Ene-yne cross-metathesis with ruthenium carbene catalysts

1,3-diene-based AIEgens: Stereoselective synthesis and applications

In recent years, significant advancements have been made in the synthesis and application of 1,3-dienes. This specific structural motif has garnered significant attention from researchers in materials science and biology due to its unique aggregation-induced emission (AIE) properties and extensive conjugation systems. The luminescent characteristics of these compounds are notably influenced by the geometry of the two double bonds. Therefore, it is essential to consolidate stereoselective synthetic strategies for 1,3-dienes. This comprehensive review seeks to elucidate the diverse techniques employed to attain stereo-control in the synthesis of 1,3-diene-based AIE luminogens (AIEgens). Particular emphasis is placed on comprehending the determinants of stereoselectivity and exploring the array of substrates amenable to these methods. Furthermore, the review underscores the AIE properties exhibited by these compounds and their extensive utility in organic light-emitting diodes (OLEDs), stimuli-responsive materials, sensors, bioimaging, and photodynamic therapy (PDT).

Synthesis of mixed musks via Eschenmoser-Tanabe fragmentation, enyne metathesis and Diels-Alder reaction as key steps

Musk analogues containing different macrocyclic ring systems as well as different annulated ring systems were synthesised by a simple and useful strategy. This strategy includes Eschenmoser-Tanabe fragmentation, enyne metathesis and Diels-Alder reaction as key steps. Starting from easily available (n) macrocyclic ketones, (n + 3) macrocyclic systems were assembled using the basic organic reactions.

Intermolecular Photocatalytic Chemo‐, Stereo‐ and Regioselective Thiol–Yne–Ene Coupling Reaction

The first example of an intermolecular thiol–yne–ene coupling reaction is reported for the one‐pot construction of C−S and C−C bonds. Thiol–yne–ene coupling opens a new dimension in building molecular complexity to access densely functionalized products. The employment of Eosin Y/DBU/MeOH photocatalytic system suppresses hydrogen atom transfer (HAT) and associative reductant upconversion (via C−S three‐electron σ‐bond formation). Investigation of the reaction mechanism by combining online ESI‐UHRMS, EPR spectroscopy, isotope labeling, determination of quantum yield, cyclic voltammetry, Stern–Volmer measurements and computational modeling revealed a unique photoredox cycle with four radical‐involving stages. As a result, previously unavailable products of the thiol–yne–ene reaction were obtained in good yields with high selectivity. They can serve as stable precursors for synthesizing synthetically demanding activated 1,3‐dienes.

Metal-Catalyzed Organic Reactions by Resonant Acoustic Mixing

We demonstrate catalytic organic synthesis by Resonant Acoustic Mixing (RAM): a mechanochemical methodology that does not require bulk solvent or milling media. Using as model reactions ruthenium-catalyzed ring-closing metathesis and copper-catalyzed sulfonamide-isocyanate coupling, RAM mechanosynthesis is shown to be faster, operationally simpler than conventional ball-milling, while also providing the first example of a mechanochemical strategy for ruthenium-catalyzed ene-yne metathesis. Reactions by RAM are readily and directly scaled-up without any significant changes in reaction conditions, as shown by the straightforward 200-fold scaling-up of the synthesis of the antidiabetic drug Tolbutamide, from hundreds of milligrams directly to 30 grams.

Cobalt-Catalyzed Z to E Geometrical Isomerization of 1,3-Dienes

An efficient cobalt-catalyzed geometrical isomerization of 1,3-dienes is described. In the combination of a CoCl₂ precatalyst with an amido-diphosphine-oxazoline ligand, the geometrical isomerization of E/Z mixtures of 1,3-dienes proceed in a stereoconvergent manner, affording (E) isomers in high stereoselectivity. This facile transformation features a broad substrate scope with good functional group tolerance and could be scaled up to the gram scale smoothly with a catalyst loading of 1 mol %.

Reactions of diborenes with terminal alkynes: mechanisms of ligand-controlled anti-selective hydroalkynylation, cycloaddition and C[triple bond, length as m-dash]C triple bond scission

The reactions of terminal acetylenes with doubly Lewis base-stabilised diborenes resulted in different outcomes depending on the nature of the ligands at boron and the conformation of the diborene (cyclic versus acyclic). N-heterocyclic carbene (NHC)-stabilised diborenes tended to undergo anti-selective hydroalkynylation at room temperature, whereas [2 + 2] cycloaddition was observed at higher temperatures, invariably followed by a C–N bond activation at one NHC ligand, leading to the ring-expansion of the initially formed BCBC ring and formation of novel boron-containing heterocycles. For phosphine-stabilised diborenes only [2 + 2] cycloaddition was observed, followed by a rearrangement of the resulting 1,2-dihydro-1,2-diborete to the corresponding 1,3-isomer, which amounts to complete scission of both the B Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> B double and CC triple bonds of the reactants. The elusive 1,2-isomer was finally trapped by using a cyclic phosphine-stabilised diborene, which prevented rearrangement to the 1,3-isomer. Extensive density functional theory (DFT) calculations provide a rationale for the selectivity observed.

The outcome of reactions between diborenes and terminal alkynes can be tuned by varying the stabilising Lewis base and/or reaction conditions, to yield either the anti-hydroalkynylation product or [2 + 2] cycloaddition-derived boron heterocycles.

A cooperative zinc/catalytic indium system for the stereoselective sequential synthesis of (E)-1,3-dienes from carbonyl compounds

A novel cooperative system based on zinc/catalytic indium for the stereoselective synthesis of highly functionalized (E)-1,3-dienes from aldehydes and ketones has been developed.

Ring-closing metathesis of prochiral oxaenediynes to racemic 4-alkenyl-2-alkynyl-3,6-dihydro-2H-pyrans

The prochiral 4-(allyloxy)hepta-1,6-diynes, optionally modified in the positions 1 and 7 with an alkyl or ester group, undergo a chemoselective ring-closing enyne metathesis yielding racemic 4-alkenyl-2-alkynyl-3,6-dihydro-2H-pyrans. Among the catalysts tested, Grubbs 1st generation precatalyst in the presence of ethene (Mori conditions) gave superior results compared to the more stable Grubbs or Hoveyda-Grubbs 2nd generation precatalysts. This is probably caused by a suppression of the subsequent side-reactions of the enyne metathesis product with ethene. On the other hand, the 2nd generation precatalysts gave better yields in the absence of ethene. The metathesis products, containing both a triple bond and a conjugated system, can be successfully orthogonally modified. For example, the metathesis product of 5-(allyloxy)nona-2,7-diyne reacted chemo- and stereoselectively in a Diels-Alder reaction with N-phenylmaleimide affording the tricyclic products as a mixture of two separable diastereoisomers, the configuration of which was estimated by DFT computations. The reported enediyne metathesis paves the way to the enantioselective enyne metathesis yielding chiral building blocks for compounds with potential biological activity, e.g., norsalvinorin or cacospongionolide B.

Cp*RuCl-Vinyl Carbenes: Two Faces and the Bifunctional Role in Catalytic Processes

Ruthenium vinyl carbenes derived from Cp/Cp*RuCl-based complexes (Cp=cyclopentadiene, Cp*=1,2,3,4,5-pentamethylcyclopentadiene) have been routinely invoked as key intermediates in tandem reactions involving a carbene/alkyne metathesis (CAM). A priori, these intermediates resemble the Grubbs-type family of catalysts, but they exhibit a completely different reactivity pattern that few, if any, other catalytic system can reproduce so far. The reactivity of these species with α-unsubstituted and α-substituted alkynals showcases the peculiarities of these intermediates. Although Z-vinyl dihydrooxazines are preferentially obtained with the former, Z-vinyl epoxypyrrolidines are obtained with the latter. A combination of spectroscopic and computational data now prove that a η³ -coordination mode of the ruthenium vinyl carbene and the presence of a Lewis basic chloride ligand give rise to two markedly different stereoelectronic faces, which are responsible for the unconventional reactivity of these species.

Silica Gel as a Promoter of Sequential Aza-Michael/Michael Reactions of Amines and Propiolic Esters: Solvent- and Metal-Free Synthesis of Polyfunctionalized Conjugated Dienes

We present an efficient, simple, metal- and solvent-free silica-gel-promoted synthesis of functionalized conjugated dienes by sequential aza-Michael/Michael reactions by starting from commercially available primary amines and propiolic esters. The scope and usefulness of the method is demonstrated for 31 examples, including a range of propiolic esters, aliphatic amines, and differently substituted aromatic amines. For aliphatic amines, the products were obtained within 0.5 to 4 h in 52 to 85 % yield, compared with 3.5 to 22 h under classical solution-phase synthesis, which proceeds with similar or lower yields. The method was found to be particularly useful for weakly nucleophilic aromatic amines, which provided products in 21 to 73 % yield over 2.5 to 9.5 h compared with yields of 0 to 49 % over 1 to 6 d under standard solution-phase conditions, and for more hydrophobic esters that gave products in yields of 47 to 79 % over 1 to 3 h compared with 0 to 45 % over 4 to 114 h in solvent.

Ring Rearrangement Metathesis in 7-Oxabicyclo[2.2.1]heptene (7-Oxanorbornene) Derivatives. Some Applications in Natural Product Chemistry

Metathesis reactions is firmly established as a valuable synthetic tool in organic chemistry, clearly comparable with the venerable Diels-Alder and Wittig reactions and, more recently, with the metal-catalyzed cross-coupling reactions. Metathesis reactions can be considered as a fascinating synthetic methodology, allowing different variants regarding substrate (alkene and alkyne metathesis) and type of metathetical reactions. On the other hand, tandem metathesis reactions such Ring Rearrangement Metathesis (RRM) and the coupling of metathesis reaction with other reactions of alkenes such as Diels-Alder or Heck reactions, makes metathesis one of the most powerful and reliable synthetic procedure.

In particular, Ring-Rearrangement Metathesis (RRM) refers to the combination of several metathesis transformations into a domino process such as ring-opening metathesis (ROM)/ring-closing metathesis (RCM) and ROM-cross metathesis (CM) in a one-pot operation. RRM delivers complex frameworks that are difficult to assemble by conventional methods constitutingan atom economic process. RRM is applicable to mono- and polycyclic systems of varying ring sizes such as cyclopropene, cyclobutene, cyclopentene, cyclohexene, pyran systems, bicyclo[2.2.1]heptene derivatives, bicyclo[2.2.2]octene derivatives, bicyclo[3.2.1]octene derivatives and bicyclo[3.2.1]octene derivatives.

In this review our attention has focused on the RRM reactions in 7-oxabicyclo[2.2.1]heptene derivatives and on their application in the synthesis of natural products or significant subunits of them.

Ethenolysis: A Green Catalytic Tool to Cleave Carbon-Carbon Double Bonds

Remarkable innovations have been made in the field of olefin metathesis due to the design and preparation of new catalysts. Ethenolysis, which is cross-metathesis with ethylene, represents one catalytic transformation that has been used with the purpose of cleaving internal carbon-carbon double bonds. The objectives were either the ring opening of cyclic olefins to produce dienes or the shortening of unsaturated hydrocarbon chains to degrade polymers or generate valuable shorter terminal olefins in a controlled manner. This Review summarizes several aspects of this reaction: the catalysts, their degradation in the presence of ethylene, some parameters driving their productivity, the side reactions, and the applications of ethenolysis in organic synthesis and in potential industrial applications.

Beyond Fischer and Schrock carbenes: non-heteroatom-stabilized group 6 metal carbene complexes – a general overview

Non-heteroatom-stabilized group 6 metal carbene complexes: versatile reagents or intermediates in stoichiometric or catalytic reactions in organic synthesis.

Tandem cross enyne metathesis (CEYM)-intramolecular Diels-Alder reaction (IMDAR). An easy entry to linear bicyclic scaffolds

A new tandem cross enyne metathesis (CEYM)-intramolecular Diels-Alder reaction (IMDAR) has been carried out. It involves conjugated ketones, esters or amides bearing a remote olefin and aromatic alkynes as the starting materials. The overall process enables the preparation of a small family of linear bicyclic scaffolds in a very simple manner with moderate to good levels of diastereoselectivity. This methodology constitutes one of the few examples that employ olefins differently than ethylene in tandem CEYM-IMDAR protocols.

N-Heterocyclic Carbene-Copper-Catalyzed Group-, Site-, and Enantioselective Allylic Substitution with a Readily Accessible Propargyl(pinacolato)boron Reagent: Utility in Stereoselective Synthesis and Mechanistic Attributes

The first instances of catalytic allylic substitution reactions involving a propargylic nucleophilic component are presented; reactions are facilitated by 5.0 mol % of a catalyst derived from a chiral N-heterocyclic carbene (NHC) and a copper chloride salt. A silyl-containing propargylic organoboron compound, easily prepared in multigram quantities, serves as the reagent. Aryl- and heteroaryl-substituted disubstituted alkenes within allylic phosphates and those with an alkyl or a silyl group can be used. Functional groups typically sensitive to hard nucleophilic reagents are tolerated, particularly in the additions to disubstituted alkenes. Reactions may be performed on the corresponding trisubstituted alkenes, affording quaternary carbon stereogenic centers. Incorporation of the propargylic group is generally favored (vs allenyl addition; 89:11 to >98:2 selectivity); 1,5-enynes can be isolated in 75-90% yield, 87:13 to >98:2 SN2'/SN2 (branched/linear) selectivity and 83:17-99:1 enantiomeric ratio. Utility is showcased by conversion of the alkynyl group to other useful functional units (e.g., homoallenyl and Z-homoalkenyl iodide), direct access to which by other enantioselective protocols would otherwise entail longer routes. Application to stereoselective synthesis of the acyclic portion of antifungal agent plakinic acid A, containing two remotely positioned stereogenic centers, by sequential use of two different NHC-Cu-catalyzed enantioselective allylic substitution (EAS) reactions further highlights utility. Mechanistic investigations (density functional theory calculations and deuterium labeling) point to a bridging function for an alkali metal cation connecting the sulfonate anion and a substrate's phosphate group to form the branched propargyl addition products as the dominant isomers via Cu(III) π-allyl intermediate complexes.

Butadiene from acetylene–ethylene cross-metathesis

Direct synthesis of butadiene from acetylene and ethylene, via enyne cross-metathesis, is demonstrated with commercial ruthenium carbene catalysts.

Terminal conjugated dienes via a ruthenium-catalyzed cross-metathesis/elimination sequence: application to renewable resources

Two-step synthesis of terminal 1,3-dienes catalyzed by two different ruthenium complexes.

Ruthenium-catalyzed cross-metathesis with electron-rich phenyl vinyl sulfide enables access to 2,3-dideoxy-d-ribopyranose ring system donors

Microwave irradiation effectively accelerates the cross-metathesis reaction of 2-deoxy-d-ribose hydroxyalkene and derivatives with electron-rich phenyl vinyl sulfide using commercially available ruthenium-based catalysts, thus providing a flexible metal-mediated route to 2,3-dideoxy-d-ribopyranose ring system donors.

Correlation between functionality preference of Ru carbenes and exo/endo product selectivity for clarifying the mechanism of ring-closing enyne metathesis

Functionality preferences of metathesis Ru carbenes to various alkenes and alkynes with electronic and steric diversity were determined by using time-dependent fluorescence quenching. The functionality preferences depend not only on the properties of multiple bonds but also on the ligands on Ru. There was a good correlation between functionality preference and the metathesis reaction outcome. The correlation between functionality preference and exo/endo product ratio offers a solution to resolve the mechanistic issue related with alkene- vs alkyne-initiated pathway in ring-closing enyne metathesis. The correlation indicates the preference is likely to dictate the reaction pathway and eventually the outcome of the reaction. The Ru catalyst favoring alkyne over alkene provides more endo product, indicating that the reaction mainly initiates at the alkyne. By changing the substitution pattern, the preference can be reversed to give an exclusive exo product.