Gold-catalyzed C-H oxidative polyacyloxylation reaction of hindered arenes

Article abstract of DOI:10.1055/s-0032-1316541

The synthesis of polyacyloxylated aromatic derivatives was achieved in moderate yields using a gold-catalyzed C-H activation strategy and di(acetoxy)iodobenzene as an oxidant. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0032-1316541

PAPER
▌2463
pGapoerld-Catalyzed C–H Oxidative Polyacyloxylation Reaction of Hindered
Arenes
Oxidative Polyacyloxylation of Hindered Arenes
Alexandre Pradal,a,b Pierre Faudot dit Bel,a,b Patrick Y. Toullec,*a,b Véronique Michelet*a,b
a
Chimie ParisTech, Laboratoire Charles Friedel (LCF), 11, rue P. et M. Curie, 75231 Paris cedex 05, France
Fax +33(1)44071062; E-mail: patrick-toullec@chimie-paristech.fr; E-mail: veronique-michelet@chimie-paristech.fr
b
CNRS, UMR 7223, 75231 Paris cedex 05, France
Received: 18.04.2012; Accepted: 10.05.2012
velopment of catalytic versions of such transformations
(Scheme 1).9
Abstract: The synthesis of polyacyloxylated aromatic derivatives
was achieved in moderate yields using a gold-catalyzed C–H acti-
vation strategy and di(acetoxy)iodobenzene as an oxidant.
Examples of such reactions using hypervalent iodine re-
agents as oxidants have been described recently in the lit-
erature. In 2007, the group of He reported direct
amidation of hindered arenes in the presence of AuCl3 cat-
alyst at room temperature using iminoiodanes as oxidant
(Scheme 2, equation 1).10 In 2008, Tse and co-workers re-
ported the homo- and hetero-coupling of arenes in the
presence of HAuCl4 as the catalyst using diacetoxyiodo-
benzene 1 (DAIB) as the oxidant (Scheme 2, equation
2).11 In 2009, the groups of Waser12 and Nevado13 inde-
pendently described the alkynylation of electron-rich
arenes and heteroarenes catalyzed by gold complexes us-
ing hypervalent iodine reagents (Scheme 2, equation 3).
Finally, in 2011, we14 and the group of Wang15 indepen-
dently reported the acyloxylation of non-activated arenes
in the presence of 1 and a gold catalyst, [(PPh3)AuCl] and
AuCl3, respectively (Scheme 2, equation 4). Selective car-
bon–oxygen bond formation starting from arenes is in-
deed particularly challenging and interesting as phenols
and derivatives are both building blocks that are frequent-
ly found in organic synthesis and structural scaffolds in
biology, material sciences and pharmaceutics.16 In line
with our research program initiated on gold-catalyzed car-
bon–oxygen bond-forming reactions,17 we decided to
investigate the gold-catalyzed polyacyloxylation of aro-
matic ring systems using hypervalent iodine reagents as
oxidizing agents.
Key words: homogeneous catalysis, arenes, esters, oxidation, gold
The use of hypervalent iodine reagents has constantly in-
creased over the years and has found widespread applica-
tions in organic chemistry.1 These oxidants have become
extremely popular because of their ready availability,
their low toxicity, and the level of selectivity they allow,
particularly in association with transition metal catalysts.
The development of synthetic strategies for the direct ox-
idative conversion of aromatic C–H bonds into carbon–
heteroatom or carbon–carbon bonds represents a critical
challenge in organic chemistry and an increasing number
of transition-metal-catalyzed methodologies in combina-
tion with mild oxidizing agents have been developed to
address this problem.2 Although a large variety of transi-
tion-metal catalysts and oxidants have been investigated,
the palladium/hypervalent iodine reagent couple plays a
central role in C–H oxidative functionalization due to its
high activity and selectively in ligand-directed transfor-
mations.3 However, its reactivity is strongly diminished
by steric hindrance on the substrate4 especially in the ab-
sence of pre-coordination by a directing group.5 As a con-
sequence, the search for new, highly active catalytic
systems allowing oxidative C–H functionalization of aro-
matic rings is of foremost importance. Gold catalysis re-
cently emerged as a new and very exciting area of
research mainly due to its unique Lewis acid carbophilic
reactivity.6 The electrophilicity of gold complexes has
also been demonstrated in the Csp2-H activation of arenes.
As early as the 1930s, initial contributions from Isbell and
Kharasch postulated auration of nonactivated aromatic
rings in the presence of Au(III) salts.7 This type of reactiv-
ity was later investigated by Braunstein et al.8 Subsequent
reductive elimination from the aryl-Au(III) complex lib-
erates a functionalized arene compound and a Au(I) spe-
cies. In the presence of an external oxidant, regeneration
of the Au(III) active intermediate occurs, allowing the de-
hypervalent
AuX3
iodine
reagent
Ar-H
X
I
X
Ph
direct
auration
oxidation
AuX
ArAuX2
reductive
elimination
Ar-X
SYNTHESIS 2012, 44, 2463–2468
Advanced online publication: 20.06.2012
0
0
3
9
-
7
8
8
1
1
4
3
7
-
2
1
0
X
Scheme 1 General postulated mechanism of Au-catalyzed arene oxi-
DOI: 10.1055/s-0032-1316541; Art ID: SS-2012-C0356-OP
© Georg Thieme Verlag Stuttgart · New York
dative C–H functionalization

2464
A. Pradal et al.
PAPER
mained moderate in all cases, and a complex mixture of
biaryl isomers resulting from the direct heterocoupling re-
action between 3a and 3b and iodobenzene represents the
major by-products of this transformation.18
N
Ns
Ph
I
NHNs
(1)
(2)
[Au] cat.
Table 1 Optimization of the Au-Catalyzed C–H Polyacetoxylation
AcO
I
OAc
Reaction of Mesitylenea
1
Ph
[Au] cat.
PPh3AuCl (x mol%)
DAIB (1; y equiv)
(OAc)n
solvent, temp, 16 h
SiiPr3
I
SiiPr3
O
2
3a n = 1
3b n = 2
3c n = 3
O
Entry [(PPh3)AuCl] Oxidant 1 Solvent
Yield (%)b
(3)
(4)
(mol%)
(equiv)
3a
3b
3c
[Au] cat.
N
N
H
H
1c
2
1
2
2
3
5
2
2
2
2
4
4
8
3
3
3
3
3
3
3
5
5
5
5
5
AcOH
AcOH
DCE
38
45
7
3
11
24
29
30
1
0
0
AcO
I
OAc
OAc
1
Ph
[Au] cat.
3
8
Scheme 2 Gold-catalyzed hypervalent iodine reagent-based C–H
4
AcOH
AcOH
DCE
12
10
34
15
11
44
1
9
functionalization of arenes
5
11
0
6d,e
7f
8
Noticeably, in the course of our studies concerning the
Au-catalyzed acyloxylation of aromatic systems,14 we ob-
served the formation of diacyloxylated arenes resulting
from the activation of two C–H bonds, as significant side-
products. We therefore anticipated that variations of the
reaction conditions would allow the preferential forma-
tion of polyacyloxylated products. In this paper, we
describe the di- and triacyloxylation of hindered nonacti-
vated aromatic rings.
DCE
23
29
10
26
22
36
4
DCE
8
9
AcOH
DCE
1
10
23
9
11
12
AcOH
DCE
9
3
28
At the beginning of our investigation, we evaluated the
degree of acyloxylation of mesitylene 2 under different
sets of conditions in the presence of DAIB 1 as the oxidant
and [(PPh3)AuCl] as a stable, easy to handle gold catalyst
precursor. As presented in Table 1, the diacetoxymesity-
lene (3b) was obtained as a major product in the presence
of [(PPh3)AuCl] (3 mol%) and DAIB 1 (3 equiv) in acetic
acid (AcOH) (compared Table 1, entry 4 to entries 1 and
a Reaction conditions: mesitylene 2 (0.2 mmol, 0.5 M), DAIB 1 (y
equiv), 110 °C unless otherwise stated. Conversions >98% unless oth-
erwise specified.
b Yields of the monoacetoxylated, diacetoxylated and triacetoxylated
products 3a, 3b and 3c determined by GC.
c 95% conversion.
d 87% conversion.
e Reaction run at 80 °C.
2). Lower catalyst loadings (entries 1 and 2) led predomi- f Reaction run at 100 °C.
nantly to the monoacetoxylated product 3a, whereas high-
er catalyst loading did not significantly improve the level
To get a better understanding of origins of the discrepancy
of C–H oxidative addition of acid (entry 5). A slightly
higher level of activation was observed in 1,2-dichloro-
ethane (DCE), only 2 mol% [(PPh3)AuCl] and 3 equiva-
lents of DAIB 1 were necessary to obtain a better yield of
3b (entry 3). Lowering the temperature of the reaction had
a strong detrimental effect on the oxidation in DCE (en-
tries 6 and 7). Increasing the oxidant to substrate ratio to
5 in acetic acid and DCE did not significantly improve the
yield or the product distribution (entry 8 versus 3 and en-
try 9 versus 2). The best yield of diacetoxylated 3b was
obtained in DCE using 8 mol% gold catalyst and 5 equiv-
alents of 1 (entry 12). Finally, a higher yield of the triace-
toxylated product 3c was obtained in the presence of 4
mol% gold catalyst and 5 equivalents of 1 (entry 10). To-
tal yields of acyloxylation products 3a–c (40–67%) re-
between the reactivities when the solvent was changed
from acetic acid to DCE, a kinetic analysis of the polyace-
toxylation of 2 in the presence of [(PPh3)AuCl] (4 mol%)
as the catalyst and 1 (3 equiv) was undertaken in both sol-
vents (Figure 1 and Figure 2). The reaction was markedly
faster in DCE; starting mesitylene 2 was completely con-
sumed within two hours, whereas full conversion was
only reached after six hours in AcOH. The final product
ratio observed in DCE for 3a/3b/3c was 1:3:1. It should
also be noticed that the formation of diacetoxylated ad-
duct 3b did not evolve in acetic acid whereas its ratio sig-
nificantly increased in 1,2-dichloroethane. As
a
consequence, one may envisage the polyacetoxylation re-
Synthesis 2012, 44, 2463–2468
© Georg Thieme Verlag Stuttgart · New York

PAPER
Oxidative Polyacyloxylation of Hindered Arenes
2465
action in 1,2-dichloroethane versus in acetic acid, starting clude the involvement of diaryl iodonium salt intermedi-
from 3b (see Table 2).
ates in gold-catalyzed direct acyloxylation reactions.
Cl–
OMe
Ph I+
MeO
1) 1 (1 equiv)
AcOH
110 °C, 3 h
MeO
OMe
2) HCl (aq)
80%
OMe
OMe
4
5
MeO OMe
[(PPh3)AuCl]
(2 mol%)
5
MeO
OMe
AcOH
110 °C, 3 h
59%
MeO OMe
6
Scheme 3 Diaryliodonium 5 formation and reactivity in the presence
of the gold(I) precatalyst
Considering the observed kinetics and polyacetoxylated
product ratio, we selected the catalytic conditions of
[(PPh3)AuCl] (2 mol%) and DAIB (3 equiv) in DCE at
110 °C for a screening of the C–H oxidation reaction of a
variety of substituted arenes (Table 2). In the case of me-
sitylene 2, diacetoxymesitylene (3b) was obtained in 31%
isolated yield as the major product, whereas the mono-
and tri-acetoxylated product 3a and 3c were isolated in 8
and 12% yield, respectively (entry 1). Tetramethyl-substi-
tuted benzenes 7 and 9 were diacetoxylated in 46 and 29%
yields, respectively (entries 2 and 3), whereas 2-bro-
momesitylene 11 was diacetoxylated in 43% yield (entry
4). The reaction seems to be strongly hampered by an ad-
ditional increase of the steric bulk of the arene substitu-
ents. When treated with DAIB (3 equiv), 1,3,5-
triethylbenzene 13 was converted in 65% yield into the
monoacetoxylated product 14a, whereas the diacetoxylat-
ed product 14b is only observed in a marginal 10% yield
(entry 5). In this case, the use of three equivalents of oxi-
dant in 1,2-dichloroethane allows the formation of the
monoacetoxylated product 14a in a significantly higher
yield compared to that described in our initial contribution
in the presence of 1.3 equivalents of DAIB in acetic acid
(65 versus 46% yield).14 These gold-catalyzed C–H oxi-
dative reactions can also be conducted in carboxylic acid
solvents, leading to the corresponding polyacyloxylated
products. When mesitylene 2 and 1,2,4,5-tetramethylben-
zene 7 were treated with DAIB (3 equiv) in propionic acid,
the diacyloxylated products 15b and 16b were formed in
39 and 41% yields, respectively (entries 6 and 7). As a test
experiment, the monoacetoxylated product 3a was treated
in the presence of two equivalents of DAIB (entry 8); un-
der these conditions the di- and tri-acetoxylated products
3b and 3c were formed in 65 and 20% yield, respectively.
Finally, the diacetoxylated product 3b was treated with ei-
ther 1.3 or 2 equivalents of 1 in the presence of 2 mol%
[(PPh3)AuCl] (entries 9 and 10); in these cases the triace-
toxylated product 3c was isolated in 40 and 49% yield, re-
spectively.
Figure 1 Plot of mesitylene 2 consumption and concomitant forma-
tion of acetyloxylated products 3a, 3b and 3c in acetic acid
Figure 2 Plot of mesitylene 2 consumption and concomitant forma-
tion of acetyloxylated products 3a, 3b and 3c in 1,2-dichloroethane
Moreover, the observed decrease in acetoxylation yields
for arenes possessing electron-donating groups in acetic
acid can be explained by the reactivity found in the case
of 1,3,5-trimethoxybenzene 4 (Scheme 3). The reaction of
4 with 1 (1 equiv) in the presence of [(PPh3)AuCl]19 (2
mol%) did not lead to any acetoxylated derivative, but in-
stead afforded a diaryliodonium salt 5.20 This salt was iso-
lated in 80% yield under the same reaction conditions in
the absence of gold catalyst. Addition of the gold catalyst
to this intermediate 5 then furnished only the direct arene
coupling product 6. These latter experiments seem to ex-
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2012, 44, 2463–2468

2466
A. Pradal et al.
PAPER
Table 2 Arene Polyacyloxylation Reactiona
Entry
1
Substrate
Product
Yield (%)b
AcO
OAc
3a, 8
AcO
AcO
AcO
AcO
OAc
2
3a
3b
3c
3b, 31
3c, 12
OAc
8a, trace
8b, 46
2
3
7
9
8a
8b
OAc
OAc
10a, trace
10b, 29
10a
10b
AcO
AcO
Br
Br
Br
12a, trace
12b, 43
4
11
13
2
12a
14a
15a
12b
14b
15b
AcO
Et
AcO
Et
OAc
Et
Et
Et
Et
14a, 65
5
14b, (10)c
AcO
AcO
OAc
Et
Et
OCOEt
Et
OCOEt
O
15a, trace
15b, 39
6d
O
OCOEt
OCOEt
16a, trace
16b, 41
7d
7
3a
3b
3b
16a
3b
3c
16b
3c
OCOEt
AcO
OAc
AcO
AcO
OAc
OAc
3b, 65
3c, 20
8e
OAc
AcO
AcO
AcO
OAc
OAc
9f
3c, 40
3c, 49
OAc
OAc
OAc
AcO
10g
3c
a Reaction conditions: arene (0.5 M, 0.5 mmol), DAIB 1 (3 equiv), 110 °C, DCE, 16 h.
b Isolated yields after column chromatography.
c Estimated yield by GC analysis.
d Reaction run in propionic acid.
e DAIB 1 (2 equiv) used.
f DAIB 1 (1.3 equiv), 110 °C, 16 h, 58% conversion.
g DAIB 1 (2 equiv), 110 °C, 16 h, 58% conversion.
Synthesis 2012, 44, 2463–2468
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Oxidative Polyacyloxylation of Hindered Arenes
2467
1,3-Diacetoxy-2,4,5,6-tetramethylbenzene (10b)
In conclusion, we have described a gold(I)-catalyzed
methodology that affords direct polyacyloxylation of non-
activated hindered arenes. The reaction proceeds in the
presence of a gold(I) precatalyst [(PPh3)AuCl] using
di(acetoxy)iodobenzene as a source of acetoxy or acyloxy
moieties. Although yields remain moderate, the present
methodology allows direct carbon–oxygen bond forma-
tion with challenging, unactivated and sterically encum-
bered arene substrates.
Yield: 37 mg (29%); white solid.
1H NMR (300 MHz, CDCl3): δ = 2.34 (s, 6 H), 2.20 (s, 3 H), 2.06
(s, 6 H), 1.93 (s, 3 H).
13C NMR (75 MHz, CDCl3): δ = 169.0, 146.0, 134.5, 126.4, 119.9,
20.5, 16.1, 13.3, 10.4.
MS (CI, NH3): m/z (%) = 268 (100) [M + NH4]+.
1,3-Diacetoxy-5-bromo-2,4,6-trimethylbenzene (12b)
Yield: 43 mg (43%); white solid.
1H NMR (300 MHz, CDCl3): δ = 2.35 (s, 6 H), 2.23 (s, 6 H), 1.91
(s, 3 H).
All starting materials were purchased from commercial sources and
used without further purification. 1H, 13C and 31P NMR spectra were
recorded with a Bruker AV 300 instrument. Signals are expressed
in ppm (δ) and internally referenced to residual protic solvent sig-
nals. Coupling constants (J) are reported in Hz and refer to apparent
peak multiplicities. Mass spectrometry analyses (direct introduction
by either chemical ionization with ammonia or electrospray ioniza-
tion) were performed at the Ecole Nationale Supérieure de Chimie
de Paris, Chimie ParisTech. Column chromatography was per-
formed with GEDURAN Si 60 silica gel (40–63 μm). Petroleum
ether (PE) refers to the fraction boiling in the 40–65 °C range.
Structure and purity of the isolated products 3a,21 6,22 8a,23 10a,14
12a,14 14a,14 15a,14 16a14 and 16b14 were confirmed by comparison
of their spectral data with those reported in the literature.
13C NMR (75 MHz, CDCl3): δ = 167.4, 145.1, 127.8, 121.5, 19.3,
16.3, 9.5.
MS (CI, NH3): m/z (%) = 334 (100) [M + NH4]+.
1,3-Bis(propionyloxy)-2,4,6-trimethylbenzene (15b)
Yield: 21 mg (39%); yellow oil.
1H NMR (300 MHz, CDCl3): δ = 6.93 (s, 1 H), 2.62 (q, J = 7.6 Hz,
4 H), 2.09 (s, 6 H), 1.92 (s, 3 H), 1.31 (t, J = 7.6 Hz, 6 H).
13C NMR (75 MHz, CDCl3): δ = 172.1, 146.4, 129.6, 127.5, 123.0,
27.4, 16.1, 9.4.
MS (CI, NH3): m/z (%) = 282 (100) [M + NH4]+.
Synthesis of 2,4,6-Trimethoxyphenyl Phenyl Iodonium Chlo-
ride (5)
Mesitylene Polyacetoxylation in DCE; Typical Procedure
1,3,5-Trimethylbenzene (2; 60 mg, 0.5 mmol) was added to a solu-
tion of di(acetoxy)iodobenzene (DAIB, 1; 483 mg, 1.5 mmol) and
[(PPh3)AuCl] (4.9 mg, 0.01 mmol) in DCE (1 mL) under an air at-
mosphere in a pressure tube. The tube was sealed and the reaction
was heated to 110 °C for 16 h. The volatiles were removed under re-
duced pressure and the crude residue was purified by flash column
chromatography (PE–EtOAc, 98:2→70:30).
1,3,5-Trimethoxybenzene (4; 168 mg, 1 mmol) was added to a so-
lution of di(acetoxy)iodobenzene (DAIB, 1; 322 mg, 1 mmol) in
glacial AcOH (1 mL) under an air atmosphere. The reaction mixture
was heated to 110 °C for 1 h. After cooling to r.t., the mixture was
diluted with EtOAc (15 mL) and treated with sat. aq NaHCO3 (15
mL). The aqueous layer was acidified with 1 M HCl and extracted
with EtOAc (3 × 30 mL). The organic layer was collected, dried
with MgSO4, and the solvents were removed under reduced pres-
sure to give 5.
Mesitylene Polyacetoxylation in AcOH; Typical Procedure
1,3,5-Trimethylbenzene (2; 60 mg, 0.5 mmol) was added to a solu-
tion of di(acetoxy)iodobenzene (DAIB, 1; 483 mg, 1.5 mmol) and
[(PPh3)AuCl] (4.9 mg, 0.01 mmol) in glacial AcOH (1 mL) under
an air atmosphere in a pressure tube. The tube was sealed and the
reaction was heated to 110 °C for 16 h. After cooling to r.t., the mix-
ture was diluted with (i-Pr)2O (15 mL) and treated with sat. aq
NaHCO3 (15 mL). The organic layer was collected, dried with
MgSO4, the solvents were removed under reduced pressure, and the
crude residue was purified by flash column chromatography (PE–
EtOAc, 98:2→70:30).
Yield: 320 mg (80%); white powder.
1H NMR (300 MHz, CDCl3): δ = 7.99–7.95 (m, 2 H), 7.44–7.38 (m,
1 H), 7.29–7.25 (m, 2 H), 6.14 (s, 2 H), 3.85 (s, 3 H), 3.84 (s, 6 H).
13C NMR (75 MHz, CDCl3): δ = 166.2, 160.4, 133.3, 131.1, 130.6,
119.7, 91.4, 90.1, 56.7, 55.8.
MS (ESI): m/z (%) = 371 (100) [C15H16O3I]+.
Acknowledgment
1,3-Diacetoxy-2,4,6-trimethylbenzene (3b)
Yield: 11 mg (31%); white solid.
This work was supported by the Centre National de la Recherche
Scientifique and the Ministère de l’Education et de la Recherche.
A.P. is grateful to the National Research Agency (ANR-09-JCJC-
0078) for a doctoral grant (2009-2012). Johnson-Matthey is
acknowledged for a generous loan of HAuCl4.
1H NMR (300 MHz, CDCl3): δ = 6.94 (s, 1 H), 2.33 (s, 6 H), 2.11
(s, 6 H), 1.95 (s, 3 H).
13C NMR (75 MHz, CDCl3): δ = 168.8, 146.4, 129.7, 127.7, 123.0,
20.4, 16.1, 10.3.
MS (CI, NH3): m/z (%) = 254 (100) [M + NH4]+.
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1,3,5-Triacetoxy-2,4,6-trimethylbenzene (3c)
Yield: 21 mg (12%); white solid.
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© Georg Thieme Verlag Stuttgart · New York
Synthesis 2012, 44, 2463–2468

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A. Pradal et al.
PAPER
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