How much do you know about the role of bases in synthesis?

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How much do you know about the role of bases in synthesis?

Generally speaking, bases play the following five roles in organic synthesis: one is to grab protons (that is, we usually say hydrogen extraction); The second is the neutralization reaction system in the proton (that is, we usually say the acid binding agent); The third is the activation catalyst (many of our palladium catalytic reactions into the catalytic cycle system are Ba (0), but we usually add Ba (2), in the middle of which is the base to activate); Fourth, promote catalyst regeneration; Five is to use itself 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. As for general inorganic bases, cations in bases mainly affect their solubility and interaction with substrates. The anions in the base mainly determine the coordination mode and stability.

Speaking of inorganic base, when we add a base to the reaction, we usually consider whether to add an inorganic base or an organic base, which reflects the different properties of inorganic base and organic base. Let’s not consider the strength of the base. From the point of view of nature, the main difference between inorganic base and organic base lies in its solubility in the reaction solvent and the magnitude of 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 bases in organic solvents is not very good, and the effect of solvents has many effects on inorganic bases, so the alkalinity of inorganic bases in solvents is quite difficult to measure. According to some experience, in most solvents, cesium carbonate is as basic as DBU and KOH is as basic as sodium tert-butanol.

In many reactions, the choice of base is often a matter of luck, sometimes it is the right one time, sometimes it is several times. This is also the case that in many literatures, when doing coupling reactions, the base is usually listed in the paper as an optimization data. In the real work, the choice of base depends on personal experience. For example, when doing Suzuki reaction, why potassium phosphate is chosen instead of potassium carbonate, cesium carbonate, potassium hydrogen phosphate, sodium phosphate, etc., is really not easy to explain if questioned, only that this base reaction is better than that one. And a lot of reactions if you have a good reaction with an inorganic base, then most of the time you don’t have a good reaction with an organic base, but if you measure the pKa of two bases and it’s not that different, it’s interesting.