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

2.4 – Mechanistic discussion
centres would have very similar g-values and the resulting signal would be a
superposition of the two. Fortunately the spectrum of i-Pr• is known which would aid
the analysis.
Sample preparation proved problematic as it required transferring into the narrow
quartz tubes at low temperature under inert conditions. Whilst this was eventually
overcome, it was not possible to obtain a homogenous sample. Once prepared,
samples were cooled in liquid nitrogen and transported to the X-band 90 spectrometer at
the University of Manchester, and one spectrum is below.
101b + s-BuLi in
DMPU/PhMe
Ph
O
N
Ph
3300 3320 3340 3360 3380 3400 3420
OMe
‡ETb
Scheme 2.26 – X-band EPR spectrum acquired at 220K (doublet, A= 20G, g=1.99)
The spectra obtained clearly show the presence of radical species in solution although
the poor resolution means little can be inferred. 91 Sample warming meant that better
spectra could not be acquired, and attempts to re-make samples failed. Karlubíková
has since obtained better resolved spectra under similar conditions showing a sextet
with a strong spin coupling of 11.5 G. 66
Whilst these results show unpaired electrons in the reaction mixture, they indicate little
else. Since EPR is a very sensitive technique, the spectrum might simply be due to a
low concentration species not on the reaction pathway. A spectrum with better
resolution would give information about the environment of the radical which might
indicate its nature. However sufficient information had been gained to continue
investigating the SET mechanism to show whether or not this paramagnetic species is
a reaction intermediate.
80