Reprinted from The Journal of Organic Chemistry, 1988, Vol. 53, page 2489 Copyright @ 1988 by the American Chemical Society and reprinted by permission of the copyright owner. Synthesis of High Carbon Materials from Acetylenic Precursors. Preparation of Aromatic Monomers Bearing Multiple Ethynyl Groups1 Thomas X. Neenan and George M. Whitesides* Department of Chemistry, Harvard University, Cambridge, Massachusetts 02138 Received October 13, 1987 The synthesis of polyethynyl aromatics as starting materials for the preparation of highly cross-linked organic solids containing high atom fractions of carbon is described. Treatment of bromo- and iodoaromatic compounds with (trimethylsily1)acetylene (TMSA) in the presence of palladium(0)and copper(1) in amine solvents yields (trimethylsily1)ethynyl-substituted aromatics. The TMS protecting groups can be removed by hydrolysis with mild base. Compounds prepared by using this technique include 1,3-diethynylbenzene,2,5-diethynylthiophene, 1,3-diethynyltetrafluorobenzene,1,4-diethynyltetrafluorobenzene, 2-ethynylthiazole, 2,4-diethynylthiazole, 2,7- diethynylnaphthalene, hexakis((trimethylsilyl)ethynyl)benzene, tetraethynylthiophene, 2,5-bis((trimethy1- silyl)ethynyl)-3,4-bis(3-hydroxy-3-methyl-l-butynyl)thiophene,2,5-diethyny1-3,4-bis(3-hydroxy-3-methyl-l-b~- tynyl)thiophene, 2,5-bis(4-(2-thienyl)butadiynyl)-3,4-bis(3-hydroxy-3-methyl-l-butynyl)thiophene,and 2,5-bis- (4-(2-thienyl)butadiynyl)-3,4-diethynylthiophene. Introduction We are engaged in a project aimed at the preparation of organic solids containing a high density of strong, di- rected carbon-carbon bonds. Our objective in this project is to relate the molecular-level structure of the solids to their macroscopic physical properties and to determine if it is possible to replicate in these materials some of the remarkable range of properties exhibited by pure allotropes of carbon (diamond: hardness, high thermal conductivity) (graphite: electrical conductivity, l~bricity).~We expect high-carbon, densely cross-linked organic solids to display high thermal stability and a high degree of hardness. Our strategy for the preparation of these materials involves the initial syntheses of low molecular weight poly(di- acetylenes); these polymers are then molded into desired shapes and processed thermally, photochemically, and/or catalytically to yield the final, highly cross-linked organic solids. This paper details the preparation of several monomeric search at DBM has developed related-materials based upon polyacetylenic aromatic compounds. Certain of these the oxidative coupling of 1,3,5-triethynylben~ene.~ monomers are the precursors to the intermediate polymers; A separate effort directed toward high-carbon solids at others were too reactive and unstable to be useful in po- Hercules concentrated on the polymerization of di- lymerizations but are of interest in their own right as un- ethynylbenzenes by a route involving cyclotrimerization usual ethynylated compounds. We will discuss the po- of the ethynyl group^.^ This procedure yielded polymers lymerization of these monomers and materials processing ostensibly containing polyphenylene units, residual ace- of the resulting polymers in later paper^.^ tylenic residues, and a small fraction of olefinic units. The commercial development of organic solids from These olefinic groups resulted from a process in which the acetylenic precursors dates from the 1960s when Hay acetylenes underwent a linear rather than a cyclo- showed that the oxidative coupling of 1,Bdiethynylbenzene trimerization polymerization. The Hercules process also (1) yielded a soluble polymers5 The molecular weight and used suitable end-capping groups such as 2 to limit mo- solubility of this; polymer could be controlled by using lecular weight. Soviet workers have also extensively de- mixtures of 1 and phenylacetylene (2) (as an end-capping veloped the cyclotrimerization polymerization of diethynyl group) in the polymerization. This procedure yielded aromatics? Commercialization of materials derived from phenyl-terminated poly(diethyny1benzene) oligomers 3 diethynylbenzene and other acetylenic precursors was that could be further processed at high temperatures to exdored at both General Electric and Hercules. The ab- form carbon films and fibers. Research at General Electric sence of an inexpensive route to the monomers, together expanded this type of chemistry to include a wide range with the limitation of a low elongation-to-break of the of acetylenic aromatic monomer^.^ More recently, re- finished materials, discouraged their full development. A parallel effort to develop acetylene-containing poly- (1) This work was supported by the National Science ~oundaGon; mers as thermoset resins resulted in the development of Grant CHE 85-08702. (2) Diercks, R.; Armstrong, J. C.; Boese, R.; Vollhardt, K. P. C. Angew. Chem.. Int. Ed. Enpl. 1986.25.268-269. (7) Dawson, D. J.; Fleming, W. W.; Lyerla, J. R.; Economy, J. In (3) ?he ~ropert'es of ~iamond;Field, J. E., Ed.;Academic Press: Reactive Oligomers; ACS Symposium Series 282; American Chemical London, 1979. Paxton, R. R. Manufactured Carbon. A Self-Lubricating Societv: Washineton. DC. 1986: DD 63-79. Economv. J.: Flandera. M. Material for Mechanical Devices; CRC: Boca Raton, FL, 1979. A.; ~ii,C.-Y. u.s:P~~:4 25'8079,1981. Economy, J.; ~i&dera,M. A. U.S. (4) Neenan, T. X.; Whitesides, G. M., unpublished. Pat. 4 273906, 1981. (5) Hay, A. S. J. Org. Chem. 1960, 25, 1275-1276. (8) Cessna, L. C., Jr. U.S. Pat. 3882073,1975. Jabloner, H. U.S. Pat. (6) Hay, A. S. U.S. Pat. 3 300456,1967. Hay, A. S. U.S. Pat. 3 332 916, 4070333. 1978. Jabloner. H. U.S. Pat. 4097460. 1978. 1967. Hay, A. S. U.S. Pat. 3519611, 1970. White, D. M.; Klopfer, H. J. (9) ~orshak,V. V.; volpin, M. E.; Sergeev, V. A.; Shitikov, V. K.; U.S. Pat. 3748305,1973. White, D. M. U.S. Pat. 3816374,1974. White, Kolomnikov, I. S. U.S. Pat. 3705 131, 1972. Korshak, V. V.; Gribova, I. D. M. U.S. Pat. 4020265, 1977. Chalk, A. J.; Gilbert, A. R. U.S. Pat. A.; Krasnov, A. P.; Sergeev, V. A,; Shitikov, V. K.; Elerdashvili, G. V. U.S. 4 108942,1978. Pat. 3 756 982, 1973. 2490 J. Org. Chem., Vol. 53, No. 11, 1988 Neenan and Whitesides acetylene-terminated imide oligomers such as the Ther- as an alternative, but deprotedion requires hydroxide base mid-600 series.1° These materials have excellent thermal in high boiling solvent. stability but their high cost and difficulty in curing have restricted their development. Results and Discussion Classical methods for the synthesis of terminal aryl- Ethynylbenzenes. We prepared l,&diethynylbenzene acetylenes involve the manipulation of preformed two- (1) by the reaction of l,&dibromobenzene with TMSA carbon units." Most useful among these are the Vilsmeier using dichlorobis(benzonitrile)palladium, triphenyl- reaction,12 the halogenation-dehydrohalogenation of vinyl phosphine, and copper(1) iodide as catalyst. The TMS- aromatics and ketones,13J4and the dehydrohalogenation protected intermediate (6) is an air- and moisture-stable of P,o-dihalo olefins.16 This latter reaction was employed for the preparation of diethynylbenzene in the research of both General Electric and Hercules. A number of re- actions also introduce ethynyl moieties directly into aro- matic rings. The most widely accepted of these methods has been the Stephens-Castr~'~.'~reaction between an solid. Treatment of compound 6 with a catalytic amount aromatic halide and a protected cuprous acetylide in re- of base in methanol removes the protecting groups. fluxing pyridine. Difficulty in removing protective groups Compound 1 is a clear colorless liquid that discolors upon has been a limitation of this method. An alternative to prolonged standing. Compound 1 may be purified by the Stephens-Castro method-the coupling between distillation at moderate temperatures and under high arylcopper reagents and (iodoethyny1)trimethylsilane at vacuum. (Note: Our initial attempt to distill 1 at tem- low temperatures18-is limited by the necessity to form the peratures of up to 110 OCand at aspirator pressures organocopper reagents. The palladium-catalyzed reaction resulted in an exothermic reaction followed by an ex- of alkynylzinc reagents with aryl halideslg yields terminal plosion. Compound 1 and all other volatile polyethynyl and internal ethynyl aromatic compounds and does not aromatics should be distilled at high vacuum and at require protection of the alkynyl groups but does require temperatures of less than 60 OCin well-shielded equip- as reactant an aryl iodide or activated aryl bromide. A ment. Only limited quantities should be distilled, stored, system of palladium(0) and copper(1) catalytically couples or manipulated as pure (undiluted) material. All of the acetylenes and aryl iodide~.~O?~lHagihara and Lau im- deprotected ethynyl aromatics mentioned in this paper proved this reaction by using (trimethylsily1)acetylene should be treated as potentially explosive material^.)^^ (TMSA).22 This improvement yields stable, readily ma- The same chemistry can be extended to the preparation nipulated trimethylsilyl-protected ethynyl aromatics as of higher poly- and persubstituted acetylenic aromatic intermediates; mild basic hydrolysis removes the TMS compounds. Reaction of hexaiodobenzene (8) with TMSA group. Electron-withdrawing groups on the aryl ring fa- cilitate the reaction; electron-donating groups are detri- mental. This method offers the advantages of simplicity, ease of workup, and high yields. Vollhardt and co-workers first