*Review* **Progress on the Cu-Catalyzed 1,4-Conjugate Addition to Thiochromones**

**Fenghai Guo 1,2,\* , Jayla A. Young <sup>1</sup> , Mina S. Perez <sup>1</sup> , Holden A. Hankerson <sup>1</sup> and Alex M. Chavez <sup>1</sup>**


**Abstract:** Carbon–carbon bond formation is one of the most important tools in synthetic organic chemists' toolbox. It is a fundamental transformation that allows synthetic chemists to synthesize the carbon framework of complex molecules from inexpensive simple starting materials. Among the many synthetic methodologies developed for the construction of carbon–carbon bonds, organocopper reagents are one of the most reliable organometallic reagents for this purpose. The versatility of organocuprate reagents or the reactions catalyzed by organocopper reagents were demonstrated by their applications in a variety of synthetic transformations including the 1,4-conjugate addition reactions. Sulfur-containing heterocyclic compounds are a much less studied area compared to oxygen-containing heterocycles but have gained more and more attention in recent years due to their rich biological activities and widespread applications in pharmaceuticals, agrochemicals, and material science. This paper will provide a brief review on recent progress on the synthesis of an important class of sulfur-heterocycles-2-alkylthiochroman-4-ones and thioflavanones via the conjugate additions of Grignard reagents to thiochromones catalyzed by copper catalysts. Recent progress on the synthesis of 2-substituted thiochroman-4-ones via alkynylation and alkenylation of thiochromones will also be covered in this review.

**Citation:** Guo, F.; Young, J.A.; Perez, M.S.; Hankerson, H.A.; Chavez, A.M. Progress on the Cu-Catalyzed 1,4-Conjugate Addition to Thiochromones. *Catalysts* **2023**, *13*, 713. https://doi.org/10.3390/ catal13040713

Academic Editors: Yongjun Ji, Liwen Xing and Ke Wu

Received: 23 March 2023 Revised: 5 April 2023 Accepted: 6 April 2023 Published: 8 April 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

**Keywords:** organocopper reagents; Cu catalysts; 1,4-conjugate addition; sulfur heterocycles; thiochromone; thioflavanones; 2-alkylthiochroman-4-ones
