Kinetische und mechanistische Untersuchungen von Übergangsmetall-Komplex-Reaktionen : XXIII. Kinetic und Mechanismus der Reaktion von 1-Diethylamino-1-propin mit übergangsmetallkoordinierten Selenoketonen

Abstract

The pentacarbonyl(arylphenylselenoketone) complexes (CO)5M[Se=C(Ph)-C 6 H 4 R-p] (M = Cr, R = H (Ia); M = W, R = H (Ib), Br (Ic), OMe (Id), NMe 2 (Ie)) react with 1-diethylamino-1-propyne, Et2NC=CMe (II), via [2 + 2]-cycloaddition across the C=C triple bond of II to the Se=C bond of I and electrocyclic ring-opening to give selenoacrylamide complexes, (CO) 5 M[Se=C(NEt 2)C(Me)=C(Ph)C6H4R-p] (III). The reaction follows second-order rate kinetics: −d[I]/dt = k2[I][II]. For the reaction of I with II in toluene, the activation enthalpy ΔH≠ is 29(1) kJ/mol for both Ia and Ib, the activation entropy ΔS≠ is −152(5) J/(mol K) for Ia and −161(2) J/(mol K) for Ib. The rate constant k2 decreases clearly with increasing donor capacity of R in Ib-e. A good positive correlation is observed between k2 and the Hammett constants σp of the R with ϱ = 2.50. In contrast, the influence of the metal and the solvent is small. At 20.6°C, k 2(W) is only 3.48 k 2(Cr) (for R = H in toluene) and k2 (nitrobenzene) = 6.8 k2(decane). A concerted biradicaloid mechanism with an early transition state is proposed. Reaction of the Se=C bond of I with 1-diethylamino-1-propyne is compared with the reaction of W=C, O=C and S=C bonds with the same alkyne.