Steric and electronic tuning of Michael addition reactions induced by pentacarbonyl metal fragments

An experimental–theoretical approach to the kinetics and mechanism of Michael type addition: α,β-Unsaturated tungsten Fischer carbene complex as the substrate

“…corresponding activation parameters were obtained and listed in Table 2. The Michael type addition reaction is characterized by low activation enthalpy (DH à = 58.4 and 52.6 kJ mol À1 ) and large activation entropy (DS à = À109.6 and À131.0 J mol À1 K À1 ), consistent with the results of van Eldik et al [15,16]. k 2 /k 1 decreases as 0.73 at 35°C > 0.69 at 40°C > 0.66 at 45°C % 0.66 at 50°C, which indicates that reaction 1 is more favored over reaction 2 as the temperature goes up.…”

“…Based on the activation parameters using the pyrrolidine as the nucleophile, van Eldik et al [14] followed a two-step mechanism with a zwitterionic intermediate involved, which was much more polar than the reactants. A small solvent dependence was observed probably due to the charge delocalization over the M(CO) 5 fragment (M = Cr, Mo or W) [15,16]. Large negative reaction constant (t = À2.95) suggested the sensitivity of Michael type addition against the electronic effects [17].…”

Kinetic Isotope Effects in the Study of Organometallic Reaction Mechanisms

“…The experimental data support a two-step process in which a zwitterionic intermediate is produced in the rds. The absence of a primary KIE for the reaction of tungsten complex 541 with deuterated pyrrolidine and imidazole together with the negligible KIE ( k H / k D = 1.2) observed in the reaction of chromium complex 542 with deuteroaniline support this proposal (Scheme ).…”