Redox-Neutral Intramolecular Dearomative Spirocyclization of Phenols Induced by Visible Light

Iodoarene Activation: Take a Leap Forward toward Green and Sustainable Transformations

Constructing chemical bonds under green sustainable conditions has drawn attention from environmental and economic perspectives. The dissociation of (hetero)aryl-halide bonds is a crucial step of most arylations affording (hetero)arene derivatives. Herein, we summarize the (hetero)aryl halides activation enabling the direct (hetero)arylation of trapping reagents and construction of highly functionalized (hetero)arenes under benign conditions. The strategies for the activation of aryl iodides are classified into (a) hypervalent iodoarene activation followed by functionalization under thermal/photochemical conditions, (b) aryl-I bond dissociation in the presence of bases with/without organic catalysts and promoters, (c) photoinduced aryl-I bond dissociation in the presence/absence of organophotocatalysts, (d) electrochemical activation of aryl iodides by direct/indirect electrolysis mediated by organocatalysts and mediators acting as electron shuttles, and (e) electrophotochemical activation of aryl iodides mediated by redox-active organocatalysts. These activation modes result in aryl iodides exhibiting diverse reactivity as formal aryl cations/radicals/anions and aryne precursors. The coupling of these reactive intermediates with trapping reagents leads to the facile and selective formation of C-C and C-heteroatom bonds. These ecofriendly, inexpensive, and functional group-tolerant activation strategies offer green alternatives to transition metal-based catalysis.

Visible-Light-Mediated Cascade 1,4-Hydrogen Atom Transfer versus Dearomative Spirocyclization of N-Benzylacrylamides: Divergent Access to Functionalized γ-Ketoamides and 2-Azaspiro[4.5]decanes

Visible-light-driven metal- and photocatalyst-free cascade 1,4-HAT and dearomative spirocyclization of N-benzylacrylamides are described for sustainable synthesis of a variety of pharmaceutically important γ-ketoamides and 2-Azaspiro[4.5]decanes in one pot in good to excellent yields. Readily accessible and nontoxic materials, expensive Ir or Ru photocatalyst-free mild conditions, excellent functional group tolerance, operational simplicity, and scalability enhance the practical value of this protocol. Mechanistic studies reveal that acyl radicals generated from α-oxocarboxylic acids trigger the rare 1,4-HAT and dearomative spirocyclization. The synthetic potential of this environmentally benign method is further showcased by late-stage functionalization of drug molecules, amino acid, and peptides.

Electron-Donor-Mediated Divergent Transformation of Br-RF via EDA Complex for the Synthesis of Fluorine-Containing Oxindoles and Amides

We have developed an unprecedented electron-donor-controlled divergent reaction between N-alkylsulfonylated acrylamides and Br-RF, facilitated by an electron donor-acceptor (EDA) complex, for the synthesis of fluorine-containing oxindoles and amides. Key to this divergent transformation is the EDA complex generation by altering electron donors. This approach has several advantages, such as broad substrate scope, being metal-catalyst- and photocatalyst-free, and having good functional group tolerance.

DFT Investigation on Palladium-Catalyzed [2 + 2 + 1] Spiroannulation between Aryl Halides and Alkynes: Mechanism, Base Additive Role, and Solvent and Ligand Effects

Transition metal-catalyzed spiroannulations are practical strategies for constructing spirocyclic skeletons of pharmaceutical and biological significance, yet the microscopic mechanism still lacks in-depth explorations. Here, the palladium-catalyzed [2 + 2 + 1] spiroannulation between aryl halides and alkynes was studied by employing the density functional theory (DFT) method. Based on comprehensive explorations on a couple of possible reaction pathways, it is found that the reaction probably experiences C-I oxidative addition, alkyne migration insertion, Cs2CO3-assisted aryl C-H activation, C-Br bond oxidative addition, C-C coupling, arene dearomatization and reductive elimination in sequence and leads to the formation of the spiro[4,5]decane pentacyclic product (P) ultimately. Among these, the C-Br bond oxidative addition step acts as the rate-determining step (RDS) of the whole reaction, featuring a practical free energy barrier of 32.4 kcal·mol-1 at 130 °C. Computationally predicted kinetics such as half-life transferred from the RDS step's barrier on the optimal reaction pathway (1.2 × 101 h) coincides well with corresponding experimental results (91% yield of the spiro[4,5]decane pentacyclic product P after reacting 10 h at 130 °C). In addition, theoretical predictions regarding the solvent/ligand effects and base additive role in the reaction, rationalized by distortion-interaction/natural population/noncovalent interaction analyses, are also in good agreement with experimental data and trend. This good agreement between experiment and theory makes sense for new designations and further experimental improvements of such Pd-catalyzed transformations.

Halogen-Bond-Promoted Direct Cross-Coupling of Trifluoromethylated Alkyl Bromides with Coumarins/Quinolinones: Unraveling Trifluoromethyl Effects

This study introduces a novel approach involving XB-mediated cross-coupling of α-trifluoromethylated alkyl bromides with coumarins and quinolinones under visible light irradiation. Both density functional theory (DFT) calculations and experimental studies converge to suggest that the noncovalent interaction between alkyl bromides and DMAP, intensified by the α-trifluoromethyl group, plays a pivotal role in facilitating this chemoselective reaction.