Synthesis of water-soluble poly(p-phenyleneethynylene) in neat water under aerobic conditions via Suzuki-Miyaura polycondensation using a diborylethyne synthon

Article abstract of DOI:10.1021/ol702758f

(Chemical Equation Presented) We report the synthesis, structure, and characterization of a novel ethyne synthon, 1,2-bis(4′,4′,5′, 5′-tetramethyl[1′,3′,2′]dioxaborolan-2′-yl) -ethyne (B2C2). We demonstrate the utility of B2C2 in the Suzuki-Miyaura polyco

Full text of DOI:10.1021/ol702758f

ORGANIC
LETTERS
2008
Vol. 10, No. 7
1341-1344
Synthesis of Water-Soluble
Poly(p-phenyleneethynylene) in Neat
Water under Aerobic Conditions via
Suzuki-Miyaura Polycondensation Using
a Diborylethyne Synthon
Youn K. Kang, Pravas Deria, Patrick J. Carroll, and Michael J. Therien*
Department of Chemistry, UniVersity of PennsylVania,
Philadelphia, PennsylVania 19104-6323
therien@sas.upenn.edu
Received November 18, 2007
ABSTRACT
We report the synthesis, structure, and characterization of a novel ethyne synthon, 1,2-bis(4′,4′,5′,5′-tetramethyl[1′,3′,2′]dioxaborolan-2′-yl)-
ethyne (B2C2). We demonstrate the utility of B2C2 in the Suzuki-Miyaura polycondensation reaction, synthesizing a water-soluble poly(p-
phenyleneethynylene) from [2,5-diiodo-1,4-bis(3-propoxy-sulfonicacid)benzene] sodium salt in neat water under an aerobic atmosphere.
Ethyne-bridged conjugated polymers impact a wide-range
of technologies; the efficacy of these species derives not only
from their established semiconducting and optical properties
but also from the facts that these rigid, rod-like structures
are readily processible and manifest high photo and thermal
stabilities.1-7 Poly(p-phenyleneethynylene)s (PPEs) define the
archetypal examples of ethyne-bridged conjugated polymers;
these species have been utilized in organic light-emitting
diodes (OLEDs)8,9 field-effect transistors (FETs),10 molecular
electronics,11 nonlinear optical materials,12 solar energy
conversion devices,13-15 and in a variety of sensory applica-
tions.4,16 As such, considerable effort has been placed in the
(8) Egbe, D. A. M.; Birckner, E.; Klemm, E. J. Polym. Sci., Part A:
Polym. Chem. 2002, 40, 2670-2679.
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10.1021/ol702758f CCC: $40.75
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Published on Web 03/12/2008

development of synthetic protocols for repeating arene-ethyne
structural motifs.
tetramethyl-1,3,2-dioxaborolane, followed by treatment with
anhydrous HCl, cleanly affords B2C2 in high yield (Scheme
1). B2C2 can be recrystallized from hexanes, giving a robust
The palladium-catalyzed Sonogashira reaction17,18 and
acyclic diyne metathesis (ADIMET)19,20 represent widely
used approaches to PPEs. Although the Sonogashira reaction
is compatible with polar functional groups and water, it
requires both dihaloarene and diethynylarene synthons; as
such, this route is susceptible to the introduction of butadiyne
defects in the PPE polymer, which are estimated to range
from 1 to 10% even under the carefully controlled reaction
conditions.2 Although an ADIMET-based synthesis circum-
vents butadiyne defect sites, this method is generally
incompatible with sensitive functional groups that include
water-soluble side chains; furthermore, the syntheses of
carbyne precursors for ADIMET protocols require inert
reaction conditions and cannot be implemented in the
presence of water.2 Despite the fact that water-soluble PPEs
have attracted increasing interest in biosensing21 and bio-
conjugation applications,22,23 relatively few such materials
have been reported.6,24-28 We report herein an environmen-
tally benign synthetic approach29 that exploits a new dibo-
rylethyne synthon that enables PPE synthesis, separation, and
purification in neat water under an aerobic atmosphere.
The Suzuki-Miyaura coupling of alkynylboranes,30 alky-
nylboronic acid or alkynylborate derivatives,31,32 and alky-
nyltrifluoroborates33,34 is well-precedented. Similar reactions
involving reagents that possess boron functionality at the 1-
and 2-carbon positions of ethyne have not yet been reported.
This fact motivated the synthesis of [1,2-bis(4′,4′,5′,5′-
tetramethyl[1′,3′,2′]dioxaborolan-2′-yl)ethyne (B2C2). The
route to B2C2 exploits a procedure developed by Brown and
Srebnik,35 in which dilithioacetylide is generated from
trichloroethylene and 3 equiv of nBuLi in THF/diethylether.36
Reaction of dilithioacetylide with 2-isopropoxy-4,4,5,5-
Scheme 1. Synthesis of
1,2-Bis(4′,4′,5′,5′-tetramethyl[1′,3′,2′]dioxaborolan-2′-yl)ethyne
crystalline solid (mp ) 270 °C) that is stable under ambient
atmosphere for at least 1 year. B2C2 exhibits high solubility
in basic water and virtually all organic solvents.
X-ray quality crystals of B2C2 were obtained via evapora-
tion of a benzene solution. During the process of structure
determination, it became obvious that there were problems
with the data: refinement of the structure was unstable and
anisotropic refinement produced several nonpositive definite
thermal parameters. A re-examination of the rotation images
revealed many reflections that did not fit the derived cell.
The crystal was found to be twinned with a total of four
components (components 1 and 2 were related by a rotation
of 180° around the normal to 110; components 1 and 3 were
related by a rotation of 180° around the normal to 001h;
components 1 and 4 were related by a rotation of 180° around
the normal to 11h0). Twin indexing and processing of twinned
data were performed by the TwinSolve37 module of Crys-
talClear.38 ORTEP representations of the two B2C2 forms
(B2C2-1 and B2C2-2) that define the asymmetric crystal-
lographic unit are depicted in Figure 1; structural factors,
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Figure 1. ORTEP views of: (a) B2C2-1 and (b) B2C2-2 with
thermal ellipsoids at 30% probability.
1342
Org. Lett., Vol. 10, No. 7, 2008

bond distances, and bond angles are tabulated in the
Supporting Information. Note that B2C2 features a standard
C-C triple bond distance of 1.20 Å.
Polymerization reactions of B2C2 with [2,5-diiodo-1,4-
bis(3-propoxy-sulfonicacid)benzene] sodium salt catalyzed
by Pd(OAc)4/tris(3-sulfonatophenyl)phosphine trisodium salt
(TPPTS) were evaluated in DMF, EtOH, and H2O at 80 °C
(Scheme 2, protocol a). Only the reactions carried out in
that realized under inert conditions (91%; see Supporting
Information for procedural details). GPC analysis of PPES
showed that the number averaged molecular weight (Mn)
ranges from 6.6 to 17.5 kD depending on the reaction
conditions, which corresponds to degrees of polymerization
(DP) of 18-47. Representative absorption and emission
spectra of PPES (DP ≈ 30) in H2O are shown in Figure 2.
Scheme 2. Synthesis of Water-Soluble PPES
DMF solvent gave appreciable conversion of polymer. This
polymerization route to PPEs is unusual; in this regard, the
synthesis of PPEs using a phenylene bisboronic acid and a
bis(2-bromoethynyl)phenylene has been accomplished,39 and
poly(arylenevinylene)s have been fabricated from arylene
bisboronate and bis(2-bromovinyl)arylene synthons40 and
through a cascade Suzuki-Heck coupling of dihaloarenes with
potassium vinyltrifluoroboronate.41
Figure 2. Absorption and emission spectra of PPES obtained via
the synthetic procedure outlined in Scheme 2, protocol b. Experi-
mental conditions: solvent ) deionized H2O, T ) 296 K, λex
390 nm. Fluorescence quantum yield (Φf) ) 0.064.
)
Two recent advances in the palladium catalyzed Suzuki-
Miyaura reaction have utilized microwave heating42-45 and
employed tris(4,6-dimethyl-3-sulfonatophenyl)phosphine tri-
sodium salt (TXPTS)46,47 as a water-soluble catalyst ligand.
Utilizing the microwave heating reaction conditions outlined
in Scheme 2 (protocol b), poly[p-{2,5-bis(3-propoxysulfonic-
acid sodium salt)}phenylene]ethynylene (PPES) can be
produced in high conversion in aqueous solvent. Interest-
ingly, this aqueous polymerization reaction can also be
accomplished under an aerobic atmosphere; in this case, the
conversion to polymer product (85%) was slightly lower than
Note that not only the polymerization reaction, but PPES
workup and purification steps can be carried out entirely in
the aqueous phase as well.48 In contrast to literature methods,6
PPES can be isolated simply via size exclusion chromatog-
raphy (Supporting Information) using only unbuffered deion-
ized water as an eluent. Given the nature of the small
molecular weight impurities present in this synthesis of
water-soluble PPES, the size exclusion stationary phase can
be regenerated for subsequent PPES purifications simply by
washing with water.
PPES 13C NMR spectra in D2O/DMSO-d6 cosolvent
showed a broad ethynyl peak at 93 ppm (Supporting Infor-
mation); notably, no resonances were evident between 75
and 85 ppm, where butadiynyl carbon peaks usually appear.49
The absence of any detectable absorption signatures in this
spectral region indicates that PPES polymer chain butadiyne
defects are suppressed in this synthesis. PPEs prepared via
Suzuki-Miyaura polycondensation of 1,4-benzenediboronic
acid derivatives and 1,4-bis(2-bromoethynyl)benzene also
show no evidence of butadiynyl signatures in 13C NMR;39
these results and those presented herein underscore the utility
(35) Brown, H. C.; Bhat, N. G.; Srebnik, M. Tetrahedron Lett. 1988,
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Org. Lett., Vol. 10, No. 7, 2008
1343

of Suzuki-Miyaura polycondensation over Sonogashira po-
lymerization conditions for the production of butadiyne
defect free PPE.
gomerization reaction conditions. Importantly, this conju-
gated rigid-rod polymer synthesis represents an example in
which polymerization, purification, and isolation steps can
be accomplished using only H2O as a solvent.
In summary, we describe the synthesis and structure of a
novel ethyne synthon, 1,2-bis(4′,4′,5′,5′-tetramethyl[1′,3′,2′]-
dioxaborolan-2′-yl)ethyne (B2C2). We show the utility of
B2C2 in the Suzuki-Miyaura polycondensation reaction,
synthesizing a water-soluble poly(p-phenyleneethynylene)
(PPES) from [2,5-diiodo-1,4-bis(3-propoxy-sulfonicacid)-
benzene] sodium salt in neat water under an aerobic
atmosphere. This environmentally benign protocol for the
preparation of ethyne-bridged conjugated polymers over-
comes key drawbacks of commonly employed Sonogashira
coupling and acyclic diyne metathesis methods, which
include: the introduction of butadiyne defects along the
polymer backbone, a requisite inert-atmosphere, and incom-
patibility of water-solubilizing functional groups with oli-
Acknowledgment. This work was supported by a grant
from the Division of Chemical Sciences, Office of Basic
Energy Research, U.S. Department of Energy (DE-FGO2-
02ER15299). We thank the MRSEC (DMR-00-79909) and
NSEC (DMR-0425780) Programs of the National Science
Foundation for infrastructural support.
Supporting Information Available: Synthetic proce-
dures, characterization, analytical, and X-ray crystallographic
data. This material is available free of charge via the Internet
at http://pubs.acs.org.
OL702758F
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Org. Lett., Vol. 10, No. 7, 2008