Iron-catalyzed regioselective cyclotrimerization of alkynes to benzenes

Article abstract of DOI:10.1016/j.jorganchem.2018.12.007

We report the synthesis and characterization of simple di(aminomethyl)pyridine ligated iron-pincer complexes, which catalyzed the regioselective [2+2+2] cyclotrimerization of terminal aryl and alkyl alkynes to provide the 1,2,4-trisubstituted benzene molecules. Interestingly, internal alkynes also exhibited similar cyclization and resulted in hexa-substituted benzene compounds. Increased steric bulk on pincer ligands diminished the selectivity for cycloaddition. Cyclotrimerization reactions proceeded at room temperature upon activation of catalyst by a Grignard reagent. EPR studies indicated thermally induced spin crossover effect in catalyst.

Full text of DOI:10.1016/j.jorganchem.2018.12.007

Accepted Manuscript
Iron-catalyzed regioselective cyclotrimerization of alkynes to benzenes
Suhas Shahaji Gawali, Chidambaram Gunanathan
PII:
S0022-328X(18)30986-0
DOI:
https://doi.org/10.1016/j.jorganchem.2018.12.007
JOM 20664
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 7 November 2018
Revised Date: 6 December 2018
Accepted Date: 6 December 2018
Please cite this article as: S.S. Gawali, C. Gunanathan, Iron-catalyzed regioselective cyclotrimerization
of alkynes to benzenes, Journal of Organometallic Chemistry (2019), doi: https://doi.org/10.1016/
j.jorganchem.2018.12.007.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Iron-Catalyzed Regioselective Cyclotrimerization of Alkynes to Benzenes
Suhas Shahaji Gawali and Chidambaram Gunanathan*
School of Chemical Sciences, National Institute of Science Education and Research (HBNI),
Bhubaneswar-752050, India.
Dedicated to Prof. V. Chandrasekhar on the occasion of his 60th birthday with our best wishes
and great admiration for his enormous and inspiring contribution to organometallic chemistry.
ABSTRACT: We report the synthesis and characterization of simple di(aminomethyl)pyridine
ligated iron-pincer complexes, which catalyzed the regioselective [2+2+2] cyclotrimerization of
terminal aryl and alkyl alkynes to provide the 1,2,4-trisubstituted benzene molecules.
Interestingly, internal alkynes also exhibited similar cyclization and resulted in hexa-substituted
benzene compounds. Increased steric bulk on pincer ligands diminished the selectivity for
cycloaddition. Cyclotrimerization reactions proceeded at room temperature upon activation of
catalyst by a Grignard reagent. EPR studies indicated thermally induced spin crossover effect in
catalyst.
KEYWORDS. cyclotrimerization; iron pincer; alkyne; epr, catalysis
INTRODUCTION
Synthesis of highly functionalized aromatic compounds finds utmost importance in
pharmaceutical as well as material synthesis. In this direction, Reppe introduced a powerful and
elegant approach for the synthesis of benzene compounds with multiple substitutions via
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[2+2+2] cyclotrimerization of alkynes.1 Using various transition metals, number of procedures
have been reported for the synthesis of arene compounds.1c,2,3 However, selective construction of
benzene rings remains a challenging task because of the poor regioselectivity, which depends on
catalyst and substrates, initial formation of metallacyclopentadiene from alkyne-dimerization and
subsequent cycloaddition of third alkyne.4 As a result, such selective trimerization of alkynes is
realized using stoichiometric amounts of zirconium5 and titanium complexes.1b,6 Alternatively,
the regioselectivity problem was addressed by introducing partially intermolecular reactions in
which the use of tethered diynes provided the required geometric and entropic restrictions, and
resulted in the selective formation of arenes.1a,6a,7
Homogeneous iron-catalysts have arguably started to challenge the dominance of noble
transition metals.8 Iron-catalyzed [2+2+2] cyclotrimerization9 of alkynes to selective formation
of benzene motifs is a challenge. In literature, different combination of iron complexes, ligands
and activators have been shown to exhibit higher catalytic efficiency with wide substrate scope
for 1,2,4-trisubstituted benzene derivatives. In addition to that, activator free and low oxidation
state (0, +1) iron complexes have also been used.10 However, in an intermolecular reaction,
simple iron salts or even iron-carbene complexes predominantly dimerize the reactive terminal
alkynes to provide a mixture (E, Z and geminal) of products11 or self-coupled products.12 Iron-
catalyzed intermolecular cyclotrimerization of terminal alkynes remains rare with poor
regioselectivity.13 Thus, development of a defined and efficient iron catalyst for regioselective
construction of 1,2,4-trisubstituted benzene compounds would be of high interest.
Transition metal pincer complexes with planar backbone are efficient catalysts for numerous
organic reactions. As the tridentate pincer ligands cover more than half of the metal coordination
sphere, they offer better control over the metal coordination sites and enforce selectivity in
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catalytic processes. Using such iron-pincer complexes, number of atom-economical
transformations14 including the Z-selective dimerization of terminal alkynes15 were reported
recently. In this report, we disclose the syntheses and structure of new NNN iron-pincer
complexes and their catalytic applications in intermolecular regioselective [2+2+2] cycloaddition
of both terminal and internal alkynes.
RESULTS AND DISCUSSION
Attempted cyclotrimerization of phenylacetylene with FeCl2 (4 mol%) and ethylmagnesium
bromide (9 mol%) as an activating reagent resulted in good conversion; but only to result in
mixture of compounds, in which the cyclotrimerization products were found in low yields (entry
1, Table 1). Thus, we envisaged utilizing easily accessible and stable iron-pincer complexes for
the catalytic cyclotrimerization of alkynes. The reaction of 2,6-bis(pyrrolidin-1-
ylmethyl)pyridine (NNNpyr), N,N'-(pyridine-2,6-diylbis(methylene))bis(N-ethylethanamine)
(NNNEt), N,N'-(pyridine-2,6-diylbis(methylene))bis(N-isopropylpropan-2-amine) (NNNiPr), and
2,6-bis((di-tert-butylphosphino)methyl)pyridine (PNPtBu) ligands with anhydrous FeCl2 provided
the paramagnetic iron(II) pincer complexes 1-4, respectively in very good yields (Scheme 1).
The single-crystal X-ray analyses of complexes [(NNNpyr)FeCl2] 1 and [(NNNEt)FeCl2] 2
revealed distorted square pyramidal geometry around the iron centers (Figure 1).
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Scheme 1. NNN- and PNP-iron complexes for alkyne trimerization.
1
4

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2
Fig. 1. Single-crystal X-ray structures of NNN-iron pincer complexes 1 and 2 (ellipsoids are
drawn at 30% probability)
Upon reaction of phenylacetylene and EtMgBr (9 mol%) with complex 1 (4 mol%) provided
excellent conversion (97%), and 76% isolated yield of trisubstituted benzene products with 93:7
(6a:7a) selectivity for 1,2,4-triphenylbenzene and 1,3,5-triphenylbenzene (entry 2, Table 1).
When similar experiment was carried out using only 4 mol% of EtMgBr (1 equiv. relative to
catalyst), both conversion of alkyne and yield of the trimerization products decreased (entry 3,
Table 1), indicating that two equivalents of activator (relative to catalyst) is essential for efficient
catalysis. Notably, the sequence of addition of reagents (alkyne, catalyst, THF and finally
activator EtMgBr) is important. When this addition sequence was altered and ethylmagnesium
bromide was added before the addition of phenylacetylene, no desired product formation
occurred (entry 4, Table 1). Use of iron-pincer [(NNNEt)FeCl2] complex 2 as a catalyst provided
similar results as that of catalyst 1 (entry 5, Table 1). However, when [(NNNiPr)FeCl2] complex
3 was used as a catalyst (4 mol%) both conversion and yield of the corresponding benzene
products were diminished (entry 6, Table 1). Further to test the role of stability and electronic
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influence of the catalyst in the cyclotrimerization reactions, the [(PNPtBu)FeCl2] complex 4 was
prepared.16 Upon using 4 as a catalyst (4 mol%), although 94% conversion of phenylacetylene
was observed, the desired products formed only in 72% yield. Thus, iron-complex 1 was found
to be a suitable catalyst for the selective [2+2+2] cycloaddition of terminal aryl alkynes to 1,2,4-
trisubstituted benzenes. Further, using 1 as a catalyst cyclotrimerization of phenylacetylene in the
presence of mercury (10 mol%) provided the products in 70% yield, indicating the involvement
of soluble molecular catalyst in the reaction. These results confirm that by suitable modification
of pincer ligands, both reactivity and selectivity in iron-catalyzed [2+2+2] cycloaddition of
terminal aryl alkynes can be controlled.
Table 1. Iron-Catalyzed Cyclotrimerization of Phenylacetylenea
entry catalyst time
(h)
conv.
(%)b
yield
(%)c
6a/7ad
1
90/10
93/7
FeCl2
16
5
95
47
76
54
0
2
97
1
1
1
2
3
4
3e
93/7
16
16
5
85
-
4f
5
18
93/7
93/7
93/7
> 99
79
75
61
72
6
5
7
5
94
aConditions: Phenylacetylene (0.5 mmol), catalyst (4 mol%), and THF (1 mL) were added in a scintillation vial and EtMgBr (9
mol%) was added at the end. bConversion determined by GC using toluene (0.5 mmol) as an internal standard. cYield of the
products (6a and 7a) after column chromatography. dRatio determined by GC. eEtMgBr (4 mol%) was used. fEtMgBr, catalyst
and THF (1 mL) was added prior to phenylacetylene.
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Next, we studied the cyclotrimerization of various substituted aryl terminal alkynes catalyzed by
complex 1 and the results are summarized in Table 2. Arylacetylenes containing electron-
donating substituents such as methyl, methoxy afforded the corresponding 1,2,4-trisubstituted
benzenes in moderate yields with very high regioselectivity (entries 2-6, Table 2). While
phenylacetylenes containing methyl substituents underwent complete conversion in 3-5 h, the
methoxy substituted phenylacetylenes required longer reaction time (14-16 h, entries 4,5, Table
2). Arylacetylenes containing electron-withdrawing groups such as 4-fluro, 2-trifluromethyl and
3,5-di(trifluormethyl) substituents provided cyclotrimerization products with the corresponding
1,2,4-trisubstituted benzenes in 58%, 51% and 67% yields, respectively at room temperature
(entries 7-9, Table 2).
Table 2. Iron-Catalyzed Cyclotrimerization of Aryl Terminal Alkynes a
ent substrate 5
ry
time
(h)
conv
(%)b
yield
(%)c
ratio:
6/7d
1
2
5
3
97
91
76
62
93/7
93/7
a
b
3
5
95
64
92/8
c
OMe
4
14
16
99
98
76
67
92/8
92/8
d
e
5
MeO
6
5
>99
64
92/8
f
7
16
5
99
92
58
51
94/6
92/8
g
F
CF3
8
h
7

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F3C
F3C
9
14
96
67
97/3
i
aConditions: Arylacetylene (0.5 mmol), catalyst 1 (4 mol%), and THF (1 mL) were added in a scintillation vial and EtMgBr (9
b
c
mol%) was added at the end. Conversion determined by GC using toluene (0.5 mmol) as an internal standard. Yield of the
products (6 and 7) after isolation from column chromatography.d Ratio determined by GC
.
Remarkably, this iron-pincer catalyzed [2+2+2] cyclotrimerization is also applicable to aliphatic
alkynes. An assortment of linear, branched alkyl as well as aryl-alkyl substituted acetylenes
underwent efficient cyclotrimerization to provide the corresponding products in moderate yields
(entries 1-8, Table 3). Notably, wide range of functional groups such as alkenes, allyl, aryl and
silyl ethers (tert-butyldimethylsilyl, TBDMS), and amines were well tolerated under these
experimental conditions; however, these functionalized acetylenes required heating of the
reaction mixture at 70 °C to obtain the moderate to quantitative conversions (entries 9-13, Table
3). Further, we have also tested catalyst 1-4 for the cyclotrimerization reactions with aliphatic 4-
(tert-butyldimethylsilyloxy)-1-butyne at 70 °C, where complex 1 provided best yield (76%, entry
13, Table 3) compared to other catalysts 2-4 (68%, 56% and 60% yields, respectively) for the
cyclotrimerized products (9m:10m). Unfortunately, all aliphatic alkynes used in Table 3
provided poor regioselectivity, and 1,3,5-trisubstituted benzenes were also formed in
considerable amount together with the major isomer of 1,2,4-trisubstituted benzenes. Out of all
the terminal alkynes tested only prop-2-yn-1-ylbenzene and but-3-yn-1-ylbenzene showed
reversal of regioselectivity; 1,3,5-trisubstituted benzenes were found to be the major
regioisomers with these two terminal alkynes (entries 7 and 8, Table 3). When trimethylsilyl
acetylene was employed for cyclotrimerization, efficient reaction occurred at room temperature
and the trisubstituted benzenes were obtained in 80% yield with moderate regioselectivity (entry
14, Table 3). Polymeric or complex mixture of side products were observed in all reactions in
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minor amount. However, electron deficient acetylene carboxylates such as methyl and ethyl
propiolate failed to provide desired benzene products.
Table 3. Iron-catalyzed cyclotrimerization of aryl terminal alkynes a
R
1 (5 mol %)
EtMgBr (11 mol %)
R
R
+
R
THF
R
R
R
8
9
10
entry substrate 8
time(h) temp conv. yield
ratio:
(%)b
(%)c
9/10d
(°C)
rt
1
2
9
93
57
54/46
55/45
a
b
c
rt
13
>99
>99
86
63
68
51
rt
rt
rt
3
4
13
9
63/37
50/50
d
5
13
>99
66
56/44
e
rt
rt
6
7
5
n.c
70
42
69/31
42/58
f
22
>99
g
rt
8
13
15
24
99
53
61
36
45/55
59/41
50/50
h
i
70
70
O
9
>99
>99
O
10
j
N
70
11
22
73
50
59/41
k
l
70
70
Si
12
9
96
99
72
76
72/28
60/40
O
13e,
18
O
m
Si
9

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rt
14
3
n.c
80
68/32
Si
n
aConditions: terminal alkyne (0.5 mmol), catalyst 1 (5 mol%), and THF (1 mL) were taken in a scintillation vial.
b
EtMgBr (11 mol%) was added and reaction was carried out at room temperature. Conversion was determined by
c
GC using toluene (0.5 mmol) as an internal standard. Yield of the products (9 and 10) after isolation from column
d
1
chromatography. Ratio determined by GC (for entries 1-8) and from H NMR (for entries 9-14) after isolation.
eReactions were also carried out with catalyst 2 (68%), catalyst 3 (56%), catalyst 4 (60%). n.c: not calculated.
Further, cyclotrimerization of internal alkynes like 1-phenyl-1-propyne, 1-phenyl-1-butyne and
1-phenyl-1-hexyne was performed using catalyst 1, which provided single regioisomer 12a, 12b
and 12c in 55%, 42% and 45% yields, respectively. Representative single-crystal structure of 6g
and 12a further confirmed cyclotrimerization of terminal and internal alkynes (Figure 2).
Although, remarkable regioselectivity was observed with arylalkyl internal alkynes, use of
dialkyl and diaryl internal alkynes (4-octyne and diphenylacetylene, respectively) failed to
provide cyclotrimerization reactions under this catalysis.
R
1 (5 mol %)
EtMgBr (11 mol %)
R
THF, 80 °C, 20 h
R
R
11a-c
12a R: CH3, 55%
12b R: Ethyl, 42%
12c R: Butyl, 45%
Scheme 2. Iron-catalyzed regioselective cyclotrimerization reaction of internal alkynes.
(a)
(b)
6g
12a
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Figure 2. Single-crystal X-ray structure of cyclotrimerized products (a) 6g and (b) 12a.
Ellipsoids are drawn at 30% probability.
When the rate of cyclotrimerization of phenylacetylene catalyzed by iron-pincer complex 1
was monitored using GC, 93% conversion occurred in 5 h indicating the rapid reaction of
alkynes (Figure 3).
Figure 3. Conversion of phenylacetylene in catalytic reaction with 1 and EtMgBr. Monitoring of
the reaction progress using GC analysis.
Upon reaction of complex 1 (0.05 mmol) with two equivalent of ethylmagnesium bromide (0.1
mmol) and stoichiometric amount of dodecene (0.05 mmol) in a closed vial under nitrogen
atmosphere, immediate color change from yellow to brown was observed; however, after 10
minutes the reaction mixture turned turbid indicating the decomposition of active species. GC
analysis of the reaction mixture after 40 min (using toluene as an internal standard) confirmed
14% formation of dodecane (Scheme 4), indicating the possible formation of an iron-hydride
intermediate 13.13b
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N
Fe
N
13
2 EtMgBr
1
N
H
8
THF, rt, 40 min
8
14%
13
Scheme 4. Stoichiometric reaction of iron complex 1 with Grignard reagent and alkene.
Electron paramagnetic resonance (EPR) measurements were carried out at 298 K in THF to
investigate the spin state of the complex 1. The complex 1 shows a single EPR signal at room
temperature, which is attributable to an S = 2 system (high spin- d6-FeII, Figure 4). The g value (g
= 2.01) of 1 is largely deviated from that (2.0023) of free electron. The width between extreme
slopes (60 G) of this EPR signal is much broader than that of conventional organic radicals, and
therefore, no hyperfine structure observed. However, EPR analysis of complex 1 at 100 K
revealed the disappearance of signal, observed at 298 K (low spin- d6-FeII). This clearly indicates
thermally induced spin crossover effect. Moreover, complex 1 has a solid-state magnetic moment
of 5.2 µB at 298 K, indicating four unpaired electrons at the iron center. The measured magnetic
moment is larger than spin-only value for an S = 2 spin state (4.89 µB), but still within the range
of the reported high spin distorted square-pyramidal iron(II) compounds.16
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Figure 4. X-band EPR of NNN-Fe catalyst 1 recorded at 298 K.
Although more studies are required, on the basis of experimental observations, a catalytic cycle
for cyclotrimerization of alkynes involving Fe(I) intermediacy was proposed17 as depicted in
Scheme 4. Reaction of catalyst 1 with Grignard reagent and alkyne generates the Fe(I)-
monohydride intermediate I. Upon insertion of coordinated alkyne into Fe−H bond and further
preliminary interaction of another alkyne motif leads to the formation of II. Coordinated alkene
and alkyne functionalities over the metal center further react to create a dienyl ligand and further
coordination of third alkyne motif results in formation of intermediate III. Subsequent [4+2]
cycloaddition between the metal coordinated dienyl ligand and alkyne leads to formation of
cyclic hexadienyl coordinated intermediate IV. A Hydride elimination from coordinated
unsaturated carbocycle liberates regioselective 1,2,4-triaryl benzene molecule and further
reaction with alkyne can regenerate the intermediate I to complete one loop of the catalytic
cycle. When aliphatic alkynes were employed in the catalysis, perhaps the steric
incompatibilities led to the formation of another regioisomer-1,3,5-trialkyl benzene in a
considerable percentage. The intermolecular cycloaddition pathway of iron-coordinated dienyl
ligand with alkynes may also be operative.
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Scheme 4. Proposed catalytic cycle for Fe-catalyzed [2+2+2] cycloaddition ceaction
CONCLUSION
In summary, we have described regioselective cyclotrimerization of aryl terminal alkynes and
internal alkynes using simple and easily accessible NNN iron-pincer catalysts to provide the
corresponding 1,2,4-trisubstituted and hexa-substituted benzene molecules. While arylalkynes
provided up to 97% regioselective 1,2,4-trisubstituted isomers, aliphatic alkynes displayed poor
regioselectivity perhaps due to the steric incompatibility on the metal coordination sphere.
Notably, the arylalkyl internal alkynes provided hexasubstituted benzene compounds as single
regioisomers. Mechanistic studies indicate the involvement of molecular catalyst and the reaction
proceeds upon activation of iron dichloride to iron(I)-hydride intermediate by a Grignard
reagent. Catalytic [2+2+2] cycloaddition is suggested to occur upon coordination of alkynes with
Fe(I) intermediates in a stepwise reaction pathway.
EXPERIMENTAL SECTION
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General Experimental. All catalytic reactions were performed under nitrogen atmosphere in an
MBraun Glove-box. Chemicals were purchased from Acros, Sigma-Aldrich, Alfa-aesar and used
1
without further purification. Dry solvents were prepared according to standard procedures. H,
29
13C, Si, and 19F NMR spectra were recorded using 400, 100.6, 79.5 and 376.5 MHz magnetic
fields, respectively. 1H and 13C{1H} NMR chemical shifts were reported in ppm downfield from
tetramethylsilane. Multiplicity is abbreviated as s, singlet; d, doublet; dd, doublet of doublet; t,
triplet; q, quartet; dq, doublet of quartet; m, multiplet; br, broad. Mass spectra were recorded on a
micro TOF-Q II spectrometer. Magnetic susceptibility was recorded on magnetic susceptibility
balance. GC-FID measurements were performed with Shimadzu series instrument. The GC-FID
instrument equipped with a capillary column coated with nonpolar stationary phase rtX was used
for molecular weight determination and identification, which allowed the separation of
hydrocarbons according to their boiling point differences.
Synthesis of 2,6-bis(bromomethyl)pyridine18: To a 100 mL one neck round-bottomed flask
equipped with a stirrer bar, guard tube and reflux condenser, pyridine-2,6-diyldimethanol (1 g,
7.05 mmol) and 60% HBr (7 mL) were added slowly. The reaction was heated at 125 °C for 7 h
then cooled to room temperature. The resulting residue was dissolved in H2O (25 mL) to give
yellow solution. To this solution saturated NaHCO3 was added to reach pH 8. The resulting
aqueous solution was extracted with CH2Cl2 (4 x 25 mL), and the combined organic layers were
dried over anhydrous sodium sulfate and filtered. The volatiles were evaporated under reduced
pressure and the resulting residue was purified by flash column chromatography (10% ethyl
acetate/hexane) to yield 2,6-bis(bromomethyl)pyridine (1.6 g, 84%) as white solid. 1H
NMR(CDCl3):
δ 7.70 (t, J = 8 Hz, 1H), 7.37 (d, J
= 8 Hz, 2H), 4.54 (s, 4H). 13C{1H}NMR
15

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(CDCl3):
δ
156.9, 138.3, 123.0, 33.6. HRMS (EI) m/z Calcd for C7H8Br2N: [M + H]+ 263.9018,
found 263.9031.
Synthesis of 2,6, bis(pyrrolidine-1-ylmethyl)pyridine19 (L1): To a 50 mL two neck round-
bottomed flask equipped with a stirrer bar, 2,6-bis(bromomethyl)pyridine (1 g, 3.8 mmol) was
dissolved in dry acetonitrile (20 mL) under an argon atmosphere. The solution was cooled to 0
°C prior to the dropwise addition of pyrrolidine (1.09 g, 15.2 mmol) over a period of 5 min. The
reaction was allowed to warm to room temperature (25 °C) and was stirred vigorously for a
period of 24 h. The volatiles were removed under reduced pressure to give a pale yellow residue,
which was diluted with 75 mL CH2Cl2 and neutralized with 1 M NaOH (1 x 30 mL) and washed
with brine (1 x 30 mL). The organic layer was then dried over sodium sulfate, filtered, and
concentrated under reduced pressure. The resulting reaction mixture was purified by alumina
column chromatography (4% ethyl acetate/hexane) to yield 2,6-bis(pyrrolidine-1-
1
ylmethyl)pyridine (0.69 g, 74%) as pale yellow oil. H NMR (CDCl3):
δ7.56 (t,
J
= 8 Hz, 1H),
7.24 (d, J δ
= 8 Hz, 2H), 3.74 (s, 4H), 2.53–2.56 (m, 8H), 1.74–1.77 (m, 8H). 13C NMR (CDCl3):
158.9, 136.7, 120.9, 62.4, 54.4, 23.6. HRMS (EI) m/z Calcd for C15H24N3: [M + H]+ 246.1965,
found 246.1957.
Synthesis of N,N’-(pyridine-2,6-diylbis(methylene))bis(N
-ethylethanamine)20 (L2): To a 50
mL two neck round-bottomed flask equipped with a stirrer bar, 2,6-bis(bromomethyl)pyridine
(0.5 g, 1.90 mmol), K2CO3(1.05 g, 7.6 mmol) was dissolved in dry acetonitrile (10 mL) under an
argon atmosphere. The solution was cooled to 0 °C prior to the dropwise addition of
diethylamine (0.55 g, 7.6 mmol) over a period of 5 min. The reaction was allowed to warm to
room temperature (25 °C) and was stirred vigorously for a period of 24 h. The reaction mixture
was concentrated under reduced pressure to give a pale yellow residue, which was diluted with
16

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60 mL CH2Cl2 and neutralized with 1 M NaOH (1 x 30 mL) and washed with brine (1 x 20 mL).
The organic layer was then dried over sodium sulfate, filtered and concentrated under reduced
pressure. The resulting residue was purified by alumina column chromatography (4% ethyl
acetate/hexane) to yield N,N’-(pyridine-2,6-diylbis(methylene))bis(N-ethylethanamine)(0.38 g,
80%) as pale yellow oil. 1H NMR (CDCl3):
(s, 4H), 2.53 (q, = 8 Hz, 8H), 1.01 (t,
δ
7.55 (t,
J
= 8 Hz, 1H), 7.29 (d,
J
= 8 Hz, 2H), 3.67
159.8, 136.5,
13
J
J
= 4 Hz, 12H). C{1H}NMR (CDCl3):
δ
120.6, 59.7, 47.3, 11.9. HRMS (EI) m/z Calcd for C15H28N3: [M + H]+ 250.2278, found
250.2279.
Synthesis of N,N'-(pyridine-2,6-diylbis(methylene))bis(N-isopropylpropan-2-amine)20 (L3):
To a 50 mL two neck round-bottomed flask equipped with a stirrer bar, 2,6-
bis(bromomethyl)pyridine (0.5 g, 1.90 mmol), K2CO3 (1.05 g, 7.6 mmol) was dissolved in dry
acetonitrile (10 mL) under an argon atmosphere. The solution was cooled to 0 °C prior to the
dropwise addition of diisopropylamine (0.77 g, 7.6 mmol) over a period of 5 min. The reaction
was allowed to warm to room temperature (25 °C) and was stirred vigorously for a period of 24
h. The volatiles were removed under reduced pressure to give a pale yellow residue, which was
diluted with 60 mL CH2Cl2 and neutralized with 1 M NaOH (1 x 30 mL) and washed with brine
(1 x 20 mL). The organic layer was then dried over anhydrous sodium sulfate, filtered and
concentrated under reduced pressure. The resulting residue was purified by alumina column
chromatography (4% ethyl acetate/hexane) to yield N,N'-(pyridine-2,6-diylbis(methylene))bis(N-
1
isopropylpropan-2-amine) (0.42 g, 72%) as pale yellow oil. H NMR (CDCl3):
δ
7.57 (t,
J
= 8
13
Hz, 1H), 7.44 (d,
J
= 8 Hz, 2H), 3.76 (s, 4H), 3.00 – 3.09 (m, 4H), 1.02 (d,
J
= 8 Hz, 24H). C
NMR (CDCl3) :
δ 161.8, 135.5, 118.2, 50.6, 47.9, 19.9. HRMS (EI) m/z Calcd for C19H36N3: [M
+ H]+ 306.2904, found 306.2909.
17

ACCEPTED MANUSCRIPT
Synthesis and Characterization of Complexes 1-4: (NNNpyr)FeCl2 (1): To the suspension of
FeCl2 (127 mg, 1 mmol) in THF (15 mL) pyrrolidine-NNN ligand (245 mg, 1 mmol) was added.
The reaction mixture was heated to 75 °C and vigorously stirred for a period of 20 h. The yellow
solution was cooled to room temperature and filtered through 0.2 µm PTFE syringe filter. The
filtrate was concentrated to 2 mL under vacuum. Hexane (15 mL) was added slowly to
precipitate a bright yellow solid, which was isolated by filtration, washed with hexane (3 x 1 mL)
and dried under vacuum yielding 352 mg (94% yield). The resulting complex is paramagnetic.
Crystals suitable of X-ray quality were obtained by slow diffusion of hexane into a solution of 1
in CH2Cl2. IR (KBr): 3413, 2963, 2918, 2847, 1609, 1579, 1473, 1456, 1438, 1381, 1349, 1334,
1262, 1225, 1108, 1083, 1024, 863, 796, 757 cm-1. HRMS (EI) m/z Calcd for C15H24Cl2FeN3:
[M +H]+ 372.0694, found 372.0664.
(NNNEt)FeCl2 (2): To the pale yellow suspension of FeCl2 (127 mg, 1 mmol) in THF (15 mL)
NNNEt ligand (249 mg, 1 mmol) was added. The reaction mixture was heated to 75 °C and
vigorously stirred for a period of 4 h. The yellow solution was cooled to room temperature and
filtered through 0.2 mm PTFE syringe filter. The filtrate was concentrated to 2 mL under
vacuum. Hexane (15 mL) was added slowly to precipitate a bright yellow solid, which was
isolated by filtration, washed with hexane (3 x 1 mL) and dried under vacuum yielding 357 mg
(95% yield). The resulting complex is paramagnetic. Crystals suitable of X-ray quality were
obtained by slow diffusion of Hexane into a solution of 2 in CH2Cl2. IR (KBr): 3408, 2983,
2855, 1605, 1579, 1464, 1402, 1380, 1360, 1262, 1168, 1150, 1133, 1035, 1025, 800, 762, 734.1
cm-1. MS (EI) m/z Calcd for C15H28Cl2FeN3: [M +H]+ 376.1, found 376.1
iPr
(NNNiPr)FeCl2 (3): To the suspension of FeCl2 (16.4 mg, 0.13mmol) in THF (4 mL) NNN
ligand (40 mg, 0.13 mmol) was added. The reaction mixture was heated to 60 °C and vigorously
18

ACCEPTED MANUSCRIPT
stirred for a period of 20 h. The yellow solution was cooled to room temperature and filtered
through 0.2 µm PTFE syringe filter. The filtrate was concentrated to 1 mL under vacuum.
Hexane (4 mL) was added slowly to precipitate off-white solid, which was isolated by filtration,
washed with hexane (2 x 1 mL) and dried under vacuum. The title compound was isolated as off-
white solid (34 mg, 61%). The resulting complex is paramagnetic. Attempts made to crystallize
the complex remain unsuccessful. IR (KBr): 3418, 2980, 1627, 1455, 1397, 1308, 1282, 1173,
1190, 1093, 983, 814 cm-1. ESI m/z Calcd for C19H36Cl2FeN3: [M + H]+ 432.1633, found
432.1568.
Complex 4 was prepared by following reported literature procedure.16
X-ray analyses and crystal data of 1, 2: Crystals suited for single crystal x-ray diffraction
measurements were mounted on a glass fiber. Geometry and intensity data were collected with a
Bruker SMART D8 goniometer equipped with an APEXCCD detector and with an
Incoatecmicrosource (Mo-Kα radiation, λ = 0.71073 Å, multilayer optics). Temperature was
controlled using an Oxford Cryostream 700 instrument. Intensities were integrated with
SAINT+21and corrected for absorption with SADABS.22 The structures were solved by direct
methods and refined on F
2with SHELXL-97.23,24
Crystal data of complexes
(NNNpyr)FeCl2(1): C15H23Cl2Fe N3, yellow block, crystal dimensions: 0.28 x 0.26 x 0.21 mm-1,
M
= 372.11, Orthorhombic with space group Pbca,
= 3378.3(4) Å3,
max = 59.318, ρcalcd = 1.463 Mg/m3, T = 100(2)K, min/max transmission factors =
a = 12.9858(8)Å, b = 12.0845(8)Å, c =
21.5276(14)Å,
1.206 mm-1, 2
α
= 90°,
β
= 90° ,
γ
= 90°,
V
Z = 8, F(000) = 1552.0, µ-(MoKα) =
θ
0.5363/0.7459, 54540 Reflections collected, 4758 unique (
R
1 = 0.0269), WR2 = 0.0627(all data).
19

ACCEPTED MANUSCRIPT
Residual electron density max/min = 0.34/-0.39e.Å-3.The structure has been deposited at the
CCDC data center and can be retrieved using the deposit number 1823256.
(NNNEt)FeCl2 (2): C15H23Cl2FeN3, yellow block, crystal dimensions: 0.18 x 0.14 x 0.12 mm-1,
M
= 376.14, monoclinic with space group P2/n,
a
= 13.8250(14) Å,
= 1886.6(3) Å3,
b
= 9.3017(10) Å,
c
=
14.8050(15) Å,
α
= 90°,
β
= 97.27(7)°,
γ
= 90°,
V
Z
= 4, F(000) = 792.0, µ-
(MoKα) = 1.080 mm-1, 2
θ
max = 54.342, ρcalcd = 1.324 Mg/m3, T = 100(2)K, min/max
transmission factors = 0.5356/0.7455, 15059 Reflections collected, 4150 unique (R1 = 0.0398),
WR2 = 0.0929(all data). Residual electron density max/min = 0.28/-0.51e.Å-3.The structure has
been deposited at the CCDC data center and can be retrieved using the deposit number 1823257.
General procedure for catalytic cyclotrimerization of alkynes: To a stirred solution of
catalyst 1 in THF (1 mL), alkyne (0.5 mmol) and then EtMgBr (11 mol%, 1 M in THF) was
charged in PTFE screw-capped reaction vial equipped with a stirrer bar inside the glovebox.
Then, the reaction mixture was brought outside and stirred at room temperature. Progress of the
reaction was monitored by GC, which indicated the completion of the reaction in specified time.
The reaction mixture was quenched by exposing it to the open atmosphere. The solvent was
evaporated under reduced pressure and the residue was purified by column chromatography to
provide cyclotrimerized product.
Kinetic studies on cylotrimerization of phenylacetylene catalyzed by 1: The conversion of
phenylacetylene versus time was monitored by GC. Within 2 minute 70% conversion was
observed by GC, for remaining 23% it has taken ~ 5 h (Figure 3). Thus, these NNN iron pincer
catalyst species were found to be highly active precursor catalysts for the phenylacetylene
cyclotrimerization with an initial TOF of 1.05 s−1 (measured at 15 min).
20

ACCEPTED MANUSCRIPT
4'-Phenyl-1,1':2',1''-terphenyl and 5'-phenyl-1,1':3',1''-terphenyl (6a and 7a): 6a:7a = 93:7;
determined by GC analysis, this product was isolated by flash column chromatography on silica
gel (Hexane). Yield 39 mg (76%), White solid; IR (DCM): 3057 (w), 3025 (w), 1472, 1441,
1
1264, 1241, 1074, 1028, 836, 754, 739, 694, 629 cm-1. H NMR (CDCl3):
δ 7.81 (s), 7.67-7.71
(m, 4H), 7.54 (d, 8 Hz, 1H), 7.46-7.50 (m, 2H), 7.37-7.41 (m, 1H), 7.21-7.26 (m, 10H). 13C{1H}
NMR (CDCl3): 141.6, 141.3, 141.1, 140.7, 140.5, 139.7, 131.2, 130.1, 130.0, 129.6, 129.0,
δ
128.1, 128.0, 127.6, 127.3, 126.7, 126.7, 126.3. HRMS (EI) m/z Calcd for C24H19: [M + H]+
307.1481, found 307.1470.
4,4''-Dimethyl-4'-(p-tolyl)-1,1':2',1''-terphenyl and 4,4''-dimethyl-5'-(p-tolyl)-1,1':3',1''-
terphenyl (6b and 7b): 6b:7b = 92:8; determined by GC analysis, this product was isolated by
flash column chromatography on silica gel (hexane). Yield 36 mg (62%), White solid; IR
(DCM): 3022 (w), 2919 (w), 2860 (w), 1512, 1479, 1451, 1239, 1110, 1019, 1006, 841, 805,
1
759, 737 cm-1. H NMR (CDCl3):
δ
7.78 (s), 7.60-7.67 (m), 7.49-7.51 (m), 7.26-7.33 (m), 7.07-
142.3, 141.5, 140.9,
7.16 (m), 2.46 (s, 3H), 2.44 (s, 3H), 2.36 (s, 6H). 13C{1H} NMR (CDCl3):
δ
140.2, 139.3, 138.9, 138.55, 138.50, 137.9, 137.4, 137.3, 136.2, 136.1, 134.3, 131.2, 129.9,
129.8, 129.7, 129.4, 128.82, 128.79, 127.3, 127.1, 125.8, 124.7, 29.8, 21.3. HRMS (EI) m/z
Calcd for C27H25: [M + H]+ 349.1951, found 349.1957.
3,3''-Dimethyl-4'-(m-tolyl)-1,1':2',1''-terphenyl and 3,3''-dimethyl-5'-(m-tolyl)-1,1':3',1''-
terphenyl (6c and 7c): 6c:7c = 93:7; determined by GC analysis, this product was isolated by
flash column chromatography on silica gel (hexane). Yield 37 mg (64%), white solid; IR (DCM):
1
3049 (w), 2923 (w), 2853 (w), 1583, 1551, 1264, 1243, 1045, 834, 782, 733, 701, 637cm-1. H
NMR (CDCl3):
δ7.67 (s), 7.52-7.56 (m), 7.38-7.42 (m), 7.23-7.27 (m), 7.07-7.12 (m), 6.82-6.86
(m), 2.36 (s, 3H), 2.34 (s, 3H), 2.19 (s, 3H), 2.18 (s, 3H). 13C{1H} NMR (CDCl3):
δ
142.5,
21

ACCEPTED MANUSCRIPT
141.7, 141.4, 141.3, 141.2, 140.8, 140.4, 139.7, 138.55, 138.51, 137.55, 137.49, 131.1, 130.70,
130.68, 129.5, 128.9, 128.3, 128.1, 127.78, 127.76, 127.4, 127.3, 127.24, 127.20, 126.1, 124.6,
124.4, 29.9, 21.7, 21.5. HRMS (EI) m/z Calcd for C27H25: [M + H]+ 349.1951, found 349.1980.
2,2''-Dimethoxy-4'-(2-methoxyphenyl)-1,1':2',1''-terphenyl
and
2,2''-dimethoxy-4'-(2-
methoxyphenyl)-1,1':2',1''-terphenyl (6d and 7d): 6d:7d = 92:8; determined by GC analysis,
this product was isolated by flash column chromatography on silica gel (EtOAc/hexane – 1/30).
Yield 50 mg (76%), yellow gel; IR (DCM): 2998 (w), 2923 (w), 2836 (w), 1528, 1515, 1478,
1462, 1287, 1243, 1173, 1115, 1031, 819, 798, 637cm-1.1H NMR (CDCl3):
δ7.73 (s), 7.63-7.65
(m), 7.47-7.49 (m), 7.33-7.37 (m), 7.18-7.20 (m), 7.08-7.16 (m), 7.02-7.07 (m), 6.85 (t, 8 Hz),
13
6.75 (t, 8 Hz), 3.87 (s, 3H), 3. 86 (s, 3H), 3.53 (s, 3H), 3.50 (s, 3H). C{1H} NMR (CDCl3):
δ
156.7, 156.4, 137.2, 131.7, 131.6, 131.3, 131.2, 130.6, 128.5, 128.4, 128.1, 120.9, 119.9, 111.2,
110.34, 110.33, 55.7, 55.6, 55.05, 55.03. HRMS (EI) m/z Calcd for C27H25O3: [M + H]+
397.1798, found 397.1775.
4,4''-Dimethoxy-4'-(4-methoxyphenyl)-1,1':2',1''-terphenyl
and
4,4''-dimethoxy-5'-(4-
methoxyphenyl)-1,1':3',1''-terphenyl (6e and 7e): 6e:7e = 92 :8; determined by GC analysis,
this product was isolated by flash column chromatography on silica gel (EtOAc/hexane – 1/30).
Yield 44 mg (67%), yellow gel; IR (DCM): 2998 (w), 2929 (w), 2834 (w), 1606, 1522, 1509,
1
1478, 1462, 1286, 1236, 1173, 1108, 1031, 819, 799, 734, 641 cm-1. H NMR (CDCl3):
δ
7.57-
7.69 (m), 7.46 (d, 8H), 7.11-7.16 (m), 7.00-7.04 (m), 6.79-6.83 (m), 3.88 (s, 3H), 3.87 (s, 3H),
3.81 (s, 3H), 3.81 (s, 3H). 13C{1H} NMR (CDCl3):
δ
159.3, 158.5, 158.4, 140.5, 139.7, 134.3,
133.9, 133.3, 131.1, 131.05, 131.01, 129.0, 128.2, 125.5, 114.4, 113.6, 113.5, 55.49, 55.47,
55.30, 55.29. HRMS (EI) m/z Calcd for C27H25O3: [M + H]+ 397.1798, found 397.1768.
22

ACCEPTED MANUSCRIPT
2,2'',4,4'',5,5''-Hexamethyl-4'-(2,4,5-trimethylphenyl)-1,1':2',1''-terphenyl
and
2,2'',4,4'',5,5''-hexamethyl-5'-(2,4,5-trimethylphenyl)-1,1':3',1''-terphenyl (6f and 7f): 6f:7f
= 92:8; determined by GC analysis, this product was isolated by flash column chromatography
on silica gel (hexane). Yield 46 mg (64%), yellow solid; IR (DCM): 2966 (w), 2918 (w), 2861
(w), 1479, 1451, 878, 842, 738, 705 cm-1.1H NMR (CDCl3):
δ7.14-7.21 (m), 7.04 (s), 6.97 (s),
6.74-6.75 (m), 2.23 (s, 4H), 2.18-2.19 (m, 7H), 2.08 (s, 3H), 2.06 (s, 3H), 2.02 (s, 3H), 1.99 (s,
3H), 1.94 (s, 6H). 13C{1H} NMR (CDCl3):
δ
141.4, 140.1, 139.2, 135.5, 134.7, 133.9, 132.9,
132.8, 132.7, 132.6, 131.9, 131.7, 131.4, 131.1, 130.5, 128.6, 127.5, 20.2, 19.4, 19.3, 19.2, 19.1.
HRMS (EI) m/z Calcd for C33H37: [M + H]+ 433.2890, found 433.2904.
4,4''-Difluoro-4'-(4-fluorophenyl)-1,1':2',1''-terphenyl
and
4,4''-difluoro-5'-(4-
fluorophenyl)-1,1':3',1''-terphenyl (6g and 7g): 6g:7g = 94:6; determined by GC analysis, this
product was isolated by flash column chromatography on silica gel (hexane). Yield 35 mg
(58%), white solid; IR (DCM): 3046 (w), 1603, 1522, 1509, 1480, 1222, 1158, 835, 820, 739 cm-
1
1. H NMR (CDCl3):
δ
7.68 (s), 7.58-7.65 (m), 7.47 (d,
J = 8 Hz, 1H), 7.11-7.19 (m), 6.93-6.98
(m). 13C{1H} NMR (CDCl3):
δ
164.0, 163.25, 163.21, 161.5, 160.80, 160.76, 141.7, 139.8,
131.55, 131.51, 131.47, 131.4, 131.2, 129.3, 128.85, 128.77, 126.3, 116.0, 115.8, 115.31, 115.27,
115.09, 115.06. 19F NMR (CDCl3):
C24H16F3: [M + H]+ 361.1, found 361.1.
δ
-115.5, -115.4, -115.0, -114.8. (ESI-MS) m/z Calcd for
2,2''-Bis(trifluoromethyl)-4'-(2-(trifluoromethyl)phenyl)-1,1':2',1''-terphenyl and 2,2''-
bis(trifluoromethyl)-5'-(2-(trifluoromethyl)phenyl)-1,1':3',1''-terphenyl (6h and 7h): 6h:7h
= 92:8; determined by GC analysis, This product was isolated by flash column chromatography
on silica gel (hexane). Yield 44 mg (51%), white solid; IR (DCM): 3067 (w), 1473, 1309, 1264,
1
1163, 1102, 1007, 765, 755, 656, 632 cm-1. H NMR (CDCl3):
δ
7.67 (d,
J
= 8 Hz, 2H), 7.54-
23

ACCEPTED MANUSCRIPT
7.59 (m), 7.47-7.50 (m), 7.29-7.41 (m), 7.18-7.25 (m), 7.01-7.16 (m), 6.99 (t,
J = 6 Hz, 2H).
13C{1H} NMR (CDCl3):
δ 140.7, 139.3, 139.0, 137.5, 137.3, 132.2, 132.0, 131.4, 131.1, 130.7,
129.64, 129.62, 129.3, 129.0, 128.7, 127.9, 127.7, 127.3, 126.3, 126.25, 126.20, 125.9, 125.8,
125.6, 123.2, 123.1, 122.9. 19F NMR (CDCl3):
δ
-57.92, -57.91, -57.19, -57.18, -57.13. HRMS
(EI) m/z Calcd for C27H16F9: [M + H]+ 511.1103, found 511.1116.
4'-(3,5-Bis(trifluoromethyl)phenyl)-3,3'',5,5''-tetrakis(trifluoromethyl)-1,1':2',1''-terphenyl
and
5'-(3,5-bis(trifluoromethyl)phenyl)-3,3'',5,5''-tetrakis(trifluoromethyl)-1,1':3',1''-
terphenyl (6i and 7i): 6i:7i = 97:3; determined by GC analysis, this product was isolated by
flash column chromatography on silica gel (hexane). Yield 80 mg (67%), white solid; IR (DCM):
1
1371, 1273, 1171, 1126, 1079, 893, 833, 740, 706 cm-1. H NMR (CDCl3)
δ
: 8.10 (s, 2H), 7.96
(s, 1H), 7.82-7.86 (m, 2H), 7.80 (s, 1H), 7.76 (d,
(s, 2H). 13C{1H} NMR (CDCl3):
J = 2 Hz), 7.70-7.72 (m, 1H), 7.60 (s, 2H), 7.57
δ
141.8, 141.7, 141.6, 141.4, 141.3, 139.7, 139.6, 139.2, 139.0,
138.5, 138.3, 132.2, 131.3, 131.2, 129.8, 129.1, 129.0, 128.23, 128.16, 127.3, 126.9, 124.6,
124.2, 121.8, 121.5, 121.2, 118.8. 19F NMR (CDCl3):
δ
-64.34, -64.30, -63.8.
1,2,4-Tripentylbenzene and1,3,5-tripentylbenzene (9a and 10a): 9a:10a = 60:40; determined
1
by H NMR, this product was isolated by flash column chromatography on silica gel (hexane).
Yield 28 mg (57%), yellow oil; IR (DCM): 2954, 2925, 2859, 1497, 1459, 1375, 829, 729 cm-1.
1H NMR (CDCl3):
1.62 (m), 1.33-1.37 (m), 0.88-0.93 (m). 13C{1H} NMR (CDCl3):
δ 7.05 (d, J = 8 Hz, 1H), 6.93-6.95 (m, 2H), 6.82 (s, 3H), 2.53-2.59 (m), 1.57-
δ
142.9, 140.5, 140.3, 137.8,
129.4, 129.1, 126.0, 125.8, 36.1, 35.7, 32.9, 32.4, 32.20, 32.19, 31.83, 31.81, 31.45, 31.43, 31.2,
22.75, 22.72, 14.2. HRMS (EI) m/z Calcd for C21H35: [M - H]+ 287.2733, found 287.2728.
24

ACCEPTED MANUSCRIPT
1,2,4-Triisopentylbenzene and 1,3,5-triisopentylbenzene (9b and 10b): 9b:10b = 60:40;
determined by GC analysis, this product was isolated by flash column chromatography on silica
gel (hexane). Yield 48 mg (63%), yellow oil; IR (DCM): 2954, 2929, 2869, 1466, 1383, 1367,
736, 704cm-1. 1H NMR (CDCl3):
2.57 (m), 1.46-1.56 (m), 0.93-0.98 (m). 13C{1H} NMR (CDCl3):
δ 7.04 (d, J = 8Hz, 1H), 6.93-6.95 (m, 2H), 6.82 (s, 3H), 2.53-
δ
143.1, 140.7, 140.5, 137.9,
129.3, 129.2, 125.8, 41.12, 41.08, 34.0, 33.5, 30.8, 30.3, 28.6, 28.5, 28.0, 22.8, 22.7. (ESI-MS)
m/z Calcd for C21H37: [M + H]+ 289.2, found 289.2.
1,2,4-Triisobutylbenzene and 1,3,5-triisobutylbenzene (9c and 10c): 9c:10c = 63:37;
determined by GC analysis, this product was isolated by flash column chromatography on silica
gel (hexane). Yield 28 mg (68%), yellow oil; IR (DCM): 2954, 2927, 2869, 1735, 1464, 1383,
1367, 1264, 1243, 737, 721cm-1. 1H NMR (CDCl3):
δ
7.01 (d,
= 7 Hz), 1.81-1.86 (m,
141.1, 139.4, 138.7, 137.0, 131.0, 129.8,
J
= 8 Hz, 1H), 6.88-6.91 (m, 2H),
6.75 (s, 3H), 2.46 (dd, J1 = 7 Hz, J2 = 2 Hz), 2.42 (d,
J
J
= 7 Hz), 0.93 (d,
J
= 7 Hz), 0.90 (d,
J
= 7 Hz). 13C{1H} NMR (CDCl3):
δ
127.5, 126.4, 45.6, 45.3, 42.2, 41.7, 30.50, 30.46, 30.0, 22.8, 22.6. HRMS (EI) m/z Calcd for
C18H29: [M - H]+ 245.2264, found 245.2232.
1,2,4-Tricyclopentylbenzene and 1,3,5-tricyclopentylbenzene (9d and 10d): 9d:10d = 50:50;
determined by 1H NMR, this product was isolated by flash column chromatography on silica gel
(hexane). Yield 24 mg (51%), yellow oil; IR (DCM): 2947, 2866, 1496, 1451, 1301, 1184, 1046,
886, 868, 712 cm-1. 1H NMR (CDCl3):
δ
7.20 (d,
J
= 8 Hz, 1H), 7.15 (d, 2 Hz, 1H), 7.05 (dd, J1
=
8 Hz, J2 = 2 Hz), 6.96 (s, 3H), 3.25-3.36 (m), 2.96 (pent,
1.61-1.70 (m). 13C{1H} NMR (CDCl3):
J
= 8 Hz), 2.03-2.08 (m), 1.80-1.82 (m),
δ
146.3, 143.8, 143.6, 141.4, 125.7, 124.7, 124.5, 123.6,
46.3, 46.0, 41.3, 41.0, 35.12, 35.09, 34.84, 34.77, 25.69. HRMS (EI) m/z Calcd for C21H29: [M -
H]+ 281.2264, found 281.2243.
25

ACCEPTED MANUSCRIPT
(Benzene-1,2,4-triyltris(methylene))tricyclohexane and 1,3,5-tris(cyclohexylmethyl)benzene
(9e and 10e): 9e:10e = 56:44; determined by GC analysis, this product was isolated by flash
column chromatography on silica gel (hexane). Yield 40 mg (66%), yellow gel; IR (DCM):
1
2918, 2848, 1447, 1372, 1262, 1239, 1046, 737 cm-1. H NMR (CDCl3):
δ 6.99 (d, J = 8 Hz,
1H), 6.86-6.88 (m, 2H), 6.73 (s, 3H), 2.41-2.45 (m), 1.48-1.69 (m), 1.16-1.18 (m), 0.89-0.98 (m).
13C{1H} NMR (CDCl3):
δ
140.7, 139.0, 138.2, 136.6, 131.2, 129.9, 127.6, 126.2, 44.3, 43.9,
40.9, 40.5, 40.0, 39.9, 39.6, 33.7, 33.4, 26.8, 26.6, 26.5. ESI-MS m/z Calcd for C27H41 [M - H]+:
365.3 found 365.3.
1,2,4-Tri-tert-butylbenzene and 1,3,5-tri-tert-butylbenzene (9f and 10f): 9f:10f = 69:31;
determined by GC analysis, this product was isolated by flash column chromatography on silica
gel (hexane). Yield 29 mg (70%), yellow oil; IR (DCM): 2956, 2868, 1470, 1391, 1362, 1259,
1201, 893, 737 cm-1. 1H NMR (CDCl3):
3H), 7.12 (dd, J1 = 8 Hz, J2 = 2 Hz), 1.56 (s), 1.54 (s), 1.33 (s), 1.31 (s). 13C{1H} NMR (CDCl3):
δ 7.62 (d, J = 8 Hz, 1H), 7.50 (d, J = 8 Hz, 1H), 7.25 (s,
δ
150.1, 147.5, 145.7, 129.3, 126.9, 122.4, 121.7, 119.6, 37.4, 35.1, 35.01, 34.97, 32.7, 32.3,
31.7, 31.5. ESI-MS m/z Calcd for C18H31: [M + H]+ 247.2, found 247.2.
(Benzene-1,2,4-triyltris(methylene))tribenzene and 1,3,5-tribenzylbenzene (9g and 10g):
9g:10g = 58:42; determined by GC analysis, this product was isolated by flash column
chromatography on silica gel (hexane). Yield 34 mg (53%), yellow oil; IR (DCM): 3058, 3025,
1
2918, 1599, 1493, 1449, 1073, 1029, 726, 699 cm-1. H NMR (CDCl3):
δ 7.23-7.30 (m), 7.14-
7.20 (m), 7.06 (d, J = 8 Hz), 6.88 (s, 3H), 3.93 (d, J δ
= 8 Hz), 3.90 (s). 13C{1H} NMR (CDCl3):
141.4, 141.35, 141.29, 140.74, 140.68, 139.4, 139.0, 137.0, 131.5, 130.8, 129.02, 128.98, 128.9,
128.8, 128.54, 128.51, 128.49, 127.6, 127.3, 126.1, 126.1, 126.0, 41.9, 41.6, 39.2, 38.7. HRMS
(EI) m/z Calcd for C27H25: [M + H]+ 349.1951, found 349.1936.
26

ACCEPTED MANUSCRIPT
(Benzene-1,2,4-triyltris(ethane-2,1-diyl))tribenzene and 1,3,5-triphenethylbenzene (9h and
10h): 9h:10h = 45:55; determined by GC analysis, this product was isolated by flash column
chromatography on silica gel (hexane). Yield 34 mg (53%), yellow oil; IR (DCM): 3025, 2927,
1
2857, 1602, 1495, 1452, 1074, 1029, 746, 699 cm-1. H NMR (CDCl3):
δ
δ
7.35-7.36 (m), 7.26-
142.1, 141.9, 139.7,
7.27 (m), 7.03-7.08 (m), 6.90 (s, 1H), 2.94 (s). 13C{1H} NMR (CDCl3):
139.5, 137.1, 129.7, 129.4, 128.6, 128.5, 128.4, 126.5, 126.4, 126.1, 126.0, 38.22, 38.17, 38.09,
37.9, 37.7, 35.0, 34.6. HRMS (EI) m/z Calcd for C30H31: [M + H]+ 391.2420, found 391.2401.
1,2,4-Tris((allyloxy)methyl)benzene and 1,3,5-tris((allyloxy)methyl)benzene (9i and 10i):
9i:10i = 60:40; determined by GC analysis, this product was isolated by flash column
chromatography on silica gel (hexane). Yield 37 mg (64%), colorless oil; IR (DCM): 2919, 2857,
1720, 1697, 1354, 1157, 1068, 990, 925, 835 cm-1. 1H NMR (CDCl3):
δ 7.34-7.36 (m, 3H), 7.24
(s, 3H), 5.89-5.96 (m), 5.26-5.30 (m), 5.17-5.19 (m), 4.56-4.57 (m), 4.50 (s), 3.99-4.02 (m).
13C{1H} NMR (CDCl3):
δ
138.8, 138.0, 136.7, 135.9, 134.84, 134.81, 129.0, 128.3, 127.1, 126.4,
117.32, 117.29, 117.2, 72.0, 71.9, 71.44, 71.37, 71.3, 71.2, 69.8, 69.6. HRMS (EI) m/z Calcd for
C18H23O3: [M – H]+ 287.1647, found 287.1645.
((Benzene-1,2,4-triyltris(methylene))tris(oxy))tribenzene
and
1,3,5-
tris(phenoxymethyl)benzene (9j and 10j): 9j:10j = 60:40; determined by 1H NMR, this product
was isolated by flash column chromatography on silica gel (hexane). Yield 24 mg (36%), yellow
1
oil; IR (DCM): 3068, 2923, 1594, 1493, 1234, 1172, 1076, 1036, 743, 696 cm-1. H NMR
(CDCl3):
δ
7.63 (s, 1H), 7.56 (d,
J
= 8 Hz, 1H), 7.50 (s), 7.46 (d,
J
= 8 Hz, 1H), 7.28-7.33 (m),
13
6.97-7.01 (m), 5.19 (s), 5.11 (s), 5.10 (s). C{1H} NMR (CDCl3):
δ 158.8, 158.7, 138.1, 137.5,
135.0, 129.67, 129.65, 129.64, 129.3, 128.1, 127.5, 126.1, 121.30, 121.27, 121.20, 121.16, 115.0,
27

ACCEPTED MANUSCRIPT
114.99, 114.97, 114.96, 69.8, 69.6, 68.0, 67.8. HRMS (EI) m/z Calcd for C27H24NaO3: [M +
Na]+ 419.1618, found 419.1608.
N,N',N''-(Benzene-1,2,4-triyltris(methylene))tris(N-methyl-1-phenylmethanamine)
and
N,N',N''-(benzene-1,3,5-triyltris(methylene))tris(N-methyl-1-phenylmethanamine) (9k and
10k): 9k:10k = 59:41; determined by GC analysis, this product was isolated by flash column
chromatography on silica gel (hexane). Yield 40 mg (50%), yellow oil; IR (DCM): 3059, 3028,
1
2931, 2783, 1493, 1452, 1363, 1129, 1026, 738, 700 cm-1. H NMR (CDCl3):
δ 7.22-7.42 (m),
3.64 (d, J δ
= 8 Hz), 3.53 (m), 3.49 (s), 2.18-2.20 (m), 2.12 (s), 2.11 (s). 13C{1H} NMR (CDCl3):
139.7, 139.4, 139.1, 138.1, 137.6, 137.0, 131.0, 130.3, 129.2, 129.13, 129.10, 128.5, 128.33,
128.28, 127.5, 127.03, 127.00, 126.98, 62.49, 62.46, 62.0, 61.93, 61.86, 61.8, 59.5, 59.2, 42.40,
42.37, 42.3. HRMS (EI) m/z Calcd for C33H40N3: [M + H]+ 478.3217, found 478.3208.
((Benzene-1,2,4-triyltris(methylene))tris(oxy))tris(tert-butyldimethylsilane)
and
1,3,5-
tris(((tert-butyldimethylsilyl)oxy)methyl)benzene (9l and 10l): 9l:10l = 69:31; determined by
GC analysis, this product was isolated by flash column chromatography on silica gel (hexane).
Yield 61 mg (72%), Yellow oil; IR (DCM): 3068, 2923, 1594, 1493, 1234, 1172, 1076, 1036,
743, 696 cm-1. 1H NMR (CDCl3):
δ
7.40 (s), 7.36 (d,
J
= 8 Hz, 1H), 7.20 (d,
J
= 8 Hz, 1H), 7.16
141.5,
(s, 3H), 4.76 (s), 4.73 (s), 0.94 (s), 0.93 (s), 0.10 (s), 0.09 (s). C{1H} NMR (CDCl3):
δ
13
126.7, 124.7, 124.6, 122.5, 65.2, 63.0, 29.8, 26.15, 26.13, 26.09, 18.6, -5.08, -5.15. (ESI-MS)
m/z Calcd for C27H54NaO3Si3: [M + Na]+ 533.3, found 533.3.
((Benzene-1,2,4-triyltris(ethane-2,1-diyl))tris(oxy))tris(tert-butyldimethylsilane) and 1,3,5-
tris(2-((tert-butyldimethylsilyl)oxy)ethyl)benzene (9m and 10m): 9m:10m = 60:40;
determined by GC analysis, this product was isolated by flash column chromatography on silica
28