Nickel-Catalyzed Allylic C(sp3)-F Bond Activation of Trifluoromethyl Groups via β-Fluorine Elimination: Synthesis of Difluoro-1,4-dienes

Article abstract of DOI:10.1021/acscatal.5b01463

The nickel-catalyzed defluorinative coupling of 2-trifluoromethyl-1-alkenes and alkynes with the aid of Et₃SiH provides 1,1-difluoro-1,4-dienes under mild reaction conditions. This reaction involves selective allylic C(sp³)-F bond ac

Full text of DOI:10.1021/acscatal.5b01463

Letter
pubs.acs.org/acscatalysis
Nickel-Catalyzed Allylic C(sp3)−F Bond Activation of Trifluoromethyl
Groups via β‑Fluorine Elimination: Synthesis of Difluoro-1,4-dienes
Tomohiro Ichitsuka, Takeshi Fujita, and Junji Ichikawa*
Division of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
S
* Supporting Information
ABSTRACT: The nickel-catalyzed defluorinative coupling of
2-trifluoromethyl-1-alkenes and alkynes with the aid of Et3SiH
provides 1,1-difluoro-1,4-dienes under mild reaction condi-
tions. This reaction involves selective allylic C(sp3)−F bond
activation via β-fluorine elimination from nickelacyclopentenes.
KEYWORDS: C−F bond activation, nickel catalysis, trifluoromethylalkenes, alkynes, 1,4-dienes, β-fluorine elimination
Scheme 1. Ni-Catalyzed Defluorinative Coupling of
2-Trifluoromethyl-1-alkenes 1 with Alkynes 2
arbon−fluorine (C−F) bond activation of the trifluoro-
Cmethyl group is rarely achieved not only because of the
high bond energy but also presumably because of the shielding
effect of the lone-pair electrons of the three fluorine atoms.1
Although defluorinative functionalization of trifluoromethyl-
bearing compounds would realize one of the most straightfor-
ward approaches to fluorine-containing compounds, harsh reac-
tion conditions have typically been required to cleave C(sp3)−F
bonds of trifluoromethyl groups.1,2
Recently, we reported the nickel-mediated [3 + 2] cyclo-
addition of 2-trifluoromethyl-1-alkenes 1 and alkynes 2 via double
C−F bond cleavage of a trifluoromethyl group under mild
reaction conditions (Scheme 1a).3 In this reaction, ring-opening
of nickelacycle A, formed via oxidative cyclization of trifluoro-
methylalkenes 1 and alkynes 2 with Ni(0), readily proceeded
via β-fluorine elimination4 to generate alkenylnickel(II) species B.
Subsequent 5-endo insertion and a second β-fluorine elimination
afforded 2-fluoro-1,3-cyclopentadienes 3 (Scheme 1, path a). The
potential advantage of β-fluorine elimination prompted us to
develop a nickel-catalyzed three-component coupling reaction of
2-trifluoromethyl-1-alkenes 1, alkynes 2, and metal species, which
would proceed via the selective cleavage of one of the C(sp3)−F
bonds.5,6 We assumed that alkenylnickel(II) fluorides B, inter-
mediates toward 2-fluoro-1,3-cyclopentadienes 3, might be trans-
metalated, for example, by an appropriate metal hydride (R = H)
to afford the corresponding 1,1-difluoro-1,4-dienes 4 along with
regeneration of Ni(0) (Scheme 1b, path b).7
To prove our hypothesis, we sought a metal hydride reagent
suitable for the coupling reaction of α-trifluoromethylstyrene
(1a) and 4-octyne (2a) in the presence of a catalytic amount
of Ni(cod)2 and PCy3 in toluene at 50 °C (Table 1). In the
absence of any hydride sources, fluorocyclopentadiene 3aa
was obtained in 3% yield, as we reported previously (Table 1,
entry 1). The use of i-PrONa as a hydride source afforded
1,1-difluoro-1,4-diene 4aa with the E configuration as the sole
product in 74% yield via cleavage of the C−F bond in the
trifluoromethyl group and formation of the C−C and C−H bonds
(entry 2).7a When 9-BBN or DIBAL-H was employed, 1a was de-
composed to give a complex mixture because of their electrophilic
reactivity (entries 3 and 4). Consequently, Et3SiH was found to be
highly effective for improving the product yield to 92% (entry 5).7b
Even 5 mol % of the Ni catalyst successfully promoted the coupling
reaction to give 4aa in 93% isolated yield (entry 6).
The scope of suitable trifluoromethylalkene substrates 1 and
alkynes 2 was then examined under the reaction conditions
Received: July 13, 2015
Revised: August 19, 2015
© XXXX American Chemical Society
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Letter
silyl-substituted trifluoropropenes 1g and 1h successfully
underwent the coupling reaction with 2a in the presence of
10 mol % ZrF4 as a cocatalyst, leading to high yields of 4ga and
4ha (entries 7 and 8), respectively.8 The use of diphenylace-
tylene (2b) with 1a resulted in the formation of the corre-
sponding coupling product 4ab in 72% yield (entry 9). In the
case of coupling between 2b and 3,3,3-trifluoropropene (1i),
use of SIMes instead of PCy3 promoted the reaction to afford
4ib in 77% yield (entry 10). Unsymmetrical 1-phenylpent-1-
yne (2c), 1-(4′-methoxyphenyl)pent-1-yne (2d), 1-(4′-ethoxy-
carbonylphenyl)pent-1-yne (2e), 1-phenylprop-1-yne (2f), and
4-methylpent-2-yne (2g) also participated in this reaction with
1a to afford the corresponding 1,1-difluoro-1,4-dienes 4ac−4ag
in good to excellent yields with good to complete regio-
selectivities (entries 11−15). The obtained regioselectivities
were in agreement with literature reports of nickel-catalyzed
coupling reactions of alkynes via oxidative cyclization.9
Furthermore, the reaction of β-trifluoromethylcinnamate 1j
(E/Z = 83:17) successfully proceeded with 3-hexyne (2h) to
afford the corresponding 1,1-difluoro-1,4-diene 4jh in 65%
yield eq 1.
Table 1. Optimization of the Reaction Conditions for
Ni-Catalyzed Defluorinative Coupling of 1a with 2a
a
a
entry
x/mol %
metal hydride
4aa/%
3aa/%
1
2
3
4
5
6
10
10
10
10
10
5
none
i-PrONa
9-BBN
0
3
0
0
0
0
0
74
15
0
DIBAL-H
Et3SiH
92
b
Et3SiH
93
a
Yield was determined by 19F NMR measurement using PhCF3 as an
b
internal standard. Isolated yield.
Table 2. Synthesis of 1,1-Difluoro-1,4-dienes 3 via
a
Ni-Catalyzed Defluorinative Coupling of 1 with 2
entry
R1 (1)
Ph (1a)
R2, R3 (2)
Pr, Pr (2a)
yield/%
4aa 93
1
2
3
4
C6H4(o-OMe) (1b)
C6H4(p-OMe) (1c)
C6H4(p-Ac) (1d)
Pr, Pr (2a)
Pr, Pr (2a)
Pr, Pr (2a)
4ba 84
4ca 80
4da 94
4ea 88
4fa 91
4ga 86
4ha 79
4ab 72
4ib 77
4ac 99
4ad 99
b
For this reaction, there are three plausible mechanisms that
can be induced by different initial steps: (i) oxidative cyclization
of 2-trifluoromethyl-1-alkenes 1 and alkynes 2 with Ni(0)
(Scheme 2, path c), (ii) oxidative addition of a C−F bond of 1
to Ni(0) (path d),10 and (iii) oxidative addition of an Si−H
bond to Ni(0) (path e).11,12 In paths c and d, the common
intermediates B are formed via an oxidative cyclization/β-fluorine
elimination or an oxidative addition/insertion sequence. Trans-
metalation of B with Et3SiH and subsequent reductive elimi-
nation afford 1,1-difluoro-1,4-dienes 4. Conversely, in path e,
oxidative addition of Et3SiH to Ni(0) initially occurs to provide
silylnickel hydride F. Subsequent hydrometalation of alkyne and
alkene insertion followed by β-fluorine elimination from the
alkylnickel complexes G gives 4.
To elucidate the mechanism, the stoichiometric reaction of
2-trifluoromethyl-1-alkene 1d and Et3SiH with a Ni(0) complex
in C6D6 at room temperature was performed and monitored by
1H, 19F, and 31P NMR eq 2. Nickelacyclopropane 5 was obtained
as the sole product in 93% yield, and neither allylnickel(II)
complex E nor silylnickel hydride F generated by oxidative
addition of 1d or Et3SiH to Ni(0) was observed. Thus, the
possibility of path e was lessened. Moreover, treatment of the
obtained reaction mixture with alkyne 2a afforded the coupling
product 4da in 70% yield from 1d along with 94% yield of
Et3SiF. These results suggest that the C−F bond activation may
proceed via an oxidative cyclization/β-fluorine elimination
sequence in this reaction (Scheme 2, path c). Note that the
oxidative cyclization process determined the stereochemistry of
alkyne-derived alkene moieties of 4 (syn addition).
5
C6H4(p-CO2Et) (1e) Pr, Pr (2a)
c
6
d
C6H4(p-Cl) (1f)
CH2CH2Ph (1g)
SiMe2Ph (1h)
Ph (1a)
Pr, Pr (2a)
,
,
e
f
7
8
9
Pr, Pr (2a)
d
Pr, Pr (2a)
g,h
Ph, Ph (2b)
Ph, Ph (2b)
Pr, Ph (2c)
i
10
11
12
13
14
15
H(1i)
g
,
,
,
j
j
j
Ph (1a)
g
g
j
Ph (1a)
Pr, C6H4(p-OMe) (2d)
Ph (1a)
Pr, C6H4(p-CO2Et) (2e) 4ae 65
Ph (1a)
Me, Ph (2f)
4af 91
k
l
Ph (1a)
i-Pr, Me (2g)
4ag 88 (95:5)
a
Reaction conditions: Ni(cod)2 (5 mol %), PCy3 (10 mol %), 1
(0.50 mmol), 2 (0.55 mmol), Et3SiH (1.0 mmol), toluene (2.5 mL),
50 °C, 3 h. 4 h. 2 h. Ni(cod)2 (10 mol %), PCy3 (20 mol %) and
ZrF4 (10 mol %) were used as catalysts. 80 °C, 15 h. RT, 2 h.
SIMes-HCI (5 mol %) and t-BuOK (5 mol %) were used instead of
b
c
d
e
f
g
h
i
PCy3. RT, 8 h. Reaction was performed using Ni(cod)2 (10 mol %),
SIMesHCI (10 mol %), t-BuOK (10 mol %), 1i (1.0 atm), 2b
(0.37 mmol), Et3SiH (0.74 mmol), toluene (1.9 mL), 80 °C, 10 h.
j
k
l
RT, 3 h. Ni(cod)2 (10 mol %), PCy3 (20 mol %), 4 h. Regioisomer
ratio was determined by 19F NMR.
obtained above (Table 2). The α-trifluoromethylstyrenes 1b
and 1c bearing electron-donating methoxy groups provided
1,1-difluoro-1,4-dienes 4ba and 4ca, in good yields (entries 2
and 3, respectively), as did the α-trifluoromethylstyrenes 1d
and 1e bearing electron-withdrawing acetyl and ethoxycarbonyl
groups (4da and 4ea, entries 4 and 5, respectively). Intriguingly,
α-trifluoromethylstyrene 1f with a chlorine substituent, which
could undergo oxidative addition to Ni(0), also participated in the
reaction without the loss of the C−Cl bond (entry 6). Alkyl- and
Organozinc and organoaluminum reagents were also employed
as a third component in the catalytic coupling reactions eqs 3 and 4.
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Scheme 2. Plausible Reaction Mechanisms for the
Ni-Catalyzed Defluorinative Coupling Reaction
with alkyne 2a was promoted by the nickel catalyst in the
presence of 2.0 equiv of Et3SiH. Allylic C−F bond cleavage
afforded the corresponding 1-fluoro-1,4-diene 9 in 82% yield
(E/Z = 50:50, eq 5). Regioselective C−F bond activation of
perfluoroalkyl alkenes was also affected by this method eqs 6
and 7. The 2-pentafluoroethyl-1-alkene 10 readily reacted with
2-butyne (2h) and Et3SiH in the presence of the nickel catalyst.
Allylic C−F bond cleavage selectively occurred to afford the
corresponding trifluoromethylated fluorodiene 11 in 97% yield
with high stereoselectivity for the Z fluoroalkene moiety (E/Z =
5:95, eq 6).14 Similarly, the reaction of 2-nonafluorobutyl-1-
alkene 12 smoothly proceeded to afford heptafluoropropylated
fluorodiene 13 in 86% yield with exclusive Z-selectivity in the
fluoroalkene moiety eq 7.14
In summary, we have developed a methodology for catalytic
C(sp3)−F bond activation of the trifluoromethyl group via
β-fluorine elimination from nickelacyclopentenes generated by
the stepwise oxidative cyclization of 2-trifluoromethyl-1-alkenes
1 and alkynes 2 with Ni(0). The metal−fluorine bond in the
intermediary alkenylnickel(II) fluoride is effectively trans-
formed into a metal−hydrogen (methyl) bond by a hydrosilane
(a dialkylzinc), regenerating Ni(0). This reaction provides a
regio- and stereoselective method for the synthesis of multis-
ubstituted difluoroalkenes, which have attracted considerable
attention as bioisosteres of carbonyl compounds in pharma-
ceutical science15 and as building blocks for further trans-
formation into fluorine-containing compounds.1,16
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.5b01463.
Experimental procedures and characterization data of
new compounds (PDF)
Crystallographic data (CIF)
On treatment with ZnMe2, 2-trifluoromethyl-1-alkene 1a and
alkyne 2a underwent the nickel-catalyzed defluorinative cou-
pling to afford methylated 1,1-difluoro-1,4-dienes 6 eq 3. Trans-
metalation of alkenylnickel(II) fluorides B with ZnMe2 caused
introduction of a methyl group into the product. In contrast,
the reaction of 1a and 2a with AlMe3 afforded triply methylated
1,4-diene 7 via cleavage of the three C−F bonds eq 4.2b,13
Furthermore, not only trifluoromethylalkenes but also difluo-
roallylic compounds underwent nickel-catalyzed defluorinative
coupling eqs 5−7. The reaction of α-difluoromethylstyrene (8)
AUTHOR INFORMATION
Corresponding Author
*E-mail: junji@chem.tsukuba.ac.jp.
■
Notes
The authors declare no competing financial interest.
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Letter
2010, 132, 2050−2057. (b) Liu, P.; Montgomery, J.; Houk, K. N. J.
Am. Chem. Soc. 2011, 133, 6956−6959.
ACKNOWLEDGMENTS
■
This work was supported by the Grant-in-Aid for Scientific
Research (B) (No. 25288016) and the Grant-in-Aid for Young
Scientists (B) (No. 26870079) from JSPS. We acknowledge
Tosoh F-Tech, Inc. for a generous gift of 2-bromo-3,3,3-
trifluoropropene.
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2006, 2006, 1397−1412. (b) Narumi, T.; Tomita, K.; Inokuchi, E.;
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3468. (c) Hazari, A.; Gouverneur, V.; Brown, J. M. Angew. Chem., Int.
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