Catalytic stereoselective addition to alkynes. Borylation or silylation promoted by magnesia-supported iron oxide and cis-diboronation or silaboration by supported platinum nanoparticles

Abstract

Iron oxide nanoparticles supported on magnesia (FeO/MgO) have been prepared by NaBH4 reduction of Fe(SO4) on MgO and spontaneous reoxidation upon storage. XPS of FeO/MgO indicates the presence of Fe(0) (16%) and Fe(II) (84%) on this solid. TEM images show that catalytically active FeO/MgO is constituted by iron oxide nanoparticles of about 25 nm dispersed on fibrous MgO. FeO/MgO in the presence of catalytic amounts of triphenylphosphine promotes highly regio- and stereoselective monoborylation of aromatic, aliphatic, terminal and internal alkynes. Chemical analysis of the liquid after the reaction and control experiments using Fe(II) salts in the absence or presence of PPh3 supports that catalysis is heterogeneous. The possibility that trace amounts of copper impurities present in the iron precursor influence the catalytic activity of FeO/MgO was studied using a commercially available high-purity Fe(SO4) as precursor (99.999% Fe purity) showing again good (but lower) activity. In addition, a control experiment using as catalyst MgO containing 30 times higher amounts of Cu than that present in low purity Fe did not lead to complete alkyne conversion, although product formation was observed in a large extent. Alkynes react with complete chemoselectivity versus alkenes. In contrast to FeO/MgO, Pt supported on MgO or active carbon efficiently promotes the stereoselective diboronation and silaboration of alkynes in the absence of triphenylphosphine at lower temperature to render the cis configured diboronated and silaborated alkene.