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-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 s-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 ? and ? 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 ?, ?, or ? position with respect to the cycloheptatrienone carbonyl group, which corresponds to conjugation of the unpaired electron through three, two, or one C?C bonds, respectively. Similar conclusions apply to a variety of substituted troponoids.