Mechanistic Insights into Copper‐Catalyzed Michael Reactions

The mechanism of the Michael reaction between 2‐(ethoxycarbonyl)cyclopentanone ( 2 ) and methyl vinyl ketone ( 3 ) catalyzed by the copper(II) salt [Cu(5‐ t Bu‐Sal) 2 ] ( 1 ) has been investigated in toluene. The kinetics of the reaction of the catalyst 1 with the oxo ester 2 has been investigated by UV/Vis spectroscopy. It has been established that [Cu(5‐ t Bu‐Sal) 2 ] ( 1 ) reacts with 2 in a first‐order reaction, which is retarded by 5‐ t Bu‐Sal‐H ( 6 ), formed in the reaction after the reversible dissociation of [Cu(5‐ t Bu‐Sal) 2 ] to [Cu(5‐ t Bu‐Sal)] + and 5‐ t Bu‐Sal – and the subsequent reversible deprotonation of 2 by 5‐ t Bu‐Sal – . The formation of the mixed complex [Cu(5‐ t Bu‐Sal)(enolate‐ 2 )] ( 5 ) is proposed and has been characterized by cyclic voltammetry. The rate constant for the overall reaction leading to [Cu(5‐ t Bu‐Sal)(enolate‐ 2 )] ( 5 ) has been determined. The reaction of the mixed complex 5 with methylvinyl ketone ( 3 ), investigated by cyclic voltammetry, is proposed as the rate‐determining step.