 1
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Journal Name
enamide derivatives 7 (Table 2). In order to overcome the hydrolytic 6 with alkyl-substituted alkenes and alkyl ynamides respectively,
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1
4
DOI: 10.1039/C9CC00537D
lability of α-chloro-enamides, these species were prepared from in- also led to complex mixtures. We next assessed the benzene ring
situ generated HCl and addition to enynamides 6, which were used of indoles. The reactions (entries 12-17, Table 2) proceeded well to
directly without further isolation.
1
3
deliver products bearing electron-donating or electron-withdrawing
groups on the aniline ring, although the yield dropped to 20% when
C-4 was substituted (entry 17). Noticeably, when en-α,β-dichloro-
enamide 9 was employed, an unusual competing C-O coupling was
Table 1. Optimization of conditions.a
Ph
15
Ph
Ph
found to give an isopropoxide 10 (entry 18).
Pd (cat.)
Cl
K2CO3
N
N
Table 2. Substrate exploration.
N
(
2 equiv.)
Ts
Ts
Ts
8
0 o
C
7a
8a
8a' Not observed
Ar2
PdCl2 (20 mol%)
TMSCl K2CO3 (2 equiv.)
4
R
Ar2
R
3
b
entry
catalyst (mol%)
solvent
1,4-dioxane
1,4-dioxane
1,4-dioxane
i-PrOH
yield (%)
2
R1
N
R1
THF, 0 o
N
C
solvent (0.1 M)
80 oC, 8 h
1
2
3
4
5
6
7
8
9
PdCl
PdCl
2
(2.5)
81
EWG
EWG
6
8
6
h
2
(20)
80
N.R.d
a
(20)c
entry
starting material 6
8
yield (%)
PdCl
PdCl
2
2
(20)
93
R
H
H
H
H
H
H
H
H
H
H
H
EWG
Ts
R1
H
Ar2
Ph
b
c
c
c
Pd(TFA)
none
2
(10)
i-PrOH
81
1
2
3d
8a
8b
79 ; 50
N.R.d
trace
N.R.d
dec.e
dec.e
60
b
i-PrOH
Ts
H
p-MeC6H4
58 ; 18
b
PdCl
2
2
2
2
2
(20)
(20)
MeOH
Ts
H
p-OMeC6H4 8c
73 ; 42
b
c
PdCl
PdCl
PdCl
PdCl
EtOH
4
Ts
H
p-ClC6H4
Ph
8d 56 ; trace
b
c
c
c
c
c
c
c
c
(20)
(20)
(10)
t-BuOH
5
Ts
Ph
8e
8f
77 ; 70
b
1
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
9
toluene
i-PrOH
6
Ts
p-MeC6H4
Ph
47 ; 50
b
7
Ts
p-t-BuC6H4
Ph
8g
8h
8i
70 ; 43
b
PdCl
PdCl
2
(5)
i-PrOH
44
8
Ms
Ns
Ts
H
H
H
H
Ph
49 ; 10
b
2
(2.5)
i-PrOH
44
9
Ph
82 ; 37
f
N.R.d
dec.e
N.R.d
N.R.d
dec.e
trace
10d
11
12d
b
PdCl
PdCl
Pd(PPh
(5), (n-Bu)
(5), (t-Bu)
2
(10), TMTU (20)
i-PrOH
o-OMeC6H4 8j
23 ; 10
g
b
2
(10), bipy (10)
i-PrOH
Ts
m-furanyl
8k
8l
24 ; 10
Ph
b
3
)
4
(5)
P (10)
P (10)
(2.5)
1,4-dioxane
1,4-dioxane
1,4-dioxane
i-PrOH
67 ; 23
Pd(PPh
Pd(PPh
Pd
3
)
4
3
N
Ts
3
)
4
3
3d,e
b
c
MeO
F
Ph
1
8
m
53 ; 11
2
(dba)
3
N
Ts
Ph
b
c
a.7a (0.15 mmol), Pd catalyst, K
b.
14
8n 51 ; N.R.
2
3
CO (0.3 mmol), solvent (4 mL), 80 °C, 6 h, N2. Isolated
c.
d.
e.
f.
yield. Without K
2
CO
3
.
N.R. = no reaction.
dec. = decomposition. TMTU =
N
g.
tetramethyl thiourea. Bipy = 2,2’-bipyridine.
Ts
Ph
Ph
b
c
1
5
8o 49 ; N.R.
In general, the one-pot, sequential cyclisation afforded 3-
benzylindoles 8 in higher yields in i-PrOH than that in 1,4-dioxane
F
N
Ts
1
6d
Cl
8p 39 ; N.R.
b
c
(
Table 2). When the styryl group contained electron-donating
groups (entries 2 and 3, Table 2) and mildly electron-deficient
groups ((entry 4, Table 2), the reductive Heck process proceeded to
deliver products 8 in moderate to good yields. As for the ynamide
fragment, terminal and internal aryl ynamides were tolerated
N
Ts
Cl
b
c
1
7
8q 20 ; N.R.
Ph
N
Ts
(
(entries 1, 5-9, Table 2). When the tosylamide was replaced by Ms-
and Ns-amides, a better yield was obtained for Ns-variant in i-PrOH
(entry 9, Table 2). However, the substrates 6 were restricted to
starting material 9r
10r
Ph
Ph
(
18
para-substituted aryl groups, cyclisation of in-situ generated 7
containing ortho-, meta-substituted aryl groups is more
complicated with slow conversion (20 h), accompanied by complex
mixtures (entries 10 and 11, Table 2). Noticeably, the substrates 6
bearing electron-poor styrene and electron-poor ynamide moieties
were incompatible with the reaction conditions, where complex
mixtures were formed. Replacing styryl and ynamidyl fragments of
Cl
N
Oi-Pr
Ts
N
5
2b; tracec
Ts Cl
a. Isolated yield. b.Isopropanol was utilized as the solvent. c.1,4-Dioxane was utilized as
the solvent. d.The reaction was conducted at 100 ℃ for 18 hours. e.Demethylated by-
product observed.
2
| J. Name., 2012, 00, 1-3
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Given that the configuration of 8o was confirmed by X-ray carbon. This indicates that before the reductive elimination of the
View Article Online
1
6
DOI: 10.1039/C9CC00537D
diffraction analysis (Figure 1), we are able to propose that a novel C-Pd(II)-D bond, palladium is located at the methylene position,
Pd(0)-catalysed reductive Heck cycloisomerisation mechanism rather than the benzylic position, which excludes the possibility of a
(Scheme 2) explains these observations. This is initiated by oxidative pathway to 8 via B’. Furthermore, this result confirmed that the
addition of Pd(0), generated by β-hydride elimination and reductive solvent was indeed involved as a hydride donor in the reduction of
1
7
6
2
elimination via coordination of PdCl with i-PrOH or 1,4-dioxane . the terminal alkylpalladium(II) species.
The intramolecular coordination of the styrene may facilitate
dissociation of the chloride anion to form the cationic Pd(II)-
Ar2
H-PdCl
enamide species A from the highly electrophilic α-chloro-enamide
R1
7
N
EWG
7
.
R
C
H-Pd
re-addition
Ar2

-H-Pd
H
PdCl
Ar2
elimination
Ar2
PdCl
H
PdH
H
R1
N
8
R
EWG
R1
i-PrOH
N
R1
D
N
R
R
EWG
B
B' EWG
allylic palladium
migration
carbopalladation
Ar2
Ar2
HO
H
Ar2
PdCl2
R
H
PdCl
R1
syn--H-Pd
elimination
R
H
PdCl
R1
N
(
II)Pd
HCl
H(H)
EWG
H
N
hydro-
N
O
R
E
EWGR1
palladation
ClPd
EWG
Figure 1. X-ray crystal structure of 8o
i-PrOH
6
-H-Pd
elimination
A
acetone
HCl
 -H-Pd
elimination
(2) reductive
elimination
This is followed by an ionic Heck enamidation of the electron-rich
styrene to afford diene C, which could be understood as arising
from the styrene acting as a Lewis base attacking the electrophilic
HPdCl
reductive
elimination
Ar2
oxidative addition
H
Ar2
Pd(0)
7
palladium(II). There is a driving force for aromatisation through a
H
pseudo-intramolecular reversible re-addition of Pd(II)-H species to
the dienyl indoline,18 which ligates Pd-H, to deliver the indolyl
Cl
N
R1
EWG
R
H
8
R
N
7
EWGR1
palladium species D. Upon alkene migration via the allylpalladium
species, E was delivered. Then, there is a preference for Pd(II) to
transfer methylene hydrogen from 1,4-dioxane or methinyl
hydrogen from i-PrOH, through its coordination with the solvent /
β-hydride elimination / reductive elimination to irreversibly afford 8.
If the cycle is not fast enough, reversible syn-β-H-Pd elimination of
Scheme 2. Proposed overall mechanistic scheme.
Ph
Ph
4
Pd(PPh3) (5 mol%)
PdCl2 (20 mol%)
Ph
N.R.
1
9
N
Ph
toluene, 80 o
overnight, 11%
i-PrOH, N , 80 oC
A and subsequent hydropalladation of ynamide 6 would occur,
C
N
Ts
2
Ts
11e
6
e
allowing the proton exchange between substrate 7 and the solvent.
(2)
Ph
D
D/H
Our next focus was to seek out potential reductants and
determine whether they contribute to the proposed reductive Heck
cycloisomerisation sequence. Firstly, the dienyl indoline 11e, acting
as the presumptive intermediate C in Scheme 1, was prepared via
cycloisomerisation of enynamide 6e. When it was subjected to
PdCl -catalysed, ligand-free conditions in i-PrOH, no reductive
2
product 8e was obtained, implying that the reductive process was
not initiated by an intermolecular H-Pd species generated from
PdCl and i-PrOH. Secondly, to determine the source of the
2
incoming hydrogen atom for the hydroenamidation of the styrene,
we conducted a labelling experiment using 12a with deuterium
labeled at the styryl moiety. Interestingly, 13a was obtained with
deuterium migrated to the benzylic position, which elucidates that
in the reduction of the styrene, the hydride source comes from the
intramolecular H-Pd species, generated by β-H-Pd elimination and
re-addition to the styrene.
Ph
PdCl2 (20 mol%)
TMSCl (stoic.)
THF, 0 oC, 6 h
K2CO3 (2 equiv.)
1,4-dioxane, 80 o
overnight, 42%
N
N
Ts
Ts
C
1
2a
13a D / H = 2.8:1
(
3)
4)
Ph
Ph
D
Ph (1) TMSCl, THF, 0 o
C
D/H
N
(2) PdCl2 (20 mol%)
K CO (2 equiv.)
Ts
N
N
2
3
Ts
14a
Ts
(CD3)2CDOD, 80 o
No
6
a
C
13a
D / H = 4.8 /1
reductive Heck
36%
(
D
Ph
Ph
Cl
TMSCl
PdCl2
Ph
Ph
(
1.2 equiv.)
(10 mol%)
O
NTs
+
D/H
D
D
2
D O (3 equiv.)
N
2 3
K CO
N
N
Ts
Ts
Ts
THF
0 oC, 6 h
D
(2.5 equiv.)
i-PrOH, 80 o
D
C
57% D / H = 3/1
14a
27%
17a
15a
16a
overnight
Scheme 3. Deuterium labelling study
Finally, isotopomer 16a, with two deuterium atoms on β-carbon
of the α-chloro-enamide, was subjected to the reductive
cycloisomerisation conditions. Interestingly, the indole 14a was
Next, from various deuterium solvent screening (1,4-dioxane-d
DMF-d ), we found that 6a was converted to the mono-deuterated
product in 2-propanol-d , without deuteration at the benzylic
8
,
7
8
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Journal Name
(a) S. Y. W. Lau, N. G. Andersen and B. A. KeaVyie,wOArrtgic.leLOentlitn.e,
6
7
delivered with one deuterium replaced by a hydrogen atom,
accompanied by a C-O coupled isopropoxide 17a. This reveals that
syn-β-D-Pd elimination of A and re-addition to the ynamide 6 occurs
reversibly, allowing D-H exchange of deuterated A with i-PrOH to
2
001, 3, 181; (b) J. A. M. Torre, P. Espinet and A. C. Albéniz,
DOI: 10.1039/C9CC00537D
Organometallics 2013, 32, 5428.
(a) J. Mo, L. Xu and J. Xiao, J. Am. Chem. Soc., 2005, 127, 751;
(b) J. Mo and J. Xiao, Angew. Chem. Int. Ed., 2006, 45, 4152;
2
0
take place.
(
c) Z. Hyder, J. Ruan and J. Xiao, Chem. Eur. J., 2008, 14, 5555;
d) J. Ruan, J. A. Iggo, N. G. Berry and J. Xiao, J. Am. Chem.
(
In conclusion, a palladium-catalysed ligand-free reductive
Heck cycloisomerisation of aromatic 1,6-enynamides has been
realised using in-situ generated 1,6-en-α-chloro-enamides in a
one-pot stepwise protocol. Deuterium isotope labeling studies
revealed that intramolecular hydride transfer, along with
intermolecular hydride donation from the solvent, were both
observed. Moreover, this indicates that there was a hydride
exchange between the chloroenamide and i-PrOH. The mild,
straightforward experimental condition will highten valuable
potential towards the synthesis of complex azacyclic target
compounds from acyclic units in both academic and industrial
research settings.
Soc., 2010, 132, 16689.
8
9
For palladium-catalysed ligand-free reactions, see(a) C. Zhu,
B. Yang and J.-E. Bäckvall, J. Am. Chem. Soc., 2015, 137,
1
1868; (b) C. Zhu; B. Yang; T. Jiang and J.-E. Bäckvall, Angew.
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Conflicts of interest
There are no conflicts to declare.
1
1
1
0 T. K. Beng, S. M. Wilkerson-Hill and R. Sarpong, Org. Lett.,
2
014, 16, 916.
Acknowledgements
This work was supported by Royal Society - Newton International
Fellowship (170322), National Natural Science Foundation of China
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Notes and references
1
(a) M. Ahlquist, G. Fabrizi, S. Cacchi and P. Norrby, J. Am. 14 For unreactive substrate table, see supporting information.
Chem. Soc., 2006, 128, 12785; (b) S. Raoufmoghaddam, S.
1
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5 For alkoxylation of 2-(chloromethyl)-1H-indole derivative,
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