Spectroscopic and Computational Study of Organocatalytic Umpolung of Bromocations: An Accelerated Stereoselective Dibromination Protocol

Abstract

Herein, the reversal of polarity of cationic bromine, organocatalytically, is presented. NBS, a proven bromocation source was converted to a superior bromoanion reagent by H/Br exchange with a secondary amine, substantiated with spectroscopic and computational evidences. The concept has further been used in a successful accelerated organocatalyzed dibromination of olefins, in a non-hazardous, commercially viable process with a wide substrate scope. The reactivity of key entities has been observed through NMR kinetics and reaction acceleration using 10 mol% of catalyst loading accounts for its major success. The nucleophilicity of the bromoanion was found to be superior in comparison to other nucleophiles such as MeOH, H2O etc. and the protocol dominates over competing allylic bromination reaction.

Structure-Reactivity relationship of olefins as Electrophiles and Nucleophiles: Factors Influencing

“…various reactivity patterns were observed with different types of substrates towards the dibromination protocol: NBS-300 mol%, pyrrolidine-10 mol% (Figure 1). 18 The olefinic bond of substituted methyl (E)-cinnamates and β-nitrostyrenes reacted well with the established protocol whereas, substituted β-methyl-β-nitrostyrenes surprisingly did not provide the corresponding dibromo product except for 4-methoxy-β-methyl β-nitrostyrene. Thus a detailed investigation was carried out to find the reactivity pattern of C⚌C functionality in β-methylβ-nitrostyrenes in comparison to substituted methyl (E)-cinnamates and β-nitrostyrenes.…”

Spectroscopic and Computational Study of the Organocatalytic Umpolung of Bromocations: An Accelerated Stereoselective Dibromination Protocol**

“…Furthermore, the above protocol can be extended Chemistry-A European Journal to the asymmetric dibromination reaction [29] employing chiral organocatalysts and/or a chiral bromocation source, which is currently underway in our laboratory. [30] Experimental Section General procedure for the dibromination of olefins: To a stirred solution of NÀ bromosuccinimide (1, 300 mol %) in dry dioxane, in an amber colored vial, were added olefin (6, 100 mol %) and pyrrolidine (2 e, 50 mol %) sequentially. The reaction mixture was stirred at rt and the reaction progress was monitored by TLC.…”