Cobalt(II)-Catalyzed Atom/Group Transfer Reactions: Stereoselective Carbene and Nitrene Transfer Reactions

Metalloporphyrins have been shown to catalyze many fundamental and practically important chemical transformations, some of which represent the first demonstrations of these catalytic processes. The most notable examples include an assortment of atom/group transfer reactions, such as oxene, nitrene, and carbene transfers. Atom/group transfer reactions allow for the direct conversion of abundant and inexpensive alkenes and alkanes into value-added functional molecules. Previous reports from our group have shown that cobalt-porphyrin based carbene and nitrene transfer reactions are some of the most selective and practical catalytic systems developed for cyclopropanation and aziridination. Backed by a family of D2-symmetric chiral cobalt porphyrins our group continues the development of stereoselective carbene and nitrene transfer reactions. Metal-catalyzed cyclopropanation of olefins with diazo reagents has attracted great research interest because of its fundamental and practical importance. The resulting cyclopropyl units are recurrent motifs in biologically important molecules and can serve as versatile precursors in organic synthesis. Supported by a family of D2-symmetric chiral cobalt porphyrins, we have demonstrated the use of succimidyl diazoacetate as carbene source for a highly diastereo- and enantioselective cyclopropanation process. The resulting cyclopropyl succinimdyl esters are highly reactive and serve as valuable synthons for generating cyclopropylcarboxamides. We have also developed the first cobalt-porphyrin based intramolecular cyclopropanation, which is able to produce the resulting bicyclic lactones in high yields and enantioselectivity. Nitrene transfer reactions are also an attractive route to produce biologically and synthetically important molecules such as amines and aziridines. Although much progress has been made in nitrene transfer reactions utilizing [N-(p-toluenesulfonyl) imino]phenyliodinane (PhI=NTs) the nitrene source suffers from several drawbacks. Consequently, there has been growing interest in developing catalytic nitrene transfer reactions using alternate nitrene sources. To this end, we have utilized arylsulfonyl azides as nitrene source to explore their use in the development of a cobalt-porphyrin catalyzed enantioselective aziridination system. The cobalt catalyzed process can proceed under mild and neutral conditions in low catalyst loading without the need of other reagents, while generating nitrogen gas as the only byproduct. We have also explored the use of arylsulfonyl azides as nitrene source in a cobalt-catalyzed intramolecular C-H amination process