NaBArF4‐Catalyzed Oxidative Cyclization of 1,5‐ and 1,6‐Diynes: Efficient and Divergent Synthesis of Functionalized γ‐ and δ‐Lactams

An efficient NaBAr^F₄‐catalyzed oxidative cyclization of readily available 1,5‐ and 1,6‐diynes has been developed. Importantly, this transition metal‐free oxidative catalysis proceeds via a presumable Lewis acid‐catalyzed S_N2’ pathway, which is distinct from the relevant oxidative rhodium and gold catalysis. This method leads to the facile and practical construction of a diverse range of synthetically useful γ‐ and δ‐lactams in mostly good to excellent yields with broad substrate scope.     

Palladium‐Catalyzed Enantioselective Reductive Heck Reactions: Convenient Access to 3,3‐Disubstituted 2,3‐Dihydrobenzofuran

The first example of highly enantioselective intramolecular hydroarylation of allyl aryl ethers was realized by palladium‐catalyzed reductive heck reactions utilizing a new chiral sulfinamide phosphine ligand (N‐Me‐ XuPhos ). N‐Me‐ XuPhos can be easily prepared on gram scale from readily available starting materials in a one‐pot synthesis approach. A series of optically active 2,3‐dihydrobenzofurans bearing a quaternary stereocenter were obtained in good yields and with excellent enantioselectivities. The practicality of this reaction was validated in the straightforward synthesis of CB2 receptor agonists. Moreover, deuterium was efficiently incorporated into the products.     

Combining Photoredox‐Catalyzed Trifluoromethylation and Oxidation with DMSO: Facile Synthesis of α‐Trifluoromethylated Ketones from Aromatic Alkenes

Trifluoromethylated ketones are useful building blocks for organic compounds with a trifluoromethyl group. A new and facile synthesis of ketones with a trifluoromethyl substituent in the α‐position proceeds through a one‐pot photoredox‐catalyzed trifluoromethylation–oxidation sequence of aromatic alkenes. Dimethyl sulfoxide (DMSO) serves as a key and mild oxidant under these photocatalytic conditions. Furthermore, an iridium photocatalyst, fac[Ir(ppy)₃] (ppy=2‐phenylpyridine), turned out to be crucial for the present photoredox process.     

Dual Hard/Soft Gold Catalysis: Intermolecular Friedel–Crafts‐Type α‐Amidoalkylation/Alkyne Hydroarylation Sequences by N‐Acyliminium Ion Chemistry

Gold catalysts have been applied in cascade‐type reactions for the synthesis of different nitrogen‐based compounds. The reactions likely proceed by a new gold‐catalyzed cascade intermolecular α‐amidoalkylation/intramolecular carbocyclization cascade process by unifying both the σ‐ and π‐Lewis acid properties of the gold salts. In the first part of this report we show that the σ‐Lewis acidity of gold(I) and gold(III) could be exploited to efficiently catalyze the nucleophilic substitution of various alkoxy‐ and acetoxylactams. The reaction was found to be applicable to a wide range of cyclic N‐acyliminium ion precursors and various nucleophiles, including allyltrimethylsilane, silyl enol ethers, arenes, and active methylene derivatives. As a logical progression of this study, a combined hard/soft binary catalytic gold system was then used to implement an unprecedented tandem intermolecular Friedel–Crafts amidoalkylation/intramolecular hydroarylation sequence allowing an expedient access to new, complex, fused polyheterocyclic structures from trivial materials.     

Hydrogenative Dearomatization of Pyridine and an Asymmetric Aza‐Friedel–Crafts Alkylation Sequence

Highly efficient synthesis of enantiomerically enriched substituted piperidines has been realized via chiral phosphoric acid catalyzed cascade hydrogenative dearomatization of substituted pyridines and aza‐Friedel‐Crafts reaction in good to excellent yields and enantioselectivity.     

Zinc‐Catalyzed Alkyne Oxidation/CH Functionalization: Highly Site‐Selective Synthesis of Versatile Isoquinolones and β‐Carbolines

An efficient zinc(II)‐catalyzed alkyne oxidation/C? H functionalization sequence was developed, thus leading to highly site‐selective synthesis of a variety of isoquinolones and β‐carbolines. Importantly, in contrast to the well‐established gold‐catalyzed intermolecular alkyne oxidation, over‐oxidation can be completely suppressed in this system and the reaction most likely proceeds by a Friedel–Crafts‐type pathway. Mechanistic studies and theoretical calculations are described.     

A Highly Efficient Friedel–Crafts Reaction of 3‐Hydroxyoxindoles and Aromatic Compounds to 3,3‐Diaryl and 3‐Alkyl‐3‐aryloxindoles Catalyzed by Hg(ClO4)2⋅3 H2O

We report a highly efficient Friedel–Crafts reaction of 3‐alkyl or 3‐aryl 3‐hydroxyoxindoles with a variety of aromatic and heteroaromatic compounds to unsymmetrical 3,3‐diaryloxindoles or 3‐alkyl‐3‐aryloxindoles, which are interesting medicinal targets and useful building blocks for the synthesis of natural products. Hg(ClO₄)₂ ? 3 H₂O was identified as a powerful catalyst for this reaction, and is significantly more efficient than other screened metal perchlorate hydrates and Brønsted acids such as HOTf and HClO₄. The high catalytic property of Hg(ClO₄)₂ ? 3 H₂O originates from the unprecedented dual activation effects of aromatic mercuration, which could generate a strong protic acid to facilitate the generation of a carbocation at the C3‐position of oxindoles and simultaneously form the more reactive nucleophilic reaction partner.     

Pd‐Catalyzed Carboamination of Alkylidenecyclopropanes: Synthesis of Congested Pyrrolidines and Piperidines

The synthesis of densely functionalized N‐heterocycles is an ongoing challenge in chemical synthesis. Herein, we report an efficient method for the construction of pyrrolidine and piperidine scaffolds using a palladium‐catalyzed carboamination of alkylidenecyclopropanes.     

Copper‐Catalyzed Azide–Ynamide Cyclization to Generate α‐Imino Copper Carbenes: Divergent and Enantioselective Access to Polycyclic N‐Heterocycles

Here an efficient copper‐catalyzed cascade cyclization of azide‐ynamides via α‐imino copper carbene intermediates is reported, representing the first generation of α‐imino copper carbenes from alkynes. This protocol enables the practical and divergent synthesis of an array of polycyclic N‐heterocycles in generally good to excellent yields with broad substrate scope and excellent diastereoselectivities. Moreover, an asymmetric azide–ynamide cyclization has been achieved with high enantioselectivities (up to 98:2 e.r.) by employing BOX‐Cu complexes as chiral catalysts. Thus, this protocol constitutes the first example of an asymmetric azide–alkyne cyclization. The proposed mechanistic rationale for this cascade cyclization is further supported by theoretical calculations.     

Rhodium(II)‐Catalyzed Cycloaddition Reactions of Non‐classical 1,5‐Dipoles for the Formation of Eight‐Membered Heterocycles

A new type of intermolecular rhodium(II)‐catalyzed [5+3] cycloaddition has been developed. This higher‐order cycloaddition between pyridinium zwitterion 1,5‐dipole equivalents and enol diazoacetates enables the formation of eight‐membered heterocyclic skeletons, which are otherwise difficult to construct. The optimized cycloaddition occurs efficiently under mild conditions with a wide range of pyridinium zwitterions and with high functional‐group tolerance.