Irradiation at 2537 Á of perfluorocyclooctatetraene yielded antiand syn-perfluorotricyclo[4.2.0.0y]octa-3,7-dienes la and lb, respectively, which reverted quantitatively to the tetraene at 150 °C. Bromine addition occurred exclusively in the exo, suprafacial fashion with both dienes, but only the anti isomer added 2 mol of bromine. The latter fact, together with analysis of the 19F NMR spectra of the bromine adducts, constitutes convincing evidence for the configurational assignments for la and lb. Perfluorobenzene underwent [2 + 2] photocycloaddition with chlorotrifluoroethylene, and the bicyclic adducts suffered photochemical electrocyclization to a stereoisomeric mixture of 7-chlorotricyclo[4.2.0.02,5]oct-3-enes. Analysis of the 19F NMR spectra of the major isomers permitted their assignment as exo,anti and endo.anti. Reduction of the former gave la, thus confirming the diene's configuration. An explanation based on "second-order" orbital symmetry effects is suggested for the syn/anti stereoselectivity found in the thermal and photochemical electrocyclic processes described above.
Cyclooctatetraene (COT)' has occupied a prominent place in the historical development of organometallic chemistry.2 It is a versatile ligand which has exhibited a flexible hapticity in its modes of bonding to transition metals, lanthanides, and actinides.'J Moreover, the elucidation of the nature of metallotropic rearrangements in COT complexes, particularly in Fe(COT)(C0)3 (l), occupied the attention of many leading research groups for a number of years.* Fluorocarbon organometallics have often proved to be more stable thermally and to exhibit different bonding modes from their hydrocarbon a n a l o g~e s .~ This communication reports examples of 1,2-7 complexes of octafluorocyclooctatetraene (OFCOT, 2)4 with iron(0) and platinum(0). These undergo intramolecular oxidative addition reactions, with formation of a metal-carbon u bond, to afford 1,2,3,6-7 bonded complexes.F F at equal areas per molecule for each of these comparisons, the inherent cratic entropy difference of R In Z5 between the racemic and enantiomeric forms should make a constant contribution to the differences in the final equilibrium states of the films." How this factor affects rates of mixing in these experiments is not clear to us. On the strength of the above experiments, the resolution of a racemic film was approached by seeding with pure crystals of either enantiomer. The racemic film was spread on 10 N H2S0, from a dilute hexane solution. After the usual 20 min for solvent evaporation was allowed, the barrier drive was started, and the film was compressed to 28 dyn/cm. The dashed curve in Figure 3 shows the surface pressure decrease caused by seeding the compressed film with pure racemic crystals. After 1.5 h the surface pressure had decreased nearly to the ESP of the racemic film.The experiment was now repeated by compressing a freshly spread racemic film, but this time crystals of pure R-(+) enantiomer were sprinkled on the surface. The solid curve of Figure 3 shows the rapid decrease of surface pressure to a point well below the ESP of the racemicfilm and approaching that of a pure enantiomeric film.These results imply the deposition of R-(+) molecules on the added crystals, leaving a partially resolved film which was composed of predominantly S-(-) molecules. Equivalent results were obtained by seeding with S-(-) crystals which would leave predominantly an R-(+) film. We have already presented evidence above that the free energy of enantiomer crystals is lower compared to liquid or film states than are racemic crystals.We have tried repeatedly to detect unseeded spontaneous resolution by leaving racemic films under compression for periods of 20 h, the present limit for maintaining reasonably constant conditions. In no case have we been able to detect a spontaneous decrease in surface pressure which could not be attributed to leaks or other artifacts.Resolution of racemates from bulk solution by seeding with enantiomers is only possible in cases, such as the present one, where the racemic crystals are more soluble than those of the en...
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