Isomerization in Olefin Metathesis: Challenges and Opportunities

Title: Isomerization in Olefin Metathesis: Challenges and Opportunities
Authors: Higman, Carolyn Sarah
Date: 2016
Abstract: The past two years have witnessed groundbreaking advances in the industrial deployment of olefin metathesis. While metathesis methodologies have been an integral part of the chemical manufacturing landscape for 60 years, implementation in pharmaceutical and specialty chemicals manufacturing represents a new frontier. The imperative to develop greener and more cost-effective manufacturing processes is anticipated to spur further improvements in sustainable synthesis. Advances in catalyst productivity, however, are critical to expansion of the uptake of metathesis methodologies in this and other manufacturing sectors. Key to increased catalyst productivity is elimination of side reactions that lower yield and errode selectivity. Among such reactions, double-bond isomerization is by far most common. Accumulating evidence suggests that unwanted isomerization during olefin metathesis is due to ruthenium species generated via catalyst decomposition. The identification of these species and how they are formed is thus of great importance. Two hydride complexes, RuHCl(CO)(H2IMes)(PCy3) and a dinuclear hydride, are known to form under some circumstances by decomposition of the second-generation Grubbs catalyst, RuCl2(H2IMes)(PCy3)(=CHPh), GII. These complexes have been widely viewed as responsible for unintended isomerization reactions. However, examination of their performance in olefin isomerization under conditions relevant to metathesis reveals that their activity is too feeble to account for the levels of isomerization observed during metathesis. Alternatively, kinetically competent culprits emerge from decomposition studies that reveal unexpected ruthenium products on decomposition of GII during metathesis; specifically, formation of ruthenium nanoparticles. The formation and catalytic non-innocence of RuNPs constitutes a new paradigm in this field, which opens the door to new approaches to prevent or to harness olefin isomerization. Key to prevention, clearly, is circumventing the decomposition pathways that enable ligand stripping from the active catalyst. New approaches to catalyst design that involve use of truncated NHC ligands are also examined. Finally, the power and utility of isomerization when coupled with metathesis is explored. The opportunities and limitations of orthogonal isomerization–metathesis catalysis are examined in the context of the two-step synthesis of cinnamates from 1-allylbenzenes abundant in essential oils. An efficient one-pot, two-catalyst protocol is developed for conversion of these biorenewable feedstocks to high-value-added chemicals.
CollectionThèses, 2011 - // Theses, 2011 -