Kinetische und mechanistische untersuchungen von übergangsmetall-komplex-reaktionen. XXVI. Kinetische untersuchungen zur cyclopropanierung von olefinen mit benzyliden(pentacarbonyl)wolfram-komplexen

The benzylidene complexes (CO)₅W=C(C₆H₄R-p)H (R = OMe (1), Me (2), H (3)) react with vinyl acetate (4a), ethyl vinyl ether (4b), 1-hexene (4c) and cyclopentene (6) by transfer of the benzylidene ligand to the CC bond of the olefin to give cyclopropanes. The reactions of 1 with these olefins and of 2 with vinyl acetate generally proceed according to an additive rate law consisting of three terms, e.g. -d[1]/dt = k₁[1] + k₂[1][olefin] + k₃[1]olefin]². The first and third order terms, however, deviate from zero only in the slow reactions and at very high concentrations of olefin. The influence of the solvent on the reaction rate is small. The activation parameters for the reaction of 1 with vinyl acetate are: ΔH{black star} = 42(1) kJ/mol and ΔS{black star} = - 143(3) J/(mol K). 2 reacts 3.8 times faster with vinyl acetate than 1. The cis/trans ratio of the cyclopropanes at high olefin concentration is independent of the olefin concentration and only slightly dependent on the polarity of the solvent. At low concentrations of olefin the reactions of 3 with 4c and 6, respectively, yield preferentially the trans-isomer in pentane and the cis-isomer in pentane/CH₂CL₂ solvent mixtures. The difference ΔS{black star}(cis)- ΔS{black star}(trans) is negative for all reactions investigated. For the reaction of 3 with 4a-c the cis/trans ratio increases with decreasing temperature, the difference ΔH{black star}(cis) - ΔH{black star}(trans) is in the range -0.73 to -6.0 kJ/mol.