Ligand-Controlled Palladium-Catalyzed Pyridylation of 1-tert-Butoxycarbonyl-3-iodoazetidine: Regioselective Synthesis of 2- and 3-Heteroarylazetidines

Visible light-mediated aza Paternò-Büchi reaction of acyclic oximes and alkenes to azetidines

The aza Paternò-Büchi reaction is a [2+2]-cycloaddition reaction between imines and alkenes that produces azetidines, four-membered nitrogen-containing heterocycles. Currently, successful examples rely primarily on either intramolecular variants or cyclic imine equivalents. To unlock the full synthetic potential of aza Paternò-Büchi reactions, it is essential to extend the reaction to acyclic imine equivalents. Here, we report that matching of the frontier molecular orbital energies of alkenes with those of acyclic oximes enables visible light-mediated aza Paternò-Büchi reactions through triplet energy transfer catalysis. The utility of this reaction is further showcased in the synthesis of epi-penaresidin B. Density functional theory computations reveal that a competition between the desired [2+2]-cycloaddition and alkene dimerization determines the success of the reaction. Frontier orbital energy matching between the reactive components lowers transition-state energy (ΔGǂ) values and ultimately promotes reactivity.

Desymmetrization of Vicinal Bis(boronic) Esters by Enantioselective Suzuki-Miyaura Cross-Coupling Reaction

The development of an enantioselective catalytic Suzuki-Miyaura reaction that applies to meso 1,2-diborylcycloalkanes is described. This reaction provides a modular route to enantiomerically enriched substituted carbocycles and heterocycles that retain a synthetically versatile boronic ester. With appropriately constructed substrates, compounds bearing additional stereogenic centers and fully substituted carbon atoms can be generated in a straightforward fashion. Preliminary mechanistic experiments suggest that substrate activation arises from the cooperative effect of vicinal boronic esters during the transmetalation step.

Synthesis of 2-Aryl Azetidines through Pd-Catalyzed Migration/Coupling of 3-Iodoazetidines and Aryl Boronic Acids

A palladium-catalyzed cross-coupling of 3-iodoazetidines and nonheteroaryl boronic acids was reported. The [1,1'-biphenyl]-2-yldicyclohexylphosphane ligand enabled the reaction that favored the formation of 2-aryl azetidines. The control experiments indicated that the reaction can proceed through either a palladium-hydride/dihydroazete complex or free dihydroazete intermediate followed by hydropalladation.

Electrochemical ammonium-cation-assisted pyridylation of inert N-heterocycles via dual-proton-coupled electron transfer

A straightforward and practical strategy for pyridylation of inert N-heterocycles, enabled by ammonium cation and electrochemical, has been described. This protocol gives access to various N-fused heterocycles and bidentate nitrogen ligand compounds, through dual-proton-coupled electron transfer (PCET) and radical cross-coupling in the absence of exogenous metal and redox reagent. It features broad substrate scope, wide functional group tolerance, and easy gram-scale synthesis. Various experiments and density functional theory (DFT) calculation results show the mechanism of dual PCET followed by radical cross-coupling is the preferred pathway. Moreover, ammonium salt plays the dual role of protonation reagent and electrolyte in this conversion, and the resulting product 9-(pyridin-4-yl)acridine compound can be used for fluorescence recognition of Fe²⁺ and Pd²⁺ with high sensitivity.

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Electrochemical NH₄⁺-assisted dual PCET followed by the radical cross-coupling

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Straightforward and practical synthetic route for N-fused heterocycles

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Fluorescence recognition of Fe²⁺ and Pd²⁺ with high-sensitivity

Palladium-Catalyzed Hiyama Cross-Couplings of Arylsilanes with 3-Iodoazetidine: Synthesis of 3-Arylazetidines

The first palladium-catalyzed Hiyama cross-coupling reactions of arylsilanes with 3-iodoazetidine were described. The protocol provides a convenient access to a variety of useful 3-arylazetidines which are of great interest in pharmaceutical laboratories in moderate to good yields (30%-88%). In addition, this strategy has the advantage of easy operation and mild reaction conditions.

Pd-Catalyzed Alkylation of (Iso)quinolines and Arenes: 2-Acylpyridine Compounds as Alkylation Reagents

The first Pd-catalyzed alkylation of (iso)quinolines and arenes is reported. The readily available and bench-stable 2-acylpyridine compounds were used as an alkylation reagent to form the structurally versatile alkylated (iso)quinolines and arenes. The method affords a convenient pathway for the introduction of alkyl groups into organic molecules.