Palladium-catalyzed dehydrogenative β-arylation of simple saturated carbonyls by aryl halides

Article abstract of DOI:10.1021/cs501326p

(Chemical Equation Presented) A versatile palladium-catalyzed synthesis of highly substituted α,β-unsaturated carbonyl compounds has been developed. In contrast to the known Heck-type coupling reaction of unsaturated carbonyl compounds with aryl halides, the present methodology allows the use of stable and readily available saturated carbonyl compounds as the alkene source. In addition, the reaction proceeds well with low catalyst loadings and does not require any expensive metal oxidants or ligands. A variety of saturated aldehydes, ketones, and esters are compatible for the reaction with aryl halides under the developed reaction conditions to afford α,β-unsaturated carbonyl compounds in good to excellent yields. A possible reaction mechanism involves a palladium-catalyzed dehydrogenation followed by Heck-type cross couplings.

Full text of DOI:10.1021/cs501326p

Research Article
pubs.acs.org/acscatalysis
Palladium-Catalyzed Dehydrogenative β‑Arylation of Simple
Saturated Carbonyls by Aryl Halides
Parthasarathy Gandeepan, P. Rajamalli, and Chien-Hong Cheng*
Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan
S
* Supporting Information
ABSTRACT: A versatile palladium-catalyzed synthesis of highly
substituted α,β-unsaturated carbonyl compounds has been
developed. In contrast to the known Heck-type coupling reaction
of unsaturated carbonyl compounds with aryl halides, the present
methodology allows the use of stable and readily available
saturated carbonyl compounds as the alkene source. In addition,
the reaction proceeds well with low catalyst loadings and does
not require any expensive metal oxidants or ligands. A variety
of saturated aldehydes, ketones, and esters are compatible for the reaction with aryl halides under the developed reaction conditions
to afford α,β-unsaturated carbonyl compounds in good to excellent yields. A possible reaction mechanism involves a palladium-
catalyzed dehydrogenation followed by Heck-type cross couplings.
KEYWORDS: palladium, dehydrogenation, alkene, aryl halide, cross coupling
Scheme 1. Pd-Catalyzed Reactions of Carbonyl Compounds
with Aryl Halides
INTRODUCTION
■
Highly substituted α,β-unsaturated carbonyl compounds are
an important class of organic compounds because of their
ubiquitous presence in natural and bioactive molecules.1
Moreover, they are also found to be potential intermediates in
numerous organic and medicinal syntheses.2 The classical
methods of obtaining these compounds including Wittig,
Horner−Wadsworth−Emmons, and Peterson olefination reac-
tions are less attractive owing to their substrate unavailability,
phosphorus byproduct, and sensitive reaction conditions.3
Similarly, aldol and Claisen−Schmidt condensation reactions
to synthesize α,β-unsaturated carbonyl compounds are less
appealing because of their basic and harsh reaction conditions.4
In addition to these methods, several metal-mediated or -catalyzed
reactions have been developed for the formation of α,β-unsaturated
carbonyl compounds.5,6
In recent years, the transition-metal-catalyzed dehydrogen-
ation of saturated C−C bonds into unsaturated CC bonds has
emerged as an effective method in organic synthesis. In
particular, the dehydrogenation of saturated carbonyl com-
pounds into unsaturated or aromatic compounds provides a new
effective synthesis approach.7 In this context, dehydrogenative
coupling reactions of saturated compounds with aryl nucleophiles
(heteroarenes and aryl carboxylic acids) to form substituted
unsaturated carbonyl compounds is limited to specific substrates
and required expensive metal oxidants.8 Alternatively, the use of
simple and largely available aryl halides in the dehydrogenative
coupling of saturated carbonyl compounds is very challenging
because of the facile α-arylation9 or β-arylation10 instead of
dehydrogenation (Scheme 1). Our continuous interest in the
development of new and efficient organic transformations using
transition-metal catalysts11 prompted us to tackle these problems.
Herein, we report a straightforward route for the synthesis of
highly substituted α,β-unsaturated carbonyl compounds via
palladium-catalyzed dehydrogenative coupling of saturated
carbonyl compounds with aryl halides.
RESULTS AND DISCUSSION
■
We started our investigations using 2-phenylpropanal (1a) and
4-iodotoluene (2a) as the substrates. After a series of systematic
investigations, we found that the reaction of 1a (0.50 mmol), 2a
(1.25 mmol), Na2CO3 (1.0 mmol), and Pd(OAc)2 (0.010 mmol,
2 mol %) in dimethyl sulfoxide (DMSO) at 120 °C for 15 h gave
arylated product (E)-2-phenyl-3-(p-tolyl)acrylaldehyde (3a) in
85% isolated yield. (See the Supporting Information for the
detailed optimization studies.) The present catalytic reaction
conditions show excellent catalytic activity requiring low catalyst
loading without the need for an external oxidant and ligand.
Next, we tested the generality of the reaction by using 1a
with various aryl iodides, and the results are presented in Table 1.
Received: September 3, 2014
Revised: October 10, 2014
© XXXX American Chemical Society
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Table 1. Scope of Diarylacrylaldehyde Formation from
2-Phenylpropanal
Table 2. Scope of the Dehydrogenative Arylation Reaction of
Ethyl Aryl Ketones with Aryl Iodides
a b c
, ,
a b
,
a
Unless otherwise mentioned, all reactions were carried out using 1a
a
Unless otherwise mentioned, all reactions were carried out using 4
(0.5 mmol), ArI (1.25 mmol), Pd(OAc)2 (0.010 mmol), and Na2CO3
(0.5 mmol), ArI (1.75 mmol), Pd(OAc)2 (0.020 mmol), and Na2CO3
b
(1.0 mmol) in DMSO (2 mL) at 120 °C for 15 h. Isolated yields.
b
(1.5 mmol) in DMSO (2 mL) at 120 °C for 24 h. Isolated yields.
c
PdCl2(PPh3)2 (0.010 mmol) was used instead of Pd(OAc)2 for 3f and 3h.
Table 3. Scope of Dehydrogenative Arylation Reaction of
α-Substituted Propiophenone 6 with Aryl Iodides
a b
,
The catalytic reaction is compatible with both electron-donating
and electron-withdrawing groups on the aryl iodides (Table 1).
It is important to mention that there is no simple route to give
unsymmetrical 1,2-diaryl acrylaldehydes.
To extend the scope of the present reaction, we tested ethyl
aryl ketones under the standard reaction conditions. Thus, the
reaction of propiophenone (4a) and 4-iodotoluene (2a) gave
β-diarylation product 5a in 65% isolated yield with only a trace
of monoarylation product 5aa (eq 1). By increasing the amount of
aryl iodide, base, reaction time, and catalyst loading, the reaction
gave 5a as the sole product in 91% isolated yield (Table 2). It is
worth mentioning that the synthesis of trisubstituted alkenes via
metal-catalyzed cross-coupling reactions is quite challenging.12
Under these modified reaction conditions, the reaction of various
substituted aryl ketones with aryl iodides to give trisubstituted
alkenes was examined, and the results are shown in Table 2. The
reaction worked well with diversely substituted ketones and aryl
iodides to afford respectively substituted chalcones in very good
yields.
Similar to the reaction of 2-phenylpropanal (1a) with aryl
iodide 2, α-aryl/alkyl substituted propiophenone 6 underwent
dehydrogenative mono β-diarylation with aryl iodides to give
various trisubstituted enone products 7a−f (Table 3). In all
cases, only E isomers were obtained. The stereochemistry of
a
Unless otherwise mentioned, all reactions were carried out using 6
(0.50 mmol), ArI (1.25 mmol), Pd(OAc)2 (0.020 mmol), and
Na2CO3 (1.0 mmol) in DMSO (2 mL) at 120 °C for 24 h. Isolated
yields.
b
products 7c and 7e was further confirmed by single-crystal
X-ray diffraction. The transition-metal-catalyzed hydroacyla-
tion of alkyne is a common strategy often employed to attain
compound 7. However, the use of unsymmetrical alkyne in
the hydroacylation reaction produces inseparable regio-
and stereoisomeric mixtures, which significantly limits their
applicability.13 The present reaction system overcomes the
limitation and allows the synthesis of different substituted
2,3-diaryl vinyl ketones.
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The present catalytic reaction can be further extended to the
dehydrogenative coupling of alkyl carbonyl compounds such as
propionaldehyde (8a), butan-2-one (8b), and methyl propionate
(8c) with aryl iodides (Table 4). Thus, the reaction of 8a with
These results suggest that the dehydrogenation step is slower than
the Heck reaction step.
The reaction of aryl bromides with 1a or 4a under the above
reaction conditions did not give the expected dehydrogenative
coupling products. However, the reaction of 2-phenylpropanal (1a)
and 4-bromotoluene (16a) proceeded smoothly in the presence of
bidentate phosphine ligand 1,4-bis(diphenylphosphino)butane
(dppb) in dimethylformamide (DMF) instead of DMSO. Unlike
the reaction of 1a with aryl iodide, aryl bromide gave both mono-
and diarylated products in good yields (Scheme 2). Under similar
reaction conditions, propiophenone (4a) also reacted with aryl
bromides 16 to give diarylation products 5 in moderate to good
yields (Scheme 2).
Table 4. Scope of Dehydrogenative Arylation Reaction of
a b
,
Alkyl Carbonyl Compounds with Aryl Iodides
To understand the nature of the present catalytic reaction,
we carried out a deuterium scrambling reaction for substrates 1a
and 4a by the addition of 10 equiv of CD3OD under standard
reactions in the absence of aryl halides (Scheme 3). Both
substrates showed 50% deuterium exchange at the α-carbon after
5 h of reaction, whereas no deuterium scrambling was observed
at β-carbon or aromatic hydrogens.
On the basis of these experimental results and the existing
literature, a possible catalytic cycle is depicted in Scheme 4. The
catalytic cycle is initiated by the oxidative addition of aryl iodide
to palladium(0) to form Ar−PdII-I. Then, the substitution of
I− into arylpalladium iodide species by carbonyl compound 1a in
the presence of Na2CO3 affords intermediate I. PdII-enolate I
could exist as either an O-bound or C-bound complex. In general,
late transition metals favor C-bound over O-bound enolates,
and thus palladium favors the formation of intermediate I. Facile
β-hydride elimination7,8 of intermediate I under the reaction con-
ditions gave unsaturated carbonyl compound II and Ar−Pd−H,
which then underwent reductive elimination to form Pd(0) and
ArH. The classical Heck reaction of intermediate II with ArI in
the presence of Pd(0) and base gave final product 3 and Pd(0).
In contrast to the known palladium-catalyzed dehydrogenation
reactions, the present reaction proceeds well without expensive
metal oxidants. ArX indirectly acts as an oxidant of palladium
under this catalytic reaction. The formation of ArH from ArX
under our reaction conditions is evidenced by the isolation of
naphthalene in the synthesis of 3ak from 1a (eq 5). Furthermore,
a
Unless otherwise mentioned, all reactions were carried out using 8
(0.5 mmol), ArI (1.75 mmol), Pd(OAc)2 (0.020 mmol), and Na2CO3
(1.5 mmol) in DMSO (2 mL) at 120 °C for 24 h. Isolated yields.
b
various aryl iodides afforded 3,3-diarylacrylaldehydes 9aa−af
in 61−82% yields. Likewise, ethyl methyl ketone 8b underwent
a dehydrogenative coupling reaction with different aryl iodides
to give 4,4-diarylbut-3-en-2-one compounds 9ba−be in
moderate yields. The syntheses of compounds 9aa−af and
9ba−be from simple acrylaldehyde or methyl vinyl ketone are
limited by facile polymerization.14
Surprisingly, β-substituted carbonyl compounds did not give
dehydrogenative coupling products under similar reaction condi-
tions (eq 2). The reaction of carbonyl compounds containing
the formation of intermediate II was evidenced by the isolation
of 1,2-diphenylprop-2-en-1-one (7A) from the reaction of 1,2-
diphenylpropan-1-one (6) and iodobenzene with a shorter reac-
tion time (eq 6). The reaction of 7A with iodobenzene under the
standard catalytic reaction conditions affords (E)-1,2,3-triphenyl-
prop-2-en-1-one 7a in 81% isolated yield (eq 7). The results
both substituted and unsubstituted β-carbons functionalized
regioselectively at the unsubstituted β-carbon (eq 3) occurred.
However, diethyl ketone reacted with 4-iodotoluene (2a) under
similar reaction conditions to form 1,1-diarylation product 15a in
17% yield and 1,1,5,5-tetraarylation product 15b in 26% yield (eq 4).
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Scheme 2. Pd-Catalyzed Dehydrogenative Coupling Reaction Using Aryl Bromides
system supports β-hydrogen elimination over reductive elimi-
Scheme 3. Deuterium Exchange Studies
nation.
CONCLUSIONS
■
We have successfully demonstrated a versatile method for
the synthesis of highly substituted α,β-unsaturated carbonyl
compounds by the palladium-catalyzed dehydrogenative
coupling reaction of saturated carbonyl compounds with
aryl halides. The reaction proceeds with broad substrate
scope and good to excellent yields. The present catalytic
system does not require expensive metal oxidants. A possible
mechanism of the catalytic reaction is proposed, and key
intermediates were isolated to support the proposed reaction
mechanism.
obtained from eqs 5−7 strongly support the proposed reaction
mechanism in Scheme 4.
The selective formation of unsaturated compound II over
the other possible α- or β-arylations from 1a (Scheme 3) is
the success of the present catalyst system. It is known that
β-hydrogen elimination from α-metal carbonyl compound I has
an activation energy that is lower than their reductive elimi-
nation.10a,15 Furthermore, the general criteria for β-hydrogen
elimination is the presence of a vacant site in a metal complex.
Under the present reaction condition, intermediate I has four
coordination sites, from which two are filled by alkyl and aryl
groups and the other two ligands most probably are filled by
DMSO. It has been reported that DMSO can coordinate to Pd
very weakly and in a reversible manner.16 Thus, the reaction
ASSOCIATED CONTENT
■
S
* Supporting Information
The following files are available free of charge on the ACS
Publications website at DOI: 10.1021/cs501326p.
General experimental procedures, characterization details,
and 1H and 13C NMR spectra of new compounds (PDF).
Crystallographic data file data_121035lt_0m (CIF).
Crystallographic data file data_mo_140935lt_0m (CIF).
Crystallographic data file data_140936LT_0m (CIF).
Scheme 4. Proposed Catalytic Cycle
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J.; Li, C.-J. Org. Lett. 2012, 14, 5606−5609. (l) Lu, Y.; Nguyen, P. L.;
AUTHOR INFORMATION
Corresponding Author
*Fax: (+886) 3572-4698. E-mail: chcheng@mx.nthu.edu.tw.
Home page: http://mx.nthu.edu.tw/∼chcheng/.
Notes
■
́
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Ministry of Science and Technology of the
Republic of China (MOST-102-2633-M-007-002) for its
support of this research.
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