Metal–Ligand Cooperation Facilitates Bond Activation and Catalytic Hydrogenation with Zinc Pincer Complexes

Michael Rauch,Sayan Kar,David Milstein,Amit Kumar,Linda J. W. Shimon,Liat Avram

doi:10.1021/jacs.0c05500

Abstract

A series of PNP zinc pincer complexes capable of bond activation via aromatization/dearomatization metal–ligand cooperation (MLC) were prepared and characterized. Reversible heterolytic N–H and H–H bond activation by MLC is shown, in which hemilability of the phosphorus linkers plays a key role. Utilizing this zinc pincer system, base-free catalytic hydrogenation of imines and ketones is demonstrated. A detailed mechanistic study supported by computation implicates the key role of MLC in facilitating effective catalysis. This approach offers a new strategy for (de)hydrogenation and other catalytic transformations mediated by zinc and other main group metals.

Highlights

In moving toward a more sustainable future, it is imperative to develop “green” chemical transformations that are both economical and free of waste generation.[1]
Access to a dearomatized zinc pincer complex is readily achieved via the reaction of the PNPtBu pro-ligand with dimethylzinc at 140 °C in toluene to generate PNPtBu*ZnMe (1) concurrent with loss of methane (Scheme 1)
This simultaneous deprotonation and complexation approach is analogous to the procedure employed to prepare lithium and potassium pincer complexes,[29] and is not uncommon in the synthesis of zinc alkyl and amido species.[8,37,38]

Summary

■ INTRODUCTION

In moving toward a more sustainable future, it is imperative to develop “green” chemical transformations that are both economical and free of waste generation.[1]. Our group is interested in leveraging metal−ligand cooperation (MLC), in which bond activation occurs across both metal and ligand, to effect transformations in an atomeconomical, waste-free fashion.[23] The MLC approach has long been demonstrated for advancements with the noble metals, and more recently with more abundant first row transition metals, for reversible bond activation resembling oxidative addition and reductive elimination (Figure 1).[1,24−27] Importantly, the metal center does not change oxidation state in this process, and as a result, this activation mode could be potentially operative with redox-innocent main group metals.[28]. This new mode of bond activation by zinc is applied toward the additive-free, catalytic hydrogenation of imines and ketones

■ RESULTS AND DISCUSSION

■ CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES