Internal Alkynes Research Articles

Electrocatalytic Semihydrogenation of Terminal Alkynes Using Ligand-Based Transfer of Protons and Electrons.

Alkyne semihydrogenation is a broadly important transformation in chemical synthesis. Here, we introduce an electrochemical method for the selective semihydrogenation of terminal alkynes using a dihydrazonopyrrole Ni complex capable of storing an H2 equivalent (2H+ + 2e-) on the ligand backbone. This method is chemoselective for the semihydrogenation of terminal alkynes over internal alkynes or alkenes. Mechanistic studies reveal that the transformation is concerted and Z-selective. Calculations support a ligand-based hydrogen-atom transfer pathway instead of a hydride mechanism, which is commonly invoked for transition metal hydrogenation catalysts. The synthesis of the proposed intermediates demonstrates that the catalytic mechanism proceeds through a reduced formal Ni(I) species. The high yields for terminal alkene products without over-reduction or oligomerization are among the best reported for any homogeneous catalyst. Furthermore, the metal-ligand cooperative hydrogen transfer enabled with this system directs the efficient flow of H atom equivalents toward alkyne reduction rather than hydrogen evolution, providing a blueprint for applying similar strategies toward a wide range of electroreductive transformations.

Palladium-catalyzed synthesis of indene-1-acetates via sequential double carbopalladation and aryloxycarbonylation

A novel palladium-catalyzed cascade reaction of o-iodostyrenes, internal alkynes and formates to access various indene-1-acetates through sequential double carbopalladation and aryloxycarbonylation under a palladium/air system was developed.

Gold-catalyzed benzannulations of 2-alkenylindoles with alkynes: a protecting-group-free regioselective approach to carbazoles.

A gold(I)-catalyzed protecting-group-free benzannulation approach to functionalized NH-carbazoles was accomplished via the hydroarylation of alkynes with 2-alkenylindoles. A broad spectrum of terminal and internal alkynes and 2-alkenylindoles successfully participated in this annulation reaction. The protocol efficiently enabled the formation of substituted NH-carbazoles with moderate to specific regioselectivities. The synthetic utility of this protocol was demonstrated by a variety of post-functionalizations.

Three-component cascade carbopalladation/Heck cyclization/borylation: facile access to boryl-functionalized indenes.

A mild Pd-catalyzed three-component cascade cyclization functionalization of o-iodostyrenes, internal alkynes and boron reagents is presented. The transformation is driven by a controlled reaction sequence of intermolecular carbopalladation, intramolecular Heck-type cyclization, and a borylation process to give versatile boryl-functionalized indene skeletons in a selective fashion. Significantly, (Bpin)2, (Bneop)2 and CH2(Bpin)2 as boron sources are all tolerated. Additionally, the synthetic utility of this approach is demonstrated by gram-scale synthesis and synthetic transformations.

Additive-free oxychlorination of unsaturated C-C bonds with tert-butyl hypochlorite and water.

Herein we report an additive-free protocol for the facile synthesis of α,α-dichloroketones and α-chlorohydrins from various aryl terminal, diaryl internal, and aliphatic terminal alkynes and alkenes, respectively. The commercially available tert-butyl hypochlorite (tBuOCl) was employed as a suitable chlorinating reagent, being accompanied by the less harmful tBuOH as the by-product. In addition, the oxygen atoms in the products came from water rather than molecular oxygen, based on the 18O-labelling experiments. Meanwhile, the diastereoselectivity of the Z- and the corresponding E-alkenes has been compared and rationalized. Using a group of control experiments, the possible mechanisms have been proposed as the initial electrophilic chlorination of unsaturated C-C bonds in a Markovnikov-addition manner in general followed by a nucleophilic addition with water. This work simplified the oxychlorination method with a mild chlorine source and a green oxygen source under ambient conditions.

Photocatalytic iodosulfonylation of internal alkynes under green conditions

Photocatalytic iodosulfonylation in ethanol allows the isolation of β-iodovinyl aryl and alkyl sulfones after dilution of the reaction mixture with water and filtration.

Ruthenium-catalyzed electrochemical C-H activation of benzamidine hydrochlorides with internal alkynes for the synthesis of 1-aminoisoquinolines.

A constant current electrochemical approach for the synthesis of 1-aminoisoquinoline derivatives through the Ru(II)-catalyzed annulation of benzamidine hydrochlorides with alkynes is reported herein. This method produces good yields of naphthyridine-based compounds via Ru(II)-catalyzed C-H bond cleavage, leading to the formation of new C-C and C-N bonds in a one-pot manner. Notably, the synergy of electricity and Ru catalysis offers broad reaction compatibility, accommodating a wide range of substrates with diverse steric and electronic properties. Additionally, this method also eliminates the use of stoichiometric amounts of external chemical oxidants.

Stereo-selective synthesis of complex dienes and eneynes by sequential hydroarylation and olefinic C–H functionalization

We present a preparation of complex dienes and eneynes by Rh-catalyzed hydroarylation of internal alkynes and Pd-catalyzed olefinic C–H alkenylation, alkynylation and allylation, enabled by the chelation assistance of pyrazinamide or picolinamide.

Vinylidene rearrangements of internal borylalkynes via 1,2-boryl migration.

Vinylidene rearrangement of alkynes is a well-established and powerful method for alkyne transformations, while use of borylalkynes has remained largely unexplored. This paper describes vinylidene rearrangements of internal borylalkynes using a cationic ruthenium complex. This rearrangement is applicable to alkynes with both tri-(B(pin), B(dan)) and tetracoordinate (B(mida)) boryl groups, and the reaction rate is dramatically affected by the Lewis acidity of the boryl group. Mechanistic study revealed that the rearrangement proceeds via 1,2-boryl migration regardless of the coordination number of the boron center. The migration mode was elucidated by theoretical calculations to indicate that the migration of the tricoordinate boryl groups is an electrophilic process in contrast to the previous vinylidene rearrangements of internal alkynes with two carbon substituents.

Ligand-controlled regiodivergent Ni-catalyzed trans-hydroboration/carboboration of internal alkynes with B2pin2

With the aid of the directing group strategy, an approach for the synthesis of alkenyl and 1-naphthyl boronates with stereo- and regioselectivity via ligand-controlled Ni-catalyzed trans-hydroboration/carboboration of internal alkynes was developed.

Green synthesis of sulfur-containing polymers by carbon disulfide-based spontaneous multicomponent polymerization

A spontaneous multi-component polymerization of activated internal alkynes, amines, and CS2 was established. Similar to the ancient “tenon and mortise” structure, CS2 acts as a “wedge” to link the other two monomers to form sulfur-containing polymers.

Iron-catalyzed fragmentation-alkynylation, -alkenylation and -alkylation cascade enabled by photoinduced ligand-to-metal charge transfer

An iron-catalyzed fragmentation alkynylation, alkenylation and alkylation enabled by photoinduced ligand-to-metal charge transfer (LMCT)processis herein reported.

Photocatalytic deuterocarboxylation of alkynes with oxalate.

Herein, a catalytic photoredox-neutral strategy for alkyne deuterocarboxylation with tetrabutylammonium oxalate as the carbonyl source and D2O as the deuteration agent was described. For the first time, the oxalic salt acted as both the reductant and carbonyl source through single electron transfer and subsequential homolysis of the C-C bond. The strongly reductive CO2 radical anion species in situ generated from oxalate played significant roles in realizing the global deuterocarboxylation of terminal and internal alkynes to access various tetra- and tri-deuterated aryl propionic acids with high yields and deuteration ratios.

Gold-catalyzed fluorination of alkynes/allenes: mechanistic explanations and reaction scope.

Since the beginning of this century, there has been a great deal of research on homogeneous gold-catalyzed alkyne fluorination due to the precious values of fluorinated scaffolds in many bioactive natural products, drugs, and agrochemicals. This area of research, which originally took advantage of gold's mild Lewis acidity and tendency to form π-complexes with alkynes, has gained new momentum after Sadighi's discovery in 2007 of Au-catalyzed hydrofluorination of internal alkynes. The methods have enabled direct access to valuable fluoroalkanes, fluoroalkenes, α-fluorocarbonyls, and fluorinated carbo- and hetero-cycles in one pot from readily available alkyne precursors. Both nucleophilic and electrophilic fluorination modes with versatile reactivity have been used to achieve several new cascade reactions. This study covers the literature reports published since 2007 and provides a comprehensive summary of the methods, applications, and mechanistic insights into gold-catalyzed alkyne fluorination using electrophilic and nucleophilic fluorinating reagents.

Copper-catalyzed syn-hydroformylation of alkynes with silanes and N,N-dimethylformamide dimethylacetal.

A copper-catalyzed syn-hydrocarbonization of internal alkynes with N,N-dimethylformamide dimethylacetal and silanes has been disclosed that offers an efficient and expedient access to (E)-α,β-unsaturated aldehydes. This highly selective process, which can be performed at gram-scale, enjoys operational simplicity, as well as syngas-free conditions.

Pd/C-catalyzed regiodivergent hydrocarboxylation and esterification of alkynes.

An unprecedented and highly reactive Pd/C catalytic system has been introduced for the regiodivergent hydrocarboxylation of terminal alkynes to selectively afford various acrylic and cinnamic acids employing oxalic acid as a CO source as well as a promoter for the formation of the active Pd-H complex. Herein, the formation of cinnamic acid is proposed to follow a unique anti-Markovnikov hydroiodination mechanism and the formation of acrylic acid might follow the traditional hydrocarboxylation pathway. Additionally, internal alkynes undergo hydrocarboxylation and carbonylative esterification with aliphatic alcohols to yield different α,β-unsaturated acids and esters respectively. The designed strategies were successfully leveraged for a diverse class of α,β-unsaturated acids and esters with excellent selectivity and yields under mild reaction conditions. Furthermore, the acid functionalization of complicated naturally derived alkynes, utilizing economical and bench-stable oxalic acid and a commercially accessible reusable catalyst with gram-scale applicability are the additional benefits of the established protocol.

Palladium-catalyzed syn-alkynylarylation of internal alkynes: rapid access to all-carbon tetrasubstituted alkenes.

Herein, a straightforward method for rapid access to all-carbon tertrasubstituted alkenes bearing alkyl, aryl and alkynyl groups is established via palladium-catalyzed three-component cross-coupling reaction of internal alkynes, haloalkynes and arylboronic acids. This protocol is characterized by a broad substrate scope and excellent chemo- and regioselectivities. The dual beneficial roles of silver salts in activating haloalkynes and inhibiting bromoalkynylation have been demonstrated by serving as both the Lewis acid and halide scavenger.

Synthesis of (E)-vinyl sulfides via copper-catalyzed hydrothiolation of internal alkynes with thiosulfonates and silanes

A copper-catalyzed syn-hydrothiolation of internal alkynes with thiosulfonates and silanes has been disclosed, which offers regio- and stereoselective access to (E)-vinyl sulfides.

Catalytic HF Shuttling between Fluoroalkanes and Alkynes.

In this paper, we report BF3 ⋅ OEt2 as a catalyst to shuttle equivalents of HF from a fluoroalkane to an alkyne. Reactions of terminal and internal aliphatic alkynes led to formation of difluoroalkane products, while diarylalkynes can be selectively converted into fluoroalkenes. The method tolerates numerous sensitive functional groups including halogen, protected amine, ester and thiophene substituents. Mechanistic studies (DFT, probe experiments) suggest the catalyst is involved in both the defluorination and fluorination steps, with BF3 acting as a Lewis acid and OEt2 a weak Lewis base that mediates proton transfer. In certain cases, the interconversion of fluoroalkene and difluoroalkane products was found to be reversible. The new catalytic system was applied to demonstrate proof-of-concept recycling of poly(vinylidene difluoride).

Catalytic HF Shuttling between Fluoroalkanes and Alkynes**

AbstractIn this paper, we report BF3 ⋅ OEt2 as a catalyst to shuttle equivalents of HF from a fluoroalkane to an alkyne. Reactions of terminal and internal aliphatic alkynes led to formation of difluoroalkane products, while diarylalkynes can be selectively converted into fluoroalkenes. The method tolerates numerous sensitive functional groups including halogen, protected amine, ester and thiophene substituents. Mechanistic studies (DFT, probe experiments) suggest the catalyst is involved in both the defluorination and fluorination steps, with BF3 acting as a Lewis acid and OEt2 a weak Lewis base that mediates proton transfer. In certain cases, the interconversion of fluoroalkene and difluoroalkane products was found to be reversible. The new catalytic system was applied to demonstrate proof‐of‐concept recycling of poly(vinylidene difluoride).