Flow 1H nmr study of the rapid nucleophilic addition of amino acids to 4-formylpyridine

Abstract

Rapid addition of primary amino acids and imidazole to 4-formylpyridine to form carbinolamine intermediates in water has been studied by means of stopped-flow 1H nuclear magnetic resonance spectroscopy. The equilibrium constant Kn for this process was determined using stopped-flow uv spectroscopy. For the amino acid D,L-alanine and its derivatives, D,L-alanine methyl ester and D,L-alanylglycine, Kn does not appear to correlate with Ka, the acid dissociation constant. Values of kn, the addition rate constant, appear to be independent of pH and buffer concentration for these nucleophiles. The limited data indicate that the variation in rates of nucleophilic addition to 4-formylpyridine cannot be accounted for in terms of pKa, Kn or related to systematic changes in structure of the nucleophile. As a group, the primary amino acid D,L-alanine and its derivatives have comparable kn values within a factor of two in spite of their different pKa values. Imidazole, an aromatic secondary diamine, has a kn value that is 10 to 20 times larger than these primary amines and has a relatively small Kn value. At the same time, its kn value is comparable to that found previously for sarcosine, a secondary acyclic amino acid having substantially larger pKa and Kn values. The nucleophilic addition step of the primary amino acid and derivatives is rapid relative to the subsequent dehydration and prototropic shift steps. It occurs in a pH range at which pyridine nitrogen protonation is negligible.