Alkylboronic acids as alkylating agents: photoredox-catalyzed alkylation reactions assisted by K3PO4

Visible-Light-Mediated Three-Component Alkene 1,2-Alkylpyridylation Reaction Using Alkylboronic Acids as Radical Precursors for the Synthesis of 4-Alkylpyridines

We report the photocatalyzed three-component alkene 1,2-alkylpyridylation reaction between alkylboronic acids, 4-cyanopyridine, and an olefin to achieve the pyridination and alkylation of the olefin and the synthesis of structurally diversified 4-alkylpyridines. The readily available and easily manipulated alkylboronic acids were used as alkyl radical precursors. The reactions take place under mild conditions with a broad substrate scope and are easy to scale up to gram level, and they are therefore of potential practical value for the synthesis and structural modification of biologically active alkylpyridine derivatives.

Visible-Light-Induced Synergistic W/Cr Catalyzed gem-Difluoroallylation of Unactivated Alkanes

Currently, the scope of the Nozaki-Hiyama-Kishi (NHK) reaction is limited to aldehydes and ketones to construct alcohol derivatives. Herein, we have described a visible-light-induced synergistic W/Cr(III)-catalyzed NHK-type gem-difluoroallylation reaction of unactivated cyclic and linear alkanes. The reaction merits feedstock materials, mild reaction conditions, and a wide functionality tolerance. Mechanistic studies imply the favorable reduction of CrCl3 to CrCl2 by reduced decatungstate W10O325-, thus closing the catalytic cycle.

Nickel/Photoredox Catalyzed Aryl-Alkyl Cross-Coupling with Alkyl Boronic Esters as Radical Precursors

Nickel/photoredox dual catalyzed cross-coupling of aryl halides with alkylboron compounds is one of the effective methodologies for the construction of C(sp2)-C(sp3) bonds. Although elegant results have been achieved by using alkyl trifluoroborates as alkyl radical precursors, the generation of alkyl radicals from readily available alkyl boronic esters is still limited due to their high oxidation potential. We disclosed here that activation of alkyl boronic esters by MeOLi is highly efficient for the generation of alkyl radicals under photocatalysis conditions. The reaction featured with a wide substrate scope, high functional group tolerance, and late-stage modification of bioactive substances. In addition, the method was also successfully extended to alkyl boronic acids. Experimental and computational mechanistic studies indicated that the cross-coupling likely proceeds via a Ni(I)-catalyzed pathway.

Photoredox-Catalyzed Synthesis of 3,3-Difluoro-γ-lactams via 1,5-Hydrogen Atom Transfer-Involved Alkyne Difunctionalization and C-N Cleavage

This manuscript describes the application of a 1,5-hydrogen atom transfer strategy in photoredox-catalyzed hydrodifluoroalkylation of alkynes. The approach utilizes a sequential cascade process of difluoroalkylation, 1,5-hydrogen atom transfer, C(sp3)-N cleavage, and intramolecular condensation cyclization to accomplish efficient [3 + 2] cycloaddition of readily available propargylamines with halodifluoroacetates (or halodifluoroacetamides). The reaction allows for the selective construction of polysubstituted 3,3-difluoro-γ-lactams with good functional group tolerance, and further transformations of the resulting 3,3-difluoro-γ-lactams into different building blocks are also accomplished.

Visible-light-driven synthesis of alkenyl thiocyanates: novel building blocks for assembly of diverse sulfur-containing molecules

The first visible-light-induced protocol for the general preparation of alkenyl thiocyanates from alkenyl bromides is presented. The reaction is simple, proceeds under very mild conditions and demonstrates broad functional group tolerance. Additionally, a flow protocol was developed to enable efficient scale-up of alkenyl thiocyanate synthesis, further enhancing the practicality and value of the method. Importantly, these alkenyl thiocyanates serve as valuable building blocks for the construction of diverse families of sulfur-containing molecules through trifluoromethylations, cycloadditions, oxidations, and C-S or P-S bond forming reactions.

Bromine radical release from a nickel-complex facilitates the activation of alkyl boronic acids: a boron selective Suzuki-Miyaura cross coupling

In this study, without utilizing any exogenous activator or strong oxidants, we successfully employed inactivated and easily accessible alkyl boronic acids (BAs) as coupling partners in a photocatalyzed Suzuki-Miyaura reaction under batch and continuous-flow conditions. Detailed mechanistic studies suggest a unique BA activation pathway, via a plausible radical transfer event between a bromine radical (formed in situ via a photo-induced homolysis of the Ni-Br bond) and the empty p-orbital on the boron atom. Subsequently, the necessity to tune the BA oxidation potential by means of hydrogen-bonding interaction with solvents or Lewis acid-base type interactions is replaced by a novel halogen radical transfer (XRT) mechanism. The mechanistic hypothesis has been supported by both control experiments and DFT calculations.

Deboronative functionalization of alkylboron species via a radical-transfer strategy

With advances in organoboron chemistry, boron-centered functional groups have become increasingly attractive. In particular, alkylboron species are highly versatile reagents for organic synthesis, but the direct generation of alkyl radicals from commonly used, bench-stable boron species has not been thoroughly investigated. Herein, we describe a method for activating C-B bonds by nitrogen- or oxygen-radical transfer that is applicable to alkylboronic acids and esters and can be used for both Michael addition reactions and Minisci reactions to generate alkyl or arylated products.

Photoredox-Catalyzed Sequential Decarboxylative/Defluorinative Aminoalkylation of CF3-Alkenes with N-Arylglycines

A photoredox-catalyzed sequential decarboxylative/defluorinative aminoalkylation of CF3-alkenes with N-arylglycines is described. This metal-free and redox-neutral protocol provided efficient access to the monofluoroalkenyl-1,5-diamines in good yields with excellent functional group compatibility. Mechanistic studies revealed that the reaction proceeds via a radical pathway with the gem-difluoroalkenyl amine as an intermediate.

Cooperative Photoredox and N-Heterocyclic Carbene Catalysis Suzuki-Miyaura-Type Reaction: Radical Coupling of Aroyl Fluorides and Alkyl Boronic Acids

An intermolecular Suzuki-Miyaura-type reaction of benzoyl fluorides with alkyl boronic acids to synthetic ketone was revealed by cooperative N-heterocyclic carbene (NHC) and photoredox catalysis. Various alkyl boric acids can be converted into alkyl radicals without external oxidants or activators. Moreover, the catalytic system was feasible for the difunctionalization of styrenes via a radical relay process. Mechanistic experiments suggested that the benzoate anion intermediate might play a unique role in this reaction system.

Solvent-Controllable C-F Bond Activation for Masked Formylation of α-Trifluoromethyl Alkenes via Organo-Photoredox Catalysis

A solvent-controllable organo-photoredox-catalyzed C-F bond activation for masked formylation of α-trifluoromethyl alkenes with low-priced 1,3-dioxolane as masked formyl radical equivalent has been described. Consequently, a diversity of masked formylated gem-difluoroalkenes and monofluoroalkenes are constructed in moderate to high yields. This approach merits readily available starting materials, mild reaction conditions, and broad substrate scope. The feasibility of this approach has been highlighted by the one-pot masked formylation/hydrolysis sequence to form γ,γ-difluoroallylic aldehydes and late-stage modification of pharmaceutical and natural product derivatives.

Formation of C-B, C-C, and C-X Bonds from Nonstabilized Aryl Radicals Generated from Diaryl Boryl Radicals

With the development of organoboron chemistry, boron-centered radicals have become increasingly attractive. However, their synthetic applications remain limited in that they have been used only as substrates for addition reactions or as initiators for catalytic reactions. We have achieved a new reaction pathway in which tetraarylborate salts are used as precursors for aryl radicals via boron radicals, by introducing a simple activation reagent. In addition, we carried out a diverse array of transformations involving these aryl radical precursors, which allowed the construction of new C-B, C-C, and C-X bonds in the presence of visible light.

Base-Mediated α-gem-Difluoroalkenylations of Aldehydes and Ketones

Herein, we present a base-mediated nucleophilic substitution reaction of α-trifluoromethylstyrenes with simple silyl enol ethers, enabling the efficient synthesis of carbonyl-substituted gem-difluoroalkenes. The merit of this protocol is exhibited by its mild reaction conditions, broad substrate scope, and scalable preparation. Notably, this method demonstrates its applicability for late-stage functionalization of structurally complex molecules. Moreover, we illustrate that the resulting products can serve as valuable precursors for the synthesis of diverse medicinally relevant compounds.