Isomerization Of Alkynes Research Articles

Remote Functionalization by Pd-Catalyzed Isomerization of Alkynyl Alcohols.

In recent years, progress has been made in the development of catalytic methods that allow remote functionalizations based on alkene isomerization. In contrast, protocols based on alkyne isomerization are comparatively rare. Herein, we report a general Pd-catalyzed long-range isomerization of alkynyl alcohols. Starting from aryl-, heteroaryl-, or alkyl-substituted precursors, the optimized system provides access preferentially to the thermodynamically more stable α,β-unsaturated aldehydes and is compatible with potentially sensitive functional groups. We showed that the migration of both π-components of the carbon-carbon triple bond can be sustained over several methylene units. Computational investigations served to shed light on the key elementary steps responsible for the reactivity and selectivity. These include an unorthodox phosphine-assisted deprotonation rather than a more conventional β-hydride elimination in the final tautomerization event.

NHC-Au-Catalyzed Isomerization of Propargylic B(MIDA)s to Allenes and Double Isomerization of Alkynes to 1,3-Dienes.

The synthesis of allenyl boronates is an important yet challenging topic in organic synthesis. Reported herein is an NHC-gold-catalyzed 1,3-H shift toward allenyl boronates synthesis from simple propargylic B(MIDA)s. Mechanistic studies suggest dual roles of the boryl moiety in the reaction: to activate the substrate for isomerization and at the same time, to prevent the allene product from further isomerization. These effects should be a result of α-anion stabilization and α-cation destabilization conferred by the B(MIDA) moiety, respectively. The NHC-Au catalyst, which is commercially available, is also found to be reactive in alkyne-to-1,3-diene isomerization reactions in an atom-economic and base-free manner.

The Alkyne Zipper Reaction: A Useful Tool in Synthetic Chemistry

The alkyne zipper reaction is an internal-to-terminal alkyne isomerization reaction with many interesting applications in synthetic chemistry, as it constitutes an efficient means of achieving acetylene functionalization. A review of its applications in synthesis processes is presented in this paper, with a brief overview of the mechanistic features of the alkyne zipper reaction, as well as a brief overview of its future potential.

Theoretical investigation of the mechanism of DMAP-promoted [4 + 2]-annulation of prop-2-ynylsulfonium with isatoic anhydride

The mechanism for DMAP-promoted [4 + 2]-annulation of prop-2-ynylsulfonium with isatoic anhydride is investigated using the M06-2X functional. The reaction comprises isomerization of prop-2-ynylsulfonium in stage 1. Stage 2 includes DMAP-promoted deprotonation, nucleophilic addition, ring opening, and decarboxylation. Three steps of intramolecular cycloaddition, DMAP-promoted protonation, and dealkylation occur in stage 3, generating methylated DMAP and neutral thioether, which undergo double-bond isomerization to yield 3-methylthio-4-quinolone. The ability of DMAP to promote the reaction lies in the barrier decrease for alkyne isomerization, deprotonation/protonation of allenes, and dealkylation as effective bases for transferring protons and methyl groups. The roles of prop-2-ynylsulfonium and isatoic anhydride were demonstrated to be C2 and C4 synthons via Multiwfn analysis on the frontier molecular orbital. An alternative path was also confirmed by the Mayer bond order of the vital transition states.

Remote methylene C(sp3)–H functionalization enabled by organophosphine-catalyzed alkyne isomerization

Non classical EWG-activated arylalkynes as C1 units in organophosphine catalyzed diverse remote sp3 C–H functionalization.

Highly Enantioselective [3 + 2] Annulation of 3-Butynoates with β-Trifluoromethyl Enones Promoted by an Amine-Phosphine Binary Catalytic System.

We report a highly enantioselective [3 + 2] annulation between 3-butynoates and β-trifluoromethyl enones, furnishing trifluoromethylated cyclopentenes with three contiguous stereogenic centers in good yields, high diastereoselectivities, and excellent enantioselectivities. A unique catalytic system consisting of a simple amine and a chiral phosphine was devised, and the synergistic play of Lewis basic amine and phosphine was crucial for alkyne isomerization and subsequent cyclization. The protocol disclosed herein allows facile activation of 3-butynoates in phosphine-mediated asymmetric transformations.

Synthesis of Allenes by 1,2-Elimination

Allenes represent an extremely important class of organic molecules, which, as a result of their twisted orthogonal π-systems can possess axial chirality. A wide array of methods for allene synthesis have been reported, such as the substitution of propargylic electrophiles, isomerization of alkynes and sigmatropic rearrangement. An alternative approach for the synthesis of allenes is 1,2-elimination of an appropriately substituted precursor. This mini-review highlights recent examples of 1,2-elimination processes, which target allenes including both polar and radical processes. The main focus is upon how control over the stereospecificity (e.g. syn- or anti-) of the 1,2-elimination process can enable the synthesis of enantioenriched axially chiral allenes. Recent developments in this field are presented including both enantiospecific and catalytic asymmetric methods.

Efficient Synthesis of α-Allylbutenolides from Allyl Ynoates via Tandem Ligand-Enabled Au(I) Catalysis and the Claisen Rearrangement

A facile construction of α-allylbutenolides from readily available allyl ynoates has been developed via tandem gold-catalyzed alkyne isomerization to allene and cycloisomerization, the Claisen rearrangement and a double bond migration. The gold catalysis is enabled by a bifunctional phosphine ligand featuring a critical remote tertiary amino group, and the reaction tolerates a range of substituents and exhibits yields up to 96%.

Chiral Bifunctional Phosphine Ligand Enabling Gold-Catalyzed Asymmetric Isomerization of Alkyne to Allene and Asymmetric Synthesis of 2,5-Dihydrofuran.

The asymmetric isomerization of alkyne to allene is the most efficient and the completely atom-economic approach to this class of versatile axial chiral structure. However, the state-of-the-art is limited to tert-butyl alk-3-ynoate substrates that possess requisite acidic propargylic C-H bonds. Reported here is a strategy based on gold catalysis that is enabled by a designed chiral bifunctional biphenyl-2-ylphosphine ligand. It permits isomerization of alkynes with nonacidic α-C-H bonds and hence offers a much-needed general solution. With chiral propargylic alcohols as substrates, 2,5-disubstituted 2,5-dihydrofurans are formed in one step in typically good yields and with good to excellent diastereoselectivities. With achiral substrates, 2,5-dihydrofurans are formed with good to excellent enantiomeric excesses. A novel center-chirality approach is developed to achieve a stereocontrol effect similar to an axial chirality in the designed chiral ligand. The mechanistic studies established that the precatalyst axial epimers are all converted into the catalytically active cationic gold catalyst owing to the fluxional axis of the latter.

A regioselectivity switch in Pd-catalyzed hydroallylation of alkynes.

By exploiting the reactivity of a vinyl-Pd species, we control the regioselectivity in hydroallylation of alkynes under Pd-hydride catalysis. A monophosphine ligand and carboxylic acid combination promotes 1,5-dienes through a pathway involving isomerization of alkynes to allenes. In contrast, a bisphosphine ligand and copper cocatalyst favor 1,4-dienes via a mechanism that involves transmetalation. Our study highlights how to access different isomers by diverting a common organometallic intermediate.

Direct Enantioselective α-Allylation of Unfunctionalized Cyclic Ketones with Alkynes through Pd-Amine Cooperative Catalysis.

The first direct enantioselective α-allylation of unfunctionalized cyclic ketones using alkynes as an economical choice of reagent is reported. This transformation uses a simple procedure with commercially available palladium, chiral bisphosphine ligand and chiral amine catalysts, and affords valuable ketones with a α-tertiary stereocenter in good to high enantiopurity. In this transformation, a chiral palladium complex containing the (S)-DIFLUORPHOS ligand catalyzes the isomerization of alkynes into an electrophilic allylpalladium species, which is attacked by the enamine generated in situ from the condensation of (R)-prolinol with the ketone substrate.

Synthesis of Carbolines via Palladium/Carboxylic Acid Joint Catalysis.

The combination of a Pd(0) complex with benzoic acid converts propargylic tryptamines to the corresponding tetrahydro-β-carbolines. The method uses unprotected indoles and affords the desired products with ample functional group tolerance. Detailed modeling studies reveal a close synergy between the organic and metal catalysts, which enables sequential alkyne isomerization, indole C-H activation, and eventual C-C reductive elimination to afford the target heterocycles.

One-step preparation of novel 1-(N-indolyl)-1,3-butadienes by base-catalysed isomerization of alkynes as an access to 5-(N-indolyl)-naphthoquinones.

A series of novel 1-(N-indolyl)-1,3-butadienes, as (1 : 1) mixtures of the (E) and (Z) dienes, was prepared in one step by base-catalysed isomerization of N-alkylindoles with a terminal butyne chain. The reaction conditions are mild, and in all cases the yields were very high (>90%). The (E) and (Z) dienes were separable by preparative TLC and could be fully characterized. This isomerization proceeded readily in the case of a butynyl chain, but didn't take place with a pentynyl chain. A mechanism was proposed for this reaction, based on previous studies on the isomerization of alkynes in basic media, and a key intermediate that supports the proposed mechanism could be isolated and fully characterized. A theoretical study of the proposed mechanism was performed by computational methods and the results validated the proposal. The reactivity of the synthesized dienes was studied in Diels–Alder reactions with p-benzoquinone, to obtain a small library of new 5-(N-indolyl)-1,4-naphthoquinones.The lack of reactivity in the case of the (Z) isomers was explained by calculation of the rotational curves of the central bond of the (Z) and (E) dienes. Finally, the cytotoxicity of the new 5-(N-indolyl)-1,4-naphthoquinones was tested against a panel of three cell lines.

ChemInform Abstract: Organomagnesium‐Catalyzed Isomerization of Terminal Alkynes to Allenes and Internal Alkynes.

AbstractThe synthesis of a dimeric organomagnesium complex (MGC) is reported.

Organomagnesium-catalyzed isomerization of terminal alkynes to allenes and internal alkynes.

Organomagnesium complexes 2 were synthesized from N,N-dialkylamineimine ligands 1 and dibenzylmagnesium by benzylation of the imine moiety. 3-Aryl-1-propynes reacted with 2 to form the corresponding tetraalkynyl complexes, which acted as catalysts for the transformation of these terminal alkynes into allenes and further to internal alkynes under mild conditions. To the best of our knowledge, this example is the first of an organomagnesium-catalyzed isomerization of alkynes. Notably, the reactions proceeded through temporally separated autotandem catalysis, thus allowing the isolation of the allene or internal alkyne species in good yields. Mechanistic experiments suggested that the catalytically active tetraalkynyl complexes consist of a tautomeric mixture of alkynyl-, allenyl-, and propargylmagnesium species.

Soft propargylic deprotonation: designed ligand enables Au-catalyzed isomerization of alkynes to 1,3-dienes.

By functionalizing the privileged biphenyl-2-ylphosphine with a basic amino group at the rarely explored 3′ position, the derived gold(I) complex possesses orthogonally positioned “push” and “pull” forces, which enable for the first time soft propargylic deprotonation and permit the bridging of a difference of >26 pKa units (in DMSO) between a propargylic hydrogen and a protonated tertiary aniline. The application of this design led to efficient isomerization of alkynes into versatile 1,3-dienes with synthetically useful scope under mild reaction conditions.

Internal Alkyne Isomerization to Vinylidene versus Stable π-Alkyne: Theoretical and Experimental Study on the Divergence of Analogous Cp*Ru and TpRu Systems

The activation of internal alkynes by Cp*Ru and TpRu complexes gives respectively π-alkyne and disubstituted vinylidene as stable species, even though both systems bear identical pyridylphosphine ligands (κ2P,N- iPr2PXPy, X = NH, CH2, S). The activation of the alkynones PhC≡CCOR (R = Me, Ph) by [TpRuCl(iPr2PXPy)] complexes allowed us to isolate and characterize metastable η1-O═C(R)C≡CPh adducts. These complexes isomerize spontaneously to vinylidene both in solution and in the solid state. Kinetic studies have been carried out in solution by 31P{1H} NMR and in the solid state by IR spectroscopy, providing the Eyring and Avrami–Erofeev parameters, respectively. The activation of internal alkynes without ketone groups provided vinylidene species as well, but without isolable intermediates. In contrast with the TpRu system, the activation of alkynones by [Cp*RuCl(iPr2PXPy)] always results in stable π-alkyne species. Representatives of both Cp*Ru−π-alkyne and TpRu–vinylidene compounds have been characterized b...

Reversibility of Disubstituted Vinylidene−Internal Alkyne Isomerization at Cationic Ruthenium and Iron Complexes

The cationic disubstituted vinylideneruthenium complexes [CpRu{═C═C(Ph)R}(dppe)][BArF4], which are prepared directly from internal alkynes via 1,2-migration of carbon substituents, are shown to undergo disubstituted vinylidene-to-internal alkyne isomerization on reaction with a monophosphine. The observation provides the first examples in which the reversible conversion between internal alkynes and disubstituted vinylidenes is experimentally confirmed, and the kinetic and mechnistic investigations indicated that an uncommon electrophilic 1,2-migration of the carbon substituents takes place in the internal alkyne−disubstituted vinylidene interconversion.

Brønsted Base‐Catalyzed Tandem Isomerization–Michael Reactions of Alkynes: Synthesis of Oxacycles and Azacycles

AbstractAn efficient synthesis of oxacycles and azacycles was developed using a Brønsted base‐catalyzed tandem alkyne isomerization–Michael reaction sequence. Functionalized 2‐alkylidenetetrahydrofurans were prepared by an intramolecular oxy‐Michael reaction on an allene that was generated in situ from an alkynoate. The aza‐Michael version using alkynylamines, alkynylamides and alkynyl carbamates led to piperidines, lactams and oxazolidinones, respectively. An enantioselective version of this reaction resulted in an axially chiral lactam with high enantioselectivity. Some alkynes, however, were unable to complete the intramolecular Michael reactions and provided enantioenriched allenes.

Isomerization of Electron-Poor Alkynes to the Corresponding (E,E)-1,3-Dienes Using a Bifunctional Polymeric Catalyst Bearing Triphenylphosphine and Phenol Groups

The use of a bifunctional non-cross-linked polystyrene bearing both phosphine and phenol groups for the organocatalytic isomerization of alkynes bearing electron-withdrawing ester substituents to afford the corresponding (E,E)-dienes in excellent yield and stereoselectivity is described. When polystyrene functionalized with only phosphine groups was used as the catalyst, either low or no yield of the desired product was obtained. Thus both of the functional groups of the bifunctional polystyrene catalyst were essential for efficient catalysis to occur. This bifunctional polymeric catalyst was also used to synthesize (E,E,E)-trienes and (E,E)-diene-substituted 2(H)-pyran-2-ones from the corresponding alkynes.