Basic additives, pyridine

In addition to pyridine, the six-membered diamine pyrimidine is also found commonly in biological molecules, particularly as a constituent of nucleic acids. With a pKa of 1.3, pyrimidine is substantially less basic than pyridine because of the inductive effect of the second nitrogen. 

The three amino-pyridines are all more basic than pyridine itself and form crystalline salts by protonation at the ring nitrogen. The a- and y-isomers are monobasic only, because charge delocalisation over both nitrogen atoms, in the manner of an amidinium cation, prevents the addition of a second proton. The effect of the delocalisation is strongest in 4-aminopyridine 9.1) and much weaker in 2-aminopyridine (p/ aH 7.2). Delocalisation is not possible for the P-isomer, which thus can form a di-cation in strong acid (p/faHS 6.6 and -1.5). " ... 

For solutes that are strong acids and bases a third component is often required in the mobile phase. In general, acidic solutes require an acidic additive (like trifluoroacetic acid). Basic solutes require a basic additive (like dimethylethylamine). These additives are mixed with the modifier at 0.1-0.5%. Polar solutes are separated on polar stationary phases (silica, amino, diol, cyano, ethyl pyridine, etc.)... 

To date, several protocols for Knoevenagel additions to protected carbohydrates or carbohydrate-derived molecules have been reported in the literature [36]. For a manganese(III)- or cerium(IV)-mediated radical addition of malonates to protected carbohydrates, see Reference 37. For early reactions of protected carbohydrates with malonic acid in basic medium (pyridine), see Reference 6j. 

The increased rate of epoxidation observed using pyridine as an additive has been studied by Espenson and Wang and was to a certain degree explained as an accelerated formation of peroxorhenium species in the presence of pyridine [62]. A stabilization of the rhenium-catalyst through pyridine coordination was also detected, although the excess of pyridine required in the protocol unfortunately led to increased catalyst deactivation. As can be seen above, MTO is stable under acidic conditions but at high pH an accelerated decomposition of the catalyst into perrhenate and methanol occurs. The Bronsted basicity of pyridine leads to increased amounts of HO2 which speeds up the formation of the peroxo-complexes and the decomposi-... 

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