ELECTRON-TRANSFER PHOTOINDUCED CLEAVAGE OF ACETALS - A MILD PREPARATION OF ALKYL RADICALS

Electron transfer from 2-alkyl- and 2,2-dialkyldioxolanes as well as from open-chain ketals to singlet excited benzene-1,2,4,5-tetracarbonitrile (TCNB) is followed by fragmentation of the donors radical cation to yield alkyl radicals and dialkoxy carbocations. The first species are trapped by TCNB to yield alkylbenzenetricarbonitriles (substitution of a second cyano group can be obtained sequentially) and in a minor path are reduced to alkanes, while the latter ones react with nucleophiles to give ortho acid derivatives. In view of the results of radical clock experiments, it is assumed that part of the process is a concerted (radical cation cleavage-addition to the aromatic) reaction, while another part involves the free-radical cation. On the other hand, intersystem crossing from the singlet radical ion pair to the triplet manifold causes cleavage of the acetal to the corresponding carbonyl derivative. This reaction offers a mild method for the preparation of alkyl radicals via C-C bond cleavage.