SCIENCE/TECHNOLOGY
Hunt for Reaction Intermediate Points to Cubical Singlet Diradical Three groups suggest that 1,4-cubadiyl is the elusive intermediate formed when substituents are removed from two opposite apexes of cubane Synthetic chemists often find themselves in pursuit of identifying shadowy intermediates in a reaction, based on the results of experiments and calculations. Recently, three independent groups have been in pursuit of the same i n t e r m e d i a r y trying to figure out what happens when one removes the two hydrogen atoms (or other substituents) at opposite apexes of the cubane skeleton. That species is called 1,4-dehydrocubane, but what is it actually? Is it an eight-carbon cube with a diagonal bond running through its center? Is it a diradical? Or is it a cubical diene generated when one of the edge bonds is cleaved? The most likely answer, according to the three groups, is that the intermediate is a singlet diradical— the unprecedented 1,4-cubadiyl. The three groups consist of professor Josef Michl, former postdoctoral research associate Karin Hassenruck, and coworkers at the University of Texas, Austin, along with a coworker at the University of Dûsseldorf in West Germany; professor Weston T. Borden and research associate David A. Hrovat of the University of Washington, Seattle; and professor Philip E. Eaton and former postdoctoral research associate John Tsanaktsidis at the University of Chicago. All three groups published their findings last month in the form of three consecutive papers [J. Am. 24
February 12, 1990 C&EN
Chem. Soc, 112, 873, 875, and 876 (1990)]. The Chicago and Austin workers settled on the diradical explanation after studying the reactions of 1,4-dihalocubanes with organolithium reagents. The products of those reactions, such as bicubyls (two cubanes linked by a carbon-carbon bond), could not be explained by invoking the known chemistry of monohalocubanes. Rather, a dehalogenated, metal-free species seemed to be lurking in the reaction mixture. Eaton's group confirmed its intermediacy by treating different 4-haloiodocubanes with excess phenyllithium and quenching with iodobenzene. In each case, 4-phenyliodocubane was formed. Eaton believes that the mechanism involves a series of exchanges and additions on the cubane framework. First, he says, lithium exchanges with the iodine. Lithium halide is lost to give 1,4-dehydrocubane, which then adds phenyllithium. Finally, the lithium is replaced with iodine from iodobenzene to give the observed product. By using a dihalocubane specifically labeled with one deuterium, the Chicago team determined that the intermediate is symmetric. The chemistry suggests that the mysterious intermediate might be 1,4-cubylane, a symmetric but hypothetical cubane with a long carboncarbon bond extending diagonally through its body. To be sure, this molecule is "enormously strained," Eaton remarks, "but after all, we're dealing with enormous reactivity." However, theoretical calculations performed by Borden's and Michl's groups indicate that 1,4-cubylane is not a realistic possibility. Its structure, Michl points out, doesn't correspond to a local minimum on the potential energy surface, and its en-
ergy is way above that of the other two alternatives. Borden says that Ç-1 and C-4 are too far apart (2.686 A) to allow the atomic orbitals on these carbons to interact substantially through space. If anything, Michl adds, the through-space interaction between these carbons is very weakly antibonding. That's because the electron population of the out-of-phase (antibonding) combination of orbitals is significantly larger than that of the in-phase (bonding) combination. The other two possibilities—the 1,4-diradical and the diene—apparently have similar energies, with the diradical's possibly being the lower of the two. Calculations suggest that cleaving a bond in the diradical to form the diene is not à favorable process, according to Borden. Because the diene has two strained, bridgehead double bonds, it would be expected to react readily with dienes such as furan in a Diels-Alder reaction. But when Eaton and Tsanaktsidis carried out the appropriate trapping experiments, they didn't snare anything. The implication, says Eaton, is that the diene isn't formed. "Given those results," he adds, 1,4-cubadiyl seems to be the most likely intermediate. Michl, in fact, believes he may have captured the elusive diradical in a cryogenic trap. He and his Austin coworkers reacted, in the gas phase, 1,4-dihalocubane with an alkali metal vapor and deposited the products in an argon matrix at about 15 K, a procedure they had previously used to abstract halogen atoms from organics. Infrared spectroscopy indicated that they had succeeded in trapping a new species containing neither halogen nor metal. Other spectroscopic evidence suggests that the trapped intermediate is not
1,4-Dehydrocubane: a tale of three suspects
1,4-Cubylane
1,4-Cubadiyl
Diene
an anion, dianion, radical, radical anion, triplet diradical, or diene. Rather, it appears to be a groundstate singlet 1,4-diradical, Michl concludes. "This may be the first 1,4-diradical that has been directly observed as a ground-state singlet," he says. Borden notes that 1,4-cubadiyl is a special kind of diradical because although C-l and C-4 are not directly bonded to each other, "there still is a strong enough interaction between them through the other bonds of the cubane skeleton such that the singlet state lies well below the triplet." In fact, both Borden's and Michl's calculations indicate that the diyl's singlet state is more stable than its triplet by more than 10 kcal per mole.
The nature of the intermediate is still not absolutely settled, but the three groups seem to be "fairly certain" that it is the cubadiyl. Whatever it is, it's incredibly unstable. When the Austin workers allow the frozen argon matrix to warm up to about 40 K, the infrared signature of the embedded intermediate disappears. At that temperature, Michl explains, molecular species diffuse quite readily in argon and react with each other. The final product is cubane, he says, presumably because 1,4-cubadiyl is a good hydrogen abstractor. Cubane is also formed in the gas phase if the intermediate is not frozen out in the argon matrix as soon as it forms. The chances of ever isolating 1,4-dehydrocubane neat under ordinary conditions appear nil. Even at -100 °C, Eaton's group had no luck in sighting it by nuclear magnetic resonance spectroscopy. Because 1,4-dehydrocubane currently is generated under "dirty" conditions—that is, in the presence of solvents, excess reactants, and byproducts—detailed study of the species is difficult, Eaton says. Therefore, his group is searching for other chemical precursors of the intermediate. It hopes such precursors will enable it to generate 1,4-dehydrocubane under cleaner conditions, perhaps photochemically. So far, he tells C&EN, attempts to do so have not been successful. Eaton remains concerned about the true nature of the intermediate. But that hasn't stopped him from using the species' reactivity to gain entry into new realms of cubane chemistry, such as the preparation of polycubyls, interesting rodlike molecules with a number of potential uses. The work of the three groups also illustrates the synergism between experiment and calculations. Experimental work led to calculations, which made predictions that led to more experiments, Borden says. The process continues, for his calculations predict that 1,3-dehydrocubane should be more stable than the 1,4 isomer and should be relatively easy to prepare. C-l and C-3 are located at opposite corners of a "cyclobutane" face of the cube, so it should be possible to connect them via a
diagonal bond, Borden notes. In 1988 he and Hrovat predicted that 1,2-dehydrocubane (cubene) should exist. And, sure enough, evidence for its formation and that of a homolog was obtained that same year, respectively, by Eaton and coworker Michèle Maggini, and by Borden and Hrovat and another group. Ron Dagani
PET bottles recycled into insulation foam As bottles of polyethylene terephthalate, or PET, continue to gain favor, the question of what to do with the empties arises. Two chemists in Switzerland believe they have come up with an economically viable answer: Convert the discarded plastic to thermal insulation foam. Their process now is being made available under license. Insulation board made from recycled PET has several advantages over conventional expanded polystyrene board, says H. Verity Smith, who with Edwin W. Trevitt, has patented a process for making it. (Smith and Trevitt are chemists at the Geneva-based recycling technology firm, Tisslam.) Foamed PET has a melting point some 100 °C higher than its polystyrene counterpart. It also has a lower degree of flammability. Besides, PET insulation board would make practical use of material that is increasingly becoming something of a social nuisance. "K, or thermal insulation, values of foamed PET over a range of densities are being determined," Smith notes. "We expect that this new foam will have insulation properties similar to expanded polystyrene. On a cost/performance basis, the PET product likely will be superior," he maintains. Until now it hasn't been possible to make a stable foam from molten PET because of the polymer's extremely low melt viscosity index, Smith explains. The foaming agent, usually a chlorofluorocarbon, simply bubbles through the liquid polymer and escapes. Molten polystyrene, on the other hand, has a much higher level of viscosity and holds the gas. Smith and Trevitt have overFebruary 12, 1990 C&EN 25
