Control of Electron Transfer Reactions on the Basis of Dynamic Intramolecular Coordination

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The present work stems from our earlier observations that the introduction of carbonyl, alkoxy, and pyridyl groups decreases the oxidation potential of tetraalkylstannanes. There is no indication of the coordination in the neutral molecule by the spectroscopy. Therefore, the decrease of the oxidation potential seems to be attributed to the dynamic intramolecular coordination to tin in the cation radical intermediates. Such coordination stabilizes the cation radical which in turn favors the electron transfer and also controls the selectivity of the bond cleavage. Pyridyl group has been found to be especially effective among the examined.Thus new class of electroauxiliary, [2-(2-pyridyl)etyl]dimethylsilyl group has been developed. Introduction of 2-(2-pyridyl)ethyl group on silicon atom of α-silylethers and sulfides gives rise to decrease of their oxidation potentials. Studies based on NMR spectra, ab initio molecular orbital calculations, and solvent effect of oxidation have suggested that the decrease of the oxidation potentials is attributed to the 'dynamic intramolecular coordination' of pyridyl group to silicon. The reactivity of this new electroauxiliary has been compared with those of trimethylsilyl group and tributylstannyl group by competition experiments. The present study indicates the feasibility of fine tuning of the reactivity of electroauxiliaries using dynamic intramolecular coordination.