A Route to Carbasugar Analogues - Jonathan Clayden - The ...

Chapter 2 – Dearomatising additions to aryl oxazolines
after ET the species are represented by the line D + A – , and the gap between the lines
represents the energy required for ET. The reaction pathway consists of solvent
reorganisation and geometric distortions that occur before ET to raise the energy of the
DA complex until it is isoenergetic with D + A – (point B), at which time ET occurs and
the species relax to their new ground state. The strong similarity between the CM
diagram and Marcus theory’s avoided crossing diagrams is deliberate – the dashed
lines were added by Pross – and the ET process if often termed outer-sphere electron
transfer.
B
E
Reaction coordinate
Scheme 2.21 – energy profile of electron transfer from species D to A
Like the balance between inner and outer sphere electron transfer, Pross proposes that
SET occurs when the stabilising interactions of synchronous bond formation do not
outweigh the energetic cost of bringing the two sets of nuclei together. The factors
determining where a reaction lies on this continuum have been widely discussed.
Since redox potentials are a direct measure of how readily a species accepts or donates
an electron they are often mooted as allowing prediction about a reaction, although this
is often limited by available data (vide infra). Viewing ET in a molecular orbital
sense, reagents with high energy HOMOs will have little interaction with low lying
LUMOs in a transition state, making a SET reaction likely.
Whilst the nature of the two reagents is important to the manner in which the reaction
proceeds, the difficulties faced when trying to make generalisations indicate that many
other factors influence how the valence electrons move. In the application of Marcus
theory to organic reactions, Eberson also emphasises that any factor that minimises
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