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

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 : RuCl 2 (H 2 IMes)(PCy 3 )(=CHPh); HII : RuCl 2 (H 2 IMes)(=CHAr), Ar = C 6 H 4 -2-O i Pr), slow salt metathesis is shown to arise from the low lability of the ancillary PCy 3 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 RuI 2 (PCy 3 )(=CHAr) HI-I 2 was prepared by treating “Grubbs I” (RuCl 2 (PCy 3 ) 2 (=CHPh), GI ) with NaI, H 2 C=CHAr ( 1a ), and a phosphine-scavenging Merrifield iodide ( MF-I ) resin. Subsequent installation of H 2 IMes 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 RuI 2 (H 2 IMes)(=CHAr′) ( nG-I 2 : Ar′ = C 6 H 3 -2-O i Pr-4-NO 2 ) was instead accessed by sequential salt metathesis of GI with NaI, installation of H 2 IMes, and finally cross-metathesis with the nitrostyrenyl ether H 2 C=CHAr′ ( 1b ), with MF-I as the phosphine scavenger. The bulky iodide ligands improve the selectivity for macrocyclization in ring-closing metathesis.

Bimolecular Coupling in Olefin Metathesis: Correlating Structure and Decomposition for Leading and Emerging Ruthenium−Carbene Catalysts

“…19b Slowly initiating CAAC-iodide metathesis catalysts may thus be of keen interest for metathesis of accessible olefinic bonds, although few such complexes have yet been developed. 8a , 33 …”

“…As illustrated in Figure c, replacing the chloride ligands in the C1 Ph derivative by iodide slows the rate of decomposition 40-fold. Iodide catalysts, long overlooked because of their lower reactivity, have recently been shown to offer productivity superior to their faster-initiating analogues in demanding contexts that require long catalyst lifetimes. ,− Retarded bimolecular decomposition is clearly an important component of this robustness, although it should be noted that coupling remains operative for nG­(I 2 ) even at micromolar catalyst concentrations . Slowly initiating CAAC-iodide metathesis catalysts may thus be of keen interest for metathesis of accessible olefinic bonds, although few such complexes have yet been developed. , …”

Recent Advances in the Domain of Cyclic (Alkyl)(Amino) Carbenes

“…In continuation of their previous studies on iodine‐based Ru catalysts, Fogg et al. have synthesized second‐generation ruthenium‐diiodide catalysts ( 17 ) with cAAC and NHC ligands for the olefin metathesis in excellent yields [68] …”

“…[67] In continuation of their previous studies on iodine-based Ru catalysts, Fogg et al have synthesized second-generation ruthenium-diiodide catalysts (17) with cAAC and NHC ligands for the olefin metathesis in excellent yields. [68] Hong et al applied Grubbs-type and Hoveyda-type, cAAC stabilized ruthenium catalysts for olefin metathesis of 2,7divinyl-9,9-di-noctylfluorene (18) and 2,2',7,7'-tetravinyl-9,9'-spirobifluorene (19) in different ratios to synthesize seven copolymerization products in good to excellent yields (Scheme 7). [69] The copolymers with spirobifluorene units are better for polymer Light-Emitting Diode (LED) devices and display improved performance like better turn-on voltage, brightness, current, and power efficiency.…”

“…have synthesized second‐generation ruthenium‐diiodide catalysts ( 17 ) with cAAC and NHC ligands for the olefin metathesis in excellent yields. [68] …”