SELECTIVE HYDROGENATION OF 1-ALKYNES TO ALKENES CATALYZED BY AN IRON(II) CIS-HYDRIDE ETA-2-DIHYDROGEN COMPLEX - A CASE OF INTRAMOLECULAR REACTION BETWEEN ETA-2-H-2 AND SIGMA-VINYL LIGANDS

The reactions of the cis-hydride eta(2)-dihydrogen complex [(PP3)Fe(H)(H2)]BPh4 (1) and of the dinitrogen derivative [(PP3)Fe(H)(N2)BPh4 (2) with a variety of 1-alkynes have been investigated. From this study, it is apparent that the insertion of the alkyne across the Fe-H bond in 1 to give a sigma-alkenyl intermediate proceeds via decoordination of a phosphine arm of PP3 rather than via H-2 decoordination. Terminal alkynes are selectively and catalytically hydrogenated to alkenes by 1 in tetrahydrofuran or 1,2-dichloroethane solutions under mild conditions. A kinetic study on the hydrogenation reaction of HC = CPh to H2C = CHPh has shown that the reduction is first order in catalyst and alkyne concentrations and zero order in hydrogen pressure. Incorporation of a large body of experimental data leads to the conclusion that (i) the dihydrogen ligand does not leave the metal prior to alkyne coordination and (ii) the reduction of the substrate most likely occurs via an intramolecular acid/base reaction involving eta(2)-H2 and sigma-vinyl ligands mutally cis disposed.