Nickel(0)-Mediated Transformation of Tetrafluoroethylene and Vinylarenes into Fluorinated Cyclobutyl Compounds

Article abstract of DOI:10.1002/anie.201610047

In the presence of Ni⁰/PCy₃, styrene was found to participate in oxidative cyclization with tetrafluoroethylene, thus leading to the corresponding nickelacycle with a unique η³-π-benzyl structure. In addition, the flexibility of the coordination mode in the η³-benzyl moiety allowed the partially fluorinated nickelacycle to undergo unprecedented amine-induced α-fluorine elimination, thus leading to the construction of a fluorinated cyclobutyl skeleton.

Full text of DOI:10.1002/anie.201610047

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International Edition: DOI: 10.1002/anie.201610047
German Edition: DOI: 10.1002/ange.201610047
Synthetic Methods
Nickel(0)-Mediated Transformation of Tetrafluoroethylene and
Vinylarenes into Fluorinated Cyclobutyl Compounds
Masato Ohashi,* Yuta Ueda, and Sensuke Ogoshi*
Abstract: In the presence of Ni0/PCy3, styrene was found to
participate in oxidative cyclization with tetrafluoroethylene,
thus leading to the corresponding nickelacycle with a unique
h3-p-benzyl structure. In addition, the flexibility of the
coordination mode in the h3-benzyl moiety allowed the
partially fluorinated nickelacycle to undergo unprecedented
amine-induced a-fluorine elimination, thus leading to the
construction of a fluorinated cyclobutyl skeleton.
fluorocarbene intermediate.[8a] Baker further demonstrated
the transformation of [(SIPr)Ni(CF2)4] into perfluorocyclo-
butene and isolated its reaction intermediate (Sche-
me 1a).[8b,c]
O
rganofluorine compounds are important components of
a variety of commercial products,[1,2] and highly fluorinated
organic compounds have recently received much attention
from the viewpoint of potential applications as agrochemicals,
medicinal drugs, and new advanced materials. Therefore,
synthetic approaches that allow the efficient preparation of
highly fluorinated organic molecules from readily available
polyfluorinated substrates have been eagerly anticipated.
One of the possible solutions would be transition-metal-
[3]
À
mediated transformation reactions by C F bond activation,
while more severe reaction conditions, in some cases, are still
À
required for the metal-mediated oxidative addition of C F
bonds. In contrast, it is well known that fluorine atoms can be
remarkably labile when they are bound to carbon atoms
which are directly ligated to metals.[3a,h,4] Some transition-
metal-catalyzed reactions which use b-fluorine elimination as
the key elementary step have been developed to yield
fluorine-containing products.[4f,5,6] In contrast, such fluorine
elimination reactions from fluorinated metallacycle species
are limited. Baker and co-workers recently demonstrated that
the addition of TMSOTf to perfluorometallacyclobutane
complexes such as LnM(CF2)3 led to the formation of the
corresponding perfluoropropenyl complexes,[5k,n] and this
reaction likely involved the formation of a transient cationic
perfluoroallyl complex[7] by b-fluoride elimination. Aside
from Bakerꢀs report, a-fluorine elimination generally occurs
to the exclusion of b-fluorine elimination in transition-metal
Scheme 1. a) TMSOTf-induced transformation of perfluorinated nickel-
acyclopentane into perfluorobutene. b) BF3·Et2O-induced transforma-
tion of 1a into 2 and its possible reaction mechanisms. Tf=trifluoro-
methanesulfonyl, TMS=trimethylsilyl.
We have recently demonstrated the potential of a fluori-
nated nickelacycle intermediate, [LnNi(CF2CF2CH2CH2)] (1),
which was generated by the oxidative cyclization of tetra-
=
fluoroethylene (TFE; CF2 CF2) and ethylene with nickel(0),
À
in nickel-catalyzed C C bond-forming reactions leading to
partially fluorinated compounds.[9,10] We next anticipated that
À
a rare C C bond formation might occur in preference to the
À
P C bond formation after the a-fluorine elimination in 1,
because the a-CH2 carbon atom would be sufficiently
nucleophilic.[9b] Based on this hypothesis, we conducted the
reaction of [(PPh3)2Ni(CF2CF2CH2CH2)] (1a) with BF3·Et2O,
and as a result, 1,2-difluorocyclobutene (2) was detected
(Scheme 1b). Based on previous studies,[8] the formation of 2
might happen through a mechanism which involves a-fluorine
elimination. However, the two possible reaction mechanisms
which are initiated by either a- or b-fluorine elimination
could not be determined definitely. Therefore, we started
investigating which olefins other than ethylene, as well as
TFE itself, could participate in the oxidative cyclization with
TFE at nickel(0) to yield a partially fluorinated nickelacycle.
The results showed that vinylarenes participated in oxidative
cyclization with TFE, thus leading to nickelacycles with
a unique h3-p-benzyl structure. We also demonstrated that the
À
species with both a and b C F bonds. Burch and co-workers
reported pioneering work wherein a-fluorine elimination in
[(PEt3)2Ni(CF2)4] was promoted by the addition of BF3·Et2O
to give (PEt3)(BF4)Ni[CF(PEt3)(CF2)3] as a result of the
migration of one of the PEt3 ligands to the transient
[*] Prof. Dr. M. Ohashi, Y. Ueda, Prof. Dr. S. Ogoshi
Department of Applied Chemistry, Faculty of Engineering
Osaka University
Suita, Osaka 565-0871 (Japan)
E-mail: ohashi@chem.eng.osaka-u.ac.jp
ogoshi@chem.eng.osaka-u.ac.jp
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201610047.
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flexibility of the coordination mode in the h3-benzyl moiety
allowed the partially fluorinated nickelacycle to undergo
unprecedented amine-induced a-fluorine elimination.
Exposure of styrene (3a) to TFE (3.5 atm) in the presence
of [Ni(cod)2] and PCy3 resulted in the formation of [(h1:h3-
CF2CF2CH2CHPh)Ni(PCy3)] (4a) in 89% yield upon iso-
lation (Scheme 2a). An X-ray diffraction study clearly
demonstrated that the phenyl ring in 4a coordinated to
nickel to possess an h3-p-benzyl structure (Figure 1).[11,12]
NMR analysis in a C6D6 solution also indicated that 4a
remained in its p-benzyl structure at room temperature on the
1
basis of a drastic upfield shift of the H and 13C resonances
Scheme 3. Reactivity of 4a towards BF3·Et2O.
attributable to one of the ortho positions of the phenyl ring.
um ylide, generated by phosphine migration to the a-carbon
atom,[8a] was not generated in this reaction. In addition,
another minor product, 3,3-difluoro-1-phenyl cyclobutene
(6a) was also formed. At the initial stage of the reaction,
a transient nickel(II) intermediate (7a) bearing a 1,2,2-
trifluoro-4-phenyl-1-cyclobutyl ligand was generated,[14] and
the quantitative conversion of 7a into cyclobutenes was also
confirmed when the reaction mixture sat at room temper-
ature. Although the bridging mode of the BF4 anion in 7a
remains unclear, treatment of 7a, which was in situ generated
at À308C, with DABCO[10d] led to a nickel(II) fluoride dimer
(8a; Scheme 3b) in 34% yield. The cyclobutenes 5a and 6a
might be generated by b-fluorine elimination from 7a and a-
fluorine elimination from 7a, respectively, followed by 1,2-
hydrogen shift.[15] It should be mentioned that thermolysis of
4a at 1008C resulted in the formation of 2,3-difluoro-1-
phenyl-1,3-butadiene (9a) in 83% yield.[16] This reaction
involved a typical conrotatory electrocyclic reaction of the
cyclobutene 5a.[17,18]
Scheme 2. Reaction of TFE with vinylarenes 3. cod=1,5-cycloocta-
diene, DABCO=1,4-diazabicyclo[2.2.2]octane.
We then explored the reactivity of 4, focusing on which
additive could facilitate the a-fluorine elimination reaction.
As a result, an a-fluorine elimination from 4a was unprece-
dentedly facilitated by the addition of an equimolar amount
of dibenzylamine (10a), thus yielding the fluoride-bridged
nickel(II) dimer complex 8a in 42% yield (Table 1, run 1). In
addition to the formation of 8a, a small amount of the
cyclobutene derivative 5a was generated concomitantly in
this reaction.[19] An X-ray diffraction study of 8a unambigu-
ously demonstrated its dimeric structure as well as the
cyclobutyl ring with the a-fluorine and phenyl substituents
in a cis configuration (Figure 2). The substitution pattern of
the cyclobutyl ring in 8a clearly supported that it was a-
fluorine elimination rather than b-fluorine elimination that
must have been involved in this reaction. The apparent
AA’BB’ coupling pattern observed for the 31P nuclei indicated
that 8a retained its dimeric structure even in solution, while
its rigorous pattern was further complicated, likely due to
Figure 1. ORTEP drawings of 4a with thermal ellipsoids at 30%
probability level. H atoms have been omitted for clarity.
Similar reactions using 2- and 1-vinylnaphthalene (3b, 3c)
gave the corresponding nickel(II) complexes (4b, 4c) in
excellent yields (Schemes 2b,c). Although the reaction with
3b had the potential to afford either of the so-called ortho-p-
naphthyl and pros-p-naphthyl isomers, the former isomer was
obtained as the sole product.[13]
When a C6D6 solution of 4a was treated with BF3·Et2O,
1,2-difluoro-3-phenyl cyclobutene (5a) was obtained as the
major product in 60% yield (Scheme 3a). As with the
reaction of 1a with BF3·Et2O, the corresponding phosphoni-
4
long-range coupling (3JPF and JPF). The characteristic
19F NMR resonances assignable to the 1,2,2-trifluorocyclobu-
tyl ligand appeared at dF = À92.6 (d, 197.4 Hz), À100.3 (d,
197.4 Hz), and À171.2 (s), and another high-field-shifted
multiplet attributable to the fluoride adjacent to nickel was
displayed at dF = À388.5 ppm. A gradual conversion of
isolated 8a into 5a at room temperature was also confirmed
(in C6D6, RT), thus indicating the occurrence of further b-
2
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Table 1: Reaction of 4a with various amines 10.[a]
was low (Table 1, run 2). In contrast, the reaction with
diphenylamine (10c) did not give 8a, and was likely due to
its lower coordinating ability (run 3). In addition, triethyl-
amine (10d) did not yield 8a, and 4a was completely
recovered in this reaction (run 4). This strongly implied that
the existence of the hydrogen atom attached to the nitrogen
atom facilitated the a-fluorine elimination of 4a. In contrast,
treatment of 4a with 2 equivalents of benzylamine (10e)
resulted in a ligand substitution reaction, thus giving [(h1:h1-
CF2CF2CH2CHPh)Ni(NBnH2)2] (11e) in 96% yield (run 5).
An X-ray diffraction study clearly showed that the coordina-
tion mode of the benzylic carbon atom was converted from h3
to h1 in exchange for the coordination of two benzylamine
ligands with the concomitant dissociation of the PCy3
ligand.[13] When a C6D6 solution of 11e stood at room
temperature for 24 hours, a-fluorine elimination took place
gradually to yield a cyclobutyl analogue.[19] This a-fluorine
elimination from 11e, however, was suppressed in the
presence of either an excess amount of 10e or the free PCy3
ligand, thus indicating that a five-membered nickelacycle
monoligated to the amine 10e would be a key reaction
intermediate.[19] In addition, treatment of 4a with bidentate
amines, such as N,N-dimethylethylenediamine (10 f;
DMEDA) and 2,2’-bipyridyl (10g), gave the corresponding
diamine and diimine complexes (11 f and 11g; runs 6 and
7),[13] and no a-fluorine elimination occurred during the
course of the reaction.
Run
Amine
t
Conv. [%]
Yield [%]
8a
5a
11
1
2
3
4
5
6
7
Bn2NH (10a)[b]
Cy2NH (10b)
Ph2NH (10c)
Et3N (10d)
72 h
72 h
72 h
72 h
18 h
10 min
10 min
100
19
0
49 (42)
8
4
–
–
–
–
–
–
–
–
15
–
–
–
–
0
–
BnNH2 (10e)[c]
DMEDA (10 f)
2,2’-bipy (10g)
100
100
100
(96)
(72)
(88)
–
[a] Yields were estimated by 19F NMR spectroscopy using a,a,a-
trifluorotoluene as an internal standard. The values within parentheses
are yields of the isolated products. [b] Conducted in toluene. [c] Used
2 equiv of 10e. bipy=bipyridine, DMEDA=N,N’-dimethylethanedia-
mine.
Based on the pKa value of the N-H group in secondary
alkyl amines (pyrrolidine; pKa = 44), it was obvious that 10a
did not act simply as a Brønsted acid to promote a-fluorine
elimination. This conclusion could also be rationalized by the
fact that treating 4a with benzoic acid (12; pKa = 11.1) led to
the quantitative formation of a nickel(II) carboxylate with
a 1,1,2,2-tetrafluoro-4-phenyl-butyl chain (13; Scheme 4). An
Figure 2. ORTEP drawings of 8a with thermal ellipsoids at 30%
probability level. H atoms have been omitted for clarity.
fluorine elimination from the cyclobutyl nickel(II) species.
Unlike 4a, neither 4b nor 4c nor 1 underwent a similar a-
fluorine elimination under the same reaction conditions.
As far as we could ascertain, this is the first example of
fluorine elimination from fluoroalkyl metal complexes by the
addition of amines. To gain deeper insights into the unique
mechanism, we further conducted the reaction of 4a with 10a
under various reaction conditions. The addition of a catalytic
amount of 10a (20 mol%) led to the formation of 8a in 17%
yield, but a prolonged reaction time was required. The
transformation was suppressed in the presence of the PCy3
ligand. These facts strongly indicated that a pre-equilibrium
process which involves a ligand exchange between PCy3 and
10a exists at the initial stage of the reaction. Indeed, an
incremental increase in the amine loading resulted in the
rapid consumption of 4a, but the yield of 8a was decreased
because of the occurrence of the PCy3/Bn2NH ligand
exchange in the product.[20]
Scheme 4. Reaction of 4a with Brønsted acids.
X-ray diffraction study of 13 clearly demonstrated that the a-
CHPh carbon atom in 4a was selectively protonated.[13] Such
reactivity of 4a towards carboxylic acids is in contrast with
that observed in [(SIPr)Ni(CF2)4], which underwent both a-
fluorine elimination and protonation.[8b]
Based on these findings, a possible reaction mechanism
for the amine-induced a-fluorine elimination from 4a is
summarized in Scheme 5. The key reaction intermediate,
which exists fleetingly before a-fluorine elimination occurs,
would be the monoamine-ligated nickelacycle A. Because the
reaction of 4a with 10a was hampered in the presence of the
additional PCy3 ligand, a pre-equilibrium process between 4a
and A exists, and can also be rationalized by the fact that a-
fluorine elimination from 11e was completely inhibited in the
The scope of the reaction with respect to amines was also
investigated. Although the use of dicyclohexylamine (10b)
promoted the a-fluorine elimination from 4a, the yield of 8a
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open a new avenue for the preparation of organofluorine
compounds.
Acknowledgments
This work was supported by Grant-in-Aid for Scientific
Research (A) (No. 21245028 and A16H02276) and (B) (No.
16KT0057), and Grant-in-Aid for Young Scientists (A) (No.
25708018) from JSPS. This work was also partially supported
by ACT-C, JST. We acknowledge the help of Prof. Dr. N.
Tohnai for collecting X-ray diffraction data. M.O. also
acknowledges The Noguchi Institute.
Scheme 5. A possible reaction mechanism.
presence of an extra amount of either 10e or PCy3. In the
intermediate A, the hydrogen attached to the nitrogen atom
of the amine intramolecularly interacts with one of the a-
fluorine atoms, and could facilitate a-fluorine elimination to
give B. Although it is unclear whether such an intramolecular
H···F interaction, as that observed in A, exists in B, the
resultant electron-deficient fluorocarbene would insert into
Conflict of interest
The authors declare no conflict of interest.
Keywords: amines · fluorine · nickel · reaction mechanisms ·
synthetic methods
À
a Ni C bond, thus leading to the construction of a cyclobutyl
ring. We also speculated that another carbocation intermedi-
ate, the tautomeric form of the fluorocarbene species, could
À
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induced a-fluorine elimination from a partially fluorinated
nickelacycle generated by the oxidative cyclization of TFE
and styrene with nickel(0). The fluoride-bridged nickel(II)
intermediate with a cyclobutyl ring was successfully isolated
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2014, 136, 15158 – 15161; g) K. Kikushima, H. Sakaguchi, H.
Saijo, M. Ohashi, S. Ogoshi, Chem. Lett. 2015, 44, 1019 – 1021.
[11] CCDC 1509216 (4a), 1509217 (4b), 1509218 (4c), 1509219 (8a),
1509220 (11e), 1509221 (11 f), 1509222 (11g), and 1509223 (13)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
[12] In Figure 1, one of the two crystallographically independent
molecules has been depicted. The other has been also depicted in
the Supporting Information.
[13] Reaction details as well as ORTEP drawings of 4b, 4c, 11e, 11 f,
11g, and 13 have been deposited in the Supporting Information.
[14] Identification of 7a was based on the comparison of its
characteristic NMR resonance with those of 8a. Isolation of 7a
was hampered by its thermal instability.
[15] A similar reaction employing 4b in toluene allowed the isolation
of the corresponding products (5b and 6b) in 17 and 26% yield,
respectively.
[16] a-Fluorine elimination did not take place when a C6D6 solution
of 4a was let stand at room temperature for over a week.
[17] Thermolysis of 5b at 1008C for 2 h lead to the quantitative
generation of 9b, while 6b remained intact under the same
reaction conditions. See also the Supporting Information.
[18] a) R. B. Woodward, R. Hoffmann, J. Am. Chem. Soc. 1965, 87,
395 – 397; b) R. B. Woodward, R. Hoffmann, The Conservation
of Orbital Symmetry, Academic Press, New York, 1970; for
reviews, see: c) W. R. Dolbier, Jr., H. Koroniak, K. N. Houk, C.
Sheu, Acc. Chem. Res. 1996, 29, 471 – 477.
[19] The crude product was found to include a considerable amount
of a nickel(II) fluoride analogue of 8a, which would be
generated by the PCy3/Bn2NH ligand exchange reaction in the
product. See the Supporting Information for details.
[20] See the Supporting Information for details.
[9] a) M. Ohashi, T. Kawashima, T. Taniguchi, K. Kikushima, S.
Ogoshi, Organometallics 2015, 34, 1604 – 1607; b) M. Ohashi, H.
Shirataki, K. Kikushima, S. Ogoshi, J. Am. Chem. Soc. 2015, 137,
6496 – 6499; see also a stoichiometric hydrogenolysis reaction;
c) R. T. Baker, R. P. Beatty, W. B. Farnham, R. L. Wallace, Jr.,
U.S. Patent 5,760,282, 1998.
Manuscript received: October 19, 2016
Revised: November 30, 2106
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Synthetic Methods
M. Ohashi,* Y. Ueda,
S. Ogoshi*
&&& — &&&
Nickel(0)-Mediated Transformation of
Tetrafluoroethylene and Vinylarenes into
Fluorinated Cyclobutyl Compounds
In the presence of Ni0/PCy3, styrene was
found to participate in oxidative cycliza-
tion with tetrafluoroethylene, thus leading
to the corresponding nickelacycle with
a unique h3-p-benzyl structure. In addi-
tion, the flexibility of the coordination
mode in the h3-benzyl moiety allowed the
partially fluorinated nickelacycle to
undergo unprecedented amine-induced
a-fluorine elimination, thus leading to the
construction of a fluorinated cyclobutyl
skeleton.
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!