Metal–Ligand Cooperative Proton Transfer as an Efficient Trigger for Rhodium-NHC-Pyridonato Catalyzed gem-Specific Alkyne Dimerization

María Galiana-Cameo,Jesús J Pérez-Torrente,Víctor Polo,Asier Urriolabeitia,Eduardo Barrenas,Andrea Di Giuseppe,Ricardo Castarlenas,Vincenzo Passarelli

doi:10.1021/acscatal.1c00602

María Galiana-Cameo, Jesús J Pérez-Torrente + Show 6 more

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https://doi.org/10.1021/acscatal.1c00602

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Abstract

The mononuclear square-planar Rh{κ2-X,N-(Xpy)}(η2-coe)(IPr) (X = O, NH, NMe, S) complexes have been synthesized from the dinuclear precursor [Rh(μ-Cl)(IPr)(η2-coe)]2 and the corresponding 2-heteroatom-pyridinate salts. The Rh-NHC-pyridinato derivatives are highly efficient catalysts for gem-specific alkyne dimerization. Particularly, the chelating N,O-pyridonato complex displays turnover frequency levels of up 17 000 h–1 at room temperature. Mechanistic investigations and density functional theory calculations suggest a pyridonato-based metal–ligand cooperative proton transfer as responsible for the enhancement of catalytic activity. The initial deprotonation of a Rh-π-alkyne complex by the oxo-functionality of a κ1-N-pyridonato moiety has been established to be the rate-limiting step, whereas the preferential protonation of the terminal position of a π-coordinated alkyne accounts for the exclusive observation of head-to-tail enynes. The catalytic cycle is closed by a very fast alkenyl–alkynyl reductive elimination.

Highlights

Organometallic catalysis is nowadays at the central core of the preparation of elaborated organic structures owing to a continuous design of new metal−ligand architectures.[1]
The synergic effect arising from metal−ligand cooperation (MLC) generally triggers an enhancement of catalytic activity and provides better control of selectivity
A particular case of MLC arises when a ligand acts as a carrier for a proton from one substrate to the other for which the term ligand assisted proton shuttle (LAPS) has been coined (Scheme 1).[3]

Summary

■ INTRODUCTION

Organometallic catalysis is nowadays at the central core of the preparation of elaborated organic structures owing to a continuous design of new metal−ligand architectures.[1]. The insertion of the alkyne into the Rh−C bond has been discarded based on previous studies on similar systems.11e,15a The obtained alkynyl-alkenyl complexes D evolve to the final products via reductive elimination via TSDAg and TSDAt, showing energetic barriers of 7.4 and 12.4 kcal·mol−1, respectively This mechanistic proposal presents an overall activation energy of 20.2 kcal·mol−1 for the gem-enyne, which is preferentially obtained due to the significantly higher barrier for the E product (23.7 kcal·mol−1). The reaction starts by coordination of a second alkyne to A, allowed by the hemilabile behavior of the pyridonato ligand.[21] As a result, a switch to a {κ1-N-(Opy)} coordination mode of this molecule is observed.[31] This process is characterized by TSAE (energetic barrier of 14.2 kcal·mol−1) leading to the intermediate E Rh{κ1-N-(Opy)}(η2-HC CPh)2(IPr), displaying a mutually trans disposition for the two π-alkyne molecules.[32] Since the pyridonato ligand is coordinated to the metal only by the nitrogen atom, free rotation about the Rh−N bond becomes possible enabling the easy approach of the basic oxo group to any terminal hydrogen of the η2-. The theoretically computed KIE for this step is 1.57, which agrees with the experimentally determined value (see Table S1 in the Supporting Information)

■ CONCLUSION

■ REFERENCES

■ ACKNOWLEDGMENTS