Synthesis with host—guest solutions

Diederich, F. and Dick, K. (1985) A Water-Soluble Tetraoxa[7.1.7.1] paracyclophane Synthesis and Host-Guest Interactions with Alicyclic and Cationic Guest Molecules in Aqueous Solution , Chem. Ber. 118,3817- 3829. 

A direct solution to the problem of enantioselective furan-carbonyl photoproduct synthesis would require a photoaddition that proceeded with enantiofacial selectivity in the aldehyde or furan component and maintained the relative face selectivity that is intrinsic to the reaction. Preliminary work used host-guest chemistry to achieve this objective.In aqueous dioxane, a 1 1 1 inclusion complex of unmodified 3-cyclodextrin, furan and benzaldehyde is formed. Upon irradiation (Hanovia 450 W lamp, Vycor filter), a rapid photoaddition occurs to afford a photoproduct of 10-20% ee. 

The attention towards electron transfer processes involving host-guest adducts of cyclodextrins (CDs) has become important with regard to their use as modifiers of organic electrode reactions. CDs, when added to solution or to electrode surfaces, can improve the selectivity of electrochemical synthesis. To elucidate the details of electron transfer reactions of guest molecules complexed inside CDs, the redox behavior of ferrocenecarboxylic acid in presence of jff-CD was studied, and this showed that the oxidation of the complexed ferrocenecarboxilic anion, FCA", must proceed via the dissociation of the host-guest adduct to form free FCA" which then transfers an electron to the electrode [81]. 

In the context of crown ether hosts, non-covalent bonds of pole-pole, pole-dipole, and dipole-dipole types can all be employed [3-6] in the formation of host-guest complexes. Where the guest species is an alkali metal (i.e. Li, Na", K", Rb, Cs ), alkaline earth metal (i.e. Mg, Ca, Sr, Ba ), or harder transition or post-transition metal (e.g. Ag", TT, Hg, Pb, La, Ce ) cation [3-6,14], an electrostatic (M" O) pole-dipole interaction binds the guest to the host whilst the (M" X ) pole-pole interaction with the counterion (X ) is often retained. The features are exemplified by the X-ray crystal structure [15] shown in Fig. la for the 1 2 complex (1) (NaPF jj formed between dibenzo-36-crown-12 (1) and NaPF. Molecular complexes involving metal cations have considerable strengths even in aqueous solution and a template effect involving the metal cation is often observed during the synthesis of crown ether derivatives. 

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