Gold-Catalyzed Intermolecular Alkyne Oxyarylation for C3 Functionalization of Benzothiophenes

Metal-Free Arylation of Benzothiophenes at C4 by Activation as their Benzothiophene S-Oxides

Benzothiophenes, activated by oxidation to the corresponding S-oxides, undergo C-H/C-H-type coupling with phenols to give C4 arylation products. While an electron-withdrawing group at C3 of the benzothiophene is important, the process operates without a directing group and a metal catalyst, thus rendering it compatible with sensitive functionalities-e.g. halides and formyl groups. Quantum chemical calculations suggest a formal stepwise mechanism involving heterolytic cleavage of an aryloxysulfur species to give a π-complex of the corresponding benzothiophene and a phenoxonium cation. Subsequent addition of the phenoxonium cation to the C4 position of the benzothiophene is favored over the addition to C3; Fukui functions predict that the major regioisomer is formed at the more electron-rich position between C3 and C4. Varied selective manipulation of the benzothiophene products showcase the synthetic utility of the metal-free arylation process.

Cp*Co(III)-Catalyzed Selective C8-Olefination and Oxyarylation of Quinoline N-Oxides with Terminal Alkynes

Herein we report Cp*Co(III)-catalyzed site-selective (C8)-H olefination and oxyarylation of quinoline N-oxides with terminal alkynes. The selectivity for C8-olefination and oxyarylation is sterically and electronically controlled. In the case of quinoline N-oxides (unsubstituted at the C2 position), only the olefination product was obtained irrespective of the nature of the alkynes. In contrast, oxyarylation was observed exclusively when 2-substituted quinoline N-oxides were reacted with 9-ethynylphenanthrene. However, alkynes with electron-withdrawing groups provided only olefination products with 2-substituted quinoline N-oxides. The developed strategy allowed a facile functionalization of quinoline N-oxides bearing natural molecules and an estrone-derived terminal alkyne to deliver the corresponding olefinated and oxyarylated products. To understand the reaction mechanism, control experiments, deuterium-labeling experiments, and kinetic isotope effect (KIE) studies were performed.

Visible Light-Promoted Synthesis of β-Keto Sulfoximines from N-Tosyl-Protected Sulfoximidoyl Chlorides

Under visible light, N-tosyl-protected sulfoximidoyl chlorides react with aryl alkynes to give β-keto sulfoximines. The reaction is characterized by a high functional group tolerance and good yields. It can be improved by the presence of a ruthenium photocatalyst. Air is the source of the ketonic oxygen in the products.

One-pot gold(I)-catalyzed synthesis of 2-pyridonyl alcohols

A highly efficient method for the synthesis of 2-pyridonyl alcohols via gold(I) catalyst was developed. The effect of methanesulfonic acid on the reaction progression with the gold catalyst was determined. The proposed mechanism involves intramolecular cyclization product formation derived from 5-exo-dig and 6-exo-dig addition of the nitrogen for 2-propargyloxypyridine and 2-(but-3-yn-1-yloxy)pyridine, respectively. The pyridinium salt formed by the gold(I) catalyst and methanesulfonic acid undergoes a rearrangement reaction in a weakly basic medium (5% aq. Na₂CO₃) to form N-alkenyl pyridonyl alcohols. N-alkenyl pyridonyl alcohols can be obtained in moderate to excellent yields using this method.

Catalyst-controlled switchable [4 + 1], [4 + 3] and [3 + 2] domino reactions of azadienes and MBH carbonates: diverse synthesis of benzothiophene fused derivatives

Catalyst-controlled switchable domino reactions between azadienes bearing a benzothiophene moiety and isatin-derived MBH carbonates were developed. The [4 + 1] annulation was triggered in the presence of DABCO, giving a variety of benzothiophene fused pyrrole derivatives, while the [4 + 3] annulation occurred when changing the catalyst to DMAP. Furthermore, the [3 + 2] annulation mode was observed with the use of catalytic Ph₂PMe. Additionally, the synthetic utility of these domino reactions was demonstrated by gram-scale experiments and simple transformations of the products. To the best of our knowledge, catalyst-controlled synthesis of benzothiophene fused or spiro derivatives has rarely been reported.