Phosphoric acid hydrogen-bonded complexes

In the above example, although the hydrogen-bond complex was formed initially, the nucleophilic addihon reaction was supposed to proceed via protonated form CPli, which clearly indicated that the phosphoric acid worked as a Bronsted-acid catalyst. 

Gridnev and Terada calculated the optimized structure of the complex derived from phosphoric acid and N-acyl imine [14], wherein the hydrogen-bond complex was observed by density functional theory (DFT) calculations. 

We propose the reaction may proceed by the following mechanism (Fig. 51). First, reductive quenching of the Ir (ppy)2(dtbpy) excited state by Hantzsch ester will afford the more reducing Ir (ppy)2(dtbpy). PCET would then occur to a hydrogen bond complex between substrate and chiral phosphoric acid to afford the neutral ketyl radical. This neutral ketyl radical would remain associated with the chiral phosphate throughout the subsequent carbon-carbon bond formation to afford... 

The asymmetric hydrogenation of C=N (Eqn. (23)), in contrast with C=0 and C=C bonds, is much less developed. Hexahydrowoquinoline was used as its phosphoric acid salt. Iridium-ferrocenyl complexes were found to be sati.sfactory. After optimisiation, Meyer et al. (1997) were able to realize an enantioselectivity of 89% ee. 

An enantioselective version of the amidoalkylation reaction has been achieved using or-acetamidostyrene and chiral phosphoric acids. The highest ee was observed with sterically demanding acids. The reaction failed when the indole nitrogen was methylated, suggesting that the hydrogen bond is an important part of the reactive complex [314]. 

Figure 6. Single crystal X-ray structure of the 1 1 chelated complex formed between monobasic phosphoric acid and sapphyrin 3c. The phosphate is held ca. 0.83 A above the sapphyrin plane by 5 hydrogen bonds. For each diprotonated 3c, there is also a non-ligated monobasic phosphate.

Examples of phosphate stractures containing hydroxonium H3O+ cations are provided by hypo-phosphoric acid dihydrate, tetrametaphosphimic acid (13.23) and hydrogen uranyl phosphate tetra-hydrate, all of which possess complex systems of H bonds in the solid state [28-30]. 

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