Reactivity of Alkyl Polyhalides toward Granular Iron: Development of QSARs and Reactivity Cross Correlations for Reductive Dehalogenation

Attempts to develop quantitative structure-activity relationships (QSARs) for reductive dehalogenation have been hindered by the unavailability of high quality predictor variables, have included relatively few compounds, and on occasion have relied on data lacking internal consistency. We herein investigate the reduction of 24 alkyl polyhalides by granular iron and the better-defined, homogeneous reductants Cr(H2O)62+ and an Fe(II) porphyrin. QSARs were constructed with a new set of computationally derived gas phase homolytic carbon-halogen bond dissociation energies and solvated one-electron reduction potentials determined using a quantum chemistry composite method (G3MP2). Reactivity cross correlations between reductant systems were also developed. Reactivity trends were generally consistent among all reductants and revealed pronounced structural influences. Compounds reduced at C-Br were orders of magnitude more reactive than analogues reduced at C-Cl; the number and identity of - (Br ~ Cl > CH3 > F > H) and -substituents (Br > Cl) also influenced reactivity. Non-linearities in QSARs and cross correlations suggest that reactions of highly halogenated alkyl polyhalides with granular iron are limited by mass transfer, as supported by estimates of mass transfer coefficients. For species not exhibiting mass transfer limitations, reasonably strong cross correlations and comparable substituent effects are consistent with dissociative electron transfer as the rate-determining step.