Chemists report progress with arynes ék
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zyne is produced yearly in the U.S., but nowhere is it listed in a handbook. This benzene ring with, as some would have it, one triple bond, appears for an instant on the way from chlorobenzene to aniline or phenol. A small amount of it appears in the laboratory of Dr. Thomas J. Barton, Iowa State University, Ames, as he seeks to detect stepwise breakdown of benzenediazonium-2-carboxylate to benzyne. Meanwhile, Dr. Daniel R. Bloch has succeeded in extending the class of arynes—aromatic molecules with one double bond replaced by a triple bond —to cy clopentadienyl anion. The chemists discussed their work at a session on molecular architecture held by the Division of Organic Chemistry. Working at the University of Illinois, Urbana, with Dr. J. C. Martin, Dr. Bloch pyrolyzes 2-diazocyclopentadienylcarboxylate at 220° C. with tetracyclone or 3,6-diphenyl-st/m-tetrazine. From tetracyclone he gets 4,5,6,7-tetraphenylindene; from the tetrazine he gets l,4-diphenyl-2H-cyclopenta[d]pyridazine. He is able to dissolve the potassium salt of his cyclopentadienyl anion starting material in organic solvents after complexing the potassium ion with dicyclohexyl-18-crown-6 (C&EN, March 2, page 26). Bow-tie hydrocarbon. From isolation of these products, Dr. Bloch con-
cludes that he has generated the aryne of cyclopentadienyl anion, which undergoes Diels-Alder reactions with the two trapping agents, followed by splitting out of carbon monoxide or nitrogen molecules. Just as benzyne itself dimerizes to form dibenzocyclobutadiene, so the Illinois chemists hope to isolate tricyclo [5.3.0.02>6] deca-l,3,5,7,9-pentaene—the ΙΟττ aromatic, so-called "bow-tie" hydrocarbon—from dimerization of their new aryne. Starting material for pyrolysis to cyclopentadienyl anion aryne is pro duced by reaction of cyclopentadienylsodium with carbon dioxide to give cyclopentadienecarboxylic acid as a Diels-Alder dimer. Esterification of the diacid with trimethylchlorosilane, followed by cracking of the dimer at 165° C , gives trimethylsilyl cyclopentadienecarboxylate. Treatment of this ester with p-toluenesulfonyl azide and piperidine, followed by hydrolysis, gives a mixture of 2- and 3-diazocyclopentadienylcarboxylates, separable by column chromatography. Stepwise. Ever since discovery of benzenediazonium-2-carboxylate py rolyses as routes to arynes by Dr. Mar tin Stiles, University of Michigan, Ann Arbor, chemists have wondered if de composition occurs in steps, perhaps with formation of 2-carboxyphenyl cation as intermediate. Dr. Barton thought he might have uncovered evi dence of this when, in work with A.
James Nelson and Dr. Jon Clardy, he pyrolyzed benzenediazonium-2-carboxylate in presence of l,l-dimethyl-2, 5-diphenylsilole and its germanium analog. Instead of the expected DielsAlder adduct, 2,3-benzo-7,7-dimethyl1,4-diphenyl-7-silanorbomadiene ( see diagram), he gets in high yield the apparent adduct of that expected prod uct with a mole of 2-carboxyphenyl cation, a siloxepinone. On careful preparation of authentic benzosilanorbornadiene, however, and reaction with benzenediazonium-2-carboxylate at low temperatures, the si loxepinone again forms. Since silox epinone and germoxepinone form at temperatures below the ones necessary to decompose benzenediazonium-2carboxylate thermally, Dr. Barton con cludes that it is benzenediazonium-2carboxylate itself which reacts with the benzosilanorbornadiene, and not an intermediate 2-carboxyphenyl cation. The Iowa chemists made the authentic benzosilanorbornadiene by reaction of the silole with benzyne made by lead (IV) acetate oxidation of 1-aminobenzotriazole. The exploding of a 2-carboxyphenyl cation intermediate theory for this adduct's formation throws doubt on sev eral collections of evidence for this in termediate in benzyne syntheses, the Ames chemists feel. On the other hand, chemists are now left with the original question: How does benzyne
form?
There's a new aryne at Illinois ( Pf!
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-CO
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N«N
- N „ -CO, 220° C.
Ph
cor \.--'-^.. .,
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... and more on an old one at Iowa State CH, X
CH •
CH,
CH;
Ph .CO,
36 C&EN SEPT. 28, 1970
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