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2.2.12 Conclusions and Future Perspectives

DOI: 10.1055/sos-SD-222-00020

Garve, L. K. B.; Werz, D. B.Science of Synthesis: Metal-Catalyzed Cyclization Reactions, (2016294.

Numerous metal-catalyzed methods to prepare cyclopropanes have been developed over the past fifty years. Notably, the taming of carbenes in the form of metal–carbene complexes has catapulted three-membered-ring syntheses back to the forefront of organic synthesis over the past decades. With the advent of the use of metals in cyclopropane synthesis, chiral ligands for diastereo- and enantiodiscrimination have also come under focus and this strategy has opened new avenues to access these three-membered rings. Apart from the use of metals to catalyze the decomposition of diazo compounds, the action of carbophilic gold(I) cations interacting with alkynes, in particular, has led to amazing syntheses of the cyclopropane moiety. However, challenges in this area still remain. Many metal–carbene complexes do not react with tetrasubstituted double bonds; thus, the development of asymmetric methods for the preparation of hexasubstituted cyclopropanes would be highly desirable. Distinct deconstruction reactions of such three-membered rings could pave the way to highly substituted heterocyclic compounds or open-chain moieties. For many metals, diazo compounds are still needed to generate the reactive complex. Synthetic routes that circumvent these potentially hazardous compounds would be a great advance. An ideal synthesis of cyclopropanes, a dream reaction, would be the ring closure of alkane subunits or the respective transformation of alkenes with alkanes, both under oxidative conditions with the release of water. Of course, these concepts suffer from unfavorable thermodynamics, but this does not mean that they are generally impossible. The smallest ring, the cyclopropane, has not yet disclosed all of its synthetic secrets.