Palladium-catalyzed oxidative direct C-H/C-H cross coupling of anilides with β-keto esters

Article abstract of DOI:10.1039/c3cc44769c

A Pd(ii)-catalyzed oxidative direct C(sp²)-H/C(sp³)-H cross coupling of anilides with α-dicarbonyl compounds with Mn(OAc) ₃·2H₂O as the oxidant is reported and this protocol provides facile access to α-aryl malonates and β-keto esters in good yields and regioselectivity.

Full text of DOI:10.1039/c3cc44769c

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ARTICLE TYPE
Palladium-Catalyzed Oxidative Direct C-H/C-H Cross Coupling of
Anilides with -Keto Esters
Wai-Wing Chan, Zhongyuan Zhou and Wing-Yiu Yu*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
A Pd(II)-catalyzed oxidative direct C(sp2)-H/C(sp3)-H cross
coupling of anilides with -dicarbonyl compounds with
formation was observed despite standing the mixture for
50 Table 1 Reaction Optimizationsa
NHPiv
NHPiv
Mn(OAc)3·2H2O as oxidant is reported and this protocol
provides a facile access to -aryl malonates and -keto esters
10 in good yields and regioselectivity.
[Pd] (10 mol%), oxidant
+
MeO2C
CO2Me
CO2Me
CO2Me
H
TFA, toluene, rt, overnight
1a
2a
TFA
(equiv.)
0.5
Entry
Pd catalyst
Malonate
(equiv.)
Oxidant
(equiv.)
Yieldb
(%)
0
Transition metal-catalyzed oxidative cross coupling reactions
for C-C bond formation via twofold direct C-H activation
(termed cross dehydrogenative coupling, CDC) are attracting
1c
2d
3
4
5
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(TFA)2
6
6
6
6
6
3
TBHP (2)
TBHP (2)
Mn(OAc)3 (1)
Mn(OAc)3 (1)
Mn(OAc)3 (1)
Mn(OAc)3 (1)
[Mn]
0.5
0.5
0.5
0.5
0
extensive
investigation.1
With
the
coupling
of
a
17
18
16
41
15 unfunctionalized hydrocarbons, the CDC reaction is
Pd(OTs)2(MeCN)2
Pd(OAc)2
promising approach for the design of highly atom-economical
transformations. In this regard, palladium has been the
catalyst of choice for many investigations. Since the
pioneering works by Fujiwara and co-workers on the Pd-
20 catalyzed oxidative aromatic C-H alkenylation,2 remarkable
advances have been made by many research groups on
6
2
7e
Pd(OAc)2
2 × 3 equiv.
2 × 3 equiv. 72
2 × 3 equiv. 80
2 × 3 equiv. 85
(2 × 50 mol%)
[Mn]
(2 × 50 mol%)
[Mn]
(3 × 50 mol%)
[Mn]
8e
Pd(OAc)2
Pd(OAc)2
none
2 × 3 equiv.
3 × 3 equiv.
3 × 3 equiv.
9e,f
10e
dehydrogenative
C(sp2)-H/C(sp2)-H
cross-coupling
of
2 × 3 equiv.
0
(3 × 50 mol%)
(hetero)arenes with alkenes3 or arenes4 via Pd(II)/Pd(0)
catalysis. However, the analogous examples of structurally
25 different hydrocarbons involving C(sp2)-H/C(sp3)-H bond
cross coupling are sparse in the literature.5
a Reaction conditions: 1a (0.2 mmol), dimethyl malonate, Pd catalyst (10
mol%), TBHP or Mn(OAc)3·2H2O as oxidant, TFA, toluene (1.5 mL) at
room temperature under N2 for overnight. b Yields are determined by 1H
NMR. c Dioxane was used as solvent. d 80 oC instead of rt. e Batchwise
55 addition interval 4 h. f Isolated yield.
Recently, we6 and others7 reported the Pd-catalyzed
oxidative arene acylations based on cross coupling of arenes
with aldehydes with tert-butyl hydroperoxide (TBHP) as
30 oxidant. The acylation reactions should involve two distinct
C-H activation processes: (1) arene C-H palladation to form
arylpalladium(II) complexes, and (2) oxyradical-mediated
hydrogen atom abstraction of the aldehyde C-H bond to form
acyl radicals. The subsequent coupling of the acyl radicals
35 with the arylpalladium complexes should bring about the C-C
bond formation. Inspired by these results, we envisioned that
the analogous cross coupling reaction of arylpalladium(II)
complex with a -dicarbonylalkyl radical [•CH(COR)2] would
lead to the formation of -aryl dicarbonyl compounds.8
40 Herein we report a mild Pd(II)-catalyzed direct α-arylation of
1,3-dicarbonyls with anilides by cross dehydrogenative
coupling reaction using Mn(OAc)3·2H2O as stoichiometric
oxidant.
overnight at room temperature or at 80 oC (Table 1, entries 1 –
2). Gratifyingly, when Mn(OAc)3·2H2O (1 equiv.) was
employed as oxidant, the “1a + dimethyl malonate” reaction
at room temperature furnished the arylmalonate 2a in 17%
60 yield (entry 3). The molecular structure of a di-tert-butyl
arylmalonate derivative 2c has been established by X-ray
crystallography (see Figure S44 in ESI). Other palladium
catalysts such as Pd(TFA)2 and Pd(OTs)2(MeCN)2 did not
give significant improvement (entries 4 – 5). After several
65 trials, we found that employing 2 equiv. of TFA would
improve the product yield to 41% (entry 6). In order to
achieve a sustained supply of the malonate radicals over the
reaction time, reagents were added to the reaction mixture in a
batchwise fashion. Thus, when 1a was treated with dimethyl
70 malonate (3
× 3 equiv./4 h), Pd(OAc)2 (10 mol%),
Mn(OAc)3·2H2O (3 × 50 mol%/4 h) and TFA (3 × 3 equiv./4
h) in toluene (1.5 mL) at room temperature, 2a was obtained
in 85% yield (entry 9). Notably, no 2a formation was
observed in the absence of the Pd catalyst (entry 10).
At the outset, we examined the reaction of 3,4-dimethyl-N-
45 pivalanilide (1a, 0.2 mmol) and dimethyl malonate (6 equiv.)
in the presence of Pd(OAc)2 (10 mol%), trifluoroacetic acid
(TFA) (0.5 equiv.) and TBHP (2 equiv.) as oxidant with
dioxane or toluene as solvent. No significant product
75
With the optimized conditions in hand, various malonates
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and keto esters were examined for the Pd-catalyzed direct C-H 35 3h in 73 and 69% yield, respectively (Scheme 1).
Table 2 Substrate scope studya,b
CO2R
NHPiv
View Article Online
conc. HCl (0.1 mL)
R2
O
DOI: 10.1039/C3CC44769C
COCH3
Pd(OAc)2 (10 mol%)
Mn(OAc)3·2H2O (3 x 50 mol%)
H
N
O
O
EtOH (5 mL), reflux, 0.5 h
N
R2
OEt
NH
OR4
H
R1
CO2R
2g, R = Bn
2h, R = Et
+
R3
OR4
R1
O
TFA (3 x 3 equiv.)
toluene, rt, N2
O
H
R5
R5
O
3g, 73%
3h, 69%
(3 x 3 equiv.)
R3
1
2
Scheme 1 Synthesis of unprotected indole 3g and 3h.
NHPiv
NHPiv
CO2R
NHPiv
2a, R = Me, 85%
2b, R = Et, 75%
2c, R = tBu, 81%
2d, R = Bn, 94%
CO2Me
The Pd-catalyzed oxidative coupling reaction exhibits a
primary KIE (kH/kD) of 3.3 based on the competitive
experiments using an equimolar amount of 1n and 1n-d5
40 (Scheme 2).10 In addition, a cyclopalladated complex Pd-1r
was prepared according to the literature.4g In this work, the
stoichiometric reaction of Pd-1r and dimethyl malonate in
presence of Mn(OAc)3·2H2O and TFA in toluene at room
temperature produced the corresponding product 2r in 44%
45 yield (Scheme 3). This result suggested that the Pd-catalyzed
coupling reaction is likely to proceed via the cyclopalladated
complex, which is formed by the rate-limiting arene C-H
activation.
CO2Et
CO2Et
Cl
O
O
CO2R
Ph
2e, 93%
NHPiv
CO2Me
2f, 52% (70%)
NHPiv
NHPiv
NHPiv
CO2Et
CO2Me
CO2R
O
F3C
O
O
Ph
O
Br
2g, R = Bn, 69%
2h, R = Et, 61%
2k, 42% (82%)
2i, 48% (68%)
2j, 51% (63%)
NHPiv
NHPiv
Ph
NHPiv
NHPiv
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
O
O
CO2Me
S
MeO
2l, 34% (89%)
Pd(OAc)2 (10 mol%)
Mn(OAc)3·2H2O (2 x 0.1 mmol /4 h)
NHPiv
2m, 20% (59%)
2n, 47% (80%)
2o, 58% (89%)
NHPiv
CO2Me
CO2Me
H5/d5
+
MeO2C
CO2Me
H5/d5
TFA (2 x 0.6 mmol/4 h)
toluene, rt, N2
20% conversion, kH/kD = 3.3
NHAc
EtO
NHAc
NHCOPh
CO2Me
CO2Me
PivO
NHPiv
CO2Me
1n : 1n-d5
(1 : 1; 0.2 mmol)
(2 x 0.6 mmol/4 h)
CO2Me
CO2Me
CO2Me
CO2Me
50
Scheme 2 Kinetic isotope effect study.
CO2Me
2p, 25% (53%)
2q, 60% (86%)
2r, 51% (86%)
2s, 48% (60%)
O
O
Pd
HN
HN
O
O
Mn(OAc)3·2H2O
(3 x 1 equiv./4 h)
NH
CO2Me
CF3
CF3
MeO2C
CO2Me
a
+
Reaction conditions: 1 (0.2 mmol), 1,3-dicarbonyls (3 × 3 equiv. / 4 h),
(3 x 6 equiv./4 h)
O
O
TFA (3 x 6 equiv./4 h)
toluene, r.t., N2
Pd
5 Pd(OAc)2 (10 mol%), Mn(OAc)3·2H2O (3 × 50 mol% / 4 h), TFA (3 × 3
equiv. / 4 h), toluene (1.5 mL) at room temperature under N2 for
CO2Me
2r, 44%
O
1r-Pd
b
overnight. Isolated yields; percentage yield based on anilide conversion
Scheme 3 Stoichiometric reaction of Pd-1r with dimethyl malonate.
is given in parentheses.
arylation (Table 2). A series of dialkyl malonates such as
10 ethyl, tert-butyl malonates would successfully transform 1a to
the corresponding arylmalonates 2b and 2c in good yields.
When malonates bearing more sterically hindered benzyl
group were used as coupling partner, 2d and 2e were obtained
in 93–94% yields. Notably, diethyl phenylmalonate was also
15 effective for the coupling reaction, and α,α-biarylmalonate 2f
bearing a quaternary carbon was obtained in 52% yield. Based
on their smaller pKa’s comparing to the malonates,9 β-keto
esters should be effective coupling partners. Notwithstanding,
the corresponding coupling with 1a produced 2g–2m in 20–
20 69% yields. The lower anilide conversion was attributed to the
incompatibility of the rate of arene C-H palladation and the
Mn-mediated radical generation.
The scope of the anilides was also investigated, and the Pd-
catalyzed direct C-H activation was found to be sensitive to
25 the substituent effects. For example, pivalanilides bearing
other meta-substituents (H, Ph and OPiv) reacted with
dimethyl malonate to give 2n–2p in 25–58% yields. As
anticipated, benzamido and acetamido are capable directing
groups for the ortho C-H coupling reactions to give 2q–2s in
30 48–60% yields.
Scheme 4 depicts a plausible mechanism for the Pd-
55 catalyzed direct C-H α-arylation of the 1,3-dicarbonyl
compounds. In the presence of an excess amount of TFA,
Pd(OAc)2 should undergo ligand exchange to form Pd(TFA)2.
Coordination of 1 with Pd(TFA)2, followed by the rate-
limiting C-H activation, afforded the six-membered
60 palladacycle with the elimination of TFA. Since oxidation of
dicarbonyl compounds by Mn(OAc)3·2H2O is known to
generate enolate radicals,11 we hypothesized that the enolate
radicals would couple with the palladacycle to give some
either the dinuclear Pd(III)12 or Pd(IV)13 complexes.
65 Reductive elimination of the high valent Pd species would
give the desired arylmalonate.
In conclusion, we have described a Pd-catalyzed direct C-
H/C-H cross coupling of 1,3-dicarbonyl compounds and
anilides. With Mn(OAc)3·2H2O as oxidant, arylmalonates
70 were obtained in good yields and regioselectivity.
A
mechanism involving the coupling of cyclopalladated
complex with carboradical is proposed. The reaction can be
performed under mild conditions without addition of base and
provides
a easy route to -aryl carbonyl compounds.
75 Comprehensive mechanistic studies are underway.
Next, we sought to apply this coupling reaction for the
synthesis of indoles. For instance, treating 2g and 2h (0.2
mmol) with concentrated HCl (0.1 mL) and EtOH (5 mL)
under reflux for 0.5 h afforded the unprotected indole 3g and
2
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This journal is © The Royal Society of Chemistry [year]

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R2
O
45
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CO2Me
CO2Me
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R1
DOI: 10.1039/C3CC44769C
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Pd(O2CCF3)2
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TFA
55
R2
L
R2
O
HN
CF3
X
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O
O
Pd
R1
Pd
O
O2CCF3
60
R1
2
CO2Me
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dinuclear Pd(III)
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oxidative
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CO2Me
CO2Me
CO2Me
CO2Me
H
7
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Mn(OAc)2
+ AcOH
70
Scheme 4 Proposed mechanism.
Notes and references
State Key Laboratory for Chirosciences and Department of Applied
5 Biology and Chemical Technology, The Hong Kong Polytechnic
University, Hung Hom, Kowloon, Hong Kong. E-mail: wing-
yiu.yu@polyu.edu.hk
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† Electronic Supplementary Information (ESI) available: Detailed
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1
10 all the compounds; crystallographic data for 2c. CCDC 946973. For ESI
and crystallographic in CIF and other electronic format see
DOI: 10.1039/b000000x/
We acknowledge the financial support of the Hong Kong Research Grants
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