Enyne Metathesis

Metal Vinylidenes and Allenylidenes in Catalysis: Applications in Anti-Markovnikov Additions to Terminal Alkynes and Alkene Metathesis

The involvement of a catalytic metal vinylidene species was proposed for the first time in 1986 to explain the regioselective formation of vinyl carbamates directly from terminal alkynes, carbon dioxide, and amines. Since this initial report, various metal vinylidenes and allenylidenes, which are key activation intermediates, have proved extremely useful for many alkyne transformations. They have contributed to the rational design of new catalytic reactions. This 20th anniversary is a suitable occasion to present the advancement of organometallic vinylidenes and allenylidenes in catalysis.

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.

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.

Theoretical Analysis of the High Versatility in PtCl2-Mediated Cycloisomerization of Enynes on a Common Mechanistic Basis

[Reaction: see text]. Transformations of enynes upon treatment with electrophilic transition-metal complexes, such as PtCl2, are strongly substrate-dependent processes and may yield a wide variety of cyclic compounds. Despite the high versatility, many of these processes could be closely related from a mechanistic point of view. Theoretical analyses of the plausible reaction mechanisms provide support for a unified mechanistic picture based on the electrophilic activation of the triple bond by the catalyst, which triggers the nucleophilic alkene attack through an endo- or exo-cyclization mode, to form the cyclopropylcarbene species. Then, these common key intermediates may evolve through alternative paths to afford a range of cyclic compounds. The preference for each path and the evolution of these intermediates are governed by the nature of the starting enyne. The effects induced by different structural motifs, such as the role played by a heteroatom directly attached to the triple bond, the tether length, the substitution on the acetylenic position, and the gem-dialkyl substitution on the tether, among others, are discussed. The proposed common mechanistic scheme can rationalize and account for the experimental observations accumulated.

Rhodium-catalyzed tandem cyclization-cycloaddition reactions of enynebenzaldehydes: construction of polycyclic ring systems

o-(1,6-Enynyl)benzaldehydes underwent a novel mode of cycloaddition using Rh(I)-precatalyst, via[3+2] cycloaddition of presumed dipolar carbonyl ylide intermediate generated by Rh-catalyst and the utility of this mechanistically intriguing enyne cyclization can be found in a number of polycyclic natural product skeletons.

Olefin Metatheses and Related Reactions Initiated by Carbene Derivatives of Metals in Low Oxidation States

Experiments carried out 24 years ago with tantalum carbenes have led to the much cited hypothesis that metals (other than ruthenium) must be in their highest oxidation states for their carbene derivatives to initiate olefin metatheses. The hypothesis legitimizes the uniqueness of high-oxidation-state molybdenum and tungsten carbenes as effective initiators, and it means that the Fischer tungsten carbenes that even earlier were found to initiate olefin metatheses and related transformations must be oxidized before they can be effective. The newer initiators have been termed "well-defined", the older "ill-defined". But what does the evidence show?

Design and synthesis of novel sugar-oxasteroid-quinone hybrids

A new class of sugar-oxasteroid-quinone hybrid molecules has been designed and synthesized involving an efficient enyne metathesis/Diels-Alder reaction strategy.

Rutheniun-Catalyzed Cycloisomerization of o-(Ethynyl)phenylalkenes to Diene Derivatives via Skeletal Rearrangement

Treatment of a series of 2',2'-disubstituted (o-ethynyl)styrenes with TpRu(PPh(3))(CH(3)CN)(2)PF(6) (10 mol %) in benzene (80 degrees C, 12-18 h) efficiently gave 2-alkenyl-1H-indene derivatives. This catalytic reaction represents an atypical enyne cycloisomerization with skeletal rearrangement of starting enyne, where the C=C bond is completely cleaved and inserted by the terminal alkynyl carbon. The reaction mechanism was elucidated by a series of deuterium and (13)C labeling experiments, as well as by changing the substituents at the phenyl moieties. The mechanism is proposed to involve the following key steps: 5-endo-dig cyclization of ruthenium-vinylidene intermediate, a nonclassical ion formation, and the "methylenecyclopropane-trimethylenemethane" rearrangement.

Synthesis of (-)-longithorone A: using organic synthesis to probe a proposed biosynthesis

We present a full report of our enantioselective synthesis of (-)-longithorone A (1). The synthesis was designed to test the feasibility of the biosynthetic proposal for 1 put forward by Schmitz involving intermolecular and transannular Diels-Alder reactions of two [12]-paracyclophane quinones. We have found that if the biosynthesis does involve these two Diels-Alder reactions, the intermolecular Diels-Alder reaction likely occurs before the transannular cycloaddition. The intermolecular Diels-Alder precursors, [12]-paracyclophanes 38, 49, 59, and 60, were prepared atropselectively, providing examples of ene-yne metathesis macrocyclization. The 1,3-disubstituted dienes produced from the macrocyclizations represent a previously unreported substitution pattern for intramolecular ene-yne metathesis. Protected benzylic hydroxyl stereocenters were used as removable atropisomer control elements and were installed by using a highly enantioselective vinylzinc addition to electron-rich benzaldehydes 26 and 27.

Palladium-catalyzed carbocyclization of 1,6-enynes leading to six-membered rings or oxidized five-membered trifluoroacetates

We describe palladium-catalyzed carbocyclization of 1,6-enynes leading to six-membered rings by water-originated hydride addition. Mechanistic features of this anomalous carbocyclization and isolation of a chiral five-membered C-Pd intermediate as an oxidized form of trifluoroacetate are also reported.

Selective domino ring-closing metathesis-cross-metathesis reactions between enynes and electron-deficient alkenes

[reaction: see text] A selective domino ring-closing metathesis (RCM)-cross-metathesis (CM) process between enynes and electron-deficient alkenes is reported. The conditions have been optimized for enynes 3 and methyl acrylate with catalyst IV. The scope and limitations of this reaction are described, and a possible mechanism is discussed.

Novel alkenylative cyclization using a ruthenium catalyst

Novel alkenylative cyclization of enyne was developed using Cp*RuCl(cod) under ethylene gas at room temperature.

Ruthenium-catalyzed ROM-RCM of cycloalkene-yne

[formula: see text] ROM-RCM (ring-opening and ring-closing metatheses) of cycloalkene-yne was demonstrated using a second-generation ruthenium complex. When cycloalkene bearing the alkyne moiety at the C-1 position was reacted with a ruthenium-carbene complex under an atmosphere of ethylene, ROM-RCM proceeded smoothly to give bicyclic compound and/or dimeric compound in good yields.

Comparative investigation of ruthenium-based metathesis catalysts bearing N-heterocyclic carbene (NHC) ligands

Exchange of one PCy3 unit of the classical Grubbs catalyst 1 by N-heterocyclic carbene (NHC) ligands leads to "second-generation" metathesis catalysts of superior reactivity and increased stability. Several complexes of this type have been prepared and fully characterized, six of them by X-ray crystallography. These include the unique chelate complexes 13 and 14 in which the NHC- and the Ru-CR entities are tethered to form a metallacycle. A particularly favorable design feature is that the reactivity of such catalysts can be easily adjusted by changing the electronic and steric properties of the NHC ligands. The catalytic activity also strongly depends on the solvent used; NMR investigations provide a tentative explanation of this effect. Applications of the "second-generation" catalysts to ring closing alkene metathesis and intramolecular enyne cycloisomerization reactions provide insights into their catalytic performance. From these comparative studies it is deduced that no single catalyst is optimal for different types of applications. The search for the most reactive catalyst for a specific transformation is facilitated by IR thermography allowing a rapid and semi-quantitative ranking among a given set of catalysts.