摘要：

Numerous electrophilic transition metal carbene complexes transfer the carbene ligand to alkenes to form cyclopropanes. Convenient synthetic procedures have been developed which allow ready access to stable carbene complexes as well as carbene complex precursors for in situ generation of less stable species. The most versatile and easily prepared reagents are complexes of the type Cp(CO) (L)Fe=CRR′+. Although synthetic chemists still employ diazo compounds or classical carbenoid reagents to convert alkenes to cyclopropanes, recent results as summarized here make these attractive reagents for certain applications. High selectivity can often be achieved. Transfers frequently occur with nearly exclusive formation of the sterically more crowded, thermodynamically less stable cyclopropane isomer; some control by variation of metal and ligand is possible. Initial experiments indicate optically pure chiral-at-metal complexes can be used to achieve high enantioselectivities. Further synthetic advantages arise in cases where classical carbenoid reagents are generally too unstable to permit efficient transfers. Stereochemical control in intramolecular additions is a promising development in the synthesis of polycyclic compounds. The current systems have obvious limitations which represent areas for future research. (1) Most transfers are successful only for nucleophilic alkenes possessing electron-donating substituents; reagents are needed which have both the stability and reactivity to transfer carbenes to electron deficient alkenes. (2) Transfer yields are often limited by rapid intramolecular rearrangements or other modes of decomposition, especially 1,2 hydride migration, and would benefit from complexes in which such decomposition was slow relative to carbene transfer. (3) All currently useful reagents involve simple methylene complexes or derivatives in which the carbene substituents are electron-donating. Stable systems involving electron-withdrawing substituents at Cα are lacking but may be accessible through use of more electron-rich LnM fragments. (4) Although chiral-at-metal complexes hold promise for enantioselective syntheses, general routes to a family of such species are not yet available. The mechanism of the transfer reaction and the way in which the metal, ligands, and carbene substituents influence reactivity and selectivity has been studied by several research groups. Certain systems, especially those in which cyclopropane formation and olefin scission occur competitively, undoubtedly involve intermediate metal-carbene-alkene complexes. Stable carbene-alkene complexes have been isolated and substantial mechanistic detail has been elucidated with regard to the mode of and necessary features required for decomposition of these species to cyclopropanes. However, the large majority of transfer reactions occur without ligand dissociation from the metal carbene complexes and without intervention of carbene-alkene complexes. In addition, there is no evidence for metallacycle formation in the course of transfer reactions. Relative reactivity studies have established the electrophilic nature of the transfer and a general picture of the transition state. However, detailed pictures of the transition states have not been developed and there are still fundamental mechanistic questions to be answered regarding transition state structures, the reasons for the observed selectivities and the modes of ring closure and metal-carbon bond cleavage. These will not be easy questions to answer and will require a careful and mechanistically sophisticated analysis of structurally well-defined systems.

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