Flexibility, π-π stacking, and ylide stabilization in organometalic catalyzation
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Recent experimental work has demonstrated the potential of mixed-ligand Rh(II) paddlewheel complexes, such as Rh2(OAc)3PhTCB, to enhance cyclopropanation yields and selectivity compared to traditional catalysts with identical ligands like Rh2(OAc)4. In this computational study using density functional theory (DFT), we explore the mechanistic basis for the improved performance of Rh2(OAc)3PhTCB. The mixed-ligand design offers increased catalyst flexibility, enabling the dissociation of the PhTCB ligand from the vacant Rh site and the formation of stabilizing π-π stacking interactions with the substrate in a three-benzene sandwich structure. Along the reaction pathway, the Rh-carbene intermediate forms an explicit C-S bond, resulting in an ylide structure that represents a deep energy minimum. The unique functionality of the Rh2(OAc)3PhTCB catalyst arises from its ability to modulate the energetic landscape and promote cyclopropanation while suppressing side reactions. These computational insights highlight the power of mixed-ligand Rh(II) catalysts and provide a framework for the rational design of improved Rh paddlewheel catalysts.