Diene,Alkyne, Alkene,and Alkyl Complexes of Early Transition Metals: Structures and Synthetic Applications in Organic and Polymer Chemistry

An unprecedented series of highly reactive alkene-and diene-complexes of the early transition metals (Groups 3A–5A of the periodic system) have been isolated recently. Diene complexes of this sort (M ? Ti, Zr, Hf, Nb, Ta) prefer, besides the (η⁴-s-cis-diene)metal structure, either a novel bent η⁴-metallacyclo-3-pentene structure or the unique (η⁴-s-trans-diene)metal structure. In bis(diene)metal complexes of Nb and Ta the η⁴-s-cis-dienes assume an unusual exo-endo (supine-prone) geometry. The M? C bonds in these diene-metal complexes generally exhibit highly polarized σ-bonding along with π-bonding character. The complexes therefore undergo a variety of regio- and stereoselective carbometalations with substrates containing C? C, C? O, or C? N multiple bonds. Examples of the products that can be obtained include ketones, vinyl ketones, unsaturated primary, secondary, and tertiary alcohols, as well as diols and unsaturated acids. Mechanistic studies on the stoichiometric and catalytic conversions of unsaturated hydrocarbons provides, inter alia, some insights into the course of polymerization reactions.