Bimolecular Coupling as a Vector for Decomposition of Fast-Initiating Olefin Metathesis Catalysts

The correlation between rapid initiation and rapid decomposition in olefin metathesis is probed for a series of fast-initiating, phosphine-free Ru catalysts: the Hoveyda catalyst HII, RuCl₂(L)(═CHC₆H₄- o-O ⁱPr); the Grela catalyst nG (a derivative of HII with a nitro group para to O ⁱPr); the Piers catalyst PII, [RuCl₂(L)(═CHPCy₃)]OTf; the third-generation Grubbs catalyst GIII, RuCl₂(L)(py)₂(═CHPh); and dianiline catalyst DA, RuCl₂(L)( o-dianiline)(═CHPh), in all of which L = H₂IMes = N,N'-bis(mesityl)imidazolin-2-ylidene. Prior studies of ethylene metathesis have established that various Ru metathesis catalysts can decompose by β-elimination of propene from the metallacyclobutane intermediate RuCl₂(H₂IMes)(κ²-C₃H₆), Ru-2. The present work demonstrates that in metathesis of terminal olefins, β-elimination yields only ca. 25-40% propenes for HII, nG, PII, or DA, and none for GIII. The discrepancy is attributed to competing decomposition via bimolecular coupling of methylidene intermediate RuCl₂(H₂IMes)(═CH₂), Ru-1. Direct evidence for methylidene coupling is presented, via the controlled decomposition of transiently stabilized adducts of Ru-1, RuCl₂(H₂IMes)L_n(═CH₂) (L_n = py _n'; n' = 1, 2, or o-dianiline). These adducts were synthesized by treating in situ-generated metallacyclobutane Ru-2 with pyridine or o-dianiline, and were isolated by precipitating at low temperature (-116 or -78 °C, respectively). On warming, both undergo methylidene coupling, liberating ethylene and forming RuCl₂(H₂IMes)L_n. A mechanism is proposed based on kinetic studies and molecular-level computational analysis. Bimolecular coupling emerges as an important contributor to the instability of Ru-1, and a potentially major pathway for decomposition of fast-initiating, phosphine-free metathesis catalysts.

Effect of Lewis acidity on the synthesis of RuHCl(CO)(phosphine)(2): subtle influence of steric and electronic effects among P(i)Pr(3), P(i)Pr(2)(3,5-(CF(3))(2)C(6)H(3)), and P(i)Pr(2)Me

To evaluate the influence of steric, electronic, and synthetic factors, the synthesis of RuHCl(CO)[P(i)Pr(2)(3,5-(CF(3))(2)C(6)H(3))](2) was carried out, and its Lewis acidity toward Cl- was compared to that of RuHCl(CO)(P(i)Pr(3))(2). In this synthesis, Na(2)CO(3) was shown to be a more effective base than NEt(3), because Na+ can better mask the nucleophilicity of the potential ligand Cl-. An X-ray structure determination of the hydride-free species RuCl(2)(CO)(P(i)Pr(2)Me)(2) shows it to be a dimer, and this solid-state structure persists in solution, but as several different isomers. The synthesis of RuHCl(CO)(P(i)Pr(2)Me)(3) shows that three of this smaller phosphine can crowd around Ru, but dynamic NMR spectra show one phosphine to be weakly bound. The rate of reaction of Me(3)SiC(triple bond)CH with this molecule is suppressed by added free P(i)Pr(2)Me, indicating phosphine dissociation to be a mechanistic component.