Alkyne metathesis with simple catalyst systems: Poly(p- phenyleneethynylene)s [1]

Full text of DOI:10.1021/ja9813865

J. Am. Chem. Soc. 1998, 120, 7973-7974
7973
Communications to the Editor
where (tBuO)3WtCtBu (3), a superb catalyst, has been used to
Alkyne Metathesis with Simple Catalyst Systems:
Poly(p-phenyleneethynylene)s
generate polymers by ring-opening of strained cyclic alkynes.8,9
We have used 3 for ADIMET (acyclic diyne metathesis) of
dipropynylated benzenes (1) to form high molecular weight PPEs
(2).10 Although high molecular weight PPEs have been obtained
by the Pd-route in some cases,11 ADIMET circumvented the
problems associated with the classic Pd-catalyzed couplings,
difficult separation of phosphorus and palladium containing
catalyst residues, ambiguity regarding the end groups (dehalo-
genation, formation of phosphonium salts),12 and the occurrence
of defect structures (butadiyne linkages formed by oxidative
coupling of terminal alkynes).13 The preparation of pyrophoric
3, however, is difficult and requires rigorous exclusion of air and
moisture. It can only be handled safely in an inert atmosphere.
These considerations render the ADIMET reaction utilizing 3 less
attractive. We therefore set out to develop an alternative, simple
ADIMET route to PPEs.
Lioba Kloppenburg, Dschun Song, and Uwe H. F. Bunz*
Department of Chemistry and Biochemistry
UniVersity of South Carolina
Columbia, South Carolina 29208
ReceiVed April 23, 1998
We describe in this communication conditions under which
an in situ catalyst system (molybdenum hexacarbonyl and
p-(trifluoromethyl)phenol) efficiently metathesizes 1,4-dipropy-
nylbenzenes (1) to poly(p-phenyleneethynylene)s (PPE, 2), at-
Despite the moderate yield, we decided to attempt the
transformation of dipropynyl benzenes (1)14,15 into PPEs, using
the easily accessible Mo(CO)6/phenol system. For polymer
formation, 1a (R ) hexyl) was reacted in 1,2-dichlorobenzene
with a molybdenum hexacarbonyl/p-(trifluoromethyl)phenol com-
bination as catalyst, (see Table 1 entry 2) for 16 h at 150 °C.
From prior experiments, we knew that p-(trifluoromethyl)phenol
was more active as cocatalyst than p-chlorophenol (entry 1). An
important experimental detail is that the byproduct butyne always
had to be removed by a constant N2 purge.16 No attempts were
made to dry or exclude O2 from the reaction mixture. Workup
tractive materials for photonic devices such as LEDs and polymer-
based lasers.1
Alkene metathesis is now an established and powerful tool for
organic, bioorganic,2 and polymer synthesis.3,4 The conceptually
(8) Zhang, X.-I.; Bazan, G. C. Macromolecules 1994, 27, 4627.
(9) McCullogh, L. M.; Schrock, R. R. J. Am. Chem. Soc. 1984, 106, 4067.
Krouse, S. A.; Schrock, R. R. Macromolecules 1989, 22, 2569.
(10) Weiss, A. M.; Auth, E.-M.; Bunz, U. H. F.; Mangel, T.; Mu¨llen, K.
Angew. Chem., Int. Ed. Engl. 1997, 36, 506.
(11) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 12593. Davey,
A. P.; Elliott, S.; O’Connor, O.; Blau, W. J. Chem. Soc., Chem. Commun.
1995, 1433.
related process of alkyne metathesis, however, has found much
less attention. Metathesis of C-C triple bonds by molybdenum/
phenol species has been known since the 1970s5 but was never
developed into an established synthetic method. A recent report
describes the metathesis of internal alkynes at elevated temper-
atures, utilizing a Mo(CO)6/p-chlorophenol mixture.6 The yields
did not exceed 80%, even if one component was added in large
excess or removed to drive the cross metathesis to completion.
The well-defined tungsten alkylidynes, introduced by Schrock,7
are active for alkyne metathesis. Scattered examples are known
(12) (a) Mangel, T.; Eberhardt, A.; Scherf, U.; Bunz, U. H. F.; Mu¨llen, K.
Macromol. Rapid Commun. 1995, 16, 571. (b) Goodson, F. E.; Wallow, T.
I.; Novak, B. M. J. Am. Chem. Soc. 1997, 119, 12441.
(13) (a) Wautelet, P.; Moroni, M.; Oswald, L.; LeMoigne, J.; Pham, A.;
Bigot, J.-Y.; Luzatti, S. Macromolecules 1996, 29, 446. (b) Li, H.; Powell,
D. R.; Hayashi, R. K.; West R. Macromolecules 1998, 31, 52. (c) Lakshmi-
kantham, M. V.; Vartikar, J.; Jen, K. Y.; Cava, M. P.; Huang, W. S.;
MacDiarmid, A. G. Polym. Prepr. (Am. Chem. Soc. DiV. Polym. Chem.) 1983,
24, 75. (d) Sanechika, K.; Yamamoto, T.; Yamamoto, A. Bull. Chem. Soc.
Jpn. 1984, 57, 752. (e) Trumbo, D. L.; Marvel, C. S. J. Polym. Sci., Polym.
Chem. 1986, 24, 2311. (f) Giesa, R.; Schulz, R. C. Makromol. Chem. 1990,
191, 857. (g) Weder, C.; Wrighton, M. S. Macromolecules 1996, 29, 5157.
(h) Wrighton, M. S.; Ofer, D.; Swager, T. M. Chem. Mater. 1995, 7, 418 (i)
Steiger, D.; Smith, P.; Weder, C. Macromol. Rapid Commun. 1997, 18, 643.
(j) Oligomeric compounds: Moore, J. S. Acc. Chem. Res. 1997, 30, 402. Tour,
J. M. Chem. ReV. 1996, 96, 537. Grubbs, R. H.; Kratz, D. Chem. Ber. 1993,
126, 149. Ho¨ger, S.; Enkelmann, V. Angew. Chem. 1996, 34, 2713.
(14) A series of substituted diiodobenzenes 5a-h was synthesized according
to literature procedures described in ref 13b and propynylated to give the
monomers 1a-h.15
(15) (a) In a 500-mL round-bottomed flask, closed with a ground glass
valve and evacuated to 1 mmHg were placed 4.98 g (10.0 mmol) of 5a,
piperidine (50 mL), PdCl2(PPh3)2 (351 mg, 0.500 mmol), and CuI (190 mg,
1.00 mmol). Propyne (520 mL, 760 mmHg) was added through the valve.
The closed flask was shaken for 4 h. Aqueous workup, chromatography (silica
gel, hexanes), and crystallization from ethanol yielded 2.65 g (82%) of 1a.
The other dipropynyls 1 were produced accordingly. (b) The optically active
1e was obtained from commercially available (S)-(+)-citronellyl bromide after
catalytic hydrogenation (no loss of the bromine functionality), transformation
into its Grignard reagent and coupling to 1,4-dichlorobenzene under standard
Kumada17 conditions. Iodination14 and propynylation yields optically active
1e. (c) Structural proof was obtained by solid-state 13C NMR spectroscopy,
which showed the three expected signals for the arene ring, one resonance
for the alkyne group, and the expected four features for the solubilizing iso-
pentyl groups.
(1) (a) Neher, D. AdV. Mater. 1995, 7, 691. (b) Kraft, A.; Grimsdale, A.
C.; Holmes, A. B. Angew. Chem. 1998, 37, 402. (c) Wang, P.-W.; Liu, Y.-J.;
Moore, J. S. AdV. Mater. 1996, 8, 237. (d) Hide, F.; Diaz-Garcia, M. A.;
Schwartz, B. J.; Heeger, A. J. Acc. Chem. Res. 1997, 30, 430.
(2) Grubbs, R. H.; Miller, S. J.; Fu, G. C. Acc. Chem. Res. 1995, 28, 446.
Miller S. J.; Blackwell, H. E.; Grubbs, R. H. J. Am. Chem. Soc. 1996, 118,
9606. Schuster, M.; Blechert, S. Angew. Chem. 1997, 36, 2036. Crowe, W.
E.; Zhang, Z. J. J. Am. Chem. Soc. 1993, 115, 10998. Crowe, W. E.; Goldberg,
D. R. J. Am. Chem. Soc. 1995, 117, 5162.
(3) Bureta, M. A.; Tilley, T. D. Organometallics 1997, 16, 1507. Weck,
M.; Mohr, B.; Maughon, B. R.; Grubbs, R. H. Macromolecules 1997, 30,
6430. Wagaman, M. W.; Grubbs, R. H. Macromolecules 1997, 30, 3978.
Grubbs, R. H.; Tumas, W. Science 1989, 243, 907. Schrock, R. R. Pure Appl.
Chem. 1994, 66, 1447. Schrock, R. R. Acc. Chem. Res. 1990, 23, 158.
(4) Lindemark-Hamberg, M.; Wagener, K. B. Macromolecules 1987, 20,
2951. Wagener, K. B.; Smith, D. W., Jr. Macromolecules 1991, 24, 6073.
Tassoni, R.; Schrock, R. R. Chem. Mater. 1994, 6, 744.
(5) Montreux, A.; Blanchard, M. J. Chem. Soc., Chem. Commun. 1974,
786. Bencheik, A.; Petit, M.; Montreux, A.; Petit, F. J. Mol. Catal. 1982, 15,
93. Villemin, D.; Cadiot, P. Tetrahedron Lett. 1982, 23, 5139. DuPlessis, J.
A. K.; Vosloo, H. C. M. J. Mol. Catal. 1991, 65, 51.
(6) Kaneta, N.; Hikichi, K.; Mori, M. Chem. Lett. 1995, 1055. Kaneta, N.;
Hirai, T.; Mori, M. Chem. Lett. 1995, 627.
(7) Schrock, R. R.; Clark, D. N.; Sancho, J.; Wengrovius, J. H.; Pederson,
S. F. Organometallics 1982, 1, 1645.
S0002-7863(98)01386-9 CCC: $15.00 © 1998 American Chemical Society
Published on Web 07/23/1998

7974 J. Am. Chem. Soc., Vol. 120, No. 31, 1998
Communications to the Editor
Table 1. Results of the ADIMET-Polymerization of Dipropynyl Benzenes 1 Utilizing Mo(CO)6/ArOH Catalyst Systems
ArOH
Pn
Pn
GPC
Mw/Mn
GPC
λmax
2 nm
entry
substituent 1,2
polymerization condition
50 mol % yield PPE 2 NMR
1
2
3
4
a hexyl
a hexyl
b-CH2CH2CH(CH3)2
c-CH2CH(CH2CH3)2
125 °C, 16 h, 5 mol % Mo(CO)6 4-Cl-phenol 85 %
150 °C, 16 h, 5 mol % Mo(CO)6 4-CF3-phenol quantitative
150 °C, 16 h, 5 mol % Mo(CO)6 4-CF3-phenol quantitative
150 °C, 16 h, 5 mol % Mo(CO)6 4-CF3-phenol 96 %
59
94 150
404
396
382
3.5
insoluble
50 soluble 3.8 barely 384
fraction soluble
180 2.6
5
6
7
8
9
d-2-(ethyl)hexyl
150 °C, 16 h, 5 mol % Mo(CO)6 4-CF3-phenol 93 %
387
396
275
440
e-[S-(+)-(CH2)2CHMe(CH2)3CH(CH3)2] 150 °C, 16 h, 5 mol % Mo(CO)6 4-CF3-phenol quantitative >150 500
f-OCH2CH2CH(CH3)2
f-OCH2CH2CH(CH3)2
g-CO2CH2CH2CH(CH3)2-CH3
2.5
130 °C, 16 h, 5 mol % Mo(CO)6 4-Cl-phenol 25 %
130 °C, 16 h, 10 mol % Mo(CO)6 4-CF3-phenol quantitative
130 °C, 16 h, 10 mol % Mo(CO)6 4-CF3-phenol
8
15 78
2.7
no polymer
gave rise to the isolation of 2a in quantitative yield.16 Soxhlet
extraction with boiling chloroform redissolved 2a completely,
regardless of its history. The degree of polymerization (Pn) and
purity were obtained from 1H NMR spectroscopy. The propyne
end groups appear as a singlet at δ 2.04, where no other signals
of 2a are observed; thus, a Pn ) 94 was estimated by end group
integration (see Table 1). We performed gel permeation chro-
matography (GPC) on this polymer, measuring Pn ≈ 150 and a
polydispersity index (Mw/Mn) of 3.5. The disparity of GPC and
NMR data is not surprising since it is known that Pn of stiff
macromolecules is overestimated by GPC, rendering the NMR
data more reliable.
To study the effect of branching with respect to solubility,
1b-d were prepared. Polymerization afforded the corresponding
PPEs 2 (see Table 1). Polymers 2b,c carrying the 3-methylbutyl
and the 2-ethylbutyl substituent, respectively, were completely
insoluble.15c 2-Ethylhexyl-substituted 1d was polymerized to 2d
with considerably increased Pn and solubility. The most impres-
sive result was obtained for ADIMET of optically active 1e. In
2e, the end groups no longer could be detected by proton NMR
(Pn g 150). GPC shows an apparent Pn of ∼500, yet the material
readily dissolved in warm chloroform to give a highly viscous
solution. All of the described PPEs are yellow (alkyl substituted)
or orange (alkoxy substituted, vide infra). As expected, they
display an intense purple (solution) or greenish-blue/yellow (solid
state) fluorescence.
ploying more Mo(CO)6 (10 mol %, entry 8) led to a dramatically
improved yield (quantitative) and increased Pn to 15. As a
consequence, the presence of ether linkages in 1 is not detrimental
to alkyne metathesis but produces polymers 2f with a lower Pn.
The catalytically active species in these polymerization reac-
tions is probably a Schrock-type carbyne of the structural formula
(ArO)3MotC-R (4), accessed by in situ oxidation of Mo(CO)6
in the presence of phenols. Such an oxophilic species, 4, would
react with an ester group, and its catalytic activity would be
attenuated by the presence of any ether linkage or another donating
functionality. We observe that behavior in our catalytic system.
Experiments to determine the nature of the active species are under
way.
In conclusion, we have shown that the ADIMET reaction can
be performed in an experimentally simple setup, utilizing inex-
pensiVe and commercially aVailable catalyst precursors without
rigid exclusion of air and moisture. Ease of process
and workup should make alkyne metathesis amenable to facile
scale-up of industrial-sized preparations. The in situ catalyst is
not only highly active but very selective and efficient for the
formation of high-molecular weight PPEs in excellent yields.
These attributes endow the described catalyst system with the
potential to be a powerful tool for the synthesis of attractive
conjugated polymers1 and novel small molecules containing
alkyne units.
To explore the scope of the ADIMET reaction, alkoxy- (1f)
and an ester-substituted monomer (1g) were prepared. The ester
1g was unreactive under different metathesis conditions, but an
initial attempt utilizing 1f (see entry 7, Table 1) was moderately
successful, resulting in low-molecular weight oligomers. Em-
Acknowledgment. We wish to thank Prof. Haskell Beckham for the
recording of the solid-state NMR of 2b and c, Adam Rawlett for GPC
measurements of 2a, d, e, and f, and Prof. Ken Shimizu (USC) for helpful
discussions. The work was financed by the University of South Carolina.
LK thanks the Deutsche Forschungsgemeinschaft for a postdoctoral
stipend.
(16) Polymerization: 1a (500 mg, 1.55 mmol), Mo(CO)6 (21 mg, 0.078
mmol), p-(trifluoromethyl)phenol (251 mg, 1.55 mmol), and 20 mL 1,2-
dichlorobenzene were placed in a 50-mL Schlenk flask, equipped with a reflux
condenser. The reaction mixture was heated for 16 h to 150 °C under N2
purge. Addition of dichloromethane, removal of the phenol by basic extraction,
precipitation into methanol, and drying at 0.1 mmHg for 12 h gave 441 mg
(>99%) of polymer 2a as yellow flakes.
(17) Tamao, K.; Sumitani, K.; Kiso, Y.; Zembayashi, M.; Fijioka, A.;
Kodama, S. I.; Nakajima, I.; Minato, A.; Kumada, M. Bull. Chem. Soc. Jpn.
1976, 49, 1958.
Supporting Information Available: Experimental details and spec-
troscopic characterization of 1-2 (10 pages, print/PDF). See any current
masthead page for ordering information and Web access instructions.
JA9813865