Iron-catalyzed direct α-arylation of α-amino carbonyl compounds with indoles

Article abstract of DOI:10.1039/c5ob02325d

A mild and general α-arylation of α-amino carbonyls with indoles catalyzed by Fe(ClO₄)₃ has been developed. C-H activation is smoothly fulfilled by using TBHP as the oxidant with good yields. Two hydrogen dissociations make this transformation more environmentally benign because of high atom efficiency.

Full text of DOI:10.1039/c5ob02325d

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Iron-catalyzed direct α-arylation of α-amino carbonyl compounds
with indoles†
Yan Zhang,a, 1 Minjie Nia, 1 and Bainian Feng*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A mild and general α-arylation of α-amino carbonyls with indoles a) Work of Stephenson with Ru catalyst
catalyzed by Fe(ClO4)3 has been developed. The C-H activation is
N
H
Ru(bpy)3Cl2 (1 mol%)
N
H
smoothly fulfilled by using TBHP as the oxidant with good yields.
Two hydrogen dissociations make this transformation more
environmentally benign because of high atom efficiency.
O
N
R1
N
(NH4)2S2O8 (5 equiv)
blue LEDs, DMF, 25oC
O
R2
R1
R2
Functionalization of
-amino carbonyl compounds is a hot topic in organic
synthesis because the -amino carbonyl motif is an important
α-C-H bonds of the carbonyl group in the
b) Work of Li with Cu catalyst
α
O
O
O
R3N
N
CuCl or CuCl2
TBHP
CuCl
α
R1
R4
R2
R4
TBHP
structure component of multitudinous natural products and
biomolecules.1-4 Many mild and general methods have been
well-established for accessing this important motif. Among
these efficient reactions for the functionalization of C-H bonds
N
CH2Cl2, r,t
air
CH2Cl2, r.t.
argon
N
R4
R1
R1
R3HN
R2
R2
O
α
to a carbonyl group, transition-metal-catalyzed α-arylation
c) Work of Li with Ru catalyst
O
R3HN
reactions are rare and are limited: the arylation reaction is
realized through deprotonation (in situ generation of carbonyl
enolate) with the aid of a base and requires the use of
expensive aryl sources with the aid of transition metal
catalysis.5-9
Ru(bpy)3Cl2
R4
R4
R3HN
N
R1
O2, 5W blue LEDs light
CH2Cl2, 40oC
O
R2
N
R1
R2
Recently, some transition-metal-catalyzed
α
-arylation of
-arylated
-C-H bond
α-
d) This work with Fe catalyst
amino carbonyls leading to nonnaturally
α
O
R3HN
Fe(ClO4)3
H
aminocarbonyl compounds from cleavage of
α
R4
TBHP
R4
without the aid of base have been researched. Stephenson10
and co-workers reported the Ru(bpy)2Cl2-catalyzed Friedel-
Crafts amidoalkylation (Scheme 1a). It was achieved by
oxidation of dialkylamides using an oxidant persulfate under
the visible light at room temperature via a reactive N-
acyliminium intermediate. In light of the pharmaceutical
activity of indole motif, almost at the same time, Li11 and co-
R3HN
N
CH3CN, r.t.
R1
N
O
R1
R2
R2
Scheme 1 Transition-metal-catalyzed
α arylation of α-amino
carbonyl compounds
workers reported a base-free copper-catalyzed
α
-amino carbonyls with indoles in the presence of tert-butyl
-arylation of
α
hydroperoxide (TBHP) through
(1H-indo-3-yl)-2-oxo-carbonyls,
not
α
-amino
carbonyl
a C-H oxidation strategy.
products, were selectively obtained as the terminal products
under argon or air atmosphere (Scheme 1b). Then, after a
series trials, they found a visible-light photoredox catalysis
strategy for 2-(1H-indo-3-yl)-2-amino-carbonyl compounds
However, 2-(1H-indo-3-yl)-2-imino-carbonyls and 2-
synthesis by direct
α-arylation between α-amino carbonyl
compounds and indoles with the aid of Ru(bpy)3Cl2, 5W LEDs
light and O2, avoiding both the uses of bases and the
conversions of the amino groups into imino groups in the
products (Scheme 1c).12
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However, to our knowledge, iron-catalyzed
Journal Name
α
arylation of
α
-
Table 1 Optimization of the Iron-catalyzed direct α-arylation
amino carbonyls using a C-H oxidation strategy has not been between
established. Based on our research, 13 herein we report a novel
α
-amino carbonyl compound and indolea
and mild route to the C-H oxidation/cross-coupling of α-amino
carbonyl compounds with indoles catalyzed by iron catalysis in
the presence of TBHP (Scheme 1d).
Our investigation began with the reaction of 1H-indole (1a
)
and 1-phenyl-2-(phenylamino)ethanone (2a) and 15 mol%
FeCl2 in CH3CN at room temperature under air atmosphere
(Table 1): a trace of desired 2-(1H-indo-3-yl)-1-phenyl-2-
(phenylamino)ethanone (3a) was observed (entry 1). These
results encouraged us to optimize the other reaction
parameters. After a series of trials, we were pleased to find
that the yield of 3a could be enhanced in the presence of
oxidants, including benzoyl peroxide (BPO), di-tert-butyl
peroxide (DTBP) and tert-butyl hydroperoxide (TBHP) (entries
2-4). Subsequently, a series of other iron catalysts, including
FeCl2, FeCl3, Fe(ClO4)3, Fe2(SO4)3, Fe(NO3)3 were tested and
Fe(ClO4)3 displayed higher catalytic efficiency for the reaction
in the presence of TBHP (entries 5-8). It is noteworthy that the
reaction cannot take place without an iron catalyst (entry 9).
Gratifying, good yields were still achieved when using either 15
Entry [Fe] (mol%)
Oxidant Solvent Temperature Yield
(%)b
1
FeCl2(15)
FeCl2(15)
FeCl2(15)
FeCl2(15)
FeCl3(15)
air
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH2Cl2
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
8
2
BPO
18
22
30
18
63
31
16
0
3
DTBP
TBHPc
TBHP
4
5
6
Fe(ClO4)3(15) TBHP
Fe2(SO4)3(15) TBHP
Fe(NO3)3(15) TBHP
7
8
9
__
TBHP
10
11
12
13
14
15
Fe(ClO4)3(10) TBHP
Fe(ClO4)3(10) TBHP
Fe(ClO4)3(10) TBHP
Fe(ClO4)3(10) TBHP
Fe(ClO4)3(10) TBHP
Fe(ClO4)3(10) TBHP
62
25
33
51
41
35
mol% or 10 mol% Fe(ClO4)3, but the latter required
a
prolonged reaction time (entry 10). Among the effect of
solvents examined, it turned out that CH3CN was the most
effective solvents examined, while both toluene and CH2Cl2
displayed a lower effect, the reaction in EtOH still gave
moderate yield (entries 11-13). Finally, the reactions at 40oC
and 60oC were tested: they could take place, but low yields
were isolated because of the synthesis of some unidentified
products (entries 14-15).
toluene r.t.
EtOH
r.t.
CH3CN
CH3CN
40oC
60oC
a
Reaction conditions: 1H-indole (1a) (0.5 mmol), 1-phenyl-2-
(phenylamino)ethanone (2a) (0.5 mmol), iron catalysis, solvent
(3mL), oxidant (1 equiv) under air atmosphere for 10-12h. b
Isolated yield. c TBHP (5.0-6.0 M in decane).
With the optimal reaction conditions in hand, the scope of
both indoles
2). In the presence of 1H-indole (1a), Fe(ClO4)3 and TBHP, a
number of other arylamino groups in -amino carbonyls were
1 and the α-amino ketones 2 was explored (Table
α
initially investigated. For 1-phenyl-2-aminoethanones having
Me-substituted phenylamino groups, the order of the
3s in 32%, 41%, 61%, 46% and 35% yields, respectively. Halo
reactivity is meta>ortho> H in terms of yields (compounds 3a
-
substituents, Cl and Br, on the indole ring were compatible
3c). No product was found with para-Me phenylamino group.
The results demonstrated that substituents, such as
phenylamino, o-tolylamino groups, in 2-amino-1-p-
tolylethanones were compatible with the optimal conditionds
with optimal conditions (compounds 3h, 3i, 3k, 3l, 3n, 3o, 3q,
3r). Gratifyingly, substituents, Me or aryl, at the 2- and 6-
position of indoles were also compatible with the optimal
conditions. In the presence of Fe(ClO4)3 and TBHP, 2-methyl-
1H-indole and 6-methyl-1H-indole successfully underwent the
arylation reaction with 1-phenyl-2-(o-tolylamino)ethanone
(compounds 3d, 3e). Interestingly, halo substituent such as Cl,
on the aryl ring of the arylethanone moiety was consistent
with the optimal conditions (compound 3f).
(
2b) leading to the desired product 3t and 3u in moderate
The optimal conditions were found to be viable for a wide
range of indloes with high substituents compatibility: several
substituents, including Me, Br, Cl and aryl groups, on the
aromatic ring of indoles were well-tolerated in the presence of
yields ( 58% and 36%). Screening revealed that indoles with a
phenyl displayed high reactivity under the same conditions,
furnishing the target product in moderate yield (compounds 3v
and 3w). Finally, substituents on the nitrogen atom of indole
moiety were tested, 1-methyl indole and 1-benzyl indole were
successfully reacted with 1-p-tolyl-2-(o-tolylamino)ethanone
1-phenyl-2-phenylaminoethanone
tolylamino)ethanone (2b), 1-phenyl-2-(m-tolylamino)ethanone
2c), 1-p-tolyl-2-(o-tolylamino)ethanone 2e and 1-(p-
chlorophenyl)-2-(o-tolylamino)ethanone 2f). For example,
treatment of 5-Me-substituted indole with 2a 2b 2c 2e 2f
3j 3m 3p
(
2a),
1-phenyl-2-(o-
(
(
)
(
2e) Fe(ClO4)3 and TBHP in good yields (compounds 3x and 3y).
The reactions of indoles and -amino esters were also
explored. -Amino esters with methyl and ethyl substituted
were all consistent with the optimal conditions (compounds
3z 3oo).
(
1
α
2
,
,
,
,
,
α
Fe(ClO4)3 and TBHP afforded the corresponding 3g
,
,
,
,
-
2 | J. Name., 2012, 00, 1-3
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Table 2 Iron-catalyzed direct
α
-arylation between
α
-amino carbonyl compounds and indolesa, b
R
O
O
O
O
HN
HN
HN
HN
N
N
H
N
H
N
H
H
3d
R = H, , 43%
3a
, 62%
3b
, 70%
3c
, 83%
3e
R = CH3, , 41%
O
O
O
O
HN
HN
O
HN
HN
HN
N
N
Ph
N
R
N
N
H
H
H
Ph
3y, 3o%
3v
R = CH3, , 39%
3u
, 58%
3x
, 34%
3t
, 58%
3w
R = Cl,
, 56%
a
Reaction conditions: indole (1) (0.5 mmol), α-amino carbonyl compound (2) (0.5 mmol), Fe(ClO4)3 (10 mmol%), CH3CN (3mL),
TBHP (1 equiv, 5.0-6.0 M in decane) under air atmosphere for 10-12h. b Isolated yield.
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Journal Name
in the absence of indole 1a. The in situ LC-MS analysis of the
resulting mixture did not show any signals corresponding to
amino peroxide. The only new species identifiable was the
iminium14 (eq 3). As expected, treatment of indole 1a with 1-
(p-tolyl)-2-(o-tolylimino)ethanone 4a15 offered the desired
product 3a in the absence of both iron catalyst and TBHP.
O
Fe(ClO4)3 (10mol%)
TBHP
HN
(1)
Ph
PhHN
N
TEMPO (2eq)
CH3CN, r.t.
O
H
N
H
1a
2a
Based on the present results and relevant literature, 16-20
a
3a, 5% yield
possible mechanism outlined in Scheme 3 was proposed. TBHP
decomposes into tert-butoxyl radical and hydroxyl anion in the
presence of the ferrous catalyst. Subsequently, a single
electron transfer (SET) from tert-butoxyl radical to 1-phenyl-2-
(phenylamino)ethanone 2a takes place to offer the radical
(2)
intermediate 5a. The radical intermediate 5a readily
undergoes the deprotonation reaction leading to imine
intermediate 6a. Finally, the reaction of indole 1a with
intermediate 6a affords the desired product. Overall, the Fe2+-
(3)
(4)
Fe3+ redox processes play key roles in the present
α-arylation
of amino carbonyl compound, which are the reductive
heterolytic cleavage of O-O bond in the peroxide, the SET
reaction of 1-phenyl-2-(phenylamino)ethanone and the
oxidation of the intermediate 5a to imine intermediate 6a
In summary, we have demonstrated a novel and efficient
protocol for the -arylation of -amino carbonyl compounds
.
α
α
via cross dehydrogenative couplings catalyzed by Fe(ClO4)3.
Scheme 2 Control experiments.
The reaction proceeds with high functional group tolerance
and broad substrate scope to give the nonnatural
α-amino
carbonyl compounds which are extremely useful synthetic
intermediates in the construction of biologically important
compounds.
We thank for the National Natural Science Foundation of
China (No. 21302067), the National Natural Science
Foundation of Jiangsu Province (No. BK20130120) and National
Undergraduate Training Programs for Innovation and
Entrepreneurship (201510295049) for financial support.
Notes and references
1
2
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Scheme 3 Possible mechanism.
To gain insight into the Fe-catalyzed
α
-arylation of
α
-amino
3
4
For reviews, see: (a) D. Culkin and J. F. Hartwig, Acc. Chem.
Res. 2003, 36, 234; (b) M. Miura and M. Nomura, Top. Curr.
Chem. 2002, 219, 211; (c) F. Bellina and R. Rossi, Chem. Rev.
2010, 110, 1082.
carbonyls, several control experiments were carried out to
elucidate the mechanism. As shown in Scheme 2, a model
reaction was performed again with an additional 2 equiv. of
radical inhibitor 2,2,6,6-tetramethylpiperidinooxy (TEMPO),
and the formation of 3a was completely inhibited (eq 1), which
implies that a radical species may be involved in this reaction.
Substrate 2g with a tertiary amine group was not viable for the
reaction under the optimal conditions (eq 2). 2e with 10 mol %
of Fe(ClO4)3 was reacted in the presence of oxidant (TBHP) but
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