Kinetische und mechanistische Untersuchungen von fibergangsmetall-Komplex-Reaktionen XXVIII : Kinetische Untersuchungen zur [2 + 2]-Cycloaddition von Vinylethern an Pentacarbonyl(thiobenzaldehyd)wolfram (CO)₅W[S=C(C₆H₄R-p)H] (R = H, Me, OMe, Br, Cl)

Abstract

The pentacarbonyl(thiobenzaldehyde)tungsten complexes (CO)5W[S=C(C6H4R-p)H][R = H (1A); Me (1B); OMe (1C); Cl (1D); Br (1E)] react with vinylethers such as ethylvinylether (2a), isobutylvinylether (2b), methylisopropenylether (2c), cis-ethyl-1-propenylether (2d), trans-ethyl-1-propenylether (2e), α-methoxystyrene (2f), α-methoxy-p- methylstyrene (2g) and α-methoxy-p-methoxystyrene by [2 + 2]-cycloaddition to form metal-coordinated thietanes (3Aa–h, 3Ba– 3Ea). The reaction follows second-order kinetics: −d[1]/dt=k2[1] [2]. For the reaction of 1A with 2a the activation enthalpy is small (ΔH★=44(1) kJ mol−1 and the activation entropy (ΔS★= −146(5) J mol−1 K−1) is strongly negative. ΔS★ is comparable to the reaction entropy of the reversible addition of pyridine to the thiocarbonyl C-atom of 1A and 1B. Electron donation groups R in (CO)5W[S=C(C6H4R-p)H] decrease the reaction rate. A positive correlation (ϱ=1.4) is observed between log k2 and the Hammett constants σp+. A substitution only in the α-position of the vinylether has remarkable effects, in this case the reaction is accelerated by electron donating substituents. The rate constant for the reaction of 1A with different vinylethers decreases in the series 2h >2g >2c >2f >2b, 2a, 2e >2d. An associative concerted non-synchronous process is proposed.