J. B. F. N. Engberts

Photochemically initiated reactions of substituted 1,3-dioxolanes and 1,3-oxathiolanes in CFCl3 : ESR study and mechanism of ring-fission

Abstract The photochemical reactions of some 2-alkyl-1,3-dioxolanes and 2-alkyl-1,3-oxathiolanes in CFCl 3 in the presence of benzophenone yield exclusively the open 2-chloroethyl carboxylic esters and S -2-chloroethyl thiocarboxylic esters respectively. Photochemically excited benzophenone abstracts the hydrogen atom at carbon between the heteroatoms from the substrate to give intermediate cyclic (thio)acetal radicals which can be trapped efficiently by 2-nitroso-2-methylpropane in inert solvents. The resulting nitroxides are identified by their ESR hfs-constants. No ring-opened (thio)ester radicals could be trapped. The course of photolysis of optically active 2 RS ,4 R -(−)-2-methyl-4-phenyl-1,3-dioxolane and other (racemic) 2,4-disubstituted-1,3-dioxolanes supports a mechanism in which a cyclic radical abstracts halogen from the solvent to form an intermediate cyclic chloro-(thio)acetal. Heterolytic cleavage of the new CCl bond gives the well stabilized cyclic carbonium ion and chloride anion. Nucleophilic attack of chloride ion at the C-4 or C-5 carbon atom (involving inversion for a chiral C-4) leads to ring rupture and formation of the final product.

Photolysis of Polyhalomethanes in the Presence of Nitrosoalkanes Formation of Acyl Alkyl Nitroxides

When tri-, di- and monohalomethyl radicals are generated photo-chemically in the presence of 2-nitroso-2-methylpropane the expected ESR spectra of the (poly)halomethyl t-butyl nitroxides are normally only observed when the halogen atom is fluorine (see chapter 2) and not when it is chlorine, bromine or iodine. These nitroxides are often very unstable. Apparently, β-halogen elimination occurs easily (except for fluorine) to give halonitrones. These halonitrones may act as precursors for the formation of new types of acyl alkyl nitroxides, which are actually observed (a N = 6.5–7.0 gauss). A mechanism is proposed in which the halonitrones undergo light-induced isomerization to halo-oxaziridines followed by a bimolecular oxidation-reduction reaction leading to the observed acyl alkyl nitroxides.