Electron-Transfer Transition States: Bound or Unbound—That is the Question!

摘要

The breathtaking advunees realized in the field of computational quantum chemistry within the past two decades have made it possible to reliably predict transition state structures for most reaction types in organic chemistry. The focus of controversyhas moved from the characterization of typical organic reaction mechanisms, such as pericyclic reactions, to more complex issues, such as reaction mechanisms in organometallic chemistry or the detailed desciiption of solvent effects. In light of this development it is quite surprising that little structural detail is available for the mos, elementary reaction step in chemistry, the exchange of electrons between two reaction partners. It was only recently realized that transition states for electron-transfer reactions might also have definitive structural requirements. Traditionally, electron-transfer (ET) reactions were qualitatively discussed in terms of outer-sphere and inner-sphere ET. These terms were originally coined for classifying ET reactionsbetween transition metal complexes, which either occur without breaking metal-ligand bonds (outer-sphere ET) or with concomitanl cleavage/formation of metal -ligand bonds (inner-sphere ET). Implicit in this categorization is the assumption that little interaction exists between the electron donor and acceptor in transition states of outer-sphere ET reactions, whereas the converse is true for inner-sphere ET reactions. Exactly this assumption was challenged by Eberson and Shaik through application of the valence-bond curve-crossing model (VBCM) to thermal ET reactions in open-shell systems. According to this qualitative model transition stales for dissociative electron transfer between radical unions and alkyl halides is stabilized by maximum overlap between the donor SOMO (singly occupied molecular orbital) and the alkyl halide LUMO (lowest unoccupied molecular orbital), whereas the most dominant interaction for the competing S_N2 transition stale is that between the donor pi HOMO (highest occupied molecular orbital) and the alkyl halide LUMO. For the combination of ketyl radical anions and alkyl halides. the corresponding orbituls are shown in Scheme 1. The most important consequences of this are the following: a) Optimization of bimolecular orbital overlap is important for transition states of formal outer-sphere electron transfer. If not prohibited by steric effects, transition slates for electron transfer should be bound more strongly than either the rcactant or the product complexes, b) Different selection rules apply for transition states of electron transfer and nucleophilic substitution.