Abstract
Bromination reactions are crucial in today’s chemical industry since the versatility of the formed organobromides makes them suitable building blocks for numerous syntheses. However, the use of the toxic and highly reactive molecular bromine (Br2) makes these brominations very challenging and hazardous. We describe here a safe and straightforward protocol for bromination in continuous flow. The hazardous Br2 or KOBr is generated in situ by reacting an oxidant (NaOCl) with HBr or KBr, respectively, which is directly coupled to the bromination reaction and a quench of residual bromine. This protocol was demonstrated by polybrominating both alkenes and aromatic substrates in a wide variety of solvents, with yields ranging from 78% to 99%. The protocol can easily be adapted for the bromination of other substrates in an academic and industrial environment.
Highlights
One of the major challenges in synthetic chemistry concerns the transformation of a relatively inert C–H bond into a more active functional group
With HBr or KBr, respectively, which is directly coupled to the bromination reaction and a quench of residual bromine
This can be done with transition metal catalysis in the so-called C–H activation [1]
Summary
One of the major challenges in synthetic chemistry concerns the transformation of a relatively inert C–H bond into a more active functional group. This is an optimal first step for most synthetic pathways since the high reactivity of this formed organobromide allows for an easy subsequent conversion into a wide variety of other functional groups [2,3,4] This reaction is, plagued by the use of toxic and corrosive reagents [5] and a high risk of runaway reactions [6]. Brominations are often performed with alternative bromination reagents, most commonly organic molecules containing a Br(I)-species This bromine(I) can be bonded to nitrogen (e.g., N-bromosuccinimide or tribromoisocyanuric acid [7,8,9]); iodine(III) [10]; carbon (e.g., CBr4 [11]), or even three bromine atoms bonded together in Br3 − , countered by an organic cation, possibly forming an ionic liquid [12,13]. These alternatives provide a number of benefits, especially for academic laboratories, they still are far from perfect—they are expensive, are often still toxic and unsustainable, produce organic by-products, and most of them use molecular bromine in their synthesis, not solving but only shifting the problem [9,14,15]
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