RESEARCH
Cubane Made from 2-Cyclopentenone in FiveThe first unequivocal preparation of a simple cubane has been carried out by scientists at the University of Chicago (C&EN, March 16, page 4 1 ) . Using a five-step synthesis, Dr. Philip E. Eaton and co-worker Thomas W. Cole, Jr., have prepared and isolated symmetrical dicarbomethoxycubane [/ACS, 86, 962 (1964)]. Key to the reaction is the preparation of a compound which readily undergoes closure into a boxlike system. The compound, a dibromodicyclopentadienedione, is converted photochemically into a cage structure. In the presence of a base, this cage product undergoes a Favorskii reaction (which in this case can be thought of as a type of benzilic acid rearrangement) to the cubane. Cubane, a C 8 cubic compound, has long been a challenge to synthetic organic chemists. Numerous attempts have been made to prepare it over the years. In 1961, Dr. H. H. Freedman of Dow Chemical reported the unexpected preparation of a dimer of tetraphenylcyclobutadiene, which was later proposed to be octaphenylcubane on the basis of x-ray, chemical, and spectroscopic evidence [JACS, 84,
PLAN. Dr. P. E. Eaton (center) explains the synthesis of cubane to graduate students Richard Hudson, William Hurt, Thomas Cole, Jr., and Kang Lin (left to right). Synthesis of other cubanes is under way 38
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Cage structure synthesis goes through a dimer that readily gives a boxlike structure 2837 (1962)]. However, the bulky phenyl groups have prevented a complete x-ray analysis of the cubic carbon skeleton. Thus the structure hasn't been fully substantiated. In contrast, the structure of the cubane synthesized by the Chicago group can be easily characterized, Dr. Eaton says. Moreover, the synthesis can be adapted to similar systems. Preparation. Starting material for preparing the cubane is 2-cyclopentenone. This compound is reacted with N-bromosuccinimide to give the 4-bromo derivative of the ketone. Further halogenation with molecular bromine yields 2,3,4-tribromocyclopentanone. The pentanone is deny drobrominated with diethylamine to give 2-bromocyclopentadienone, which spontaneously undergoes a Diels-Alder dimerization. Ultraviolet irradiation of the dimer obtained (a dibromodicyclopentadienedione) leads to polymerization rather than to the desired cage closure reaction. So the dimer is converted to the 8-hemiketal by dissolving it in methanolic hydrogen chloride. When it's irradiated by UV, the 8-hemiketal readily undergoes closure into the cage system, which, in turn, forms a bishemiketal. Heating the bis compound with water, then drying it, removes the ketal group to form the diketone. The diketone is then refluxed for several hours with 50% aqueous potassium hydroxide. Under these conditions, it undergoes the Favorskii reaction to give the cubane diacid. The diacid is reacted with diazomethane to convert it to dicarbomethoxycubane. Dicarbomethoxycubane is a stable, white crystalline solid. Over-all yield
through all the steps from 2-cyclopentenone is about 10%. The cubane melts at 161° to 162° C. Analysis. Several dimers of 2-bromocyclopentadienone are possible. However, only one dimer forms. Its structure can be verified in several ways, Dr. Eaton says. For instance, it has an endo configuration. If the dimer were exo, ring closure couldn't take place. Also, the nuclear magnetic resonance spectrum shows only three vinyl protons. This confirms the substitution of a bromine for a hydrogen on the oj-vinyl position of the cyclopentenone segment. Finally, body diagonal substitution of cubane can only be obtained if the dimer has this structure. NMR and other spectra plus analytical data confirm that the compound produced is the dimethyl ester of cubane. In NMR studies, only two resonance peaks appear. Both are equal in area, and are less than half a cycle wide at half-height. One signal comes from the hydrogens on the methyl ester groups, the other from the cubyl hydrogens. The analytical figures obtained agree well with theory. A determination of the cubane's molecular weight, by melting point depression, for instance, gave 229. The calculated value is 220. To determine the hybridization involved in the cubane skeleton, Dr. Eaton, with the help of Dr. Gerhard L. Closs, measured the spin-spin coupling of the skeletal hydrogens attached to the small amount of carbon13 naturally present in the cubane system. The studies show that there is about 32% s character in the ring C—H bond. Cyclobutane and cyclopropane, for instance, have 27% and 32% s character, respectively. Therefore, the C—C bonds are bent to a large extent to meet the geometric needs of the cubane skeleton, Dr. Eaton concludes. Future. Next step in the program is the synthesis of cubane itself. "However, with dicarbomethoxycubane, we already have one of the most useful compounds for examining the chemistry of cubane," Dr. Eaton points out.
been started. Chicago's Dr. Everly Fleischer is now doing a definitive xray study. Raman and infrared analyses are slated. Dr. Eaton plans to prepare cubyl carbinol tosylate (the cubane analog of the neopentyl system) and study its solvolysis. One possible result of solvolysis would be a simple displacement reaction. The more likely reaction, though, is skeletal rearrangement into a variety of compounds, he says. The Chicago chemists also will try to introduce an electron into the molecular cavity of cubane. This would amount to a synthesis of the quantum mechanical "electron in a box." The method used would be one worked
Step Synthesis Work is also being done on the preparation of other cubane derivatives and on the chemical reactions of such systems. Proposed synthetic routes include the use of substituted cyclopentenones as starting materials and substitution of the carbomethoxy group on the symmetrical dicarbomethoxycubane. A more extensive study of the bonding in the cubane system has already
Dimer Formation Is Key to Cubane Synthesis 9v.
Br
/Br
Br2
N-Bromosuccinimide
II
II
O
O
8r
2,3,4-Tribromocyclopentanone
4-Bromocyclopentenone
Diethylamine' Spontaneous Diels-Alder dimerization
o
u
6r
II 0
Br
2-Bromocyclopentadienone
A dibromodicyclopentadienedione
hu
Br
0 =
50% K0H (Favorskii reaction)
>-COOR
BK-^J
Rooc' Dicarboxylic acid cubane (R = H)
Diketone of the dimer
Dicarbomethoxycubane (R = CH3)
i Can be thought of as a type of benzilic acid rearrangement
0 *°
C0OH
out by Dr. F. D. Greene of Massachusetts Institute of Technology, who has shown that an electron can be transferred from sodium-potassium alloy to adamantane, a polycyclic decane of high symmetry. The back lobes of the C—H bonds of cubane are perfectly positioned for exchange interactions with an electron located in the cavity. However, Dr. Eaton stresses, only the experiment will tell whether the electron will accept confinement.
Radiation Extends Beyond Van Allen Belts High-energy radiation far beyond that previously known for the earth's radiation belts has been detected, according to Dr. Kinsey A. Anderson of the University of California, Berkeley. The region has been located by detectors on the interplanetary monitoring platform (IMP) satellite. The region seems to fan out beyond the belts on either side of the earth, and trail off in a wake in a direction away from the sun. Extent of the radiation wake suggests to the Berkeley physicist that the moon may be peppered with high-energy radiation particles. Whether the new region of radiation is part of the Van Allen radiation belts is still undetermined, Dr. Anderson told a special symposium about the IMP satellite. The symposium was arranged by the National Aeronautics and Space Administration, and held at NASA's Goddard Space Flight Center, Md. The high-energy radiation within and beyond the Van Allen belts (lying about 40,000 miles from the earth) seems to be created in the collision of the earth's magnetic forces with particles (mostly protons and lowerenergy electrons) streaming from the sun that make up what is called the solar wind. Mechanism for this electron interaction isn't known, he adds. About half of the high-energy electrons find their way into the trapped regions of the Van Allen belts. They are later lost into the earth's atmosphere, Dr. Anderson believes. Other high-energy electrons, he says, remain outside the belts as transient radiation. These are then "sloughed" off the sides of the radiation belts and trail out in the wake that is carried in the current of the solar wind. High-energy electrons of this kind have been detected at the outermost limit of the IMP satellite's orbit, exMAR. 2 3, 1964 C&EN
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cept in the direction of the sun. It's likely, Dr. Anderson adds, that the moon may encounter them as it passes through the radiation wake each month. The amount of radiation probably does not hold any serious hazard for space travelers, he notes. The 138-pound IMP satellite was launched Nov. 26 from Cape Kennedy, Fla. It travels on a four-day orbit in an extremely eccentric orbit; the apogee (long end of the orbit) extends about 120,000 miles into space. The orbit itself is also revolving slowly about the earth. The satellite was launched toward the sun. The long end of the orbit will reach the farthest point away from the sun early in May, and will return to the sunward launch position at the end of the first year of flight. Thus the satellite is giving space scientists an extended opportunity to learn about the main region of the Van Allen belts, and to map near and distant radiation patterns around the earth. BRIEFS Standard samples of H3B03 are available from the National Bureau of Standards, Washington, D.C., and the central bureau of nuclear measurements (CBNM) of Euratom, Geel, Belgium. The nuclear cross section group of the American Energy Commission and the European American Nuclear Data Committee urge that measurement of boron neutron cross sections be done with boron from the standard stock materials. The standard material has a boron-10 content of about 19.8 atomic %. To obtain samples, U.S. scientists should write to Dr. R. S. Caswell at NBS. European workers should write to Dr. G. H. Debus at CBNM.
Two stable and two metastable forms of pure bismuth oxide have been found by E. M. Levin and R. S. Roth of the National Bureau of Standards, Washington, D.C. The two NBS scientists have also constructed partial phase diagrams showing the effect of 33 selected oxide impurities on bismuth oxide. Bismuth oxide is becoming more important to the ceramics industry as a constituent in high refractive index glasses, better bonding glazes, and ceramics for nuclear or electronic uses.
