ELECTROCHEMICAL OXIDATION OF ORGANOSILICON COMPOUNDS .7. THE ORIGIN OF BETA-SILICON EFFECT IN ELECTRON-TRANSFER REACTIONS OF SILICON-SUBSTITUTED HETEROATOM COMPOUNDS - ELECTROCHEMICAL AND THEORETICAL-STUDIES

Silyl substitution at the carbon adjacent to the oxygen causes significant decrease in the electrochemical oxidation potentials of ethers and alcohols, and this effect strongly depends upon the geometry of the molecule. Two explanations are possible; (a) the cation radical intermediate isstabilized by overlap of the filled C-Si bond with the half-vacant 2p orbital of the oxygen; (b) overlap of 2p orbital of the oxygen with the filled C-Si bond in the neutral molecule raises its HOMOlevel which in turn favors electron transfer. Theoretical studies using MO calculations indicate that the second effect is dominant. The geometric requirements can also be explained in terms of the HOMO level. The HOMO level varies with the torsion angle of Si-C-O-H(C), and when the C-Si bond achieves coplanar orientation with the p orbital of the oxygen, the HOMO level becomes the maximum. The plots of experimental oxidation potentials of some cyclic silyl-substituted ethers in which rotation around the C-0 bond is restricted vs the torsion angle fit well to the plots of HOMO energy. Although the most stable conformation does not seem to be suitable for the electron transfer in acyclic systems, significant silicon effects are observed even for such systems in the electrochemical studies. The electron transfer seems to take place not in the most stable conformation but in the higher energy conformation, which is favorable for the oxidation. Temperature dependence of oxidation potentials has been examined in order to reveal the effect of conformation. The electrochemical oxidation of silyl-substituted ethers having an additional silyl group or oxygen atom has also been studied. The β-silicon effect for the electron-transfer reaction of sulfides, amines, and carbamates has been discussed, and the energy level of the p orbital of the heteroatom relative to that of the C-Si bond is suggested to be very important. © 1990, American Chemical Society. All rights reserved.