The recent uptake of molecular metathesis catalysts in specialty-chemicals and pharmaceutical manufacturing is reviewed.

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, RuCl(L)(═CHCH- o-O Pr); the Grela catalyst nG (a derivative of HII with a nitro group para to OPr); the Piers catalyst PII, [RuCl(L)(═CHPCy)]OTf; the third-generation Grubbs catalyst GIII, RuCl(L)(py)(═CHPh); and dianiline catalyst DA, RuCl(L)( o-dianiline)(═CHPh), in all of which L = HIMes = 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 RuCl(HIMes)(κ-CH), 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 RuCl(HIMes)(═CH), Ru-1. Direct evidence for methylidene coupling is presented, via the controlled decomposition of transiently stabilized adducts of Ru-1, RuCl(HIMes)L(═CH) (L = py ; 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 RuCl(HIMes)L. 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.

The inverse relationship between NHC donicity and catalyst initiation.

Two ruthenium hydride complexes commonly proposed as agents of unintended isomerization during olefin metathesis are examined for their activity in isomerization of estragole, a representative allylbenzene. Neither proves kinetically competent to account for the levels of isomerization observed during cross‐metathesis of estragole by the second‐generation Grubbs catalyst. A structure–activity analysis of selected ruthenium hydride complexes indicates that higher isomerization activity correlates with a more electrophilic metal center.