Palladium(II) Acetate Catalyzed Reductive Heck Reaction of Enones; A Practical Approach

Article abstract of DOI:10.1002/cctc.201500760

A surprisingly practical Pd(OAc)₂ or Pd(TFA)₂-catalyzed reductive Heck reaction between aryl iodides and α,β-unsaturated ketones is described using N,N-diisopropylethylamine (DIPEA, Hünigs base) as the reductant. In general, 1 mol % of Pd(OAc)₂ is sufficient to afford good yields using electron-rich or halogen-substituted aryl iodides. Electron-deficient aryl iodides preferentially give homocoupling. Enones containing aryl or bulky substituents on the β-carbon react smoothly, producing mainly reductive Heck product, whereas enones with alkyl substituents on the β-carbon afford a mixture of reductive Heck and Heck product. Deuterium labeling experiments show that the reaction is a bona fide reductive Heck reaction and exclude a Heck reaction-conjugate reduction cascade. Deeply DIPEA about... A palladium-catalyzed reductive Heck reaction of aryl iodides to α,β-unsaturated ketones is described using N,N diisopropylethylamine (DIPEA, Hünigs base) as the reductant. A deuterium labeling study shows that the reaction is a bona fide reductive Heck reaction and excludes a Heck reaction-conjugate reduction cascade.

Full text of DOI:10.1002/cctc.201500760

DOI: 10.1002/cctc.201500760
Full Papers
Palladium(II) Acetate Catalyzed Reductive Heck Reaction
of Enones; A Practical Approach
Subramaniyan Mannathan,[a] Saeed Raoufmoghaddam,[b] Joost N. H. Reek,[b] Johannes G. de
Vries,*[a, c] and Adriaan J. Minnaard*[a]
A surprisingly practical Pd(OAc)2 or Pd(TFA)2-catalyzed reduc-
tive Heck reaction between aryl iodides and a,b-unsaturated
ketones is described using N,N-diisopropylethylamine (DIPEA,
Hünigs base) as the reductant. In general, 1 mol% of Pd(OAc)2
is sufficient to afford good yields using electron-rich or halo-
gen-substituted aryl iodides. Electron-deficient aryl iodides
preferentially give homocoupling. Enones containing aryl or
bulky substituents on the b-carbon react smoothly, producing
mainly reductive Heck product, whereas enones with alkyl sub-
stituents on the b-carbon afford a mixture of reductive Heck
and Heck product. Deuterium labeling experiments show that
the reaction is a bona fide reductive Heck reaction and exclude
a Heck reaction-conjugate reduction cascade.
Introduction
Transition-metal catalyzed conjugate addition of organometal-
lic reagents to enones, and other electron-deficient alkenes is
an efficient method to form carbon-carbon bonds in organic
synthesis.[1] A variety of organometallic reagents, for example,
organoboron, -zinc, -aluminum, -silicon, and -magnesium
(Grignard) have been used with a plethora of Michael accept-
ors.[2–6] Whereas the addition of alkyl, and to a lesser extent al-
kenyl and alkynyl, fragments is dominated by copper catalysis,
the addition of aryl groups has been reported mainly using
rhodium and palladium catalysis.
chiometric amounts of organometallic reagents and anhydrous
conditions, but uses the parent aryl halides instead. Possibly
because the reaction is reminiscent of the Heck reaction and
more readily designed in an intramolecular fashion, most re-
ports on the reductive Heck reaction concern the latter pro-
cess, recently also with high enantioselectivity.[13] We recently
studied the intermolecular reductive Heck reaction with aryl
halides and were able to dissect the different factors that lead
to Heck reaction versus reductive Heck reaction.
Mechanistically both reactions proceed through a common
aryl-Pd intermediate A, but to convert A into the reductive
Heck product either protonation with subsequent reduction of
PdII to Pd0 or formation of a hydrido-palladium-alkyl complex
followed by a reductive elimination is necessary (Scheme 1). In
1983, Cacchi and coworkers reported the first example of palla-
dium-catalyzed reductive Heck reaction of phenyl iodides to
enones and enals in the presence of triethylamine, tetrabuty-
lammonium iodide, and formic acid.[10] In the absence of
formic acid, the reaction still proceeded but much less effi-
cient. We described the reductive Heck reaction of phenyl io-
dides to enones using an N-heterocyclic carbene (NHC) palladi-
um complex as the catalyst in the presence of tributylamine as
the reductant.[11] This reaction is very effective, makes the addi-
tion of tetrabutylammonium iodide, and formic acid redundant
and affords the reductive Heck products exclusively and in
high yields. More recently, the groups of Deng[14a,b] and Dou-
cet[14c] developed a desulfitative palladium-catalyzed conjugate
addition reaction to enones. In the reaction, benzenesulfonyl
chloride and arylsulfonyl hydrazides are used as the arylating
agent to obtain selectively the conjugate addition product.
As our initially reported reaction[11] was rather slow, we con-
tinued studying this reaction. This resulted in a novel catalytic
system reported here, based on readily available Pd(OAc)2 and
Hünigs base (DIPEA) that is in several aspects as effective as
the Pd-NHC catalyst system and considerably faster.[19]
On the other hand, little effort has been devoted to value
the conjugate addition of the corresponding aryl halides,[7–11]
although this pathway is often observed as a side reaction in
the corresponding Mizoroki–Heck reaction.[12] In this process,
not the reagent but the catalyst is subjected to umpolung
and, therefore, a stoichiometric reductant has to be added.
Nevertheless, in particular from a practical point of view, this
conjugate addition (also known as reductive Heck) reaction is
much more convenient as it does not involve the use of stoi-
[a] Dr. S. Mannathan, Prof. Dr. J. G. de Vries, Prof. Dr. A. J. Minnaard
Stratingh Institute for Chemistry
University of Groningen
Nijenborgh 7, 9747 AG, Groningen (The Netherlands)
Fax: (+31)50-363-429
E-mail: A.J.Minnaard@rug.nl
[b] Dr. S. Raoufmoghaddam, Prof. Dr. J. N. H. Reek
Van ‘t Hoff Institute for Molecular Sciences
University of Amsterdam
Science Park 904, 1098 XH, Amsterdam (The Netherlands)
[c] Prof. Dr. J. G. de Vries
Leibniz-Institut für Katalyse e.V. an der
Universität Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
E-mail: Johannes.deVries@catalysis.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500760.
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1a, if any, was observed (entries 2, 3, and 5), indicat-
ing a remarkable effect of the precise nature of the
amine (vide infra).[19a] Already in an early stage of the
research we noted a very low conversion with trie-
thylamine. With DIPEA, 3a was obtained in a much
better yield (55%) (entry 4) along with a minor
amount of Heck product 4a and a large amount of
homocoupling product 7. Notably, however, side
products originating from Heck reaction with oxi-
dized DIPEA were not formed. Replacing DMF with
NMP made the reaction medium homogeneous and
afforded 3a in 50% yield (entry 6). Use of other sol-
vents including THF and acetonitrile gave 3a in low
yield (entries 7 and 8). A slight increase in the ratio of benzala-
cetone (1a) to 4-iodoanisole (2a) considerably improved the
yield of 3a (entry 9) in a shorter reaction time of 2 h, and sup-
pressed formation of 7 to traces.
Scheme 1. Mizoroki–Heck versus conjugate addition reaction.
Results and Discussion
As reported before, with the combination of 1,3-bis(2,4,6-trime-
thylphenyl)imidazol-2-ylidene (1,4-naphtho-quinone) palladi-
um(0) dimer (Pd0-NHC), Bu3N and DMF as solvent, we observed
the side products (E)-but-1-en-1-ylbenzene (5) and 1-phenylbu-
tan-1-one (6) as a result of Heck reaction of 4-iodoanisole (2a)
with the oxidized Bu3N (Table 1). Moreover, the reaction is
probably slowed down due to the immiscibility of Bu3N and
DMF, at least at room temperature. To overcome these down-
sides, we studied the reaction of benzalacetone (1a) and 2a
with various combinations of alkyl amines and solvents using
Pd0-NHC as the catalyst. In the presence of tri-n-propylamine,
triisobutylamine, and N-methylmorpholine, a low conversion of
During this study, it was surprisingly found that Pd(OAc)2, in
particular in combination with DIPEA, and NMP as the solvent
also catalyzes the reductive Heck reaction of 2a to 1a with
similar reactivity as the Pd0-NHC system (entry 10). With
1 mol% of Pd(OAc)2, the reaction was completed in 6 h afford-
ing 3a in 78% yield together with minor amounts of Heck
product 4a (<10%) (entry 12). It had been reported by Cacchi
et al. that the same reaction with Pd(OAc)2 as catalyst and trib-
utylamine as reductant in DMF, gave in low yield a mixture of
reductive Heck and Heck prod-
uct in a 4:1 ratio.[10e]
Table 1. Variation in the catalyst, the reductant and the solvent.[a]
The role of DIPEA is of particu-
lar note here, as this amine is
crucial for a rapid reaction and
high yield, especially in combina-
tion with NMP as the solvent
that ensures a homogeneous re-
action. As noted before, the en-
amine formed upon oxidation of
tributylamine elicits a competing
Heck reaction. This was not ob-
served when DIPEA was applied.
McAlees et al. compared the re-
action of PdCl2·2PhCN with Et3N,
Bu3N, and DIPEA, and found that
DIPEA reacts considerably faster,
an observation that is congruent
with our observations on the
rate of the overall reaction with
these amines.[19a] In that reaction,
DIPEA exclusively reacts at the
ethyl substituent, as also ob-
served in a mechanistic study by
Peters et al.[19b] who showed that
selective oxidation of the ethyl
Entry
Catalyst
X
Reductant
Solvent
t
[h]
Yield [%][c]
[mol%]
3a
83[b]
0
20
55
<3
4a[g]
1[d]
2[d]
3[d]
4[d]
5[d]
Pd0-NHC
Pd0-NHC
Pd0-NHC
Pd0-NHC
Pd0-NHC
1.5
1.5
1.5
1.5
1.5
Bu3N
(iBu)3N
(nPr)3N
DMF
DMF
DMF
DMF
DMF
12
12
12
12
12
0
0
0
(iPr)2NEt
N-methyl
morpholine
(iPr)2NEt
(iPr)2NEt
(iPr)2NEt
(iPr)2NEt
(iPr)2NEt
Bu3N
<10
0
6
7
Pd0-NHC
Pd0-NHC
Pd0-NHC
Pd0-NHC
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
1.5
1.5
1.5
1.5
3
3
1
0.5
1
NMP
THF
12
12
12
2
50
22
<3
<10
<10
–
10
9
4
7
10
24
8[d]
9[e]
10[e]
11[e]
12[f]
13[f]
14[h]
CH3CN
NMP
NMP
NMP
NMP
NMP
NMP
[b]
85 (81)
3
6
6
73
18
(iPr)2NEt
(iPr)2NEt
(iPr)2NMe
78 (75)[b]
61
12
9
60
[a] For the entries 1 to 8, reactions were carried out using 1a (1.14 mmol), 2a (2.72 mmol), catalyst (1.5–
3.0 mol%), amine (4.5 equiv), 1.5 mL of solvent and decane as internal standard for 3–12 h. [b] Isolated yields.
[c] GC yields. [d] Biphasic reaction medium. [e] For the entries 9 to 11, the reactions were carried out using 1a
(1.65 mmol), and 2a (1.10 mmol). [f] For the entries 12 and 13, the reactions were carried out using 1a
(1.10 mmol), 2a (1.32 mmol) and 5.0 equiv of DIPEA. [g] 4a was obtained as an E/Z mixture. [h] The reaction
was carried out using 1a (1.10 mmol), 2a (1.32 mmol) and 5.0 equiv of DIPMA (N,N-diisopropylmethylamine).
substituent
occurred
when
DIPEA was treated with a zwitter-
ionic palladium complex. Iridi-
um-catalyzed selective hydride
elimination of the ethyl-substitu-
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ent has been reported as well, producing bis-isopropylamino
ethene.[18a] Other reports propose[19c–d] the requirement of
water or hydroxide for the reduction of palladium in this way,
however these studies focused on the reduction of Pd-phos-
phine complexes (and led to the oxidation of the phosphine).
Upon performing the reaction with N,N-diisopropylmethyla-
mine (DIPMA, Table 1, entry 14), a mixture of 3a and 4a in
60% and 24% yield, respectively, was obtained in an incom-
plete reaction after 9 h. This poor selectivity and moderate re-
action rate further confirm that it is most probably the ethyl
moiety in DIPEA that is oxidized, and that the bulky isopropyl
groups apparently inhibit Heck reaction of the resulting bis-iso-
propylamino ethene. Thus, it was highly significant to explore
the scope of the reaction using Pd(OAc)2, a stable, an inexpen-
sive palladium catalyst, in the reductive Heck reaction.
used for further transformations. Finally, the effect of a substitu-
ent at the different positions of the benzene ring was also ex-
amined. Thus, m-tolyl iodide (2i) gave 3i in 82% yield whereas
o-tolyl iodide (2j) afforded 3j in only 32% yield, the latter due
to incomplete conversion. The heteroaromatic iodides 3-iodo-
thiophene (2j) and 5-iodoindole (2k) also furnished their corre-
sponding reductive Heck products 3k and 3l in good yields.
Also the scope in terms of the Michael acceptor was studied
using 2a (Table 3). In addition to benzylidene acetone, also
chalcone (1b) and the benzylidene derivatives 1c and 1d par-
ticipated well in the reaction to give the reductive heck prod-
ucts in good yields. The reaction with some of the b-alkyl sub-
stituted enones gave a low yield of reductive Heck product,
however, due to formation of the Heck product in considerable
amounts. The formation of the reductive Heck product increas-
es upon the introduction of bulky substituents on the b-
carbon of the enone. For example, in the series 3p, 3q, 3r,
yields increase from 32% to 60%.
Initially, we studied the scope in aryl iodides using benzala-
cetone (1a) as the substrate (Table 2). In general, electron-rich
Gratifyingly, also 2-cyclohexenone (1h) reacted
Table 2. The Pd(OAc)2-catalyzed reductive Heck reaction of aryl iodides 2b-j to benza-
lacetone 1a.[a,b,g]
smoothly, in this case with a catalytic amount of
Pd(TFA)2, giving 3s in 74% yield. This is an improve-
ment compared to the use of the Pd0-NHC/Bu3N/
DMF system (56% yield). The scope was extended to
(E)-1-methoxy-4-(2-nitrovinyl)benzene, 1i, which af-
forded solely the reductive Heck product 3t in a mod-
erate 33% yield. Unsaturated esters and nitriles did
not give reductive Heck products but furnished
solely the Heck products 4b and 4c in high yields. In
general, for enones the selectivity of reductive Heck
versus Heck product is 12:1 to 15:1, except for b-
alkyl substituted enones. In comparison with our ear-
lier reported palladium carbene catalyst system, the
current yields are slightly lowered by 5 to 10%. How-
ever, for cyclohexenone and halo-substituted arenes,
we observed higher yields.
Mechanistic studies by Friestad and Branchaud[16]
on the Cacchi system suggested that the absence of
acid would result in the initial formation of the Mizor-
oki–Heck product 4a, followed by its reduction to 3a
by accumulated PdÀH species formed from NBu3. To
study this hypothesis in our present catalyst system,
we carried out a reaction with deuterated benzyli-
dene acetone ([D1]-1a) under the optimized reaction
conditions (Scheme 2). Either the absence of deuteri-
um or deuterium scrambling at the b-carbon of 3a,
will support the reaction pathway via Heck reaction
[a] Unless otherwise mentioned, all reactions were carried out using 1a (1.10 mmol), 2
(1.32 mmol), Pd(OAc)2 (1.0 mol%) and 5 equiv of DIPEA in 1.5 mL of NMP. [b] Isolated
yields. [c] homocoupling of 1 was found to be predominant. [d] 2.4 equiv of 2 was
used. [e] 5.0 equiv of 2 was used. [f] The reaction was carried out for 24 h. [g] In all re-
actions, the selectivity of reductive Heck versus Heck product is >11:1.
and electron neutral phenyl iodides participate well in the re-
action and afford the product in good yields, whereas elec-
tron-deficient phenyl iodides preferentially give homocoupling.
For instance, p-tolyl iodide (2b) and iodobenzene (2c) gave
the reductive Heck products in good yields, but 4-iodo methyl-
benzoate (2d) afforded the product in low yield and the reac-
tion with 4-iodonitrobenzene (2e) was completely ineffective.
A separate class is formed by the halogen substituted phenyl
iodides (2 f–h) that furnish the desired products in good yields
(62–77%). It is advantageous that bromo- and chloro substitu-
ents on the benzene ring remain unaffected, so these can be
followed by reduction. However, the reaction gave reductive
Heck product [D1]-3a in 80% yield with>97% remaining deut-
eration at the b-carbon. This rejects the hypothesis of Friestad
and Branchaud and supports the pathway in which the Pd-
alkyl complex A (Scheme 1) is reduced by DIPEA to a palladium
hydride species followed by reductive elimination. Alternatively
A tautomerizes to an O-bound palladium enolate that is subse-
quently reduced by DIPEA.[20]
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Experimental Section
Table 3. The Pd(OAc)2-catalyzed reductive Heck reaction of p-iodo anisole (2a) to
enones 1b–i.[a,b]
General procedure for the conjugate addition of aryl io-
dides 1 to enones 2: An oven-dried Schlenk tube,
equipped with stopper and stirrer bar, was placed
under nitrogen, and charged with aryl iodide
2
(1.32 mmol), and alkene 1 (1.10 mmol), followed by
Pd(OAc)2 (1.0 mol%) as a stock solution (0.011 mmol in
0.5 mL NMP) using syringe. To this mixture, N,N-diiso-
propylethylamine (DIPEA) (5.5 mmol), and N-methylpyr-
rolidone (NMP, 1.0 mL) were added sequentially while
stirring. The Schlenk tube was placed into a preheated
oil bath at 808C. Upon completion (after 6 h, as judged
by GC/MS), the reaction mixture was cooled down to
RT, diluted with either diethyl ether or ethyl acetate,
and filtered through a Celite and silica gel pad. The fil-
trate was concentrated and the residue was purified on
silica gel chromatography to afford the desired product
3.
Acknowledgements
This research has been performed within the frame-
work of the CatchBio Program. The authors gratefully
acknowledge the support of the Smart Mix Program
of the Netherlands Ministry of Economic Affairs, Agri-
culture and Innovation and the Netherlands Ministry
of Education, Culture and Science.
[a] Unless otherwise mentioned, all reactions were carried out using 1a (1.10 mmol), 2
(1.32 mmol), Pd(OAc)2 (1.0 mol%) and 5 equiv of DIPEA in 1.5 mL of NMP. [b] Isolated
yields. [c] 100% conversion of 2a as determined by GC using n-decane as internal
standard and 4 was obtained as an E/Z mixture [d] 1 mol% of Pd(TFA)2 was used in-
stead of Pd(OAc)2.
Keywords: alkenes
· aryl iodides · conjugate
addition · N,N-diisopropylethylamine · palladium
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Received: July 10, 2015
[15] This strategy of switching between reaction modes was reported more
recently also for the oxidative Heck reaction versus conjugate addition,
Published online on September 30, 2015
ChemCatChem 2015, 7, 3923 – 3927
www.chemcatchem.org
3927
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