1,3-Chlorine Shift to a Vinyl Cation: A Combined Experimental and Theoretical Investigation of the E-Selective Gold(I)-Catalyzed Dimerization of Chloroacetylenes

Gold-Catalyzed Synthesis of 5H-Benzo[b]indeno[2,1-d]silines by Insertion of Vinyl Carbocations into the Si-H Bond

We have developed a gold-catalyzed cascade reaction of aryldiynes bearing a hydrosilyl group to afford a variety of unexplored 5H-benzo[b]indeno[2,1-d]silines. The reaction system is applicable to the synthesis of bidirectionally π-extended silacycles from tetra(alkynyl)aryl compounds. Computational studies suggest that 5H-benzo[b]indeno[2,1-d]silines are formed via the insertion of a vinyl carbocation intermediate into the Si-H bond.

Indium(III)-Catalyzed Haloalkynylation Reaction of Alkynes

Green chemistry strives for sustainability at the molecular level and is gaining increasing relevance in the development of chemical reactions. The haloalkynylation reaction is a highly atom-economical C-C coupling reaction that was previously only achieved using transition metal catalysts. It enables the introduction of an alkyne unit and a halogen atom into the target molecule. Herein, we present a haloalkynylation reaction catalyzed by indium(III) halides. The use of indium(III) bromide as a catalyst leads exclusively to the cis addition products with yields up to 86 %. In addition, iodoacetylenes can be applied for the first time for the haloalkynylation reaction of internal alkynes which is an important step forward in the development of industrially relevant and sustainable catalysts. In contrast to gold catalysis, which proceeds via a similar mechanism, the use of alkyl-substituted haloacetylenes as reagents is also possible. Based on 13C labeling experiments and quantum chemical calculations, we postulate two possible mechanisms for the indium(III)-catalyzed haloalkynylation reactions.

Gold(I)-Catalyzed Bis-Cyclization of Allenynes for the Synthesis of Strained and Planar Polycyclic Compounds

A gold-catalyzed cascade cyclization of naphthalene-tethered allenynes gave strained fused phenanthrene derivatives. The reaction proceeds through the nucleophilic reaction of an alkyne with the activated allene to generate a vinyl cation intermediate, followed by arylation with a tethered naphthalene ring to form the 4H-cyclopenta[def]phenanthrene (CPP) scaffold. When using aryl-substituted substrates on the alkyne terminus, the gold-catalyzed reaction produced dibenzofluorene derivatives along with the CPP derivatives. Selective formation of CPP and dibenzofluorene derivatives depending on the reaction conditions is also presented.

Palladium-Catalyzed Cross Haloalkynylation of Haloalkynes

Herein, we have developed a robust Pd-catalyzed haloalkynylation of haloalkynes for the synthesis of dihaloalkenyne derivatives. This cross-haloalkynylation reaction proceeds in a highly chemo- and regioselective manner under mild conditions in the presence of two slightly structurally different haloalkynes and shows strong functional group tolerance under ligand- and base-free conditions.

Controlling the Gold(I)-Catalyzed 1,5-Allenene Reaction: Construction of Fused Rings with Excellent Diastereoselectivity

In the present study, the gold(I)-catalyzed reaction of 1,5-allenenes was controlled in such a way that instead of a [2 + 3] cycloaddition, a 5-exo-cyclization with the formation of a carbocation occurred. The latter could be trapped with both oxygen and carbon nucleophiles. In the investigated system, fused tricyclic frameworks with three contiguous stereocenters with excellent chemo- and diastereoselectivity in up to 95% yield were obtained.

Metal-Catalyzed Haloalkynylation Reactions

Metal catalysis has revolutionized synthetic chemistry, leading to entirely new, very efficient transformations, which enable access to complex functionalized molecules. One such new transformation method is the haloalkynylation reaction, in which both a halogen atom and an alkynyl unit are transferred to an unsaturated carbon‐carbon bond. This minireview summarizes the development of metal‐catalyzed haloalkynylation reactions since their beginning about a decade ago. So far, arynes, alkenes and alkynes have been used as unsaturated systems and the reactivities of these systems are summarized in individual chapters of the minireview. Especially, the last few years have witnessed a rapid development due to gold‐catalyzed reactions. Here, we discuss how the choice of the catalytic system influences the regio‐ and stereoselectivity of the addition.

Construction of Tricyclic Nitrogen Heterocycles by a Gold(I)-Catalyzed Cascade Cyclization of Allenynes and Application to Polycyclic π-Electron Systems

A novel approach to the direct construction of tricyclic nitrogen heterocycles based on gold-catalyzed cascade cyclization of aminoallenynes is described. The expected biscyclization reaction of hydroxyisobutyryl-protected aminoallenynes was efficiently promoted by a catalytic amount of BrettPhosAuNTf₂ in the presence of iPrOH to produce 1,2-dihydrobenzo[cd]indole derivatives in good yields. When the reaction was combined with Friedel-Crafts acylation or palladium-catalyzed N-arylation, the resulting tricyclic products were efficiently converted into nitrogen-containing polycyclic aromatic compounds (N-PACs) with highly conjugated π-electron systems. A newly obtained hexacyclic indolium salt showed characteristic concentration-dependent absorption and emission properties.

Gold Catalysis of Non-Conjugated Haloacetylenes

Gold-catalyzed reactions of conjugated haloacetylenes are well known and usually result in the formation of addition or dimerization products. Herein, we report a gold-catalyzed reaction of non-conjugated­ haloacetylenes, which leads exclusively to the halogenated cyclization products. Remarkable is the gold-catalyzed reaction of tritylhaloacetylenes to haloindene derivatives, as mechanistic studies reveal that an 1,2-aryl shift occurs in the initially formed gold complex. The potential functionalization at the halogen atom and the wide scope of these cyclization reactions make them an attractive method for the construction of cyclic systems.

Cyclopropenylmethyl Cation: A Concealed Intermediate in Gold(I)-Catalyzed Reactions

The last years have witnessed many gold-catalyzed reactions of alkynes. One of the most prominent species in the reaction of two alkyne units is the vinyl-substituted gold vinylidene intermediate. Here, we were able to show that the reaction of a haloacetylene and an alkyne proceeds via a hitherto overlooked intermediate, namely the cyclopropenylmethyl cation. The existence and relative stability of this concealed intermediate is verified by quantum chemical calculations and 13 C-labeling experiments. A comparison between the cyclopropenylmethyl cation and the well-known vinylidene intermediate reveals that the latter is more stable only for smaller cycles. However, this stability reverses in larger cycles. In the case of the smallest representative of both species, the vinylidene cation is the transition state en route to the cyclopropenylmethyl cation. The discovery of this intermediate should help to get a deeper understanding for gold-catalyzed carbon-carbon bond-forming reactions of alkynes. Furthermore, since enynes can be formed from the cyclopropenylmethyl cation, the inclusion of this intermediate should enable the development of new synthetic methods for the construction of larger cyclic halogenated and non-halogenated conjugated enyne systems.

Stereodivergent Alkyne Hydrofluorination Using Protic Tetrafluoroborates as Tunable Reagents

The discovery of safe, general, and practical procedures to prepare vinyl fluorides from readily available precursors remains a synthetic challenge. The metal-free hydrofluorination of alkynes constitutes an attractive though elusive strategy for their preparation. Introduced here is an inexpensive and easily handled reagent that enables the development of simple and scalable protocols for the regioselective hydrofluorination of alkynes to access both the E and Z isomers of vinyl fluorides. These reaction conditions were suitable for a diverse collection of alkynes, including several highly functionalized pharmaceutical derivatives. Computational and experimental mechanistic studies support C-F bond formation through vinyl cation intermediates, with the E- and Z-hydrofluorination products forming under kinetic and thermodynamic control, respectively.

Gold(I)-Catalyzed Haloalkynylation of Aryl Alkynes: Two Pathways, One Goal

Haloalkynylation reactions provide an efficient method for the simultaneous introduction of a halogen atom and an acetylenic unit. For the first time, we report a gold(I)‐catalyzed haloalkynylation of aryl alkynes that delivers exclusively the cis addition product. This method enables the simple synthesis of conjugated and halogenated enynes in yields of up to 90 %. Notably, quantum chemical calculations reveal an exceptional interplay between the place of the attack at the chloroacetylene: No matter which C−C bond is formed, the same enyne product is always formed. This is only possible through rearrangement of the corresponding skeleton. Hereby, one reaction pathway proceeds via a chloronium ion with a subsequent aryl shift; in the second case the corresponding vinyl cation is stabilized by a 1,3‐chlorine shift. ¹³C‐labeling experiments confirmed that the reaction proceeds through both reaction pathways.

A K-arylacetylide complex for catalytic terminal alkyne functionalization using KOtBu as a precatalyst

Herein we report a transition metal free catalytic terminal alkyne functionalization across the C-X triple bond (X = CH and N) with E-selective homo (alkyne-alkyne) and head-to-tail selective hetero (alkyne-nitrile) dimerization. A series of stoichiometric reactions enabled us to crystallize a reactive organometallic intermediate K-arylacetylide complex which was characterized by X-ray crystallography, indicating that an ionic mechanism is operative.

Mechanistic Insight Into the AuCN Catalyzed Annulation Reaction of Salicylaldehyde and Aryl Acetylene: Cyanide Ion Promoted Umpolung Hydroacylation/Intramolecular Oxa-Michael Addition Mechanism

The detailed mechanism of the AuCN-catalyzed annulation of salicylaldehyde (SA) and phenyl acetylene leading to isoflavanone-type complexes has been investigated via density functional theory (DFT) calculations. Reaction pathways and possible stationary points are obtained with the combined molecular dynamics and coordinate driving (MD/CD) method. Our calculations reveal that the cyanide ion promoted umpolung hydroacylation/intramolecular oxa-Michael addition mechanism is more favorable than the Au(I)/Au(III) redox mechanism proposed previously. In the umpolung mechanism, the hydroxyl of SA is found to strongly stabilize the cyanide ion involved intermediates and transition states via hydrogen bond interactions, while the Au(I) ion always acts as a counter cation. The overall reaction is exergonic by 41.8 kcal/mol. The hydroacylation of phenyl acetylene is the rate-determining step and responsible for the regioselectivity with a free energy barrier of 27.3 kcal/mol. These calculated results are in qualitative accord with the experimental findings.

Dimerization of substituted 4-aryl-1,3-diacetylenes – quantum chemical calculations and kinetic studies

Thermal dimerization of 4-aryl-1,3-diacetylenes resulting in diradical intermediates were investigated by experimental and quantum chemical studies.