A versatile synthesis for new 9,10-Bis(4-alkoxyphenyl)-2,7- diiodophenanthrenes: Useful precursors for conjugated polymers

Article abstract of DOI:10.1021/ol0713584

A practical synthesis for 9,10-bis(alkoxyphenyl)-2,7-diiodophenanthrenes is described. 9,10-Bis(4-hexyloxyphenyl)-2,7-diiodophenanthrene was selectively obtained by a one-pot reaction of 10,10-bis-(4-hexyloxy-phenyl)-2,7-diiodo-10H- phenanthren-9-one with

Full text of DOI:10.1021/ol0713584

ORGANIC
LETTERS
2007
Vol. 9, No. 16
3149-3152
A Versatile Synthesis for New 9,10-
Bis(4-alkoxyphenyl)-2,7-diiodophenanthrenes:
Useful Precursors for Conjugated
Polymers
Roberto Grisorio, Gian Paolo Suranna,* Piero Mastrorilli, and
Cosimo Francesco Nobile
Department of Water Engineering and of Chemistry, Polytechnic of Bari,
Campus UniVersitario, Via Orabona 4-Bari, Italy
surannag@poliba.it
Received June 8, 2007
ABSTRACT
A practical synthesis for 9,10-bis(alkoxyphenyl)-2,7-diiodophenanthrenes is described. 9,10-Bis(4-hexyloxyphenyl)-2,7-diiodophenanthrene was
selectively obtained by a one-pot reaction of 10,10-bis-(4-hexyloxy-phenyl)-2,7-diiodo-10H-phenanthren-9-one with 1 equiv of triethylsilane in
trifluoroacetic acid. The target molecular architecture was demonstrated to be a useful precursor for obtaining the corresponding poly-
(phenanthrene) and poly(phenanthrylene-vinylene).
In the field of organic light emitting diodes (OLEDs), a
considerable interest in obtaining high band gap π-conjugated
polymers as stable blue-emitting materials continues to be
addressed. In order to limit the delocalization of the π density
on the conjugated framework, much attention has been
focused on high resonance energy systems. Poly(p-phe-
nylenes) (PPPs) and their derivatives1 have represented the
most widely investigated class of π-conjugated polymers.
However, the mandatory introduction of solubilizing groups
in the polymer backbone of PPPs causes an additional torsion
of the phenylene-phenylene angles,2 which shifts the
emission of these polymers to the violet region. The idea of
linking two phenylene units by a methine bridge3 turned out
to be successful in terms of color emission, but it introduces
easily oxidizable sites, which are responsible for the spectral
instability of the corresponding polymers.4 From a structural
point of view, a viable alternative is constituted by phenan-
threne, one of the most versatile fused aromatic compounds
in the light of its easy functionalizability.
After a report highlighting its potential,5 a very recent work
has pointed out the possibility of a simple introduction of
several functional groups in six positions of the phenanthrene
system.6 Furthermore, its high resonance energy and conse-
quent stability7 grants this molecule the suitable requisites as
monomer precursor for obtaining stable blue-emitting poly-
(3) (a) Neher, D. Macromol. Rapid Commun. 2001, 22, 1366-1385. (b)
Setayesh, S.; Marsitzky, D.; Mu¨llen, K. Macromolecules 2000, 33, 2016-
2020. (c) Jacob, J.; Sax, S.; Gaal, M.; List, E. J. W.; Grimsdale, A. C.;
Mu¨llen, K. Macromolecules 2005, 38, 9933-9938. (d) Scherf, U. J. Mater.
Chem. 1999, 9, 1853-1864.
(4) (a) Becker, K.; Lupton, J. M.; Feldmann, J.; Nehls, B. S.; Galbrecht,
F.; Gao, D.; Scherf, U. AdV. Funct. Mater. 2006, 16, 364-371. (b) Sims,
M.; Bradley, D. D. C.; Ariu, M.; Koeberg, M.; Asimakis, A.; Grell, M.;
Lidzey, D. G. AdV. Funct. Mater. 2004, 14, 765-781.
(5) Suh, H.; Jin, Y.; Park, S. H.; Kim, D.; Kim, J.; Kim, C.; Kim, J. Y.;
Lee, K. Macromolecules 2005, 38, 6285-6289.
(6) Boden, B. N.; Jardine, K. J.; Leung, A. C. W.; MacLachlan, M. J.
Org. Lett. 2006, 8, 1855-1858.
(1) (a) Rehahn, M.; Schlu¨ter, A.-D.; Wegner, G.; Feast, W. J. Polymer
1989, 30, 1054-1060. (b) Child, A. D.; Reynolds, J. R. Macromolecules
1994, 27, 1975-1977. (c) Balanda, P. B.; Ramey, M. B.; Reynolds, J. R.
Macromolecules 1999, 32, 3970-3978.
(2) Lamba, J. J. S.; Tour, J. M. J. Am. Chem. Soc. 1994, 116, 11723-
11736.
10.1021/ol0713584 CCC: $37.00
© 2007 American Chemical Society
Published on Web 07/13/2007

mers. In this framework, it has been ascertained that the pres-
ence of aryl groups in the 9,10 positions of phenanthrene is
crucial for preserving the optical properties of the correspond-
ing macromolecules in the solid state.8 The synthesis of 9,-
10-diaryl-2,7-dibromophenanthrenes has been recently de-
scribed by Mu¨llen8 et al. and the proposed approach, sketched
in Scheme 1, envisages as key step the McMurry-type
90% yield. Remarkably, the carbonyl compound 6 was
obtained in one pot, bypassing the adoption of the known
multistep procedures10 which use Grignard reagents that are
incompatible with the presence of iodides in the molecule.11
The plausible mechanism yielding 6 is shown in Scheme
3. The aromatic electrophilic substitution of the protonated
Scheme 3. Plausible Mechanism for Obtaining 6
Scheme 1. Synthesis of 9,10-Diaryl-2,7-dibromophenanthrenes
According to Ref 8
coupling of carbonyl groups in 2 generating the phenanthrene
scaffold. The introduction of the leaving groups (in this case
bromides) in the less activated 2,7 positions of phenanthrene
is pursued by a selective reductive homocoupling of 1, exploit-
ing the higher reactivity of iodides toward the Ullmann-type
reaction, leading to 2. However, the potentialities of this syn-
thetic approach are limited by the forced presence on phenan-
threne of halogen leaving groups less reactive than iodides.
In this paper, we propose a very practical alternative syn-
thetic method for the preparation of 9,10-diaryl-2,7-diiodo-
phenanthrenes. The reaction sequence is outlined in Scheme
2. The commercially available phenanthrenequinone was con-
form of phenanthrenequinone onto phenol affords the inter-
mediate 9, possessing two sites for the subsequent proton
attack: the carbonyl and the hydroxyl group. The protonation
of the carbonyl would lead to the formation of the correspond-
ing pinacol compound 10, which transforms into 6 through
acid-promoted pinacol/pinacolone rearrangement. On the
other hand, following protonation of the C-9 hydroxyl group,
the ketonic product 6 could be directly obtained. It is reason-
able to suppose that the two pathways are both plausible
and that the pinacol/pinacolone rearrangment is strongly
favored in the acidic reaction medium. It is remarkable how,
notwithstanding the excess of phenol used, the reaction does
not lead to the tetrasubstituted derivative, probably because
of the severe steric hindrance exerted by the two aryl groups,
hampering further aromatic electrophilic substitutions.
As illustrated in Scheme 2, the presence of the hydroxyl
functionalities in 6 ensured a straightforward introduction
of alkyl chains of the desired length by means of a
nucleophilic substitution on the appropriate alkyl bromide.
In our case, compound 7 was obtained in 73% yield by
reaction of 6 and hexyl bromide in methyl-isobutylketone
in the presence of K2CO3 as base.
Scheme 2. Synthesis of 8
At this point of the synthetic sequence, the construction
of the 9,10-diarylphenanthrene scaffold, according to litera-
ture approaches,10 should require the reduction of the ketone
(7) (a) Tian, H.; Wang, J.; Shi, J.; Yan, D.; Wang, L.; Geng, Y.; Wang, F.
J. Mater. Chem. 2005, 15, 3026-3033. (b) Tian, H.; Wang, J.; Shi, J.; Yan,
D.; Wang, L.; Geng, Y.; Wang, F. Chem. Commun. 2006, 3498-3500.
(8) Yang, C.; Scheiber, H.; List, E. J. W.; Jacob, J.; Mu¨llen, K.
Macromolecules 2006, 39, 5213-5221.
(9) Ciszek, J. W.; Tour, J. M. Tetrahedron Lett. 2004, 45, 2801-2803.
(10) Lai, Y.-H. J. Amer. Chem. Soc. 1985, 107, 6678-6683.
(11) (a) Zakharkin, L. I.; Okhlobystin, O. Y.; Bilevitch, K. A. J.
Organomet. Chem. 1964, 2, 309-313. (b) Tamborski, C.; Moore, G. J. J.
Organomet. Chem. 1971, 26, 153-156.
verted into the corresponding 2,7-diodo derivative 5 using
N-iodosuccinimide in triflic acid, according to a literature pro-
cedure.9 Compound 5 was then submitted to a Friedel-Crafts
reaction with an excess of phenol and methanesulfonic acid
in CCl4 at 80 °C to afford the corresponding product 6 in
3150
Org. Lett., Vol. 9, No. 16, 2007

in 7 to the -OH group followed by a rearrangement of the
aryl groups and subsequent dehydration catalyzed by proton
sources to afford 8. In order to avoid the use of LiAlH4 for
obtaining the -OH group in this highly bulky substrate (its
employment might have led to a simultaneous dehalogenation
of the substrate,12 forming impurities that are extremely
difficult to remove) we chose milder conditions for the
reductive step in the synthesis of 8.
We were pleased to find that reaction of 7 with one
equivalent of triethylsilane in trifluoroacetic acid at 80 °C
did not simply afford the ketone reduction product but the
target molecule 8, with unexpected high selectivity, in a fast
(3 h), high-yielding (80%) one-pot reaction. The product
could be straightforwardly purified and was fully character-
ized by NMR, IR, and elemental analysis. The selectivity of
this reaction is quite surprising, considering all the possible
byproducts that could be obtained during the reaction. It is
known that triethylsilane formally acts as a hydride source
for carbocations,13 and its versatility has been demonstrated
for the conversion of ketones to methylene groups.14 The
use of triethylsilane requires a protic source (trifluoroacetic
acid) necessary to generate the suitable carbocations.
For these reasons, it is realistic to consider two simulta-
neous reaction pathways for the obtainment of 8, as depicted
in Scheme 4.
carbocation 14. The latter intermediate would then undergo
the aryl shift yielding the more stable carbocation 15 and
subsequent aromatization leading to the target molecule 8.
On the other hand, another reaction pathway can start from
the rearrangement of 11 yielding the more stable carbocation
16, which, in turn, can act as hydride acceptor forming 17.
Dehydration of 17 leads again to the intermediate 15,
followed by aromatization through proton elimination, giving
the target molecule 8. The high selectivity of the reaction is
explained by assuming that the rearrangement of carbocation
14 and the aromatization of 15 are faster processes with
respect to reaction of these species with triethylsilane.
The advantages of having built up an iodophenanthrene
building block were demonstrated by carrying out poly-
merization tests for the obtainment of poly(2,7-phenan-
threne)s and poly(2,7-phenanthrylenevinylene)s.
As a first test, the polymerizaton of 8 by Ni(COD)2-
promoted reductive homocoupling was carried out affording
the corresponding polymer PPhen in 76% yield (Scheme
5). The obtained polymer was soluble in CHCl3, CH2Cl2,
Scheme 5. Synthesis of PPhen and PPhenV from 8
Scheme 4. Plausible Mechanisms for Obtaining 8
1
and THF and was characterized by H NMR, FT-IR, and
gel permeation chromatography (GPC).
Its number average molecular weight (against polystyrene
standards) was estimated as 31 000 Da (∼60 repeating units)
and, noteworthy, could be obtained with fairly low polydis-
persity (Mw/Mn ) 1.5).
In another polymerization experiment, monomer 8 was
submitted to a cascade Suzuki-Heck reaction, a method re-
cently reported by us for obtaining poly(arylenevinylene)s,15
which employs the use of potassium vinyltrifluoroborate16 as
ethylene equivalent. The macromolecule PPhenV was
obtained in a one-pot reaction with potassium vinyltrifluo-
roborate, palladium(II) acetate/tri(o-tolyl)phosphane as cata-
lyst, and triethylamine as base in toluene/DMF at 120 °C.
This polymer was obtained in 71% yield as a green powder,
(13) (a) Carey, F. A.; Tremper, H. S. J. Amer. Chem. Soc. 1968, 2578-
2583. (b) Carey, F. A.; Tremper, H. S. J. Org. Chem. 1971, 36, 758-761.
(14) West, C. T.; Donnelly, S. J.; Kooistra, D. A.; Doyle, M. P. J. Org.
Chem. 1973, 38, 2675-2681.
(15) (a) Grisorio, R.; Mastrorilli, P.; Nobile, C. F.; Romanazzi, G.;
Suranna, G. P. Tetrahedron Lett. 2005, 46, 2555-2558. (b) Grisorio, R.;
Mastrorilli, P.; Nobile, C. F.; Romanazzi, G.; Suranna, G. P.; Gigli, G.;
Piliego, C.; Ciccarella, G.; Cosma, P.; Acierno, D.; Amendola, E.
Macromolecules 2007, 40, 4865-4873.
Protonation of 7 leads to the carbocation 11, which can
act as hydride acceptor. The alcohol 12 can be further
protonated yielding, after loss of a water molecule, the
(16) (a) Molander, G. A.; Rivero, M. R. Org. Lett. 2002, 4, 107-109.
(b) Molander, G. A.; Bernardi, C. R. J. Org. Chem. 2002, 67, 8424-8429.
(12) Karabatsos, G. J.; Shone, R. L. J. Org. Chem. 1968, 33, 619-621.
Org. Lett., Vol. 9, No. 16, 2007
3151

A preliminary investigation of the physical properties of
the two polymers was carried out by means of UV-vis and
photoluminescence both in chloroform solution and in the
solid state (Figure 1). The optical band gap of the two
polymers was estimated by the onset of the absorption as
2.97 eV (PPhen) and 2.63 eV (PPhenV). Consequently,
PPhen shows a blue-emission (λmax ) 409 nm) while
PPhenV emits in the blue-green region (λmax ) 461 nm) in
solution. In the solid state, structured bands can be observed
with maxima at 433 nm (PPhen) and 493 nm (PPhenV).
As expected, the emission features of both polymers show
no evidence for aggregation phenomena (broadening of
emission band) in the solid state because of the beneficial
effect of the two phenyl groups in the phenanthrene 9,10-
positions, orthogonally placed with respect to the conjugated
backbone, preserving the molecules from strong intermo-
lecular interactions.
In conclusion, we have synthesized a 2,7-diiodo-9,10-
diaryl-phenanthrene by means of a versatile synthetic ap-
proach, envisaging as key step a reaction of the suitable
carbonyl precursor with triethylsilane in trifluoroacetic acid.
This reaction proceeds with a simultaneous hydride donation,
dehydration and an aryl 1,2-shift to construct the phenan-
threne scaffold in high yield and selectivity. In addition to
these synthetic aspects, the approach grants the obtainment
of monomers affording poly(2,7-phenanthrene)s and poly-
(2,7-phenanthrylenevinylene)s with relatively high molecular
weigths and low polydispersities. Our current efforts are
devoted to testing the scope width of the reaction and to the
use of our monomer typology for obtaining suitable phenan-
threne-based copolymers for applications as active layers in
OLEDs.
Figure 1. UV-vis and photoluminescence spectra of PPhen (A)
and PPhenV (B) in chloroform solution and as a thin film on quartz.
soluble in chlorinated solvents, which was characterized by
1H NMR and FT-IR spectroscopy. Its number average molec-
ular weight was found to be 7500 Da (∼14 repeating units)
with a polydispersity of 2.2. Noteworthy, this is the poly-
(2,7-phenanthrylene-vinylene) with the highest reported
relative molecular weights.6
Supporting Information Available: Synthetic procedures
and characterization data of the compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL0713584
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Org. Lett., Vol. 9, No. 16, 2007