Electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part X. Pyridine N-oxide

Abstract

All three 1-acetoxyethylpyridine N-oxides have been prepared and the rates of elimination of acetic acid from these esters and from 1-phenylethyl acetate have been measured over a 50° range for each compound at temperatures between 286 and 402°. The rates lead to σ+ values of +0·81 and +0·016 for the 3-and 4-positions respectively, in good agreement with both theoretical predictions and the qualitative observations of electrophilic substitution. The reactivity of the 4-position lends strong support to the belief that nitration of pyridine N-oxide does not take place on the free base. The 2-compound is very much more reactive than the other isomers (contrary to theoretical prediction) and gives 2-acetylpyridine instead of the expected 2-vinylpyridine N-oxide as the elimination product. 2-Acetylpyridine is also obtained on elimination of water from 2-(1-hydroxyethyl)pyridine N-oxide which proceeds at almost the same rate as elimination of acetic acid from the acetate. The high reactivity is attributed either to acceleration of the loss of the functional group in the side chain through a neighbouring group effect (followed by rearrangement) or to an initial insertion of the oxygen into the side chain α-C–H bond followed by rapid elimination from the 1,1-diol (and its acetyl derivative). The literature mechanism for the 3-nitration of quinoline N-oxide is shown to be highly unlikely, and an alternative is suggested.