REDUCTIVE ACCELERATION OF THE MIGRATORY INSERTION REACTION - EVIDENCE THAT INSERTION OCCURS IN A 19-ELECTRON INTERMEDIATE

The electrochemical reduction of the cobaltacycle Cp(CO)CoC12H8 (1) has been studied in tetrahydrofuran/ 0.1 M Bu4NPF6 by cyclic voltammetry, bulk coulometry, and rotating-ring-disk voltammetry. This complex reduces in an irreversible one-electron process at ca. -1.7 V vs SCE. Within the time frame of a CV scan, about 1/3 equiv of the fluorenone anion, Fl-, is produced per equiv of 1-. Since 1 produces FI only slowly when heated under CO, it is clear that the reduction of 1 results in an enormous increase in the rate of alkyl to acyl migratory insertion for this complex. Since the insertion must occur immediately after uptake of an electron by 1, a 19e intermediate is responsible for the enhanced rate of migratory insertion. A body of earlier work on redox acceleration of migratory insertion reactions has led to controversy over whether insertion occurs at the 17e or 19e stage. The present results offer definitive evidence that 19e species may show very large enhancements in their rates of migratory insertions. © 1990, American Chemical Society. All rights reserved.