One-pot catalysis of dehydrogenation of cyclohexanones to phenols and oxidative Heck coupling: Expedient synthesis of coumarins

Article abstract of DOI:10.1039/c3cc41296b

One-pot reactions leading to highly functionalized coumarins have been developed via a Pd(ii)-catalyzed dehydrogenation-oxidative Heck-cyclization process.

Full text of DOI:10.1039/c3cc41296b

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One-pot catalysis of dehydrogenation of
cyclohexanones to phenols and oxidative Heck
coupling: expedient synthesis of coumarins†
Cite this: Chem. Commun., 2013,
49, 4021
Received 19th February 2013,
Accepted 24th March 2013
Donghee Kim, Minsik Min and Sungwoo Hong*
DOI: 10.1039/c3cc41296b
www.rsc.org/chemcomm
One-pot reactions leading to highly functionalized coumarins have
been developed via a Pd(II)-catalyzed dehydrogenation–oxidative
Heck–cyclization process.
The direct functionalization of C–H bonds in (hetero)arenes is a
highly desirable process for enhancing the efficiency of formation of
new C–C and C–X bonds in the organic synthesis.1 Recently, the
palladium(II)-catalyzed dehydrogenation of cyclohexanones has
been demonstrated to be a highly efficient process for generating
substituted phenol derivatives.2 This route offers a promising
alternative to classical procedures because electrophilic aromatic
substitutions of phenols are limited to ortho- and para-positions due
to the influence of strong electron directing effects. A one-pot
procedure is synthetically and environmentally advantageous for
carrying out multistep reactions in one synthetic operation, thereby
reducing synthesis time, cost and undesired waste. The develop-
ment of one-pot catalysis of tandem processes is a desirable strategy
for generating molecular complexity from simple starting materials
by employing a single catalyst in a single reaction vessel.3
Coumarin derivatives exhibit a broad range of biological
activities4 and also comprise one of the most widely used classes of
fluorophores for their outstanding optical properties.5 Extensive
synthetic efforts have therefore been made to introduce a range of
groups into the coumarin core.6 Among them, an efficient synthesis
of coumarin from phenol has also been developed.7 We speculated
that the phenol generated in situ from the Pd(II)-catalyzed dehydro-
genation could be utilized for further oxidative cross-coupling via
the electrophilic palladation of phenol using the same Pd(II) catalyst
system.8 If successful, the one-pot reaction sequence would represent
a practical and exceptionally efficient method for accessing diversely
substituted coumarins (Scheme 1). Herein, we report the first
example of a sequential Pd(II)-catalyzed dehydrogenation–oxidative
Heck9–cyclization process using readily available cyclohexanones
and alkenes, providing an efficient and straightforward protocol
for preparing a variety of coumarins.
Scheme 1 Strategy for a one-pot process involving dehydrogenation–oxidative
Heck–cyclization starting from cyclohexanones.
We explored the potential of the proposed sequential reactions by
investigating the reactivity of 4-phenylcyclohexanone (1a) and butyl
acrylate (2a) as model substrates (Table 1). The starting material 1a
underwent dehydrogenation to provide appreciable amounts of a
phenol intermediate; however, no coumarin product was isolated
under 1 atm O2 in the presence of a Pd(II) catalyst (Table 1, entry 1).
Table 1 Optimization studies of the one-pot process of sequential dehydro-
genation–oxidative Heck–cyclizationa
Entry
Pd
Oxidant
Additive (equiv.)
Yieldb (%)
1
2
3
4
5
6
7
8
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
—
DDQ
K2S2O8
Cu(OAc)2
CuCO3ÁCu(OH)2
Cu(OAc)2
Cu(OAc)2
CuCO3ÁCu(OH)2
CuO
TsOH (0.2)
—
—
—
—
Trace
Trace
63
69
47
23
80
68
Trace
81
—
NaOAc (1)
Ag2CO3 (1)
—
—
—
—
9
10
11
CuCl2
Cu(Oac)2
a
Department of Chemistry, Korea Advanced Institute of Science and Technology
(KAIST), Daejeon, 305-701, Korea. E-mail: hongorg@kaist.ac.kr;
Fax: +8242-350-2810; Tel: +8242-350-2811
Reactions were conducted with 1a (1.0 equiv.), 2a (1.2 equiv.), PdL2
(0.2 equiv.), and oxidant (1.0 equiv.) in PivOH at 110 1C for 20 h under
1
b
atm O2. Yield of the isolated product. DDQ
=
2,3-dichloro-
5,6-dicyano-1,4-benzoquinone, TFA = trifluoroacetate.
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cc41296b
c
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Table 2 Cyclic ketone substrate scopea
Table 2 (continued )
Entry Substrate
Product
Yieldc [%]
67
Entry Substrate
Product
Yieldc [%]
15
3o
56
1
3b
3b
3c
a
Conditions: 1 (1.0 equiv.), 2a (1.2 equiv.), Pd(TFA)2 (0.2 equiv.),
Cu(OAc)2 (1.0 equiv.), PivOH, 110 1C, 17–35 h (see the ESI for details),
2
3
4
72
59
56
b
1 atm O2. Reaction was conducted with PivOH–1,2-dioxane (1 : 5) as a
c
solvent. Yield of the isolated product.
We therefore examined the ability of other oxidizing agents, such as
Cu(II), Ag(II), OXONE, DDQ, and K2S2O8, to facilitate the re-oxidation of
Pd(0) to Pd(II). The copper source was critical to the efficiency of the
coupling reaction between the resulting phenol and the butyl acrylate.
With a selected set of copper sources, the desired 7-phenylcoumarin
(3a) was readily produced, proving that the sequential process indeed
proceeded (entries 4 and 5). A variety of Cu species were then evaluated,
and Cu(OAc)2 was found to be the most effective and economical
oxidant for promoting the reactions.10 Among the palladium sources
tested, Pd(TFA)2 displayed the best catalytic reactivity. Under optimized
reaction conditions, the sequential dehydrogenation–oxidative Heck–
cyclization process of 1a (1 equiv.), in the presence of butyl acrylate
(1.2 equiv.), Pd(TFA)2 (0.2 equiv.), and Cu(OAc)2 (1 equiv.) in pivalic acid
at 110 1C, proceeded to provide the best isolated yield of 82%.
With this one-pot procedure in hand, the scope and generality of
this methodology were explored using a variety of cyclic ketones
(Table 2). A variety of cyclic ketone substrates were compatible with
the one-pot conditions, affording modest to good yields of the desired
coumarin products. A slightly higher product yield was obtained when
cyclohexenone was used as a substrate (entries 1 and 2). The reaction
of the C3-substituted cyclohexanones, which are readily accessed via
the 1,4-addition to the cyclohex-2-enone proceeded with complete
regioselectivity to provide the C7-functionalized coumarins (Table 2,
entries 3–7). The introduction of electron-donating groups, such as
OMe, at position 7 on the coumarin core triggers a bathochromic shift
with strong charge transfer character. The methodology was success-
fully applied to 3-methoxycyclohexanone to enable the C7 installation
of a methoxy group on the coumarin core under slightly modified
reaction conditions in which dioxane was employed as a co-solvent
(Table 2, entry 8). The scope of the reactions could be expanded
from the cyclohexanones to bicyclic ketone systems, leading to the
formation of coumarins 3n and 3o (entries 14 and 15).11
3d
3e
5
6
7
52
70
46
3f
3g
8b
3h
3i
56
77
9
10
11
12
3j
78
81
85
3k
3l
Encouraged by the successful transformation of cyclohexanones
to coumarins, we turned our attention to the alkene substrate scope
to introduce more structural diversity around the coumarin core. We
were pleased to observe that alkene substrate variations were viable
under optimized conditions, leading to the formation of synthetically
and biologically useful C3- or C4-phenyl coumarins in moderate to
good yields (Table 3). Notably, expanding the scope from a phenyl to
a naphthyl substituted system 4n, it was also possible to facilitate the
process, and highlight the generality of this method.
13
14
3m 38
As an extension of these studies, we preliminarily investi-
gated the potential of this catalytic system to catalyze further
3n
48
c
4022 Chem. Commun., 2013, 49, 4021--4023
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Table 3 Alkene substrate scopea
Scheme 2 Strategy for a one-pot process involving dehydrogenation–oxidative
Heck–cyclization–alkenylation starting from cyclohexanones.
available cyclohexanones in a single synthetic operation. The
substrate scope is broad and permits the construction of a
variety of substituted coumarins. The methodology is syntheti-
cally useful and presents considerable advantages in both
simplicity and efficiency.
This work was supported by the Research Center Program of
Institute for Basic Science (IBS) in Korea and by National
Research Foundation of Korea (NRF) through general research
grants (NRF-2011-0016436, 2011-020322).
Notes and references
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a
Conditions: cyclohexanone (1.0 equiv.), alkene (1.2 equiv.), Pd(TFA)2
(0.2 equiv.), Cu(OAc) (1.0 equiv.), PivOH, 110 1C, 15–30 h, 1 atm O2.
Yield of the isolated product.
´
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sequential transformations. Our group recently reported the
Pd(II)-catalyzed direct alkenylation of coumarins at the C3 position
to extend the p-electron system.6f Based on the mechanistic path-
way, we envisaged that a further consecutive reaction would be
possible because the resulting coumarins were susceptible to
electrophilic palladation at the C3 position (Scheme 2). Indeed,
we were delighted to observe that the further C3-alkenylation
process worked well by sequential addition of alkene (2 equiv.)
and K2CO3 (3 equiv.) to provide 3-vinylcoumarins 5a and 5b.
In summary, we presented the first example of a one-pot 10 For other substrates, higher yields (4–11%) were obtained with the
use of Cu(OAc)2 in place of CuCO3ÁCu(OH)2.
process involving dehydrogenation–oxidative Heck–cyclization
in the presence of the Pd(II) catalyst system, offering an efficient
11 The parent cyclohexanone substrates bearing an ortho substituent
decomposed under the optimized one-pot conditions, and a two-
route to highly functionalized coumarins starting from readily
step procedure was required to yield the coumarins.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 4021--4023 4023