Enol ethers are alkenes with alkoxy substituents. Enol ethers and enamines are so-called activated olefins or electron-rich olefins because oxygen atoms provide electrons to the double bond by forming a resonant structure with the corresponding oxonium ion. This property makes them become substrates for some organic reactions such as Diels-Alder reaction. Enol ether can be considered as the ether of the corresponding enolate. Two simple enol ethers are methyl vinyl ether and 2,3-dihydrofuran. Enol ethers are intermediate in their chemical properties between olefins and enamines. Wislicenus, who was the first to prepare ethyl vinyl ether, also reported on the halogenation, hydrolysis, and polymerization of this simple enol ether.
Applications
Polymerization reaction: The enol ether monomers undergo facile and rapid UV-induced photopolymerization by using diaryliodonium salt photoinitiators. The polymerization rate usually depends on the presence and steric bulk of the substituents. While the photopolymerization of 2,3-dihydropyran yields high polymers, 4,5-dihydro-l,3-dioxepin and 1-methoxy-1 -cyclohexene, respectively, undergo oligomerization and no polymerization. The differences observed in the polymerizability of cyclic enol ethers appear to be mainly due to steric effects.
Hydrolysis reaction: The hydrolysis of 1-cyclopropylvinyl methyl ether has been studied. The hydrolysis of this compound shows all characteristics of a reaction that involves a slow proton transfer. It is also shown that cis- and trans-2-arylvinyl ethers are not interconverted during hydrolysis; therefore, with these compounds too, the rate-determining step is protonation of the carbon-carbon double bond.
Addition reaction: The addition of chlorine and bromine to vinyl ethers has been studied. In the case of alkyl vinyl ethers, the reaction is so vigorous that cooling is necessary. The reaction is generally carried out at about 0°C. Bromination occurs without a catalyst, whereas peroxides are added in chlorinations to prevent polymerization. Trihalogeno ethers are often farmed as byproducts together with the 1,2-dihalogeno ethers. Elimination, the initial step in by-product formation, is favored to such an extent by the resonance-stabilized ionic intermediate that it undergoes even under very mild conditions. Attempts to iodinate enol ethers with elemental iodine are unsuccessful and yield polymers. Under the conditions of the iodoform reaction, alkyl vinyl ethers are converted into iodoform and potassium formate; 1,2-diiodo ethers and iodoacetaldehyde are assumed to occur as intermediates. Enol ethers have also been treated with N-bromoamides instead of bromine. For example, 2,3-dihydropyran reacts with N-bromophthalimide predominantly by a polar mechanism. The ionic intermediate can be intercepted by addition of alcohol to form phthalimide.
Fig.1. Photopolymerization of cyclic enol ethers.
