EPR observation of cathodically-generated radical anions of colchicides and isocolchicides, and a comparison with the radical anions of troponoids. A general rationalization of the spin-density distribution in these systems

Cathodic reduction of 10-ethylthiocolchicide (2), 9-ethoxyisocolchicide
(3), 9-methylthioisocolchicide (4), 9-n-butylthioiso-colchicide
(5), and 9-phenylthioisocolchicide (6) at a platinum electrode in DMF
under N2 gave the corresponding radical anions for which
EPR spectra were recorded. Voltammetric analysis of these compounds
revealed a reversible wave at minimum scan rate 0.1–0.5 V s 2 1 .
Hyperfine coupling constants (hfcc) for these radical anions—assigned by
spectral simulation and DFT calculations of electron spin densities—are
highest at the position adjacent to the carbonyl group. Decreasingly small
hfccs were assigned, in the given order, to the g and b positions with
respect to the carbonyl group. This trend—which is only marginally
affected by oxygen, amino, and sulfur substituents—can be qualitatively
rationalized on the basis of classical resonance structures. These afford
stability to the system in diminishing order for the unpaired electron at
the a, g,orb position with respect to the cycloheptatrienone carbonyl
group, which corresponds to conjugation of the unpaired electron through
three, two, or one CvC bonds, respectively. Similar conclusions apply to a
variety of substituted troponoids.