Visible light induced ‘on water’ benzylic bromination with N-bromosuccinimide (original) (raw)

Abstract

Benzylic bromination of various 4-substituted toluenes (Me, tert-Bu, COOEt and COMe) was effectively conducted with NBS in pure water and with a 40 W incandescent light-bulb as an initiator of the radical chain process, while electron donating groups (OMe and NHAc) directed the reaction to electrophilic aromatic substitution.

Figures (3)

Scheme 1.

Scheme 1.

Yields were determined by 'H NMR spectroscopy of the crude mixture after phase separation and the products were determined by comparison with literature data, yields in parentheses refer to yields of compounds isolated by column chromatography. Accompanied by a small amount of benzyl dibromides in the following isolated yields: 2b (4%), 5b (3%), 15b (3%), 17b (6%) and 19b (3%). “Experiment on a larger scale (50 mmol). ‘Ambient light and 20 mL of water. third mode of activation of the radical process—light. Reaction under ambient conditions was selective for the radical reaction and benzyl bromination was the only reaction that took place (line 7). When we placed an incandescent 40 W light bulb 15cm from the flask, the yield of benzyl bromide increased (line 8). Similar results were obtained with a ‘solar’ lamp (high-pressure mercury lamp, OSRAM HQL 125 W), the reaction rate increased and bromination occurred in three and a half hours (line 9, react. temp. = 30 °C). We found that simple visible ight (40 W incandescent ight-bulb) induced radical reaction with NBS in water was superior in yield to the other modes of activation hat were investigated. Therefore, we used the following reaction conditions for benzylic bromination of various methyl benzene derivatives (2 mmol substrate, 2 mmol NBS and 10 mL water were stirred at 500 rpm and illu- minated with a 40 W incandescent light-bulb, react. emp. = 27°C). Methyl benzenes have a lower density han water and during the reaction form a layer ‘on water’, but as soon as they are brominated they become denser and sink to the bottom of the flask. As he residue of NBS is soluble in water and the organic phase is only comprised of product(s), the isolation procedure is very simple and consists of only phase sepa- ration or filtration. In cases where clear phase separa- ions did not occur, such as in small-scale experiments, iquid—liquid extraction was necessary. Following this procedure, toluene 4 was converted selectively to benzyl bromide 5a (Scheme 2). Visible light induced bromina- ion of p-xylene 6 afforded mainly a product preferen- ially brominated at only one of the methyl groups 7a/7b = 6.5). This is in contrast to the reaction with bromine in water, where the reactivity was the opposite 7a/7b = 0.2).'* Mesitylene 8, as well as 4-methylanisole 10 and 4-methylacetanilide 12 reacted solely at the aro- matic ring, while ethyl 4-methylbenzoate 14 reacted at he methyl group. Interestingly, 4-methylacetophenone 16 was exclusively brominated by a free radical process at the benzyl position leading to benzyl bromide 17a. On

Yields were determined by 'H NMR spectroscopy of the crude mixture after phase separation and the products were determined by comparison with literature data, yields in parentheses refer to yields of compounds isolated by column chromatography. Accompanied by a small amount of benzyl dibromides in the following isolated yields: 2b (4%), 5b (3%), 15b (3%), 17b (6%) and 19b (3%). “Experiment on a larger scale (50 mmol). ‘Ambient light and 20 mL of water. third mode of activation of the radical process—light. Reaction under ambient conditions was selective for the radical reaction and benzyl bromination was the only reaction that took place (line 7). When we placed an incandescent 40 W light bulb 15cm from the flask, the yield of benzyl bromide increased (line 8). Similar results were obtained with a ‘solar’ lamp (high-pressure mercury lamp, OSRAM HQL 125 W), the reaction rate increased and bromination occurred in three and a half hours (line 9, react. temp. = 30 °C). We found that simple visible ight (40 W incandescent ight-bulb) induced radical reaction with NBS in water was superior in yield to the other modes of activation hat were investigated. Therefore, we used the following reaction conditions for benzylic bromination of various methyl benzene derivatives (2 mmol substrate, 2 mmol NBS and 10 mL water were stirred at 500 rpm and illu- minated with a 40 W incandescent light-bulb, react. emp. = 27°C). Methyl benzenes have a lower density han water and during the reaction form a layer ‘on water’, but as soon as they are brominated they become denser and sink to the bottom of the flask. As he residue of NBS is soluble in water and the organic phase is only comprised of product(s), the isolation procedure is very simple and consists of only phase sepa- ration or filtration. In cases where clear phase separa- ions did not occur, such as in small-scale experiments, iquid—liquid extraction was necessary. Following this procedure, toluene 4 was converted selectively to benzyl bromide 5a (Scheme 2). Visible light induced bromina- ion of p-xylene 6 afforded mainly a product preferen- ially brominated at only one of the methyl groups 7a/7b = 6.5). This is in contrast to the reaction with bromine in water, where the reactivity was the opposite 7a/7b = 0.2).'* Mesitylene 8, as well as 4-methylanisole 10 and 4-methylacetanilide 12 reacted solely at the aro- matic ring, while ethyl 4-methylbenzoate 14 reacted at he methyl group. Interestingly, 4-methylacetophenone 16 was exclusively brominated by a free radical process at the benzyl position leading to benzyl bromide 17a. On

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References (16)

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