Effect of Cyclodextrins on Electrophilic Aromatic Bromination in Aqueous Solution

Abstract

Cyclodextrins act as molecular reactors to change the ratios of the products of reactions of anisole, acetanilide, 3methylanisole, and 3-methylacetanilide with pyridinium dichlorobromate. With anisole and acetanilide, bromination at the para position is favoured over ortho substitution, and the effect is greatest with α-cyclodextrin. In the reactions of the methylanisole and methylacetanilide, the cyclodextrins afford higher yields of monobrominated products and less of the di-and tribromides, and β-cyclodextrin has the greatest effect. These outcomes can be attributed to inclusion of the substrates within the cyclodextrins restricting access of the reagent adjacent to the methoxy and acetamido groups. The yields of 4-bromoanisole, 4-bromoacetanilide, 4-bromo-3-methylanisole, and 4-bromo-3methylacetanilide are thus increased from 73 to 94, 55 to 98, 37 to 86, and 39 to 72%, respectively. Perhaps more significantly, the quantities of the corresponding by-products are substantially reduced, from 27 to 6, 45 to 2, 63 to 14, and 61 to 28%. Since the reactions occur readily in water at ambient temperature, the cyclodextrins make them very efficient.

Molecular Reactors and Machines: Applications, Potential, and Limitations

“…Some of the most straightforward molecular reactors of this type involve a change in the regioselectivity of reaction of a substrate as a result of access of a reagent being restricted 11–14. In a representative example from our own work,11 which builds on earlier studies of aromatic chlorination by Breslow et al,12, 13 α‐ and β‐cyclodextrin have been shown to change the ratios of products of bromination of anisole 7 and acetanilide 9 with pyridinium dichlorobromate (Scheme ). Substitution at the para ‐position is preferred, presumably because the ortho ‐positions are shielded (Figure 2), and the effect is greatest with α‐cyclodextrin.…”

Green Analysis of the Bromination Reaction of Propiophenone Derivatives Mediated by Cu2+ Complexes

“…It is very important to note that propiophenones have multiple active sites in its structures. Therefore, the search for novel reaction conditions that allow to obtain products in a regio‐ or chemoselective way, is always interesting …”

Reversal of Regioselectivity and Enhancement of Rates of Nitrile Oxide Cycloadditions through Transient Attachment of Dipolarophiles to Cyclodextrins

“…Keywords: cycloaddition · cyclodextrins · inclusion compounds · molecular reactors · regioselectivity fied hosts, including allylations, [47][48][49] Diels-Alder cycloadditions, [50][51][52] oxidations, [53,54] hydrolyses, [55][56][57][58][59][60][61] enolizations, [62,63] reductions, [64,65] aldol condensations, [66] indigoid dye synthesis, [67,68] aromatic substitutions, [69][70][71] and aliphatic hydroxylations. [72][73][74][75][76][77] Their scope is indicated by the activity of a ditelluride b-cyclodextrin dimer as a glutathione peroxidase mimic, which catalyzes the reduction of cumene peroxide by an aryl thiol 200 000 times more effectively than diphenyl diselenide.…”