Fast-Initiating, Ruthenium-based Catalysts for Improved Activity in Highly E-Selective Cross Metathesis

Ruthenium-based olefin metathesis catalysts bearing dithiolate ligands have been recently employed to generate olefins with high E-selectivity (>99% E) but have been limited by low to moderate yields. In this report, ^1H NMR studies reveal that a major contributing factor to this low activity is the extremely low initiation rates of these catalysts with trans olefins. Introducing a 2-isopropoxy-3-phenylbenzylidene ligand in place of the conventional 2-isopropoxybenzylidene ligand resulted in catalysts that initiate rapidly under reaction conditions. As a result, reactions were completed in significantly less time and delivered higher yields than those in previous reports while maintaining high stereoselectivity (>99% E)

A Highly Efficient Synthesis of Z-Macrocycles using Stereoretentive, Ruthenium-Based Metathesis Catalysts

A highly efficient, Z-selective ring-closing metathesis system for the formation of macrocycles using a stereoretentive, ruthenium-based catalyst supported by a dithiolate ligand is reported. The catalyst is remarkably active as observed in initiation experiments showing complete catalyst initiation at −20 °C within 10 minutes. Macrocyclization reactions generated Z-products from easily accessible diene starting materials bearing a Z-olefin moiety. This approach provides a more efficient and selective route to Z-macrocycles relative to previously reported systems. Reactions were completed within shorter reaction times, and turnover numbers of up to 100 could be achieved. Macrocyclic lactones ranging in size from twelve- to seventeen-membered rings were synthesized in moderate to high yields (67–79 %) with excellent Z-selectivity (95–99 %)

Z-Selective ruthenium metathesis catalysts: Comparison of nitrate and nitrite X-type ligands

Two new Ru-based metathesis catalysts, 3 and 4, have been synthesized for the purpose of comparing their catalytic properties to those of their cis-selective nitrate analogues, 1 and 2. Although catalysts 3 and 4 exhibited slower initiation rates than 1 and 2, they maintained high cis-selectivity in homodimerization and ring-opening metathesis polymerization reactions. Furthermore, the nitrite catalysts displayed higher cis-selectivity than 2 for ring-opening metathesis polymerizations, and 4 delivered higher yields of polymer

Stereoretentive Olefin Metathesis: An Avenue to Kinetic Selectivity

Olefin metathesis is an incredibly valuable transformation that has gained widespread use in both academic and industrial settings. Lately, stereoretentive olefin metathesis has garnered much attention as a method for the selective generation of both E- and Z-olefins. Early studies employing ill-defined catalysts showed evidence for retention of the stereochemistry of the starting olefins at low conversion. However, thermodynamic ratios E/Z were reached as the reaction proceeded to equilibrium. Recent studies in olefin metathesis have focused on the synthesis of catalysts that can overcome the inherent thermodynamic preference of an olefin, providing synthetically useful quantities of a kinetically favored olefin isomer. These reports have led to the development of stereoretentive catalysts that not only generate Z-olefins selectively, but also kinetically produce E-olefins, a previously unmet challenge in olefin metathesis. Advancements in stereoretentive olefin metathesis using tungsten, ruthenium, and molybdenum catalysts are presented

Kinetics and Mechanism of Indene C–H Bond Activation by [(COD)Ir(μ_2-OH)]_2

The hydroxy-bridged dimer [(COD)Ir(μ_2-OH)]_2 (COD = 1,5-cyclooctadiene) cleanly cleaves C–H bonds in indene and cyclopentadiene to produce (COD)Ir(η^3-indenyl) and (COD)Ir(η^5-C_(5)H_5), respectively. The kinetics of the formation of (COD)Ir(η^3-indenyl) are consistent with a mechanism that involves coordination of indene to [(COD)Ir(μ_2-OH)]_2 followed by rate-determining C–H activation from the iridium dimer–indene unit. Transition-state analysis of the Ir and Rh hydroxy dimers indicates that the C–H activation proceeds through a direct deprotonation of indene by the M–OH unit rather than a stepwise oxidative addition/reductive elimination mechanism. The crystal structure of [(COD)Ir]_5(μ_4-O)(μ_3-O)(μ_2-OH), a dehydration product of [(COD)Ir(μ_2-OH)]_2, is presented

Concise Syntheses of Δ^(12)-Prostaglandin J Natural Products via Stereoretentive Metathesis

Δ^(12)-Prostaglandin J family is recently discovered and has potent anticancer activity. Concise syntheses of four Δ^(12)-prostaglandin J natural products (7–8 steps in the longest linear sequences) are reported, enabled by convergent stereoretentive cross-metathesis. Exceptional control of alkene geometry was achieved through stereoretention

Ru-Catalyzed, cis-Selective Living Ring-Opening Metathesis Polymerization of Various Monomers, Including a Dendronized Macromonomer, and Implications to Enhanced Shear Stability

An unsaturated polymer’s cis/trans-olefin content has a significant influence on its properties. For polymers obtained by ring-opening metathesis polymerization (ROMP), the cis/trans-olefin content can be tuned by using specific catalysts. However, cis-selective ROMP has suffered from narrow monomer scope and lack of control over the polymerization (giving polymers with broad molecular weight distributions and prohibiting the synthesis of block copolymers). Herein, we report the versatile cis-selective controlled living ROMP of various endo-tricyclo[4.2.2.0^(2,5)]deca-3,9-diene and various norbornene derivatives using a fast-initiating dithiolate-chelated Ru catalyst. Polymers with cis-olefin content as high as 99% could be obtained with high molecular weight (up to M_n of 105.1 kDa) and narrow dispersity (<1.4). The living nature of the polymerization was also exploited to prepare block copolymers with high cis-olefin content for the first time. Furthermore, owing to the successful control over the stereochemistry and narrow dispersity, we could compare cis- and trans-rich polynorbornene and found the former to have enhanced resistance to shear degradation

Using stereoretention for the synthesis of E-macrocycles with ruthenium-based olefin metathesis catalysts

The synthesis of E-macrocycles is achieved using stereoretentive, Ru-based olefin metathesis catalysts supported by dithiolate ligands. Kinetic studies elucidate marked differences in activity among the catalysts tested, with catalyst 4 providing meaningful yields of products in much shorter reaction times than stereoretentive catalysts 2 and 3. Macrocycles were generated with excellent selectivity (>99% E) and in moderate to high yields (47–80% yield) from diene starting materials bearing two E-configured olefins. A variety of rings were constructed, ranging from 12- to 18-membered macrocycles, including the antibiotic recifeiolide

Using stereoretention for the synthesis of E-macrocycles with ruthenium-based olefin metathesis catalysts

The synthesis of E-macrocycles is achieved using stereoretentive, Ru-based olefin metathesis catalysts supported by dithiolate ligands. Kinetic studies elucidate marked differences in activity among the catalysts tested, with catalyst 4 providing meaningful yields of products in much shorter reaction times than stereoretentive catalysts 2 and 3. Macrocycles were generated with excellent selectivity (>99% E) and in moderate to high yields (47–80% yield) from diene starting materials bearing two E-configured olefins. A variety of rings were constructed, ranging from 12- to 18-membered macrocycles, including the antibiotic recifeiolide

Concise Syntheses of Δ^(12)-Prostaglandin J Natural Products via Stereoretentive Metathesis

Δ^(12)-Prostaglandin J family is recently discovered and has potent anticancer activity. Concise syntheses of four Δ^(12)-prostaglandin J natural products (7–8 steps in the longest linear sequences) are reported, enabled by convergent stereoretentive cross-metathesis. Exceptional control of alkene geometry was achieved through stereoretention