A Rh­(III)-catalyzed annulative coupling of 3,5-diarylisothiazoles and alkynes is reported. The N–S bond in the isothiazole ring acts as an internal oxidant to regenerate the Rh­(III) species in combination with an external Cu­(II) oxidant, and the corresponding 1:2 coupling products are obtained. The remarkable difference in the reaction outcome between isothiazoles and the relevant isoxazoles has been investigated by DFT calculations, revealing that the relative stability of the enolate intermediates dictates the product selectivity.

A rhodium-catalyzed cascade annulative coupling of 3,5-di­arylisoxazoles with three equivalents of an alkyne proceeds smoothly in the presence of a Cu(II) oxidant, where the sequential construction of isoquinoline and naphtho[1,8-bc]pyran frameworks connected by a biaryl linkage is achieved by a single operation. Most of the obtained polycyclic compounds exhibit visible fluorescence in both the solution and the solid state. The hexaphenylated isoquinoline-naphthopyran conjugate (R = Ph) as a representative product shows a green emission which can be turned off by making an isoquinolinium salt with an acid. The emission is also reversibly turned on by treatment with a base.

Direct annulative coupling of 3-aryl-1,2-benzisoxazoles and alkynes efficiently proceeds in the presence of a Cp*Rh(III) catalyst to produce 2-(1-isoquinolinyl)phenols of interest in medicinal chemistry as well as materials chemistry. The products may also be useful precursors of quinoline-based bidentate ligands.

No abstract