Symmetric diarylacetylenes: One-pot syntheses and solution photoluminescence

Article abstract of DOI:10.1016/j.tetlet.2011.02.062

Bis(tri-n-butylstannyl)acetylene was synthesized and used to create a series of symmetric diarylacetylenes via a one-step Stille coupling protocol with Pd(PPh₃)₄ as the catalyst. In many cases, the product simply crystallized in good yields from the reaction mixture upon cooling after reflux at 100 °C or upon removal of solvent. The diarylacetylenes were studied using UV-vis and fluorescence spectroscopies, which showed that naphthyl- and biphenyl-substituted acetylenes had very high solution-state fluorescence quantum yields.

Full text of DOI:10.1016/j.tetlet.2011.02.062

Tetrahedron Letters 52 (2011) 1960–1963
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Symmetric diarylacetylenes: one-pot syntheses and
solution photoluminescence
a
b,
⇑
Amy E. Brown , Barrett E. Eichler
a
Ozarks Technical Community College, 1001 E. Chestnut Expressway, Springfield, MO 65802, USA
Department of Chemistry, Augustana College, 2001 S. Summit Ave., Sioux Falls, SD 57197, USA
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Bis(tri-n-butylstannyl)acetylene was synthesized and used to create a series of symmetric diarylacetyl-
enes via a one-step Stille coupling protocol with Pd(PPh ) as the catalyst. In many cases, the product
3 4
simply crystallized in good yields from the reaction mixture upon cooling after reflux at 100 °C or upon
removal of solvent. The diarylacetylenes were studied using UV–vis and fluorescence spectroscopies,
which showed that naphthyl- and biphenyl-substituted acetylenes had very high solution-state fluores-
cence quantum yields.
Received 1 February 2011
Accepted 14 February 2011
Available online 19 February 2011
Keywords:
Bis(tri-n-butylstannyl)acetyene
Stille coupling
Ó 2011 Elsevier Ltd. All rights reserved.
Organotin reagents
Optoelectronic materials
Pi-conjugation
Carbon–carbon bond formation through palladium catalysis is a
1
useful tool in the organic chemistry toolbox. Palladium-catalyzed
organotin Stille coupling2 offers gentle reaction conditions and
good yields. Diarylacetylenes are of interest in the fields of natural
products and materials science,4 especially in the field of elec-
tronic materials because of extended conjugation between the aryl
groups through the acetylene. In this investigation, a versatile, one-
pot Stille coupling method is used to create symmetric diarylacet-
ylenes. This work describes facile work-up procedures en route to
3
Scheme 1. Synthesis of diarylacetylenes using Sonogashira method.
the isolation of a series of
p-conjugated diaryl-substituted acety-
lenes and the investigation of the relationship between the diaryl-
acetylene structures and their solution-state absorbance and
optoelectronic properties.
Stille et al.,9a first showed that the three steps of the Sonogash-
ira protocol could be combined into a one-step reaction involving
bis(tri-n-butylstannyl)acetylene (1), an aryl iodide and a palladium
A traditional synthesis of a symmetric diarylacetylene involves
3 2 2
catalyst (CH CN) PdCl . This opened investigations into the utility
5
Sonogashira coupling, completed in three steps. The first involves
of 1 as a reagent for the synthesis of other symmetric diarylacetyl-
coupling an aryl halide to trimethylsilylacetylene using a palla-
dium catalyst (Scheme 1), followed by desilylation, and a second
Sonogashira coupling of the aryl halide. A recent study by Hua
and co-workers described a similar one-pot, three-step synthesis
9
9b
enes. Cummins demonstrated that a less expensive catalyst,
Pd(PPh
and
3
)
4
, could be used to synthesize two bis(anisolyl)acetylenes
bis(benzoyloxyphenyl)-acetylene as precursors to
a
bis(hydroxyphenyl)alkynes. The Pd-catalysis reaction is tolerant
of numerous functional groups and has been used in the synthesis
of acetylenes containing ferrocenyl-type transition metal com-
6
of diarylacetylenes starting from 2-methyl-3-butyn-2-ol. Another
method to make symmetric diarylacetylenes utilizes acetylene gas
under Sonogashira conditions in one step, although the aryl halides
were limited to aryl iodides, E-2-bromostyrene, and bromopyri-
9
a
9b
plexes, as well as organic functional groups such as esters
9
c
10
and amides. Compound 1 has been used extensively as a re-
agent for the synthesis of a number of symmetric diarylacetylenes,
but rarely has there been more than one compound synthesized
per investigation and a systematic study of this coupling reaction
is desired. In this study, we expand the scope of the previously re-
ported Stille cross-coupling reaction with 1 to include a number of
functionalities on the aryl halide precursors.
5
a,7
dine.
More recently, Therien and co-workers described a one-
step diaryl addition to a diboronate-substituted acetylene under
Suzuki–Miyaura conditions.8
⇑
Corresponding author.
E-mail address: barrett.eichler@augie.edu (B.E. Eichler).
0
040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.02.062

A. E. Brown, B. E. Eichler / Tetrahedron Letters 52 (2011) 1960–1963
1961
Using palladium catalysis, we found that 1 was a useful starting
material in synthesizing a library of diarylacetylenes in one-step
reactions. Although it is commercially available, 1 is moderately
expensive and, therefore, does not lend itself to large scale synthe-
ses. We sought an inexpensive, large scale synthetic route to com-
pound 1. Jones and Lappert11 first described the synthesis of 1 from
acetylene gas. We used a modified method by Barton et al., to
affordably synthesize 1. Trichloroethylene was treated with
n-butyllithium to create an intermediate dilithioacetylene, and
compound 1 was produced by reaction of the dilithioacetylene
with tri-n-butylstannyl chloride (Scheme 2). The product was iso-
lated by vacuum fractional distillation in 70–90% yield, routinely in
ð1Þ
were heated to 100 °C with 1 and 1–2 mol % Pd(PPh ) for 1–6 h. All
reactions were monitored by GC–MS and gave air- and room-tem-
perature-stable diarylacetylenes 2–14 in yields from 37% to 96%
Table 1). In general, reactions at 100 °C with aryl iodides were com-
plete in 1 h, whereas most reactions with aryl bromides required at
least 2 h. The only reactants studied that did not yield acetylenes
were chlorobenzene, 1-bromo-2,4,6-trimethylbenzene (bromome-
sitylene) and bromopentafluorobenzene, even after heating at
3 4
1
2
(
5
0 g batches, with the main by-product being the lower boiling
1
00 °C in p-dioxane for 6 h. Aryl chlorides are less reactive than their
monosubstituted tin acetylene (Bu SnC„CH).
3
2
bromo and iodo counterparts and this result is not unexpected. The
mesityl substituent is sterically hindered at the ortho-positions, thus
preventing the addition to the acetylene. In the case of bromopenta-
fluorobenzene, the electronegative fluorines on the bromobenzene
most likely reduce the reactivity toward Stille coupling reactions,
although the fluorine atoms in the 2- and 6-positions are more ste-
rically hindered than the hydrogens in other reactions.
The diarylacetylenes were synthesized utilizing Stille coupling
palladium-catalysis (Eq. 1). Two equivalents of the aryl halide
A major benefit of this general procedure is that many of the
reaction mixtures gave solid, even crystalline products upon cool-
ing of the reaction mixture or upon evaporation of the p-dioxane.
The solids could then be rinsed with hexanes to remove the liquid
tin by-product, Bu
from toluene or CH
could be easily separated from the tin by-product by acidification,
3
SnX (X = Br, I), and subsequently recrystallized
2
Cl /hexanes. Nitrogen-containing acetylenes
2
Scheme 2. Synthesis of bis(tri-n-butylstannyl)acetylene.
Table 1
Solution photophysical data for diarylacetylene 2–14
Yielda (%)
k
max (nm, abs)
b
kmax (nm, em)
c
Ud
Compound
Ar
X
2
3
Cl, Br, I
Br
X=Cl, 0X=Br, 37X=I, 80
96
299
–
369
372
0.007
0.016
4
Br
68
303
370
0.027
5
6
7
8
9
Br
Br
I
78
49
82
74
60
64
308
359, 377
358, 378
357, 373
362
0.061
0.161
0.029
0.369
0.708
0.764
313
313
Br
Br
Br
303, 324
–
–
360, 377
367, 386
1
1
0
1
Br
Br
87
77
337, 359
317, 338
368, 387
350, 368
0.928
0.846
1
1
2
3
Br
Br
45
76
347, 364
375, 393
357, 373
0.370
0.119
1
4
321, 335, 349
a
b
c
Isolated yields.
ꢁ6
Measured in toluene (7.7 ꢀ 10 M).
ꢁ6
Measured in toluene (7.7 ꢀ 10 M) and were excited at 330 nm.
d
ꢁ5
15
Determined with quinine sulfate (1.0 ꢀ 10 M in H
2 4
SO ).

1
962
A. E. Brown, B. E. Eichler / Tetrahedron Letters 52 (2011) 1960–1963
separation and basicification. If the product did not solidify upon
removal of solvent, the reaction mixture was taken up in diethyl
ether and washed with saturated KF(aq), which turned the soluble
tin bromide or iodide into the insoluble tri-n-butyltin fluoride that
could be filtered away. As a last resort to separate the acetylene
from the tin by-product, it was passed through silica gel, although
this was only necessary for the very soluble compounds 2 and 5.
Quantum yields increase in both cases of adding a para-trimethyl-
silyl group to the parent phenyl- or biphenyl-substituted acety-
lenes. The biphenyl derivatives 9 and 10 have quantum yields of
U
PL = 0.708 and 0.764, respectively. These are significantly larger
than the analogous one-ring compounds 2 and 6, and most likely
are a result of extended -conjugation. Compound 11 had the
highest quantum yield ( PL = 0.928) emission and was the most
red-shifted of the strong emitters. This large quantum yield was
similar to that of compound 12 ( PL = 0.846), but by shifting the
p
U
Attempts were made to make bis(4-bromophenyl)acetylene,
a–d
7
,4
a potentially useful starting material for other materials be-
U
cause the bromoaryl moiety is amenable to many further reactions.
Initial attempts to make 7 from 1,4-dibromobenzene resulted in a
mixture consisting of unreacted 1,4-dibromobenzene, bis(4-
bromophenyl)acetylene, bis(4-bromophenylethynyl)benzene, and
insoluble phenylethynylene oligomers. When the starting material
was switched to 1-bromo-4-iodobenzene and the reaction temper-
ature was greater than 80 °C, 7 was efficiently synthesized at 82%
isolated yield, with no evidence of coupling to produce larger
phenylethynylenes. This result illustrates the reactivity difference
between aryl bromides and iodides.
acetylene from the 1- to the 2-position on the naphthyl ring, the
emission kmax decreases by 19 nm. Due to the high quantum yields,
11 and 12 are attractive precursor targets for optoelectronic de-
vices. Upon adding a nitrogen to the naphthylenyl moiety to create
quinolinyl-substituted acetylenes 13 and 14, the quantum yields
decrease significantly (UPL = 0.370 and UPL = 0.119, respectively),
although 13 has the longest wavelength emission kmax of any
acetylene studied here. The quantum yield is higher for the 1-
substituted quinolinyl derivative than it is for the 2-substituted
derivative, as it is for the naphthylenyl-substituted derivatives.
In summary, synthesis of compound 1 was described and it was
shown to be a useful reagent in the synthesis of a variety of sym-
metric diarylacetylenes in moderate to high yields using a one-step
Pd-catalysis reaction from aryl bromides and iodides. Crystalliza-
tion from the reaction mixture in many cases makes this an attrac-
tive synthetic protocol. Solution-state emission spectra of the
acetylenes show trends in intensity and wavelengths that may be
exploited in future design for optoelectronic materials. The
quantum yields of the 1-naphthyl, 2-naphthyl, 4-biphenyl, and
The optoelectronic properties of diarylacetylenes are of interest
1
3
because of the p-conjugation resulting from their structures and
their optoelectronic properties are of interest to researchers in the
fields of organic light-emitting diodes and fluorescent sensors. To
the best of our knowledge, very few systematic investigations of
diarylacetylenes have compared structure to optoelectronic prop-
1
4
erties. The diarylacetylenes absorb and emit primarily in the
ultraviolet region, so our focus was not only to determine which
substituents gave red-shifted emissions, but also to observe which
ones had the highest quantum yields in solution photolumines-
cence. The solution absorbance and photoluminescence properties
of the compounds synthesized in this study are summarized in
Table 1. For this discussion, ‘one-ring’ aromatic systems represent
acetylenes with only one aryl ring (2–8) on either side of the acet-
ylene triple bond such as pyridyl, 4-trimethylsilylphenyl, etc., and
0
4 -trimethylsilyl-4-biphenyl substituted acetylenes were very high
(UPL >0.7), which may be of use to investigators in the field of or-
ganic light-emitting diodes or fluorescent sensing.
Acknowledgments
‘
(
two-ring’ aromatic systems represent those with two aryl rings
9–14) such as naphthyl, quinolinyl, etc.
The apparent first trend in reviewing the UV–vis data is that the
We would like to thank the American Chemical Society—Petro-
leum Research Fund, 41287-B1 for funding and Dr. Steve Lorimor
at Missouri Western State University for recording the 300 MHz
NMR spectra.
one-ring acetylenes absorb light at shorter wavelengths than the
two-ring acetylenes. The one exception is the cyano-substituted
8, which showed two clear absorption maxima, whereas the others
Supplementary data
only exhibited one (or none >295 nm, in the case of 3). The biphe-
nyl-substituted 9 and 10 showed no maxima, although they ab-
sorbed out to approximately 370 nm. The other two-ring
acetylenes showed two or three absorbtion maxima. The absorp-
tion by the two-ring acetylenes also tailed to longer wavelengths
than by the one-ring acetylenes.
Supplementary data (experimental procedures, spectroscopic
data for all new compounds, UV–vis absorption and fluorescence
spectra in toluene) associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.02.062.
1
5
The diarylacetylene solutions were excited with 330 nm light,
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