Nickel-Catalyzed Facile [2+2+2] Cyclotrimerization of Unactivated Internal Alkynes to Polysubstituted Benzenes

Article abstract of DOI:10.1002/asia.201601469

A catalytic [2+2+2] cyclotrimerization of unactivated internal alkynes providing cyclotrimerization products in excellent yields with high regioselectivity is described. The transformation is accomplished by using a simple catalytic mixture comprising Ni(acac)₂, an imidazolium salt and a Grignard reagent at room temperature or 60 °C for 20 min or 1 h.

Full text of DOI:10.1002/asia.201601469

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Nickel-Catalyzed Facile [2+2+2] Cyclotrimerization of Unactivated
Internal Alkynes to Polysubstituted Benzenes
Authors: Fei Xue, Ying Kai Loh, Xiaolu Song, Wei Jie Teo, J. Y.
Darrence Chua, Jin Zhao, and T. S. Andy Hor
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10.1002/asia.201601469
Chemistry - An Asian Journal
FULL PAPER
Nickel-Catalyzed Facile [2+2+2] Cyclotrimerization of Unactivated
Internal Alkynes to Polysubstituted Benzenes
Fei Xue,[a] Ying Kai Loh,[b] Xiaolu Song,[b] Wei Jie Teo,[a] J. Y. Darrence Chua,[b,c] Jin Zhao,*[a,b] and T. S.
Andy Hor*[a,b,d]
Abstract: Catalytic [2+2+2] cyclotrimerization of unactivated internal
alkynes giving cyclotrimerization products in excellent yields with
high regioselectivity is accomplished through the use of simple and
common catalytic mixture comprising Ni(acac)2, imidazolium salt and
Grignard reagent at r.t. or 60 °C for 20 min or 1 h.
of Ni(acac)2/imidazolium salt/Grignard reagent in an effective
catalysis of [2+2+2] cyclotrimerization of unactivated internal
alkynes under mild conditions. The easily accessible NHC
precursors, in form of imidazolium salts, IMes∙HX (1,3-bis(2,4,6-
trimethylphenyl)imidazolium chloride or bromide), IBz∙HX (1,3-
dibenzylimidazolium chloride or bromide), IPr∙HCl (1,3-bis-(2,6-
diisopropylphenyl)l)imidazolium chloride chloride) and IPr*∙HCl
(1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazolium
chloride) were used in this study.
Introduction
Transition metal-catalyzed [2+2+2] cyclotrimerization of
alkynes is an effective tool for constructing polysubstituted
benzenes which are useful building blocks for synthesis of new
materials and biologically active compounds.[1] In spite of the
progress, cyclotrimerization of unactivated internal alkynes is still
challenging in terms of catalytic efficiency and regioselectivity. In
many known protocols of internal alkynes cyclotrimerization,[2]
high catalyst loading, high reaction temperature and/or
prolonged reaction time are generally required, and the internal
alkyne substrates have been often restricted to those bearing
small or activated substituents such as ethyl phenylpropiolate.
Since Reppe’s pioneering cyclotrimerization of alkyne using a Ni
based catalyst,[3] many Ni(0/II) compounds supported by
different ligands have been found to be efficient in catalyzing
inter- and intramolecular cyclotrimerization of alkynes and the
related reactions.[1,2a,2j,4] Nickel NHC (N-heterocyclic carbene)
complexes are catalytically active for many organic
transformations.[5] The combination of Ni(cod)2/NHC could
catalyze the cyclotrimerization of unsymmetrical internal alkynes
with ester substituent,[2j] the cyclotrimerization of diynes with
nitriles, isocynates or CO2,[4m] and the [2+2+2] cycloaddition of
1,n-enyne to form substituted 1,3-cyclohexadiene.[4n] As part of
our ongoing investigations in Grignard-assisted Ni catalyzed C-C
bond formation reactions,[6] we herein report a combinative use
Results and Discussion
[2+2+2] Cyclotrimerization of 1-phenyl-1-propyne catalyzed
by Ni(acac)2/NHC∙HX/nBuMgCl
Preliminary screenings of reaction conditions were carried out
on the [2+2+2] cyclotrimerization of unsymmetrical internal
alkyne, viz. 1-phenyl-1-propyne (1a) by using the catalyst
system containing Ni(acac)2 (1 mol%), imidazolium salt and
nBuMgCl in a ratio of 1:3:20. The results are summarized in
Table 1. Among the imidazolium salts used, the system
containing IMes∙HX showed the highest catalytic activity,
yielding the cyclotrimerization products, 1,2,4-isomer 2a and
1,3,5-isomer 2a’ in quantitative yield (2a: 2a’ = 80: 20) after 20
min at r.t. in the mixed-solvent of THF/toluene (entry 1). In all
THF solvent, the system showed similarly high catalytic activity.
However, no reaction occurred when toluene was replaced with
CH2Cl2 or CH3CN. In the absence of either IMes∙HX or Grignard
reagent, no reaction took place (entries 2, 3). Use of IPr*∙HCl
could give a high total yield of 2a and 2a’, however, with poor
regioselectivity (entry 4). The use of IPr∙HCl led to both low total
yield and poor selectivity (entry 5). The combination of
Ni(acac)2/IBz∙HX/nBuMgCl gave the highest regioselectivity (2a:
2a’ ≥ 99: 1) and a quantitative yield of 2a could be obtained at
60 °C after 1 h (entries 6, 7). This catalyst system is therefore
more efficient than Hilt’s cobalt diimine catalyst (CoBr2(Cy-
diimine) (5 mol%)/Zn/ZnI2) which produced 2a and 2a’ in 98% of
yield (2a: 2a’ = 96: 4) at 80 °C for 15 h.[2d] Other alkyl or aryl
Grignard reagents combined with Ni(acac)2 and IMes∙HX could
catalyze the reaction as efficiently as nBuMgCl, whilst alkenyl
Grignard reagent was much less active and alkynyl Grignard
reagent was inactive (Table S1 in the Supporting Information).
The influence of the basicity of Grignard reagent on the catalytic
activity is thus evident.
[a]
[b]
Dr. F. Xue, W.-J. Teo, Dr. J. Zhao, Prof. Dr. T. S. A. Hor
Department of Chemistry
National University of Singapore
3 Science Drive 3, Singapore 117543 (Singapore)
Dr. X. Song, Y.-K. Loh, J. Y. D. Chua, Dr. J. Zhao, Prof. Dr. T. S. A.
Hor
Institute of Materials Research and Engineering
Agency for Science, Technology and Research
2 Fusionopolis Way, Innovis, Singapore 138634 (Singapore)
E-mail: zhaoj@imre.a-star.edu.sg
[c]
[d]
J. Y. D. Chua
School of Chemical and Biomedical Engineering
Nanyang Technological University
62 Nanyang Drive, Singapore 637459 (Singapore)
Prof. Dr. T. S. A. Hor
Ni(0) species is known to be an active catalyst in the
commonly accepted mechanism of cyclotrimerization of alkynes
and other related reactions.[1] As expected, no reaction took
Department of Chemistry
The University of Hong Kong
Pokfulam, Hong Kong SAR (China)
E-mail: andyhor@hku.hk
t
place when BuOK, commonly used to form free NHC ligand in
situ in the synthesis of metal NHC complexes, replaced Grignard
reagent (entry 8), while the system containing BuLi was found
n
Supporting information for this article is given via a link at the end of
the document.
to be active with 82% yield of 2a and 2a’ (entry 9). Neither
Ni(acac)2/IPr or Ni(cod)2/IPr∙HCl could catalyze the reaction
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10.1002/asia.201601469
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(entries 10, 11), while the catalytic activity of Ni(cod)2/IPr was
similar to that of Ni(acac)2/IPr∙HCl/nBuMgCl (entry 12 cf. entry 5).
The Grignard reagent used herein thus serves both as a base
and reducing agent and the catalytically active Ni(0)-NHC
species is generated in situ. This is further supported by the
observation that simple nickel salts, NiCl2∙6H2O and
Ni(OAc)2∙4H2O could catalyze the reaction as efficiently as
Grignard reagent to activate NiCl2(IMes)2 under the catalytic
reaction conditions. In the catalytic reaction mixture the active
Ni(0)-NHC complex could be also formed through the direct
reaction between the in situ formed NHC ligand and Ni(0)
species obtained from the reduction of Ni(II) precursor by
Grignard reagent.
n
Ni(acac)2 when mixing with IMes∙HCl and BuMgCl (entries 13,
Table 1. [2+2+2] Cyclotrimerization of 1-phenyl-1-propyne[a]
14). Although the definitive explanations are not forthcoming, the
results above also suggest some general trends of the effect of
NHC ligand on the catalytic performance. The catalytic activity
increases when the donor ability of the NHC ligand[7a-c]
decreases. On the other hand, when the steric bulk of the NHC
ligand[7d] decreases (IPr>IMes>IBz), the regioselectivity of the
1,2,4-isomer (2a) increases. The similar steric effect of NHC was
also indicated in the Ni(cod)2/NHC catalyzed cyclotrimerization
of ethyl but-2-ynoate where the less bulky SIMes (1,3-bis-(2,4,6-
trimethylphenyl)imidazolidin-2-ylidene) ligand gave higher
regioselectivity to 1,2,4-isomer than SIPr (1,3-bis-(2,6-
diisopropylphenyl)imidazolidin-2-ylidene) ligand.[2j] It was, thus
expected that the most bulky IPr* ligand could give the highest
selectivity of the 1,3,5-isomer (2a’). However its regioselectivity
was found to be comparable to that of IPr ligand. It might be
attributed to the ability of IPr* to adjust its steric surrounding.[7e]
Under the conditions applied in this work, no reaction took place
when using phosphine ligands such as PPh3, dppe or dppb or
CoCl2 as metal source (entries 15, 16).
Entry
Metal salt or
complex
NHC∙HX
or other
ligand
RMgX or
other
additive
Yield[b] of
2a+2a’
(%)
Ratio[c] of
2a : 2a’
IMes∙HCl/
IMes∙HBr
80:20/
79:21
1
Ni(acac)2
Ni(acac)2
Ni(acac)2
Ni(acac)2
Ni(acac)2
Ni(acac)2
Ni(acac)2
Ni(acac)2
Ni(acac)2
Ni(acac)2
Ni(cod)2
nBuMgCl
nBuMgCl
-
98/99
NR[d]
NR
93
-
-
2
3
IMes∙HCl
IPr*∙HCl
IPr∙HCl
-
4
nBuMgCl
nBuMgCl
nBuMgCl
nBuMgCl
tBuOK
nBuLi
57 : 43
5
40
52 : 48
IBz∙HCl/
IBz∙HBr
IBz∙HCl/
IBz∙HBr
6
29/34
98/99
NR
82
99 : 1
7[e]
8
99 : 1
The synthesis of nickel(II) NHC complexes, Ni(acac)2(NHC)[8]
[9]
IMes·HCl
IMes∙HCl
IPr
-
and NiCl2(NHC)2 (NHC = IPr, IMes) have been reported
previously. The presence of Ni(II) precursor, Grignard reagent
and imidazolium salt in the catalyst system implies that a Ni(II)-
NHC complex formed in situ could be a precatalyst. Using the
mixture of Ni(acac)2(IPr)/nBuMgCl as catalyst in which
Ni(acac)2(IPr) was obtained from the reaction of Ni(acac)2 with
free IPr ligand, the [2+2+2] cyclotrimerization products of 1a
could be obtained in 41% of yield (2a:2a’= 59:41) under similar
conditions given in Table 1. It suggests that Ni(acac)2(NHC)
could be responsible for the catalytic activity of
Ni(acac)2/IPr∙HCl/nBuMgCl (cf. entry 5). However, attempt to
isolate Ni(acac)2(NHC) from the reaction between Ni(acac)2 and
IMes.HCl or IPr.HCl treated by nBuMgCl (1.1 equiv) was futile. In
contrast, NiCl2(IMes)2 could be isolated by treating anhydrous
9
80 : 20
10
11
12
13
14
15
16
-
NR
NR
42
-
IPr∙HCl
IPr
-
-
Ni(cod)2
-
60 : 40
NiCl2·6H2O
Ni(OAc)2·4H2O
Ni(acac)2
CoCl2·6H2O
IMes∙HCl
IMes∙HCl
nBuMgCl
nBuMgCl
nBuMgCl
nBuMgCl
90
75 : 25
99
80 : 20
dppe/dppb
/PPh3
NR
<1
-
-
IMes∙HCl
n
NiCl2 (1 equiv.) and IMes∙HCl (2 equiv.) with BuMgCl (2 equiv.)
[a] Conditions: to a mixture of 1a (0.50 mmol), metal salt or complex (0.005
mmol, 1 mol%) and NHC∙HX (0.015 mmol, 3 mol%) in toluene (1.0 mL) was
added RMgX (0.1 M in THF, 1.0 mL, 0.10 mmol) dropwise within 30 sec. The
resulted mixture was stirred under N2 at r.t. for 20 min. [b] Isolated yield. [c]
Determined by GC-MS and 1H NMR analyses. [d] NR = no reaction. [e] At
60 °C, for 1h.
in THF at r. t. (see the Supporting Information). However the
mixture of NiCl2(IMes)2/nBuMgCl could not catalyze the [2+2+2]
cyclotrimerization of 1a. When NiCl2(IMes)2 (1 equiv.) was
treated with nBuMgCl (2 equiv.) in THF at r.t., no color change of
the reaction solution was observed even after 12 h. Interestingly,
from the reaction mixture of IMes∙HCl (3 equiv.), NiCl2 (1 equiv.),
diphenylacetlylene (2 equiv.) and nBuMgCl (10 equiv.) at r.t.
NiCl2(IMes)2 was isolated and identified by single-crystal X-ray
diffraction analysis. The structural features of NiCl2(IMes)2 (see
the Supporting Information), which have not been reported in the
literature, are similar to those of the bis(carbene) Ni(II)
Herrmann et al. reported that a mixture of Ni(acac)2/IPr∙HX
(1:1) or Ni(acac)2/IMes∙HX (1:1) was an efficient catalyst system
for the cross-coupling of aryl Grignard reagents with aryl
chlorides.[11] The catalyst systems Ni(cod)2/SIPr and
Ni(cod)2/SIMes in 1:1 ratio could catalyze the cyclotrimerization
of unsymmetrical internal alkynes with ester substituent.[2j] These
results prompted us to examine the catalytic activity of the
[9]
complexes, such as NiCl2(IPr)2 and NiCl2(SIMes)2[10]. On the
basis of these observations, the lack of the catalytic activity of
NiCl2(IMes)2/nBuMgCl could be attributed to the failure of
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mixture of Ni(acac)2 and NHC∙HX in 1:1 ratio in the presence of
nBuMgCl. The results are very similar to those obtained in the
mixtures of Ni(acac)2 and NHC∙HX in 1:3 ratio (see Table S2 in
the Supporting Information). The ratio of 1:1 has therefore been
used in the subsequent study.
[2+2+2] Cyclotrimerization of diarylacetylenes catalyzed by
Ni(acac)2/IMes∙HCl/nBuMgCl
The cyclotrimerization of an array of diarylacetylenes could be
catalyzed efficiently when Ni(acac)2/IMes∙HCl/nBuMgCl (1:1:20)
was used as catalyst (Scheme 1). At r. t. hexaphenylbenzene
2m was obtained in nearly quantitative yield after 20 min. Using
[2+2+2]
Cyclotrimerization
of
alkyl(aryl)acetylenes
catalyzed by Ni(acac)2/IBz∙HBr/nBuMgCl
cobalt diimine (CoBr2(Cy-diimine)/Zn/ZnI2)[2d] or Ni(cod)2/PPh3
[2j]
as catalyst, 2m could be obtained in good yields (76% and 83%,
respectively), however, longer reaction time (15 and 24 h,
respectively) and higher temperature (80C) were required. For
the substrates with phenyl ring containing electron donating
substituent at the para- or meta-position, a longer reaction time
The
catalytic
[2+2+2]
cyclotrimerization
of
other
unsymmertrical alkyl(aryl)acetylenes was examined using the
mixture of Ni(acac)2/IBz∙HBr/nBuMgCl (1:1:20) as catalyst (at 60
C, 1 h). The results are summarized in Table 2. The catalytic
system applied well to 1-aryl-1-propyne with either electron
donating or withdrawing group in either para- or meta-position of
the phenyl ring (entries 1-9). However, when 1-(hex-1-yn-1-yl)-2-
methylbenzene was used, no cyclotrimerization product was
formed.
High yield and high regioselectivity could be also obtained
when the methyl group of 1a was replaced with n-butyl group or
ethyl group (entries 10-11). When (cyclohexylethynyl)benzene or
methyl phenylpropiolate was used as substrate, no
cyclotrimerization product was observed. The present catalyst
system is not active for the cyclotrimerization of dialkylalkynes.
of
1 h gave 2n-2p in high yields. Cyclotrimerization of
diarylacetylenes with electron withdrawing group on the phenyl
ring needed to be conducted at elevated temperature (60 C) to
produce 2q and 2r in high yield. The preparation of 2n has been
reported previously using well-known cobalt catalyst, Co2(CO)8
under reflux condition in dioxane for
3 . Hexakis(4-
h.[12]
fluorophenyl)benzene (2q), a precursor to hexa-fluoro-hexa-peri-
hexabenzocoronene used for n-type organic field-effect
transistors, could not be obtained from Co2(CO)8 catalyzed
cyclotrimerization of 4,4’-difluorodiphenylacetylene.[13] It was
hence prepared from
a multi-step synthesis with harsh
conditions such as at 260 C and long reaction time of 9 h, and
even so with low total yields.[13] When unsymmetrical
diarylacetylene was used, the cyclotrimerization products with
the same high yield could be obtained. The method developed in
this work, thus, provides a convenient synthetic route to various
π-conjugated systems.
Table 2. [2+2+2] Cyclotrimerization of alkyl(aryl)acetylenes[a]
Entry
1
Ar
R
Yield of 2 (%)[b]
99 (2b)
86 (2c)
92 (2d)
96 (2e)
90 (2f)
Ratio of 2 :2’
98 : 2
4-MeC6H4
4-MeOC6H4
4-tBuC6H4
4-FC6H4
3-MeOC6H4
3-MeC6H4
3-FC6H4
4-PhOC6H4
6-MeONaphthyl
Ph
Me
Me
Me
Me
Me
Me
Me
Me
Me
nBu
Et
2
98 : 2
3
98 : 2
4
98 : 2
5
97 : 3
6
95 (2g)
98 (2h)
95 (2i)
97 : 3
7
99 : 1
8[c]
9[c]
10
11
98 : 2
Scheme 1. [2+2+2] cyclotrimerization of diarylacetylenes. (Conditions:
nBuMgCl (0.1 M in THF, 1.0 mL, 0.10 mmol) was added dropwise to a mixture
of alkyne (0.50 mmol), Ni(acac)2 (0.005 mmol, 1 mol%) and IMes.HCl (0.005
mmol, 1 mol%) in toluene (1.0 mL) within 30 seconds. The resulted mixture
was stirred under N2 at r.t. for 1h, Isolated yield. [a] Unsymmetrical substrate
was used and the ratio of the 1,2,4 isomer to the 1,3,5 isomer was not
determined.)
92 (2j)
98 : 2
98 (2k)
92 (2l)
98 : 2
Ph
97 : 3
[a] Conditions: nBuMgCl (0.1 M in THF, 1.0 mL, 0.10 mmol) was added
dropwise to a mixture of alkyne (0.50 mmol), Ni(acac)2 (0.005 mmol, 1 mol%)
and IBz.HBr (0.005 mmol, 1 mol%) in toluene (1.0 mL) within 30 seconds.
The resulted mixture was stirred under N2 at 60 °C for 1h. [b] Isolated yield.
[c] nBuMgCl (0.1 M in THF, 2.0 mL, 0.20 mmol) was added.
Conclusions
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139.2, 138.7, 135.8, 134.9, 134.8, 133.9, 132.0, 131.6, 130.11, 130.09,
129.3, 129.1, 128.03, 127.99, 21.2, 21.1, 19.5, 18.3, 18.1; HRMS (EI)
m/z calcd for C30H30 390.2342, found: 390.2353.
We have developed a facile and effective catalyst system
comprising simple nickel complex, easily accessible imidazolium
salt and common Grignard reagent for the [2+2+2]
cyclotrimerization of simple unactivated alkyl(aryl)acetylenes
and diarylacetylenes under mild conditions. This methodology
provides a new and operationally simple synthetic method of
polysubstituted benzenes. This work also offers an opportunity
to tune the activity and regioselectivity of the catalyst system by
using different NHC ligands.
1,2,4-Trimethyl-3,5,6-tri-(4-methoxyphenyl)benzene (2c)
1H NMR (500 MHz, CDCl3)  7.15 (d, J = 8.6 Hz, 2H), 6.98 (d, J = 8.7 Hz,
2H), 6.90 (d, J = 8.6 Hz, 2H), 6.86 (d, J = 8.7 Hz, 2H), 6.70-6.66 (m, 4H),
3.87 (s, 3H), 3.75 (s, 3H), 3.73 (s, 3H), 2.04 (s, 6H), 1.72 (s, 3H); 13C
NMR (125 MHz, CDCl3)  158.1, 157.4, 157.3, 140.9, 140.5, 139.1, 134.8,
134.22, 134.18, 132.24, 132.17, 131.24, 131.20, 130.4, 113.8, 112.80,
112.77, 55.2, 55.03, 55.00, 19.6, 18.3, 18.2; HRMS (EI) m/z calcd for
C30H30O3 438.2190, found: 438.2204.
Experimental Section
1,2,4-Trimethyl-3,5,6-tri-(4-t-butylphenyl)benzene (2d)
General Methods
1H NMR (500 MHz, CDCl3)  7.45 (d, J = 8.2 Hz, 2H), 7.18 (d, J = 8.1 Hz,
2H), 7.09-7.05 (m, 4H), 6.86 (d, J = 8.1 Hz, 2H), 6.82 (d, J = 8.2 Hz, 2H),
2.11 (s, 3H), 2.04 (s, 3H), 1.80 (s, 3H), 1.39 (s, 9H), 1.21 (s, 9H), 1.20 (s,
9H); 13C NMR (125 MHz, CDCl3)  149.1, 148.1, 148.0, 141.2, 140.9,
139.6, 139.4, 138.6, 134.0, 131.7, 131.5, 130.01, 129.98, 129.95, 129.0,
125.1, 123.8, 123.8, 34.5, 34.23, 34.21, 31.5, 31.30, 31.28, 19.6, 18.4,
18.3; HRMS (EI) m/z calcd for C39H48 516.3751, found: 516.3762.
All preparations and manipulations were performed using standard
Schlenk techniques under a nitrogen atmosphere. NMR spectra were
measured at 25 °C (unless stated otherwise) using a Bruker ACF 500
MHz NMR spectrometer. GC-MS analyses were recorded on Agilent
6890N/5973N system. HRMS was conducted on Bruker MicrOTOF-QII
spectrometer. Solvents were dried according to standard procedures. All
Grignard reagents were obtained from Sigma-Aldrich. All Grignard
reagents were diluted to 0.1M by THF before use. The imidazolium salts,
IMes∙HX[14a-c], IPr∙HCl[14a,c], and IPr*∙HCl[14d] IBz∙HX[14e] and the alkynes[15]
were prepared according to the literature methods.
1,2,4-Trimethyl-3,5,6-tri-(4-fluorophenyl)benzene (2e)
1H NMR (500 MHz, CDCl3)  7.20-7.13 (m, 4H), 6.94-6.80 (m, 8H), 2.02
(s, 6H), 1.69 (s, 3H); 13C NMR (125 MHz, CDCl3)  161.7 (d, JC-F = 243.7
Hz), 161.10 (d, JC-F = 243.5 Hz), 161.06 (d, JC-F = 243.2 Hz), 140.6
140.0, 138.4, 138.0 (d, JC-F = 3.5 Hz), 137.32 (d, JC-F = 3.5 Hz), 137.26 (d,
JC-F = 3.5 Hz), 134.5, 132.3, 131.7, 131.6 (d, JC-F = 7.8 Hz), 130.8 (d, JC-F
= 7.7 Hz), 115.4 (d, JC-F = 21.0 Hz), 114.5 (d, JC-F = 21.1 Hz), 114.4 (d,
JC-F = 21.1 Hz), 19.4, 18.3, 18.1; HRMS (EI) m/z calcd for C27H21F
402.1590, found: 402.1601.
Representative procedure of [2+2+2] cyclotrimerization of alkyne (Entry 7,
Table 1)
After standard cycles of evacuation and refill with pure N2, Ni(acac)2 (1.3
mg, 0.005 mmol), 1-phenyl-1-propyne (58 mg, 0.50 mmol), IBz.HBr (4.9
mg, 0.015 mmol) and toluene (1.0 mL) were introduced into a 25 mL-
Schlenk tube equipped with a magnetic stir bar. Butylmagnesium chloride
solution (0.1 M in THF) (1.0 mL, 0.10 mmol) was then added to the
mixture dropwise within 30 seceonds. The reaction mixture was stirred at
60 °C. After 1 h, the reaction was diluted with hexane (20 mL). The
mixture was passed through a pad of silica gel with hexane as eluent.
The resulting solution was concentrated under vacuum and then
subjected to a column chromatography (silica gel) with hexane as eluent
to give 1,2,4-trimethyl-3,5,6-triphenylbenzene (2a: 57.5 mg, 99%).
1,2,4-Trimethyl-3,5,6-tri-(3-methoxyphenyl)benzene (2f)
1H NMR (500 MHz, DMSO-d6, 100 °C),  7.40-7.38 (m, 1H), 7.05-6.93 (m,
3H), 6.77-6.50 (m, 8H), 3.81 (s, 3H), 3.63 (s, 6H), 2.01, 2.00 (s, total 6H),
1.69 (s, 3H); 13C NMR (125 MHz, DMSO-d6, 100 °C),  159.2, 158.2,
142.8, 142.1, 142.0, 140.3, 139.4, 137.9, 132.4, 130.6, 129.5, 128.9,
127.6, 122.0, 120.9, 115.5, 115.4, 114.4, 111.9, 111.4, 54.7, 54.5, 18.1,
17.0, 16.9; HRMS (EI) m/z calcd for C30H30O3 438.2190, found:
438.2203.
1,2,4-Trimethyl-3,5,6-triphenylbenzene (2a)[2d]
1H NMR (500 MHz, CDCl3)  7.47-7.44 (m, 2H), 7.37-7.34 (m, 1H), 7.27-
7.25 (m, 2H), 7.15-6.97 (m, 10H), 2.055, 2.051 (s, total 6H), 1.73 (s, 3H);
13C NMR (125 MHz, CDCl3)  142.4, 141.63, 141.60, 141.4, 140.6, 139.2,
133.9, 131.9, 131.2, 130.30, 129.4, 128.4, 127.30. 127.28, 126.5, 125.7,
125.6, 19.4, 18.3, 18.1; HRMS (EI) m/z calcd for C27H24 348.1873, found:
348.1876.
1,2,4-Trimethyl-3,5,6-tri-(3-methylphenyl)benzene (2g)
1H NMR (500 MHz, DMSO-d6, 100 °C)  7.37-7.34 (m, 1H), 7.18-7.17 (m,
1H), 7.02-6.99 (m, 4H), 6.88-6.76 (m, 6H), 2.38 (s, 3H), 2.18，2.16 (s,
total 6H), 1.98, 1.97 (s, total 6H), 1.64 (s, 3H); 13C NMR (125 MHz,
DMSO-d6, 100 °C)  141.4, 140.7, 140.60, 140.3, 139.6, 138.2, 136.9,
135.4, 132.2, 130.4, 130.2, 130.15, 130.08, 129.4, 129.1，127.6, 126.4,
126.3, 125.54, 125.48, 20.2, 20.04, 20.02, 18.3, 17.1, 16.9; HRMS (EI)
m/z calcd for C30H30 390.2342, found: 390.2357.
1,3,5-Trimethyl-2,4,6-triphenylbenzene (2a’)[2d]
1H NMR (500 MHz, CDCl3)  7.44-7.41 (m, 6H), 7.34-7.31 (m, 3H), 7.24-
7.23 (m, 6H), 1.72 (s, 9H); 13C NMR (125 MHz, CDCl3)  142.1, 139.8,
133.2, 129.4, 128.4, 126.4, 19.4.
1,2,4-Trimethyl-3,5,6-tri-(3-fluorophenyl)benzene (2h)
1H NMR (500 MHz, CDCl3)  7.45-7.40 (m, 1H), 7.16-6.95 (m, 5H), 6.82-
6.67 (m, 6H), 2.03 (s, 6H), 1.71 (s, 3H); 13C NMR (125 MHz, DMSO-d6,
100 °C)  161.9 (d, JC-F = 243.6 Hz), 161.1 (d, JC-F = 242.2 Hz), 143.4 (d,
JC-F = 6.8 Hz), 142.8 (d, JC-F = 7.2 Hz), 142.6 (d, JC-F = 7.5 Hz), 139.6,
138.6, 136.9, 133.1, 130.9, 129.9 (d, JC-F = 8.6 Hz), 129.6, 128.6 (d, JC-F
1,2,4-Trimethyl-3,5,6-tri-(4-methylphenyl)benzene (2b)
1H NMR (500 MHz, CDCl3)  7.27-7.25 (m, 2H), 7.14-7.12 (m, 2H), 6.96-
6.84 (m, 8H), 2.42 (s, 3H), 2.25 (s, 3H), 2.23 (s, 3H), 2.04, 2.03 (s, total
6H), 1.71 (S, 3H); 13C NMR (125 MHz, CDCl3)  141.2, 140.6, 139.5,
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= 8.5 Hz), 125.7, 124.7, 116.2 (d, JC-F = 21.1 Hz), 115.4 (d, JC-F = 21.5
Hz), 113.0 (d, JC-F = 20.6 Hz), 112.22 (d, JC-F = 20.2 Hz), 112.18 (d, JC-F
= 21.2 Hz), 18.0, 17.0, 16.8; HRMS (EI) m/z calcd for C27H21F 402.1590,
found: 402.1598.
1H NMR (500 MHz, CDCl3)  6.67 (d, J = 8.1 Hz, 12H), 6.62 (d, J = 8.1
Hz, 12H), 2.35 (t, J = 7.4 Hz, 12H), 1.42-1.36 (m, 12H), 1.17-1.10 (m,
12H), 0.83 (t, J = 7.3 Hz, 18H); 13C NMR (125 MHz, CDCl3)  140.3,
139.0, 138.3, 131.3, 126.5, 35.0, 33.4, 21.8, 13.9; HRMS (EI) m/z calcd
for C66H78 870.6098, found: 870.6104.
1,2,4-Trimethyl-3,5,6-tri-(4-phenoxyphenyl)benzene (2i)
Hexakis(4-methylphenyl)benzene (2p)
1H NMR (500 MHz, CDCl3)  7.39-7.36 (m, 2H), 7.32-7.28 (m, 4H), 7.21-
7.19 (m, 2H), 7.15-7.04 (m, 7H), 6.97-6.89 (m, 8H), 6.85-6.81 (m, 4H),
2.11 (s, 3H), 2.09 (s, 3H), 1.81 (s, 3H); 13C NMR (125 MHz, CDCl3) 
157.7, 157.2, 156.0, 154.92, 154.85, 140.9, 140.4, 138.9, 137.2, 137.0,
134.4, 132.1, 131.73, 131.69, 130.7, 129.8, 129.67, 129.65, 123.3,
122.85, 122.83, 119.1, 118.7, 118.34, 118.31, 19.6, 18.4, 18.2; HRMS
(EI) m/z calcd for C45H36O3 624.2659, found: 624.2661.
1H NMR (500 MHz, CDCl3)  6.67-6.63 (m, 24H), 2.09 (s, 18H); 13C NMR
(125 MHz, CDCl3)  140.3, 138.0, 134.0, 131.3, 127.2, 21.0; HRMS (EI)
m/z calcd for C48H42 618.3281, found: 618.3286.
Hexakis(4-fluorophenyl)benzene (2q)[13]
1H NMR (500 MHz, CDCl3)  6.74-6.70 (m, 12H), 6.63-6.59 (m, 12H); 13
NMR (125 MHz, CDCl3)  160.7 (d, JC-F = 244.3 Hz), 139.9, 135.9 (d, JC-F
= 3.5 Hz), 132.5 (d, JC-F = 7.9 Hz), 114.1 (d, JC-F = 21.2 Hz); HRMS (EI)
m/z calcd for C42H24F6 642.1777, found: 642.1781.
C
1,2,4-Trimethyl-3,5,6-tri-(6-methoxynaphthyl)benzene (2j)
1H NMR (500 MHz, DMSO-d6)  7.93 (d, J = 8.4 Hz, 1H), 7.86 (d, J = 9.0
Hz, 1H),7.71-7.46 (m, 7H), 7.39-7.36 (m, 2H), 7.22-6.97 (m, 7H), 3.90 (s,
3H), 3.78 (s, 3H), 3.76 (s, 3H), 2.02 (s, 3H), 1.99 (s, 3H), 1.65 (s, 3H);
13C NMR (125 MHz, DMSO-d6, 100 °C)  157.0, 156.7, 156.6, 140.5,
139.9, 138.4, 136.6, 132.8, 132.6, 131.9, 131.8, 131.1, 130.3,
128.7,128.4, 128.34, 128.28, 127.6, 127.5, 126.8, 126.3, 124.9, 117.96,
117.54, 117.49, 106.1, 105.8, 54.9, 54.7, 54.6, 18.5, 17.3, 17.1; HRMS
(EI) m/z calcd for C42H36O3 588.2659, found: 588.2670.
Hexakis(3-fluorophenyl)benzene (2r)
1H NMR (500 MHz, CDCl3)  6.90-6.88 (m, 6H), 6.63-6.55 (m, 18H); 13C
NMR (125 MHz, DMSO-d6, 100 °C)  160.5 (d, JC-F = 242.4 Hz), 140.9 (d,
JC-F = 8.1 Hz), 138.4, 127.9 (d, JC-F = 8.5 Hz), 126.4 (d, JC-F = 2.4 Hz),
117.0 (d, JC-F = 21.6 Hz), 112.1 (d, JC-F = 20.7 Hz); HRMS (EI) m/z calcd
for C42H24F6 642.1777, found: 642.1780.
1,2,4-Tri-(n-Butyl)-3,5,6-triphenylbenzene (2k)
1,2,4-Triphenyl-3,5,6-tri-(4-methylphenyl)benzene (2s) and 1,3,5-
Triphenyl-2,4,6-tri-(4-methylphenyl)benzene (2s’)
1H NMR (500 MHz, CDCl3)  7.42-7.39 (m, 2H), 7.34-7.30 (m, 3H), 7.11-
6.97 (m, 10H), 2.41-2.35 (m, 4H), 2.05-2.02 (m, 2H), 1.44-1.33 (m, 4H),
1.16-1.03 (m, 6H), 0.73-0.66 (m, 8H), 0.38-0.35 (m, 3H); 13C NMR (125
MHz, CDCl3)  141.6, 141.5, 141.24, 141.18, 139.2, 138.3, 136.5, 136.2,
130.6, 130.5, 130.0, 127.6, 126.87, 126.83, 126.3, 125.51, 125.45, 33.4,
32.8, 31.1, 30.5, 30.2, 23.23, 23.16, 22.7, 13.51, 13.48, 13.1; HRMS (EI)
m/z calcd for C36H42 474.3281, found: 474.3294.
1H NMR (500 MHz, CDCl3)  6.86-6.82 (m, 15H), 6.70-6.63 (m, 12H),
2.09 (s) and 2.08 (s) (total 9H); 13C NMR (125 MHz, CDCl3)  141.02,
141.00, 140.97, 140.5, 140.4, 140.36, 140.23, 140.20, 140.08, 137.70,
137.68, 134.31, 134.26, 131.5, 131.3, 127.2, 126.5, 124.94, 124.88,
21.02, 20.98; HRMS (EI) m/z calcd for C45H36 576.2817, found:
576.2808.
1,2,4-Triethyl-3,5,6-triphenylbenzene (2l)
1H NMR (500 MHz, CDCl3)  7.46-7.43(m, 2H), 7.39-7.36 (m, 3H), 7.14-
7.02 (m, 10H), 2.54-2.46 (m, 4H), 2.16-2.12 (m, 2H), 1.04-0.98 (m, 6H),
0.66 (t, J = 6.95 Hz, 3H); 13C NMR (125 MHz, CDCl3)  141.6, 141.4,
141.3, 141.2, 141.1, 139.4, 139.3, 137.7, 137.3, 130.6, 130.5, 130.0,
127.7, 126.94, 126.90, 126.4, 125.6, 125.5, 24.6, 23.6, 23.4, 15.6, 15.3;
HRMS (EI) m/z calcd for C30H30 375.2113, found: 375.2108.
Acknowledgements
We are grateful to the Agency for Science, Technology and
Research (A*Star) of Singapore (WBS No: R-143-000-566-305
and IMRE/16-1C0131) and GlaxoSmithKline (WBS No: R-143-
000-492-592) for financial support. We thank Mr. Bruno
Donnadieu for X- Ray Diffractometry assistance and Professor
Han Vinh Huynh for helpful discussion.
Hexakis(phenyl)benzene (2m)[2d]
1H NMR (500 MHz, CDCl3)  6.84 (s, 30H); 13C NMR (125 MHz, CDCl3) 
140.6, 140.3, 131.40, 126.6, 125.2; HRMS (EI) m/z calcd for C42H30
534.2342, found: 534.2342.
Hexakis(3-methylphenyl)benzene (2n)[12]
Keywords: catalytic [2+2+2] cyclotrimerization • internal alkynes
• nickel • Grignard reagents • N-heterocyclic carbene (NHC)
1H NMR (500 MHz, CDCl3)  6.73-6.60 (m, 24H), 1.99 (pseudo-triple,
18H); 13C NMR (125 MHz, CDCl3)  140.6, 140.1, 135.53, 135.46, 135.39,
132.40, 128.36, 128.33, 126.15, 126.10, 126.04, 125.5, 21.0; 1H NMR
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Entry for the Table of Contents
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Fei Xue, Ying Kai Loh, Xiaolu Song, Wei
Jie Teo, J. Y. Darrence Chua, Jin Zhao,*
and T. S. Andy Hor*
Page No. – Page No.
Nickel-Catalyzed Facile [2+2+2]
Cyclotrimerization of Unactivated
Internal Alkynes to Polysubstituted
Benzenes
Catalytic [2+2+2] cyclotrimerization of unactivated internal alkynes giving
cyclotrimerization products in excellent yields with high regioselectivity is
accomplished through the use of simple and common catalytic mixture comprising
Ni(acac)2, imidazolium salt and Grignard reagent at r.t. or 60 °C for 20 min or 1 h.
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