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Transition-Metal-Free Decarboxylative Bromination of Aromatic Carboxylic Acids† Cite This: Chem

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Transition-Metal-Free Decarboxylative Bromination of Aromatic Carboxylic Acids† Cite This: Chem Chemical Science View Article Online EDGE ARTICLE View Journal | View Issue Transition-metal-free decarboxylative bromination of aromatic carboxylic acids† Cite this: Chem. Sci.,2018,9, 3860 Jacob M. Quibell, ‡a Gregory J. P. Perry, ‡ab Diego M. Cannas a and Igor Larrosa *a Methods for the conversion of aliphatic acids to alkyl halides have progressed significantly over the past century, however, the analogous decarboxylative bromination of aromatic acids has remained a longstanding challenge. The development of efficient methods for the synthesis of aryl bromides is of great importance as they are versatile reagents in synthesis and are present in many functional molecules. Herein we report a transition metal-free decarboxylative bromination of aromatic acids. The reaction is applicable to many electron-rich aromatic and heteroaromatic acids which have previously Received 2nd March 2018 proved poor substrates for Hunsdiecker-type reactions. In addition, our preliminary mechanistic study Accepted 24th March 2018 suggests that radical intermediates are not involved in this reaction, which is in contrast to classical DOI: 10.1039/c8sc01016a Hunsdiecker-type reactivity. Overall, the process demonstrates a useful method for producing valuable Creative Commons Attribution 3.0 Unported Licence. rsc.li/chemical-science reagents from inexpensive and abundant starting materials. Introduction primary, secondary and tertiary carboxylic acids can be con- verted into the corresponding alkyl bromides in the presence of Aryl bromides are the substrate of choice when performing either an iridium or silver catalyst. transition metal-catalysed cross-coupling reactions1 or Conversely, the decarboxylative bromination of aromatic preparing Grignard and organolithium reagents.2 They are also acids has suffered from a slower pace of development (Scheme used in a variety of other transformations, such as nucleophilic 1B). Early ndings demonstrated that the bromination of This article is licensed under a substitution and HalEx reactions,3 and are the core structures in aromatic acids could indeed proceed, however, the reaction with many natural products and dyes.4 Consequently, developing electron-rich substrates was unselective and electron-decient efficient methods for the synthesis of aryl bromides remains an aromatics gave varying yields (Scheme 2).11 Many have looked Open Access Article. Published on 26 March 2018. Downloaded 9/28/2021 7:55:31 PM. important objective.5 The ability to directly substitute a carboxyl to solve these issues, but current procedures still suffer as they group with a bromo group has interested the synthetic (a) require stoichiometric transition metals, (b) are of poor community for many years. This transformation was rst generality, and/or (c) are unselective.12 This is disappointing as demonstrated by Borodine over a century ago, but came to bear decarboxylative bromination holds potential as an economic the name “The Hunsdiecker Reaction” during the 1940's and is route for aryl halide formation;13 benzoic acids are inexpensive now a fundamental reaction in organic synthesis.6,7 The reac- and abundant, and the bromide group is a handle for selective tion involves the mixing of an aliphatic carboxylic acid with transformations. We were eager to reinvestigate the aromatic bromine in the presence of a silver salt to produce the desired Hunsdiecker reaction in order to establish a more efficient and alkyl halide. Various developments in this area were made selective route for aryl bromide formation. Herein we reveal the during the latter half of the 20th century,8 however, the appli- development of a transition metal-free decarboxylative bromi- cability of all these methods was limited due to the requirement nation that is applicable to a variety of electron-rich aromatic of stoichiometric transition metal salts and/or poor generality. acids. In addition, preliminary investigations begin to shed light It is only recently that signicant progress has been made in the on the mechanism of this reaction. decarboxylative bromination of aliphatic acids (Scheme 1A). Namely, the groups of Glorius9 and Li10 have demonstrated that Results and discussion aSchool of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, We have recently reported a transition-metal-free decarbox- UK. E-mail: [email protected] ylative iodination of aromatic acids.14 The success of this bInstitute of Transformative Bio-Molecules (WPI-ITbM) and Graduate School of procedure lies in the ability to prepare a variety of aryl iodides, Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan † Electronic supplementary information (ESI) available. See DOI: simply by heating a benzoic acid in the presence of I2.We 10.1039/c8sc01016a therefore questioned whether a similarly efficient and low-cost ‡ G. J. P. P. and J. M. Q. contributed equally to this work. decarboxylative bromination could be developed. We began 3860 | Chem. Sci.,2018,9,3860–3865 This journal is © The Royal Society of Chemistry 2018 View Article Online Edge Article Chemical Science Scheme 1 Timeline of the contrasting progress in (A) aliphatic vs. (B) aromatic decarboxylative bromination. Hunsdiecker reaction shown in Scheme 2 and demonstrates the challenges for achieving a selective bromination. By lowering the equivalents of Br2 the selectivity could be improved, however, a large amount of the brominated acid 2a0 was still produced (entry 2). We then investigated less electrophilic bromine sources, such as NBS (N-bromosuccinimide) and DBH (1,3-dibromo-5,5-dimethylhydantoin), however a mixture of Scheme 2 Current status of the aromatic Hunsdiecker reaction. products was still obtained and the desired product was formed Creative Commons Attribution 3.0 Unported Licence. in low yield (entries 3 and 4). We then turned to the use of tri- bromide reagents as bromine sources for this transformation. our investigation by exposing the benzoic acid 1a to our Although pyridinium tribromide performed poorly in this reac- previous conditions, but switching the halogen source from I 2 tion (entry 5), we found that tetraalkyl ammonium tribromide to Br (Table 1). Unfortunately, this resulted in the undesired 2 salts, N(Me )Br and N(nBu )Br , displayed good reactivity and formation of the brominated acid 1a0 and dibrominated 4 3 4 3 high selectivity for the desired decarboxylative bromination product 2a0 and none of the desired product 2a (Table 1, (entries 6 and 7). N(nBu )Br was chosen as the brominating entry 1). This reactivity is comparable with the aromatic 4 3 reagent of choice, allowing the product to be isolated in 90% yield. Further control experiments revealed that the reaction This article is licensed under a Table 1 Optimisation of the transition metal free decarboxylative does not proceed in the absence of a base (entry 8), but that a bromination performing the reaction in the dark or adding one equivalent of water had little effect on the reaction (entries 9 and 10).15 ffi Open Access Article. Published on 26 March 2018. Downloaded 9/28/2021 7:55:31 PM. Having demonstrated an e cient transition metal-free decarboxylative bromination, we then turned to exploring the scope of this reaction (Scheme 3). Previously, the decarbox- ylative bromination of 4-methoxybenzoic acid under Hunsdiecker-type conditions resulted in a mixture of products (Scheme 2), therefore we were impressed to observe the formation of the aryl bromide 2b in high yield and high selec- tivity. This clearly demonstrates the advantages of our proce- 0 0 Entry [Br] 1a 2a 1a 2a dure over previous techniques. The desired product, 2c, was not b observed when using 3-methoxybenzoic acid, suggesting the 1 Br2 Trace 0 23 72 position of decarboxylation must be sufficiently nucleophilic for 2Br2 826563 3 NBSc 58 21 9 Trace the decarboxylative bromination to occur. Other highly 4 DBHd 640327electron-rich substrates (1d–h) could also undergo the desired e 5 PyHBr3 39 6 54 0 transformation, including non-ortho-substituted benzoic acids 6 N(Me4)Br3 68552 n f (1b, 1h), which are generally unreactive in transition metal- 7N(Bu4)Br3 5 91 (90) 1 Trace g n mediated decarboxylative functionalisations. The procedure 8 N( Bu4)Br3 100 0 0 0 h n 9 N( Bu4)Br3 11 84 2 Trace can also be applied on a large scale, thus, 5.5 g of the bromi- i n 10 N( Bu4)Br3 10 85 Trace 3 nated product 2e was prepared using the standard conditions a on the bench top at room temperature, without requiring Reaction conditions: 1a (0.2 mmol), [Br] (0.2 mmol, 1.0 equiv.), K3PO4 b (0.2 mmol, 1.0 equiv.), MeCN (1.0 mL), 100 C, 4 h. Br2 (0.6 mmol, 3.0 column chromatography. Polymethylated benzoic acids are c d equiv.). N-bromosuccinimide. 1,3-Dibromo-5,5-dimethylhydantoin. poorly reactive substrates in transition metal-mediated decar- e Pyridinium tribromide. f Yield in parenthesis is of isolated material. 0 0 g boxylations, but they, and even simple toluic acid, showed good Isolated as a mixture with 2a (2a : 2a , >150 : 1). No K3PO4 added. h i Performed in the dark. 1.0 equiv. H2O added. reactivity under our conditions (2i–2l). The procedure could also This journal is © The Royal Society of Chemistry 2018 Chem. Sci.,2018,9,3860–3865 | 3861 View Article Online Chemical Science Edge Article Creative Commons Attribution 3.0 Unported Licence. Scheme 4 Scope of the decarboxylative bromination of hetero- aromatic acids. Reactions carried out on a 0.5 mmol scale aRatios in brackets indicate mono : dibrominated material by NMR analysis before separation. Asterisk indicates position of dibromination. bIso- lated as mixture. c50 C. reactivity in this reaction and we are currently investigating the cause of this unique behaviour. This article is licensed under a The procedure could also be applied to a range of hetero- aromatic acids (Scheme 4). The bromination of heteroaromatic acids is highly limited as elevated temperatures (>160 C) and 16 Open Access Article.
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