Mechanism of Oxidatively Induced Migratory Insertion of Carbon Monoxide: Evidence for a 19-Electron Intermediate
The mechanism of oxidatively induced CO insertion for CpFe(CO)(PPh3)CH3has been probed by transient electrochemical techniques. the data presented herein suggest that the alkyl-to-acyl migration at 17-electron CpFe(CO)-(PPh3)CH3+proceeds by a mechanism consisting of two chemical steps. the first step is a second-order process, first order in both the metal radical and entering pyridine nucleophile (Nu), that produced a 19-electron species CpFe(CO)(PPh3)(Nu)CH3+. This complex can be observed directly at low temperature in cyclic voltammetry measurements and in the EPR and IR spectra. the rate of nucleophilic attack at CpFe(CO)(PPh3)CH3+depends on the a-basicity of the nitrogen Lewis base as seen from second-order constants, kl, that vary over 5000-fold from 200 (3,4-dimethylpyridine) to 0.04 (3-cyanopyridine) M-1s“-lat 20 °C. Hammett analysis of the rate data shows that log klcorrelates well with the σ-meta and σ-para values for eight 3-and 4-substituted pyridines. the second chemical step that occurs is a nucleophile concentration independent first order process that converts the intermediate CpFe(CO)(PPh3)(Nu)CH3+to the acyl product, 17-electron CpFe(PPh3)(Nu)(COCH3)+. We rule out a mechanism for the oxidatively induced migratory insertion that proceeds via a 15-electron intermediate, where insertion occurs before nucleophile coordination at CpFe(CO)(PPh3)CH3+. © 1987, American Chemical Society. All rights reserved.
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