Controlling the Activity of a Caged Cobalt-Porphyrin-Catalyst in Cyclopropanation Reactions with Peripheral Cage Substituents

In this study, three novel cubic cages were synthesized and utilized to encapsulate a catalytically active cobalt(II) meso-tetra(4-pyridyl)porphyrin guest. The newly developed caged catalysts (Co-G@Fe₈(Zn-L ⋅ 1)₆, Co-G@Fe₈(Zn-L ⋅ 2)₆ and Co-G@Fe₈(Zn-L ⋅ 3)₆) can be easily synthesized and differ in exo-functionalization, which are either none, polar or apolar groups. This leads to a different polarity of the peripheral environment surrounding the cage, which affects the (relative) local concentration of the substrates surrounding the cage and hence indirectly influences the substrate availability of the catalysis embedded in the active site of the caged catalyst systems. The resulting increased local substrate concentrations give rise to higher catalytic activities of the respective caged catalyst in metalloradical catalyzed cyclopropanation reactions. Interestingly, the catalytic activity is the highest when the apolar cage catalyst (Co-G@Fe₈(Zn-L ⋅ 1)₆) is used, and lowest with the polar analog (Co-G@Fe₈(Zn-L ⋅ 3)₆). In addition, the catalytic activity of the cage without exo-functionalities (Co-G@Fe₈(Zn-L ⋅ 2)₆) is nearly two times lower than that of Co-G@Fe₈(Zn-L ⋅ 1)₆ and three times higher than that of Co-G@Fe₈(Zn-L ⋅ 3)₆, which further demonstrates the effect of the peripheral functionalities on the cyclopropanation reaction.