Ethylene metathesis of sulfur-containing alkynes

Ene-Yne Metathesis of Allylphosphonates and Allylphosphates: Synthesis of Phosphorus-Containing 1,3-Dienes

A variety of ene-yne cross metathesis reactions were performed using unsaturated phosphonate and phosphate reagents, affording the corresponding phosphorylated 1,3-diene products in good to excellent yields. These difficult ene-yne metatheses employed a Grubbs catalyst bearing a cyclic amino alkyl carbene ligand. A variety of terminal alkynes of varying substitution underwent the reaction, and different phosphorus-containing alkenes were found to give the conjugated diene products in high yields. The resulting dienes were further transformed by Horner-type Wittig reactions and a Diels-Alder cycloaddition.

Activation by allylic alcohols of sulfur-containing dienes to ruthenium-catalysed ring-closing metathesis

The presence of an allylic alcohol facilitates ring-closing metathesis of sulfur-containing substrates with Grubbs’ dihydroimidazole ruthenium complex.

Allyl Sulfides: Reactive Substrates for Olefin Metathesis

Allyl sulfides have gained traction in recent years as promoters for olefin metathesis. The high reactivity of allyl sulfides in olefin metathesis is remarkable, given that many sulfur-containing substrates are incompatible with ruthenium-based olefin metathesis catalysts. In stark contrast, allyl sulfides actually enhance the rate of metathesis in comparison with other alkenes, when matched with a suitable catalyst. This review examines how the high reactivity of allyl sulfides in olefin metathesis has been harnessed in diverse areas of synthesis. In the cases examined, allyl sulfides have been explicitly incorporated into substrates to promote olefin metathesis. Recent insights into catalyst considerations, applications in chemical and biochemical synthesis, and future opportunities are discussed.

Transition-metal-catalyzed laboratory-scale carbon-carbon bond-forming reactions of ethylene

Ethylene, the simplest alkene, is the most abundantly synthesized organic molecule by volume. It is readily incorporated into transition-metal-catalyzed carbon-carbon bond-forming reactions through migratory insertions into alkylmetal intermediates. Because of its D2h symmetry, only one insertion outcome is possible. This limits byproduct formation and greatly simplifies analysis. As described within this Minireview, many carbon-carbon bond-forming reactions incorporate a molecule (or more) of ethylene at ambient pressure and temperature. In many cases, a useful substituted alkene is incorporated into the product.

Ene-yne cross-metathesis with ruthenium carbene catalysts

Conjugated 1,3-dienes are important building blocks in organic and polymer chemistry. Enyne metathesis is a powerful catalytic reaction to access such structural domains. Recent advances and developments in ene–yne cross-metathesis (EYCM) leading to various compounds of interest and their intermediates, that can directly be transformed in tandem procedures, are reviewed in this article. In addition, the use of bio-resourced olefinic substrates is presented.

Recent Progress on Enyne Metathesis: Its Application to Syntheses of Natural Products and Related Compounds

Olefin metathesis using ruthenium carbene complexes is a useful method in synthetic organic chemistry. Enyne metathesis is also catalyzed by these complexes and various carbo- and heterocycles could be synthesized from the corresponding enynes. Dienyne metathesis, cross enyne metathesis and ring-opening enyne metathesis have been further developed. Various complicated compounds, such as the natural products and the related biologically active substances, could be synthesized using these metatheses reactions. Skeletal reorganization using the transition metals and metallotropic rearrangement are also discussed.

Advanced approach to polycyclics by a synergistic combination of enyne metathesis and Diels-Alder reaction

Enyne metathesis (EM) has widely been used to prepare various synthetic and natural products. Further, a sequential use of EM and Diels-Alder (DA) reactions generates highly functionalized and intricate polycarbocycles and heterocyclic frameworks. In this critical review we describe the application of a unique combination of EM and DA reactions to prepare various amino acid derivatives, natural products and heterocycles. Some of the heterocyclic targets include cyclic peroxides, siloxanes and various nitrogen (or oxygen) containing compounds. Use of ethylene and cyclooctadiene (COD) for improving the yield of EM products are also discussed (69 references).

Structure and absolute stereochemistry of the anticancer agent EBC-23 from the Australian rainforest

EBC-23 (2), a prostate anticancer agent, was isolated from the fruit of Cinnamomum laubatii (family Lauraceae) in the Australian tropical rainforest. Extensive NOE experiments enabled the relative stereochemistry of the proposed EBC-23 (2) structure to be determined. Total synthesis of both enantiopodes over nine linear steps, involving challenging RCM and spiroacetal cyclizations, confirmed the gross structure and relative and absolute stereochemistry.

Efficient Method for the Synthesis of Chiral Pyrrolidine Derivatives via Ring-Closing Enyne Metathesis Reaction

[reaction: see text] A series of new pyrrolidine derivatives were prepared directly in very good yields, from the substrates containing a basic or nucleophilic N atom via ring-closing enyne metathesis reaction under mild reaction conditions. Moreover, the reaction occurs smoothly without the presence of ethylene gas.

Encapsulation and Functionalization of Nanoparticles in Crosslinked Resorcinarene Shells

Two resorcinarene-derived tetrathiols with terminal alkene sidechains (tetraarylthiol cavitand 3 and tetrabenzylthiol cavitand 4) were determined to be efficient at extracting colloidal gold nanoparticles from aqueous solutions and stabilizing their dispersion in organic solvents. Treatment of these nanoparticle dispersions with the Grubbs olefin metathesis catalyst resulted in crosslinked resorcinarene shells that were highly resistant to alkanethiol-induced desorption at high temperatures. Nanoparticles in crosslinked shells of tetrabenzylthiol cavitand 4 were particularly robust, and could be precipitated and redispersed many times with minimal attrition. These shells could also withstand oxidative conditions and were amenable to synthetic modifications involving epoxidation and dihydroxylation.

Ring closing enyne metathesis: control over mode selectivity and stereoselectivity

Ring closing enyne metathesis to form 10-15-membered rings was achieved by using a tartrate-derived linker to attach ene and yne subunits. The exo/endo selectivity of the ring closure reaction of these substrates was found to be a function of ring size, whereby larger rings (12-15) give endo-products selectively, while smaller rings (5-11) give exo-products. The E/Z selectivity of the resultant macrocyclic 1,3-dienes was not predictable except for 10- and 11-membered rings. However, both the exo/endo-mode selectivity of the ring closure and the E/Z selectivity of the 1,3-dienes were improved by performing these reactions under ethylene atmosphere. The presence of ethylene induces a selective cross metathesis between the alkyne moiety and ethylene to generate an acyclic 1,3-diene which can undergo ring closing diene metathesis between the isolated olefin and the distal monosubstituted double bond of the 1,3-diene to generate exclusively the endo-product with high E-selectivity.

Ethylene-promoted intermolecular enyne metathesis

[reaction: see text] Intermolecular enyne metathesis between functional group-rich alkynes and vinyl ethers was promoted by ethylene cometathesis. The concentration of ethylene was optimized to suppress the competing formation of butadiene through background ethylene metathesis. The role of ethylene appears to be both protective and rate enhancing.

Simple Thiazocine-2-acetic Acid Derivatives via Ring-Closing Metathesis

A new protocol for synthesis of 2-heterocylylacetic acid derivatives involving conjugate addition of allyl mercaptan to an acrylate containing a tethered olefinic site followed by RCM (ring-closing metathesis) is described. In this series, sulfanyl derivatives were unreactive, while sulfoxide and sulfone analogues provided the corresponding thiazocines in fair to excellent yields. Use of the sulfoxide oxidation state as a protecting group for sulfides inert to RCM is demonstrated also. Thus, oxidation of sulfide 9 [N-allyl-N-[2-(allylthio)-4-(1H-indol-1-yl)-4-oxobutyl]-4-methylbenzenesulfonamide] followed by cyclization yielded the corresponding thiazocine sulfoxide 12. Deprotection (deoxygenation) of 12 was accomplished using Lawesson's reagent, producing 1-[[4-[4-(methylphenyl)sulfonyl]-3,4,5,8-tetrahydro-2H-1,4-thiazocin-2-yl]acetyl]-1H-indole (21) in 67% unoptimized yield.

A stereoselective enyne cross metathesis

[reaction: see text] Intermolecular enyne metathesis reaction of alkynes with olefins catalyzed by second-generation Grubbs catalyst (1) proceeded stereoselectively under ethylene atmosphere to produce 1,3-disubstituted butadienes with E stereochemistry.

Synthesis of oxazines and N-arylpyrroles by reaction of unfunctionalized dienes with nitroarenes and carbon monoxide, catalyzed by palladium-phenanthroline complexes

The reaction between an unfunctionalized conjugated diene and a nitroarene under CO pressure and at 100 degrees C, catalyzed by [Pd(Phen)2][BF4]2 (Phen = 1,10-phenanthroline), affords the corresponding hetero-Diels-Alder adduct (oxazine) in up to 91% yields in one pot. If the reaction mixture is then heated to 200 degrees C, the oxazines are converted into the corresponding N-arylpyrroles in good yields. Pressures as low as 5 bar can be employed, and 0.08% catalyst is sufficient to effect the transformation. The reaction can be equally run by employing the nitroarene or the diene as limiting agent and works well for nitroarenes bearing either electron-withdrawing or mildly electron-donating substituents. A moderate steric hindrance on the nitroarene (o-methyl) is well tolerated, but 1,4-disubstituted-1,3-dienes are not suitable substrates.

Efficiency of a ruthenium catalyst in metathesis reactions of sulfur-containing compounds

[reaction: see text] 1,3-Dimesitylimidazol-2-ylidene ruthenium benzylidene catalyst (RuCl2(=C(H)Ph)(PCy3)(IMes)) has been successfully employed in ring-closing metathesis reactions of acyclic diene sulfides, disulfides, and dithianes and in self-cross metathesis reactions of ene-sulfides, thioethers, and thiols.