Cooperative: hydrogen bonding origins

The great majority of transitions observed for iron(III) are gradual and the observation of thermal hysteresis associated with them is relatively rare. In the only instances where features indicative of significant cooperativity have been reported, extensive hydrogen-bonding networks (formed in some thiosemicarbazone compounds [111, 115,118, 119]) or n-n stacking interactions (operative in several compounds of N20 Schiff base systems [135-138, 164, 165]) have been invoked as the origin of the cooperativity. 

In 1959, Eberson (1959, 1992) found that a family of derivatives of succinic acid shows a remarkably large negative cooperativity, i.e., gjj < < 1, which is difficult to explain on the basis of electrostatic theories only. We shall discuss these compounds in Subsection 4.8.6. At present, there is no satisfactory molecular interpretation of these findings. One of the more popular ideas, originally suggested by Jones and Soper (1936) and further elaborated upon by McDaniel and Brown (1953), is that an intramolecular hydrogen bond would facilitate the first dissociation of the proton, i.e., Kj becomes smaller (or Kj becomes larger). Also, the second proton will dissociate with more difficulty. The net effect would be a... 

DeChancie, J., Houk, K. N. The origins of femtomolar protein-ligand binding hydrogen-bond cooperativity and desolvation energetics in the biotin-(strept)avidin binding site. J. Am. Chem. Soc. 2007, 129, 5419-5429. 

Many of the properties of n interconnected hydrogen bonds are different than that of M-times the sum of each individual bond. This effect is called cooperativity, and originates in the polarization that is induced in the donor and acceptor groups of one molecule due to the participation of this molecule in other hydrogen bonds. Such an effect makes that the strength estimated for the bonds in the isolated dimers (or isolated motifs) is smaller than that found in aggregates where multiple bonds can be found. 

A novel organocatalytic asymmetric cyclopropanation of y3,y-unsaturated a-ketoesters with stabilized sulfur ylides using C(2)-symmetric urea (6) as a hydrogen-bond catalyst has been described (Scheme 4). This reaction allows an efficient access to 1,2,3-trisubstituted cyclopropane derivatives in good yields with up to 16 1 dr and 90 10 er under mild reaction conditions. The mechanism study proved that the high stereoinduction originated from the cooperative effect of the hydrogen-bond catalyst. 

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