Highly chemoselective carbon-carbon σ-bond activation: Nickel/Lewis acid catalyzed polyfluoroarylcyanation of alkynes

Article abstract of DOI:10.1002/anie.201207880

Selective C-C bond cleavage: A catalyst consisting of nickel(0)/DPEphos and BPh₃ is highly effective for the addition of polyfluorobenzonitriles to alkynes through selective activation of the C-CN bond over the C-H or C-F bonds. The addition reaction is applicable to the sequential insertion of two different alkynes into C-CN and C-H bonds (see scheme). Copyright

Full text of DOI:10.1002/anie.201207880

Angewandte
Chemie
DOI: 10.1002/anie.201207880
À
C C Activation
À
Highly Chemoselective Carbon Carbon s-Bond Activation: Nickel/
Lewis Acid Catalyzed Polyfluoroarylcyanation of Alkynes**
Yasunori Minami,* Hirofumi Yoshiyasu, Yoshiaki Nakao,* and Tamejiro Hiyama*
À
Metal-catalyzed activation of saturated C C bonds has
a great potential in economical and ecofriendly transforma-
5-cyano-4-octene (3aa) was formed. However, the use of
DPEphos as a ligand and BPh3 as a Lewis acid in cyclo-
pentylmethylether (CPME) at 1008C was highly effective,
and arylcyanation proceeded to afford 3aa in 90% yield after
purification by preparative TLC [Eq. (1)]. The Z configura-
tions.[1] For example, the reaction involving cleavage of the C
C s bond and subsequent addition to unsaturated bonds is
ideal for the simultaneous formation of two C C s bonds
without the generation of by-products (100% atom econ-
À
À
À
À
omy). On the other hand, C F and C H activation of
polyfluoroarenes by various metal complexes has been well
investigated for synthetic organic chemistry.[2,3] However, the
À
catalytic functionalization of polyfluoroarenes through C C
activation remains to be explored. In this respect, the work by
1
À
Gunay and Jones is seminal: the C(sp ) C6F5 bond in
bis(pentafluorophenyl)acetylene is shown to add oxidatively
to a platinum(0) complex under UV irradiation.[4] Our group
demonstrated that the nickel(0)/Lewis acid catalyst system is
highly effective for the carbocyanation of alkynes and alkenes
using various organic nitriles.[1b,5] On the basis of these studies,
tion was unambiguously confirmed by NOESY NMR anal-
À
À
we expected that C CN activation by nickel(0)/Lewis acid
ysis. Of note, all the C F bonds remained intact during the
reaction. The use of BPh3 was found to be crucial to enhance
À
À
catalyst is preferred rather than C H and C F activation.
À
À
Herein, we report the selective cleavage of the C CN bond in
polyfluorobenzonitriles by the nickel(0)/DPEphos complex
(DPEphos = bis(2-diphenylphosphinophenyl)ether)
the reactivity of the C CN bond. Other Lewis acids, such as
BF3, B(C6F5)3, ZnCl2, and AlMe3, were less effective. When
the optimum catalyst system was applied to 2,3,5,6-tetrafluor-
and
À
À
À
À
BPh3; all other reactive C H and C F bonds are unaffected,
thus resulting in the smooth addition of the polyfluorophenyl
obenzonitrile 1b, which has an acidic C H bond, only the C
CN bond participated in the reaction to give 3ba in 98%
ꢀ
=
À
and cyano moieties to C C and C C bonds.
yield. On the other hand, C H activation of 1b by [Ni(cod)2]/
First, we tested the typical conditions ([Ni(cod)2], PPhCy2,
and AlMe3 as catalysts; cod = 1,5-cyclooctadiene) for the
arylcyanation of 4-octyne (2a, 1.5 mmol)[5d] with pentafluoro-
benzonitrile (1a, 1.0 mmol) and were disappointed to find
that no trace of the desired adduct (Z)-4-pentafluorophenyl-
PCyp3 catalyst[3a,c] was not observed. These results clearly
À
show that the C CN bond in 1b is exclusively activated in the
À
À
presence of the C H and C F bonds in 1b and 3ba.
Competition experiments between 1a and benzonitrile 4
were carried out to get a deeper insight into the reactivity of 1.
Substrates 1a, 4, and 2a were added to a solution of
[Ni(cod)2], DPEphos, and BPh3 in CPME [Eq. (2)]. The
[*] Dr. Y. Minami, H. Yoshiyasu, Prof. Dr. T. Hiyama
Research and Development Initiative, Chuo University
Kasuga, Bunkyo-ku, Tokyo 112-8551 (Japan)
E-mail: yminami@kc.chuo-u.ac.jp
thiyama@kc.chuo-u.ac.jp
Dr. Y. Nakao
Department of Material Chemistry, Graduate School of Engineering,
Kyoto University
Kyoto 615-8510 (Japan)
E-mail: nakao.yoshiaki.8n@kyoto-u.ac.jp
[**] This research was supported financially by a Grant-in-Aid for
Scientific Research (S) (21225005 to T.H.) and Scientific Research
on Innovative Areas “Molecular Activation Directed toward
Straightforward Synthesis” (Y.N., No. 22105003) from JSPS and
MEXT, respectively. Y. M. acknowledges JSPS for the postdoctoral
scholarship. The authors thank Prof. Dr. Youichi Ishii, Dr. Yuichiro
Mutoh, Masahiro Sakigawara, Kota Ikenaga, and Dr. Makoto
Yamashita for their kind assistance in X-ray crystallographic
analyses.
reaction mixture was stirred at 1008C for 1 h and assayed by
GC, which showed that 3aa was selectively formed in 91%
yield without a trace of 5. Thereafter, 4 gradually participated
in the reaction, and the yield of 5 reached 91% within 3 h
(Figure 1).[6] The original arylcyanation conditions ([Ni-
(cod)2], PPhMe2, and AlMe2Cl as catalysts)[5d] gave minute
amounts of 3aa and 5 in a nonselective manner. Possibly, 1a
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207880.
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883

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Figure 1. Time course of the competition reaction between 1a and 4
~
*
catalyzed by [Ni(cod)2], DPEphos, and BPh3. : yield of 3aa. : yield
of 5.
and AlMe2Cl interacted to render the Ni species inactive for
the desired catalysis.[7]
Next, we carried out stoichiometric reactions in order to
shed light on the polyfluoroarylcyanation mechanism. The
reaction of 1b with [Ni(cod)2] and DPEphos (1:1:1 ratio) was
monitored by 1H, 19F, and 31P NMR spectroscopy using
hexafluorobenzene as the internal standard [Eq. (3)].[8]
Figure 2. ORTEP diagram of cis-7a (top) and trans-7b (bottom).
À
spectroscopy, thus indicating that the C CN bond is much
À
more reactive than the C H bond.
When the reaction was carried out at 228C in the presence
When cis-7a was allowed to react with 2a (10 equiv) in
CPME/C6D6 (1:1) at 1008C for 1 h [Eq. (4)], the adduct 3ba
À
of BPh3, an oxidative addition under cleavage of the C CN
bond occurred to give cis-7a ([cis-Ni(2,3,5,6-F4-C6H)-
(CNBPh3)(DPEphos)], 31P NMR: d = 18.3 ppm (s)) in 64%
yield along with an h2-nickel(0) complex 6a ([Ni(h2-2,3,5,6-F4-
C6HCN)(DPEphos)], 31P NMR: d = 19.8 (d, J = 41 Hz),
1
28.4 ppm (d, J = 41 Hz)) in 25% yield, as determined by H,
19F, and 31P NMR spectroscopy and X-ray crystallographic
analysis (Figure 2).[9] Of note, no hydrido or fluoro complex
was produced, in sharp contrast to the oxidative addition of
polyfluoroarenes to a metal complex,[2,3] thus indicating that
fluorine atoms bound to the aryl group promote the oxidative
À
addition of the C CN bond through their high electronega-
was produced in 86% yield (determined by NMR spectro-
sopy), together with the h2-alkyne nickel(0) complex 8 (79%
by 31P NMR spectrospopy: d = 27.2 ppm (s)).[9,11] On the
other hand, trans-7b did not react with 2a. Thus, DPEphos
was clearly shown to be effective for the alkyne insertion.
We carried out competition experiments between 1b and
4 for the oxidative addition and the following insertion of 4-
octyne [Eq. (5)]. Oxidative addition at 228C for 1 h resulted
in the sole formation of cis-7a. This result showed that the
formation of cis-7a was thermodynamically favored over that
of [Ni(Ph)(CNBPh3)(DPEphos)] 7c, possibly because of the
tivity. In contrast to our previous observations,[8a] no BPh3-
bound h2-nitrile nickel(0) complex was observed. In the
absence of BPh3, 6a was quantitatively formed, but 7a’ was
not formed at all, showing that BPh3 dramatically facilitated
the oxidative addition of the C CN bond from 6a. More-
over, when PCyp3 was used as a ligand, oxidative addition
[10]
À
À
immediately proceeded to give the C CN bond adduct trans-
7b (31P NMR: d = 24.2 ppm (s)) quantitatively; no hydrido
nickel complex was formed according to 1H, 19F, and 31P NMR
884
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polyfluoropyridyl group lowered the Lewis acidity of BPh3
throught coordination (Table 1, entry 9).
Using 1c as the substrate, we examined the scope of the
alkynes (Table 2). Symmetrical alkynes, such as 3-hexyne 2b,
Table 2: Reaction of 1c with various alkynes.[a]
Entry R1, R2 (2)
t
[h]
Product
Yield 3:3’[c]
[%][b]
electron deficiency of the tetrafluoroaryl group. When 2a
(1 equiv) in CPME was added to the solution, and the
resultant mixture was heated to 1008C for 14 h, only 3ba was
produced in 81% yield.[12] These results demonstrated that
the thermodynamics of the oxidative addition of 1 and 4 is the
key to the difference in the reactivity of nitriles.
With the information obtained by the mechanistic anal-
ysis, we examined the scope of the polyfluorobenzonitriles
under the optimum conditions (Table 1). The reaction of
2,3,4,5-tetrafluorobenzonitrile 1c with 2a proceeded
smoothly to give 3ca in 98% yield (Table 1, entry 1). Mono-
, di-, and trifluorobenzonitriles 1d–1j gave the corresponding
adducts 3da–3ja in over 90% yield (Table 1, entries 2–8).
Similarly, 2,3,5,6-tetrafluoro-4-pyridinecarbonitrile 1k
reacted smoothly with 2a to give 3ka in 93% yield, although
a stoichiometric amount of BPh3 was required for this
conversion. A catalytic amount of BPh3 failed to promote
the reaction, possibly because the nitrogen atom in the
1
Et, Et (2b)
8
87
72
71
3cb
CH2TMS,
CH2TMS (2c)
2
26
23
8
3cc
3[d]
4[e]
5[f]
6
Ph (2d)
3cd
3ce (E/Z=4:96), 3ce’
3cf, 3cf’
Me, Ph (2e)
Me, tBu (2 f)
H, TIPS (2g)
87
83
55
54:46
86:14
>99:1
52
24
Table 1: Reaction of polyfluorobenzonitriles with 4-octyne 2a.[a]
3cg
[a] Unless otherwise noted, a mixture of 1c (1.0 mmol), 2 (1.5 mmol),
[Ni(cod)2] (0.02 mmol for entries 1, 2, and 5; 0.03 mmol for entries 3 and
4; and 0.05 mmol for entry 6), DPEphos (0.02 mmol for entries 1, 2, and
5; 0.03 mmol for entries 3 and 4; and 0.05 mmol for entry 6), BPh3
(0.08 mmol for entries 1, 2, and 5; 0.12 mmol for entries 3 and 4; and
0.20 mmol for entry 6), and CPME (1.0 mL) was heated to 1008C for 8–
52 h. [b] Yields of isolated products. [c] Determined by 1H NMR
spectroscopy. [d] 2d (3.0 mmol). [e] 808C. [f] 1208C. TIPS=triisopro-
pylsilyl.
Ent.
1
1
Yield [%][b] Ent.
1
Yield [%][b]
1c
3ca, 98
3da, 98
3ea, 98
3 fa, 97
5
1g
3ga,90
2[c] 1d
bis(trimethylsilylmethyl)acetylene 2c, and diphenylacetylene
2d, gave the corresponding adducts 3cb, 3cc, and 3 cd in good
yields (Table 2, entries 1–3). 1-Phenyl-1-propyne 2e gave the
corresponding adducts 3ce and 3ce’ in 87% combined yield
with poor selectivity (54:46); this result stands in sharp
contrast to that observed for the arylcyanation of 1-(4-
methoxyphenyl)-1-butyne with 4-chlorobenzonitrile (Table 2,
entry 4).[5d,13] The structures of 3 cd and 3ce’ were unambig-
uously established by X-ray crystallographic analysis.[9] 4,4-
Dimethyl-2-pentyne 2 f gave 3cf and 3cf’ in 86:14 ratio,
demonstrating that the CN group prefers a more-hindered
alkyne carbon center, similar to the arylcyanation reaction
(Table 2, entry 5).[5d] Of note, the addition of 1c to triisopro-
6
7
8
1h o-F
1i m-F
1j p-F
3ha, 97
3ia, 98
3ja, 98
3
4
1e
1 f
9[d] 1k
3ka, 93
[a] Unless otherwise noted, a mixture of 1 (1.0 mmol), 2a (1.5 mmol),
[Ni(cod)2] (0.02 mmol), DPEphos (0.02 mmol), BPh3 (0.08 mmol), and
CPME (1.0 mL) was heated to 1008C for 8 h. [b] Yield of isolated product.
[c] [Ni(cod)2] (0.03 mmol), DPEphos (0.03 mmol), and BPh3
(0.12 mmol) were used. [d] [Ni(cod)2] (0.03 mmol), DPEphos
(0.03 mmol), and BPh3 (1.00 mmol) were used.
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pylsilylacetylene 2g gave 3cg with excellent regioselectivity,
albeit in modest yield (Table 2, entry 6). Presumably, the use
of DPEphos, which is a weak donor ligand, and/or an
electron-withdrawing polyfluoroaryl group allows the poly-
fluoroarylcyanation of terminal alkynes. Facile trimerization
and oligomerization of terminal alkynes are prone to take
place in the presence of nickel catalysts with electron-
donating phosphine ligands, as described in the previous
carbocyanation results.[5d]
The nickel/BPh3 catalyst could also be used for the
polyfluoroarylcyanation of norbornene 9a and norborna-
diene 9b.[5d,14] Both alkenes reacted with 1c to afford
(2R*,3S*)-3-aryl-2-cyanobicyclo[2.2.1]heptane (10a) and
(2R*,3S*)-3-aryl-2-cyano-bicyclo[2.2.1]hept-5-ene
(10b),
Scheme 1. A plausible mechanism for the reaction of polyfluorobenzo-
respectively, in high yields [Eq. (6)].
nitrile with alkyne.
tive elimination of 3 from 15 generates a nickel(0) complex to
complete the catalytic cycle. The low regioselectivity in the
reaction with 1-phenyl-1-propyne 2e may be ascribed to
a plausible p-stacking interaction between the electron-
deficient ArF and the phenyl ring (R1) in 14, followed by
arylnickelation toward the alkyne.[16] This interaction appa-
rently competes with the regioselectivity caused by the steric
repulsion.
Site-selective polyfluoroarylcyanation of tetrafluorotere-
phthalonitrile 1l was easily achieved by using 2a (1.5 equiv):
only the monoalkenylation adduct 3la was obtained in 65%
yield (73% conversion of 1l), without cleavage of the other
À
Encouraged by the selective C CN activation of poly-
À
fluorobenzonitriles, we examined the orthogonal C H acti-
vation of the adducts for the reaction of 3ba with 2c
À
C CN bond. These results indicate that it is possible to use
(Scheme 2).[3a,c] A catalyst system consisting of nickel(0),
two different alkynes to react with 1l. Thus, 11a and 11b were
isolated in 72% and 65% yields, respectively, from the
reaction of 3la with 2b and 2c [Eq. (7)].
Scheme 2. Aryl cyanation/hydroarylation of alkynes.
PCyp3, and BPh3 was effective for the production of the
desired hydropolyfluoroarylation product 16 in 90% yield.
The effect of BPh3 remains to be carefully examined, but it is
assumed that ligation of the cyano group to the boron atom
A plausible mechanism for the reaction of polyfluoro-
benzonitrile with an alkyne is shown in Scheme 1. The
À
enhances the reactivity of the C H bond by lowering the
catalytic reaction should be initiated by the formation of the
electron density of the polyfluoroaryl moiety. In the absence
2
À
h -complex 12 or 12’. Subsequent oxidative addition of the C
of BPh3, the yield of the double adduct 16 significantly
À
CN bond to nickel(0) affords 13 after the cyano group
nitrogen atom is bound to BPh3. Insertion of alkyne 2 into the
decreased. The feasibility of one-pot sequential C CN and
À
C H activation was also confirmed: 16 was obtained in 72%
À
ArF Ni bond in 13 gives 15 via the alkyne-coordinated
yield without isolation of the intermediate 3ba.
complex 14. Steric repulsion between the bulkier group R2
and the polyfluorophenyl group on the Ni center in 14 is
In conclusion, the present study has substantiated the role
of a nickel/BPh3 catalyst system in the addition of polyfluoro-
assumed to be minimal.[15] Finally, C C bond-forming reduc-
benzonitriles to alkynes through selective C CN activation
À
À
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À
À
main product. Hydrido–nickel and fluoro–nickel complexes are
not observed. All products are inactive for the insertion of
alkynes.
without C H and C F bond cleavages. This method allows
access to a variety of fluorinated organic compounds as
functional materials for liquid crystals and organic light-
emitting diodes.[17,18] The fluorine atoms bound to the aryl
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[9] CCDC 898224 (cis-7a), 898225 (trans-7b), 903425 (8), 866661
(3 cd), and 866662 (3ce’) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
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www.ccdc.cam.ac.uk/data_request/cif.
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À
groups significantly enhance the reactivity of the C CN bond
toward the oxidative addition to the nickel(0) complex. Site-
selective alkenylation between the C CN and C H bond also
proceeds successfully. Efforts to extend the present poly-
fluoroarylcyanation reaction to various other substrates are
underway.
À
À
Received: September 29, 2012
Published online: November 20, 2012
Keywords: alkynes · fluorides · insertion · nickel · nitriles
.
[11] To confirm the generation of 8 by NMR spectroscopy, 8 was
synthesized by reaction of [Ni(cod)2], DPEphos, and 4-octyne
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=
yphenyl)-1-butyne gives Z-(4-chlorophenyl)(Me)C C(CN)(4-
=
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[6] The reaction of 4 with 2a affords 5 in 97% yield under the
present reaction conditions. Compare with reference [5d].
[7] NMR analysis of the mixture of 1b, [Ni(cod)2], PMe2Ph, and
AlMe2Cl in [D6]benzene shows a mixture that apparently
includes [Ni(2,3,5,6-F4-C6H)(CNAlMe2Cl)(PMe2Ph)2] as the
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