Hydrogen complex formation with water

Klein, R. A. Ab initio conformational studies on diols and binary diol-water systems using DPT methods. Intramolecular hydrogen bonding and 1 1 complex formation with water, J. Comput. Chem. 2002, 23, 585-599. 

Unmodified poly(ethyleneimine) and poly(vinylpyrrolidinone) have also been used as polymeric ligands for complex formation with Rh(in), Pd(II), Ni(II), Pt(II) etc. aqueous solutions of these complexes catalyzed the hydrogenation of olefins, carbonyls, nitriles, aromatics etc. [94]. The products were separated by ultrafiltration while the water-soluble macromolecular catalysts were retained in the hydrogenation reactor. However, it is very likely, that during the preactivation with H2, nanosize metal particles were formed and the polymer-stabilized metal colloids [64,96] acted as catalysts in the hydrogenation of unsaturated substrates. 

Complex formation with substrate (S) can proceed directly, by route A, to yield a relaxed a-cyclodextrin with all six 0(2) -0(3 ) hydrogen bonds engaged (as in the a-cyclodextrin methanol complex, Fig. 18.8), or the macrocycle can first open up to a relaxed form, route B, with the enclosed water molecules disordered over several sites so as to fill, statistically, the 5 A diameter a-cydodextrin cavity (as observed in the a-cyclodextrin 7.57H20 crystal struc- ture, Fig. 18.6 b). The water is now in an activated form and can be replaced directly by the j substrate. In a third possible mechanism, route C, the substrate aggregates first at the periphery of tense a-cyclodextrin, and in a second step replaces the two enclosed water molecules. 

This equation shows that even for uncharged molecules with no net dipole moment may be significant owing to the quadrupole term. A detailed treatment of the theory has been presented by Abraham and Bretschneider (1974). The reaction-field model has been tested for a number of conformational equilibria, and usually gives excellent results, but is limited to solutions in which no specific interaction exists between solute and solvent, such as hydrogen bonding and charge-transfer complex formation. Thus water and alcohols are excluded, and aromatic solvents such as benzene and toluene also often show anomalous behaviour. Solvent mixtures can in principle be treated by the theory but such a treatment is usually avoided. 

Du XZ, Miao W, Liang YQ (2005) IRRAS studies on chain orientation in the monolayers of amino acid amphiplriles at the air-water interface depending on metal complex and hydrogen bond formation with the headgroups. J Phys Chem B 109(15) 7428-7434. doi 10.1021/ Jp0441700... 

The use of chiral ruthenium catalysts can hydrogenate ketones asymmetrically in water. The introduction of surfactants into a water-soluble Ru(II)-catalyzed asymmetric transfer hydrogenation of ketones led to an increase of the catalytic activity and reusability compared to the catalytic systems without surfactants.8 Water-soluble chiral ruthenium complexes with a (i-cyclodextrin unit can catalyze the reduction of aliphatic ketones with high enantiomeric excess and in good-to-excellent yields in the presence of sodium formate (Eq. 8.3).9 The high level of enantioselectivity observed was attributed to the preorganization of the substrates in the hydrophobic cavity of (t-cyclodextrin. 

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