Oligomers as Intermediates in Ring-Closing Metathesis

Abstract

Oligomerization is kinetically favored in RCM reactions catalyzed by RuCl2(PCy3)(IMes)(CHPh), for a range of unhindered α,ω-dienes leading to large or medium-sized rings, even at dilutions designed to minimize intermolecular reaction. Reversible metathesis (i.e., ethenolysis) is inhibited by rapid volatilization of ethylene. At appropriately high dilutions, however, the RCM products are efficiently liberated by backbiting.

Mapping the potential energy surfaces for ring-closing metathesis reactions of prototypical dienes by electronic structure calculations

“…Our solution experimental work quantified the efficiency of formation of some substituted cyclooctenes by RCM, 21 where we measured the first effective molarities for RCM reactions. We also showed that the medium-ring products formed directly from acyclic diene precursors without the detectable intermediacy of oligomeric species, suggesting strongly that we were following a kinetically-controlled process (meaning that the product formed arose from the fastest reaction), and not a de-oligomerisation via backbiting 22 in which the most stable product formed. During the course of our work, we became aware that there was relatively little underpinning computational work dealing with the RCM of prototypical a,w-dienes.…”

Time as a Dimension in High‐Throughput Homogeneous Catalysis

“…[34] It is now widely acknowledged that no single best catalyst exists for all substrates, [35,36] and the burgeoning number of potential catalysts (of which a selection is shown in Figure 2) means that HT screening is increasingly desirable, particularly in industrial R&D. Reaction profiles are important to assess activity, lifetime, and optimum conditions, particularly concentration, where ring-chain equilibria can partition products between the desired cyclic products and (cyclo)-A C H T U N G T R E N N U N G oligomeric species. [37,38] Although the robustness of the Grubbs class ruthenium metathesis catalysts limits the pool of potential quenching agents, several effective poisons have been reported. Included in this study (Figure 3) are ethyl vinyl ether 6, [39,40] methylimidazole 7, [41,42] acetonitrile 8, [43][44][45] and selected bidentate or tridentate proligands (9-13) with potential to deactivate the catalysts by coordination and oxidation or protonolysis.…”

“…Of key relevance from the perspective of catalyst (re)design are the structure-activity relationships that can be extracted; for example, the effect of ligands on activity and lifetime. A wide range of other parameters could be screened: for specific substrates, important examples may include the influence of protecting groups on RCM efficiency and optimal concentrations, [37,38] the influence of ligands or additives on enantioselectivity in asymmetric RCM, etc.…”

Metathetic Synthesis of Common and Medium-Sized Lactones: The State of the Art

“…In fact, the unsubstituted α,β-unsaturated lactone 5,6,7,8-tetrahydro-(3Z )-H-oxocin-2-one (136, 2-heptenolide) was obtained from the corresponding acrylate ester 135 by using a second-generation Grubbs catalyst (5 mol%) and isolated in high yield (92%) without apparent limitations with respect to the smaller α,β-unsaturated seven-138 and six-membered 140 lactones (Table 8) [62]. Although this result may appear in contrast with the already mentioned lack of formation of the γ,δ-unsaturated analog 132 [203], it is very likely that conjugation between the carbonyl and the double bond in 2-heptenolide acts as a stabilizing factor during the transition state of ring closure. To the best of our knowledge, the unsubstituted β,γ- [200], δ,ε-, or ε,ζ- [207] unsaturated eightmembered lactones have not been prepared by RCM reactions.…”

“…It was found that the activation energy involved in the formation of lactones is highest for eight-and ninemembered rings [202] and derives essentially from ring strain due to imperfect In such unfavorable cases, formation of oligomers is highly competitive. In the specific case of RCM of unsubstituted diene esters 129, 131 and 133, values of EM were compared with the molar percentages of oligomers and lactone for rings with n ¼ 7, 8, 10 in the presence of catalyst G2 [203]. Indeed, the formation of the expected γ,δ-unsaturated eight-membered lactone 3,4,7,8-tetrahydro-H-oxocin-2-one 132 from 3-buten-1-yl-4-pentenoate 131 failed even at substrate concentration as low as 0.05 mM, whereas lactones 130 (n ¼ 7) and 134 (n ¼ 10) formed in good yields (Table 7).…”