Reactions of Grubbs Catalysts with Excess Methoxide: Formation of Novel Methoxyhydride Complexes

Abstract

On exposure to NaOMe (≥3 equiv) in CH2Cl2–MeOH at 23 °C, the first-generation Grubbs catalyst RuCl2(PCy3)2(CHPh) (1a) is immediately transformed into the six-coordinate methoxyhydride complexes RuH(OMe)(CO)2(PCy3)2 (4a) and RuH(OMe)(CO)(H2)(PCy3)2 (5a). Complex 5a can be recycled into 4a under conditions conducive to removal of H2. The second-generation catalyst RuCl2(IMes)(PCy3)(CHPh) (1b; IMes = N,N′-bis(mesityl)imidazol-2-ylidene) reacts more slowly, requiring several hours even at 20 equiv of NaOMe, and terminates at five-coordinate RuH(OMe)(CO)(IMes)(PCy3) (3b). Experiments in the presence of added PCy3 reveal that consumption of 1a, but not 1b, proceeds via the four-coordinate intermediate formed by equilibrium loss of phosphine, a function of the lability of the PCy3 ligand at ambient temperatures. The poor accessibility of such an intermediate for 1b at 23 °C retards salt metathesis and inhibits further reaction of 3b. For the bis(PCy3) analogue 3a, fast transformation into 4a is proposed to involve reversible loss of PCy3, coordination of methanol, σ-metathesis of methanol at the hydride site to liberate H2, and β-elimination/decarbonylation of bound methoxide. Competitive uptake of H2 by 3a yields six-coordinate 5a (the dihydrogen adduct of 3a). Independent routes to RuH(OMe)(CO)2(L)(PCy3) (4a/b; a, L = PCy3; b, L = IMes) were developed: these involved sequential transformation of RuHCl(CO)(L)(PCy3) (2a/b) into the bis-carbonyl adducts RuHCl(CO)2(L)(PCy3) (7a/b) under CO, conversion of 7a/b into the more reactive triflates RuH(OTf)(CO)2(L)(PCy3) (8a/b), and reaction of 8a/b with equimolar NaOMe. Dihydride 6b was also prepared, by reaction of 8b with NaH.

Isomerization During Olefin Metathesis: An Assessment of Potential Catalyst Culprits

“…16 Most commonly cited in recent reports is the potential involvement of two hydride complexes ([Ru 2 (μ‐C)Cl 3 (H)(H 2 IMes) 2 ] ( Ru‐1 ) and [RuCl(CO)H(H 2 IMes)(PCy 3 )] ( Ru‐2 )], accessible by decomposition of GII (Scheme ) 17. Dinuclear Ru‐1 was shown by Hong and Grubbs to form on thermolysis of the resting‐state methylidene complex (albeit slowly),16 whereas several reports describe routes to hydridocarbonyl complexes of type Ru‐2 from GII 18–20. Notably, Percy and co‐workers observed Ru‐2 during ring‐closing metathesis of 1,7‐octadiene by GII ,21 and the Grubbs and Mol groups found that closely related carbonyl complexes form on exposure of the Grubbs catalysts to oxygen 17f,h.…”

Highly Active Ruthenium Metathesis Catalysts Exhibiting Unprecedented Activity and Z-Selectivity

“…18 Reactions were run utilizing the standard conditions described above. Of the three catalysts, 9 gave the best results, providing the homodimerization products in high conversions (>95% and 77% for 13 and 14 , respectively) with >95% Z- selectivity for both substrates.…”

Ru-based Z-selective metathesis catalysts with modified cyclometalated carbene ligands

“…10-Methyl undecenoate ( 32 , Table 3) was selected because the ester functionality is far removed from the double bond, while 4-pentenol ( 34 , Table 4) has been linked to decomposition pathways of prior generations of Ru-metathesis catalysts. 27 Furthermore, pronounced Z -content degradation has been observed in the homodimerization of 4-pentenol with previous Z -selective metathesis catalysts 7 and 8 . Metathesis reactions were executed utilizing the standard conditions described above.…”