Substrate Directed Regioselective Monobromination of Aralkyl Ketones Using N-Bromosuccinimide Catalysed by Active Aluminium Oxide: α-Bromination versus Ring Bromination

Reddy Bodireddy Mohan,G Trivikram Reddy,N C Gangi Reddy

doi:10.1155/2014/751298

Abstract

Bromination of aralkyl ketones using N-bromosuccinimide in presence of active Al2O3 provided either α-monobrominated products in methanol at reflux or mononuclear brominated products in acetonitrile at reflux temperature with excellent isolated yields depending on the nature of substrate employed. The α-bromination was an exclusive process when aralkyl ketones containing moderate activating/deactivating groups were subjected to bromination under acidic Al2O3 conditions in methanol at reflux while nuclear functionalization was predominant when aralkyl ketones containing high activating groups were utilized for bromination in presence of neutral Al2O3 conditions in acetonitrile at reflux temperature. In addition, easy isolation of products, use of inexpensive catalyst, short reaction time (10–20 min), and safe operational practice are the major benefits in the present protocol.

Highlights

Nowadays, researchers are focusing on the development of more acceptable bromination protocols to accomplish increasing demands for “organohalogen” chemistry and to achieve higher efficiency and selectivity of the bromination reactions which include α-bromination and nuclear bromination
A popular and superior brominating agent such as N-bromosuccinimide [14] was utilized for α-bromination of carbonyl compounds using a radical initiator such as azobisisobutyronitrile (AIBN) or dibenzoyl peroxide (BPO) [15] and, later, it has been demonstrated that the reactivity of NBS could be modulated with ionic liquids [16], photochemical energy [17]; sonochemical energy [18], solvent free reaction conditions (SFRC) [19] and various catalysts such as Mg(ClO4)2 [20], NH4OAc [21], amberlyst-15 [22], silica supported NaHCO3 [23], sulfonic acid functionalized silica [24], FeCl3 [25], montmorillonite-K10 [26], and Silica gel [27]
Two types of substrates were selected for optimization of reaction conditions using N-bromosuccinimide in presence of either acetophenone and 4󸀠

Summary

Introduction

Researchers are focusing on the development of more acceptable bromination protocols to accomplish increasing demands for “organohalogen” chemistry and to achieve higher efficiency and selectivity of the bromination reactions which include α-bromination and nuclear bromination. On deep investigation it is found that aralkyl ketones with moderate activating/moderate or high deactivating groups undergo exclusively α-bromination in the presence of acidic alumina in methanol, while substrates with high activating groups undergo nuclear bromination predominantly in the presence of neutral alumina in acetonitrile at reflux temperature, as shown in Scheme 1. Aralkylketones containing high electron donating groups may be susceptible to rapid activation of the aromatic ring of substrate and rapid release of bromonium ion from NBS via surface interaction with neutral Al2O3 which leads to exclusive formation of nuclear brominated product (4) in acetonitrile based on substrate employed and the catalyst may assist in abstracting the proton during the course of bromination as shown in Scheme 3

Results and Discussion

Conclusion

Experimental

Physical and Spectral Characterization Data