Claisen rearrangements and cyclisations in phenyl propargyl ethers under electron impact conditions

Abstract

An interesting Claisen rearrangement has been observed in phenyl propargyl ether, yielding significant amounts of the [M – CO] ion under electron impact conditions. The other competing pathway is the loss of a hydrogen radical from M˙⁺ furnishing the most abundant ion in its mass spectrum. Expulsion of a hydrogen radical from the propargylic carbon leads to the propargylic cation species, while ejection of the ortho hydrogen gives rise to a benzopyrylium cation structure. Substituents on the phenyl ring do not favour the Claisen rearrangement, whereas ejection of the substituents producing benzopyrylium cations gains importance. The mechanisms and ion structures proposed are supported by high-resolution data, B/E and B²/E linked-scan spectra. Collision Activated Decomposition (CAD)-B/E linked-scan spectra and deuterium isotope labelling.