Copper- and phosphine-ligand-free palladium-catalyzed direct allylation of electron-deficient polyfluoroarenes with allylic chlorides

Article abstract of DOI:10.1002/chem.201202824

Simply id(all)ylic: A copper- and phosphine-ligand-free Pd-catalyzed direct allylation of electron-deficient polyfluoroarenes with allylic chlorides and the reaction mechanism are described (see scheme). The simple catalytic system, broad substrate scope,

Full text of DOI:10.1002/chem.201202824

COMMUNICATION
DOI: 10.1002/chem.201202824
Copper- and Phosphine-Ligand-Free Palladium-Catalyzed Direct Allylation
of Electron-Deficient Polyfluoroarenes with Allylic Chlorides
Yan-Bo Yu, Shilu Fan, and Xingang Zhang*[a]
Transition-metal-catalyzed allylation reactions of aryl
metals have been used extensively in organic synthesis as a
method to synthesise the allylated arene motif that is found
in numerous natural products and biologically active com-
pounds,[1] and because of the versatility of the double
carbon–carbon bond for further synthetic transformations.[2]
Nevertheless, this strategy is constrained by the need for
multiple steps to prepare arylmetals.[3] The Friedel–Crafts-
type allylation of arenes represents a complementary strat-
egy; however, the reaction is limited by the substrate scope
of p-electron-rich arenes.[4] Therefore, developing new and
efficient reactions to access allylated arenes is still highly de-
sirable. Over the past few years, impressive progress has
lyzed allylation reaction of electron-deficient polyfluoroar-
enes with allylic chlorides. The notable features of this pro-
tocol are its simple catalytic system (copper- and phosphine-
ligand free, only [Pd2ACHTNUGTRNEUNG(dba)3] as the catalyst precursor), a
broad substrate scope that includes difluorobenzene (an
ꢀinertꢁ substrate), and an excellent functional-group compat-
ibility, which includes bromide. This protocol provides a
useful and rapid access to allylated polyfluoroarenes that
are of interest in both life and materials science. Mechanistic
studies reveal that the reaction proceeds through the attack
of a polyfluoroarene anion that is generated in situ at the
(p-allyl)palladium–carboxylate complex, which is different
from previous studies and is complementary to the tradition-
al mechanism for the Pd-catalyzed allylation reaction of aryl
metals.[3,11]
been made towards the transition-metal-catalyzed direct
[5]
À
functionalization of C H bonds. This strategy provides an
attractive alternative to traditional techniques because the
necessary preactivation of arenes is omitted. However, little
attention has been given to the direct allylation of electron-
deficient arenes by using this strategy. To the best of our
knowledge, only isolated examples have been reported so
far.[6] Although these achievements are promising, these
processes are restricted by their low tolerance of important
functional groups, limited substrate scope, and/or the re-
quirement for multiple-component catalytic systems (e.g.,
We began this study by examining different leaving
groups on the allylated compound 2 (Table 1). However, the
desired product 3a was not obtained when tert-butylcinna-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
PPh3 (10 mol%), and Cs2CO3 in toluene (Table 1, entries 1–
3). Considering that the phosphine ligand may play an im-
portant role in the reaction efficiency, control experiments
were conducted in the absence of PPh3 to further under-
stand the reaction. To our surprise, a 70% yield of 3a was
obtained when 2c was used as a substrate (Table 1, entry 6).
PdACHTUNGTRENNUNG(OAc)2/PPh3 and CuI/Phen (Phen=1,10-phenanthroline)
co-catalyzed systems). Hence, the development of a new cat-
alytic system for widespread application and that will over-
come these limitations is still highly desirable.
Owing to the importance of polyfluoroarenes in materials
and life sciences,[7] it is of great interest to install a fluoroar-
yl group on the organic molecules.[8–10] However, for less re-
active fluoroarenes, such as 1,2,3,4-tetrafluorobenzene and
difluorobenzene, it is difficult to obtain their corresponding
Table 1. Effects of leaving group on the allylated compound 2 on the Pd-
catalyzed allylation of pentafluorobenzene 1.[a]
À
allylated compounds by direct C H bond functionalization
À
due to the low reactivity of the C H bond, which needs to
be activated. From the new practical and academic stand-
point, we herein present a highly efficient palladium-cata-
2, X
PPh3
[mol%]
Yield of 3a
[%]
N
1
2
3
4
2a, OCO2tBu
2b, Br
2c, Cl
2a, OCO2tBu
2b, Br
2c, Cl
20
20
20
–
–
–
trace
trace
trace
56[b]
[a] Y.-B. Yu, S. Fan, Prof. Dr. X. Zhang
Key Laboratory of Organofluorine Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Science
5
trace
6
70[b]
345 Lingling Lu, Shanghai 200032 (P.R. China)
Fax : (+86)21-6416-6128
7[c]
2c, Cl
–
NR
E-mail: xgzhang@mail.sioc.ac.cn
[a] Reaction conditions (unless otherwise specified):
1 (2.0 equiv), 2
(0.6 mmol), toluene (2.5 mL), 10 h. [b] Yield of isolated product. [c] Re-
action conducted in the absence of Pd catalyst. NR=no reaction.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202824.
Chem. Eur. J. 2012, 18, 14643 – 14648
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14643

Table 2. Representative results for the optimization of the Pd-catalyzed
The reaction of compound 2a also gave the corresponding
product in 56% yield (Table 1, entry 4), whereas only a
trace amount of product was formed when 2b was examined
(Table 1, entry 5). The results demonstrate that the reaction
can be inhibited by PPh3 under these reaction conditions,
and that the reactivity of the leaving groups on the allylated
compound 2 increase in the order: bromide<tert-butyl car-
bonate<chloride. No reaction occurred in the absence of a
Pd catalyst, indicating that a palladium catalytic cycle is in-
volved in the reaction (Table 1, entry 7).
Encouraged by these results, cinnamyl chloride (2c) was
employed as a model substrate for further optimization of
the reaction conditions (Table 2). Taking into account that a
(p-allyl)palladium complex would be generated during the
reaction, and that the addition of carboxylic acid may affect
the reactivity of the palladium intermediate,[12] different
acids were examined (Table 2, entries 1–7). We found that
the nature of the acid plays an important role for the reac-
tion efficiency. Pivalic acid (PivOH) showed a superior reac-
tivity, providing an 82% yield of 3a (Table 2, entry 4). The
reaction was also sensitive to the solvent (see Table S1 in
the Supporting Information). Reactions conducted in polar
solvents, such as DMSO, DMF, DMA, and NMP, failed to
give the product, and only the reaction in diox-
allylation of pentafluorobenzene (1).[a]
Pd catalyst,
[mol%]
Cs2CO3
[equiv]
Acid,
ACHTUNGTRNE[NUNG equiv]
Yield
[%][b]
G
U
1
2
3
4
5
6
7
8
9
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
N
2.4
2.4
2.4
2.4
2.4
2.4
2.4
1.2
1.2
1.2
1.2
1.2
CH3COOH, 1.2
CF3COOH, 1.2
C2H5COOH, 1.2
tBuCOOH, 1.2
AdCOOH, 1.2
PhCOOH, 1.2
TsOH, 1.2
tBuCOOH, 0.1
tBuCOOH, 0.1
tBuCOOH, 0.1
none
75 (62)
28 (25)
70 (53)
87 (82)
79 (68)
78 (67)
62
90 (85)
62 (57)
(96)
10
11
12
A
CHTUNGTRENNUNG
A
C
(79)
NR
tBuCOOH, 0.1
[a] Reaction conditions (unless otherwise specified): 1 (2.0 equiv), 2c
(0.6 mmol), toluene (2.5 mL), 10 h. [b] NMR yield was determined by
19F NMR spectroscopy using fluorobenzene as an internal standard;
number in parenthesis is the isolated yield.
ane gave 3a in 36% yield. Further decreasing
the loading of PdACHTUNGTRENNUNG(OAc)2 from 10 mol% to
Table 3. Pd-catalyzed allylation of pentafluorobenzene 1 with various allylic chlorides 5.
5 mol% and the use of a catalytic amount of
PivOH (0.1 equiv) gave 3a in a slightly higher
yield (Table 2, entry 8).[13] Given that a phos-
phine-ligand-free Pd0 species must be generated
to facilitate the present reaction, [Pd2ACTHNUTRGNEN(UG dba)3]
was investigated to further improve the reaction
efficiency. To our delight, the highest product
yield (96%) was obtained when the reaction
was carried out with [Pd2ACHTUNRTGNEUNG(dba)3] (2.5 mol%)
and PivOH (0.1 equiv; Table 2, entry 10). How-
ever, the absence of PivOH in the reaction was
less effective (Table 2, entry 11), indicating the
important role of pivalate for the reaction effi-
ciency. In contrast to [Pd2ACHTUNGTRENNUG(dba)3], PdACHTUNGTREN(NUGN PPh3)4
showed no catalytic activity (Table 2, entry 12).
Thus, a palladium(0) complex without phos-
phine ligands is most favorable as the catalyst
precursor for this reaction.
This method allowed the direct allylation of
pentafluorobenzene 1a with a variety of allylic
chlorides (Table 3). For substrates with an elec-
tron-withdrawing group, moderate regioselec-
tivities were observed due to the relatively
easier 1,3-hydrogen shift that takes place under
basic conditions. Branched allylic chloride is
also a suitable substrate and a good product
yield and moderate regioselectivity were ob-
tained (Table 3, 3j). It is noteworthy that versa-
tile functional groups such as ester, nitrile, alde-
hyde, and chloride are compatible in the reac-
Reaction conditions (unless otherwise specified): 1 (2.0 equiv), 5 (0.6 mmol), toluene
(2.5 mL), 10 h. All reported reaction yields are isolated yields; all the ratios of isomers
were determined by 19F NMR spectroscopy before column chromatography. [a] Ratio of
E-3/other isomers (Z-3 and/or regioisomer 4)>20:1. [b] Ratio of E-3/other isomers (Z-3
tion, providing a platform for further transfor- and/or regioisomer 4)=13:1. [c] A 3% yield of the branched product was obtained.
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Allylation of Polyfluoroarenes
COMMUNICATION
Table 4. Pd-catalyzed allylation of fluoroarene 6 with allylic chlorides 5.
mations without the need for protection/depro-
tection sequences (Table 3, 3d, 3g–i). This is in
sharp contrast to previous results[6] and high-
lights the advantages of the present method. Im-
portantly, even for substrates with a bromide
substituent, which is reactive in the presence of
a Pd0-catalyst, a synthetically useful yield was
still obtained without the formation of the di-
pentafluorophenylated side product (Table 3,
3e). In addition, less-reactive aliphatic allylic
chlorides also underwent the reaction smoothly
to give products in moderate yields and stereo-
selectivities (E/Z from 15:1 to 6:1, Table 3, 3k–
l), and for a linear aliphatic allylic chloride, only
a minor branched product was formed (Table 3,
3l). Thus, these findings suggest that a (p-allyl)-
palladium complex is generated during the reac-
tion.
The reactions of the Z-allylic chloride 2c’ and
Z-allylic carbonate 2a’ were also tested. As il-
lustrated in Scheme 1a, the linear E-allylated
product 3a was obtained in high stereo- and re-
gioselectivity with no observed formation of the
Z-allylated isomer. However, for the Z-allylic
carbonate 2a’, the desired product was not ob-
served under these optimized reaction condi-
tions. When the reaction was carried out with
Reaction conditions (unless otherwise specified): 6 (3.0 equiv), 5 (0.6 mmol), toluene
(2.0 mL), 10 h. All reported reaction yields are isolated yields; E-7/other isomers (Z-7 and/
or regioisomer)>20:1, determined by 19F NMR spectroscopy before column chromatogra-
phy. [a] 2.0 equiv of 6 were used. [b] Minor disubstituded products were also detected by
19F NMR spectroscopy (see the Supporting Information). [c] 1.2 equivalents of 6 were
used.
PdACHTUNGTRENNUNG(OAc)2 (10 mol%) and Cs2CO3 (1.2 equiv) in
toluene at 1408C, 48% of the E-allylated prod-
uct 3a was obtained with no formation of the Z-
allylated isomer (Scheme 1b).
To further ascertain the scope of this method-
À
ology, various fluoroarenes 6 containing 2–4 fluorine atoms
were tested and representative results are illustrated in
Table 4. Substrates containing 3–4 fluorine atoms have more
than one reaction site (Table 4, 7a–e, 7i) and good yields of
tivities. In this case, the most acidic C H bond, which is lo-
cated between the two fluorine atoms, is the primary reac-
tion site (Table 4, 7j–l). Other useful functional groups, such
as chloride and bromide, also tolerated the reaction condi-
tions well (Table 4, 7h, 7k). Furthermore, for the reaction of
an aryl-substituted fluoroarene, a good yield could be ob-
tained by using only 1.2 equivalents of the fluoroarene
(Table 4, 7 f), thus providing an efficient way to use this un-
usual substrate.
the monoACHTUNGTRENNUNG
allylated products were provided.[14] In particular,
even reactions with an ꢀinertꢁ substrate such as 1,2,3,4-tetra-
fluorobenzene gave the corresponding products efficiently
(Table 4, 7d–e). It is also noteworthy that 1,3-difluoroben-
zene, which was previously demonstrated to be an unsuita-
ble substrate,[6] underwent smooth reactions to provide the
corresponding products in good yields and high regioselec-
It has been demonstrated that polyfluoroarene–thiophene
and -azine (e.g., pyridine, quinoline) structures constitute a
distinct class of materials in electronic devices, such as or-
ganic light-emitting diodes (OLEDs) and filed-effect transis-
tors (FETs).[7d] To demonstrate the utility of this protocol,
two types of allylated polyfluoroarenes that contain a thio-
phene or quinoline motif were prepared. As shown in
À
Scheme 2, after the selective Pd-catalyzed direct C H bond
heteroarylation of 1,2,4,5-tetrafluorobenzene,[9a,c] the result-
ing compounds 8 and 9 were directly allylated by using the
present strategy to furnish the allylated polyfluoroarene–thi-
ophene and -quinoline structures in a highly efficient
manner. This provides a good opportunity for the further
application of these highly functionalized structures in elec-
tronic devices.
Scheme 1. Pd-catalyzed allylation of pentafluorobenzene (1) with linear
Z-allylic chloride 2c’ and Z-allylic carbonate 2a’.
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14645

X. Zhang et al.
To further probe the reac-
tion mechanism shown in
Scheme 5, the following experi-
ments were conducted. Firstly,
as a model complex for I, com-
pound I-1 was employed
(Scheme 6). When I-1 was
treated with pentafluoroben-
zene 1 (4.0 equiv) and Cs2CO3
(4.8 equiv) in the presence of
of PivOH (2.4 equiv), 3a was
produced in 30% yield (as de-
termined by 19F NMR; Sche-
me 6a). However, the absence
of PivOH led to a lower yield
(Scheme 6b), indicating that a
more active (p-allyl)palladium
pivalate II may be generated
in the reaction. Accordingly, a
simplified dimer of (p-allyl)-
palladium acetate II-1[17] was
then investigated as a model
complex for II to further un-
derstand the reaction mecha-
nism. However, when stoichio-
À
Scheme 2. Synthesis of allylated polyfluoroarene-thiophene and -quinoline structures through sequential C H
bond functionalization. Cy-JohnPhos=2-(dicyclohexylphosphino)biphenyl, Ad=adamantanyl.
To gain some mechanistic
insight into the present reac-
tion, kinetic isotope effect
(KIE) and H/D-exchange ex-
periments were performed.
The KIE experiments between
pentafluorobenzene and its
Scheme 3. Kinetic isotope effect study.
deuterated derivative with 2c
show a primary KIE of 1.0
À
(Scheme 3), which implies that the C H bond cleavage of
poly
A
tion–deprotonation pathway,[15] because it is not a turnover-
limiting step.[16] H/D-exchange experiments revealed that
À
the C H bonds of polyfluoroarenes can be directly cleaved
by the sole use of Cs2CO3, thus implying that a polyfluoroar-
ene anion is generated during the reaction (Scheme 4; for
details, see the Supporting Information).
Scheme 4. H/D-exchange experiment on pentafluorobenzene (1).
On the basis of these results, and the fact that a phos-
phine-ligand-free Pd0 species as well as PivOH is important
for the reaction efficiency, we proposed the following mech-
anism for the Pd-catalyzed direct allylation of polyfluoroar-
enes with allylic chlorides (Scheme 5). The catalytic cycle
begins with the oxidative addition of the allylic chloride to a
Pd0 species to produce the (p-allyl)palladium chloride I,
which subsequently undergoes chloride and pivalate ex-
change to give the (p-allyl)palladium pivalate II. Intermedi-
ate II reacts with the polyfluoroarene anion that is generat-
ed in situ by the deprotonation of polyfluoroarene with
Cs2CO3 to afford the key intermediate Pd(polyfluoroaryl)-
ACHTUNGTRENNUNG(allyl)ACHTUNGTRENNUNG(p-allyl) III. Finally, the reductive elimination of III
provides the allylic polyfluoroarene as the product and re-
Scheme 5. Mechanistic proposal for direct Pd-catalyzed allylation of poly-
fluoroarenes with allylic chlorides. Arf =polyfluoroaryl.
generates the active catalyst species Pd0.
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Allylation of Polyfluoroarenes
COMMUNICATION
lyl)palladium carboxylate complex by a polyfluoroarene
anion that is generated in situ is reasonable. This is different
from previous studies, and provides a complementary ap-
proach to the traditional techniques.
In conclusion, a copper- and phosphine-ligand-free [Pd2-
AHCTUNGRTEG(NUNN dba)3]-catalyzed method for the direct allylation of elec-
tron-deficient polyfluoroarenes with allylic chlorides has
been developed. The PivOH additive plays an important
role in the reaction efficiency. Because of the high reaction
efficiency, simple catalytic system, broad substrate scope
that encompasses even ꢀinertꢁ fluoroarenes, and the excel-
lent functional group compatibility, this protocol provides a
useful and facile access to allylated polyfluoroarenes that
are of interest in the fields of both life and materials science.
In particular, the successful synthesis of allylated poly
ACHTUNGTRENNUNGfluoro-
AHCTUNGTREGaNNUN rene–heteroarene structures by a highly efficient sequential
À
C H functionalization highlights the potential of this cross-
coupling for further development and applications.
Experimental Section
General procedure: [Pd2ACHTUNRGTNE(UNG dba)3] (2.5 mol%) and Cs2CO3 (1.2 equiv) were
added to a septum-capped sealed tube (25 mL) under N2, followed by the
addition of toluene (2.5 mL) with stirring. Polyfluoroarene (1.2 mmol,
2.0 equiv), allyl chloride (0.6 mmol, 1 equiv), and PivOH (0.06 mmol,
0.1 equiv) were then added. The sealed tube was screw-capped and
heated at 1408C in an oil bath. After stirring for 10 h, the reaction mix-
ture was cooled to room temperature and diluted with ethyl acetate,
washed with brine, dried over Na2SO4, filtered, and concentrated. The
residue was purified by silica gel column chromatography to provide the
pure product.
Scheme 6. Experiments for mechanistic studies. Yields of 3c were deter-
mined by 19F NMR spectroscopy.
metric amounts of complex II-1 were reacted with penta-
ACHTUNGTRENNUNGfluoroACHTUNGTRENNUNGbenzene in toluene at 1408C, palladium black was
formed immediately, and 3a was only obtained in 12 and
17% yield for the reactions with and without PivOH, re-
spectively, as determined by 19F NMR. To avoid the decom-
position of the palladium complex II-1, a catalytic amount
of II-1 was then used, which provided 3a in higher yields
(Scheme 6c and d). The reactions in which PivOH was pres-
ent was more efficient than those conducted without
PivOH. Hence, on the basis of these findings, the formation
of a (p-allyl)palladium pivalate in the reaction is reasonable.
We next turned our attention to the formation of the key
intermediate III. We supposed that it may be generated by
an attack of the polyfluoroarene anion at the palladium
center of (p-allyl)palladium pivalate II. Thus, a room tem-
perature reaction of II-1 with the pentafluorophenyl anion
that was generated in situ by the deprotonation of penta-
fluoroarene with Cs2CO3 in toluene at 1408C was performed
(Scheme 6e). However, the desired key intermediate III was
not obtained due to its instability. Different solvents
(CH3CN, DMSO) and phosphine (PPh3) were then used to
trap the intermediate III. To our delight, a 16% yield of a
mixture of III-1 and III-2 was obtained when PPh3 was em-
ployed. These results demonstrate that the polyfluoroarene
anion can be generated by the use of Cs2CO3 alone under
the present reaction conditions, and that the catalytic cycle
illustrated in Scheme 5, which involves attack at the (p-al-
Acknowledgements
The NSFC (No. 21172242, 20902100, and 20832008), the National Basic
Research Program of China (973 Program) (No. 2012CB821600), and
SIOC are greatly acknowledged for funding this work.
À
Keywords: allylic compounds · C H activation · palladium ·
pivalic acid · polyfluoroarenes · synthetic methods
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Received: August 6, 2012
Revised: September 15, 2012
Please note: Minor changes have been made to this manuscript since its
publication in Chemistry—A European Journal Early View. The Editor.
Published online: October 10, 2012
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