Electronic Delocalization through Phosphorus
A R T I C L E S
Chart 1
tion of the spectroscopic and electrochemical properties of these
novel polymers supports electronic delocalization through
phosphorus along the backbone of the polymer upon chemical
or electrochemical oxidation.
Results
Synthesis of Monomers for Polymerization. The preparation
of PPPP-PANI copolymers can be conducted by either N-C
or P-C bond-forming reactions. Efficient palladium-catalyzed
C-N bond-forming reactions have recently been developed by
Hartwig23 and Buchwald,24 while the related palladium-catalyzed
P-C bond-forming reactions have also been reported.25 We
attempted to prepare PPPP-PANI copolymers via both N-C
and P-C bond-forming reactions and discovered that condensa-
tion polymerization via P-C bond formation is more effective
for polymerization reactions. While palladium-catalyzed C-N
bond-forming reactions have been used to generate poly-
(N-arylamine)s,18 the polymerization procedures typically utilize
hindered phosphine ligands, which allow the generation of
reactive Pd(0) monophosphine complexes.26 Attempts to prepare
PPPP-PANI copolymers using similar conditions were inef-
fective. The problems encountered during attempts to prepare
PPPP-PANI copolymers via C-N bond-forming reactions are
likely due to the presence of high concentrations of unhindered
phosphine comonomers. Alternative attempts to conduct C-N
bond-forming polymerizations with PdBINAP or PdDPPF
generated only low molecular weight oligomers. Therefore,
palladium-catalyzed C-N and C-P bond-forming reactions
were used to prepare the appropriate monomers for polymeri-
zation, and P-C bond-forming reactions were used for polym-
erization reactions.
The preparation of 1,4-bis(N-4′-iodophenyl,-N-4′′-n-butyl-
phenylamino)benzene (2) followed a two-step procedure. First,
reaction of 1,4-dibromobenzene with excess (5 equiv) 4-butyl-
aniline catalyzed by Pd(DPPF)Cl2 (2.5 mol %) in toluene with
2.2 equiv of NaOt-Bu was conducted for 2 days at 100 °C.
Purification via column chromatography yields light yellow
crystals of 1,4-bis(N-4′-n-butylphenylamino)benzene (1, 65%
yield). Conversion of diamine 1 to 2 is achieved by reaction of
1 with excess 1,4-diiodobenzene catalyzed by Pd(DPPF)Cl2
(5 mol %) in toluene with 2.2 equiv of NaOt-Bu for 7 days at
100 °C. This provides 2 as a yellow solid (20% yield).
Following related procedures N,N-bis-p-bromophenyl-p-anis-
idine (3) was prepared via the palladium-catalyzed cross
coupling of p-anisidine with dibromobenzene. Compound 3 was
isolated as a white crystalline material in 61% yield. Attempts
to prepare N,N-bis-p-bromophenyl-p-n-butylaniline via pal-
ladium-catalyzed cross coupling of dibromobenzene with n-
butylaniline provided only low yields of the monosubstituted
product. While 3 is a good starting material for the preparation
Scheme 1
amples where the lone pair on phosphorus is involved in
extended delocalization.14 Other examples supporting overlap
of the lone pair on phosphorus with p orbitals on carbon include
phosphanylcarbenes reported by Bertrand and co-workers.15 The
X-ray structure of a phospanyl(mesityl)carbene contains a
phosphorus atom in a planar environment consistent with
electronic donation of the lone pair into the vacant orbital of
the carbene. These results suggest that the lone pair on
phosphorus has good overlap with the electron-deficient p orbital
on carbon. However, further investigation is required to
determine the extent of delocalization of the lone pair of
electrons on phosphines and related phosphorus(III) species.
The preparation and investigation of oligo(N-phenyl-
aniline),16,17 poly(N-arylaniline)s,18 and poly(phenylenesulfide)-
polyaniline alternating copolymers19-21 have been reported. In
most cases the electronic and spectroscopic properties are similar
to polyaniline. However, the substituted polymers have good
solubility in organic solvents, providing superior processability.
Incorporation of poly(phenylenesulfidephenyleneamine) as a
hole transport layer in trilayer light-emitting diodes has been
shown to promote hole injection from ITO into the emissive
layer.22 In this article we describe the synthesis of poly-
(p-phenylene phosphine)-polyaniline alternating copolymers
(PPPP-PANI, Scheme 1). PPPP-PANI copolymers are pre-
pared via palladium-catalyzed C-P or C-N bond-forming
reactions. Depending on the structure of the comonomers used
for polymerization, PPPP-PANI copolymers have been
prepared with main-chain alternating units comprised
of -(-N-C6H4-P-C6H4-)-, -(-N-C6H4-N-C6H4-P-
C6H4-)-, or -(-N-C6H4-P-C6H4-P-C6H4-)-. Investiga-
(14) (a) Lucht, B. L.; St. Onge, N. O. Chem. Commun. 2000, 2097. (b) Jin, Z.;
Lucht, B. L. J. Organomet. Chem. 2002, 653, 167.
(15) Buron, C.; Gornitzka, H.; Romanenko, V.; Bertrand, G. Science 2000, 288,
834.
(16) (a) Sadighi, J. P.; Singer, R. A.; Buchwald, S. L. J. Am. Chem. Soc. 1998,
120, 4960. (b) Singer, R. A.; Sadighi, J. P.; Buchwald, S. L. J. Am. Chem.
Soc. 1998, 120, 213. (c) Louie, J.; Hartwig, J. F.; Fry, A. J. J. Am. Chem.
Soc. 1997, 119, 11695.
(17) Strohriegl, P.; Jesberger, G.; Heinze, J.; Moll, T. Makromol. Chem. 1992,
193, 909-919.
(23) (a) Louie, J.; Hartwig, J. F. Tetrahedron Lett. 1995, 36, 3609. (b) Driver,
M. S.; Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 7217. (c) Hartwig, J.
F. Angew. Chem., Int. Ed. Engl. 1998, 37, 2046.
(24) (a) Guram, A. S.; Rennels, R. A.; Buchwald, S. L. Angew. Chem., Int. Ed.
Engl. 1995, 34, 1348. (b) Wolfe, J. P.; Wagaw, S.; Buchwald, S. L. J. Am.
Chem. Soc. 1996, 118, 7215. (c) Wolfe, J. P.; Wagaw, S.; Marcoux, J.-F.;
Buchwald, S. L. Acc. Chem. Res. 1998, 31, 805.
(25) (a) Cai, D.; Payack, J. F.; Bender, D. R.; Hughes, D. L.; Verhoeven, T. R.;
Reider, P. J. J. Org. Chem. 1994, 59, 7180. (b) Hillhouse, J. H. U.S. Patent
5,550,295, 1996. (c) Herd, O.; Hessler, A.; Hingst, M.; Tepper, M.; Stelzer,
O. J. Organomet. Chem. 1996, 522, 69.
(26) (a) Hartwig, J. F.; Kawatsura, M.; Hauck, S. I.; Shaughnessy, K. H.; Alcazar-
Roman, L. M. J. Org. Chem. 1999, 64, 5575-5580. (b) Wolfe, J. P.;
Tomori, H.; Sadighi, J. P.; Yin, J.; Buchwald, S. L. J. Org. Chem. 2000,
65, 1158-1174.
(18) (a) Goodson, F. E.; Hartwig, J. F. Macromolecules 1998, 31, 1700. (b)
Goodson, F. E.; Hauck, S. L.; Hartwig, J. F. J. Am. Chem. Soc. 1999, 121,
7527.
(19) (a) Wang, L. X.; Soczka-Guth, T.; Havinga, E.; Mullen, K. Angew. Chem.,
Int. Ed. Engl. 1996, 35, 1495-1497. (b) Leuninger, J.; Wang, C.; Soczka-
Guth, T.; Enkelmann, V.; Pakula, T.; Mullen, K. Macromolecules 1998,
31, 1720-1727.
(20) Leuninger, J.; Uebe, J.; Salbeck, J.; Gherghel, L.; Wang, C.; Mullen, K.
1999, 100, 79-88.
(21) Zhu, K.; Wang, L.; Jing, X.; Wang, F. Macromolecules 2001, 34, 8453-
8455.
(22) Tak, Y.-H.; Bassler, H.; Leuninger, J.; Mullen, K. J. Phys. Chem. B 1991,
102, 4887-4891.
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