PI-ACCEPTOR INDUCED REACTIONS - RADICALS VS IONS IN THERMALLY INDUCED ETHER CLEAVAGE REACTIONS

The reactions of benzyl ether (1a) and 4,4'-dicyanobenzyl ether (1b) with a wide range of pi-acceptors in acetonitrile at 190-degrees-C have been studied. The pi-acceptors were chosen in order to systematically vary the DELTA-G-degrees for hydride or electron transfer from the ethers to the pi-acceptors. The products of thermal activation of the bond cleavage reaction are the aldehyde and the alcohol derived from the starting ethers. A linear free energy correlation of the relative thermal reaction rates of la and lb with the reduction potentials of the quinones was obtained. This relationship indicates that the rate of radical pair separation becomes increasingly important relative to the rate of the second electron transfer and subsequent product formation through the hydride transfer intermediate as the electron affinity of the quinone decreases.