Ring Closing Metathesis Reaction Planning
Ring closing metathesis (RCM) has evolved into one of those coveted “predictable” reactions in organic synthesis
Ring-closing metathesis in peptides - ScienceDirect
The ruthenium catalysts containing monodentate, chiral NHC's (2a and 2b) had the same air and moisture stability of the parent catalyst 1a, as well as a similar level of reactivity. Additionally, we achieved a single example of 90% ee using 2b. Based on this initial discovery, and because ARCM remains challenging for 6a and 6b, we decided to modify 2a and 2b to enhance enantioselectivity and expanded the substrate scope of ARCM. The synthesis of new chiral ruthenium catalysts for asymmetric olefin metathesis, as well as their reactivity and selectivity in expanding the scope of ARCM, is reported herein. Catalysts containing substitution on the aryl ring para to the ortho-isopropyl group (3a, 3b, 4a, and 4b) showed very similar enantioselectivities to that of the parent chiral catalysts 2a and 2b. However, substitution on the same side of the ring as the ortho-isopropyl group (5a and 5b) caused an increase in enantioselectivity to the extent that in a number of substrates, the dichloride catalyst 5a could be used at very low catalyst loadings to obtain high enantiomeric excesses and conversions.
The development of well-defined catalysts has made olefin metathesis an important, reliable, and widespread method for constructing carbon-carbon double bonds. Since the initial report of asymmetric olefin metathesis for small molecule synthesis, a variety of chiral, ruthenium- and molybdenum-based alkylidene catalysts have been developed.- The molybdenum catalysts have been shown to give excellent enantioselectivities in asymmetric ring-closing metathesis (ARCM), asymmetric ring-opening/ring-closing metathesis (ARORCM), and asymmetric ring-opening/cross metathesis (AROCM).- Our interest in ruthenium-based olefin metathesis catalysts stems from their increased functional group tolerance compared to the molybdenum systems, as well as their stability to air and moisture. Two classes of chiral ruthenium catalysts have been explored (): those containing monodentate N-heterocyclic carbenes (NHCs) with chirality in the backbone of the carbene (2a and 2b) developed in our laboratory, and those containing chiral, bidentate NHC/binaphthyl ligands (6a and 6b) developed by Hoveyda et al. While catalysts of the latter type gave good enantioselectivities and yields for AROCM reactions, they exhibited reduced reactivity and selectivity toward ARCM relative to those of the former class.
Z-selective ring-closing metathesis reactions - Google …
3 A brief history of ruthenium-catalyzed RCM Grubbs, R. H. Angew. Chem., Int. Ed. 2006, 45, 3760. representative ring closing metathesis (RCM) Ru reacts with soft Lewis bases and π-olefins more functional group tolerant very low reactivity highly active toward metathesis highly oxophilic (low functional group tolerance)
11 Representitive olefin metathesis transformations Ring closing metathesis (RCM) Acyclic diene metathesis polymerization (ADMET) Cyclodepolymerization metathesis (CDP) Ring opening metathesis polymerization (ROMP) Monfette, S.; Fogg, D. Chem. Rev. 2009, 109, 3783.
Ring-closing metathesis | Wiki | Everipedia
Silyl ethers proved to be excellent substrates, as both 6- and 7-membered rings (14 and 22) formed in good conversions and 92% ee's with 5a. Catalyst loadings of 1 mol % or less could be used in these reactions. The ring-closing of 13 illustrated that the high yield and enantiomeric excess were maintained when the reaction was scaled up to almost 1 g of substrate. Using the analogous diiodide catalyst 5b for ARCM of 13 and 21 resulted in 6- and 7-membered rings with the same enantiomeric excesses but lower conversions than with the dichloride catalyst 5a. Substrate 23, which placed the prochiral center further from the trisubstituted olefins, also underwent ARCM to form a 7-membered ring in good enantiomeric excess and excellent yield with a substitution pattern different than 22.
The ability of the chiral ruthenium catalysts 2-5 to give high conversions and enantioselectivities for the ARCM reactions of 11 and 13 encouraged us to examine other substrates which afford 5–8 membered rings upon ARCM. Based on the results from our initial screen, catalysts 2b and 5a were used. Two types of prochiral trienes were explored: alkenyl ethers and alkenyl silyl ethers (). Conversions >90% and 90% ees were obtained when generating 5- and 6-membered rings 12 and 16. The isolated yields were moderately reduced due to the volatility of the products during the purification procedure. The synthesis of a 7-membered ring from 17 occurred in 85% ee when 2b was used. Unfortunately the formation of the desired product was only ∼5%, with the major species being unreacted starting material and homocoupled product. The yield for the same reaction could be drastically increased when catalyst 5a was used, and because 5a is more selective than the other catalysts in the absence of NaI (65% ee for 18 using 2a), the desired product could be obtained in 92% yield and 76% ee. Synthesis of an 8-membered ring was more challenging, and 2b catalyzed the ARCM reaction of 19 to 20 in 85% ee, but with only a ∼2% yield. Attempts to increase the yield using 5a resulted in a similar yield and a reduced enantiomeric excess. This result was not surprising, as generating 8-membered rings via ring-closing olefin metathesis is a challenging problem.
Ring-Closing Metathesis Polymerization (RCM)
Using the ring-closing metathesis of 11 to form 12 as a ..
This review highlights developments in the field of ring-closing metathesis applied to the synthesis of cyclic peptides
Ring-opening Metathesis Polymerization
Ring Closing Metathesis Reactions: Organic Chemistry Tutorial - YouTube
Synthesis of N -Heterocycles - Organic Chemistry Portal
Ring-opening metathesis polymerization (ROMP) uses metathesis catalysts to generate polymers from cyclic olefins.
Name Reactions - Organic Chemistry Portal
12 Ring closing metathesis exists in an equilibrium Fogg, D. and coworkers. J. Am. Chem. Soc. 2007, 129, 1024. RCM efficiency limited by competition between pathways fully reversible product distribution is “living” -- known as equilibrium ring closing metathesis (ERCM)
Diels–Alder reaction - Wikipedia
2 Various methods for macrocyclization Stang, E.; Christina White, M. Nat. Chem. 2009, 1, 547. Kurti, L, Czako, B. Strategic Applications of Named Reactions in Organic Synthesis, 1st ed,; Elsevier: Amsterdam, 2005. Macrolactonization: Nozaki-Hiyama-Kishi Macrolactamization: Nozaki-Hiyama-Kishi C-H oxidation Enyne metathesisRing-closing metathesis
Thorpe–Ingold effect - Wikipedia
Ring closing metathesis (RCM) has evolved into one of those coveted “predictable” reactions in organic synthesis. Sure, there are things that can go wrong, but for forming unstrained rings, it’s a great way to go.
Asymmetric Total Syntheses of Colchicine, β …
Ring-opening metathesis polymerization (ROMP) uses metathesis catalysts to generate polymers from cyclic olefins. ROMP is most effective on strained cyclic olefins, because the relief of ring strain is a major driving force for the reaction – cyclooctene and norbornenes are excellent monomers for ROMP, but cyclohexene is very reluctant to form any significant amount of polymer. Norbornenes are favorite monomers for ROMP, as a wide range of monomer functionalities are easily available through Diels-Alder reactions.
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Secondary metathesis reactions (controlled by catalyst choice and reaction conditions) also affect the product distribution. Recoordination of an alkene on the growing polymer chain with the catalyst can lead to cyclic oligomers through a ring-closing metathesis reaction (“backbiting”). Chain transfer (cross metathesis) between a growing polymer unit and an adjacent polymer alkene also leads to broadened molecular weights. Chain transfer can also be used to improve processability of the resulting polymer – addition of an acyclic olefin (chain-transfer agent) can limit chain molecular weights and introduce terminal functional groups.
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