Acta Chimica Sinica ›› 2019, Vol. 77 ›› Issue (9): 889-894.DOI: 10.6023/A19050173 Previous Articles     Next Articles

Special Issue: 有机自由基化学

Communication

路易斯碱-硼自由基促进的邻苯二甲酰亚胺类羧酸酯的Giese反应和Barton脱羧反应研究

靳继康, 张凤莲*(), 汪义丰*()   

  1. 中国科学技术大学化学系 合肥 230026
  • 收稿日期:2019-05-13 出版日期:2019-09-15 发布日期:2019-07-09
  • 通讯作者: 张凤莲,汪义丰 E-mail:zfl9@ustc.edu.cn;yfwangzj@ustc.edu.cn
  • 基金资助:
    项目受国家自然科学基金(21672195);项目受国家自然科学基金(21702201);中央高校基本科研业务费专项资金资助

Lewis Base-Boryl Radical Enabled Giese Reaction and Barton Decarboxylation of N-Hydroxyphthalimide (NHPI) Esters

Jin, Jikang, Zhang, Fenglian*(), Wang, Yifeng*()   

  1. Department of Chemistry, University of Science and Technology of China, Hefei 230026
  • Received:2019-05-13 Online:2019-09-15 Published:2019-07-09
  • Contact: Zhang, Fenglian,Wang, Yifeng E-mail:zfl9@ustc.edu.cn;yfwangzj@ustc.edu.cn
  • Supported by:
    Project supported by the National Natural Science Foundation of China(21672195);Project supported by the National Natural Science Foundation of China(21702201);the Fundamental Research Funds for the Central Universities

Decarboxylation of N-hydroxyphthalimide (NHPI) esters represents a powerful tool to generate carbon radicals, which has wide applications in the construction of C—C bonds and C—X bonds. Traditionally, the radical decarboxylation of NHPI esters has been enabled by transition-metal catalysis and photoredox catalysis. Recently, several visible light-mediated photosensor-free decarboxylation reactions have been reported with the use of special electron-donor systems. While notable, it’s still highly desirable to develop transition-metal-free, mild, and general methods to realize the radical decarboxylation of NHPI esters. Herein, we report 4-dimethylaminopyridine (DMAP)-boryl radical enabled Giese reaction and Barton decarboxylation of NHPI esters. The reaction starts from the generation of DMAP-boryl radical in the presence of a radical initiator, which then adds specifically to the carbonyl oxygen atom of NHPI ester 2, followed by β-fragmentation to give a nucleophilic carbon radical intermediate. Addition of the carbon radical to electron-deficient alkenes affords the Giese reaction product 4. On the other hand, hydrogen atom transfer from thiol to the nucleophilic carbon radical results in the Barton decarboxylation products 5. The reactions exhibit a broad substrate scope and excellent functional group tolerance. NHPI esters of primary, secondary, and tertiary alkyl carboxylic acids, including bio-active natural products and drugs, proceed smoothly to give the corresponding products in moderate to good yields. A variety of electron-deficient alkenes, such as vinyl esters, vinyl amides and vinyl sulphones, can be used as the Michael acceptors. A general procedure for the Giese reaction is as following: a solution of NHPI ester 2 (0.5 mmol), 4-dimethylaminopyridine-borane (0.6 mmol), AIBN (0.1 mmol) and electron- deficient alkenes 3 (0.4 mmol) in toluene (4.0 mL) was stirred at 80 ℃ for 4 h under nitrogen atmosphere. After evaporation of solvent, the crude residue was purified by flash column chromatography on silica gel (petroleum ether/ethyl acetate) to afford Giese reaction product 4. A general procedure for the Barton decarboxylation is as following: a solution of NHPI ester 2 (0.5 mmol), 4-dimethylaminopyridine-borane (0.55 mmol), TBHN (0.1 mmol) and PhSH (0.1 mmol) in benzotrifluoride (5.0 mL) was stirred at 80 ℃ for 1 h under nitrogen atmosphere. After evaporation of solvent, the crude residue was purified by flash column chromatography on silica gel (petroleum ether/ethyl acetate) to afford decarboxylative reduction product 5.

Key words: 4-dimethylaminopyridine-boryl radical, radical decarboxylation, Giese reaction, Barton decarboxylative reduction, radical chemistry