Metallocene Complexes of Group 3 Metals : Structure and Reactivity

Metallocene and ansa-metallocene complexes of group 3 metals and the Lanthanides have shown to be effective catalysts for a number of chemical reactions. These include alkene polymerization, hydrosilation, hydroboration, hydrogenation and hydroamination. The coordinatively unsaturated, monomeric permethylyttrocene complexes in particular have proven to be highly effective for carrying out these transformations with high turnover numbers. Using Cp*2LnE(SiMe3)2 (Ln = Sc, Y or Lanthanide; E = CH or N) as a model system this paper will describe how ligand modifications alter the structure of the catalyst at the metal center and the reactivity in hydroamination reactions. Introduction Complexes of scandium, yttrium and the lanthanides are competent catalysts for a number of useful chemical transformations. An exceptionally useful transformation is the addition of nitrogen and hydrogen across unsaturated carbon-carbon bonds (eq 1). HNR2 + H2C=CH2 R2NCH2CH2H Catalyst (1) This reaction is general for primary and secondary amines as well as carbon-carbon unsaturations including alkenes, allenes, alkynes and dienes. Hydroamination reactions are atom economical and provide a means to access nitrogen containing molecules such as heterocycles, alkaloids and unnatural amino acids. A number of transition metals spanning the periodic table are capable of facilitating hydroamination reactions including group 3 complexes. Group 3 elements have several features which make them attractive for the development of catalytic transformations. While other oxidation states are possible, generally group 3 metals have an oxidation state of (III) giving them a d electron configuration. In addition lanthanide contraction causes a greater decrease in atomic radii than expected moving from left to right across the lanthanide series. As a result tunability of the coordination at the metal center in complexes without changing ligands can be accomplished by choosing a metal of the appropriate ionic radius. The high electrophilicity and kinetic lability result in group 3 metal centers being predisposed to two reactivity patterns, sigma-bond metathesis and olefin insertion (Scheme 1), both of which are advantages for hydroamination. Scheme 1. Olefin Insertion and Sigma-bond Metathesis Pathways Utilizing complexes with the general formula Cp2LnE[Si(CH3)2] (Cp* = C5Me5, Ln = Group 3 element, E = CH or N) as well as ansa-metallocene complexes with the Cp* ligands tethered, results in catalyst activity with high turnover numbers and selectivity. Catalytic Cycle for Group 3 Metal Hydroamination In the proposed catalytic cycle (Scheme 2) for Ln catalyzed hydroamination reactions the pre-catalyst enters the catalytic cycle via sigma-bond metathesis, is rapidly protonated and eliminates CH2(SiMe3). The amido complex generated then undergoes the turnover-limiting alkene insertion through a four centered transition state. A second sigma-bond metathesis by another substrate molecule regenerates the catalytically active species. Scheme 2 General Catalytic Cycle for Hydroamination of Aminoalkenes With alkene insertion being the limiting step of the catalytic cycle both the size and shape of the catalytic pocket should have effects on both the rate and selectivity of the hydroamination reaction. As stated previously with the group 3 metals, size may be altered without changing the ligands by simply choosing a metal of the appropriate ionic radius. If a new stereocenter is being created in the substrate then the shape of the catalytic pocket can be tuned by altering the ligand structure to "block" one face of the alkene from approaching the metal center due to sterics, favoring one enantiomer. Simple Achiral Metallocene Complexes Following work by Bercaw and Thompson as well as Marks on permethyllanthocene derivatives, Jan Teuben and co-workers synthesized and characterized two yttrocene complexes corresponding to the formulas Cp2YN(SiMe3)2 (1) and Cp2YCH(SiMe3)2 (2). These complexes are thermally stable but quite sensitive to air and water. Figure 1. PLUTO drawings of Cp2YN(SiMe3)2 and Cp2YCH(SiMe3)2