Oxoammonium salts are catalysing efficient and selective halogenation of olefins, alkynes and aromatics

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.

Iron(III)‐Mediated Nucleophilic Halogenation of Phenols Using an Amido Directing Group

“…[2] However, such strategy always shows poor selectivity in substitution sites or mono and multi functionalization under hypertoxic halogens and corrosive acids conditions. Thereafter, various electrophilic halogenating systems [3][4][5][6][7][8] have been developed, among which Et 2 SX • SbCl 5 X, [5] Palau'chlor, [6] NXS/DMSO, [7] and DXDMH/ TEMPO [8] exhibited high efficiency and selectivity for halogenation of aromatics.…”

Regioselective Synthesis of Pyrazolo[1,5-a]pyridine via TEMPO-Mediated [3 + 2] Annulation–Aromatization ofN-Aminopyridines and α,β-Unsaturated Compounds

“…Replaced TEMPO with [TEMPO]­[OTf] (0.2 and 1.2 equiv) under otherwise identical conditions gave similar results (43% yield, C4:C6 = 4.8:1 and 53% yield C4:C6 = 3.6:1, respectively) (Scheme B, entries 2, 3); however, the quick consumption of 7i (less than 1 h even the reaction was performed at −20 °C) was observed in both reactions. This combined with longer reaction time by postaddition of TEMPO (Scheme A) made us believe that the oxoammonium cation formed in situ during the reaction may serve both as a Lewis acid to accelerate the first [3 + 2] annulation reaction and as an oxidant in the aromatization steps. Additionally, TEMPO was also replaced by TEMPOH under the identified conditions (Scheme B, entries 4, 5).…”

Photoinduced metal-free borylation of aryl halides catalysed by an in situ formed donor–acceptor complex

“…In the first step, almost equivalent regioselective bromination is accomplished using N -bromosuccinimide (NBS) as the bromination reagent, controlled by the electrophilicity and steric hindrance effect of the aromatic ring. 23 Subsequently, a series of aryl bromides without further purification were transformed into the corresponding boronic esters with high regioselectivities and efficiencies ( e.g. , 13a, 73% yield; rr up to 40 : 1).…”