Abstract
Electrophilic halogenation reactions have been a reliable approach to accessing organohalides. During the past decades, various catalytic systems have been developed for the activation of haleniums. However, there is still a short of effective catalysts, which could cover various halogenation reactions and broad scope of unsaturated compounds. Herein, TEMPO (2,2,6,6-tetramethylpiperidine nitroxide) and its derivatives are disclosed as active catalysts for electrophilic halogenation of olefins, alkynes, and aromatics. These catalysts are stable, readily available, and reactive enough to activate haleniums including Br+, I+ and even Cl+ reagents. This catalytic system is applicable to various halogenations including haloarylation of olefins or dibromination of alkynes, which were rarely realized in previous Lewis base catalysis or Lewis acid catalysis. The high catalytic ability is attributed to a synergistic activation model of electrophilic halogenating reagents, where the carbonyl group and the halogen atom are both activated by present TEMPO catalysis.
Highlights
Electrophilic halogenation reactions have been a reliable approach to accessing organohalides
In order to overcome the limitations of electrophilic halogenation with molecular halogen (X2, X = Cl, Br, I)[7], various electrophilic halogenating reagents have been developed (Fig. 1a)[8,9,10]
A catalytic asymmetric approach was established by Yamamoto et al.[57] utilizing a dual-role Lewis base/Brønsted acid catalyst
Summary
The calculated atomic charges show that the positive charge on the Cl atom of the halogenating reagent TEMPO+ complex is larger than that of isolated halogenating reagents, while the positive charge on the Br atom changes slightly These results suggest that TEMPO+ could form a synergistic activation model of electrophilic halogenating reagents, where the carbonyl group and the halogen atom were both activated by TEMPO+ catalysis. We speculated the oxygen atom on TEMPO+ could function like a Lewis base to form a polarized complex with halogenating reagents, which synergistically activates the haleniums. Substrate S30–S47 (0.50 mmol), DBDMH (0.75 mmol), TEMPO (0.10 mmol), and DCE (2.0 ml) was added to a reaction tube with a magnetic bar. Substrate S48–S69 (0.50 mmol), NCS (80.2 mg, 0.60 mmol), [TEMPO][OTf] (30.5 mg, 0.10 mmol), and CHCl3 (2.0 ml) was added to a reaction tube with a magnetic bar. The reaction mixture was quenched with saturated Na2SO3 aqueous solution (2 ml). The residue was purified over silica gel chromatography to afford 48–69
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