Alkynyl Radicals, Myths and Realities

Abstract

This Perspective deals with the organic chemistry of alkynyl radicals, a species that is ultimately still little known in the synthetic community. Starting with the first observations and characterizations of alkynyl radicals generated by various methodologies in the gas phase, we then particularly turned our attention to the implications of these highly reactive intermediates in organic synthesis and materials science. Mechanistic considerations have been provided, in particular, for the key steps of generating alkynyl radicals, which are mainly based on photochemical or thermal activation and single electron transfer processes. This Perspective should serve as a roadmap for the synthetic chemist in order to plan more reliably alkynylation reactions based on alkynyl radicals.

Scheme 1. Typical Alkynylation Pathways

Scheme 2. Diverse Precursors Used in the Formation of Alkynyl Radicals under Photolysis or Pyrolysis Conditions

Scheme 3. (a) Reactivity of Ethynyl Radical with Isoprene; (b) Reactivity of Ethynyl Radical with 1,3-Butadiene

Scheme 4. Addition Channels of C₂H and N₂O Reaction

Scheme 5. (a) Major Product Channels from the Reactions of Methyl-Acetylene (CH₃CCH) and Allene (H₂CCCH₂) with Phenylethynyl Radical; (b) Phenanthrene Synthesis from Phenylethynyl Radical; (c) Photolysis of Spiropentane

Scheme 6. Homolytic Aromatic Substitution by Phenyethynyl Radicals

Scheme 7. Photoinduced Alkynylation of Amines and Alcohols

Scheme 8. Photoinduced Head to Tail Coupling of Bromoalkynes

Scheme 9. Photoinduced Inverse Sonogashira Reaction

Scheme 10. Rh Catalyzed Photoinduced Ortho-Alkynylation

Scheme 11. Photoinduced Rh Catalyzed P(III)-Directed C–H Activation

Scheme 12. EnT-Initiated Alkylynative Cascade

Scheme 13. Modeling Studies for the Photosensitized Gold(I) Catalyzed Alkynylative Cyclization of o-Alkylnylphenols: An Alkynyl Radical Based Pathway vs the Nonradical Pathway Proposed Previously

Scheme 14. Synthesis of Alkynylated Indoles via the Photoactivation of in Situ Generated Organogold Species

Scheme 15. Alkynylation of Aldehydes with Alkynyliodonium Salts

Scheme 16. Alkynylsamariums Reactants

Scheme 17. Gold-Catalyzed Photoredox Cross-Coupling of Tertiary Alkylamines with Iodoalkynes

Scheme 18. Chalcogen-Bonding Catalysis for the Generation of Alkynyl Radicals

Scheme 19. Urea Promoted Generation of Alkynyl Radicals

Scheme 20. Alkynylation of 2-Oxindoles through Radical–Radical Coupling

Scheme 21. Cobalt Catalyzed Synthesis of Pyrrolidinones from Aliphatic Amides and Terminal Alkynes

Scheme 22. New Mechanism for Glaser Coupling Involving the Formation of Alkynyl Radical

Scheme 23. (a) Grafting of Alkynyl Radicals on Carbon Surfaces; (b) Electrochemical Reduction of Alkynyliodonium Salt for the Synthesis of Alkynyl Radicals

Scheme 24. Intermolecular HAT Reaction on Metal Surface

Scheme 25. Experimental Results on the Blue LED Irradiation of Haloalkynes

Scheme 26. (a) Alternative SOMOphilic Pathway; (b) Alternative Mechanism for the Formation of 1,1-Dibromo-1-en-3-yne Products