Homolytic displacement at carbon. Part 3. First example of α-attack on the allenyl- and prop-2-ynyl-cobaloximes

Abstract

Allenylcobaloxime reacts with substituted benzenesulphonyl chlorides in an inert atmosphere under irradiation with tungsten lamps to give good yields of allenyl sulphones. However, the same reactions carried out in Srinivasan's photoreactor using a 400 W medium-pressure mercury lamp or under thermal conditions give exclusively prop-2-ynyl sulphones, by regiospecific rearrangement, in good yields. The corresponding reactions of (3,3-disubstituted allenyl) cobaloximes afford 1,1-disubstituted prop-2-ynyl sulphones irrespective of the conditions used. Similarly, prop-2-ynyl sulphones are formed exclusively in the reactions of prop-2-ynylcobaloxime with organosulphonyl chlorides under thermal and photochemical conditions. The reactions are believed to take place through a chain mechanism in which cobaloxime(II), present adventitously or formed by partial homolysis of the substrate cobaloxime, abstracts a chlorine atom from the organosulphonyl chloride to give the organosulphonyl radical RS^·O₂. Depending upon the reaction conditions and the nature of cobaloxime, RS^·O₂ attacks the α- or the γ-carbon atom of the axial organic group of the cobaloxime, thereby displacing cobaloxime(II) and giving the observed organic product.