Chemistry of 4-alkylaryloxenium ion "precursors": Sound and fury signifying something?

Abstract

Quinol esters '2b', '2c', and '3b' and sulfonamide '4c' were investigated as possible precursors to 4-alkylaryloxenium ions, reactive intermediates that have not been previously detected. These compounds exhibit a variety of interesting reactions, but with one possible exception, they do not generate oxenium ions. The 4-isopropyl ester '2b' predominantly undergoes ordinary acid- and base-catalyzed ester hydrolysis. The 4-tert-butyl ester '2c' decomposes under both acidic and neutral conditions to generate tert-butanol and 1-acetyl-1,4-hydroquinone, '8', apparently by an S(N)1 mechanism. This is also a minor decomposition pathway for '2b', but the mechanism in that case is not likely to be S(N)1. Decomposition of '2c' in the presence of N₃⁻ leads to formation of the explosive 2,3,5,6-tetraazido-1,4-benzoquinone, '14', produced by N₃⁻-induced hydrolysis of '8', followed by a series of oxidations and nucleophilic additions by N₃⁻. No products suggestive of N₃⁻-trapping of an oxenium ion were detected. The 4-isopropyl dichloroacetic acid ester '3b' reacts with N₃⁻ to generate the two adducts 2-azido-4-isopropylphenol, '5b', and 3-azido- 4-isopropylphenol, '11b'. Although '5b' is the expected product of N₃⁻ trapping of the oxenium ion, kinetic analysis shows that it is produced by a kinetically bimolecular reaction of N₃⁻ with '3b'. No oxenium ion is involved. The sulfonamide '4c' predominantly undergoes a rearrangement reaction under acidic and neutral conditions, but a minor component of the reaction yields 4-tert-butylcresol, '17', and 2-azido-4- tert-butylphenol, '5c', in the presence of N₃⁻. These products may indicate that '4c' generates the oxenium ion '1c', but they are generated in very low yields (ca. 10%) so it is not possible to definitively conclude that '1c' has been produced. If '1c' has been generated, the N₃⁻-trapping data indicate that it is a very short-lived and reactive species in H₂O. Comparisons with similarly reactive nitrenium ions indicate that the lifetime of '1c' is ca. 20-200 ps if it is generated, so it must react by a preassociation process. Density functional theory calculations at the B3LYP/6-31G*//HF/6-31G* level coupled with kinetic correlations also indicate that the aqueous solution lifetimes of '1a-c' are in the picosecond range.