Targeting an Achilles heel in olefin metathesis: A strategy for high-yield synthesis of second-generation Grubbs methylidene catalysts

The first clean, high-yield route is presented to methylidenes RuCl 2(L)(PCy 3)(=CH 2) (L = H 2IMes or IMes), key vectors for catalysis and deactivation in many olefin metathesis reactions. (H 2IMes = N,N′-bis(mesityl)imidazolin-2-ylidene; IMes = N,N′-bis(mesityl)imidazol-2-ylidene)

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

On exposure to NaOMe (≥3 equiv) in CH₂Cl₂–MeOH at 23 °C, the first-generation Grubbs catalyst RuCl₂(PCy₃)₂(CHPh) (<b>1a</b>) is immediately transformed into the six-coordinate methoxyhydride complexes RuH­(OMe)­(CO)₂(PCy₃)₂ (<b>4a</b>) and RuH­(OMe)­(CO)­(H₂)­(PCy₃)₂ (<b>5a</b>). Complex <b>5a</b> can be recycled into <b>4a</b> under conditions conducive to removal of H₂. The second-generation catalyst RuCl₂(IMes)­(PCy₃)­(CHPh) (<b>1b</b>; IMes = <i>N,N</i>′-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) (<b>3b</b>). Experiments in the presence of added PCy₃ reveal that consumption of <b>1a</b>, but not <b>1b</b>, 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 <b>1b</b> at 23 °C retards salt metathesis and inhibits further reaction of <b>3b</b>. For the bis­(PCy₃) analogue <b>3a</b>, fast transformation into <b>4a</b> 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 <b>3a</b> yields six-coordinate <b>5a</b> (the dihydrogen adduct of <b>3a</b>). Independent routes to RuH­(OMe)­(CO)₂(L)­(PCy₃) (<b>4a/b</b>; <b>a</b>, L = PCy₃; <b>b</b>, L = IMes) were developed: these involved sequential transformation of RuHCl­(CO)­(L)­(PCy₃) (<b>2a/b</b>) into the bis-carbonyl adducts RuHCl­(CO)₂(L)­(PCy₃) (<b>7a/b</b>) under CO, conversion of <b>7a/b</b> into the more reactive triflates RuH­(OTf)­(CO)₂(L)­(PCy₃) (<b>8a/b</b>), and reaction of <b>8a/b</b> with equimolar NaOMe. Dihydride <b>6b</b> was also prepared, by reaction of <b>8b</b> with NaH