Cyclotrimerization of alkynes catalyzed by a self-supported cyclic tri-nuclear nickel(0) complex with α-diimine ligands

Article abstract of DOI:10.1039/c9dt00819e

A cyclic tri-nuclear α-diimine nickel(0) complex [{Ni(μ-L^Me-2,4)}₃] (2) was synthesized from a “pre-organized”, trimerized trigonal LNiBr₂-type precursor [Ni₃(μ²-Br)₃(μ³-Br)₂(L^Me-2,4)₃]·Br (1; L^Me-2,4 = [(2,4-Me₂C₆H₃)NC(Me)]₂). In complex 2, the α-diimine ligands not only exhibit the normal N,N′-chelating mode, but they also act as bridges between the Ni atoms through an unusual π-coordination of a C═N bond to Ni. Complex 2 is able to catalyze the cyclotrimerization of alkynes to form substituted benzenes in good yield and regio-selectivity for the 1,3,5-isomers, which is found to vary with the nature of the alkyne employed. This complex represents a convenient self-supported nickel(0) catalyst with no need for additional ligands and reducing agent.

Full text of DOI:10.1039/c9dt00819e

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Cyclotrimerization of alkynes catalyzed by a self-
supported cyclic tri-nuclear nickel(0) complex
with α-diimine ligands†
Cite this: DOI: 10.1039/c9dt00819e
a
a
b
b
a
Lingyi Shen, Yanxia Zhao, Qiong Luo, Qian-Shu Li, Bin Liu,
a,c
a
a
Carl Redshaw,
Biao Wu
and Xiao-Juan Yang
*
Me-2,4
A cyclic tri-nuclear α-diimine nickel(0) complex [{Ni(µ-L
3
)} ] (2) was synthesized from a “pre-orga-
2
3
Me-2,4
Me-2,4
nized”, trimerized trigonal LNiBr
2
-type precursor [Ni
3
(µ -Br)
3
2
(µ -Br) (L
)
3
]·Br (1; L
= [(2,4-
Me )NC(Me)] ). In complex 2, the α-diimine ligands not only exhibit the normal N,N’-chelating
2
C
6
H
3
2
mode, but they also act as bridges between the Ni atoms through an unusual π-coordination of a CvN
bond to Ni. Complex 2 is able to catalyze the cyclotrimerization of alkynes to form substituted benzenes
in good yield and regio-selectivity for the 1,3,5-isomers, which is found to vary with the nature of the
alkyne employed. This complex represents a convenient self-supported nickel(0) catalyst with no need for
additional ligands and reducing agent.
Received 23rd February 2019,
Accepted 13th March 2019
DOI: 10.1039/c9dt00819e
rsc.li/dalton
2
g,h,k
while selective formation of the 1,3,5-products is rare.
Nonetheless, C -symmetric molecules such as benzene-1,3,5-
Introduction
3
The transition-metal-catalyzed [2 + 2 + 2] cycloaddition of tricarboxamide (BTA) and derivatives are useful building
alkynes is one of the most efficient synthetic methods for blocks for a variety of applications including in supramolecu-
benzene derivatives because of the high atom-efficiency and lar assembly, nanotechnology, liquid crystals, gels, polymer
8
–10
variety of substrates that can be used. Since Reppe reported and biomedical science.
the nickel-catalyzed [2 + 2 + 2] cycloaddition of alkynes in In the nickel catalysts for alkyne cyclotrimerization, e.g.
1
1
1
948, a plethora of metal (Ni, Rh, Pd, Co, Ti, Ru, Nb, In, U, Ir, Reppe’s nickel–carbonyl–phosphine complexes and Dieck’s
1
1
Fe, Cr, Ge, Ta) systems have been developed to catalyze the α-diimine-nickel(0) catalysts, the active Ni(0) center is nor-
1
1,12
intermolecular cycloaddition of three individual alkyne mole- mally stabilized by unsaturated molecules (e.g. COD,
2
1,13
14
cules. In general, the cyclotrimerization of asymmetric CO,
nitriles etc.) which are π-bonded to the [LNi] frag-
1
2
alkynes (R CuCR ) produces two regio-isomers, namely the ment. Alternatively, the Ni(0) center can also be in-situ gener-
,3,5- and 1,2,4-isomers, in which the same substituents (R or ated by using an extra reducing agent such as zinc, Mg, BuLi,
R ) are positioned differently on the resulting benzene ring. DIBAH (diisobutylaluminum hydride) and Grignard reagents
1
1
2
6
,15
II
16
Although a mixture of these two isomers is often observed to reduce nickel salts
or Ni precursors. Nevertheless,
higher regio- these additives may require more polar organic solvents such
selectivity has also been achieved. However, in the majority of as THF, CH CN, toluene, DMF etc.
2
e,f,3–5
with a slight excess of one over the other,
3
2
j,6,7
the examples, the less symmetric 1,2,4-isomers are favored,
We have been interested in the redox non-innocent
α-diimine ligands and have synthesized a series of dinuclear
metal–metal-bonded compounds with (reduced) α-diimines.
These complexes displayed excellent reactivity towards unsatu-
a
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the
Ministry of Education, College of Chemistry and Materials Science,
rated organic species, such as azobenzenes, alkynes, and
Northwest University, Xi’an 710069, China. E-mail: yangxiaojuan@nwu.edu.cn
17,18
alkenes.
ligand L
In the current work, we designed an α-diimine
b
Center for Computational Quantum Chemistry, South China Normal University,
Me-2,4
Me-2,4
(L
= [(2,4-Me C H )NC(Me)] ), which bears
Guangzhou 510631, China
2 6 3 2
c
Department of Chemistry and Biochemistry, University of Hull, Cottingham Road,
Hull, HU6 7RX, UK
methyl groups at the 2,4-positions of the N-aryl ring instead of
R-2,6
the commonly employed ‘symmetric’ ligands L
(with 2,6-
†
Electronic supplementary information (ESI) available: Experimental pro-
19
substituents). The LNiBr -type complex of this ligand shows
an unusual trimerized structure with bromide bridges,
2
cedures, analytical data, and X-ray crystallography (PDF), and X-ray crystallo-
graphic information (CIF). CCDC 1856112–1856114 for compounds 1, 2, 4a. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c9dt00819e
[
{LNiBr } ], and thus it was expected to give a Ni cluster upon
2
3
3
reductive elimination of the bromide ions. Indeed, a tri-
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Scheme 1
Me-2,4
nuclear complex [{Ni(µ-L
)} ] (2, Scheme 1) was obtained, while each two adjacent Ni atoms are bridged by a μ -Br atom,
3
2
which features nickel(0) centers and neutral α-diimine ligands and the three Ni atoms are further bridged by two μ
3
-Br atoms,
serving as both N,N-donor ligands and bridges to link the thus resulting in six-coordinate Ni centers adopting distorted
metals. Very promisingly, complex 2 can catalyze the cyclotri- octahedral geometries. The last bromide (Br ) acts as a
3
merization of both terminal and internal alkynes. The syn- counter-anion in the crystal lattice. This tri-nuclear structure is
thesis, structure, and catalytic properties of this nickel(0) different from most of the mono- or di-nuclear [LNiBr
2
]-type
trimer are reported herein.
complexes with α-diimine ligands but is similar to the
Me,Cl-2,4
Me,Cl-2,4
complex [Ni
)NC(Me)]
ization of ethylene.
,4-positions on the N-phenyl of the α-diimine may “pre-
organize” the triangular structure, which prompted us to
3
(µ-Br)
5
(L
)
3
]·Br (L
= [(2-Me-4-Cl-
C
6
H
3
2
) that was reported as a catalyst for the polymer-
1
9b
We speculated that substitution at the
Results and discussion
Synthesis and structure of [Ni (µ -Br) (µ -Br) (L
2
Me-2,4
)3]·Br (1)
3
2
3
3
2
Me-2,4
employ the ligand L
in this work.
The bromo-nickel(II) precursor 1 was synthesized from [(DME)
Me-2,4
NiBr
2
] and the ligand L
as a highly symmetric (space
Me-2,4
Synthesis and structure of [{Ni(µ-L
The bromo-complex 1 was then reduced by sodium metal
6 equiv.) in order to yield Ni clusters through salt elimi-
nation. Unexpectedly, the tri-nuclear complex 2 was isolated
as deep purple crystals). In the molecular structure of 2, the
)} ] (2)
3
group P6
3
m) trimer, in which the three Ni atoms are arranged
in a triangle (Fig. 1). Each Ni atom is chelated by a ligand,
(
3
(
three Ni atoms, each chelated by a ligand through the N,N′
donors, form a triangle as in precursor 1 (Fig. 2a). Upon
reduction by Na metal, all of the bromide ions in 1 have been
eliminated, giving the cyclic [Ni L ] complex self-bridged by
3 3
the coordination of Ni to a CvN bond of another ligand.
Neither the expected Ni3 cluster nor the Ni–Ni-bonded
complex [Ni
cing the dibromo species [L
2 2
L ], the latter of which was resulted when redu-
iPr-2,6
20
2
NiBr ], was obtained. These
results further suggest that 2,4-substitution might be essential
to “pre-organize” the tri-nuclear structure of 2.
The C–N bonds that coordinate to nickel are stretched to
1.423(4)–1.435(4) Å, which are significantly longer than those
in the neutral ligand. The other, free C–N (av. 1.313 Å) and the
C–C (av. 1.449 Å) bond lengths are close to those in the neutral
ligand (C–N: 1.29 Å, C–C: 1.47 Å). Thus, the ligands can best
be described as the neutral form in 2, while the Ni centers
show a formal oxidation state of zero. Each Ni atom is three-
coordinate (with two N atoms and a CvN bond) and resides in
Fig. 1 Molecular structure of 1 (thermal ellipsoids are set at the 30%
probability level; hydrogen atoms and the free Br anion are omitted for
clarity). Selected bond lengths (Å): N1–C1, 1.286(10); C1–C1A, 1.503(18);
Ni1–N1, 2.030(6); Ni1–Br1, 2.529(2); Ni1–Br2, 2.6061(15). Symmetry
code: A, 1 − y, x − y, z; B, 1 − x + y, 1 − x, z.
21
−
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Fig. 3 HR ESI-MS spectrum of complex 2. The m/z peaks at 1052.4467
Me-2,4
+
and
1193.3668
correspond
to
[Ni
3
(L
)
3
]
and
Me-2,4
+
[Ni
3
(L
) ·2MeCN·2H O·Na] . Inset: Experimental and simulated iso-
3 2
topic splitting pattern of the two peaks.
phenylpropiolate in hexane did not yield the anticipated
metal-alkyne adducts as in the cases of other complexes
2
4
bearing related diimine ligands. Instead, benzene derivatives
were isolated, indicating possible cyclotrimerization of the
alkyne molecules.
Catalytic studies
To confirm (and better understand) the catalytic effect of 2 on
Fig. 2 (a) Molecular structure of 2; (b) simplified coordination environ- the cyclotrimerization of alkynes, we studied the reactions of 2
ment of Ni atoms. Selected bond lengths (Å): Ni⋯Ni, 2.718–2.735; N1– with alkynes in more detail. The identification of optimal con-
C1, 1.305(4); C1–C2, 1.452(5); C2–N2, 1.423(4); N3–C21, 1.316(3); C21–
C22, 1.446(4); C22–N4, 1.435(4); N5–C41, 1.319(4); C41–C42, 1.452(5);
ethyl propiolate as the substrates, and the results are summar-
C42–N6, 1.425(4); Ni1–N1, 1.908(3); Ni1–N2, 1.996(3); Ni2–N3, 1.908(3);
ditions was carried out by using ethyl phenylpropiolate and
ized in Table 1. Notably, among the solvents tested, the non-
Ni2–N4, 1.997(3); Ni3–N5, 1.920(3); Ni3–N6, 1.990(3); Ni1–C22,
1
.983(3); Ni2–C42, 1.984(3); Ni3–C2, 1.987(3).
polar, less toxic solvent n-hexane was found to be a much
better medium for the 1,3,5-product than more polar solvents
both for ethyl phenylpropiolate and ethyl propiolate (Table 1,
a trigonal geometry. The Ni⋯Ni distances (2.72 to 2.74 Å) are entries 4 and 12). This is different from previous literature
significantly longer than the Ni–Ni bond lengths among reports where the catalytic reaction was mostly found to
α-diimine Ni–Ni-bonded complexes (up to 2.4649(8) Å) and the proceed better in toluene or benzene for which the 1,2,4-
2
2
sum of the covalent radii of Ni (2.492 Å), indicating the lack isomers were the main product.
of Ni–Ni bonding.
It should be mentioned that the reaction of the terminal
The NMR spectrum of complex 2 implies a diamagnetic ethyl propiolate (3b) containing an electron-withdrawing group
species, and the HR ESI-MS spectrum shows the molecular ion at room temperature was quite fast (0.5 h) and there was about
Me-2,4
+
of [Ni
3
(L
)
3
] (appearing at m/z 1052.4467, calc. 1052.3846; 20% of the tetra-substituted cyclooctatetraene as determined
1
Fig. 3). Moreover, optimization of complex 2 by DFT compu- by H NMR spectroscopy. Raising the temperature had little
tations reproduced the X-ray crystal structure. Natural bonding effect on the reaction/reaction products. In contrast, in the
orbital (NBO) analysis gave a natural charge of about 0.56 for case of ethyl phenylpropiolate (3a), when the reaction was
each nickel center (and −0.56 for each ligand), which is carried out at room temperature, the yield for the trimerized
similar to previously reported nickel(0) complexes with dienes benzene products was very low even for a prolonged time of
2
3
(
0.555). This partial positive charge may indicate the π-back 5 h. This lower reactivity may be due to the steric effect of the
bonding of nickel(0) to ligand L in 2.
substrate. However, when the temperature was increased to
As a continuing exploration of the reactivity of metal–metal- 100 °C (at which point the solution started boiling), the total
bonded (and related) compounds towards small molecules, yield of benzene products increased dramatically from 5% up
reactions of the tri-nuclear complex 2 with alkynes were to 96%, and the selectivity for the 1,3,5-substituted product
carried out. Surprisingly, treatment of 2 with an excess of ethyl was also improved to about 3 : 2 (Table 1, entries 1–4).
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a
Table 1 Optimization of the reaction conditions
b
c
Entry
Substrate
Solvent
Time (h)
Temp. (°C)
Yield of 4 + 5 (%)
4 : 5
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3b
3b
3b
3b
n-Hexane
n-Hexane
n-Hexane
n-Hexane
Benzene
Toluene
DMF
Acetonitrile
Dioxane
DME
5
5
5
5
5
5
5
5
5
5
5
0.5
0.5
0.5
0.5
r.t.
60
80
5
n.d.
58
94
96
93
87
15
45
68
55
54
78
42
80
73
58 : 42
69 : 31
69 : 31
64 : 36
66 : 34
21 : 79
26 : 74
56 : 44
44 : 45
55 : 45
71 : 29
55 : 45
34 : 66
38 : 62
d
100
100
100
100
100
100
100
0
1
2
3
4
5
d
THF
100
n-Hexane
Toluene
Diethyl ether
Acetonitrile
r.t.
r.t.
r.t.
r.t.
a
−1
Reactions were performed in a sealed pressure tube under an argon atmosphere using 2.5 × 10 mmol of the alkyne in 0.5 mL n-hexane and
−3
b
1
c
7
.5 × 10 mmol of complex 2 (9 mol% of Ni). Yield determined by H NMR spectroscopy using CH I as an internal standard. Ratio deter-
2 2
1 d
mined by H NMR. At this temperature the solution in the sealed pressure tube started to boil.
The optimized conditions were then applied to explore the monomer. For terminal alkynes, those (3b and 3e) containing
scope of the reaction with a variety of alkynes. The cyclotrimer- EWGs were readily cyclotrimerized at room temperature within
ization was found to be catalyzed by 3 mol% of complex 2 in 0.5 h in good yields (78–79%). In contrast, the alkyl- (3g and
moderate to excellent yields (Table 2). The results demon- 3h) and phenyl-substituted (3f) terminal alkynes required
strated that different functionalities such as ester (substrates heating to 100 °C for 5 h to give benzene derivatives in moder-
3
a–3d), ketone (3e), phenyl (3f and 3i) and alkyl (3g and 3h) ate to good yields (38–83%), which may be attributed to the
groups were well tolerated, establishing the generality of this EDGs that lower the reactivity. These results are similar to the
catalytic [2 + 2 + 2] cyclization process. The reaction rate was trend observed in related catalytic alkyne trimerization reac-
2
d,f
found to be dependent on the electronic properties of the tions.
On the other hand, with EWGs, even the internal
a
Table 2 Cyclotrimerization of alkynes catalyzed by 2
b
c
Entry
R
1
R
2
Substrate
Time (h)
Temp. (°C)
Yield of 4 + 5 (%)
4 : 5
1
2
3
4
5
6
7
8
9
Ph
H
Me
CO
H
H
H
H
Ph
CO
CO
CO
CO
(CO)CH
Ph
TMS
n-Bu
Ph
2
2
2
2
Et
Et
Et
Me
3a
3b
3c
3d
3e
3f
3g
3h
3i
5
0.5
5
0.5
0.5
5
5
5
5
100
r.t.
100
r.t.
93
78
86
89
79
83
52
38
n.d.
69 : 31
71 : 29
9 : 91
—
75 : 25
50 : 50
8 : 92
33 : 67
n.d.
2
Me
3
r.t.
100
100
100
100
a
Reactions were performed under Ar using 2.5 × 10 mmol of the alkyne in 0.5 mL hexane and 7.5 × 10−3 mmol of 2 (9 mol% of Ni). Isolated
−1
b
c
1
yield. Ratio determined by H NMR.
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alkynes (3a and 3d, Table 2) can display high reactivities, while
the diphenylacetylene 3i is almost unreactive due to both elec-
tronic and steric factors.
As mentioned above, cyclotrimerization of asymmetric
alkynes may generate both the 1,3,5- and 1,2,4-isomers. In our
experiments, good regio-selectivity for the 1,3,5-isomer was
observed in three cases (entries 1, 2 and 5), which appears to
arise from both the electronic properties and the poor solubi-
lity of the 1,3,5-products in n-hexane. This latter factor can
promote both the reaction to proceed and the separation of
products. As shown in Table 2, alkynes 3b and 3e that contain
the EWG ester or ketone (entries 2 and 5) gave mostly the
head-to-tail cyclotrimerized 1,3,5-isomer, with a regio-selecti-
vity of 4/5 = 71 : 29 and 75 : 25, respectively, while alkynes 3g Fig. 4 Plots of [4a + 5a] vs. time with variable catalyst concentrations.
2
2
1
% equiv. (slope = 5.51, R = 0.98), 2% equiv. (slope = 7.76, R = 0.99),
and 3h with electron-donating substituents gave predomi-
nantly the 1,2,4-product (ratio of 4/5 = 8 : 92 and 33 : 67,
respectively).
2
and 3% equiv. (slope = 9.60, R = 0.99) of catalyst relative to the con-
centration of 3a. Reaction conditions: 0.5 mL of n-hexane, 500 mM of
3
a (41 μL, 0.25 mmol), 100 °C. Each point represents the average of two
1
In the reaction of catalyst 2 with ethyl phenylpropiolate 3a,
independent catalytic reactions and each is analyzed by
H NMR
excellent yields of trimerized products and high selectivity of spectroscopy.
the 1,3,5-isomer 4a were observed (the substitution pattern
was further confirmed by X-ray diffraction, Fig. S9†). In this
3
case, the preference for the C -symmetric 1,3,5-substituted lyst 2] (Fig. S10†), which implies that the catalyst resting state
product is possibly determined mainly by the poorer solubility is not trimeric. Based on this, the mechanism of the catalytic
of 4a in n-hexane versus the 1,2,4-isomer 5a. For ethyl propio- reaction reported here may be proposed to be similar to that
late 3b, about 35% of the product precipitates from the reac- commonly accepted for the cyclotrimerization of alkynes by
2
a,27
tion solution after standing overnight, which contains only the mononuclear complexes (Scheme S1†).
,3,5-substituted benzene. Another 14% of this isomer can be
1
precipitated by concentrating the filtrate, while the remaining
solution consists of about 6% of 1,3,5- and 23% of 1,2,4-substi-
tuted benzene and about 22% of tetrameric products, thus
Conclusions
equating to a total of 55% of the 1,3,5-isomer. Such a differ- In summary, a cyclic tri-nickel(0) complex (2) bearing the N-
Me-2,4
ence in solubility is advantageous to the separation and purifi- 2,4-dimethylphenyl substituted α-diimine ligand L
dis-
cation of the 1,3,5-product as desired. Moreover, yields of plays high catalytic activity towards alkyne cyclotrimerization
1
,3,5-product from the cyclotrimerization of ethyl phenylpro- to form benzene derivatives. The aggregation of the Ni
1
1a,25
piolate greater than 20% have not been reported
and the complex provides stabilization for the nickel(0) center.
substrates are often restricted to alkynes bearing small or acti- Moreover, the reactions can be performed in the less toxic,
1
6a
vated substituents.
Also, to our knowledge, for most non-polar solvent n-hexane, which also facilitates the separ-
transition metal systems, such cyclotrimerization reactions ation of the 1,3,5-products due to the different solubility of
work inefficiently using hexane as the medium due to the poor 1,3,5- versus 1,2,4-isomers.
3
solubility of the catalyst.
According to previous reports by Dieck et al., the nickel(0)
H
iPr-2,6
compound with a similar α-diimine ( L
) behaves as a
In other cases,
Experimental
1
1a
catalyst for cyclotetramerization of alkynes.
iPr-2,6
Synthesis and characterization of complex [Ni
3
(µ
2
-Br)
3
(µ
3
-
for nickel complexes of L
, only Ni-diimine-alkyne adducts
Me-2,4
Br) (L
)3]·Br (1)
2
were obtained from the reaction with alkynes, indicating poor
2
4
Me-2,4
catalytic activity of these complexes. Thus the current ligand (DME)NiBr
2
(1.55 g, 5.00 mmol) and
L
(1.46 g,
Me-2,4
L
and the self-supporting nature of the nickel(0) species 5.00 mmol) were added to 50 mL of CH Cl in a Schlenk flask.
2 2
may be the key to the activity of 2 toward alkynes.
The colour changed from orange to brown immediately, and
the mixture was stirred for 18 h at room temperature. The
mixture was concentrated and the precipitate was filtered and
Kinetics study
The kinetics of the catalytic reaction was investigated using washed with ether to yield complex 1 (2.19 g, 86%). Brown
2
6
established procedures. To study the dependence on catalyst crystals were grown from CH
2
Cl
concentration, we determined vobs from plots of ln[catalyst] at cm ): 2951(m), 2918(m), 2864(m), 1638(m), 1595(m), 1493(s),
varying catalyst concentrations of 5.0–15.0 mM (Fig. 4) at 1377(s), 1229(s), 1165(m), 849(s), 735(m). C60 Ni Br
00 °C. The plots gave a linear relationship with slope n = 0.5 (1531.89): Calcd (found): C, 47.02 (46.88); H, 4.73 (4.71); N
2
. M.p.: 291–293 °C. IR (KBr, ν/
−
1
H
72
N
6
3
6
1
of the catalyst, consistent with half-order dependence on [cata- 5.48 (5.49) %.
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Me-2,4
Synthesis and characterization of complex [{Ni(µ-L
3
)} ] (2)
Notes and references
Under strictly anhydrous and anaerobic conditions, the bromo
complex 1 (0.766 g, 0.50 mmol) and sodium metal (0.069 g,
1
W. Reppe and W. J. Schweckendiek, Justus Liebigs Ann.
Chem., 1948, 560, 104–116.
3
.00 mmol) were added to 30 mL of Et O at room temperature.
2
2
(a) S. Saito and Y. Yamamoto, Chem. Rev., 2000, 100,
The mixture was stirred for 8 h whereupon the colour changed
to dark purple. The mixture was filtered and the filtrate was
concentrated to about 6 mL and stored at room temperature
2901–2916; (b) P. R. Chopade and J. Louie, Adv. Synth.
Catal., 2006, 348, 2307–2327; (c) G. Domínguez and
J. Pérez-Castells, Chem. Soc. Rev., 2011, 40, 3430–3444;
(d) S. Pal and C. Uyeda, J. Am. Chem. Soc., 2015, 137,
for several days to yield complex 2 as purple crystals (0.342 g,
1
6
5%). M.p.: 173–174 °C. H NMR (400 MHz, C
6
D
6
, 298 K): δ =
8042–8045; (e) S. Okamoto, T. Yamada, Y. Tanabe and
0
.69 (s, 9H, N–CCH ), 0.76 (s, 9H, N–CCH ), 2.18 (s, 18H, Ar–
3
3
M. Sakai, Organometallics, 2018, 37, 4431–4438;
(f) V. Cadierno, S. E. Garcίa-Garrido and J. Gimeno,
CH
3
), 2.60 (s, 9H, Ar–CH
3
), 2.73 (s, 9H, Ar–CH
3
),
1
3
1
6
2
2
.44–7.89 ppm (18H, Ar–H). C{ H} NMR (100.6 MHz, C D ,
98 K): δ = 16.7 (N–CCH ), 18.4 (Ar–o-CH ), 19.6 (Ar–o-CH ),
0.4 (Ar–p-CH
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6
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3
3
(
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), 62.9 (N–CCH
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−
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CCH ), 123.6–150.3 (aryl-C) ppm. IR (Nujol, ν/cm ): 2968(m),
3
2
1
920(s), 2859(m), 1641(m), 1606(m), 1490(s), 1448(m), 1367(w),
185(m), 1124(m), 1031(m), 829(m), 719(w). C60 Ni
3
8
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j) D. Brenna, M. Villa, T. N. Gieshoff, F. Fischer,
H
72
N
6
(
(
1053.37): Calcd (found): C 68.42 (68.54), H 6.89 (6.87), N 7.98
7.95) %.
(
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propiolate and ethyl phenylpropiolate (optimization of the
reaction conditions)
Under an argon atmosphere, complex 2 (7.9 mg, 7.5 × 10−3
mmol) and ethyl propiolate or ethyl phenylpropiolate were
mixed in 0.5 mL of solvent (see Table 1 for details) at 25 °C
11369–11377.
(ethyl propiolate) or 100 °C (ethyl phenylpropiolate). The
3
4
R. L. Hollingsworth, A. Bheemaraju, N. Lenca, R. L. Lord
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resulting mixture was stirred at the temperature for 0.5 h (ethyl
propiolate) or 5 h (ethyl phenylpropiolate). The reaction was
complete and the yield and isomeric ratio were determined by
1
analyzing the H NMR spectra.
1383–1390.
5
6
7
8
9
V. Cadierno, J. Francos, S. E. García-Garrido and J. Gimeno,
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the cyclotrimerization products
In a 15 mL sealed pressure tube were added the alkyne
13330.
(
2.5 mmol) and catalyst 2 with 5 mL of n-hexane. The mixture
was stirred at 100 °C for 5 h or at room temperature for 0.5 h
depending on the alkyne substrate) under Ar. After cooling
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(
down to room temperature, the volatiles were removed under
reduced pressure and the pure 1,3,5- and 1,2,4-isomers were
obtained by flash chromatography on silica gel (gradient ethyl
acetate/petroleum ether = 1/20 (v/v) or petroleum ether). For
details of each individual reaction and characterization data of
the trimerized products please see ESI.†
1
1
1
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2 (a) T. Schaub and U. Radius, Z. Anorg. Allg. Chem., 2006,
32, 807–813; (b) H. D. Srinivas, Q. Zhou and M. P. Watson,
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant No. 21771144). CR thanks the 14 C. Crisóstomo, M. G. Crestani and J. J. García, J. Mol. Catal.
EPSRC (Grant No. EP/R023816/1) for a travel grant.
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