Nickel-catalyzed synthesis of N-aryl-1,2-dihydropyridines by [2+2+2] cycloaddition of imines with alkynes through T-shaped 14-electron aza-nickelacycle key intermediates

Article abstract of DOI:10.1002/chem.201304830

Despite there being a straightforward approach for the synthesis of 1,2-dihydropyridines, the transition-metal-catalyzed [2+2+2] cycloaddition reaction of imines with alkynes has been achieved only with imines containing an N-sulfonyl or -pyridyl group. Considering the importance of 1,2-dihydropyridines as useful intermediates in the preparation of a wide range of valuable organic molecules, it would be very worthwhile to provide novel strategies to expand the scope of imines. Herein we report a successful expansion of the scope of imines in nickel-catalyzed [2+2+2] cycloaddition reactions with alkynes. In the presence of a nickel(0)/PCy₃ catalyst, a reaction with N-benzylidene-P,P-diphenylphosphinic amide was developed. Moreover, an application of N-aryl imines to the reaction was also achieved by adopting N-heterocyclic carbene ligands. The isolation of an (η²-N-aryl imine)nickel(0) complex containing a 14-electron nickel(0) center and a T-shaped 14-electron five-membered aza-nickelacycle is shown. These would be considered as key intermediates of the reaction. The structure of these complexes was unambiguously determined by NMR spectroscopy and X-ray analyses. Oxidative cyclization: The title reaction has been achieved for the first time by employing nickel(0)/N-heterocyclic carbene (NHC) catalyst. In addition, isolation of a T-shaped 14-electron aza-nickelacycle, which would be a key reaction intermediate, is also reported (see scheme).

Full text of DOI:10.1002/chem.201304830

DOI: 10.1002/chem.201304830
Full Paper
&
Organic Synthesis
Nickel-Catalyzed Synthesis of N-Aryl-1,2-dihydropyridines by
[2+2+2] Cycloaddition of Imines with Alkynes through T-Shaped
14-Electron Aza-Nickelacycle Key Intermediates
Yoichi Hoshimoto,[a, b] Tomoya Ohata,[a] Masato Ohashi,[a] and Sensuke Ogoshi*[a, c]
Abstract: Despite there being a straightforward approach
for the synthesis of 1,2-dihydropyridines, the transition-
metal-catalyzed [2+2+2] cycloaddition reaction of imines
with alkynes has been achieved only with imines containing
an N-sulfonyl or -pyridyl group. Considering the importance
of 1,2-dihydropyridines as useful intermediates in the prepa-
ration of a wide range of valuable organic molecules, it
would be very worthwhile to provide novel strategies to
expand the scope of imines. Herein we report a successful
expansion of the scope of imines in nickel-catalyzed
[2+2+2] cycloaddition reactions with alkynes. In the pres-
ence of a nickel(0)/PCy3 catalyst, a reaction with N-benzyli-
dene-P,P-diphenylphosphinic amide was developed. More-
over, an application of N-aryl imines to the reaction was also
achieved by adopting N-heterocyclic carbene ligands. The
isolation of an (h2-N-aryl imine)nickel(0) complex containing
a 14-electron nickel(0) center and a T-shaped 14-electron
five-membered aza-nickelacycle is shown. These would be
considered as key intermediates of the reaction. The struc-
ture of these complexes was unambiguously determined by
NMR spectroscopy and X-ray analyses.
Introduction
The formation of metalacycles by the reaction of unsaturated
compounds with low valent transition metals is a key reaction
in transition-metal-catalyzed cycloaddition reactions.[1] The de-
velopment of efficient methods to generate a variety of met-
alacycles would provide us more opportunities to access cyclic
organic compounds. Nickel is a highly promising candidate as
a catalyst for cycloaddition reactions since a number of oxida-
tive cyclization reactions of two unsaturated compounds with
nickel(0) giving nickelacycles have been reported.[2] During the
course of studying hetero-nickelacycles,[2b,3] we reported that
the reaction of N-benzylidene benzenesulfonamide (N-BBSA)
and alkynes with [Ni(cod)2] (cod=1,5-cyclooctadiene) and PCy3
gave aza-nickelacycle compounds, which we proposed as
a key intermediates in the [2+2+2] cycloaddition of an imine
(1) with two alkynes (2) to yield a 1,2-dihydropyridine (3)
(Scheme 1a).[3b,c] Similar [2+2+2] cycloaddition reactions giving
dihydropyridines have been reported by Yoshikai[4a] and
Scheme 1. Formation of 1,2-dihydropyridines by [2+2+2] cycloaddition of
imines with alkynes in the presence of nickel(0).
Gandon and Aubert;[4b] thus far, however, the substituent
groups on imine nitrogen atom have been limited to sulfonyl
and pyridyl groups. This result indicates that the chelate coor-
dination of the heteroatom on the N-substituent group to the
metal center is key to success in the catalytic reaction. In the
nickel catalysis, electron-withdrawing substituents would be re-
quired for oxidative cyclization by promoting back donation
from nickel(0) to imines. The coordination of a heteroatom to
the nickel(II) center would stabilize the aza-nickelacycle inter-
mediate through the occupation of a vacant coordination
site.[2b,3,4a] Based on these points of view, we propose two strat-
egies. Considering the importance of 1,2-dihydropyridines as
useful intermediates in the preparation of a wide range of val-
uable organic molecules, it would be very worthwhile to pro-
vide novel strategies to expand the scope of imines. One is the
development of a substituent group on nitrogen, such as sul-
fonyl and pyridyl groups, which can decrease the electron den-
[a] Dr. Y. Hoshimoto, T. Ohata, Dr. M. Ohashi, Prof. Dr. S. Ogoshi
Department of Applied Chemistry, Faculty of Engineering
Osaka University, Suita, Osaka 565-0871 (Japan)
E-mail: ogoshi@chem.eng.osaka-u.ac.jp
[b] Dr. Y. Hoshimoto
Frontier Research Base for Young Researchers
Graduate School of Engineering, Osaka University, Suita
Osaka 565-0871 (Japan)
[c] Prof. Dr. S. Ogoshi
JST, ACT-C, Suita, Osaka 565-0871 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304830.
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sity on the imine and act as an intramolecular coordination
group. The other is employing N-heterocyclic carbenes (NHCs)
instead of tertiary phosphines since a stronger electron-donat-
ing and more steric-demanding ligand might allow us to con-
struct a catalytic reaction with N-aryl imines by stabilizing a T-
shaped 14-electron aza-nickelacycle intermediate by covering
a vacant site. In fact, we reported the preparation of T-shaped
14-electron hetero-nickelacycles with NHCs.[3d,f] Here we report
the formation of a T-shaped 14-electron five-membered aza-
nickelacycle by the oxidative cyclization of an N-aryl imine and
an alkyne with nickel(0)/NHC. Moreover, a successful expansion
of the imine scope to include N-aryl imines in transition-metal-
catalyzed [2+2+2] cycloaddition with alkynes is also shown for
the first time, which would proceed through the five-mem-
bered aza-nickelacycle intermediate (Scheme 1b).
Results and Discussion
First, N-benzylidene-P,P-diphenylphosphinic amide (1a) was
employed in the reaction with 2-butyne (2a) (1 equiv),
[Ni(cod)2] (1 equiv), and PCy3 (1 equiv) (Scheme 2). The reaction
was completed in 24 h to afford aza-nickelacycle 4 in 87%
yield with the concomitant formation of complex 5 in 13%
Figure 1. Molecular structures of a) 4’ and b) 5 with thermal ellipsoids at the
30% probability level. H atoms are omitted for clarity. Selected bond lengths
[ꢁ] and angles [8]: a) NiÀN 1.893(4), NiÀC3 1.894(5), NiÀP2 2.198(1), NiÀO
2.094(4); N-Ni-O 75.2(1), O-Ni-P2 100.6, P2-Ni-C3 104.3(1), C3-Ni-N 84.0(1);
b) NiÀN 1.903(5), NiÀC1 1.904(5), C1ÀN 1.40(1).
In the presence of [Ni(cod)2] (10 mol%) and PCy3 (20 mol%),
the treatment of 1a with 2a (2 equiv) at 1008C resulted in the
formation of 1,2-dihydropyridine 3aa in 64% yield (Scheme 3).
Scheme 2. The stoichiometric reactions with 1a. Yields of products deter-
mined by H NMR spectroscopy are given in parenthesis. [a] the ratio of syn/
anti is 42:58. [b] The ratio of syn/anti is 18:82.
1
yield. An analogous aza-nickelacycle complex 4’ was prepared
from 1a and diphenylacetylene, with a molecular structure of
4’ that was unambiguously determined by X-ray crystallogra-
phy (Figure 1a).[5] Complex 4’ has a planar tetracoordinate nick-
el(II) center with an intramolecular coordination of oxygen to
nickel. In addition, the formation of g-lactam derivative 6 by
the carbonylation of 4 would support the five-membered nick-
elacycle skeleton of complex 4. Complex 5 existed as a mixture
of syn/anti isomers in solution. In the solid state, only an anti
isomer was observed, as shown in Figure 1b (please see the
Supporting Information for details of syn/anti isomers). No re-
action took place when an excess amount of 2a was added at
room temperature to 5, which was prepared by the reaction of
1a with one equivalent of [Ni(cod)2] and PCy3 in C6D6. These
results suggest that 5 would be highly stabilized through the
intramolecular coordination of oxygen to nickel, and thus the
simultaneous coordination of 1a and 2a might be inhibited.
Scheme 3. Nickel(0)-catalyzed [2+2+2] cycloaddition of 1a with 2a. Yield of
3aa was determined by H NMR spectroscopy.
1
Next, we turned our attention to utilizing NHCs as a ligand
to test our hypothesis that NHCs can help the formation of
aza-nickelacycle compounds by the oxidative cyclization of al-
kynes and imines without a chelation group.
We examined the reaction of N-benzylidene-4-trifluorometh-
yl aniline (1b) or N-benzylidene-2-aminopyridine (1c) with
a stoichiometric amount of [Ni(cod)2] and IPr (IPr=1,3-bis(2,6-
diisopropylphenyl)imidazole-2-ylidene) (Scheme 4). Both reac-
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Scheme 4. The stoichiometric reactions using N-aryl imines and alkynes with
Ni0 and IPr. Yields were determined by 1H NMR spectroscopy. [a] The reaction
was carried out in toluene. Isolated yields after recrystallization are shown.
tions were completed within 10 min to give [Ni(IPr)(h2-imine)]
complexes in 95 (7) and 92% (8) yields, respectively. The mo-
lecular structure of 7 was confirmed by X-ray crystallography
(Figure 2a). The C1ÀN bond length is 1.37(1) ꢁ, which is clearly
elongated compared with a typical C=N bond length (ca. 1.27–
1.30 ꢁ)[6] due to the back donation from the nickel(0) center.
Moreover, 7 was found to have a 14-electron nickel(0) center
while the previously reported [Ni(PCy3)2(h2-PhCH=NSO2Ph)] had
a 16-electron center.[3b] This structural difference might be
caused by the steric bulkiness of IPr, that is, a more bulky IPr
would stabilize the highly reactive 14-electron nickel(0) com-
plex by covering its vacant coordination sites. The reaction of
N-BBSA with [Ni(cod)2] and IPr did not afford the correspond-
ing [Ni(IPr)(h2-PhCH=NSO2Ph)] complex at all, and unidentified
white precipitates were observed.[7]
Treatment of 7 with 2a or 4-octyne (2b) in C6D6 at room
temperature gave five-membered aza-nickelacycles 9a (56%
isolated yield) or 9b (96% NMR spectroscopic yield). The mo-
lecular structure of 9a was determined by X-ray crystallogra-
phy, showing its T-shaped 14-electron nickel(II) center (Fig-
ure 2b). The sum of the bond angles around Ni along the C3,
N, and C4 is 359.08. Thus, Ni and these three atoms are on the
same plane. A space-filling model of 9a clearly indicates that
such a geometry is due mostly to the bulkiness caused by the
aryl group on the imine nitrogen atom together with the
bulky IPr ligand.[5] On the other hand, the structure of aza-nick-
elacycles 10a and 10b, which were prepared by the reaction
of 8 with either 2a or 2b, would have a planar tetracoordinate
nickel(II) center with an intramolecular coordination of the N-
pyridine ring.[4a] The crystal structure of 10a is shown in Fig-
ure 2c.
Figure 2. Molecular structures for a) 7, b) 9a, and c) 10a with thermal ellip-
soids at the 30% probability level. Hydrogen atoms are omitted for clarity.
Selected bond lengths [ꢁ] and angles [8]: a) NiÀN 1.85(1), NiÀC1 1.94(1), NiÀ
C2 1.84(1), C1ÀN 1.37(1), N-Ni-C2 172.8(6); b) NiÀN 1.859(6), NiÀC3 1.861(7),
NiÀC4 1.877(8), N-Ni-C3 85.7(3), C3-Ni-C4 101.3(3), N-Ni-C4 172.0(3); c) NiÀN1
1.872(9), NiÀN2 2.116(8), NiÀC3 1.906(9), NiÀC4 1.876(7), N1-Ni-N2 63.6(4),
N1-Ni-C3 84.9(5), C3-Ni-C4 105.5(4), N2-Ni-C4 106.5(3).
to yield 1,2-dihydropyridines 3bb in 94% yield. The formation
We reported that the reaction of the planar tetracoordinate
five-membered aza-nickelacycle prepared from N-BBSA, diphe-
nylacetylene, [Ni(cod)2], and PCy3, with another equivalent of
the alkyne, afforded a seven-membered aza-nickelacycle;[3b,c]
however, 9b reacted with the second 2b at room temperature
of the corresponding seven-membered aza-nickelacycle was
1
not observed at all by H NMR spectroscopy. This result might
indicate that the rate of reductive elimination from the seven-
membered aza-nickelacycle to give 1,2-dihydropyridine is
faster than that of the insertion of the second alkyne into the
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Scheme 5. Nickel(0)/NHC-catalyzed [2+2+2] cycloaddition reaction of N-aryl
imines with alkynes. General conditions: imines (1.00 mmol), alkynes
(2.00 mmol) and [Ni(cod)2/IMes] (2 mol%) were reacted in THF (1.0 mL) at
408C for 24 h. Yields of isolated products are given. [a] 5mol% of [Ni(cod)2]
and IPr was used. [b] 2 mol% of [Ni(cod)2] and IPr was used (48 h). [c] Total
yield of the four products after isolation. [d] 10 mol% of [Ni(cod)2] and IPr
was used in 1,4-dioxane at 1008C (72 h). Total yield of 3bd and 3bd’ is
given.
Scheme 6. The stoichiometric reaction of 1b and 2d with [Ni(cod)2] and IPr.
Isolated yield of 11 is given. Molecular structure of 11 (thermal ellipsoids at
the 30% probability level) is also shown. Hydrogen atoms are omitted for
clarity. Selected bond lengths [ꢁ]: NiÀN 1.965(6), NiÀC3 1.932(7), NiÀC4
2.045(8), NiÀC5 2.024(8), NiÀC6 1.936(6).
hand, the reaction of 1b with 2-methyl-1-hexen-3-yne (2d) at
1008C for 72 h gave a mixture of two products (3bd and
3bd’) in a total yield of 58%. The ratio of 3bd/3bd’ was 83:17.
This result can be rationalized by the following results shown
in Scheme 6. The stoichiometric treatment of 1b with 2d,
[Ni(cod)2], and IPr at room temperature resulted in the forma-
tion of aza-nickelacycle 11 in 97% NMR spectroscopic yield,
which was isolated in 77% yield. The crystal structure of 11 is
also shown in Scheme 6. Because an h3-butadienyl coordina-
tion can highly stabilize nickelacycles,[3 h,8] the oxidative cycliza-
tion of 1b with 2d, which was found to be reversible, would
take place regioselectively.[9] The transition state of the inser-
tion of the second 2d into 11, which can proceed at 1008C,
might also be stabilized by the assistance of h3-butadienyl co-
ordination. Thus, 3bd was formed as a major product while
the regioselectivity of the insertion of the second 2d was not
perfectly controlled at 1008C. Employing terminal alkynes such
as phenyl- and trimethylsilylacetylene gave a complicated mix-
ture, and the formation of the corresponding dihydropyridines
was not observed at all by GC analysis.
five-membered complex. In sharp contrast, complex 10b did
not react with 2b at room temperature. This might be due to
the suppression of the coordination of 2b by the intramolecu-
lar coordination of the N-pyridine.
Catalytic [2+2+2] cycloaddition reactions of N-aryl imines
with alkynes were carried out (Scheme 5). The reaction of 1b
with 2b proceeded efficiently with 5 mol% of [Ni(cod)2] and
IPr to give 3bb in 91% yield. Moreover, the catalyst loading
can be decreased to 2 mol% without loss of efficiency by em-
ploying 1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene (IMes)
as a ligand (3bb; 92% NMR yield, 83% isolated yield). A sub-
strate with an N-3-CF3C6H4 group (1d) gave the corresponding
1,2-dihydropyridines 3da (from 2a) or 3db (from 2b) in 76
and 86% yields, respectively, by using 2 mol% of [Ni(cod)2]
and IMes; however, an imine with an N-2-CF3C6H4 (1e) did not
afford the product at all under the same reaction conditions. It
is noteworthy that the present reaction conditions were suc-
cessfully applied to a simple N-phenyl imine 1 f and gave 3 fb
in 43% yield in the presence of [Ni(cod)2] and IPr (5 mol%). In
addition, 1g containing a 4-fluorophenyl group also reacted
with 2b to afford 3gb in a moderate yield. While the reaction
time had to be extended to 48 h, imine 1h was successfully
converted into 3hb in 88% yield in the presence of 2 mol%
[Ni(cod)2] and IPr. Employing an unsymmetrical alkyne 2c af-
forded a mixture of four 1,2-dihydropyridines with a ratio of
30:29:21:20 and a total product yield of 79%. On the other
Nickel(0)-catalyzed [2+2+2] cycloaddition of imines with al-
kynes would proceed through the steps proposed in
Scheme 1: 1) oxidative cyclization of an imine and an alkyne
giving
a five-membered aza-nickelacycle, 2) formation of
a
seven-membered aza-nickelacycle by the insertion of
a second alkyne, and 3) reductive elimination. In our previous
report using N-BBSA, the formation of the seven-membered
aza-nickelacycle was observed at room temperature, and re-
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ductive elimination to give 1,2-dihydropyridine took place at
1008C, which indicates that reductive elimination significantly
influenced the reaction rate. In addition, Yoshikai also pro-
posed that, based on DFT calculations, reductive elimination
would be the rate-limiting step of the reaction with N-pyridyl
imines.[4a] On the other hand, the reaction rate of the [2+2+2]
cycloaddition of N-aryl imines with alkynes in the presence of
a nickel(0)/NHC catalyst might be determined by the insertion
of the second alkyne into the five-membered aza-nickelacycle,
as mentioned above.
General procedure for catalytic reactions (Scheme 5)
The reaction was conducted in a pyrex test tube equipped with
a magnetic stirrer bar. An imine (1.0 mmol) was added to a solution
of [Ni(cod)2] (5.5 mg, 0.02 mmol, 2 mol%) and IMes (6.1 mg,
0.02 mmol, 2 mol%) in THF (1 mL). The solution was stirred for
5 min, and then an alkyne (2.0 mmol, 2 equiv) was added. The re-
action mixture was heated at 408C and stirred for the indicated
time. After cooling to room temperature, the resulting mixture was
filtered through a silica gel short column (eluted with AcOEt). Then
all volatiles were removed under reduced pressure, and the residue
was purified by silica gel column chromatography. Further purifica-
tion was carried out by Kugelrohr distillation or recycle HPLC.
Conclusion
Acknowledgements
In summary, an expansion of the scope of N-substituents of
imines in the nickel(0)-catalyzed [2+2+2] cycloaddition with al-
kynes giving 1,2-dihydropyridines was successfully achieved. In
the presence of a catalytic amount of nickel(0) and PCy3, the
[2+2+2] cycloaddition reaction of N-benzylidene-P,P-diphenyl-
phosphinic amide with alkynes was developed, and the isola-
tion of a planar tetracoordinate five-membered aza-nickelacy-
cle complex was achieved. In addition, an application of N-ary-
limines to the reaction was demonstrated by adopting NHCs
as a ligand. The isolation of an (h2-N-aryl imine)nickel complex
containing a 14-electron nickel(0) center and a T-shaped 14-
electron five-membered aza-nickelacycle complex was present-
ed. Based on the results of stoichiometric reactions, the inser-
tion of a second alkyne into the five-membered aza-nickelacy-
cle might control the rate of this catalytic reaction.
This work was supported by Grants-in-Aid for Scientific Re-
search (A) (21245028), Scientific Research on Innovative Area
“Molecular Activation Directed toward Straightforward Synthe-
sis” (23105546), and Grants-in-Aid for Young Scientists (A)
(25708018) from MEXTand by ACT-C from JST and by the Asahi
Glass Foundation. Y.H. acknowledges support from the Frontier
Research Base for Global Young Researchers, Osaka University,
on the Program of MEXT.
Keywords: 1,2-dihydropyridine
· cycloaddition · nickel ·
nickelacycle · oxidative cyclization
[1] For selected recent reviews on transition-metal-catalyzed [2+2+2] cyclo-
addition reactions, see: a) S. Saito, Y. Yamamoto, Chem. Rev. 2000, 100,
2901; b) J. A. Varela, C. Saꢂ, Chem. Rev. 2003, 103, 3787; c) S. Kotha, E.
Brahmachary, K. Lahiri, Eur. J. Org. Chem. 2005, 4741; d) P. R. Chopade, J.
Louie, Adv. Synth. Catal. 2006, 348, 2307; e) B. Heller, M. Hapke, Chem.
Soc. Rev. 2007, 36, 1085; f) T. Shibata, K. Tsuchikama, Org. Biomol. Chem.
2008, 6, 1317.
[2] For reviews on nickel-catalyzed reactions via nickelacycle intermediate,
see: a) J. Montgomery, Angew. Chem. 2004, 116, 3980; Angew. Chem. Int.
Ed. 2004, 43, 3890, and references therein; b) S. Ogoshi, Yuki Gosei
Kagaku Kyokaishi 2013, 71, 14, and references therein.
[3] For examples of our related works, see, a) S. Ogoshi, M. Oka, H. Kurosa-
wa, J. Am. Chem. Soc. 2004, 126, 11802; b) S. Ogoshi, H. Ikeda, H. Kurosa-
wa, Angew. Chem. 2007, 119, 5018; Angew. Chem. Int. Ed. 2007, 46, 4930;
c) S. Ogoshi, H. Ikeda, H. Kurosawa, Pure Appl. Chem. 2008, 80, 1115; d) T.
Tamaki, M. Nagata, M. Ohashi, S. Ogoshi, Chem. Eur. J. 2009, 15, 10083;
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131, 9160; f) T. Tamaki, M. Ohashi, S. Ogoshi, Chem. Lett. 2011, 40, 248;
g) Y. Hoshimoto, M. Ohashi, S. Ogoshi, J. Am. Chem. Soc. 2011, 133, 4668;
h) A. Nishimura, M. Ohashi, S. Ogoshi, J. Am. Chem. Soc. 2012, 134,
15692.
[4] Transition-metal-catalyzed approaches for the synthesis of 1,2-dihydro-
pyridines, and the utilities of the 1,2-dihydropyridines are found: a) L.
Adak, W. C. Chan, N. Yoshikai, Chem. Asian J. 2011, 6, 359; b) M. Amatore,
D. Leboeuf, M. Malacria, V. Gandon, C. Aubert, J. Am. Chem. Soc. 2013,
135, 4576; the formation of 1,2-dihydropyridine as a byproduct was re-
ported, see: c) P. A. Wender, T. M. Pederson, M. J. C. Scanio, J. Am. Chem.
Soc. 2002, 124, 15154; a cycloaddition of 3H-indoles with dimethylacety-
lenedicarboxylate was reported, see: d) D. J. Le Count, A. P. Marson, J.
Chem. Soc. Perkin Trans. 1 1988, 451.
Experimental Section
Isolation of 9a
Compound 2a (15.7 mL, 0.20 mmol) was added to a solution of
complex 7 (139.3 mg, 0.20 mmol) in toluene (5 mL) at room tem-
perature. The solution changed from dark green to dark brown.
After stirring for 10 min, the reaction mixture was concentrated in
vacuo. The resulting brown solid was reprecipitated from toluene/
pentane to give 9a as a brown solid in 56% yield (84.07 mg,
0.11 mmol). An analytical sample and a single crystal for X-ray dif-
fraction analysis were prepared by recrystallization from toluene/
hexane at À308C. 1H NMR (400 MHz, C6D6): d=0.65 (s, 3H; CH3),
0.91–0.94 (m, 12H; IPr), 1.16 (d, J=6.6 Hz, 6H; IPr), 1.29 (s, 3H;
CH3), 1.31 (d, J=6.6 Hz, 6H; IPr), 2.59 (sept, J=6.6 Hz, 2H; IPr), 3.13
(sept, J=6.6 Hz, 2H; IPr), 4.38 (s, 1H; CHPh), 5.66 (d, J=8.8 Hz, 2H;
2-(4-C6H4CF3)), 6.53 (s, 2H; IPr), 7.00–7.24 (m, 9H), 7.36 (t, J=7.8 Hz,
2H; Ar-H), 7.47 ppm (d, J=6.4 Hz, 2H; Ar-H); 13C{1H} NMR
(100 MHz, C6D6): d 12.1 (NiC(CH3)), 22.6, 23.3, 23.7 (NiC(CH3)=
C(CH3)), 25.6, 26.2, 29.4, 29.8, 75.0 (CHPh), 109.4 (2-(4-C6H4CF3)),
110.4 (NiC(CH3)=C(CH3)), 111.7 (q, JCF =32.0 Hz; CCF3), 124.9, 125.0,
125.1, 125.7, 126.2, 126.3 (q, JCF =3.0 Hz; 3-(4-C6H4CF3)), 127.5 (q,
J
CF =266.3 Hz; CF3), 129.3, 130.7, 136.0, 145.6, 145.7, 145.7, 146.8
(NiC(CH3)), 159.5 (ipso-Ph), 183.9 ppm (NCN); elemental analysis
calcd (%)for C45H52F3N3Ni: C 72.01, H 6.98, N 5.60; found: C 71.89, H
[5] See the Supporting Information for details on the experimental proce-
dures and full identification of the products. CCDC-957850 (4’), -957849
(5), -960074 (7) -957734 (9a), 957735 (10a), and 957736 (11) contain the
supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
¯
6.93, N 5.57; X-ray data for 9a: M=750.62, brown, triclinic, P1 (#2),
a=11.244(1), b=11.9223(9), c=16.212(2) ꢁ, a=73.459(2), b=
73.333(8),
g=81.203(3)8,
V=1989.8(3) ꢁ3,
Z=2,
1calcd =
1.253 gcmÀ3, T=À1508C, R1 (wR2)=0.0859 (0.2735).
Chem. Eur. J. 2014, 20, 4105 – 4110
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Full Paper
[6] a) J. Kovach, M. Peralta, W. W. Brennessel, W. D. Jones, J. Mol. Struct.
2011, 992, 33; b) F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G.
Orpen, R. Taylor, J. Chem. Soc. Perkin Trans. 2 1987, S1.
[9] Heating 11 in C6D6 at 1008C in the presence of 2d (10 equiv) gave a mix-
ture of 1b, dihydropyridines (3bd+3bd’), trimer of 2d, and unidentified
products.
[7] The precipitates were not fully identified; however, the product might
be an imine–IPr adduct or its enamine-type isomer. See, a) D. A. DiRocco,
K. M. Oberg, T. Rovis, J. Am. Chem. Soc. 2012, 134, 6143; b) M. He, J. W.
Bode, Org. Lett. 2005, 7, 3131.
[8] A stabilization effect of h3-butadienyl coordination in the oxidative cycli-
zation on nickel(0) was calculated, see: P. Liu, P. McCarren, P. H.-Y.
Cheong, T. F. Jamison, K. N. Houk, J. Am. Chem. Soc. 2010, 132, 2050.
Received: December 10, 2013
Published online on February 27, 2014
Chem. Eur. J. 2014, 20, 4105 – 4110
www.chemeurj.org
4110
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim