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Metallocene carbene chemistry
Authors:Erker  G  Kehr  G  Fröhlich  R
Institution:(1) Institute of Organic Chemistry, Münster University, 48149 Münster, Germany
Abstract:The Group IV bent metallocenes Cp2M (M = Ti, Zr, Hf) were involved in carbene-related chemistry in various ways. Examples from four different areas are used to illustrate this. In situ generated Cp2M: species exhibit some carbene-like character. They add to olefins, and their addition products can incorporate additional unsaturated organic reagents, e.g., alkyne, to form five-membered metallacycles. The high oxophilicity of the Group IV metals helps the addition of alkene-, aryne-, eegr2-ketone-, eegr2-aldehyde-, or butadiene-containing reagents to a great variety of metal carbonyl compounds to form the Fischer-type carbene complexes. The resulting zirconoxycarbene complexes have found some application in organometallic chemistry and in organic synthesis. Reactive Cp2MIVR]+ cations can be stabilized by the addition of the Arduengo carbenes that serve as bulky two-electron donor ligands. First examples were structurally characterized. Dialkylimidazol-2-ylidenes also add to the electrophilic MX4 compounds yielding stable trans-bis(imidazol-2-ylidene)MX4 systems. Several examples are presented where ruthenium carbene complexes are used for carrying out catalytic olefin metathesis reactions at pendant olefinic substituents at the bent metallocenes. These reactions have led to the formation of novel bimetallic metallocene systems, as well as to new ansa-metallocenes. These catalytic reactions have helped in the current efforts to develop a functional group chemistry at the sensitive Group IV bent metallocene frameworks.
Keywords:carbenes  metallocenes  catalysis  reactivity
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