Regio- and stereoselective dimerization of arylacetylenes and optical and electrochemical studies of (E)-1,3-enynes

Article abstract of DOI:10.1002/adsc.201400062

The N-heterocyclic carbene palladium complex (SIPr)Pd(cinnamyl)Cl [SIPr=N,N'-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene) promotes regio- and stereospecific dimerization of a variety of arylalkynes to give (E)-1,3-enynes in good to excellent yields. An efficient and practical procedure for their synthesis was developed using a biphasic aqueous alkali/heptane system. Optical and electronic properties of (E)-1,3-enynes are highly tunable. Depending on the nature of the substituents, HOMO energies vary in the range 5.3-6.0eV. (E)-1,3-Enynes can exhibit intense photoluminescence in the spectral region 350-500nm.

Full text of DOI:10.1002/adsc.201400062

FULL PAPERS
DOI: 10.1002/adsc.201400062
Regio- and Stereoselective Dimerization of Arylacetylenes and
Optical and Electrochemical Studies of (E)-1,3-Enynes
Oleg S. Morozov,a Andrey F. Asachenko,a Denis V. Antonov,b Vitaly S. Kochurov,b
Dmitry Yu. Paraschuk,b and Mikhail S. Nechaeva,c,*
a
A. V. Topchiev Institute of Petrochemical Synthesis, RAS, Moscow 119991, Russia
International Laser Center and Department of Physics, M. V. Lomonosov Moscow State University, Moscow 119991,
b
Russia
c
Department of Chemistry, M. V. Lomonosov Moscow State University, Leninski gory, 1(3), GSP-1, Moscow 119991,
Russia
Fax : (+7)-495-939-2777; e-mail: m.s.nechaev@org.chem.msu.ru
Received: January 17, 2014; Revised: April 9, 2014; Published online: July 17, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400062.
Abstract: The N-heterocyclic carbene palladium highly tunable. Depending on the nature of the sub-
complex (SIPr)Pd
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
dene) promotes regio- and stereospecific dimeriza- minescence in the spectral region 350–500 nm.
tion of a variety of arylalkynes to give (E)-1,3-
enynes in good to excellent yields. An efficient and
practical procedure for their synthesis was developed Keywords: alkynes; biphasic catalysis; cyclic voltam-
using a biphasic aqueous alkali/heptane system. Op- metry; enynes; N-heterocyclic carbenes; palladium;
tical and electronic properties of (E)-1,3-enynes are UV/vis spectroscopy
Introduction
Transition metal-catalyzed dimerization reactions of
À
terminal alkynes are attractive, atom-economic C C
bond forming reactions that can give conjugated 1,3-
enynes which have been investigated for their antimi-
crobial activity[1] and are important building blocks
for organic synthesis.[2] Enynes have found increasing
application in the construction of electronic and opti-
cal materials[3] and conjugated oligo- and polymers[4].
ꢀ
Dimerization of RC CH is able to produce differ-
ent isomeric products. The most common ones in-
ꢀ
clude the linear (E)-RCH=CHC CR (A), (Z)-RCH=
ꢀ
CHC CR (B), and branched vinyl derivatives CH2=
ꢀ
C(R)C CR (C). Less common are cumulated tri- activity, selectivity, and broad substrate scope are
enes[5] cis-RHC=C=C=CHR (D), and trans-RHC=C= rare. Pd-based catalysts promote the dimerization of
C=CHR (E).
terminal alkynes with high selectivity with moderate
For practical applications in organic synthesis, it is catalyst loadings. A highly selective head-to-tail palla-
necessary to develop a catalytic system that provides dium-catalyzed dimerization of terminal alkynes was
only the desired isomer for a wide range of substrates. developed by Trost[7l,m] using
a combination of
Many organometallic catalysts, employing early,[6] late Pd
ACTHUNTRGNE(UGN OAc)2 (2 mol%) and the bulky electron-rich
transition metals,[4,7] or rare earth metals[8] were re- ligand tris(2,6-dimethoxyphenyl)phosphine (TDMPP).
ported to catalyze the dimerization of terminal alk- Head-to-head palladium-catalyzed dimerization of
ACHTUNGTRENNUNGynes. Examples of catalysts that combine high catalyst terminal alkynes has been achieved with high regio-
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Oleg S. Morozov et al.
Table 1. The choice of Pd catalyst for phenylacetylene di-
and (E)-stereoselectivity, using the Pd2ACTHUNTRGNE(UGN dba)3·CHCl3/
merization.[a]
TDMPP catalytic system.[7g] Mixtures of regio- and
stereoisomeric enynes with dominant E-isomer were
obtained when using a PdACHTNUTRGENNG(U OAc)2/imidazolium chlo-
ride catalytic system.[7f] However, these catalytic sys-
tems give good yields only in the case of ortho-unsub-
stituted arylacetylenes. An improved Pd-mediated
protocol using a bis-N-heterocyclic carbene palladium
complex (2 mol%) in the presence of TDMPP was re-
cently reported.[7a] In this contribution, we report on
a practical Pd-catalyzed regio- and stereoselective
synthesis and studies of the optical and electrochemi-
cal properties of the resulting (E)-1,3-enynes.
Results and Discussion
Synthesis
Previously we reported that NHC palladium com-
plexes efficiently catalyze Suzuki–Miyaura reactions
in water as a solvent.[9] We attempted to utilize a relat-
ed approach in the Sonogashira reaction, the coupling
of acetylenes with aryl halides.[10]
Surprisingly, we found no formation of coupling
products in reaction of phenylacetylene and phenyl
bromide in water. Instead, exclusive formation of the
head-to-head dimer of phenylacetylene with high (E)-
stereoselectivity was observed. Thus, we decided to
study this reaction in more detail. Initially, we investi-
gated the activity of a series of NHC and expanded
ring NHC palladium complexes. A catalytic system
Entry
[Pd]
Isolated yield
1
2
3
4
5
6
A
E
E
E
G
A
N
53%
77%
67%
12%
63%
31%
ACHTUNGTRENNUNG
[a]
Reaction conditions: phenylacetylene (5 mmol), Pd
(0.25 mol%), KOH (15 mmol) in water (5 mL) at reflux
temperature for 1 h.
Abbreviations:
4,5-dihydroimidazol-2-ylidene, SIMes=N,N’-bis(2,4,6-tri-
methylphenyl) 4,5-dihydroimidazol-2-ylidene, THP-
consisting of 0.25 mol% [(NHC)PdACTHNUGRTENUNG(cinn)Cl] and
3 equiv. of KOH in refluxing water was used
(Table 1).
[b]
SIPr=N,N’-bis(2,6-diisopropylphenyl)-
Complexes with SIPr, THP-Mes and THD-Mes li-
gands promote the dimerization of phenylacetylene to
give the trans-1,4-disubstituted enyne as the major
product. Reactions, catalyzed by complexes bearing
SIMes, THP-Dipp and THD-Dipp ligands, lack selec-
tivity: the reaction products are mixtures of dimer,
trimer and heavier oligomers. Notably, the formation
of the dimer is regio- and stereoselective in all cases.
The only dimer product is (E)-1,4-diphenylbut-1-en-3-
yne. No formation of (Z)- (head-to-head) (B) or gem-
(head-to-tail) (C) dimers was observed.
Dipp=1,3-bis(2,6-diisopropylphenyl)-3,4,5,6-tetrahydro-
pyrimidin-2-ylidenes, THP-Mes=1,3-bis(2,4,6-trimethyl-
phenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidenes,
Dipp=1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydro-
diazepin-2-ylidenes, THP-Mes=1,3-bis(2,4,6-trimethyl-
phenyl)-3,4,5,6-tetrahydrodiazepin-2-ylidenes.
THD-
dimer fall in the range 62–82% upon variation of the
Further optimization of catalytic system was done catalyst loading from 0.125 to 1 mol% (entries 7–11).
using (SIPr)Pd(cinn)Cl as precatalyst. Variation of the A mixture of phenylacetylene and water is a two-
ACHTUNGTRENNUNG
alkali and catalyst loadings, reaction times and addi- phase system. It was decided to add ethanol for better
tives of a different nature was performed (Table 2). It penetration of phenylacetylene into the aqueous
was found that addition of an appropriate amount of phase. However, this led to a significant drop in yield
alkali is obligatory for the reaction to proceed. No re- of the (E)-enyne and predominant formation of oligo-
action was observed when 0.1 equiv. of KOH was mers (entry 13).
used (entry 1). Variation of the alkali amount from
Analogously, addition of 10 mol% of a phase-trans-
0.2 to 5 equiv. leads to variation of product yield from fer catalyst (Bu4NBr) resulted in full conversion of
63% to 82%. The maximum yield was reached when phenylacetylene into oligomers. It was supposed that
3 equiv. of KOH were used (entries 2–6). Yields of the reaction does not stop at the formation of a (E)-
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Regio- and Stereoselective Dimerization of Arylacetylenes
Table 2. Optimization of the reaction conditions for the Pd-
H2O:heptane=5: 3), acetylenes of different natures
are dimerized regio- and stereoselectively in good to
excellent yields. High performance of catalytic system
was found in reactions of sterically crowded (entry 3),
donor- (entries 4, 5, 9) and acceptor- (entries 6–8) sub-
stituted, polycyclic aromatic (10–12) and heteroaro-
matic (entries 13 and 14) acetylenes. Notably, ortho-
substituted substrates (entries 3, 5, 7, 10–12) are di-
merized efficiently.[7g] Unlike arylacetylenes, aliphatic
alkynes did not give the corresponding head-to-head
dimers. NMR analysis showed only traces (<1%) of
such products in the cases of 1-heptyne, 1-octyne and
2-methylbut-3-yn-2-ol. Comparison of our results with
previous works shows that the use of our new catalyt-
ic system enables us to obtain products in significantly
higher yields at lower (0.25 mol% vs. 2 mol%) cata-
lyst loadings.
catalyzed phenylacetylene dimerization.[a]
Entry
[Pd]
[mol%]
KOH
[equiv.]
Additive
Isolated
yield [%]
G
N
1
2
3
4
5
6
7
8
0.5
0.5
0.5
0.5
0.5
0.5
0.125
0.25
0.375
0.5
1,0
0.5
0.5
0.5
0.25
0.25
0.1
0.2
0.5
1
3
5
3
3
3
3
3
3
3
3
3
3
–
–
–
–
–
–
–
–
–
–
–
–
0[b]
65
69
75
82
63
62
77
78
82
64
67[c]
9
9
10
11
12
13
14
15
16
1 mL of EtOH
10 mol% TBAB
3 mL of heptane
3 mL of toluene
0[d]
>99
97
Optical Data
[a]
Reaction
conditions:
phenylacetylene
(5 mmol),
(SIPr)PdACHTUNGTRENNUNG(cinn)Cl (n mol%), KOH (m equiv.) in water
(5 mL) at reflux temperature for 30 min.
No reaction was observed.
Reaction time increased to 6 h.
Total conversion of phenylacetylene, complex mixture of
oligomers was observed.
All enynes under study show optical absorption and
photoluminescence (PL) in the UV and the blue part
of the visible spectral range. Figure 1a shows absorp-
tion and PL spectra of the parent enyne 2a in ODCB
(ortho-dichlorobenzene) and THF. Optical data in
ODCB are similar to those of a structurally analogous
conjugated oligomer, 1,4-diphenyl-1,3-butadiene,[11]
[b]
[c]
[d]
enyne and proceeds further to form oligomers. which has nominally the same conjugation length.
Indeed, increasing the catalyst loading from 0.5 to Specifically, absorption and PL spectra are vibration-
1 mol% leads to a reduction in yield from 82% to ally structured and demonstrate nearly mirror symme-
64% (entries 10 and 11). Similarly, with increasing re- try at a wavelength of 350 nm. Interestingly, the opti-
action time from 30 min to 6 h, the yield drops from cal spectra of 2a are strongly different in THF
82% to 67% (entries 10 and 12). Evidently, the de- (broken lines in Figure 1a): the PL is blue shifted at
sired product should be isolated from the catalytic ~0.7 eV and becomes structureless; the main absorp-
system. The use of high-boiling non-polar solvents tion band at ~320 nm in ODCB almost disappears
might solve this problem. In fact, addition of heptane and shifts in blue by ~1 eV. Very similar absorption
or toluene leads to an increase in yields to virtually and PL spectra were observed for enynes 2b, 2d, 2e,
quantitative (entries 15 and 16).
and 2i bearing donor substituents (not shown, see the
As a result, we developed a catalytic system that Supporting Information).
promotes the regio- and stereoselective dimerization
The optical spectra of 2c in ODCB and THF are
of terminal acetylenes to give 1,4-disubstituted (E)- similar (Figure 1b). PL in THF shows a small blue
enynes with the lowest Pd-catalyst loading published shift (~60 meV), and absorption demonstrates a blue
to date. Additionally, the developed protocol has sev- shift of ~0.2 eV comparing to ODCB solution. We
eral significant advantages: (i) the reaction is held could assign the strongly blue shifted optical spectra
under aerobic conditions, the use of inert atmosphere of the diarylenynes with donor substituents to their
and absolute solvents is not necessary; (ii) inexpensive strong aggregation in THF. One might suggest that
and easily recoverable aqueous alkali KOH is used, enynes form H-type aggregates so that their transition
for comparison, expensive and irrecoverable TDMPP dipole moments are almost cancelled. As a result, the
[tris(2,6-dimethoxyphenyl)phosphine] was used pre- lowest optical transition is strongly suppressed, and its
viously;[7a] (iii) due to utilization of biphasic media oscillator strength shifts to the blue region.
isolation of product in high purity is simple.
The much weaker solvent effect in mesityl-substi-
To determine the scope and limitations of the de- tuted enyne 2c could be explained by its bulky struc-
veloped protocol we performed catalytic tests for a va- ture, which may impede the aggregation. ODCB
riety of arylacetylenes (Table 3). It was found that seems to be a good solvent for the enynes as com-
under optimized reaction conditions [0.25 mol% pared to THF because of the aromatic nature of the
(SIPr)PdACHTUNGTRENNUNG(cinn)Cl, 3 equiv. of KOH, mixture of ODCB molecule. One should note that the absorption
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Oleg S. Morozov et al.
Table 3. Dimerization of terminal alkynes to (E)-enynes catalysed by SIPrPdACTHUNTGRENNG(U cinn)Cl/
KOH system in H2O:heptane.
[a]
Isolated yield. Reaction conditions: arylacetylene (3 mmol), (SIPr)PdACTHNUGRTNEUNG(cinn)Cl (0.25–
0.5 mol%), KOH (3 equiv.) in the mixture water (5 mL)-heptane (3 mL) at reflux
temperature.
[b]
[c]
Previously reported[7a] isolated yields with catalyst loading of 2 mol% Pd are in paren-
theses.
Previously reported[7a] isolated yields of (E)-1,4-di(2,6-dimethylphenyl)butenyne are
in parentheses.
[d]
[e]
5 mL of toluene were used instead of heptanes
0.5 mol% of (SIPr)PdACTHNUGRTNEUNG(cinn)Cl was used
of 2c in ODCB does not show any fine structure in absorption of 2c in THF is fine structured. This sug-
contrast to the that of parent enyne 2a (Figure 1a). gests that the rigidity of enyne 2c in the electronic
This could be due to a softer ground state geometry ground state is noticeably different in ODCB and
of 2c than 2a because of steric repulsion between the THF because of aggregation.
hydrogen atoms of methyl and vinyl groups. However,
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Regio- and Stereoselective Dimerization of Arylacetylenes
Figure 1. Absorption (thick lines) and photoluminescence (thin lines) spectra of enynes 2a (a) and 2c (b). Solid and broken
lines show data for solutions in ODCB and THF, respectively. Absorption in ODCB is shown for wavelengths longer than
300 nm because of strong solvent absorption. Concentration is below 15 mM. The maximum extinction of 2a is about 30,000
(Mcm)À1.
Interestingly, the optical spectra of enyne 2f (with aggregated and individual species (Figure 1Sb in the
Cl substituents in the para position) in THF are very Supporting Information). Enyne 2n (with pyridine
similar to those of the parent enyne 2a in ODCB (see end groups) in THF demonstrates a smooth absorp-
Figure 1Sa in the Supporting Information), that is, tion edge that is similar to the parent enyne 2a and
enyne 2f does not show signatures of noticeable ag- a very weak structureless PL centered at 380 nm.
gregation in THF. On the other hand, optical spectra
The optical absorption and PL spectra of enynes 2j
of enyne 2g (not shown, see the Supporting Informa- and 2k are presented in Figure 2. The optical spectra
tion), which is an isomer of 2f, indicate considerable inherit the main features of the end groups. Indeed,
aggregation in THF.
the absorption spectrum of 2j in Figure 2a shows two
This behaviour of chloro-substituted derivatives of maxima at 230 and 350 nm and resembles the naph-
2a in different solvents deserves further studies. The thalene spectrum, which has a strong band at
optical spectra of enyne 2m with thiophene end ~220 nm and a weak band at ~270 nm.[12] However,
groups also do not show any essential aggregation in the weak band of 2j is red shifted and the extinction
THF (Figure 1Sd in the Supporting Information). per naphthalene moiety is about one order of magni-
Nearly the same spectra were recorded in ODCB. tude higher than that of the naphthalene band. In ad-
The optical spectra of enyne 2h with nitro-substitu- dition, the PL is red shifted by about 1 eV as com-
ents in the phenyl ring corresponds to a mixture of pared to naphthalene.[13] These differences indicate ef-
Figure 2. Absorption (thick lines) and photoluminescence (thin lines) spectra in THF (solid) and DCB (broken). (a) Enyne
2j at a concentration of 6.6 mM. Molar extinction at 230 and 350 nm is 2.6·105 and 0.55 105 (Mcm)À1, respectively. (b) Enyne
2k at a concentration of 5 mM. Molar extinction at 257 and 420 nm is 2.7·105 and 0.32 105 (Mcm)À1, respectively.
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Table 4. HOMO energies of enynes evaluated from CV
data.
ficient p-electron communication between the naph-
thalene moieties through the enyne bridge. The sol-
vent effect in the optical spectra of 2j is weak as seen
in Figure 2a. Similar spectral features demonstrate
that enyne 2k has anthracene end groups. The absorp-
tion spectrum of 2k (Figure 2b) also shows weak and
Entry
#
ÀEHOMO [eV]
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
5.93
5.77
5.71
5.43
5.46
5.90
6.02
5.40
5.41
5.88
5.29
5.36
5.42
5.44
strong bands similar to those of anthracene.[14]
A
strong band at 255 nm is nearly at the same position
as for anthracene, whereas the weak band is red shift-
ed by about 0.3 eV. Similarly to 2j, the molar extinc-
tion of the weak band per anthracene moiety is con-
siderably higher in 2k than in anthracene. The PL
spectrum of 2k is also red shifted by 0.2 eV relative to
that of anthracene.[14] Analogous trends are exhibited
in the UV and PL spectra of enyne 2l bearing phenan-
threne end groups (Figure 1Sc in the Supporting In-
formation). Thus, it can be concluded that absorption
and PL spectra of enynes with the condensed aromat-
ic end groups are similar to those of the free aromatic
compounds (naphthalene, anthracene, and phenan-
9
10
11
12
13
14
threne). The observed red shifts can be assigned to ef- 2n) show higher redox potentials than 2a, their
ficient p-electron communication between the end HOMO energy is ~5.4 eV.
groups through the enyne bridge.
Enynes bearing condensed aromatic groups (2j–l)
Of note, all enynes, except for heteroaromatically have higher HOMO energies than the parent 2a. This
substituted 2m and 2n, show high PL intensity. can be assigned to higher p-electron delocalization.
Indeed, PL intensities are at least one order of magni- The highest HOMO energy (5.29 eV) was found in 2l
tude higher than the background solvent signals even bearing anthracenyl groups. The HOMO energy in 2l
at micromolar concentrations. If not aggregated, is higher than that in free anthracene by about 0.5 eV
enynes 2a–i, bearing substituted phenyl end groups, due to p-electron delocalization via the enyne bridge.
have PL in the spectral range 350–450 nm. Enynes
bearing condensed aromatic end groups (2j–l) show
PL in the blue-green spectral region.
Conclusions
In conclusion, N-heterocyclic carbene palladium(p-
cinnamyl) chloride complexes catalyze the regio- and
stereoselective dimerization of terminal arylalkynes in
CV Data
To evaluate the HOMO energies of enynes, one-elec- aqueous alkali to give 1,4-disubstituted (E)-enynes in
tron electrochemical oxidation was studied using the good yields. A protocol using biphasic water/heptane
CV technique. All compounds show reversible one- system and (SIPr)PdACTHNUGRTNEUNG(cinn)Cl as a precatalyst is highly
electron oxidation (Table 1S in the Supporting Infor- efficient for the dimerization of a variety of aryl and
mation). The HOMO energies calculated from the heteroaryl terminal alkynes.
CV data are in the range 5.29–6.02 eV (Table 4).
Enynes with substituted phenyl end groups have
Introduction of donor substituents (Me, OMe) in the main absorption band in the UV range and show
2b–e leads to an increase of their HOMO energies by strong PL intensity in the spectral range 350–450 nm.
up to 0.5 eV compared to the parent enyne 2a. How- The optical spectra of enynes with condensed aromat-
ever, introduction of acceptor substituents (Cl and ic end groups inherit the main features of the end
NO2) in 2f–h does not lead to the expected lowering groups but the spectra are red-shifted due to efficient
of their HOMO energies. Thus, only the o-Cl-substi- p-electron communication via the enyne bridge.
tuted enyne (2g) demonstrates the lower HOMO HOMO energies of enynes evaluated from the CV
energy (by 0.1 eV) than the reference, 2a. On the data vary in the range 5.3–6 eV depending on the
other hand, the HOMO energy of p-Cl-substituted 2f nature of the end groups.
is about that of the parent 2a, and p-NO2-substituted
The deep-blue and UV luminescence of enynes and
enyne (2h) is higher by 0.53 eV. This counterintuitive adjustable HOMO energies could be promising for
effect of acceptor groups could be attributed to a high light emitting devices such as organic light-emitting
contribution of the mesomeric factor, which results in diodes (OLEDs) and organic light-emitting transistors
extended p-conjugation and an increased HOMO (OLETs). From the optical spectra we suggest that
energy. Enynes with heterocyclic end groups (2m and some enynes with substituted phenyl end groups can
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Regio- and Stereoselective Dimerization of Arylacetylenes
100 mM) as the electrolyte and the sample (10 mM) in ace-
tonitrile at room temperature. The platinum electrode fabri-
cated in a glass rod (0.8 mm diameter) was used as the
working electrode. The counter electrode was a platinum
wire. Ag/AgCl electrode in 3.5M solution of KCl was used
as the reference electrode. CVs were recalculated relative to
Fc/Fc+. HOMO energies were found using the calculated
be strongly aggregated in solution at low concentra-
tions. This could be interesting for the design of self-
assembling supramolecular structures for various ap-
plications.
Experimental Section
oxidation potential (Eonset vs
. Fc/Fc+) and the Fc/Fc+ as an ex-
ternal redox standard with the equation: EHOMO =e Eonset vs
.
*
General Information
+ EFc/Fc+vsc, where EFc/Fc+vac =4.8 eV.
FC/FC+
Unless otherwise stated, all manipulations were carried out
under ambient atmosphere. Water was distilled prior to use.
Other chemicals and solvents were obtained from commer-
cial sources and used without further purification. Palladium
complexes were prepared by the method published earlier.[9]
Analytical thin-layer chromatography was performed using
silica gel 60 F254, 0.25 mm pre-coated TLC plates. Com-
pounds were visualized using 254 nm and/or 365 nm UV
wavelength. NMR spectra were acquired on Bruker Avance
II 600 spectrometer equipped with a 5 mm inverse probe-
heads with Z-gradient coil. All deuterated solvents were
stored over molecular sieves (4 ꢁ). Chemical shifts are re-
ported in parts per million relative to residual solvent sig-
nals. Coupling constants J are given in Hertz as positive
values regardless of their real individual signs. Multiplicity
of signals is indicated as “s”, “d”, or “m” for singlet, dou-
blet, or multiplet, respectively. Abbreviation “br” is given
for broadened signals. Elemental analyses were performed
on a Carlo Erba EA1108 CHNS-O elemental analyzer at
the Institute of Petrochemical Synthesis, Russian Academy
of Sciences.
Acknowledgements
We thank E.A. Shirhin and G.S. Budylin for their help in re-
cording the optical data.
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General Procedure for Dimerization of Terminal
Alkynes
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All manipulations were carried out under ambient atmos-
phere. To 3 mmol of ethynylarene, 4–8 mg (0.25–0.5 mol%,
see Table 3) of SIPrPdACTHNUTRGNE(UNG cinn)Cl and 504 mg of KOH
(9 mmol) were added 5 mL of water and 3 mL of heptane
(or toluene, see Table 3). The mixture was stirred and re-
fluxed for the time reported in Table 3. After cooling to am-
bient temperature 20 mL of dichloromethane were added to
the reaction mixture, the organic phase was separated and
the solvents were evaporated under reduced pressure. The
thus obtained crude product was purified by column chro-
matography on silica gel.
Optical Data
Optical transmission spectra were recorded using a double
beam spectrophotometer “Lambda 25” (Perkin–Elmer) with
slit widths of 2 nm. Photoluminescence spectra were mea-
sured using a spectrofluorimeter “Fluoromax-4” (Horiba) at
slit widths of 5 nm. The excitation wavelengths were usually
250 or 300 nm. All optical spectra were recorded using an
optical cell with a thickness of 1 cm.
Cyclic Voltammetry
CV was measured using a potentiostat-galvanostat mAUTO-
LAB Type III with a three-electrode cell at a scan rate of
100 mVsÀ1 with a solution of lithium perchlorate (LiClO4,
Adv. Synth. Catal. 2014, 356, 2671 – 2678
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