Coupling of 3,8-dibromo-1,10-phenanthroline with 3,5-diethynylheptyloxybenzene: A Suzuki/Miyaura versus a Sonogashira perspective

Article abstract of DOI:10.1081/SCC-120023989

We report a new application of the Suzuki-Miyaura reaction whereas two bifunctional reactants, 3,8-dibromo-1,10-phenanthroline and 3,5-diethynylheptyloxylbenzene (9), yield 3,8-bis(3-ethynyl-5-heptyloxyphenylethynyl)-1,10-phenanthroline (2) efficiently (7

Full text of DOI:10.1081/SCC-120023989

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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 19, pp. 3317–3325, 2003
Coupling of 3,8-Dibromo-1,10-phenanthroline
with 3,5-Diethynylheptyloxybenzene:
A Suzuki/Miyaura Versus a
Sonogashira Perspective
Jinhua Yang,1 Woon Su Oh,1 Ian A. Elder,1 Nicholas Leventis,2
and Chariklia Sotiriou-Leventis1,
*
1Department of Chemistry, University of
Missouri-Rolla, Rolla, Missouri, USA
2NASA Glenn Research Center, Materials Division,
Cleveland, Ohio, USA
ABSTRACT
We report a new application of the Suzuki-Miyaura reaction whereas
two bifunctional reactants, 3,8-dibromo-1,10-phenanthroline and 3,5-
diethynylheptyloxylbenzene (9), yield 3,8-bis(3-ethynyl-5-heptyloxy-
phenylethynyl)-1,10-phenanthroline (2) efficiently (74% yield)
without polymerization. This was achieved by reacting a stoichio-
metric amount of 9 and (Me3Si)2NLi to obtain quantitatively the
*Correspondence: Chariklia Sotiriou-Leventis, Department of Chemistry,
University of Missouri-Rolla, 21000 Brookpark Road, Rolla, MO 65409, USA;
Fax: (573)341-6033; E-mail: cslevent@umr.edu.
3317
DOI: 10.1081/SCC-120023989
Copyright & 2003 by Marcel Dekker, Inc.
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3318
Yang et al.
monoacetylide anion of 9 (10). The latter was activated with
B-methoxy-9-BBN and reacted in analogy to the alkynyl copper
complex of a Sonogashira route. However, in the Sonogashira
reaction, the alkynyl copper complex is present in small equilibrium
concentrations and polymerization takes place even when reagents
are mixed slowly. Actually the Sonogashira route gave no desired
product 2, as the latter polymerizes easily via homo-coupling in the
presence of air and Cu(I).
Key Words: Suzuki; Miyaura; Sonogashira; Coupling; 3,8-
Dibromo-1,10-phenanthroline.
Sonogashira coupling involves the palladium(0) catalyzed reaction of
terminal alkynes with vinyl or aryl halides in the presence of CuI or CuBr
and an amine such as triethyl or diisopropyl amine.[1] Mechanistically,
oxidative addition of the organic halide gives a Pd(II) intermediate,
which undergoes transmetallation with the alkynyl copper (generated in
small equilibrium quantities from the terminal alkyne, base, and copper
iodide). Reductive elimination with coupling of the two organic ligands
gives the product and regenerates the Pd(0) catalyst. For convenience,
oftentimes the palladium catalyst is introduced as a stable, soluble Pd(II)
derivative such as Pd(PPh3)2Cl2. This complex is rapidly reduced by the
terminal alkyne in situ to give the catalytic Pd(0) species. Due to the mild
conditions usually employed (e.g., reaction at room temperature) and the
use of inexpensive reagents, the Sonogashira coupling reaction has been a
very convenient synthetic method for the preparation of aromatic alkynes.
Despite its convenience, the Sonogashira reaction cannot be used
for coupling of dialkynes with dibromoaromatic compounds. First,
bifunctionality on both reactants generally leads to polymerization.
Furthermore, even though it might be considered that by using slow
syringe-pump addition of the dialkyne might lead to the desirable
product, unfortunately during work-up, the bisacetylenic product itself
tends to undergo oxidative coupling in the presence of air and Cu(I),
known as homocoupling,[2] leading again to polymerization.
In brief, the main causes of polymerization during the Sonogashira
coupling reaction are: (a) the generation of only a small equilibrium
amount of the alkynyl anion, and (b) the presence of catalytic amounts
of copper (related to homocoupling). It seems that if the alkynyl anion
could be generated quantitatively and activated by means other than
copper, polymerization would be avoided.
An alternative procedure that also leads to aryl alkynes is the
Suzuki/Miyaura coupling reaction.[3] According to that procedure,

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Suzuki/Miyaura Vs. Sonogashira Perspective
3319
the alkynyl anion is formed quantitatively by the reaction of a terminal
alkyne with a stoichiometric amount of a strong base (e.g., (Me3Si)2NLi).
Subsequently, the alkynyl anion is activated via reaction with B-methoxy-
9-borabicyclo[3.3.1]nonane (B-methoxy-9-BBN) to form a stable methoxy
(alkynyl)borate complex, which subsequently reacts with an aryl bromide
in the presence of Pd(0) to yield identical products with the correspond-
ing Sonogashira coupling reaction that involves the same alkyne and aryl
bromide starting materials. The Suzuki/Miyaura route uses harsher con-
ditions (strong base) and a more expensive reagent (B-methoxy-9-BBN).
However, it involves quantitative formation of the alkynyl anion and it
does not use copper salts, so polymerization with bifunctional terminal
halides and dibromoaromatic compounds should not be an issue.
These inferences were confirmed by comparing the synthesis of 3,8-
bis(4-isonicotynoylphenylethynyl)-1,10-phenanthroline (1) with the
synthesis of 3,8-bis(3-ethynyl-5-heptyloxyphenylethynyl)-1,10-phenan-
throline (2). Compound 1 was designed as a ligand for intramolecular
electron transfer quenching, while metal complexes of compound 2 were
designed as building blocks of larger conjugated systems.
O
C7H15
C7H15O
O
O
N
N
N
N
N
N
2
1
H
H
Sonogashira coupling of 4-( p-bromobenzoyl)pyridine (3) with 3,8-
bisethynyl-1,10-phenanthroline (4, prepared also via Sonogashira cou-
pling of TMS-protected acetylene and 3,8-dibromo-1,10-phenanthroline)
proceeds uneventfully giving 1 in 32% yield (Sch. 1).[4] On the other hand,
directSonogashira coupling of 3,8-dibromo-1,10-phenanhtroline wiht
3,5-diethynylheptyloxybenzene (9) under the same conditions led to an
intractable polymer.
Compound 9 was synthesized according to Sch. 2. Pentabromophenol
(5) was dehalogenated in benzene by AlCl3 to give 3,5-dibromo-
phenol (6),[6] which then reacted with 1-bromoheptane to yield 3,5-
dibromoheptyloxylbenzene (7). A directSonogashira coupling of 7 with
O
Pd(PPh ) Cl , CuI
3 2 2
+
1 (32%)
Br
N
N
HN(i-Pr) , THF
2
reflux, 24 h
N
3
4
Scheme 1.

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3320
Yang et al.
OH
OCH (CH ) CH
OH
2
2 5
3
Br
Br
Br
Br
AlCl , Benzene
3
BrCH (CH ) CH
3
2
2 5
K CO , acetone
Br
Br
reflux, 3h
Br
Br
(88%)
2
3
Br
reflux, overnight
5
6
7 (84%)
TMS
Pd(PhCN) Cl ,
2
2
CuI, P(t-Bu)
3
HN(i-Pr) , dioxane,
2
R.T., 2h
OC H
7
15
OCH (CH ) CH
2
2 5
3
NaOH
MeOH, Acetone
H
H
TMS
TMS
8 (89%)
9 (98%)
Scheme 2.
-
H
OCH
3
B
OC H
7
15
(Me Si) NLi
3
2
+
9
Li
OC H
7 15
H CO
3
0
0
-78 C, THF
B
-78 C, THF
H
Li
10
11
Pd(PPh )
THF, reflux 24 h
3 4
Br
Br
N
N
2 (74%)
Scheme 3.
trimethylsilylacetylene catalyzed by Pd(PhCN)2Cl2 with P(t-Bu)3 in the
presence of CuI and diisopropylamine at room temperature gave
compound 8, which was subsequently deprotected in the presence of
NaOH in CH2Cl2/MeOH to give compound 9 in 65% overall yield from 5.
Of course, protection of one of the acetylenic groups of 9 could be a
viable route for the synthesis of 2 via the Sonogashira reaction, however
protection would introduce two more steps and would most certainly
compromise the overall yield. Interestingly, both the first step of a
possible protection route for the synthesis of 2, and the first step of the
Suzuki/Miyaura route (Sch. 3) involve quantitative formation of the
monoacetylide 10 by using a stoichiometrric amount of a strong base.

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Suzuki/Miyaura Vs. Sonogashira Perspective
3321
In the Suzuki/Miyaura route, 10 is activated with B-methoxy-9-BBN
to form a stable methoxy(alkynyl)borate complex 11, which is then
added to a solution of 3,8-dibromo-1,10-phenanthroline and a catalytic
amountof Pd(PPh ) in THF. Intermediate 11 reacts in analogy to the
3 4
alkynyl copper complex of the Sonogashira route yielding 2 in very good
yield (74%).
In conclusion, we have shown that coupling via the Suzuki/Miyaura
route of two bifunctional reagents, namely 3,8-dibromophenanthroline
and 3,5-diethynylheptyloxybenzene (9) gives compound 2 efficiently
without polymerization.
EXPERIMENTAL
General. 1H and 13C NMR spectra were recorded on a Varian
INOVA 400 NMR spectrometer and are reported as parts per million
(ppm) from TMS (ꢀ). Melting points were uncorrected. The
Electrospray Mass Spectometry (ESI) was conducted using a TSQ7000
(ThermoFinnigan) with an API2 source and the ‘‘performance pack’’
(wider aperture on the skimmer and extra turbo pump on the source).
The electrospray source is equipped with a stainless steel needle; the
voltage was 4.5 kV. Sample solutions were injected using a 500 mL
Gastight syringe (Hamilton) in a Pump 11 syringe pump (Harvard
Apparatus).
Materials. All starting materials, reagents and solvents were from
Aldrich or Acros and used as received, unless noted otherwise. P(t-Bu)3
was purchased from Strem Chemicals. For coupling reactions, all the
solvents were anhydrous and were obtained directly from Aldrich or
Acros.
3,8-bis(4-Isonicotinoylphenylethynyl)-1,10-phenanthroline (1). 4-bromo-
benzoylpyridine[7] (3, 0.68 g, 2.6 mmol), 3,8-diethynyl-1,10-phenan-
throline[8] (4, 0.2 g, 0.88 mmol), Pd(PPh3)2Cl2 (60 mg, 0.085 mmol), and
CuI (64 mg, 0.34 mmol) were added to anhydrous diisopropylamine
(8 mL) and anhydrous DMF (40 mL) under nitrogen and the reaction
mixture was refluxed for 24 h. After evaporation of the solvent under
vacuum, the residue was diluted with CH2Cl2, washed with 5% aqueous
KCN solution (3 ꢀ 20 mL), water (5 ꢀ 100 mL), and saturated aq. NaCl
(100 mL). The organic phase was dried over anhydrous sodium sulfate,
concentrated in vacuo followed by the addition of diethyl ether. The solid
thus obtained was recrystallized from CH2Cl2/THF to give the product
(0.17 g, 32%). M.p. 266ꢁC (dec.). H NMR (400 MHz, CDCl3) 9.32 (d,
1
J ¼ 1.83 Hz, 2H), 8.85 (d, J ¼ 5.68 Hz, 4H), 8.45 (d, J ¼ 1.83 Hz, 2H), 7.89

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3322
Yang et al.
(s, 2H), 7.86 (d, J ¼ 5.49 Hz, 4H), 7.76 (d, J ¼ 8.06 Hz, 4H), 7.60
(d, J ¼ 5.68 Hz, 4H). MS (ESIþ) 591.1816 (M þ H)þ, (theoretical ionic
mass: 590.1743).
3,5-Dibromophenol (6).[6] Aluminum chloride (2.67 g, 0.02 mol)
was added to a solution of pentabromophenol (5, 5.0 g, 0.01 mol) in
anhydrous benzene (20 mL) and the reaction mixture was heated to
reflux for 3 h under nitrogen. After cooling to room temperature, the
reaction mixture was poured into a mixture of HCl and ice water. The
organic layer was separated and the water layer was extracted with
diethyl ether (3 ꢀ 20 mL). The combined organic layers were extracted
with 10% aqueous NaOH (3 ꢀ 20 mL). The combined aqueous layers
were made strongly acidic with HCl. The product was then extracted
with methylene chloride (3 ꢀ 20 mL) and the combined organic layers
were washed with water until the aq. layer was neutral and then dried
over anhydrous Na2SO4. Evaporation of the solvent in vacuo, gave the
crude product which was recrystallized from hexane to give off-white
needles (2.3 g, yield: 88%). M.p. 81–82ꢁC (Lit.[6a] m.p. 80ꢁC, Lit.[6b]
m.p. 76–79ꢁC). 1H NMR (400 MHz, CDCl3) 4.85 (s, 1H), 6.95 (d,
J ¼ 1.65 Hz, 2H), 7.25 (t, J ¼ 1.65 Hz, 1H). 13C NMR (100 MHz,
CDCl3) 156.4, 126.8, 123.1, 117.8.
3,5-Dibromoheptyloxylbenzene (7). A mixture of 3,5-dibromophenol
(6, 5.0 g, 0.02 mol), potassium carbonate (2.76 g, 0.02 mol), and
1-bromoheptane (3.1 mL, 0.02 mol) in acetone (50 mL) was stirred at
reflux overnight. After cooling to room temperature, water (50 mL) was
added and the reaction mixture was extracted with diethyl ether
(3 ꢀ 20 mL). The combined organic layers were washed firstwith aqueous
10% NaOH, followed by water, dried over anhydrous Na2SO4 and
evaporated to dryness under vacuum. The oil thus obtained was distilled
under vacuum to give a colorless liquid (5.4 g, 84%). B.p. 145ꢁC/0.5 torr.
1H NMR (400 MHz, CDCl3) 0.89 (t, J ¼ 6.96 Hz, 3H), 1.29–1.46 (m, 8H),
1.72–1.79 (m, 2H), 3.90 (t, J ¼ 6.50 Hz, 2H), 6.97 (d, J ¼ 1.65 Hz, 2H),
7.22 (t, J ¼ 1.65 Hz, 1H). 13C NMR (100 MHz, CDCl3) 160.6, 126.3,
123.3, 117.1, 68.8, 32.1, 29.3, 26.2, 22.9, 14.4.
3,5-bis(Trimethylsilylethynyl)heptyloxylbenzene (8). Pd(PhCN)2Cl2
(46 mg, 0.12 mmol) and CuI (15.2 mg, 0.08 mmol) were added to a dry,
septum-capped round bottom flask, purged with nitrogen. Anhydrous
dioxane (2 mL), P(t-Bu)3 (0.5 mL, 0.25 M solution in dioxane),
HN(i-Pr)2 (0.68 mL, 4.8 mmol), 7 (0.7 g, 2 mmol) in 0.5 mL dioxane
were then added via syringe to the stirred reaction mixture. Finally,
trimethylsilylacetylene (0.66 mL, 4.6 mmol) was added to the flask. In a
few minutes, a precipitate was formed. After stirring the reaction mixture
at room temperature for 2 h, the solvent was evaporated to dryness and

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Suzuki/Miyaura Vs. Sonogashira Perspective
3323
the crude product was purified on a silica gel column using 2% CH2Cl2 in
hexane as eluent. The product was further purified by recrystallization
from methanol to give a yellow solid (0.68 g, yield 89%). M.p. 47ꢁC.
1H NMR (400 MHz, CDCl3) 7.16 (t, J ¼ 1.47 Hz, 1H), 6.94 (d,
J ¼ 1.28 Hz, 2H), 3.91 (t, J ¼ 6.50 Hz, 2H), 1.71–1.79 (m, 2H), 1.28–1.46
(m, 8H), 0.89 (t, J ¼ 5.78 Hz, 3H), 0.22 (s, 18H).
3,5-bis(Ethynyl)heptyloxylbenzene (9). 8 (3.35 g, 8.7 mmol), acetone
(30 mL), methanol (60 mL), and NaOH (0.12 g, 3 mmol) were mixed
together and stir at room temperature for 2 h. Water (100 mL) was
then added, followed by extraction with diethyl ether (3 ꢀ 50 mL). The
combined ether extracts were washed with water until the aqueous layer
was neutral, dried with anhydrous Na2SO4 and evaporated to dryness in
vacuo to give a yellow solid (2.05 g, 98%). M.p. 32ꢁC. 1H NMR
(400 MHz, CDCl3) 7.20 (t, J ¼ 1.28 Hz, 1H), 6.99 (d, J ¼ 1.28 Hz, 2H),
3.93 (t, J ¼ 6.59 Hz, 2H), 3.05 (s, 2H), 1.72–1.80 (m, 2H), 1.28–1.46 (m,
8H), 0.89 (t, J ¼ 6.95 Hz, 3H). 13C NMR (100 MHz, CDCl3) 158.6, 128.0,
123.2, 118.8, 82.6, 77.5, 68.2, 31.7, 29.1, 29.0, 25.9, 22.6, 14.1. MS (ESIþ)
241.1587 (M þ H),þ (theoretical ionic mass: 240.1514).
3,8-bis(3-Ethynyl-5-heptyloxyphenylethynyl)-1,10-phenanthroline (2).
Lithium bis(trimethylsilyl)amide (2.25 mmol, 2.25 mL of 1 M solution in
THF) was added under nitrogen to a well stirred solution of 9 (0.86 g,
2.25 mmol) in anhydrous THF (10 mL) at ꢂ78ꢁC. After 30 min, B-meth-
oxy-9-BBN (2.25 mmol, 2.25 mL of 1 M solution in hexane) was added to
the reaction mixture via a septum with a syringe. After stirring for 2 h at
ꢂ78ꢁC, the solution was transferred via a cannula to a second flask con-
taining Pd(PPh3)4 (60 mg, 0.052 mmol) and 3,8-dibromo-1,10-phenan-
throline (0.25 g, 0.75 mmol) in dry THF (30 mL) under nitrogen. The
reaction mixture was then heated to reflux for 24 h and cooled to room
temperature. Methylene chloride (50 mL) was added and the organic layer
was washed with water (20 ꢀ 100 mL) dried with anhydrous Na2SO4 and
evaporated to dryness. The crude product thus obtained was purified on
silica gel column. Initial elution with CH2Cl2 gave unreacted starting mate-
rial. Subsequent elution with diethyl ether gave the product. After removal
of the solvent in vacuo a light yellow solid was obtained (0.36 g, 74%).
M.p. 85–86ꢁC. H NMR (400 MHz, CDCl3) 9.25 (d, J ¼ 2.01 Hz, 2H),
1
8.34 (d, J ¼ 2.01 Hz, 2H), 7.78 (s, 2H), 7.33 (t, J ¼ 1.28 Hz, 2H), 7.12
(t, J ¼ 1.46 Hz, 2H), 7.05 (t, J ¼ 1.28 Hz, 2H), 3.98 (t, J ¼ 6.5 Hz, 4H),
3.10 (s, 2H), 1.78–1.82 (m, 4H), 1.48 (m, 8H), 1.28–1.46 (m, 8H), 0.91
(t, J ¼ 6.87 Hz, 6H). 13C NMR (100 MHz, CDCl3) 158.6, 152.1, 144.2,
137.9, 127.8, 127.5, 126.7, 123.4, 123.3, 119.2, 118.9, 118.0, 92.9, 86.4,
82.5, 77.6, 68.2, 31.7, 29.0, 28.9, 25.8, 22.5, 14.0. MS (ESIþ) 657.3476
(M þ H),þ (theoretical ionic mass: 656.3403).

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Yang et al.
ACKNOWLEDGMENTS
We gratefully acknowledge support by the Petroleum Research
Fund, administered by the ACS (Grant No. 35154-ACS), and from the
National Cancer Institute (Grant No. 1 R15 CA82141-01A2 to C. S.-L.).
We also thank Dr. Nathan D. Leigh at the University of Missouri-
Columbia for obtaining the MS spectra.
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Received in the USA April 3, 2003

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