Impact of Ligands and Metals on the Formation of Metallacyclic Intermediates and a Nontraditional Mechanism for Group VI Alkyne Metathesis Catalysts.

Richard R Thompson,Madeline E Rotella,Frank R Fronczek,Semin Lee,Xin Zhou,Osvaldo Gutierrez

doi:10.1021/jacs.1c01843

Richard R Thompson, Madeline E Rotella + Show 4 more

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https://doi.org/10.1021/jacs.1c01843

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Abstract

The intermediacy of metallacyclobutadienes as part of a [2 + 2]/retro-[2 + 2] cycloaddition-based mechanism is a well-established paradigm in alkyne metathesis with alternative species viewed as off-cycle decomposition products that interfere with efficient product formation. Recent work has shown that the exclusive intermediate isolated from a siloxide podand-supported molybdenum-based catalyst was not the expected metallacyclobutadiene but instead a dynamic metallatetrahedrane. Despite their paucity in the chemical literature, theoretical work has shown these species to be thermodynamically more stable as well as having modest barriers for cycloaddition. Consequentially, we report the synthesis of a library of group VI alkylidynes as well as the roles metal identity, ligand flexibility, secondary coordination sphere, and substrate identity all have on isolable intermediates. Furthermore, we report the disparities in catalyst competency as a function of ligand sterics and metal choice. Dispersion-corrected DFT calculations are used to shed light on the mechanism and role of ligand and metal on the intermediacy of metallacyclobutadiene and metallatetrahedrane as well as their implications to alkyne metathesis.

Highlights

The catalytic formation of carbon−carbon bonds remains one of the most crucial applications in organometallic chemistry
Cross-coupling, olefin metathesis, and polymerization reactions have all garnered extensive attention with myriad studies on optimizing conditions, improving substrate scope, and expanding the applications of these transformations following their initial discoveries.[1−4] On the other hand, alkyne metathesis has been a relatively dormant field until recent advances by Bunz,[5] Fürstner,[6] Tamm,[7] Moore,[8] Zhang,[9,10] Buchmeiser,[11−13] and Jia.[14]. These studies have demonstrated that alkyne metathesis is an incredibly powerful tool for generating pharmaceuticals,[15] complex natural products,[16−24] supramolecular structures,[25−37] and polymers.[5,38−44] The currently accepted mechanism for alkyne metathesis consists of a [2 + 2]/retro-[2 + 2] cycloaddition mechanism (Scheme 1) akin to those implicated in olefin metathesis
A sign of a marked difference between the two ligands is the chemical shift of the O-H with those of SiPPh coming in at 4.45 ppm while those associated with SiPEt are upfield by nearly 1 ppm at 3.60 ppm

Summary

■ INTRODUCTION

The catalytic formation of carbon−carbon bonds remains one of the most crucial applications in organometallic chemistry. Similar to the tungsten system, we explored the traditional [2 + 2]/retro-[2 + 2] mechanism in which the MCBD [Mo]-B forms product prior to isomerization to [Mo]-ent-B, but, akin to W-system, we found that the barrier for this pathway is insurmountable and is likely not operative for this system The formation of the metallocyclobutadiene or metallatetrahedrane intermediates using these podand ligands appears instead to be dependent on the nature of the metal with the favorable electrostatic energy between the ligand and substrate leading to its preference for the MCBD intermediate in the case of tungsten and the MTd intermediate in the case of molybdenum

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES