Rate-determining processes in acid-catalyzed decarboxylation reactions
نویسندگان
چکیده
The acid-catalyzed decarboxylation reactions of indoleand pyrrole-carboxylic acids require the addition of one equivalent of water to the carboxyl group and a proton to the heterocyclic ring carbon at the position α to the carboxyl. Where α-protonation is thermodynamically favoured over β-protonation, the magnitude of the observed 12C/13C kinetic isotope effect (CKIE) is greater than where the β-position is protonated. This can be understood in terms of a mechanism involving a protonated hydrated precursor to carbon-carbon bond cleavage, where the difference in energy of intermediates and transition states control the proportioning of the intermediates. The intrinsic CKIE on the carbon-carbon bond-breaking step that produces protonated carbonic acid (PCA) is independent of the site of protonation. The interpretation of the observed CKIEs can be generalized based on intermediates from isomeric carboxylic acids whose energetics vary predictably with their sites of protonation. The relative free energy barriers to reversion and formation of PCA control the magnitude of the observed CKIEs and correlate with reactivity. The reported data implicate the formation of PCA as the initial product of carbon-carbon bond cleavage. Application of the principle of microscopic reversibility implies that electrophilic aromatic substitution based on PCA should be an accessible route to carboxylation of aromatic substrates. Over the course of
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