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N-Heterocyclic Carbene Catalysis
Published in Andrew M. Harned, Nonnitrogenous Organocatalysis, 2017
Xinqiang Fang, Yonggui Robin Chi
Chi and coworkers developed a similar strategy for the synthesis of optically pure β-hydroxyesters.31 Compared to Rovis’ work,30 a different oxidant of 109 was used, and chiral catalyst ent-13′ was the best choice for both the yields and ee. Two equivalents of enals were necessary to get higher yields. Excellent enantioselectivity (~95% ee for most products) was observed (Figure 7.26). To be noted, the radical clock experiment (Figure 7.27) supported the conclusion that the radical spin is more likely delocalized between the enal formyl carbon and the triazolium NHC unit (as showed in 113, Figure 7.27), because no ring-opening products were detected.
Hydro-trifluoromethyl(thiol)ation of alkenes: a review*
Published in Journal of Sulfur Chemistry, 2022
Seyedeh Bahareh Azimi, Manzarbanou Asnaashariisfahani, Bayan Azizi, Elham Mohammadi, Abdol Ghaffar Ebadi, Esmail Vessally
Although a number of synthetic approaches have been described for hydrotrifluoromethylation of alkenes, too little attention has been paid to hydrotrifluoromethylthiolation reactions. In 2015, Yang, Lu, and Shen reported the first hydrotrifluoromethylthiolation of unactivated alkenes 32 employing trifluoromethylthiolating reagent 33 as an unprecedented synthetic entry to alkyl(trifluoromethyl)thioethers 34 [71]. After due screening of various iron catalysts such as Fe(NO2)3•9H2O, Fe(SO4)3, Fe(acac)3, FeCl3, Fe2(ox)3 and hydrogen sources such as NaBH4, BH3•THF, PhSiH3, and Et3SiH; 1.5 equiv. of Fe(NO2)3•9H2O and 5 equiv. of BH3•THF were found to be ideal for each equivalent of alkene at 0°C. Using this synthetic strategy, various alkyl(trifluoromethyl)thioethers 34 were synthesized in moderate to excellent yields within 30 min (Scheme 19). Unfortunately, no product could be isolated when sterically hindered trisubstituted alkenes were employed as the reactants. However, the authors elegantly solved this limitation using NPhth-SCF3 as SCF3 source and replacing Fe(NO2)3•9H2O with Fe2(SO4)3, which afforded decent yields of the corresponding trifluoromethylthiolated products. In both systems, all the terminal olefins tested showed excellent Markovnikov selectivity, while the only investigated internal olefin provided the addition product with a ratio of 1:1. Regarding the reaction mechanism, as evidenced by radical-trapping and radical clock experiments, a radical process is probably involved in this reaction. Three years later, Matcha and Antonchick applied a similar principle to the hydrotrifluoromethylthiolation of alkynes [72].