Alkynes as Privileged Synthons in Selected Organic Name Reactions

Abstract

Background: Alkynes are actually basic chemicals, serving as privileged synthons for planning new organic reactions for assemblage of a reactive motif, which easily undergoes a further desirable transformation. Name reactions, in organic chemistry are referred to those reactions which are well-recognized and reached to such status for being called as their explorers, discoverers or developers. Alkynes have been used in various name reactions. In this review, we try to underscore the applications of alkynes as privileged synthons in prevalent name reactions such as Huisgen 1,3-dipolar cycloaddtion via Click reaction, Sonogashira reaction, and Hetero Diels-Alder reaction. Objective: In this review, we try to underscore the applications of alkynes as privileged synthons in the formation of heterocycles, focused on the selected reactions of alkynes as a synthon or impending utilization in synthetic organic chemistry, which have reached such high status for being included in the list of name reactions in organic chemistry. Conclusion: Alkynes (including acetylene) are an unsaturated hydrocarbon bearing one or more triple C-C bond. Remarkably, alkynes and their derivatives are frequently being used as molecular scaffolds for planning new organic reactions and installing reactive functional group for further reaction. It is worth mentioning that in general, the terminal alkynes are more useful and more frequently being used in the art of organic synthesis. Remarkably, alkynes have found different applications in pharmacology, nanotechnology, as well as being known as appropriate starting precursors for the total synthesis of natural products and biologically active complex compounds. They are predominantly applied in various name reactions such as Sonogashira, Glaser reaction, Friedel-crafts reaction, Castro-Stephens coupling, Huisgen 1.3-dipolar cycloaddtion reaction via Click reaction, Sonogashira reaction, hetero-Diels-Alder reaction. In this review, we tried to impress the readers by presenting selected name reactions, which use the alkynes as either stating materials or precursors. We disclosed the applications of alkynes as a privileged synthons in several popular reactions, which reached to such high status being classified as name reactions. They are thriving and well known and established name reactions in organic chemistry such as Regioselective, 1,3-dipolar Huisgen cycloaddtion reaction via Click reaction, Sonogashira reaction and Diels-Alder reaction.

Recent Strides in the Transition Metal-Free Cross-Coupling of Haloacetylenes with Electron-Rich Heterocycles in Solid Media

“…The combination of the electron-rich, five-membered aromatic heterocyclic nucleus with highly reactive carbon-carbon triple bond in one molecule allows using these compounds for the targeted synthesis of various complex heterocyclic systems. Commonly, ethynylation of heterocycles is implemented via Sonogashira reaction employing the halogenated heterocycles and terminal alkynes [16][17][18][19]. In 2004, as a complementation to the existing cross-coupling protocols, the direct palladium-and copper-free ethynylation of the pyrrole ring with haloacetylenes in the Al 2 O 3 medium (room temperature, no solvent) was discovered [20].…”

Decatungstate as Direct Hydrogen Atom Transfer Photocatalyst for SOMOphilic Alkynylation

“…Thus, when 1,3-dioxolane (1d) was employed as the H-donor, the reaction proceeded to deliver a 1:1 mixture of isomers 6 A and 6 B (67% overall yield), resulting from the functionalization at the acetalic and ethereal positions, respectively. Differently, the selectivity diverted to the methine C(sp 3 )−H in the case of 2-methyl-1,3-dioxolane (1e), affording products 7 A and 7 B in 75% overall yield with a 3.3:1 ratio. This selectivity pattern has been previously reported and can be rationalized based on the stability of the involved radical intermediates.…”

“…In fact, both internal and terminal alkynes are frequently used in drugs due to their capability to modulate the activity of several proteins, as well as for the late-stage manipulation of organic molecules through click chemistry, − nowadays routinely used for bioconjugation. What is more, alkynes are precious building blocks that offer direct access to a plethora of functionalities, including alkenes, alkanes, halides, and carbonyl compounds, among the others. − Finally, the CC triple bond plays a key role in materials science as well, where the use of Mo- and W-based complexes has been reported to catalyze alkyne metathesis polymerization …”

“…8 A more controlled activation manifold is offered by photocatalyzed HAT, which has secured its place as one of the most convenient and sustainable ways for the generation of C-centered radicals. 9 Thus, the excited state of the photocatalyst (PC) generated upon light absorption can be exploited 15 However, a general platform for the alkynylation of strong aliphatic C(sp 3 )−H bonds remains an unmet goal. Building upon our expertise in the use of TBADT (tetrabutylammonium decatungstate, (Bu 4 N) 4 [W 10 O 32 ]) as photocatalyst for the functionalization of organic substrates via HAT, 16,17 we evaluated the use of this polyoxometalate to promote the alkynylation of a variety of aliphatic H-donors, including ethers, acetals, aldehydes, amides, a silane and even hydrocarbons.…”

“…We also achieved the functionalization of the α-to-S C(sp 3 )−H bond of tetrahydrothiophene to give product 10 in 72% isolated yield, while no ring-opening of the sulfurheterocycle was observed. 15 Silane 1i (see Chart S1 in the Supporting Information) was likewise a competent substrate in the reaction, as demonstrated by the isolation of 11 in 62% yield.…”