Kinetische and mechanistische Untersuchungen von Uebergangsmetall-Komplex-Reaktionen; XXIV. Kinetische Untersuchungen zur -Cycloaddition von Diene an Uebergangsmetallkoordinierte Thioaldehyde, Selenoaldehyde und Selenoketone

Article abstract of DOI:10.1016/0022-328X(88)80235-3

The pentacarbonyl(thioaldehyde), -(selenoaldehyde), and -(selenoketone) complexes (CO)5M (I) (M = Cr, X = Se, R = Ph; M = W, X = S, R = H; M = W, X = Se, R = H, Ph, C6H4Br-p, C6H4Me-p, C6H4OMe-p, C6H4NMe2-p) react with dienes such as cyclopentadiene, trans-1-methyl-1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, and 1,3-cyclohexadiene via <4 + 2>-cycloaddition to give metal-coordinated thia- and selenacycles.The reaction follows second-order kinetics: -d/dt=k2*.The activation enthalpy ΔH^<*> is small (46 to 28.7 kJ/mol) and decreases along the series selenoketone, thioaldehyde, selenoaldehyde complex.The activation entropy ΔS^<*> is strongly negative (-144 to -147 J/(mol K)) and is indipendent of the type of complex.The rate constant k2 for the reaction with cyclopentadiene decreases clearly with increasing donor capacity of Y in (CO)5W.A good positive correlation is observed between k2 and the Hammett constants ρ_p of Y (ρ = 2.5).Exchange of R = Ph in (CO)5W for R = H accelerates the reaction with cyclopentadiene by a factor of 73500.In contrast the influences of X (k2(Se)=19.0*k2(S)), M (k2(W) = 1.87*k2(Cr)), and the solvent are small.The rate constant for the reaction of dienes with (CO)5W increases significantly along the series 1,3-cyclohexadiene ca. trans-1-methyl-1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, cyclopentadiene.A concerted biradicaloid mechanism is proposed.