The Catalytic Potential of 4-Guanidinylpyridines in Acylation Reactions

Abstract

A series of 3-alkyl-4-guanidinylpyridines with variable alkylation pattern have been synthesized and characterized with respect to their catalytic potential in acylation reactions of alcohols. The ability of the substitution pattern to stabilize acylpyridinium cations, which act as critical intermediates in the catalytic cycle of pyridine-catalyzed acylation reactions, has been assessed at the MP2(FC)/6-31+G(2d,p)//B98/6-31G(d) level of theory and inclusion of solvent effects in chloroform using the PCM continuum solvation model. The most active 4-guanidinylpyridines are among those having the most electron-rich pyridine ring. The influence of the type and concentration of the auxiliary base on the catalytic activity has also been studied. While the change from triethylamine to N,N-diisopropylethylamine as the auxiliary base does not lead to a systematic increase or decrease in the catalytic rates, the complete absence of auxiliary base leads to a 27-fold reduction in reaction rate.

Key words

esterification - Lewis bases - ab initio calculations - kinetics­

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31

Use of the data points describing acetylation of the guanidinyl group leads to an inferior correlation described by the equation ΔH ₂₉₈ = -3.960 ln(1/t _1/2) - 91.13 kJ/mol; R^² = 0.28.