Charge-Transfer Mechanism for Electrophilic Reactions. S_E2 Cleavage of Alkylmetals with Iodine

The absorption spectra of transient charge transfer (CT) complexes are observed immediately upon mixing iodine and various organometals RM, where M = tin, lead, and mercury. The formation constants Act and the transition energies h i'ct of these CT complexes vary with the ionization potentials and the steric properties of the alkylmetals. The subsequent disappearance of the CT absorption band is accompanied by the cleavage of the alkylmetal by iodine (iodinolysis). The kinetics of the iodine disappearance are consistent with a preequilibrium formation of the CT complex followed by the rate-limiting iodinolysis of the alkylmetal. The selectivity in the iodinolysis of unsymmetrical tetraalkyltin compounds is determined by product analysis and shown to be strongly dependent on the solvent polarity. The solvent effect is also shown to affect the formation constant of the CT complex and the rate constant for iodinolysis in a parallel manner. A charge-transfer mechanism is proposed for iodinolysis in which the rate-limiting step involves the unimolecular decomposition of the CT complex by electron transfer from the alkylmetal donor to the iodine moiety to form the ion pair [RM+ I2-]. This activation process is akin to the charge-transfer interaction, as formulated in the Mulliken theory. Accordingly, the difference DE in the CT transition energy hvci of a [RM I2] complex relative to that of a reference alkylmetal (either Me4Sn or Me2Hg) is used to evaluate the interaction energy of the ion pair. The change in the overall driving force AGr for electron transfer in the CT complex is determined from DE and the ionization potential of the alkylmetal. The activation free energy DG,-* for electron transfer is developed from the rate data by a similar comparative procedure, and shown to respond directly to the free-energy change, i.e., DGt* = DGr. This linear free energy relationship, together with a pronounced macroscopic solvent effect on DGr* based on Kirkwood's equation, supports a highly polar transition state for iodinolysis in accord with Scheme II. The same CT formulation can be quantitatively applied to the solvent effect on the relationship between the selectivity and the rate constants for iodinolysis in Figure 8, as well as the relationship between the selectivity and the formation constant of the CT complexes in Figure 9. It correctly predicts the inverse relationship often observed between selectivity and rate. Importantly, the charge-transfer formulation provides a quantitative foundation for the description of electrophilic processes, heretofore provided only in qualitative forms.