Reactivity and reaction pathways of electrochemically generated 17‐electron tricarbonyl steroid chromium cations

Electrochemical oxidation of α‐ and β‐diastereomers of a range of steroid hormone receptor marker chromium tricarbonyl complexes, (steroid)Cr(CO)₃, have been examined at platinum electrodes in dichloromethane. Data confirm the general nature of previously published conclusions on the oxidation of (arene)Cr(CO)₃ complexes (arene = benzene or steroid). That is, with 0.1 M Bu₄NPF₆ as the electrolyte, and in the absence of nucleophiles, a reversible oneelectron process, (steroid)Cr(CO)₃ ⇌ [(steroid)Cr(CO)₃]⁺ + e⁻, is observed, followed by an irreversible one‐electron process at considerably more positive potentials. The reversible half‐wave potentials (approximately E°‐values) calculated for the [(steroid)Cr(CO)₃]⁺/(steroid)Cr(CO)₃ redox couple are shown to be dependent on whether the α‐ or β‐diastereomer is oxidized. Similarly the rate of nucleophilic attack on the 17‐electron cation [(steroid)Cr(CO)₃]⁺ by nucleophiles such as ClO 4−, PPh₃ and bis(diphenylphosphine)methane confirms a previous observation that the stereochemistry of this class of compound is important with respect to redox, kinetic and hormone receptor properties. The nature of the electrochemical data obtained on the (arene)Cr(CO)₃ complexes in the presence of nucleophiles suggests that reactions associated with the nucleophilic attack on the 17‐electron cations are complex and that a range of reaction pathways occur simultaneously. Electrochemical studies on the oxidation of (benzene)Cr(CO)₂PPh₃ and (oestradiol)Cr(CO)₂PPh₃ confirm some aspects of the proposed mechanisms, although it is clear that a great deal still has to be learned concerning mechanistic aspects of nucleophilic attack on these 17‐electron complexes.