RESEARCH
Cyclopropenes lead to new cyclic compounds Chemists make stable radialenes and diquinoethylenes and find that some highly strained Diels-Alder adducts rearrange to new structures New and unusual cychc compounds have been synthesized from the reactive three-membered ring halocarbon, tetrachlorocyclopropene. Dr. Robert West of the University of Wisconsin, Madison, and Dr. David Zecher (now at Hercules) have prepared a series of remarkably stable radialene- [3] (trimethylenecyclopropane) derivatives plus a new type of electronic structure which they call diquinoethylene. In research into DielsAlder reactions of tetrachlorocyclopropene and related halocarbons, Dr. Stephen W. Tobey at Dow Chemical's eastern research laboratory, Wayland, Mass., and Dr. David C. F. Law, a postdoctoral fellow (now at Dow Corning) found that some highly strained adducts undergo rearrangements to interesting new structures. Details of both the Wisconsin and Dow investigations were presented at the Symposium International sur la Chimie des Petis Cycles et ses Applications, held in Louvain, Belgium. Work now under way at Wisconsin leads ultimately to compounds which contain no halogen. Tetrachlorocyclopropene reacts with a strong Lewis acid such as aluminum chloride to give a stable salt of the trichlorocyclopropenium ion, C3CI3+. This ion, Dr. West and Dr. Zecher have found, exhibits a variety of interesting reactions. For example, C3CI3 + reacts with aromatic hydrocarbons to give monoaryldichloro- or diarylmonochlorocyclopropenium ions—the higher the temperature, the greater the substitution. Using highly reactive aromatic compounds such as anisole, all three chlorine atoms can be replaced by aromatic rings to give triarylcyclopropenium ions. This is a good, general way of synthesizing triarylcyclopropenium ion salts, according to Dr. West. If C3CI3+ reacts with a phenol, a triarylcyclopropenium ion also forms. However, this ion easily loses a proton to become a neutral diarylquinocyclopropene. Quinocyclopropenes have been synthesized before, but the new route is much simpler, the Wisconsin chemists say. If a dihydroxyarylquinocyclopropene—a quinocyclopropene with the other two aromatic rings each contain44 C&EN OCT. 2, 1967
ing a hydroxyl group—is prepared, it can be oxidized with K 3 Fe(CN) 6 or Pb0 2 . "Initially we thought that the oxidation might lead to a diradical, but in fact, the product is a totally new kind of compound," Dr. West told the meeting. The compound obtained is a triquinocyclopropane in which all electrons are paired. He described the synthesis of a triquinocyclopropane
earlier this year [J. Am. Chem. Soc, 89, 152 (1967)]. But since then, a series of these compounds have been prepared with diflFerent alkyl groups substituted ortho to the oxygen atoms. The new compounds can be looked at as derivatives of radialene-[3] but are much more stable than such radialenes isolated to date. The oxidation reaction is a quantitative and revers-
ible two-electron transfer—hydroquinone reduces the triquinocyclopropane back to quinocyclopropene. Of the different triquinocyclopropanes synthesized by Dr. West, the most stable compound isolated so far is the one with six tert-butyl groups flanking the three oxygens. This derivative is stable to 280° C. All the compounds are purple solids and all have unusually strong absorption— about 770 millimicrons—in the near infrared. "Apparently there are very low-energy, highly permitted electronic transitions for all of these molecules," he points out. Another new series of compounds, called diquinoethylenes, have been prepared by the Wisconsin chemists starting with C 3 C1 3 +. When the ion reacts with phenol at 0° C , only two chlorines are substituted and the product, after hydrolysis, is a dihydroxyarylcyclopropenone. Upon irradiation such compounds lose carbon monoxide to yield dihydroxyaryl acetylenes. Oxidation of these acetylene derivatives gives diquinoethylenes. Diquinoethylenes can also be prepared by oxidizing dihydroxyarylcyclopropene to form a violet-colored diquinocyclopropanone. This compound is unstable and spontaneously
loses carbon monoxide to form the ethylene derivative. Currently, Dr. West is studying the mechanisms for forming these various new compounds. Intermediate monoradicals, anion radicals, and dianions have been identified and are now being investigated. Dienophilic reactivity. Previous work had shown that cyclopropene and its alkyl- and phenyl-substituted derivatives react readily with 1,3dienes to give Diels-Alder addition products. So Dow's Dr. Tobey and Dr. Law decided to investigate the reactions of halogenated cyclopropenes with 1,3-dienes, and the effect of the different halogens on cyclopropene dienophilic reactivity. The 1,3-dienes used in the study were cyclopentadiene, furan, and 1,3butadiene. Reacting tetrachlorocyclopropene or tetrabromocyclopropene with excess cyclopentadiene at room temperature gives bicyclic adducts in very good yields. Apparently an unstable tricyclic intermediate forms in each reaction and it spontaneously rearranges to form the bicyclic adduct. Similar bicyclic products are obtained when tetrachlorocyclopropene or tetrabromocyclopropene reacts with furan. On the basis of these results Dr.
Tobey and Dr. Law predicted that 1,2,3-trichloro-3-fluorocyclopropene should react with furan to give two products—one, an initial DielsAlder adduct and the other, a rearranged product. This is actually the case. The reaction yields a tricyclic compound and a rearranged bicyclic product in the ratio of 1.5 to 1. What happens, Dr. Tobey explained to the symposium, is that two intermediates are formed—one with a fluorine atom syn to the double bond, the other with a chlorine atom syn to the double bond. According to the Woodward-Hoffmann rules governing electrocyclic reactions, only the halogen syn to the double bond can ionize. Because of the strength of the carbonfluorine bond, the compound with fluorine syn to the double bond simply doesn't rearrange. However, the carbon-chlorine bond ionizes and the other adduct rearranges as soon as it forms. With 1,3-butadiene, fluorine-substituted cyclopropenes react readily at 100° C. to form stable Diels-Alder adducts. Furthermore, tetrachlorocyclopropene and tetrabromocyclopropene under the same conditions also yield unrearranged products. Therefore, the instability of the intermediates in the reactions of cyclopropenes with cyclopentadiene and furan must stem from the high ground-state strain in the adducts created by the methylene or oxygen bridges, Dr. Tobey told the meeting. 'These bridges distort the Diels-Alder adducts precisely in the direction required to initiate disrotatory opening of their cyclopropane rings," he explains. In investigating the relative reactivity of the tetrahalocyclopropenes, Dr. Tobey and Dr. Law learned that replacing chlorine with bromine at any position on the cyclopropene ring enhances the reactivity of the cyclopropene. In contrast, replacing chlorine or bromine with fluorine decreases cyclopropene's reactivity. Since steric effects would give just the opposite order of reactivity, the electronic effect exerted by the halogen substituents must be the more important. This order of decreasing reactivity— Br > CI > F—is opposite to that predicted by the Alder rule which asserts that Diels-Alder reaction rates should be increased by electron-donating substituents in the diene and electronwithdrawing substituents in the dienophile. The rule doesn't hold with cyclopropenes, Dr. Tobey believes, because replacing chlorine or bromine with fluorine changes the hybridization in the cyclopropene sigma bonds. Fluorine substitution relieves the cyclopropene strain energy, thus lowering the driving force for the Diels-Alder reaction. OCT. 2, 1967 C&EN
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