Bu4NI-Catalyzed, Radical-Induced Regioselective N-Alkylations and Arylations of Tetrazoles Using Organic Peroxides/Peresters

Metal-Free Bisamidation of N-Tosylhydrazones with Carboxylic Acids Promoted by Tetrabutylammonium Iodide and tert-Butyl Hydroperoxide

A versatile reaction between N-tosylhydrazones and carboxylic acids to access bisamides is reported. This metal-free, room-temperature reaction was catalyzed by TBAI, while TBHP served as the oxidant. Broad substrate scope and good functional group tolerance are the key features of the strategy. Subsequent intramolecular N-arylation of suitably substituted bisamides readily afforded functionalized 3-indazolones.

Microwave-promoted radical addition/cyclization of biaryl vinyl ketones with diacyl peroxides in water under metal-free conditions

This communication reports an efficient microwave-promoted radical addition/cyclization reaction of biaryl vinyl ketones with diacyl peroxides in water under metal-free conditions. A series of 10-methyl-10-benzyl(alkyl)phenanthren-9(10H)-ones were obtained in high yields with good functional group tolerance.

Copper-Catalyzed Intermolecular Cross-dehydrogenative C-N Coupling at Room Temperature via Remote Activating Group Enabled Radical Relay Strategy

Employing N-fluorobenzenesulfonimide (NFSI) as a nitrogen-centered radical (NCR) precursor, an intermolecular C(sp²)-N coupling on heteroarenes or substituted benzenes with remote activated aniline derivatives via copper catalyzed N-N radical relay strategy at room temperature is developed. Good to excellent yields are acquired, and no ligand or additive is required. Reaction scope investigation and preliminary mechanistic studies demonstrate that the remote activating strategy and delicate control on the reactivities of active NCR species are essential to guarantee satisfactory chemo- and site-selectivity.

Synergistic Organoboron/Palladium Catalysis for Regioselective N-Allylations of Azoles with Allylic Alcohols

A method for regioselective palladium-catalyzed allylic alkylation of ambident nitrogen heterocycles, employing simple allylic alcohols as electrophile precursors, is described. An organoboron co-catalyst serves both to activate the azole-type nucleophile toward selective N-functionalization and to accelerate the formation of a π-allylpalladium complex from the allylic alcohol. The method can be applied to various heterocycle types, including 1,2,3- and 1,2,4-triazoles, tetrazoles, pyrazoles, and purines, and can be extended to substituted allylic alcohol partners.

Metal-Free Regioselective N2-Arylation of 1H-Tetrazoles with Diaryliodonium Salts

We describe a simple, metal-free regioselective N²-arylation strategy for 5-substituted-1H-tetrazoles with diaryliodonium salts to access 2-aryl-5-substituted-tetrazoles. Diaryliodonium salts with a wide range of both electron-rich and previously challenged electron-deficient aryl groups are applicable in this method. Diversely functionalized tetrazoles are tolerable also. We have devised a one-pot system to synthesize 2,5-diaryl-tetrazoles directly from nitriles. The synthetic utility of this method is furthered extended to late-stage arylation of two biologically active molecules.

Regio- and enantioselective formation of tetrazole-bearing quaternary stereocenters via palladium-catalyzed allylic amination

A general and efficient method via catalysis by readily available Pd(0)/DACH-naphthyl catalyst under mild conditions, unlocks a new platform that permits the synthesis of elusive quaternary N2-allylic tetrazoles, even in the context of late-stage functionalization.

Understanding the regioselectivity of 5-substituted 1H-tetrazoles alkylation

A novel rational to account for the selectivity of 5-substituted 1H-tetrazole alkylation.

Intermolecular CDC amination of remote and proximal unactivated Csp3 -H bonds through intrinsic substrate reactivity - expanding towards a traceless directing group

An intermolecular radical based distal selectivity in appended alkyl chains has been developed. The selectivity is maximum when the distal carbon is γ to the appended group and decreases by moving from γ → δ → ε positions. In –COO– linked alkyl chains, the same distal γ-selectivity is observed irrespective of its origin, either from the alkyl carboxy acid or alkyl alcohol. The appended groups include esters, N–H protected amines, phthaloyl, sulfone, sulfinimide, nitrile, phosphite, phosphate and borate esters. In borate esters, boron serves as a traceless directing group, which is hitherto unprecedented for any remote C_sp³–H functionalization. The selectivity order follows the trend: 3° benzylic > 2° benzylic > 3° tertiary > α to keto > distal methylene (γ > δ > ε). Computations predicted the radical stability (thermodynamic factors) and the kinetic barriers as the factors responsible for such trends. Remarkably, this strategy eludes any designer catalysts, and the selectivity is due to the intrinsic substrate reactivity.

An intermolecular amination at the distal methylene carbon has been realized in an appended alkyl chain with electron withdrawing groups. Traceless remote C_sp³–H functionalization has been accomplished using borate esters.

Flash chemistry enables high productivity metalation-substitution of 5-alkyltetrazoles

Tetrazoles play a prominent role in medicinal chemistry due to their role as carboxylate bioisosteres but have largely been overlooked as C-H functionalisation substrates. We herein report the development of a high-yielding and general procedure for the heterobenzylic C-H functionalisation of 5-alkyltetrazoles in up to 97% yield under batch conditions using a metalation/electrophilic trapping strategy. Through the use of thermal imaging to identify potentially unsafe exotherms, a continuous flow procedure using a flash chemistry strategy has also been developed, allowing products to be accessed in up to 95% yield. This enabled an extremely high productivity rate of 141 g h-1 to be achieved on an entry-level flow system.

Alkylation of 5-Substituted 1H-Tetrazoles via the Diazotization of Aliphatic Amines

A new alkylation reaction of monosubstituted tetrazoles via the diazotization of aliphatic amines is reported. This method enables preferential formation of 2,5-disubstituted tetrazoles. A one-pot 1,3-dipolar cycloaddition/diazotization sequence starting from widely available nitriles is also described. Azide residues are quenched in the second step with the nitrite reagent, thus limiting the intrinsic risk associated with trimethylsilyl azide. The reaction conditions were compatible with several functional groups, including thiocyanates, which afford preferentially disubstituted 2-alkyl-5-(substituted-thio)tetrazoles.

Diacyl peroxides: practical reagents as aryl and alkyl radical sources

Diacyl peroxides, which can be easily synthesized from corresponding carboxylic acids, are commonly utilized as radical initiators and one electron oxidants. Under thermal, transition-metal catalysis or irradiation conditions the cleavage of relatively weak O-O bonds would occur followed by CO₂ extrusion to generate the corresponding aryl or alkyl radicals. Thus, diacyl peroxides can be employed as ideal arylating and alkylating reagents in organic synthesis, including C-H/N-H arylation/alkylation, aryl/alkyl radical addition to unsaturated bonds, hetero arylation/alkylation, eliminative arylation/alkylation, perfluoroalkylation etc. Moreover, these arylation/alkylation protocols have been successfully utilized in the synthesis and late-stage functionalization of natural products as well as bioactive molecules. In this review, recent advances on arylation and alkylation using diacyl peroxides as aryl and alkyl radical sources are summarized and discussed.

Recent developments in selective N-arylation of azoles

Transition-metal based carbon-heteroatom (C-X) bond formation has attracted the attention of synthetic chemists over the past few years because the resultant aryl/heteroaryl motifs are important substructures in many natural products, pharmaceuticals, etc. Several efficient protocols such as Buchwald-Hartwig amination, Ullmann coupling, Chan-Lam coupling and metal-free approaches have proved beneficial in C-X bond formation. Selective arylation of one hetero-centre over other centres without protection/deprotection thus allowing minimum synthetic manipulation has been achieved for several substrates using these protocols. Azoles are one such novel five-membered heterocyclic core with huge pharmaceutical applications. Though N-arylation on azole-bearing analogues has been extensively practised, selective N-arylation either on one N-centre or the exocyclic N-site of the azole ring in competition with other hetero-centres in the framework has been recently explored for azole-carrying systems. Thus, this review would focus on recent advances in chemo- and regio-selective N-arylation (either on one N-centre or the exocyclic N-site of the azole ring) on azole-containing frameworks.

Photocatalytic Visible-Light-Induced Nitrogen Insertion via Dual C(sp3)-H and C(sp2)-H Bond Functionalization: Access to Privileged Imidazole-based Scaffolds

Here we have demonstrated a visible-light-mediated metal-free organic-dye-catalyzed dehydrogenative N-insertion leading to highly substituted imidazoles and privileged dihydroisoquinoline-based imidazole derivatives via C(sp³)-H and C(sp²)-H bond functionalization. A sustainable, convenient, metal-free azidation/C-H aminative cyclization approach in the absence of stoichiometric oxidants is presented. This protocol involves a rare photoinduced iminyl radical as a key intermediate for the "N" insertion.

Recent advances in reactions using diacyl peroxides as sources of O- and C-functional groups

This review summarizes recent advances in reactions utilizing diacyl peroxides as O- and C-sources, with examples illustrating how the reactivity of diacyl peroxides in organic reactions can be controlled.