Air-stable 18-electron adducts of Schrock catalysts with tuned stability constants for spontaneous release of the active species

Eddy Martin,Imre Pápai,Márk Szabó,Jordi Benet-Buchholz,Shigetaka Hayano,Ágota Bucsai,Henrik Gulyás,Ádám Madarász

doi:10.1038/s42004-021-00503-4

Abstract

Schrock alkylidenes are highly versatile, very active olefin metathesis catalysts, but their pronounced sensitivity to air still hinders their applications. Converting them into more robust but inactive 18-electron adducts was suggested previously to facilitate their handling. Generating the active species from the inactive adducts, however, required a high-temperature Lewis acid treatment and resulted in an insoluble by-product, limiting the practicality of the methodology. Herein, we introduce an approach to circumvent the inconvenient, costly, and environmentally taxing activation process. We show that 18-electron adducts of W- and Mo-based Schrock catalysts with finite stability constants (typically K = 200–15,000 M−1) can readily be prepared and isolated in excellent yields. The adducts display enhanced air-stability in the solid state, and in solution they dissociate spontaneously, hence liberating the active alkylidenes without chemical assistance.

Highlights

Schrock alkylidenes are highly versatile, very active olefin metathesis catalysts, but their pronounced sensitivity to air still hinders their applications
14-electron Mo(VI) and W(VI) alkylidenes, are highly efficient catalysts for olefin metathesis (OM) reactions. The understanding of their chemistry has steadily been growing over the course of the last thirty years, leading to the development of newer and newer generations of these remarkable catalysts, and tackling such challenges as the efficient synthesis of macrocyclic olefins[1], enantioselective metatheses[2,3], Z and E selective metatheses[4,5,6,7], including the formation of Z-disubstituted enol ethers[8], Z-alkenyl halides[9], and Z-trifluoromethyl-substituted alkenes[10]
XiMo researchers showed that embedding Schrock alkylidenes into paraffin pellets provided effective physical protection from air[16], while the Fürstner group developed a general approach towards their chemical protection[17,18]

Summary

Introduction

Schrock alkylidenes are highly versatile, very active olefin metathesis catalysts, but their pronounced sensitivity to air still hinders their applications. 1234567890():,; Schrock complexes, 14-electron Mo(VI) and W(VI) alkylidenes, are highly efficient catalysts for olefin metathesis (OM) reactions. XiMo researchers showed that embedding Schrock alkylidenes into paraffin pellets provided effective physical protection from air[16], while the Fürstner group developed a general approach towards their chemical protection[17,18]. The 18-electron alkylidenes 2 and 3 were not active as olefin metathesis catalysts, the active 14-electron Mo(VI)-alkylidene 1 could be liberated prior to performing the desired OM reactions by the addition of Lewis acidic reagents, such as ZnCl2, which at elevated temperatures was capable of capturing the protecting Lewis base from the coordination sphere of the molybdenum[17] (Fig. 1). From Fürstner’s findings, Schrock and Hoveyda showed that the bipyridine adducts of highly unstable molybdenum imido alkylidene bispyrrolide complexes displayed enhanced chemical and air stability. One of the possible reasons for this is the necessity of the high-temperature Lewis acid treatment to liberate the active

Methods

Results

Conclusion