Manganese-Catalyzed Oxidative Azidation of Cyclobutanols: Regiospecific Synthesis of Alkyl Azides by CC Bond Cleavage

Palladium-Catalyzed Stagewise Strain-Release-Driven C-C Activation of Bicyclo[1.1.1]pentanyl Alcohols

A palladium-catalyzed chemoselective coupling of readily available bicyclo[1.1.1]pentanyl alcohols (BCP-OH) with various halides is reported, which offers expedient approaches to a wide range of cyclobutanone and β,γ-enone skeletons via single or double C-C activation. The chemistry shows a broad substrate scope in terms of both the range of BCP-OH and halides including a series of natural product derivatives. Moreover, dependency of reaction chemodivergence on base additive has been investigated through experimental and density functional theory (DFT) studies, which is expected to significantly enrich the reaction modes and increase the synthetic potential of BCP-OH in preparing more complex molecules.

Selective Degradation of Styrene-Related Plastics Catalyzed by Iron under Visible Light*

Efficient degradation of plastics, the vital challenge for a sustainable future, stands in need of better chemical recycling procedures that help produce commercially valuable small molecules and redefine plastic waste as a rich source of chemical feedstock. However, the corresponding chemical recycling methods, while being generally restricted to polar polymers, need improvement. Particularly, degradation of chemically inert nonpolar polymers, the major constitutes of plastics, suffers from low selectivity and very harsh transformation conditions. Herein, an efficient method was developed for selective degradation of styrene-related plastics under gentle conditions through multiple oxidation of sp³ C-H bonds and sp³ C-C bonds. The procedure was catalyzed with inexpensive iron salts under visible light, using oxygen as green oxidant. Furthermore, simple iron salts could be used to degrade plastics in the absence of solvent under natural conditions, highlighting the potential application of iron salts as additives for degradable plastics.

Enantioselective Cleavage of Cyclobutanols Through Ir-Catalyzed C-C Bond Activation: Mechanistic and Synthetic Aspects

The Ir-catalyzed conversion of prochiral tert-cyclobutanols to β-methyl-substituted ketones proceeds under comparably mild conditions in toluene (45-110 °C) and is particularly suited for the enantioselective desymmetrization of β-oxy-substituted substrates to give products with a quaternary chirality center with up to 95 % ee using DTBM-SegPhos as a chiral ligand. Deuteration experiments and kinetic isotope effect measurements revealed major mechanistic differences to related RhI -catalyzed transformations. Supported by DFT calculations we propose the initial formation of an IrIII hydride intermediate, which then undergoes a β-C elimination (C-C bond activation) prior to reductive C-H elimination. The computational model also allows the prediction of the stereochemical outcome. The Ir-catalyzed cyclobutanol cleavage is broadly applicable but fails for substrates bearing strongly coordinating groups. The method is of particular value for the stereo-controlled synthesis of substituted chromanes related to the tocopherols and other natural products.

Photocatalytic Aerobic Oxidative Ring Expansion of Cyclic Ketones to Macrolactones by Cerium and Cyanoanthracene Catalysis

We describe a cerium-catalyzed aerobic oxidative ring expansion for the expedient construction of synthetically challenging macrolactones under visible-light conditions. Cyanoanthracene has been employed as co-catalyst to accelerate the turnover of the cerium cycle leading to a fast conversion within 20 min of irradiation. Taking advantage of the high efficiency and operationally simple conditions, a collection of over 100 macrolactones equipped with ring systems ranging from 9- to 19-membered macrocycles have been prepared from simple building blocks. Moreover, the enabling potential of this strategy to simplify the generation of molecular complexity has been demonstrated through the concise synthesis of sonnerlactone.

TFA-Catalyzed [3+2] Spiroannulation of Cyclobutanols: A Route to Spiro[cyclobuta[a]indene-7,1'-cyclobutane] Skeletons

A straightforward method for the synthesis of spiro[cyclobuta[a]indene-7,1'-cyclobutane] derivatives from cyclobutanols has been developed via one-pot [3+2] spiroannulation. A series of new spiro[cyclobuta[a]indene-7,1'-cyclobutane] derivatives are facilely synthesized in good yields under mild reaction conditions.

DMSO-Enabled Selective Radical O-H Activation of 1,3(4)-Diols

Control of selectivity is one of the central topics in organic chemistry. Although unprecedented alkoxyl-radical-induced transformations have drawn a lot of attention, compared to selective C-H activation, selective radical O-H activation remains less explored. Herein, we report a novel selective radical O-H activation strategy of diols by combining spatial effects with proton-coupled electron transfer (PCET). It was found that DMSO is an essential reagent that enables the regioselective transformation of diols. Mechanistic studies indicated the existence of the alkoxyl radical and the selective interaction between DMSO and hydroxyl groups. Moreover, the distal C-C cleavage was realized by this selective alkoxyl-radical-initiation protocol.

Radical Dehydroxymethylative Fluorination of Carbohydrates and Divergent Transformations of the Resulting Reverse Glycosyl Fluorides

A mild and convenient method for the synthesis of reverse glycosyl fluorides (RGFs) has been developed that is based on the silver-promoted radical dehydroxymethylative fluorination of carbohydrates. A salient feature of the reaction is that furanoid and pyranoid carbohydrates furnish structurally diverse RGFs bearing a wide variety of functional groups in good to excellent yields. Intramolecular hydrogen atom transfer experiments revealed that the reaction involves an underexploited radical fluorination that proceeds via β-fragmentation of sugar-derived primary alkoxyl radicals. Structurally divergent RGFs were obtained by catalytic C-F bond activation, and our method thus offers a concise and efficient strategy for the synthesis of reverse glycosides by late-stage diversification of RGFs. The potential of this method is showcased by the preparation and diversification of sotagliflozin, leading to the discovery of a promising SGLT2 inhibitor candidate.

Iron-Catalysed Remote C(sp3 )-H Azidation of O-Acyl Oximes and N-Acyloxy Imidates Enabled by 1,5-Hydrogen Atom Transfer of Iminyl and Imidate Radicals: Synthesis of γ-Azido Ketones and β-Azido Alcohols

In the presence of a catalytic amount of iron(III) acetylacetonate [Fe(acac)₃ ], the reaction of structurally diverse ketoxime esters with trimethylsilyl azide (TMSN₃ ) afforded γ-azido ketones in good to excellent yields. This unprecedented distal γ-C(sp³ )-H bond azidation reaction went through a sequence of reductive generation of an iminyl radical, 1,5-hydrogen atom transfer (1,5-HAT) and iron-mediated redox azido transfer to the translocated carbon radical. TMSN₃ served not only as a nitrogen source to functionalise the unactivated C(sp³ )-H bond, but also as a reductant to generate the catalytically active Fe^II species in situ. Based on the same principle, a novel β-C(sp³ )-H functionalisation of alcohols via N-acyloxy imidates was subsequently realised, leading, after hydrolysis of the resulting ester, to β-azido alcohols, which are important building blocks in organic and medicinal chemistry.

Alkylation of Allyl/Alkenyl Sulfones by Deoxygenation of Alkoxyl Radicals

A challenging deoxygenation of alkoxyl radicals from readily accessible alcohol derivatives was developed, affording facile synthesis of functionalized alkenes with good functional group tolerance under mild reaction conditions. Because alkoxyl radicals can easily undergo β-fragmentations or hydrogen abstractions, this new strategy for deoxygenation of alkoxyl radicals is highly valuable. Moreover, mechanistic studies revealed that the electron-neutral phosphine acts as the deoxygenation reagent.

Synthesis of Chlorotrifluoromethylated Pyrrolidines by Electrocatalytic Radical Ene-Yne Cyclization

The stereoselective synthesis of chlorotrifluoromethylated pyrrolidines was achieved using anodically coupled electrolysis, an electrochemical process that combines two parallel oxidative events in a convergent and productive manner. The bench-stable and commercially available solids CF₃ SO₂ Na and MgCl₂ were used as the functional group sources to generate CF₃^. and Cl^. , respectively, via electrochemical oxidation, and the subsequent reaction of these radicals with the 1,6-enyne substrate was controlled with an earth-abundant Mn catalyst. In particular, the introduction of a chelating ligand allowed for the ene-yne cyclization to take place with high stereochemical control over the geometry of the alkene group in the pyrrolidine product.

Photoinduced Remote Functionalisations by Iminyl Radical Promoted C-C and C-H Bond Cleavage Cascades

A photoinduced cascade strategy leading to a variety of differentially functionalised nitriles and ketones has been developed. These reactions rely on the oxidative generation of iminyl radicals from simple oximes. Radical transposition by C(sp3 )-(sp3 ) and C(sp3 )-H bond cleavage gives access to distal carbon radicals that undergo SH 2 functionalisations. These mild, visible-light-mediated procedures can be used for remote fluorination, chlorination, and azidation, and were applied to the modification of bioactive and structurally complex molecules.

Tertiary-Alcohol-Directed Functionalization of Remote C(sp3 )-H Bonds by Sequential Hydrogen Atom and Heteroaryl Migrations

Reported for the first time is a tertiary-alcohol-guided heteroarylation of remote C(sp³ )-H bonds. The mild and direct generation of alkoxyl radicals from alcohols is enabled by visible-light photocatalysis. A remote hydrogen atom and heteroaryl migration sequence are involved in the reaction. Many sensitive groups remain intact in the reaction, thus illustrating wide functional-group compatibility. This protocol provides a practical strategy for the late-stage modification of alkyl ketones.

Metal-Catalysed Azidation of Organic Molecules

The azide moiety is a desirable functionality in organic molecules, useful in a variety of transformations such as olefin aziridination, C-H bond amination, isocyanate synthesis, the Staudinger reaction and the formation of azo compounds. To harness the versatility of the azide functionality fully it is important that these compounds be easy to prepare, in a clean and cost-effective manner. Conventional (non-catalysed) methods to synthesise azides generally require quite harsh reaction conditions that are often not tolerant of functional groups. In the last decade, several metal-catalysed azidations have been developed in attempts to circumvent this problem. These methods are generally faster, cleaner and more functional-group-tolerant than conventional methods to prepare azides, and can sometimes even be conveniently combined with one-pot follow-up transformations of the installed azide moiety. This review highlights metal-catalysed approaches to azide synthesis, with a focus on the substrate scopes and mechanisms, as well as on advantages and disadvantages of the methods. Overall, metal-catalysed azidation reactions provide shorter routes to a variety of potentially useful organic molecules containing the azide moiety.