Palladium-Catalyzed Intermolecular Arylcarbonylation of Unactivated Alkenes: Incorporation of Bulky Aryl Groups at Room Temperature

Article abstract of DOI:10.1002/anie.201810405

A palladium-catalyzed intermolecular arylcarbonylation of unactivated alkenes has been developed. Unsymmetrical diaryliodonium salts (DAISs) were used as arylation reagents, the bulky aryl group (Ar_L) of which was exclusively incorporated into the arylcarbonylated products, which contained the Ar_L group and a carboxylic ester group at the α- and β-carbon position, respectively, of the original terminal C?C double bond. The reaction features excellent chemo- and regioselectivity, high functional-group tolerance, and very mild reaction conditions.

Full text of DOI:10.1002/anie.201810405

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Accepted Article
Title: Pd-Catalyzed Intermolecular Arylcarbonylation of Unactivated
Alkenes: Incorporation of Bulky Aryls at Room Temperature
Authors: Guosheng Liu, Xiang Li, and Pinhong Chen
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10.1002/anie.201810405
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Pd-Catalyzed Intermolecular Arylcarbonylation of Unactivated
Alkenes: Incorporation of Bulky Aryls at Room Temperature
Xiang Li, Pinhong Chen* and Guosheng Liu*
Dedicated to Prof. Xiyan Lu on the occasion of his 90th birthday
Abstract:
A Pd-catalyzed intermolecular arylcarbonylation of
unactivated alkenes has been established herein. Unsymmetric
diaryliodonium salts (DAIS) are used as arylation reagents, in which
the bulky aryls (ArL) are exclusively incorporated into the
arylcarbonylated products bearing the ArL group and a carboxylic
ester group at the - and-carbon position on the terminal C-C
double bond, respectively. The reaction features excellent chemo-
and regio-selectivities and high functional group tolerance under
very mild reaction conditions.
Transition metal-catalyzed arylation of alkenes such as Heck
(type) reaction, arylamination and arylhalogenation has been
extensively studied and widely applied in organic synthesis over
the last several decades.[1] However, only small or moderate
sterically hindered aryl groups (ArS) are introduced into alkene
substrates in these reactions,[2] and few examples of the
introduction of sterically congested ArL (i.e., aryls bearing two
ortho-substituents) into alkenes have been reported to date,
which however generally suffers from harsh conditions and low
yields.[3]
Diaryliodonium salts (DAIS) are nontoxic, moisture- and air-
stable, and have therefore received much attention in recent
years.[4] Compared to commonly-used aryl iodides, the
diaryliodonium salts are more reactive to give ArM intermediates
by oxidative addition to transition metal catalysts.[4a] For
unsymmetric DAIS bearing two different aryls (ArL and ArS),
however, the oxidation process exclusively yielded (ArS)M
species, thus leading to the incorporation of the ArS into
products (Scheme 1a).[5-7] For instance, Gaunt and coworkers
reported an elegant method for Cu(I)-catalyzed arylation of
alkenes with unsymmetric DAIS, giving the arylation products
with the ArS incorporated selectively (Scheme 1b).[7c-e] On the
other hand, reactions of unsymmetric DAIS with various
Scheme 1. TM-catalyzed arylation of alkenes with unsymmetric DAIS.
intermolecular arylcarbonylation of unactivated alkenes at room
temperature, using various unsymmetric DAIS as arylation
reagents, in which a bulky aryl group is exclusively introduced
into the products (Scheme 1c).
As part of our continuous efforts toward difunctionalization of
alkenes, recently we have demonstrated the intermolecular oxy-,
azido- and fluoro-carbonylation reactions of unactivated alkenes
under a CO atmosphere, using various hypervalent iodine
reagents such as PhI(OAc)2, ArI(N3) and ArIF2.[11] On the basis
of these studies, we envisaged that, if the aforementioned I(III)
reagents were replaced with diaryliodonium salts (DAIS) as an
arylation reagent, a novel arylcarbonylation of alkenes might be
feasible.
To test the above-mentioned hypothesis, we initially tested
the reaction of terminal alkene 1a with DAIS 2a in the presence
of Pd(OAc)2 under a CO atmosphere. As shown in Table 1,
gratifyingly, the reaction indeed occurred and gave the
arylcarbonylation product 3a in 40% yield, into which only the
bulky mesityl (Mes) group was incorporated (entry 1). This
unexpected result prompted us to further explore the reaction
conditions. In addition, several side reactions, such as
isomerization (1a-1 in 25% yield), Heck-type reaction (1a-2 in
5% yield) and dicarbonylation reaction (1a-3 in 12%) occurred
as well. We reasoned that the isomerization was probably
triggered by palladium hydride species. Sen and coworkers
reported that the PdH species can be trapped by radical
scavengers, which impeded the isomerization.[12a] Pleasingly, the
isomerization reaction was indeed prevented by adding TEMPO,
galvinoxyl or DPPH, and TEMPO proved to be the best, giving
the product 3a in 70% yield (entry 2-4);[12] meantime, the Heck-
type reaction was inhibited as well. Further palladium catalyst
screening showed that Pd(PhCN)2Cl2 and Pd(TFA)2 were not
suitable for the reaction (entries 5-7), but Pd(hf-acac)2 slightly
improved a yield of 3a to 73% (entry 8). Although the
dicarbonylation of alkenes could not be prohibited in all cases
(1a-3 in 6~26% yields), the side reaction could be significantly
inhibited by adding small amounts of water (entry 9).[13]
Interestingly, when degassed solvent was used, a yield of 3a
heteroatom
nucleophiles
under
metal-free
conditions,
exclusively forging ArL-heteratom bonds through reductive
elimination at a I(III) center, have also been investigated.[4,8]
However, the arylation of alkenes with DAIS, as a feasible
method for the efficient construction of ArL-C bonds, is quite
rare.[9,10] The incorporation of sterically congested aryl (ArL)
groups into the C-C double bond still remains an extremely
challenging task in organic synthesis, especially under mild
conditions. Herein, we communicate this palladium-catalyzed
X. Li, Dr. P. Chen, Prof. Dr. G. Liu,
State Key Laboratory of Organometallic Chemistry, Center for
Excellence in Molecular Synthesis, Shanghai Institute of Organic
Chemistry, University of Chinese Academy of Sciences, Chinese
Academy of Sciences.
345 Lingling Road, Shanghai, China, 200032
E-mail: gliu@mail.sioc.ac.cn; pinhongchen@sioc.ac.cn
Homepage: http://guoshengliu.sioc.ac.cn/
Supporting information for this article is given via a link at the end of the
document.((Please delete this text if not appropriate))
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was further increased to 87%, and the dicarbonylation reaction
was completely inhibited (entry 10). These experimental results
revealed that the reaction was probably initiated by a Pd(0)
catalyst; in fact, Pd(dba)2 could catalyze the reaction efficiently
to give the product 3a in 81% yield (entry 11). Decreasing the
catalyst loading from 10 mol% to 5 mol% didn't affect reaction
efficiency (entry 12, 88%), but further decreasing the catalyst
loading to 2 mol% resulted in a lower yield (entry 13, 75%).
When EtOH or iPrOH instead of MeOH were used, the reactions
also proceeded smoothly to give the corresponding esters in
77% and 43% yield, respectively (entries 14-15).
alkenyl moieties, the reaction only happened on the electron-rich
double bond, affording the product 3s in 82% yield. Importantly,
styrenes bearing different functional groups on the aromatic ring
could be employed as suitable substrates as well, yielding the
corresponding products 3t-3z in good yields. To demonstrate the
preparative utility, our reaction was performed on a 5-mmol
scale and 1.43 g of the product 3b was obtained in 78% isolated
yield. It is noteworthy that our reaction features excellent chemo-
(only mesityl group migration) and regio-selectivity (> 20:1 in
most cases, and 15~18:1 in few cases).
Then, we turned our attention to examining the scope of
different unsymmetric DAIS 2 bearing phenyl and bulky aryl (ArL)
groups (Table 2B). The DAIS bearing 2,4,6-triethylphenyl and tri-
isopropylphenyl were employed as arylation agents, giving the
desired products 4a and 4b in good yields (81% and 72%). The
diaryliodonium salts bearing 2,6-disubstituted phenyl groups
were also suitable for the reaction to afford the products 4c-4i in
moderate to good yields (45-80%). Again, these reactions
exhibited excellent regio- and chemo-selectivities.
Table 1. Optimization reaction conditions.a,b
Next, we further explored DAIS 2 bearing one substituent at
the ortho position on the bulky aromatic ring. The DAIS 2
bearing ortho phenyl or CF3 groups on the ArL ring exhibited
good reactivities to give the desired products 4j-4k. In addition, a
1-naphthyl group in DAIS 2 could also be incorporated into the
product 4l in 69% yield. Furthermore, a variety of heteroarenes
in DAIS 2, such as indole (4m, 4p), benzothiophene (4n, 4q)
and thiophene (4o), could be smoothly transferred into the
desired products in good yields (48-83%) with good to excellent
regioselectivities (>10:1), while
a 5:1 regioselectivity was
obtained in the case of 4o. Importantly, a variety of complex
alkene substrates were also tested under the current reaction
conditions (Table 2C). Reactions of substrates bearing sugar,
cholesterol, lithocholic acid and estrone moieties proceeded well
to give the products 4r-4u in good yields (62-83%).
Unfortunately, DAIS 2, in which ArL does not bear ortho
substituents, gave only trace arylcarbonylation products, but the
direct arylation ArCO2Me as a major product[14] (see SI).
Notably, our arylcarbonylation reaction could be conducted
in a one-pot fashion by directly using bulky arenes (ArLH) as
arylation reagents. For instance, diaryliodonium salt 2a could be
in situ generated from commercially available PhI(OAc)2 and
mesitylene,[7a,15] which was further reacted with alkene 1b to
Having established the optimal reaction conditions, we
started to examine the substrate scope of alkenes. As revealed
in Table 2A, the reaction appeared quite general for aliphatic
terminal alkenes, affording the corresponding arylcarbonylated
products 3a-3n in good to excellent yields. A wide variety of
functional groups, such as imide (3a-3c), halide (3d and 3e),
ester (3j and 3m), ether (3k), ketone (3l), carbazole (3o) and
indole (3p), were well tolerated under the mild reaction
conditions. In addition, reactions of alkenes bearing primary,
secondary alcohols (3h and 3i) and the amidyl moiety (3n) also
worked nicely, but the oxidation and arylation of alcohols or
amides with DAIS 2a didn't happen. Notably, alkenes bearing
the boronic ester moiety was reacted with 2a to give the product
3q, albeit with a significant yield drop (32%). Furthermore, for
unconjugated diene 1r, the reaction exclusively occurred on the
terminal double bond, delivering the product 3r in 83% yield. For
unconjugated diene 1s bearing both electron-rich and -deficient
afford the arylcarbonylated product 3b in 85% yield (eq 1). More
importantly, the arylcarbonylation products could be further
synthetically transformed (Scheme 2). For example, the product
3b could be converted into -lactam 5b through sequential
deprotection of phthalimides/amination of esters. In addition, the
carboxylic acid 5a by the hydrolysis of 3b could undergo radical
decarboxylative chlorination, azidation and boronation, providing
the corresponding products 5c-5e in moderate yields (for the
reaction conditions, see SI).[16]
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Table 2. Substrate Scope.a
catalytic system. Obviously, the chemoselective palladium
oxidation by unsymmetric DAIS 2 was tunable by CO gas to
generate the major ArLPd(II) and minor ArSPd(II) species in our
current reaction (Scheme 4). In addition, the reactivity of the
ArLPd(II) species was also variable by different CO pressure;
the alkene insertion by ArLPd(II) exclusively occurred at 1 atm
CO pressure, while the direct carbonylation of ArLPd(II) was
remarkably promoted by increasing CO pressure (eq 5).
Furthermore, DIAS 2r was employed to examine electronic
effects, and the reaction provided the product 3a as a major
product, along with 4v as a minor product (eq 6), indicating the
incorporation of electron-rich aryl groups into the
arylcarbonylation products was preferred. Compared to
favorable reductive elimination of electron-defficient aryl groups
at an iodine(III) center,[4] the current results (eq 6) were again
inconsistent with our previously reported I(III)-mediated alkene
activation process. In addition, stereochemistry of the reaction
showed the cis-addition of both aryl and carboxylate ester
groups across alkenes, which is consistent with the sequential
alkene insertion ArLPd speiceis and CO insertion sp3 C-Pd
species (int-III) processes (eq 7).[20]
Based on these control experiments, a plausible mechanism
for the selective arylcarbonylation of alkenes is described in
Scheme 4. The palladium catalyst was initially oxidized by DAIS
reagents to give the major ArLPd(II) and minor ArSPd(II) species
in the presence of CO gas.[21] Subsequently, the resultant
ArLPd(II) underwent exclusively alkene insertion (path a) rather
than direct carbonylation (path b, k1 << k3), affording the
Scheme 2. Synthetic applications.
To get more insight into a plausible mechanism, some
control experiments were performed (Scheme 3). First, in order
to address the chemoselective issue of diaryliodonium salts, the
reaction of 1a with unsymmetric DAIS 2q as a limiting reagent
was conducted in deuterium solvent (eq 2). The ArL group was
selectively transferred into alkenes to give the desired product
3a-d3 in 48% yield, together with trace amounts of the direct
carbonylation product 6a-d3; notably, the ArS group exclusively
underwent the direct carbonylation to give the product 6b-d3 in
18% yield (eq 2). The reaction in the absence of alkene
substrates gave the products 6a-d3 in 42% yield and 6b-d3 in
15% yield (eq 3); the ratio of 3a-d3 (ArL) to 6b-d3 (ArS) in eq 2 is
close to that of 6a-d3 to 6b-d3 in eq 3.[14] These outcomes
indicated that, unlike our previously reported I(III)-meidated
alkene activation process, the arylcarbonylation reaction might
proceed through an initial palladium oxidation by DAIS to
generate both ArLPd(II) and ArSPd(II) species;[17] the weakly
Lewis-acidic DAIS presumably could not activate alkenes.[18]
However, when the reaction was conducted in the absence
of CO gas, only the ArS group (Ph group from 2a) was
selectively incorporated into the Heck-type product 7a (eq 4),[19]
indicating the involvement of the sole ArSPd(II) species in the
arylcarbonylation products
3
(eq 2); While the direct
carbonylation of the ArLPd(II) species (path b) could occur under
increased CO pressure (eq 5). On the contrary, the ArSPd(II)
species proceeded through a fast direct carbonylaton rather than
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Acknowledgements
We are grateful for financial support from the National Basic
Research Program of China (973-2015CB856600), the National
Nature Science Foundation of China (Nos. 21532009, 21672236
21790330 and 21761142010), the Science and Technology
Commission of Shanghai Municipality (Nos. 17XD1404500 and
17JC1401200), and the strategic Priority Research Program (No.
XDB20000000) and the Key Research Program of Frontier
Science (QYZDJSSW-SLH055) of the Chinese Academy of
Sciences.
70
product 3a
product 6a
60
50
40
30
20
10
0
Keywords: palladium • alkenes • arylcarbonylation •
unsymmetric diaryliodonium • bulky aryls incorporation
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Scheme 4. Proposed mechanism.
In summary, we have developed a novel Pd-catalyzed
intermolecular arylcarbonylation of unactivated alkenes. Various
unsymmetric diaryliodonium salts, in which bulky aryls are
exclusively and regioselectively incorporated into the
arylcarbonylated products, are used as an arylation resource.
The reaction features excellent yields and good functional group
tolerance under very mild reaction conditions.
[7]
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10.1002/anie.201810405
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[20] In our previous studies on oxidative oxycarbonylation of alkenes via a
I(III)-activated process (ref. 8a), the trans-addition product was
observed, which is also inconsistent with our current results.
[21] For the CO effect on the chemoselective palladium oxidation by DAIS,
it might be attributed to the CO coordination to a Pd center, although a
detailed mechanism is still unclear at this moment.
[12]
a) A. C. Albérniz, P. Espinet, R. López-Fernández, A. Sen, J. Am.
Chem. Soc. 2002, 124, 11278-11279. For more examples involving the
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A. Studer, Angew. Chem. Int. Ed. 2009, 48, 4235-4238; c) Z. He, S.
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Angewandte Chemie International Edition
COMMUNICATION
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COMMUNICATION
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COMMUNICATION
Xiang Li, Pinhong Chen,* and Guosheng
Liu*
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Pd-Catalyzed Intermolecular
Arylcarbonylation of Unactivated
Alkenes: Incorporation of Bulky Aryls
at Room Temperature
Pd-catalyzed intermolecular arylcarbonylation of unactivated alkenes has been
established herein. Unsymmetric diaryliodonium salts (DAIS), in which bulky aryls
(ArL) are exclusively incorporated into terminal alkenes at the -position, are used
as arylation reagents. In addition, the reaction features excellent chemo- and regio-
selectivities and high functional group tolerance under very mild reaction conditions.
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