Hydrogen-bonding acceptors, formate

Recently, Kamlet and Taft introduced new elaborate parameters in order to explain the linear energy relationship for the formation of the hydrogen bond between HBA (hydrogen bond acceptor) and HBD (hydrogen bond donor). They treated several sulphoxides as HBA. The detailed presentations and actual treatments of these parameters have been described in their recent review article72. 

A similar quantitative treatment of sulphoxides as hydrogen bonding acceptors has been obtained by comparing the IR frequency shift AvOH of the C—I bond in an acetylenic iodide such as IC=CI (Avc j) due to formation of a C—T complex with phenol in various bases. This investigation suggests that sulphoxides belong to the same family as carbonyls, phosphine oxides, arsine oxides and their derivatives90. 

With this separation of the hydrogen bond-acceptor template and the sites for the stopper attachment, the steric effects due to shielding by the wheel are avoided making the synthesis of the rotaxane 48 more efficient (in some of the systems the yields of rotaxane formation increase from 5 to 30% when the functionalised centre-piece is added). 

Further studies by the same authors have demonstrated that PFj acts a hydrogen bond-acceptor template in the assembly of several other interwoven structures. In an extensive study aimed at using a combination of hydrogenbonding motifs to self-assemble pseudorotaxanes into more complex structures it was discovered that PFj assists on the organization of the components that yield the final superstructure [74]. Particularly, it was found that this anion dictates the orientation of the two carboxylic acid groups of the [3]pseudorotaxanes 58 and 59 (see Schemes 27 and 28) when these groups are co-directional with respect to each other the formation of discrete hydrogen-bonded dimers is observed. 

Because warming can often disperse protein-polyphenol hazes, it is clear that covalent bonding is not involved in their formation. Asano et al. demonstrated that protein-polyphenol haze formation is inhibited by the nonpolar solvent dioxane and the hydrogen bond acceptor dimethylforma-mide (DMF), but not by a solution of sodium chloride (Asano et al., 1982). 

Beijer FH, Sijbesma RP, Kooijman H, Spek AL, Meijer EW. Strong dimerization of ureidopyr-imidones via quadruple hydrogen bonding. Self-assembly mediated by the donor-donor-acceptor acceptor-acceptor-donor (DDA-AAD) hydrogen-bonding motif formation of a robust hexameric aggregate. J Am Chem Soc 1998 120 6761-6769. 

Koga et al. complexed oxo acids like methyl phenylphosphonate with bis(resorcinol) quinoline derivative 57 [83], Anslyn and coworkers presented a more rigid polyazacleft containing hydrogen bond acceptor and donor sites in form of pyridine rings and amino groups, respectively. The formation of the 3 1 complex 58 with dibenzyl phosphate is assumed, in which the four components are spatially fixed by a net of H-bonds [84]. 

Banks JT, Ingold Kl, Lusztyk J (1996) Measurement of the equilibrium constants for complex formation between phenol and hydrogen-bond acceptors by kinetic laser flash photolysis. J Am Chem Soc 118 6790-6791... 

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