The mechanism of thermal eliminations. Part 20. The relative rates of pyrolysis of the 2-ethoxy, 2-isopropoxy, and 2-t-butoxy derivatives of pyrazine and pyrimidine to pyrazin-2-one and pyrimidin-2-one, respectively: polarity of the transition states and the importance of nucleophilic attack

Abstract

We have measured rates of thermal elimination of the title compounds to give the corresponding cyclic amides, between 587.0 and 698.5 K. The relative rates (primary : secondary : tertiary) at 600 K are 1 : 27.0 : 3720 for the pyrazines, and 1 : 26.4 : 4150 for the pyrimidines. These ratios are somewhat larger than for the corresponding 2-alkoxypyridines and suggest that C–O bond breaking is kinetically more significant in the pyrazines and pyrimidines, leading to a transition state with greater carbocationic character. This is consistent with electron withdrawal provided by the aza ‘substituent’ facilitating C–O bond cleavage. It does not however lead to a general increase in reactivity (as would be the case for pyrolysis of comparable esters) because this electron withdrawal reduces the nucleophilicity of the nitrogen involved in the elimination. This latter is particularly important for the primary and secondary compounds (which have more E_i- like transition states) than for the tertiary compounds, which therefore show a normal reactivity vs. rate spread pattern. The importance of the nucleophilicity of the nitrogen in the alkoxy-heterocycles, and its ability to be modified, may stem in part from the dual pathway available for transmission of substituent effects in the aromatic ring.