Polar Radicals. 14. On the Mechanism of Trialkylstannane Reductions. Positive ρ Values for the Tri-n-butylstannane Reduction of Benzyl Halides. A Correlation with ?-

Article abstract of DOI:10.1021/ja00522a042

The relative reactivities towards reduction by tri-n-butylstannane of a series of substituted benzyl halides have been determined.The order of reactivity was shown to be I > Br > Cl, the same as that reported for alkyl halide reductions.Under the reaction conditions (90 deg C, benzene as solvent, benzoyl peroxide initiation), both aralkyl and alkyl fluorides were shown to be completely unreactive.Activation energies were estimated for the direct abstraction of halogen by tri-n-butyl radicals, and on the basis of these estimates all of the halogens would be predicted to readily undergo abstraction.To explain the non-reactivity of the fluorides several other mechanisms were considered: reversible tin radical addition to the halogen to form an intermediate with an expanded octet, and a free radical chain mechanism involving an electron-transfer reaction between the trialkyltin radical and the benzyl halide. The benzyl halide reductions showed excellent Hammett correlations with positive ρ values; all of these correlations were obtained using ?^- substituent constants.The demonstration (the first ever reported) that ?^- substituent constants correlate with the relative reactivities for reduction infers that bond breaking takes place in the transition state for the reaction.The magnitudes of the ρ values were not found, as were expected, to be in the inverse order of the relative reactivities: ρ for the iodides (+0.81), for the bromides (+0.17), and for the chlorides (+0.34).To explain the anomalously high value of ρ found for the iodide reduction, it was suggested that the benzyl iodides underwent reduction by a different mechanism than the chlorides and the bromides.The relative rates of reduction of the benzyl iodides in a more polar solvent than benzene, acetonitrile, showed decreased sensitivity to substituents, whereas the reduction of the bromides and chlorides was relatively insensitive to solvent effects.