Observation of Electrochemically Generated Radical Anions of Colchicides and Isocolchicides, and a Comparison with the Radical Anions of Troponoids

Cathodic reduction of 10-ethylthiocolchicide (2), 9-ethoxyisocolchicide (3), 9-methylthioisocolchicide (4), 9-n-butylthioisocolchicide
(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 s21.
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, or b 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.