Heptane-soluble ring-closing metathesis catalysts

Terminally vinyl-functionalized polyisobutylene (PIB) oligomers can be easily transformed into end-functionalized PIB-bound Ru metathesis catalysts. The nonpolar catalysts so prepared can be used as solutions in heptane and recycled by a gravity-based extraction after addition of a heptane-immiscible polar solvent. This paper describes the synthesis and the recycling of a PIB-supported second-generation Hoveyda-Grubbs catalyst for Ru-catalyzed ring-closing metathesis and ring-opening metathesis polymerizations.     

Highly active chiral ruthenium catalysts for asymmetric ring-closing olefin metathesis

The synthesis of olefin metathesis catalysts containing chiral, monodentate N-heterocyclic carbenes and their application to asymmetric ring-closing metathesis (ARCM) are reported. These catalysts retain the high levels of reactivity found in the related achiral variants (1a and 1b). Using the parent chiral catalysts 2a and 2b and derivatives that contain steric bulk in the meta positions of the N-bound aryl rings (catalysts 3-5), five- through seven-membered rings were formed in up to 92% ee. The addition of sodium iodide to catalysts 2a-4a (to form 2b-4b in situ) caused a dramatic increase in enantioselectivity for many substrates. Catalyst 5a, which gave high enantiomeric excesses for certain substrates without the addition of NaI, could be used in loadings of < or =1 mol %. Mechanistic explanations for the large sodium iodide effect as well as possible mechanistic pathways leading to the observed products are discussed.     

Activated pyridinium-tagged ruthenium complexes as efficient catalysts for ring-closing metathesis

New pyridinium-tagged ruthenium catalysts have been synthesised to perform olefin metathesis in several media including both organic and aqueous solvents and room temperature ionic liquids (RTILs). High activity was obtained in the ring-closing metathesis (RCM) of a variety of di- or tri-substituted and/or oxygen-containing dienes. However, only fair levels of recycling combined with low to moderate residual ruthenium levels (25–173 ppm) have been observed showing clearly the difficulty of associating high activity and recyclability.     

Group-selective ring-closing enyne metathesis

The ring-closing metathesis (RCM) of dienynes represents a powerful methodology for the construction of mono- and bicyclic systems containing 1,3-diene functionality. Despite its synthetic potential, the utility of dienyne RCM is significantly reduced due to poor group selectivity. To circumvent this shortcoming, several strategies utilizing steric hindrance, electronic variation, relay metathesis and ring-closure kinetics have been implemented to exert control over the reaction pathways. This article highlights a variety of methods to achieve group-selective enyne RCM of dienynes.     

Enantioselective ruthenium-catalyzed ring-closing metathesis

[reaction: see text] The first enantioselective ruthenium olefin metathesis catalysts have been prepared, and high enantiomeric excesses (up to 90%) are observed in the desymmetrization of achiral trienes. A model consistent with the stereochemical outcome of the reactions is described and suggests side-on olefin binding and reorganization of the halide ligands.     

Molybdenum-catalyzed ring-closing metathesis of allenynes

A ring-closing metathesis reaction of allenynes occurred at room temperature in the presence of a molybdenum alkylidene complex to give ring-closed vinylallenes. The vinylallene skeletons were constructed by a metathesis-type reaction between the alkyne moiety and the proximal carbon-carbon double bond of the allene moiety. [reaction: see text]     

Synthesis of spirocyclic thiazolidinediones using ring-closing metathesis and one-pot sequential ring-closing/cross metathesis

A novel synthetic route to spirocyclic thiazolidinediones is reported by utilizing ring-closing metathesis (RCM). A selective cross metathesis (CM) of N-allyl azaspiro derivatives with different olefins has been demonstrated to prepare substituted azaspiro-[4.4]nonenediones. The X-ray crystal structure of a spirocyclic thiazolidinedione dimer is described, which has been prepared in two steps from thiazolidinedione using a one-pot sequential ring-closing and self metathesis. Cross metathesis proceeds smoothly with both electron rich and poor olefins. The symmetrical bis-thiazolidinedione spirocyclic system can be used as CM coupling partner with olefins. One-pot sequential RCM-CM has been developed for the synthesis of substituted spirocyclic compounds. The methodology allows a quick access to thia-azaspiro-[4.4]nonene and -[4.5]decene-dione ring systems from readily available starting materials which are not otherwise accessible.     

Highly efficient ruthenium catalysts for the formation of tetrasubstituted olefins via ring-closing metathesis

[reaction: see text] A series of ruthenium-based metathesis catalysts with N-heterocyclic carbene (NHC) ligands have been prepared in which the N-aryl groups have been changed from mesityl to mono-ortho-substituted phenyl (e.g., tolyl). These new catalysts offer an exceptional increase in activity for the formation of tetrasubstituted olefins via ring-closing metathesis (RCM), while maintaining high levels of activity in ring-closing metathesis (RCM) reactions that generate di- and trisubstituted olefins.     

Solvents for ring-closing metathesis reactions

A study of the influence of eight diverse solvents on a Grubbs II-catalysed ring-closing metathesis (RCM) reaction reveals a complex dependence of the different reaction steps on the solvent and suggests acetic acid as a useful solvent for RCM reactions.     

Some allylic substituent effects in ring-closing metathesis reactions: allylic alcohol activation

[formula: see text] Dienes 2a-e were used to study allylic substituent effects in the ring-closing metathesis reaction (RCM). Both the steric and electronic character of the allylic substituents were found to influence alkene reactivities. Free allylic hydroxyl groups exert a large activating effect on the RCM reaction rates.     

A ring-closing yne-carbonyl metathesis of ynamides

[reaction: see text] An acid-catalyzed ring-closing ynamide-carbonyl metathesis is described here. This hetero RCM methodology is applicable to the construction of carbocycles as well as heterocycles such as chromenes, quinolizidines, indolizidines, and pyrrolizidines.     

Stereoselectivity of macrocyclic ring-closing olefin metathesis

[reaction: see text] Macrocyclic ring-closing olefin metathesis using ruthenium catalyst 3 was performed to produce a 14-membered lactone. The E/Z ratio of lactone was high regardless of the R group (auxiliary) or the initial alkene stereochemistry. A kinetic study demonstrates that the high E/Z ratio is due to secondary metathesis reactions that isomerize the product to the thermodynamic E/Z ratio.     

Silica-supported catalysts for ring-closing metathesis: effects of linker group and microenvironment on recyclability

The interesting effects of the linker and microenvironment on the recyclability of well-defined silica-supported catalysts were examined, which demonstrated the excellent activity and reusability for the ring-closing metathesis (RCM) of a number of substrates.     

Synthesis of cyclitols via ring-closing metathesis

A convenient synthesis of enantiomerically pure and differentially protected l-chiro- and myo-inositols as well as conduritols B and F from 2,3,4-tri-O-benzyl-d-xylopyranose via ring-closing metathesis is reported. The facile synthesis of conduritol B constitutes a short formal synthesis of (−)-cyclophellitol.     

Polycyclic aromatic hydrocarbons by ring-closing metathesis

A strategy for the synthesis of polycyclic aromatic hydrocarbons (PAHs) by the ring-closing olefin metathesis (RCM) of pendant olefins on a phenylene backbone has been developed. RCM of 2,4',6',2' '-tetravinyl-[1,1';3',1' ']terphenyl and 2,2',5',2' '-tetravinyl-[1,1';4',1']terphenyl affords in high yield the isomeric [a,j] and [a,h] dibenzanthracenes, respectively. In contrast with other intramolecular annulation methods, such as Friedel-Crafts acylations, this reaction is completely regioselective. Since RCM is reversible and PAHs are often thermodynamic sinks, this strategy is an effective and general method for the preparation of PAHs. Density functional theory calculations support these results. Carbon disulfide is a suitable solvent for these reactions.     

Enyne ring-closing metathesis on heteroaromatic cations

Cationic heteroaromatic enynes have been employed as substrates in enyne ring-closing metathesis, under an atmosphere of ethylene and using the Hoveyda-Grubbs catalyst, for the first time; the reaction affords new 1-vinyl- and 2-vinyl-substituted 3,4-dihydroquinolizinium salts, useful precursors for biologically relevant cations based on the quinolinizium system.     

A ring-closing metathesis strategy to phosphonosugars

[reaction: see text]. Syntheses of cyclic phosphonate (phostone) analogues of carbohydrates containing a phosphorus atom at the anomeric position are described. The ring-closing metathesis reaction of mixed allylic phenyl esters of allylphosphonic acid 2 and 22 generates the six-membered allylic phosphonates 3 and 23 in excellent yields. Introduction of the polyhydroxy functionality in these cyclic phosphonates provides facile entry into an array of phostone sugar analogues.