Exploratory studies on reactions of cobaloxime π-cations with C-nucleophiles: Irreversible alkene decomplexation versus nucleophilic capture

Article abstract of DOI:10.1021/jo951319i

(β-Hydroxyalkyl)- and (β-acetoxyalkyl)cobaloximes can undergo facile acid-catalyzed β-heteroatom exchange with oxygen and nitrogen nucleophiles in an S_N1-like mechanism via cationic metal-alkene π-complex intermediates (cobaloxime π-cations). The reaction of cobaloxime π-cations with carbon nucleophiles has not been previously reported. The results reported in this paper demonstrate that cobaloxime π-cations are reasonably good electrophiles. They are sufficiently reactive to add to the electron-rich sp² centers in allyltrimethylsilane and pyrrole. A major side reaction for intermolecular reactions is irreversible alkene decomplexation. An intramolecular pyrrole cyclization (22 to 23, 83% yield) significantly raised the yield for nucleophilic addition compared to that of the analogous intermolecular couplings (12b tb 13b, 32% yield and 17 to 18, 29% yield). The results of these studies provide a foundation for the design and evaluation of modified cobalt ligands with the goal of suppressing alkene decomplexation and enhancing reaction of cobaloxime π-cations with C-nucleophiles.