Organometallics in acidic media: Catalytic dimerization of ethylene by (perfluoroalkyl)phosphine complexes of platinum and palladium in trifluoroacetic acid

Catalytic ethylene dimerization by (dfepe)Pt(Me)X complexes in aprotic and trifluoroacetic acid solvents is described. In CB2Cl2, catalyst deactivatio n due to acid loss and generation of (dfepe)Pt(eta(2)-C2H4) is observed. In contrast,long-term ethylene dimerization activity takes place in trifluoro acetic acid at 80 degrees C and produces 2-(trifluoroacetato)butane. as the sole organic product. Under these reaction conditions, the catalyst restin g state is (dfepe)Pt(Et)(O2CCF3). At 20 degrees C, reversible acid eliminat ion from (dfepe)Pt(Et)(O2CCF3) and ethylene ligand exchange results in cata lytic vinylic H+/D+ exchange with CF3CO2D (t(1/2) approximate to 40 min). A nalogous palladium systems exhibit enhanced dimerization activity at 25 deg rees C (340 turnovers/ h, 100 psi C2H4) and form (dfepe)(2)Pd as a catalyst resting state. Inhibition of catalytic activity in the presence of added d fepe was noted. Comparisons between catalytic runs in CF3CO2H and CF3CO2D g ave an apparent solvent kinetic effect of 3.5(2), based on an initial secon d order rate dependence on ethylene. In CF3CO2D, regioselective acid additi on to give only CH3CH(O2CCF3)CH(D)CH3 indicates that isomerization to 2-but ene has occurred prior to 1,2-acid addition. A general olefm dimerization m echanism incorporating protonation preequilibria is presented.