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Metal carbenes have for a long time been classified as Fischer or Schrock carbenes depending on the oxidation state of the metal. Since the introduction of N-heterocyclic carbene complexes this classification needs to be extended because of the very different electronic character of these ligands. The electronic structure of these different kinds of carbene complexes is analysed and compared to analogous...
The insertion of an alkyne into an α,β-unsaturated Fischer metalcarbene complex leads to a 1-metalla-1,3,5-hexatriene. This usually undergoes subsequent insertion of a carbon monoxide molecule, and the resulting dienylketene complex, in a 6π-electrocyclization, yields an alkoxycyclohexadienone or its tautomeric hydroquinone monoether. The overall process is a [3+2+1] cocyclization and constitutes...
Group 6 heteroatom-stabilised carbene complexes (Fischer carbene complexes) offer many interesting possibilities to build rings (carbocycles and heterocycles) not readily accessible through conventional methods. In this chapter, a summary of cycloaddition reactions involving group 6 Fischer carbene complexes is presented. Firstly, two-component coupling reactions where a substrate reacts with the...
Since its discovery the chromium-mediated benzannulation reaction has been developed into a unique and useful tool in organic synthesis. In this review, topical aspects of this reaction concerning its mechanism and the chemo-, regio- and stereoselectivity are summerised and discussed in detail. Special attention is paid to the asymmetric benzannulation reaction and, finally, the importance of this...
The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cycloaddition reactions presented include reaction with imines to form β-lactams, with alkenes to form cyclobutanones,...
The dirhodium(II) core is a template onto which both achiral and chiral ligands are placed so that four exist in a paddle wheel fashion around the core. The resulting structures are effective electrophilic catalysts for diazo decomposition in reactions that involve metal carbene intermediates. High selectivities are achieved in transformations ranging from addition to insertion and association. The...
For many years after its discovery, olefin metathesis was hardly used as a synthetic tool. This situation changed when well-defined and stable carbene complexes of molybdenum and ruthenium were discovered as efficient precatalysts in the early 1990s. In particular, the high activity and selectivity in ring-closure reactions stimulated further research in this area and led to numerous applications...
With the commercial availability of well-defined ruthenium metathesis catalysts which combine high stability and broad functional group compatibility, metathesis has firmly established itself in the toolbox of target-oriented chemists. RCM is now routinely integrated in the retrosynthetic planning of natural product syntheses. The availability of metathesis catalysts with different activity, and hence...
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