Iodine atom transfer addition reactions with alkynes. Part 2; α-iodocarbonyls

Stereodivergent atom transfer radical addition of α-functionalized alkyl iodides to alkynes: a strategy for selective synthesis of both E- and Z-iodoalkenes

The geometrical control of atom transfer radical addition (ATRA) reactions to alkynes poses significant challenges. Herein, we present a uniform solution by developing a stereodivergent synthetic method for both isomers of the resulting alkene products, starting from the same materials. The synthesis of the thermodynamically more stable isomer utilizes the strategy of uphill catalysis while the accumulation of the less stable isomer is facilitated by a manganese-catalyzed iodo-abstraction/radical rebound process, taking advantage of its reversibility. Various substituted alkyl iodides can be used to provide easy access to both isomers of iodoalkene products with valuable functional groups such as CF3, CF2H, CN, ester, or amide at the allylic position.

Intermolecular Addition of Carbon-Centered Radicals to Ynamides. A Regio- and Stereoselective Route to Persubstituted α-Iodo-enamides

Rather surprisingly, C-C bond formation through "intermolecular" radical addition to internal ynamides has never been reported. Actually, ynamides are excellent acceptors for "electrophilic" carbon-centered radicals. These processes enable the introduction of functionalized alkyl chains at Cβ, groups that have not yet been introduced via the addition of organometallics. Radical carboiodination affords persubstituted α-iodo-enamides in moderate to high yield. The addition is totally stereoselective. Theoretical support to the mechanism and the scope and limitation of the reaction are discussed.

Visible-Light-Promoted Manganese-Catalyzed Atom Transfer Radical Cyclization of Unactivated Alkyl Iodides

An expedient visible-light-promoted atom transfer radical cyclization (ATRC) reaction of unactivated alkyl iodides facilitated by earth-abundant and inexpensive manganese catalysis is described. The practical protocol shows a broad substrate scope and good functional-group tolerance, allowing for the preparation of synthetically valuable alkenyl iodides and diquinanes under simple and mild reaction conditions. Notably, the method provides a net redox-neutral strategy for ATRC reactions that avoids classic hydrogen atom transfer mechanism.

Photocatalytic Synthesis of γ-Lactones from Alkenes: High-Resolution Mass Spectrometry as a Tool To Study Photoredox Reactions

A mild photocatalytic manifold for the synthesis of γ-lactones has been developed. Utilizing Ru(bpy)₃Cl₂ as the photocatalyst, a cheap and reproducible synthetic protocol for γ-lactones has been introduced. Mechanistic studies revealed the successful monitoring of photocatalytic reactions and radical intermediates via high-resolution mass spectrometry.

Copper-catalysed direct radical alkenylation of alkyl bromides

A copper-catalysed direct radical alkenylation of benzyl bromides and α-carbonyl alkyl bromides has been developed. Compared with recent radical alkenylations which mostly focused on secondary or tertiary alkyl halides, this transformation shows good reactivity towards primary alkyl halides and tertiary/secondary alkyl halides are also tolerated.

Visible light-mediated atom transfer radical addition via oxidative and reductive quenching of photocatalysts

Herein, the development of visible light-mediated atom transfer radical addition (ATRA) of haloalkanes onto alkenes and alkynes using the reductive and oxidative quenching of [Ir{dF(CF(3))ppy}(2)(dtbbpy)]PF(6) and [Ru(bpy)(3)]Cl(2) is presented. Initial investigations indicated that the oxidative quenching of photocatalysts could effectively be utilized for ATRA, and since that report, the protocol has been expanded by broadening the scope of the reaction in terms of the photocatalysts, substrates, and solvents. In addition, further modifications of the reaction conditions allowed for the efficient ATRA of perfluoroalkyl iodides onto alkenes and alkynes utilizing the reductive quenching cycle of [Ru(bpy)(3)]Cl(2) with sodium ascorbate as the sacrificial electron donor. These results signify the complementary nature of the oxidative and reductive quenching pathways of photocatalysts and the ability to predictably direct reaction outcome through modification of the reaction conditions.

Intermolecular atom transfer radical addition to olefins mediated by oxidative quenching of photoredox catalysts

Atom transfer radical addition of haloalkanes and α-halocarbonyls to olefins is efficiently performed with the photocatalyst Ir[(dF(CF(3))ppy)(2)(dtbbpy)]PF(6). This protocol is characterized by excellent yields, mild conditions, low catalyst loading, and broad scope. In addition, the atom transfer protocol can be used to quickly and efficiently introduce vinyl trifluoromethyl groups to olefins and access 1,1-cyclopropane diesters.

Radical and radical-ionic multicomponent processes

All in one pot! Radical, radical-ionic, and radical-organometallic MCR are highly convergent processes, representing a useful pathway to molecular and structural diversity (see scheme). This concept article highlights recent developments in the field and shows the potential of the strategy for the economical elaboration of various kinds of organic substrates.Progress in multicomponent reactions (MCRs) based on ionic and organometallic processes has recently allowed the synthesis, with high efficiency, of large collections of drug-like molecules. In contrast, MCRs based on cascade radical reactions appear to have attracted, so far, much less interest. This concept article highlights the latest progress in the field and underlines the potential of such an approach. The collected examples demonstrate that radical and radical-ionic MCRs are highly convergent processes leading to useful structural diversity.

Facile 5-Endo Amidyl Radical Cyclization Promoted by Vinylic Iodine Substitution

[reaction: see text] BF(3).OEt(2)-catalyzed atom-transfer radical addition of iodoacetamides to alkynes yielded the corresponding vinyl iodides, which upon treatment with (t)BuOCl and I(2) afforded gamma-lactam derivatives in moderate to good yield. The mechanism was proposed to be 5-endo amidyl radical cyclization, and vinylic iodine substitution provided the driving force for the cyclization.

Thiophenol-Mediated Hydrogen Atom Abstraction: An Efficient Tin-Free Procedure for the Preparation of Cyclopentane Derivatives

[reaction: see text] An efficient procedure for running a cascade reaction involving 1,5-abstraction of a hydrogen atom followed by a radical cyclization is reported. Alkenyl radicals are generated from easily available terminal alkynes and thiophenol. This procedure eliminates the need of using the toxic tributyltin hydride and gives a greater amount of radical translocation products.

Synthesis of Aromatic Triynes as Precursors to Helicene Derivatives

A general and versatile synthetic approach to a broad series of aromatic triynes as precursors to helicene derivatives has been developed. Employing a set of simple tools, triynes comprising the (phenylethynyl)benzene, 1-(phenylethynyl)naphthalene, and 1-(1-naphthylethynyl)- naphthalene moiety have been prepared in good to excellent yields throughout the whole reaction sequence. The methodology allows constructing various types of a junction between the central diarylacetylene moiety and the attached acetylene units to get the target triynes of general formula R-C≡C-CH2-X-CH2-Ar-C≡C-Ar'-CH2-X-CH2-C≡C-R or R-C≡C-CH2CH2-Ar-C≡C-Ar'-CH2CH2-C≡C-R (R = H, CH3, TMS, or TIPS; X = O, NTs, or C(CO2CH3)2; Ar/Ar' = 2-phenylene or 2-naphthylene).

Substituent Effects on Vinyl Radical Cyclizations onto Aryl Rings

Regioselective toluenesulfanyl radical addition to the C-1 carbon of propynyl benzyl ethers 2a-i leads to vinyl radicals 3a-j which mainly give methyl sulfides 9a-j and methyl ethers 8a-j through stereoselective 5-(pi-endo)exo and 5-(pi-exo)exo cyclization, respectively. Additionally, minor amounts of pi-endo 6-membered cyclization products 5a-j and hydrogen abstraction products 4a-j are also formed. The role of steric and stereoelectronic factors in the 5-membered vs 6-membered and pi-endo vs pi-exo cyclization as well as the role of stabilization and polar factors has been studied. The substituent effect on the 5-(pi-exo)exo cyclization has been estimated by the relative rate constants, k(R), calculated for several substituents in the 4- and 3-position. Results show that stabilization and polar factors slightly affect the rate of the vinyl radical cyclization onto arene rings, which appears to be rather unselective with respect to the nature of the substituent. The nature of polar effects indicates that vinyl radicals are slightly electrophilic in character.