Metallozirconocenes in Organic Synthesis

Many of the most important achievements in organic chemistry in the last 20—25 years have been associated in some way with the use of transition metal complexes. Among these complexes, an increasingly important place is occupied by zirconium compounds, which have a number of unique properties enabling them to be used as highly reactive reagents in organic synthesis [1—9]. 

The impetus for the development of gem-bimetallics was initially to discover alkylidene-transfer reagents akin to Tebbe s reagent [14]. Schwartz prepared bimetallic aluminum—zirconocene derivatives by the hydrometallation of various vinyl metallic compounds [15—17]. Knochel has developed zinc—zirconium gem-bimetallics by hydrozircona-tion of vinylzincs and has used them as alkylidene-transfer reagents [18]. More recently, other gem-bimetallics have been developed that exhibit different reactivities of the two carbon-metal bonds. Thus, Normant and Marek have reported the allylmetallation of vinyl metals to afford zinc—magnesium and zinc—lithium gem-bimetallics, which react selectively with various electrophiles such as ClSnBu3, H20, etc. [19, and references cited therein]. However, selective and sequential cleavage of the two carbon—metal bonds 

This chapter is intended to highlight the synthesis and use of geminal bimetallic compounds such as gem-metallozirconocenes, but we will concentrate on the chemistry of aluminum, boron, lithium, gallium, germanium, tin, zinc, and zirconium. 

Synthesis of Stable Planar Tetracoordinate Carbon Zr/AI Compounds 

In 1874, varft Hoff [26] and Le Bel [27] independently surmised that tetracoordinate carbon is surrounded by substituents in a tetrahedral geometry. This perception marked the very beginning of modern organic chemistry, which is increasingly being determined by stereochemical argumentation. Some time ago, attempts were made to synthesize stable planar tetracoordinate carbon compounds [28—30]. 

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