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

2.4 – Mechanistic discussion
orbital overlap as the transition state is approached will disfavour the polar mechanism
or inner-sphere electron transfer. 78 These can be factors such as steric or geometrical
strain, solvent interactions or simply that a weak bond is being formed.
A number of methods have been developed to study the nature of electron transfer in
organic reactions. The mechanism of halogen-lithium exchange has been a topic of
much investigation and controversy 79 since 1956 when the first observations hinting at
radical intermediates were made. 14 Whilst spectral data confirmed the presence of
radicals, they were not shown to be reaction intermediates until a number of
experiments led to a consensus such as those performed on norbornenes 130 by Ashby
(Scheme 2.22).
Fast
X
130X
t-BuLi
Li
Slow at −78 °C
131
X = I at −78 °C
X = Br at 0 °C
Scheme 2.22 – Ashby’s norbornene radical trapping experiments 80
These and other results strongly support the presence of radical intermediates in
bromine-lithium exchange, 81 and their absence in iodine lithium exchange. 82 This is in
disagreement with predictions based upon electrode potentials (alkyl bromides E ½ –2.5
V and iodides E ½ –1.6 V 83 ) and emphasises the lack of predictability using this
method. Very little electrochemical data is available for organolithiums, presumably
due to electrode decomposition, as reported by Breslow, who was only able to obtain
potentials for allyllithium (–1.63 V) and phenyllithium (–1.43 V). 71
The investigations above highlight that whilst it might be relatively easy to indicate
that a reaction proceeds via SET, it can be particularly difficult to prove so. Ashby
suggests a number of general methods; spectroscopic evidence showing that the rate of
decay of a paramagnetic species is in line with the rate of reaction; loss of
stereochemistry during reaction; and use of a cyclisable probe, preferably of known
lifetime (a radical clock). 79 Rate data from UV-vis spectra of the reduction of
76