Synthesis of homoconjugated oligomers derived from 7,7-diphenylnorbornane

Article abstract of DOI:10.1021/ol071161b

A methodology for the synthesis of monodisperse homoconjugated oligomers (dimer, trimer, and tetramer) derived from cofacial 7,7-diphenylnorbornane (DPN) is described. Extended aromatic homoconjugation is observed in these oligomers as revealed by the ele

Full text of DOI:10.1021/ol071161b

ORGANIC
LETTERS
2
007
Vol. 9, No. 15
943-2946
Synthesis of Homoconjugated
Oligomers Derived from
2
7,7-Diphenylnorbornane
Noelia Caraballo-Mart ´ı nez, Mar ´ı a del Rosario Colorado Heras,
Myriam Mba Bl a´ zquez, Jos e´ Os ´ı o Barcina,* Antonio Garc ´ı a Mart ´ı nez, and
†
Mar ´ı a del Rosario Torres Salvador
Departamento de Qu ´ı mica Org a´ nica, Facultad de Ciencias Qu ´ı micas, UniVersidad
Complutense de Madrid, Ciudad UniVersitaria s/n, 28040 Madrid, Spain
josio@quim.ucm.es
Received May 18, 2007
ABSTRACT
A
methodology for the synthesis of monodisperse homoconjugated oligomers (dimer, trimer, and tetramer) derived from cofacial
7,7-diphenylnorbornane (DPN) is described. Extended aromatic homoconjugation is observed in these oligomers as revealed by the electronic
spectra. The effective homoconjugation length (EHL) is in the range of 4−5 DPN subunits.
4
5
6
Monodisperse conjugated oligomers are currently of great
interest because of their applications in molecular electronics
and optoelectronics due to their properties such as electrical
conductivity, electroluminescence, and nonlinear optical
spiro-conjugation, toroidal-conjugation, π-stacking, and
homoconjugation.
7
Until now, very few examples of electron delocalization
in polymers and oligomers by aromatic homoconjugation
have been reported. In a previous work, we described the
1
7
properties. These organic bridges also play an important role
in studies directed to the understanding of photoinduced
electron- and energy-transfer processes. Modulation of the
first example of a soluble polymer with alternating conjuga-
tion-homoconjugation in which homoconjugated segments
were 7,7-diphenylnorbornane (DPN) subunits. In DPN, aryl
rings adopt a cofacial conformation forced by the norbornane
2
electronic and optical properties as well as of the applications
of molecular wires can be achieved by using alternative
3
8
modes of electron delocalization, such as cross-conjugation,
framework. Now we report the synthesis and characteristics
of new homoconjugated molecular wires derived from DPN
†
Laboratorio de difracci o´ n de Rayos X, Facultad de Ciencias Qu ´ı micas,
(10). The preparation of these oligomers is shown in Schemes
Universidad Complutense, Ciudad Universitaria, 28040 Madrid. E-mail:
mrtorres@quim.ucm.es.
(
1) (a) Electronic Materials: The Oligomer Approach; M u¨ llen, K.,
(4) Saragi, T. P. I.; Spehr, T.; Siebert, A.; Fuhrmann-Lieker, T.; Salbeck,
Wegner, G., Eds.; Wiley-VCH: New York, 1998. (b) Handbook of
Conducting Polymers; Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J.
R., Eds.; Taylor & Francis: London, 2006.
J. Chem. ReV. 2007, 107, 1011.
(5) (a) Rosokha, S. V.; Neretin, I. S.; Sun, D.; Kochi, J. K. J. Am. Chem.
Soc. 2006, 128, 9394. (b) Lambert, C. Angew. Chem., Int. Ed. 2005, 44,
7337.
(
2) (a) Welter, S.; Salluce, N.; Belser, P.; Groeneveld, M.; de Cola, L.
Coord. Chem. ReV. 2005, 249, 1360. (b) Benniston, A. C.; Harriman, A.
(6) (a) Morisaki, Y.; Chujo, Y. Angew. Chem., Int. Ed. 2006, 45, 6430.
(b) Mataka, S.; Thiemann, T.; Taniguchi, M.; Sawada, T. Synthesis 2000,
1211.
Chem. Soc. ReV. 2006, 35, 169.
(3) Klokkenburg, M.; Lutz, M.; Spek, A. L.; Klokkenburg, M.; Lutz,
M.; Spek, A. L.; van der Maas, J. H.; van Walree, C. A. Chem.sEur. J.
003, 9, 3544.
(7) Garc ´ı a Mart ´ı nez, A.; Os ´ı o Barcina, J.; de Fresno Cerezo, A.; Schl u¨ ter,
A.-D.; Frahn, J. AdV. Mater. 1999, 11, 27.
2
1
0.1021/ol071161b CCC: $37.00
© 2007 American Chemical Society
Published on Web 06/23/2007

Scheme 1. Synthesis of DFN Dimers 1 and 2
Scheme 2. Synthesis of DFN Trimer 3 and Tetramer 4
1
8
and 2. Alkyl chains (endo-2-C H17) in the cases of 2, 3,
and 4 are introduced in the structure of norbornane in order
to increase the solubility of the oligomers since an unsub-
stituted trimer is quite insoluble.
The methodology described in Schemes 1 and 2 allows
the preparation of monodisperse homoconjugated oligomers
by simply reacting the corresponding dilithium compounds
(prepared from 1,4-diiodobenzene or the diiodides 12 or 14)
with 7-norbornanones 5 or 6, followed by reaction of the
resulting diols 7, 8, 13, or 15 with benzene/TfOH. Addition
reactions of aryllithiums to ketone 6 are in agreement with
the facial selectivity described for the reaction of alkyl-
substituted 7-norbornanones with nucleophiles, and alcohols
9
derived from the anti attack are formed as major products.
Oligomers with the desired length can be obtained in good
yields using this synthetic route. The length of the oligomers
1
can be easily obtained by H NMR from the relation between
aromatic and bridgehead protons.
In solution, two stable conformers of dimers 1 and 2 (U-
shaped and Z-shaped) are in equilibrium, both with almost
the same energy (isoenergetics), according to semiempirical
calculations (PM3 and AM1) (Figure 1). This situation can
be extrapolated to trimer 3 and tetramer 4 considering that
the number of stable conformations increases with chain
10
length. Therefore, unlike oligobenzyls, in which the rotation
of the C-aryl bond is not restricted, DPN oligomers can be
considered as open chain but preorganized structures. As a
consequence, these oligomers could be used as receptor
molecules (molecular clips or tweezers) capable of binding
+
11
metal cations, such as Ag , or neutral electron-deficient
1
2
molecules since the 1-U conformer (or the analogous closed
conformers in the cases of 3 and 4) affords an electron-rich
environment to the coordination of the guests.
In the solid state, X-ray diffraction analysis (Figure 2)
shows that 1 crystallizes, adopting the Z-shaped (zigzag)
conformation. This crystal packing is stabilized by CH-π
(
8) (a) Garc ´ı a Mart ´ı nez, A.; Os ´ı o Barcina, J. Encyclopedia of Supramo-
lecular Chemistry; Atwood, J. L., Steed, J. W., Eds.; Marcel Dekker: New
York, 2004. (b) Garc ´ı a Mart ´ı nez, A.; Os ´ı o Barcina, J.; de Fresno Cerezo,
A.; Guti e´ rrez Rivas, R. J. Am. Chem. Soc. 1998, 120, 673. (c) Garc ´ı a
Mart ´ı nez, A.; Os ´ı o Barcina, J.; Albert, A.; Cano, F. H.; Subramanian, L.
R. Tetrahedron Lett. 1993, 34, 6753.
(
9) Mehta, G.; Chandrasekhar, J. Chem. ReV. 1999, 99, 1437.
(11) (a) Garc ´ı a Mart ´ı nez, A.; Os ´ı o Barcina, J.; Colorado Heras, M. R.;
de Fresno Cerezo, A.; Torres Salvador, M. R. Chem.sEur. J. 2003, 9, 1157.
(b) Rathore, R.; Chebny, V. J.; Abdelwahed, S. H. J. Am. Chem. Soc. 2005,
127, 8012.
(10) (a) Strassner, T. Can. J. Chem. 1997, 75, 1011. (b) Feigel, M. J.
Mol. Struct. 1996, 366, 83. (c) Montaudo, G.; Finocchiaro, P.; Caccamese,
S.; Bottino, F. J. Am. Chem. Soc. 1971, 93, 4208.
2944
Org. Lett., Vol. 9, No. 15, 2007

Figure 1. Z- and U-shaped conformers of dimer 1.
interactions between the para hydrogen atom of one molecule
1
3
and the central aryl ring of the neighboring dimer,
a
disposition that resembles the T-shaped edge-to-face aromatic
interaction in the benzene dimer.14 Both aryl rings in 1 are
almost perpendicular (93.9°), and the distance between the
centroids of the arenes is 5.14 Å. The CH-π distance is
Figure 3. Electronic spectra in hexane of DPN, 1, 3, and 4.
2.69 Å, well below the sum of the corresponding van der
Waals radii (1.2 Å for C-H plus 1.7 Å for a half-thickness
of the aryl ring). All these values are typical of CH-π
interaction.13 However, no π-π interactions are observed
between two adjacent parallel aromatic rings since the inter-
ring distance (3.63 Å) is clearly larger than the sum of the
van der Waals radii of aryl rings (3.40 Å).15
of the homoconjugation between the aromatic systems.8,16
Consequently, electronic delocalization by means of aromatic
homoconjugation in DPNs can be evaluated by UV spec-
7
troscopy.
Table 1. Electronic Spectra Data in Hexane of DPN, 1, 3, and
4
ꢀ 10-
3
ꢀ 10-3/n
λmax
a
b
DPN
1
228
242
12.6
19.5
12.6 (6.3)
a
b
9.75 (6.5)
(
222)
(11.0)
27.0
(21.0)
32.0
a
b
b
3
4
248
(
250
9.0 (6.7)
232)
a
8.0 (6.4)
(
(
231)
228)
(28.0)
(27.8)
a
Considering n ) number of DPN subunits. b Considering n ) number
of aryl rings.
Figure 2. Crystal packing of dimer 1.
Figure 3 shows the electronic spectra of dimer 1, trimer
3
, and tetramer 4 as well as DPN. As can be seen, the
characteristic homoconjugation band of DPN, centered at 228
nm (ꢀ ) 12 600), is bathochromically and hyperchromically
shifted as the chain length of the oligomers increases.
Extension of chromophore homoconjugation shifts the long-
est wavelength absorption maximum (λmax) at 242 nm (ꢀ )
We have previously described that the electronic spectra
of DPN and its derivatives show an additional band which
is not observed in diphenylmethanes (Figure 3, Table 1). The
origin of this band is the preferred cofacial disposition of
the aryl rings. Therefore, this absorption is a consequence
1
9 500) in 1 (∆λ ) 14 nm), at 248 nm (ꢀ ) 27 000) in 3
(
(
∆λ ) 6 nm), and 250 nm (ꢀ ) 32 000) in 4 (∆λ ) 2 nm)
Table 1).
From the absorption increments of the homoconjugation
(
12) (a) Kl a¨ rner, F.-G.; Kahlert, B. Acc. Chem. Res. 2003, 36, 919. (b)
Garc ´ı a Mart ´ı nez, A.; Os ´ı o Barcina, J.; de Fresno Cerezo, A. Chem.sEur.
J. 2001, 7, 1171.
(13) (a) Nishio, M. Tetrahedron 2005, 61, 6923. (b) Steiner, T. Angew.
bands of the oligomers 1-4, it can be concluded that
saturation of the bathochromic shift is reached for oligomers
n ) 4 and 5 since, on going from the trimer 3 to the tetramer
Chem., Int. Ed. 2002, 41, 48. (c) Tsuzuki, S.; Honda, K.; Uchimaru, T.;
Mikami, M.; Tanabe, K. J. Am. Chem. Soc. 2002, 124, 104. (d) Umezawa,
Y.; Tsuboyama, S.; Takahashi, H.; Uzawa, J.; Nishio, M. Tetrahedron 1999,
5
5, 10047.
14) Meyer, E. A.; Castellano, R. K.; Diederich, F. Angew. Chem., Int.
Ed. 2003, 42, 1210.
15) Yu, L.; Schneider, H.-J. Eur. J. Org. Chem. 1999, 1619.
(
(16) Garc ´ı a Mart ´ı nez, A.; Os ´ı o Barcina, J.; de Fresno Cerezo, A.; Rojo,
(
G.; Agull o´ -L o´ pez, F. J. Phys. Chem. B 2000, 104, 43.
Org. Lett., Vol. 9, No. 15, 2007
2945

4
, an increase of only 2 nm is observed. The ꢀ values also
In summary, an efficient and convenient synthetic meth-
odology for the preparation of new monodisperse homocon-
jugated molecular wires, derived from cofacial DPN, has
been developed. The effective homoconjugation length
(EHL) in these oligomers has been estimated to be in the
range of 4-5 DPN subunits (5-6 homoconjugated cofacial
aryl rings). We are currently investigating the applications
of these wires in photoinduced electron- and energy-transfer
processes and compounds with NLO properties, as well as
its use as open chain preorganized ligands able to form stable
complexes with cations and neutral guests.
increase with chain length of the oligomers, but ꢀ/n remains
3
in the range of (9.0 ( 1.0) × 10 (considering n as number
3
of DPN subunits) or (6.5 ( 0.2) × 10 (considering n as
number of aryl rings). Therefore, the limiting value of the
wavelength in this type of system can be estimated to be in
the range of 4-5 monomeric units (tetramer or pentamer).
In conjugated oligomers and polymers, the number of
repeat units required to furnish size-independent properties
(redox, optical, electronic) defines the effective conjugation
17
length (ECL) of the molecular wire. By analogy with
conjugated oligomers, the effective homoconjugation length
(
EHL) for homoconjugated oligomers can be also defined
Acknowledgment. Thanks are due to the Ministerio de
Educaci o´ n y Ciencia of Spain (Research Project CTQ2004-
07244-C02-01) for the support of this investigation.
for the first time. Therefore, for the homoconjugated mo-
lecular wires described in this work, the EHL can be
estimated to be in the range of 4-5 DPN subunits. This is
important information in order to achieve further studies on
photoinduced electron- and energy-transfer processes or
NLO16 properties employing homoconjugated wires.
Supporting Information Available: Detailed experi-
mental procedures, full spectroscopic data and crystal-
lographic data of dimer 1. This material is available free of
charge via the Internet at http://pubs.acs.org.
(17) (a) Rissler, J. Chem. Phys. Lett. 2003, 395, 92. (b) Meier, H.;
Stalmach, U.; Kolshorn, H. Acta Polym. 1997, 48, 379.
OL071161B
2946
Org. Lett., Vol. 9, No. 15, 2007