Synthesis of alkoxy-substituted ortho-phenylene ethynylene oligomers

Article abstract of DOI:10.1021/ol0352254

(Matrix presented) This Letter describes the first published synthesis and characterization of alkoxy-substituted ortho-phenylene ethynylene (o-PE) oligomers. Sonogashira coupling was used to assemble discrete chain lengths, using a key monomer with ortho

Full text of DOI:10.1021/ol0352254

ORGANIC
LETTERS
2003
Vol. 5, No. 18
3297-3299
Synthesis of Alkoxy-Substituted
ortho-Phenylene Ethynylene Oligomers
Ticora V. Jones, Richard A. Blatchly,† and Gregory N. Tew*
Polymer Science and Engineering, UniVersity of Massachusetts,
Amherst, Massachusetts 01002
tew@mail.pse.umass.edu
Received July 2, 2003
ABSTRACT
This Letter describes the first published synthesis and characterization of alkoxy-substituted ortho-phenylene ethynylene (o-PE) oligomers.
Sonogashira coupling was used to assemble discrete chain lengths, using a key monomer with orthogonal groups. Deprotection or activation
allowed stepwise coupling to produce the dimer, trimer, and tetramer, while convergent coupling of appropriately substituted trimers produced
the hexamer. The placement of alkoxy side chains renders these oligomers soluble in common organic solvents permitting solution
characterization. Absorption and emission spectra of the trimer, tetramer, and hexamer are provided.
The study of supramolecular chemistry and, in particular,
foldamers, which mimic the folding behavior of biological
molecules with abiotic molecules, has produced considerable
interest.1-7 The need for a wide variety of structurally sound
scaffolds upon which to build diverse molecular architectures
is vital. At the same time, the study of defined-length
oligomers with interesting optical and electronic properties
remains important.8
The scaffold can adopt either extended or helical conforma-
tions; para-substituted derivatives are considered rigid rods.
A number of meta derivatives have been made, including
those designed to adopt helices in solution or the solid
state.9-14 Recently, we patterned meta derivatives with polar,
cationic, and nonpolar groups to generate facially amphiphilic
extended structures.15,16
Both the meta- and ortho-PEs are capable of forming a
helical structure with six and three rings per turn, respec-
tively, but little is known of the ortho conformations. Of
the two isomers, meta-PEs have been studied more
thoroughly10,12,17-19 than ortho isomers for their folding
Phenylene ethynylene (PE) oligomers represent an im-
portant class of scaffolding backbones due to their ease of
synthesis and compatibility with a variety of substituents.
† Department of Chemistry, Keene State College, Keene, New Hampshire
03435.
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10.1021/ol0352254 CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/07/2003

Scheme 1. Synthesis of Pivotal Monomer 5a
Scheme 2. Synthesis of Oligomersa
a Legend: (a) (S)-2-methyl-1-butanol, DIAD, PPh3, THF, 0-20
°C 30 min. (b) FeCl3, N2H4‚H2O, carbon black, CH3OH, 70 °C, 2
h. (c) (1) HCl, CH3CN, H2O, 0 °C; (2) NaNO2, H2O, 0 °C, 30
min; (3) K2CO3, Et2NH, H2O, 0-20 °C, 15 min. (d) PdCl2(PPh3)2,
CuI, TMS-acetylene, TEA, 45 °C, 12 h.
behavior. The few known syntheses of o-PEs have not
reported the framework with side chain substituents and have
discussed difficulties with Sonogashira methods.20-23 This
work is significant because it establishes the utility of known
reactions for preparing new o-PE frameworks that are
soluble. Computational studies suggest that these oligomers
can adopt helical structures as minimum energy conforma-
tions.24 These oligomers are decorated with the (S)-2-methyl-
butoxy side chain to facilitate the study of potential helical
conformations and chiral aggregates.25-27
The synthesis of oligomers involves production of pivotal
monomer 5 (Scheme 1). Scheme 2 shows the cycle of
acetylene deprotection, triazene activation, and Sonogashira
coupling used to produce dimer 8a, trimer 8b, and tetramer
8c. Convergent coupling of appropriate trimer molecules (6b
and 7c) produces hexamer 8d.
a Legend: (a) CH3I, 110 °C, 6 h. (b) K2CO3, CH3OH, THF, rt,
0.25-3 h. (c) PdCl2(PPh3)2, CuI, 6a or 6b, TEA, 45 °C, 12 h.
reduction of the nitro group in the presence of the aryl
iodide.28 In our hands, a traditional method for reducing the
nitro compound (SnCl2) also reduced the aryl iodide, but
alternate conditions gave excellent yield of 3. Formation of
the triazene from aniline 3 requires vigorous mixing to
achieve good yield. The orthogonally protected 5 yields the
two fundamental starting materials, 6a and 7a, for stepwise
oligomer synthesis in separate steps shown in Scheme 2.
Despite the ortho proximity of the triazene and acetylene,
the overall synthesis of 5 proceeds in very good yield.29-32
Elaboration of the monomer to produce a series of
oligomers was carried out in satisfactory yield. The ether
side chain, ortho-diacetylene moieties, and the TMS protect-
ing group are easily able to withstand the triazene depro-
Beginning with the commercially available 3-nitro-4-
iodophenol 1, the synthesis of monomer 5 is notable for the
(17) Nelson, J. C.; Saven, J. G.; Moore, J. S.; Wolynes, P. G. Science
1997, 277, 1793-1796.
(18) Prince, R. B.; Saven, J. G.; Wolynes, P. G.; Moore, J. S. J. Am.
Chem. Soc. 1999, 121, 3114-3121.
(19) Brunsveld, L.; Meijer, E. W.; Prince, R. B.; Moore, J. S. J. Am.
Chem. Soc 2001, 123, 7978-7984.
(20) Grubbs, R. H.; Kratz, D. Chem. Ber. 1993, 126, 149-157.
(21) Orita, A.; Alonso, E.; Yaruva, J.; Otera, J. Synth. Lett. 2000, 1333-
1335.
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J. Chem. Eur. J. 1999, 5, 1355-1363.
(23) Wong, M. S.; Nicoud, J. F. Tetrahedron Lett. 1994, 35, 6113-
6116.
(24) Blatchly, R. A.; Tew, G. N. J. Org. Chem. 2003, submitted for
publication.
(25) Schenning, A.; Jonkheijm, P.; Peeters, E.; Meijer, E. W. J. Am.
Chem. Soc. 2001, 123, 409-416.
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E. W. J. Am. Chem. Soc. 2000, 122, 1820-1821.
(27) LangeveldVoss, B. M. W.; Janssen, R. A. J.; Christiaans, M. P. T.;
Meskers, S. C. J.; Dekkers, H.; Meijer, E. W. J. Am. Chem. Soc. 1996,
118, 4908-4909.
(28) Clive, D. L. J. J. Org. Chem. 1987, 52, 1339-1342.
(29) Haley, M. M. Org. Lett. 2000, 2, 969-972.
(30) Haley, M. M.; Bell, M. L.; Brand, S. C.; Kimball, D. B.; Pak, J. J.;
Wan, W. B. Tetrahedron Lett. 1997, 38, 7483-7486.
(31) Haley, M. M.; Bell, M. L.; English, J. J.; Johnson, C. A.; Weakley,
T. J. R. J. Am. Chem. Soc. 1997, 119, 2956-2957.
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3507-3520.
3298
Org. Lett., Vol. 5, No. 18, 2003

tection conditions (methyl iodide, 110 °C, 12 h). Moreover,
the key deprotection, activation, and coupling steps are not
1
hindered by sterics related to ortho substitution. H NMR
data for compounds 8b-d show excellent resolution even
in the aryl region of the hexamer; nearly aryl all resonances
are assignable at 300 MHz in chloroform. All oligomers,
including the hexamer, are soluble in common organic
solvents, including THF, ethyl acetate, chloroform, and
hexanes.
The UV-vis spectra for compounds 8b-d in chloroform
display bands at 280, 290, and 300 nm, consistent with the
three-peak pattern of a diphenylacetylene chromophore.29 The
band near 350 nm appears to be consistent with other o-PE
oligomers reported, although these do not contain side chain
substituents.20 Further, the longer-wavelength band at 350
nm is not associated with the triazene, as the trimer 6b still
contains this band (not shown).
Figure 1. Normalized UV (left curves) and fluorescence (right
curves) spectra of oligomers 8b (blue), 8c (green), and 8d (red) in
chloroform.
Emission from the oligomers is found near 400 nm with
a red shift from the trimer to tetramer to hexamer. Until
longer oligomers are prepared, it will remain unclear whether
the red shift has saturated in these oligomers, although
previous reports indicate that the λmax saturates around the
hexamer for unsubstituted oligomers.20 In summary, we have
synthesized oligomers 8b-d, the first ether-substituted o-PEs
through a series of deprotection, activation, and Sonogashira
coupling steps. These oligomers are readily soluble so that
solution studies can be performed. We are currently optimiz-
ing the synthesis to prepare and study longer oligomers,
which will be reported in due course.
Acknowledgment. We are grateful for support from NSF
RSEC grant CHE-0223643 (R.A.B.), the ONR Young
Investigator Award (G.N.T.), and NIH-NRSA from NHLBI
(T.V.J.).
Supporting Information Available: Synthetic procedures
and NMR data are available for all compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL0352254
Org. Lett., Vol. 5, No. 18, 2003
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