RESEARCH
Free Radicals Rearrange Cyclopropanols Reaction involves attack on alcohol's Ο—Η bond with ring opening taking place at the same time Cyclopropanols go through a fast, ring-opening rearrangement by a free radical mechanism, according to chemists at Iowa State University, Ames. The reaction is done in carbon tetrachloride or chloroform at 80° C. With cyclopropanol itself, the product is propionaldehyde; 1-methylcyclopropanol gives methyl ethyl ketone; and /rans-2-phenylcyclopropanol gives 2phenylpropionaldehyde. Iowa State's Dr. C. H. DePuy, G. M. Dappen, and Dr. J. W. Hausser say (JACS, July 20, 1961) that the rearrangement is "remarkable." For example, cyclopropyl acetate and methyl cyclopropyl ketone, by con-
trast, are completely inert under the same reaction conditions. This fact, coupled with earlier findings by other workers that radical abstractions from cyclopropanes are difficult, suggests that the reaction involves attack on the alcohol's oxygen-to-hydrogen bond; ring opening probably takes place at the same time. If the reaction definitely takes this path, the Iowa State workers say, then it's unusual. For if a hydrogen atom were abstracted from the cyclopropane ring instead, it would be removed most readily from a carbon that's adjacent to the hydroxyl group. Such an abstraction would probably give
Rearrangement of cyclopropanols gives propionaldehydes
Cyclopropanol
Propionaldehyde
Reaction may involve radical attack on 0—Η bond
Free
^
Radical
C
«2
Cyclopropanol
Η
*|
Propionaldehyde 4?
If the hydrogen came from the ring, attack would occur next to the OH, giving acetone instead
Free Radical
. * Cyclopropanol
n
. x Acetone
Χ
acetone, not propionaldehyde, they explain. Other unusual aspects of the reaction : • It's very solvent dependent, going rapidly in carbon tetrachloride or chloroform but not in other solvents like benzene, methylene chloride, cyclohexane, or water. • It isn't catalyzed by azo-bis ( isobutyronitrile), a standard free radical ini tiator, although several peroxides and hydroperoxides effectively catalyze the reaction. • •'It is completely inhibited by ditert-butyl-p-cvesol, which has been shown in the past to be effective against peroxyradicals but never against alkyl radicals; thus, the identity of the chain-carrying radical isn't clear. With their results so far, the Iowa State scientists cannot completely rule out a radical-catalyzed build-up of some kind of electrophilic species that is especially reactive toward cyclopro panols; but a direct radical attack on the alcohol seems most likely. They're now doing kinetic, stereochemical, and tracer studies of the rearrangement. Sealed Tubes. The rearrangement is done in a sealed tube containing 0.5 ml. of 0.5M cyclopropanol in carbon tetrachloride, for instance. The tube is sealed under air or oxygen, then heated to 80° C. The reaction's progress is tracked by gas chromatography and infrared and nuclear magnetic reso nance spectroscopy. Cyclopropanol subsequently disappears (half life is about 30 minutes) and the only de tectable product is propionaldehyde. If oxygen is kept out while the tube is being sealed, or if di-tert-butyl-pcresol is added to samples sealed in air, little reaction takes place even after heating for a long time. However, cyclopropanols sealed without oxygen but containing added peroxides are re arranged quickly to propionaldehydes. The reaction rate isn't changed by add ing triethylamine or acid. In some ad ditional experiments, 1-methylcyclopropanol and ifûHs-2-phenylcyclopropanol reacted the same way.
UNUSUAL. Kinetic, stereochemical, and tracer studies are being made on this unusual rearrangement of cyclopropanols. The direct radical attack on the alcohol, though, seems most likely since hydrogen abstraction from the cyclopropane ring is difficult
