What do you know about the role of bases in synthesis?

Many organic reactions require the addition of bases, and the classification of bases is varied (acid-base ionization theory; Acid-base proton theory; But what role do bases play in organic reactions, as if they have different roles in different reactions

In general, in organic synthesis, alkali mainly plays the following five functions: first, to grab protons (namely, we usually call hydrogen extraction); The other is to neutralize the protons in the reaction system (what is commonly called an acid binding agent); The third is the activation catalyst (we have a lot of palladium-catalyzed reaction into the catalytic cycle system is BA (0), but we usually add ba(2), which is the activation of the base in the middle); The fourth is to promote catalyst regeneration; Five is itself used as a catalyst.

And alkali how these effect in the reaction, we all know that some add different alkali reaction difference is not big, but some even join with series of alkali reaction are also different, it has a great relationship and mechanism of alkali, alkali in the reaction and the influence of many factors, such as the kinds of alkali, alkali, alkaline strength, the size of the degree of solubility in different solvent settlement and so on. For inorganic base in general, the cation in base mainly affects its solubility and the interaction with substrate. The anions in the base mainly determine the coordination mode and stability.

Speaking of inorganic base, we add alkali in reaction, is usually considered adding inorganic alkali or organic alkali, it reflects the different properties of inorganic alkali and organic alkali, regardless of the strength of the basic problems, in nature, inorganic alkali and organic alkali main difference lies in its solubility in the solvent and different size on the size of the space steric hindrance.

That speak alkali, alkali, alkaline size is a problem not around the past, there is no question of comparison between organic alkali, alkaline in the organic solvent comparison problem between inorganic base is large, the biggest problems are organic alkali and alkaline in the organic solvent comparison between inorganic base, but this is we often involves synthesis problems. Because the solubility of inorganic base in organic solvent is not very good, and the effect of the solvent on inorganic base also has a lot of influence, so the alkalinity of inorganic base in the solvent is quite difficult to measure. According to some experience, cesium carbonate and DBU are equally basic, and KOH and sodium tert-butanol are equally basic in most solvents.

In many reactions, the choice of base is often a matter of luck, sometimes it’s right once, sometimes it’s several times, and this is also the case in a lot of literature when you do coupling reactions, you usually include base as an optimization data in the article. For example, when doing the Suzuki reaction, why the reaction chooses potassium phosphate rather than potassium carbonate, cesium carbonate, potassium hydrogen phosphate, sodium phosphate, etc., this is not easy to explain, except that this base reaction is better than that one. And a lot of the reactions if the inorganic base is doing well, then you don’t do well with the organic base most of the time, but if you measure the PkA of the two bases, it’s kind of interesting.