Kinetics and mechanism of the bromination of phenols in aqueous solution. Evidence of general base catalysis of bromine attack

Abstract

This paper is dedicated to Professor Ross Stewart ot1 tlze occasion of his 65th birthda!.OSWALD S. TEE and N. RANI IYENGAR. Can. J. Chem. 68, 1769 (1990).The reactions of bromine with phenol, 4-bromophenol, and 4-methylphenol (p-cresol) in aqueous solution are catalyzed by carboxylate anions, confirming the suggestions of earlier work. The results are consistent with deprotonation of the phenol hydroxyl group by a general base occurring at more or less the same time as electrophilic attack by molecular bromine. 1769 (1990) Les rCactions du brome avec le phenol, le 4-bromophCno1 et le 4-mCthylphCno1 (p-cresol), en solution aqueuse, sont catalyskes par les anions carboxylates; cette observation confirme les suggestions faites anter~eurement. Les resultats sont en accord avec un mCcanisme impliquant une dCprotonation du groupement hydroxyle du phCnol par une base gCnCrale qui se produirait pratiquement en m&me temps que I'attaque Clectrophile par le brome moleculaire. On d~scute des origines possible de la catalyse basique gCnCrale. Si I'on combine les rCsultats actuels avec ceux obtenus antkrieurement, on peut suggerer qu'une I cyclohexadi&none protone'e n'est pas un intermediaire nCcessaire dans la bromation du phCnol; cet intermidiaire peut &tre evite par la formation et par l'enolisation de l'intermediaire cyclohexadi6none par une catalyse gCnCrale.Mots cle's : bromation, phenol, mkcanisme, catalyse, cinktique.[Traduit par la revue]The importance of acid-base catalysis for a large number of reactions occurring in aqueous solution is firmly established (1). Many reactions which are sluggish or non-existent in other media are facilitated by the ease of proton transfers in water, 1 thereby allowing highly unfavorable intermediates to be stabil-I ized or avoided (1,2). As reported herein, such may be the case I with phenol bromination in aqueous solution. ISeveral recent studies in this laboratory have been concerned with the mechanism of bromination of phenol derivatives in I aqueous solution (3-9). Some of these studies dealt mainly with electrophilic bromine attack (3, 4), while others were devoted to the enolization or debromination of transient bromocyclohexadienones (5-9). Three studles, in particular, suggested that bromine attack may be catalyzed by general bases.Firstly, we found that salicylate (o-hydroxybenzoate) anions (1) have an enhanced reactivity towards bromine and a reduced selectivity (less negative p+), when compared to analogous, monosubstituted phenols (36). As a result, we argued that the attack of bromine on 'salicylate anions is intramolecularly catalyzed by their ortho carboxylate groups (reaction [l]). Complementary to these conclusions, we also found that debromination of the ipso-dienone 5 by bromide ion is strongly catalyzed by the internal carboxyl group (reaction [3]) (36, 6,9).'Since three, quite distinct studies strongly suggested that the attack of bromine on phenols is catalyzed by general bases, we have looked for buffer catalysis in the aqueous bromination of phenol and tw...

Theoretical study on the mechanism of thieno[3,2‐b]benzofuran bromination: the importance of Lewis and non‐Lewis type NBOs interactions along the reaction path

“…It is known that the structure of attacking species in electrophilic bromination depends on the reaction conditions: when a strong Lewis acid is used with the bromine, the attacking electrophile may be a positive bromonium cation Br + . In the presence of polar solvents, the attacking electrophile may be a neutral Br 2 molecule polarized by the solvent . In the absence of a catalyst or polar solvents, and using nonpolar solvents, the attacking bromine molecule can be polarized by the π‐electron‐rich aromatic rings system of heterocycle .…”

“…In the presence of polar solvents, the attacking electrophile may be a neutral Br 2 molecule polarized by the solvent . In the absence of a catalyst or polar solvents, and using nonpolar solvents, the attacking bromine molecule can be polarized by the π‐electron‐rich aromatic rings system of heterocycle . Bromination of thieno[3,2‐ b ]benzofuran takes place under mild reaction conditions in the absence of catalysis using nonpolar solvents .…”

Chiral Pyridin-3-ones and Pyridines: Syntheses of Enantiopure 2,4-Disubstituted 6-Hydroxy-1,6-Dihydro-2H-Pyridin-3-ones, 2,3-Disubstituted 4-Iodopyridines, and Enantiopure 2,3-Disubstituted 4-Pyridinemethanols

“…However, when the same reaction was carried out in presence of NBS alone at −5 °C → RT, the reaction progressed smoothly to completion in about 4 h to furnish 4-substituted 1,6-dihydro-2 H -pyridin-3-one 12a in 66% yield . That the transformation of crowded 2,3-disubstituted furan 11a to the pyridin-3-one 12a in the presence of NBS/NaOAc·3H 2 O was unsuccessful could be attributed to the weaker reactivity of in situ generated brominating agent acetyl hypobromite (CH 3 COOBr) from NBS and the buffer anion compared with that of NBS alone successfully utilized for the same transformation …”