MASS-SPECTRA OF ALKENYL METHYL ETHERS

Electron impact ionization mass spectra of numerous alkenyl methyl ethers C(n)H2n-1OCH3 (n = 3-6) recorded under normal (4 kV, 70 eV, 175-degrees-C) and low-energy, low-temperature (8 kV, 12 eV, 75-degrees-C) conditions are reported. The influence of the position and stereochemistry of the double bond on the dissociation of ionized alkenyl methyl ethers is discussed. The mechanisms by which these ethers fragment after ionization have been further investigated using extensive H-2-labelling experiments and by studying the energy dependence of the reactions. Ethers of allylic alcohols show spectra that are distinct from those of the isomeric species in which the double bond is separated by one or more sp3 carbon atoms from the carbon atom carrying the methoxy group. Three principal primary fragmentations are observed. The most common process, especially for ionized ethers of allylic alcohols, is loss of an alkyl group. This reaction often occurs by simple alpha-cleavage of radical-cations of the appropriate structure; however, alkyl groups attached to either end of the double bond are also readily lost. These formal beta- and gamma-cleavages are explained in terms of rearrangements via distonic ions and, at least in the case of gamma-cleavages, ionized methoxycyclopropanes. Ionized homoallyl methyl ethers tend to eliminate an allylic radical, particularly at high internal energies, with formation of an oxonium ion (CH3 + O = CH2 or +O = CHCH3). The esthers of linear pentenols and hexenols show abundant [M - CH3OH]+. ions in their spectra, especially when a terminal methoxy group is present. Methanol loss also takes place from ionized ethers of allylic alcohols in which there is a delta-hydrogen atom; this process is significantly favoured by cis, rather than trans, stereochemistry of the double bond.