Exchange reactions between amino protons

Anionic ring-opening polymerization of POx initiated by partially deprotonated dipropyleneglycol (DPG) in the presence of the phosphonium cation tetrakis[cyclohexyl(methyl) amino jphosphonium as counterion (P Scheme 14) has been reported by Rexin and Miilhaupt. An alcohol/phospho-nium alkoxide mixture (95/5) was utilized to control reactivity, making use of a fast proton exchange reaction between dormant hydroxy compounds and active phosphonium alkoxide ends (Scheme 16). 

LebriUa has developed a chiral mass spectrometry method that has potential to determine enantiomeric excess of mixtures. The method involves an ion—molecule reaction between a neutral amine and a noncovalent charged complex (formed via ESI) consisting of a selector (cyclodextrin) and the desired selectand (chiral substrate, typically an amino acid). The protonated selector—selectand complex undergoes a guest—ligand exchange reaction with the amine. The rate of this reaction is sensitive to the chirality of the bound selectand and thus can be used to the quantify enantiomeric excess of other mixtures using suitable calibration curves. 

The existence of bimolecular proton exchange was demonstrated in aqueous solutions of some amino-acids (36, 42) and nucleotides (381, 3) through ultrasonic absorption measurements. These exchanges occur between differently ionized forms of the same amino-acid or nucleotide. With nucleotides the bimolecular exchange occurs between the secondary phosphoric acid function of the phosphate moiety and a protonatable group of the base ring, and is coupled with a fast stacking process, as well as with protolysis reactions at each of the two protonatable sites (38b,U3). 

As the exchange reactions which involve the nitrogen atom of the various amino derivatives strongly depend on the medium, the borderline between intermolecular effects and chemical reactions may become blurred. This is particularly so for proton exchange reactions which occur as a function of pH in >N-H derivatives. However, such processes will be considered in Sect. 8, and only conventional medium effects along with protonation effects will be dealt with now. 

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