脱氢[4 + 2]环化是构建苯并稠合六元环的一种步骤和原子经济的方法,苯并稠合六元环在天然产物、药物和农用化学品中普遍存在。在此,我们公开了苯并硫酰胺和炔烃的可见光诱导脱氢[4 + 2]成环反应,该反应在温和有氧条件下进行,并由无金属异质光催化剂石墨氮化碳(gC 3 N 4)催化。多相光催化操作简单、条件温和,进展顺利,并以良好的收率产生了多种异硫色烯。gC 3 N 4在光化学反应中表现出优异的稳定性和可回收性。详细的机制研究表明,gC3 N 4在可见光照射下通过单电子转移过程促进了以硫为中心的自由基的形成。自由基中间体进一步介导与炔烃的[4+2]环化,产生异硫色烯。
Exogenous Photosensitizer-, Metal-, and Base-Free Visible-Light-Promoted C–H Thiolation via Reverse Hydrogen Atom Transfer
作者:Ze-Ming Xu、Hong-Xi Li、David James Young、Da-Liang Zhu、Hai-Yan Li、Jian-Ping Lang
DOI:10.1021/acs.orglett.8b03679
日期:2019.1.4
This reaction induces the cyclization of thiobenzanilides to benzothiazoles. The substrate absorbs visible light, and its excited state undergoes a reverse hydrogen-atom transfer (RHAT) with 2,2,6,6-tetramethylpiperidineN-oxyl to form a sulfur radical. The addition of the sulfur radical to the benzene ring gives an aryl radical, which then rearomatizes to benzothiazole via RHAT.
Three-componentreactions involving amines, aldehydes, and elemental sulfur powder are reported to afford thioamides in a simple one-pot procedure in the absence of a catalyst. A variety of thioamides can be obtained in good to excellent yields up to 88 %.
An aerobic visible-light driven photoredox catalytic formation of 2-substitutedbenzothiazoles through radical cyclization of thioanilides has been accomplished. The reaction features C–H functionalization and C–S bond formation with no direct metal involvement except the sensitizer. The reaction highlights the following: (1) visible-light is the reaction driving force; (2) molecular oxygen is the
A metal-free approach to N-aryl thioamides fromBuntesalts and anilines in DMSO has been developed. This method tolerated a wide range of functional groups on the aromatic ring, providing an ideal way to N-aryl thioamides in good to excellent yields from cheap and easily available starting materials. A plausible mechanism was also proposed based on the X-ray single crystal diffraction, NMR and MS
Riboflavin as Photoredox Catalyst in the Cyclization of Thiobenzanilides: Synthesis of 2-Substituted Benzothiazoles
作者:Lydia M. Bouchet、Adrián A. Heredia、Juan E. Argüello、Luciana C. Schmidt
DOI:10.1021/acs.orglett.9b04384
日期:2020.1.17
Benzothiazoles are synthesized from thiobenzanilides using riboflavin as a photosensitizer and potassium peroxydisulfate as a sacrificial oxidizing agent under visible light irradiation. The methodology accepts a broad range of functional groups and affords the 2-substitutedbenzothiazoles by transition-metal-free organic photoredoxcatalysis under very mild conditions.