Free electron transfer mirrors rotational conformers of substituted aromatics: Reaction of benzyltrimethylsilanes with n-butyl chloride parent radical cations

Abstract

The rotation motion of a larger substituent of an aromatic ring is accompanied by the electron density fluctuation of the highest occupied molecular orbitals. For benzyltrimethylsilanes p-R₃–C₆H₄––CR₁R₂(SiMe₃) [R₃ = H,PhCO; R₁, R₂ = H,H; H,Me; Me,Me; H,Ph; Ph,Ph] the silyl containing substituent rotates on the axis between the aromatic moiety and the benzylic carbon as indicated in the formula. In the course of this rotation a diversity of various conformers is passed. For the sake of simplicity, we reduce this diversity to two (extreme) borderline structures, one with the substituent in plane with the aromatic ring (transient), and one where the substituent is twisted by 90° (stable structure). In the very rapid and non-hindered electron transfer from the conformer mixture to n-butyl chloride parent radical cations, the singlet ground state conformers are converted into two kinds of radical cations. The cations tending to the planar type are stable whereas those of the twisted type dissociate immediately into benzyl type radicals (p-R₃–C₆H₄–C˙R₁R₂) and trimethylsilyl cations. The above mentioned product pattern and quantum-chemical calculations on characteristic parameters of the ground state conformers and the product cations, show that the electron transfer occurs completely unhindered after diffusional encounter, i.e. after each approach of the reactants. Therefore the statistics of the rotation conformers mixture is reflected by the pattern and ratio of the product transients such as metastable radical cations and radicals derived from a dissociative radical cation. Up to now this is the only case where a diffusion-controlled electron transfer reaction proceeding in the nanosecond scale is influenced by intramolecular rotation motions within the electron donor.