Abstract
Exciting new variations in Fischer-type carbene complex composition and reactivity have been realised by following or modifying well-established synthetic approaches such as metal carbonyl functionalization and modification of existing carbene ligands. The formation of targeted complexes for organic synthesis, carbene-containing chelates, and polynuclear carbene complexes, by employing 'click chemistry', warrants discussion. Transmetallation and α,α-dehydrogenation of ethers and amines have come into their own as viable synthetic methods to access carbene complexes with unique properties and activities. Successful mediation of carbene complex formation with pincer ligands has proved its worth. Quantum chemistry has become essential for supporting or initiating mechanistic proposals, but heuristic approaches such as invoking the vinylology principle to describe substituted phenylcarbene complexes are still valuable in the interpretation of bonding properties and the classification of complex types. Electrochemical studies now also constitute a powerful part of the experimental characterization tool kit.
Highlights
Ever since the ground-breaking publication by Fischer and Maasböl fifty years ago,[1] introducing stable, isolable and fully characterized carbene complexes of group 6 metals, the versatility of the carbene ligand class has not failed to fascinate the chemical community
Many other types of carbene ligands have been exposed in detail – see Table 1 for examples.[3]
The research group of Bernasconi has published numerous articles on nucleophilic substitution at the carbene carbon atom. Their latest contribution[87] elaborates on two important results: (i) in the reaction of thiolate anions with Fischer type alkoxycarbene complexes, the leaving group expulsion is rate limiting with thiolates that are weak bases, whereas the nucleophilic attachment step is rate determining when such bases are strong; and (ii) weak base thiolates are inferior leaving groups compared to aryloxides of comparable basicities
Summary
Fischer carbene complexes remain favourite targets, and vehicles for new discoveries. Exciting new variations in Fischer-type carbene complex composition and reactivity have been realised by following or modifying well-established synthetic approaches such as metal carbonyl functionalization and modification of existing carbene ligands. The formation of targeted complexes for organic synthesis, carbene-containing chelates, and polynuclear carbene complexes, by employing ‘click chemistry’, warrants discussion. Transmetallation and α,α-dehydrogenation of ethers and amines have come into their own as viable synthetic methods to access carbene complexes with unique properties and activities. Successful mediation of carbene complex formation with pincer ligands has proved its worth. Electrochemical studies constitute a powerful part of the experimental characterization tool kit
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