Crown ethers as molecular bromine carriers for bromination reactions

Abstract

Macrocyclic polyethers (crown ethers) form molecular complexes with bromine that may be used as reagents for the bromination of olefins and acetylenes. We have used 12-crown-4,15-crown-5,18-crown-6, polydibenzo-18-crown-6, dibenzo-18-crown-6, and 6-crown-2(dioxan) as carriers for bromine, and have investigated the thermodynamics of complex formation and the kinetics and stereochemistry of bromine addition to cis- and trans-β-methylstyrene. Formation constants for the ethers are similar implying that all interactions are similar to the known 6-crown-2·Br₂ polymeric structure, and do not involve encapsulation by the macrocycle. Kinetic results also indicate very little difference between the rates of bromination as the ethers are varied. The stereoselectivity of addition, however, is significantly changed as a function of the ether. The solid dibenzo- and polydibenzo-18-crown-6 ether complexes exhibit very high stereoselectivity for anti-bromination, whilst the other ethers and carriers such as silica gel, alumina, and montmorillonite show enhanced formation of syn-addition, compared with other modes of bromine delivery (e.g. free Br₂, pyr·Br₂). The use of poly-dibenzo-18-crown-6 as a stationary phase column-packing, enabling stereospecific or stereoselective bromination, is the preferred technique for such brominations.