Palladium-Catalyzed Olefination and Arylation of Polyfluoroarenes Using Molecular Oxygen as the Sole Oxidant

Article abstract of DOI:10.1002/adsc.201500632

The palladium-catalyzed cross-dehydrogenative-coupling of polyfluoroarenes with alkenes or simple arenes has been developed. The approach is characterized by using atmospheric pressure of molecular oxygen as the sole oxidant, and provides a straightforward and environmentally benign route to functionalized polyfluoroarenes.

Full text of DOI:10.1002/adsc.201500632

UPDATES
DOI: 10.1002/adsc.201500632
Palladium-Catalyzed Olefination and Arylation of
Polyfluoroarenes Using Molecular Oxygen as the Sole Oxidant
Qiufeng Huang,a,b,* Xinyu Zhang,a Lin Qiu,a Jiajun Wu,a Hanbing Xiao,a
Xiaofeng Zhang,a and Shen Linb
a
College of Chemistry & Chemical Engineering, Fujian Normal University, Fuzhou 350007, and Key Laboratory of Coal to
Ethylene Glycol and Its Related-Technology, Chinese Academy of Sciences, Fuzhou 350002, Peopleꢀs Republic of China
Fax: (+86)-591-22867399; e-mail: qiufenghuang@fjnu.edu.cn
Fujian Key Laboratory of Polymer Materials, Fuzhou 350007, Peopleꢀs Republic of China
b
Received: June 30, 2015; Revised: August 12, 2015; Published online: October 29, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500632.
Abstract: The palladium-catalyzed cross-dehydro-
genative-coupling of polyfluoroarenes with alkenes
or simple arenes has been developed. The approach
is characterized by using atmospheric pressure of
molecular oxygen as the sole oxidant, and provides
a straightforward and environmentally benign route
to functionalized polyfluoroarenes.
try and materials science.[6] Therefore, there is a grow-
ing demand for an efficient synthetic methodology
toward this structural target.[7] Recent developments
in CDC reactions showed that synthetic targets incor-
porating polyfluoroarenes can be efficiently synthe-
sized by reacting polyfluoroarenes with alkenes or
simple arenes using palladium catalysts.[8] For exam-
ple, Zhang et al. reported the Pd(OAc)2-catalyzed
direct olefination of polyfluoroarenes by using
Ag2CO3 (2.0–3.0 equiv.) as the oxidant.[8a,b] The same
group later reported an improved method in which
a catalytic amount of Ag2CO3 (0.1 equiv.) was used as
a co-catalyst oxidant with O2 as a terminal oxidant.[8c]
Liu et al. developed a Pd-catalyzed olefination of per-
À
Keywords: arylation; C H activation; molecular
oxygen; olefination; palladium
Transition metal-catalyzed cross-dehydrogenative- fluoroarenes with allyl esters by using AgOAc
coupling (CDC) reactions have emerged as one of the (2.0 equiv.) as the oxidant.[8d] Su et al. and Shi et al.
most efficient and straightforward methods for the reported Pd(OAc)2-catalyzed CDC reactions of poly-
À
formation of C C bonds, and have received consider- fluoroarenes with simple arenes by using Cu(OAc)2
able attention and intensive study in recent years.[1] In (2.0 equiv.) or Ag2CO3 (1.5 equiv.) as the oxidant.[8e,f]
spite of the great progress made in this field, several However, each of these procedures suffers from the
key challenges still remain. Regioselectivity is one requirement of a hazardous and toxic oxidant such as
À
major challenge for C H activation; selective CDC Ag2CO3, AgOAc and Cu(OAc)2 [Figure 1, Eqs. (1)
reactions usually require a pre-installed directing and (2)]. The development of a new catalytic system
group to coordinate with the metal catalysts which that uses O2 as oxidant would be highly desirable.
cannot be easily removed from the products.[2] Anoth- Herein, we report an efficient Pd(OAc)2-catalyzed
er urgent challenge is to develop CDC reactions CDC reaction of polyfluoroarenes with alkenes or
which can utilize atmospheric molecular oxygen as simple arenes by using atmospheric O2 as the sole oxi-
a terminal oxidant[3] or the sole oxidant,[4] as most re- dant [Figure 1, Eqs. (3) and (4)].
ported CDC reactions require hazardous and stoichio-
Our investigation started with pentafluorobenzene
metric oxidants, such as copper(II) salts and silver(I) (1a) and ethyl acrylate (2a) as the standard substrates
salts.[5] Using molecular oxygen is advantageous over with Pd(OAc)2 as the catalyst to optimize the reaction
À
other metal oxidants because C C bond formation conditions. Based on our recent work dealing with the
occurs with only the generation of water as a by-prod- Pd(OAc)2-polyoxometalate-catalyzed Fujiwara–Mori-
uct. However, only a few cases of CDC reactions tani reaction,[9] several polyoxometalates were firstly
using molecular oxygen as an oxidant have been re- examined as co-catalyst for the reaction and afforded
ported.
the corresponding desired product in relatively low
Building blocks which contain polyfluoroarenes are yield (15–44%; Table 1, entries 1–4). To further im-
of high significance in areas such as medicinal chemis- prove the yield, a variety of mono-protected amino
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Qiufeng Huang et al.
Table 1. Optimization of the reaction conditions.[a]
Figure 1. Pd(OAc)2-catalyzed CDC reactions of polyfluor-
oarenes
acids (MPAAs) which are known to positively influ-
À
ence palladium-catalyzed C H activation reactions
were tested extensively.[10] Gratifyingly, by adding
0.1 equiv. of dl-pGlu-OH, the desired product 3a was
formed in 91% yield (E/Z=20:1; Table 1, entry 9)
even in the absence of heteropoly acid. Other
MPAAs such as Boc-Val-OH, Z-Phe-OH, Boc-Leu-
OH and Fmoc-Ile-OH were less effective for this re-
action (Table 1, entries 5–8). It is worth mentioning
that PivOH is crucial for this reaction, removing
PivOH from the reaction mixture resulted in a lower
yield (50%; Table 1, entry 10). In addition, the cou-
pling of 1a with 2a afforded the desire product in
65% yield when the reaction was run only with
PivOH as additive (Table 1, entry 11). Furthermore,
replacing Pd(OAc)2 with Pd(OPiv)2 gave a comparable
yield (90%; Table 1, entry 12). These facts indicate
[a]
Reaction conditions (unless otherwise specified): 1a
(0.75 mmol, 2.5 equiv.), 2a (0.3 mmol, 1 equiv.), solvent
(3 mL), O2 (1 atm) at 1208C for 16 h. DMA=N,N-dime-
thylacetamide, PivOH=pivalic acid.
Yield according to GC based on the amount of 2a used.
Number in parenthesis is isolated yield.
Pd(OPiv)2, instead of Pd(OAc)2 was used. In the absence
of PivOH, only 58% of product 3a was obtained.
The reaction was carried out under a nitrogen atmos-
phere.
[b]
[c]
[d]
[e]
Using air as the oxidant.
that the Pd-PivOH combination could lower the
[11]
À
energy of C H bond cleavage. In terms of solvent,
DMA proved to be the most efficient, other common-
With the optimized reaction conditions in hand, we
ly used solvents such as DMSO, toluene, 1,4-dioxane began to explore the scope of the substrates; the re-
and CH3CN proved to be either less effective or total- sults are shown in Table 2. The reactions of penta-
ly ineffective (Table 1, entry 13–16). In a control ex- fluorobenzene (1a) with acrylates of various alcohols
periment, no product was observed in the absence of gave the corresponding products in good yields (see
O2 (Table 1, entry 17), and only 30% of the desired 3b–d). Electron-deficient olefins bearing amide, phos-
product was produced when air was used as the oxi- phonate, sulfone were all tolerated, albeit product
dant instead of oxygen (Table 1, entry 18). If taken to- yields were rather modest (see 3e–g). When N,N-di-
gether, these results indicated the following as the op- methylacrylamide or methyl vinyl sulfone was em-
timal reaction conditions: pentafluorobenzene ployed, a moderate yield (54% or 40%) of the E-
(2.5 equiv.), ethyl acrylate (1 equiv.), Pd(OAc)2 isomer was formed selectively (see 3f and 3g). Im-
(10 mol%),
(10 mol%) in DMA under an atmosphere of O2 substituents could also undergo this transformation
(1 atm) at 1208C. with good yields and excellent stereoselectivity (see
PivOH
(1.5 equiv.),
dl-pGlu-OH pressively, styrene or styrenes bearing different para
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Palladium-Catalyzed Olefination and Arylation of Polyfluoroarenes
Table 2. Olefination of polyfluoroarenes with alkenes.[a]
[a]
Reaction conditions (unless otherwise specified): 1 (0.75 mmol, 2.5 equiv), 2 (0.3 mmol, 1 equiv), DMA (3 mL), O2
(1 atm) at 1208C for 16 h. Isolated yield. E/Z ratio of product determined by GC.
3h–k). It is worth noting that the presence of bromide effect of the substituent on the benzene ring is very
or ester in the phenyl ring of styrene did not change significant. As for monosubstituted arenes, a mixture
the reaction efficiency, which offers the opportunity of meta-and para-arylation products was obtained
for further functionalization. Tetrafluoroarenes con- (see 4b–g); for 1,2-dimethylbenzene (see 4h), the re-
taining an electron-donating group were more reac- action selectively occurred at 4-position. Other poly-
tive and stereoselective than those with electron-with- fluoroarenes, such as 1,2,4,5-tetrafluoro-3-methylben-
drawing groups (see 3l–n). In addition, other poly- zene and 1,2,4,5-tetrafluoro-3-methoxybenzene also
fluoroarenes such as tri- and tetrafluorinated ben- worked, and gave the products 4i and 4j. Because of
zenes and pyridine were also efficient coupling part- the diarylation of 1,2,4,5-tetrafluorobenzene under
ners, and good yields of the products were achieved. the standard conditions, the moderate yield of 4k was
(54–74%, see 3o–q).
obtained. Moreover, 2,3,5,6-tetrafluoropyridine af-
To demonstrate the versatility of this novel method- forded the corresponding coupled product in a syn-
À
ology for C H activations of polyfluoroarenes, we fur- thetically useful yield (see 4l).
ther explored the arylation of polyfluoroarenes with
In conclusion, we have developed a simple and effi-
À
simple arenes. To our delight, this novel catalytic cient direct C H olefination and arylation of poly-
system and our optimized reaction conditions offered fluoroarenes using a novel catalytic system. This
the desired cross-dehydrogenative-coupling products system features use of molecular oxygen as the sole
successfully. Table 3 summarizes the substrate scope oxidant at atmospheric pressure and proves to be
of the arylation. Various arenes such as benzene, tolu- practically convenient and environmentally friendly.
ene, o-xylene, electron-rich or electron-deficient ben- With a great generality of substrates, our new proto-
zene derivatives were efficiently coupled with penta- col also offers a general and efficient method for the
fluorobenzene (1a) to give the arylated products in syntheses of fluorinated arenes and derivatives.
moderate to good yields (see 4a–h), and the steric
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UPDATES
Qiufeng Huang et al.
Table 3. Arylation of polyfluoroarenes with arenes.[a]
column chromatography [eluting with petroleum ether/ethyl
acetate=100:1] to afford 3a; yield: 80%. The GC analyses
of the reaction mixture disclosed the formation of 3a in
91% yield (E/Z=20:1).
Acknowledgements
We thank Dr. Zhe Zhang (University of Houston) for proof-
reading the manuscript and Prof. Ruihu Wang (Fujian Insti-
tute of Research on the Structure of Matter, Chinese Acade-
my of Sciences for assistance with 19F NMR characterization.
Financial support from NSFC (Grant No. 21103022), Foun-
dation of Fujian Educational Committee (Grant No.
JK2014010) is gratefully acknowledged.
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