Synthesis and Free Radical Decarbonylation of Some Small-Ring Aldehydes

Abstract

A number of small-ring radicals have been generated by peroxide-initiated decarbonylation of the appropriate aldehydes, neat and in diphenyl ether, and that extent of rearrangement in these reactions has been determined in each cases. The results are compared with known examples of free radical rearrangements and with rearrangements which occur in carbonium ion reactions in the same small-ring systems. The results are found to be in accord with the predictions of simple molecular orbital (L.C.A.O) theory. No decarbonylation of cyclopropanecarboxaldehyde was observed. 1-Methyl- and 1-phenylcyclopropanecaroxaldehyde gave exclusively methyl- and phenylcyclopropane, respectively, on decarbonylation. Cyclobutanecarboxaldehyde, cyclopropylacetaldehyde and dimethyl-cyclopropylacetaldehyde afforded exclusively cyclobutane, 1-butene and 2-methyl-2-pentene, respectively, on decarbonylation. A mixture of 2-methyl-2-pentene and isopropylcyclopropane was obtained on decarbonylation of dimethylcycloprpylacetaldehyde in benzyl mercaptan. The possible role of non-classical homoallylic radical intermediates in the free radical reactions of cyclopropylcarbinyl derivatives is discussed. A decision could not be made on the basis of available evidence between mechanisms involving classical and non-classical radical intermediates in these reactions. The evidence does suggest that non-classical bicyclobutonium radical intermediates are not important in these reactions. The methods of synthesis of the small-ring aldehydes in this study are presented and discussed.