Routes to High-Performing Ruthenium-Iodide Catalysts for Olefin Metathesis: Ligand Lability Is Key to Efficient Halide Exchange.

Christian O Blanco,Daniel L Nascimento,Deryn E Fogg

doi:10.1021/acs.organomet.1c00253

Christian O Blanco, Daniel L Nascimento + Show 1 more

Open Access

https://doi.org/10.1021/acs.organomet.1c00253

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Journal: Organometallics	Publication Date: Jun 16, 2021
Citations: 10	License type: CC BY 4.0

Affiliation: University of Ottawa, University of Bergen

Abstract

Clean, high-yielding routes are described to ruthenium–diiodide catalysts that were recently shown to enable high productivity in olefin metathesis. For the second-generation Grubbs and Hoveyda catalysts (GII: RuCl2(H2IMes)(PCy3)(=CHPh); HII: RuCl2(H2IMes)(=CHAr), Ar = C6H4-2-OiPr), slow salt metathesis is shown to arise from the low lability of the ancillary PCy3 or ether ligands, which retards access to the four-coordinate intermediate required for efficient halide exchange. To exploit the lability of the first-generation catalysts, the diiodide complex RuI2(PCy3)(=CHAr) HI-I2 was prepared by treating “Grubbs I” (RuCl2(PCy3)2(=CHPh), GI) with NaI, H2C=CHAr (1a), and a phosphine-scavenging Merrifield iodide (MF-I) resin. Subsequent installation of H2IMes or cyclic (alkyl)(amino)carbene (CAAC) ligands afforded the second-generation iodide catalysts in good to excellent yields. Given the incompatibility of the nitro group with a free carbene, the iodo-Grela catalyst RuI2(H2IMes)(=CHAr′) (nG-I2: Ar′ = C6H3-2-OiPr-4-NO2) was instead accessed by sequential salt metathesis of GI with NaI, installation of H2IMes, and finally cross-metathesis with the nitrostyrenyl ether H2C=CHAr′ (1b), with MF-I as the phosphine scavenger. The bulky iodide ligands improve the selectivity for macrocyclization in ring-closing metathesis.

Highlights

Diiodide catalysts that were recently shown to enable high productivity in olefin metathesis
Olefin Metathesis Catalysts and N-heterocyclic carbene (NHC) or CAACa Ligands Discussed such as nG-I2 have recently been shown to offer improved productivities in the synthesis of macrocycles via ring-closing metathesis,[10] and increased selectivity for metathesis of terminal versus internal olefins.[5]
Their tolerance for ethylene[11,12] is striking: it is due in part to relatively slow bimolecular decomposition.[6,12]

Summary

MeOH MeOH MeOH THF THF

NaI with a series of Grubbs-class catalysts, for which the PCy3 lability spans 5 orders of magnitude.[16] In situ yields of the diiodide products at 1 h declined in the order GI > GII > GII′ ≫ GIIm (Table 1, entries 5−8) These yields correspond to the established[16] trend in rates of PCy3 dissociation, consistent with our proposal that salt metathesis is mediated by fourcoordinate RuCl2(L)( CHPh).15c Of note, the corresponding experiment with the first-generation complex HI effected complete halide exchange, versus 6% for its H2IMes analogue HII (entries 9 and 10).

■ ACKNOWLEDGMENTS

Findings

■ REFERENCES