Mechanism Involves Thiirene Dioxides a,a- Dichlorosulfones in base rearrange through thiacyclopropene dioxides
151ST
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MEETING
Organic Chemistry In base, a,a:-dichlorosulfones rear range mainly through thiirene dioxide intermediates. Thiirene dioxides, un saturated three-membered rings con taining a suif one group, represent the first group of three-membered heterocycles with potential aromaticity in volving d orbitals on the hetero atom. Their stability, like the cyclopropenones, apparently increases with sub stitution, according to Dr. Leo A. Paquette of Ohio State University. Dr. Paquette and his co-worker, Lawrence S. Wittenbrook, have been unable to isolate the thiirene dioxides, however. This is because thiirene di oxides are unstable under the alkaline reaction conditions. During the reac tion, hydroxide ion opens the thiirene
dioxide rings to produce a vinylsulfonic acid. And some of the thiirene dioxide usually decomposes thermally to sulfur dioxide and an acetylene. The less stable monosubstituted thiirene diox ides decompose thermally more readily than do the more stable disubstituted ones. The Ohio State chemists base this generalization on the relative amounts of acetylenes and vinylsul fonic acids among the products. In an independent study, Dr. Louis A. Carpino and Dr. Louis V. McAdams, III, at the University of Massa chusetts, have synthesized diphenylthiirene dioxide and found it stable at room temperature. At its melting point, it decomposes to diphenylacetylene and sulfur dioxide. Dr. Carpino has not measured the aromaticity of the system, but he has found that diphenylthiirene dioxide is considerably more stable than its dihydro derivative. Mechanism. Dr. Paquette believes
that the base-induced rearrangement of α,α-dichlorosulfones begins with the removal of an alpha proton by base. The resulting carbanion cyclizes by expelling a chloride ion from the other alpha carbon atom. It does this by a 1,3 intramolecular displacement. The resulting saturated cyclic halosulfone loses hydrogen chloride in the presence of base and forms a thiirene dioxide. (Some of the molecules lose sulfur dioxide to form a vinyl chlo ride.) The thiirene dioxide, then, either opens to a vinylsulfonic acid or decomposes to an acetylene. This sequence has been verified by control experiments and by submitting an authentic sample of chlorothiirane di oxide (synthesized from chlorosulfene and diazomethane) to the same reac tion conditions. On the basis of the isolation of an exclusively trans vinylsulfonic acid, the Ohio State group postulates that the thiirene dioxide opens stereospecifically. Furthermore, Dr. Paquette says, it opens to generate the more stable of two possible carbanions. Finally, he notes, when tert-butoxide ion is the base instead of hydroxide ion, no sulfonic acids form. Almost all the products are acetylenes.
Hydroxide Ion Cleaves Thiirene Dioxides to Vinylsulfonic Acids
STABILITY. Dr. Leo Paquette (left) and Lawrence Wittenbrook at Ohio State believe that substituents stabilize thiirene dioxide intermediates, as they do the cyclopropenones 46
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NMR Shows No Favored Cyclooctane Form 151ST
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Cyclooctanes Have Many Low-Energy Forms
MEETING
Organic Chemistry
The nuclear magnetic resonance spectra of methylcyclooctane and tertbutylcyclooctane depend on temperature and show narrow methylene bands at room temperature. This means, says Dr. Frank A. L. Anet of the University of California, Los Angeles, that these compounds cannot exist exclusively (or even largely) in one conformation. The acetonide of cyclooctane-fran.S'-l,2-diol behaves the same way. By contrast, cyclohexanes usually have one preferred conformation. Dr. Anet and his co-worker, Maurice St. Jacques, also studied the NMR spectra of some deuterated cyclooctanes: d-s-l,2-dihydro-, tran.s-l,2-dihydro-, and 1,1-dihydroperdeuteiOcyclooctane. These spectra are consistent with the crown, the stretch crown, or the twist crown conformations, provided the second and third of these undergo rapid averaging to effect a regular crown symmetry. The boat-chair conformation also agrees with Dr. Anet's data, again provided that some averaging process is available. One such process is pseudorotation. Another is a wagging of a methylene group, resulting in the reversible formation of a crown.
If wagging is fast at —140° C , a mixture of the three crown forms and the boat-chair would have only two equal sets of protons. If pseudorotation is fast, too, the twist boat-chair form may also be present. The twist boat-chair is an intermediate when the boat-chair pseudorotates. Dr. Anet feels that it is significant that calcula-
tions by Dr. J. B. Hendrickson [/. Am. Chem.'Soc, 86, 4854 (1964)] show that these five forms are the lowest energy forms of cyclooctane, and that their energies are similar. Dr. Anet concludes that cyclooctane and alkylcyclooctanes contain up to about 30% crown forms and the rest boat-chair forms.
Three New Carboranes Complete B3-B10 Series 151ST
ACS
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MEETING
Organic Chemistry
MIXTURE. Dr. Frank Anet (right) and Maurice St. Jacques find that alkylcyclooctanes are a mixture of perhaps five different conformations 48
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Dr. M. Frederick Hawthorne and his co-workers have synthesized three new carboranes, B 7 H 7 ( C H ) 2 and two isomers of B S H S ( C H ) 2 . (A carborane is a polyhedral boron hydride containing two CH groups within the polyhedron.) This completes the carborane series from B a through B 1 0 . Now that these new carboranes are all available in good yields, Dr. Hawthorne says, it will be possible to explore their derivative chemistry and perhaps to make new compounds that have unusual chemical properties and high kinetic stabilities. The group at the University of California, Riverside, has also made another new compound—a "missing link" between trialkylboranes and boron hydrides. This compound is the dicarbanonaborane B 7 H 9 ( C H 2 ) 2 .
PRECURSOR. Dr. Hawthorne (back row, left), Philip M. Garrett (clockwise), Donald C. Young, Dr. James A. Manning, and Robert C. Stafford make the new carboranes from a dicarbanonaborane
It is a precursor for the new carboranes. Dr. Hawthorne calls the dicarbanonaborane a missing link because each carbon atom has an intermediate number (two) of bonds to boron. A trialkylborane carbon atom has one bond to boron; a carborane
carbon has three. This is the first time methylene groups have been observed in carborane chemistry. The new carboranes were prepared by pyrolyzing the dimethyl derivative of the dicarbanonaborane at 200° C. The reaction products are hydrogen and three dimethylcarboranes: B (j H 6 ( C C H 3 ) 2 , B 7 H 7 ( C C H 3 ) 2 , and one of the new B S H S ( CCH 3 ) 2 isomers. Add ing diborane to the reaction raises the yield of the B s carborane at the ex pense of the other two. At 350° C , the B s carborane rearranges to its iso mer in 95% yield. Dr. Hawthorne prepares the dicarbanonaborane by oxidizing a B 9 carborane with chromic acid.
"Missing Link" in Carborane Chemistry Contains Methylenes
Vinylcyclopropane Forms Cyclobutane in Light 151ST
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Organic Chemistry
The first case of a vinylcyclopropane rearranging photochemically to a cy clobutane has been found by Dr. Margaret J. Jorgenson of the University of California, Berkeley. Until now, the major products from her vinylcy clopropane photolyses have been cy-
clopropenes, furans, and cyclopentenes. A cyclopropane ring on ethyl acrylate shifts the ultraviolet absorp tion maximum of the acrylic ester to a higher wave length by about 35 η\μ. (An ordinary alkyl group shifts it by about 10 π\μ.) When the molecule is photochemically excited, some of the absorbed energy goes into the cyclo propane ring. This excess energy usu ally ruptures the ring. In her efforts to determine the na ture of the ring cleavage, Dr. Jorgen son, in collaboration with Dr. Clayton Heathcock, has found that one major pathway involves a carbene which col lapses to a furan and a cyclopropene. Another path leads to cyclopentenes. In her new work, she finds that a cy clopropane can also expand photo chemically to a cyclobutane. She photolyzed the ethyl ester of 2-methyl-3-cyclopropylacrylic acid, ex pecting to get a cyclopropene and a cyclopentene. These formed only in minor amounts. The major products (21%) were two isomeric cyclobu tanes. The cyclobutanes form in al most equal amounts and are thermally interconvertible. They are isomeric with the starting material, but contain no olefinic unsaturation. She found that these isomers are the endo and exo isomers of ethyl 1,5-dimethylbicyclo [2.1.0]pentane-5-carboxylates. The isomers inter convert by breaking and reforming the fusion bond. Significant. This work is significant in that it indicates how electronic charge is distributed in the excited molecule. Expansion of a cyclopro pane to a cyclobutane is generally con ceded to be an exclusive property of the cyclopropyl carbinyl cation or the bicyclobutonium ion. Although allylcarbinyl and cyclopropylcarbinyl sys tems can also inter convert through radicals and anions, Dr. Jorgenson's work strongly suggests that the cyclo propylcarbinyl carbon responsible for rearrangement is electron deficient in the excited state.
New Photochemical Rearrangement Probably Involves a Carbonium Ion
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