A chiral, bimetallic cobalt catalyst was discovered that is highly active and enantioselective for epoxide polymerization. The enantiomerically pure catalyst system exhibits a stereoselectivity factor (s = k(fast)/k(slow)) of 370 for propylene oxide, allowing enantiomerically pure epoxide to be recovered in nearly the maximum theoretical yield. In addition, the racemic catalyst forms highly isotactic poly(propylene oxide) in quantitative yield. The catalyst is active and selective for other epoxides, such as 1-butene oxide, 1-hexene oxide, and styrene oxide.
N-aryl, N-alkyl N-heterocyclic carbene (NHC) ruthenium metathesis catalysts are highly selective toward the ethenolysis of methyl oleate, giving selectivity as high as 95% for the kinetic, ethenolysis products over the thermodynamic, self-metathesis products. The examples described herein represent some of the most selective NHC-based ruthenium catalysts for ethenolysis reactions to date. Furthermore, many of these catalysts show unusual preference and stability toward propagating as a methylidene species, and provide good yields and turnover numbers (TONs) at relatively low catalyst loading (<500 ppm). A catalyst comparison showed that ruthenium complexes bearing sterically hindered NHC substituents afforded greater selectivity and stability, and exhibited longer catalyst lifetime during reactions. Comparative analysis of the catalyst preference for kinetic versus thermodynamic product formation was achieved via evaluation of their steady-state conversion in the cross-metathesis reaction of terminal olefins. These results coincided with the observed ethenolysis selectivities, in which the more selective catalysts reach a steady-state characterized by lower conversion to cross-metathesis products compared to less selective catalysts, which show higher conversion to cross-metathesis products.
A highly active enantiopure bimetallic cobalt complex was explored for the enantioselective polymerization of a variety of monosubstituted epoxides. The polymerizations were optimized for high rates and stereoselectivity, with s-factors (k(fast)/k(slow)) for most epoxides exceeding 50 and some exceeding 300, well above the threshold for preparative utility of enantiopure epoxides and isotactic polyethers. Values for mm triads of the resulting polymers are typically greater than 95%, with some even surpassing 98%. In addition, the use of a racemic catalyst allowed the preparation of isotactic polyethers in quantitative yields. The thermal properties of these isotactic polyethers are presented, with many polymers exhibiting high T(m) values. This is the first report of the rapid synthesis of a broad range of highly isotactic polyethers via the enantioselective polymerization of racemic epoxides.
A highly active bimetallic cobalt catalyst system is reported for the polymerization of racemic terminal epoxides to yield isotactic polyethers.Stereoregular polymers often display superior mechanical and thermal properties compared to their atactic analogues. 1 Polyethers are an important class of polymers with industrial and biological applications. 2 Much progress has been made in controlling the regiochemistry and molecular weight of polyethers, 3 but controlling the stereochemistry of polyethers synthesized from racemic epoxides remains a challenge. 4 Previous isoselective polymerization systems for racemic epoxides produced mixtures of atactic and isotactic materials, or generated only moderately isotactic polymers. 5 We previously reported a highly active and isoselective cobalt catalyst for the polymerization of racemic propylene oxide (rac-PO). 6 Using the information gained by studying the mechanism of this catalyst, 7 we have recently designed a highly active and enantioselective polymerization catalyst system that exhibits greater substrate scope. It consists of a bimetallic cobalt complex (1) and a bis(triphenylphosphine)iminium (PPN) acetate cocatalyst (4, Scheme 1). This system selectively polymerizes one enantiomer of racemic terminal epoxides to produce highly isotactic polyethers and enantiopure epoxides. 8 Unfortunately, this limits the theoretical maximum yield of polymer at ca. 50% unless the racemic form of 1 is used. Attempts to synthesize rac-1 (equimolar mixture of (R 4 S)-1 and (S 4 R)-1) from racemic starting materials produced inseparable diastereomers, which led us to develop a new catalyst that is active and isoselective while being derived from inexpensive racemic and/or achiral starting materials.Insight into the origin of enantioselectivity of 1 was gained through the reactivity of the diastereomeric cobalt complex 2, which possesses all S stereochemistry. When activated with PPN compound 4, complex 2 displayed lower activity than 1 for the polymerization of neat rac-PO (T rxn = 0 1C), but preferred the same enantiomer (S) with a k S /k R selectivity factor of 210. 9 This result confirmed that the stereochemistry of the binaphthol linker determines which enantiomer of monomer is enchained, possibly allowing simplification of 1 and 2 by removal of the chiral diamine linker.Having determined that the axial chirality of the binaphthol linker is responsible for the stereoselectivity of the polymerization, we synthesized complex 3, which incorporated ethylene diamine and a rac-binaphthol linker.w All attempts at crystallizing 3 from non-coordinating solvents were unsuccessful, however crystals were grown from pyridine and toluene giving black needles that were analyzed by single crystal X-ray diffraction. Fig. 1 shows the structure of 3 with hydrogens omitted and pyridine ligands truncated for clarity. Two pyridines are bound to each cobalt center displacing the chlorides giving a pseudo C 2 symmetric complex. 10 The Co-Co distance is 6.774 Å and the endo naphthol-naphthol dihedral...
The relative TONs of productive and nonproductive metathesis reactions of diethyl diallylmalonate are compared for eight different ruthenium-based catalysts. Nonproductive cross metathesis is proposed to involve a chain-carrying ruthenium methylidene. A second morechallenging substrate (dimethyl allylmethylallylmalonate) that forms a trisubstituted olefin product is used to further delineate the effect of catalyst structure on the relative efficiencies of these processes. A steric model is proposed to explain the observed trends.The widespread application of olefin metathesis in various fields of chemical synthesis has fueled the continued search for transition metal catalysts that exhibit high reactivity, selectivity, and stability.1 Studies have described the effect on reactivity upon modifying every ligand of ruthenium-based catalysts. Generally, activity is reported as yield or turnover number (TON) for the reaction of a substrate of interest, which does not account for nonproductive metathesis events.The role of nonproductive metathesis in the cross metathesis (CM) of terminal olefins has been studied in detail for early hetero-and homogeneous molybdenum and tungsten catalysts.2 In general, the rate of degenerate metathesis greatly exceeds that of the productive metathesis reactions and evidence suggests the chain-carrying intermediate is a metal alkylidene (M=CHR), not a methylidene (M=CH 2 ). Thus, the TON determined from the amount of product formed is less than the total number of metathesis events that the catalyst has accomplished. An efficient catalyst must perform many turnovers and be selective for productive pathways. Although degenerate reactions do not result in a net change in concentration of the catalyst or the substrate, they can provide additional opportunities for catalyst decomposition, and therefore can decrease efficiency. Recently, Hoveyda and Schrock reported that degenerate processes in asymmetric ring-closing metathesis (RCM) reactions are both prevalent and key to achieving high levels of enantioselectivity.3While measuring conversion of substrate is a common and straightforward method of assessing catalyst activity in olefin metathesis, and other catalytic reactions, degenerate Diethyl diallylmalonate (9) and allylmethallylmalonate (15) have become benchmark substrates for evaluation of olefin metathesis catalysts in RCM.5 Besides the productive pathway, there are at least two potential nonproductive pathways. The first begins with a ruthenium methylidene (10), and forms a β-substituted metallacycle (11); breakdown of this intermediate regenerates the starting material, but exchanges the methylene termini (eq 1). Alternatively, methylene exchange could take place via a α,α-disubstituted metallacycle (13) by coordination of a substrate molecule to ruthenium alkylidene 12 (eq 2).(1)To investigate these productive and nonproductive pathways, diethyl d 2 -diethyldiallylmalonate (9-d 2 ) was prepared and subjected to catalysts 1-8. The conversion to cyclopentene 14 was monitore...
Telechelic polyisoprene was synthesized via the ring-opening metathesis polymerization (ROMP) of 1,5-dimethyl-1,5-cyclooctadiene (DMCOD) in the presence of cis-1,4-diacetoxy-2-butene as a chain transfer agent (CTA). This method afforded telechelic polymer in excellent yield, and the acetoxy groups were successfully removed to yield R,ω-hydroxy end-functionalized polyisoprene with potential for subsequent reactions. Efficient, quantitative incorporation of CTA was achieved, and NMR spectroscopy was utilized to confirm the chemical identity of the polymer end groups. Polymerization of discrete DMCOD monomer generated polyisoprene with excellent regioregularity in the polymer backbone. Successful ROMP of sterically challenging DMCOD in the presence of a CTA for chain end-functionalization was borne out through screening of a variety of Ru-based olefin metathesis catalysts.
Highly thermally stable N-aryl,N-alkyl N-heterocyclic carbene (NHC) ruthenium catalysts were designed and synthesized for latent olefin metathesis. These catalysts showed excellent latent behavior toward metathesis reactions, whereby the complexes were inactive at ambient temperature and initiated at elevated temperatures, a challenging property to achieve with second generation catalysts. A sterically hindered N-tert-butyl substituent on the NHC ligand of the ruthenium complex was found to induce latent behavior toward cross-metathesis reactions, and exchange of the chloride ligands for iodide ligands was necessary to attain latent behavior during ring-opening metathesis polymerization (ROMP). Iodide-based catalysts showed no reactivity toward ROMP of norbornene-derived monomers at 25 °C, and upon heating to 85 °C gave complete conversion of monomer to polymer in less than 2 hours. All of the complexes were very stable to air, moisture, and elevated temperatures up to at least 90 °C, and exhibited a long catalyst lifetime in solution at elevated temperatures.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.