Schiff bases, formation hydration

Aldehyde 82 was extremely reactive and was best isolated as the hydrate 84a. Indeed, recrystallization of the aldehyde 82 from ethanol gave 3-(l-ethoxy-l-hydroxymethyl)fervenulin 84b, while reaction with ethylene glycol gave the cyclic acetal 76a. The reactivity of the aldehyde 82 was exploited by easy Schiff base formation upon reaction with /i-aminobenzoylglutamic acid, a process that was followed by reduction to give the fervenulin-based folic acid analogue 85 <1996JHC949>. 

Carbonyl addition reactions include hydration, reduction and oxidation, the al-dol reaction, formation of hemiacetals and acetals (ketals), cyanohydrins, imines (Schiff bases), and enamines [54]. In all these reactions, some activation of the carbonyl bond is required, despite the polar nature of the C=0 bond. A general feature in hydration and acetal formation in solution is that the reactions have a minimum rate for intermediate values of the pH, and that they are subject to general acid and general base catalysis [121-123]. There has been some discussion on how this should be interpreted mechanistically, but quantum chemical calculations have demonstrated the bifunctional catalytic activity of a chain of water molecules (also including other molecules) in formaldehyde hydration [124-128]. In this picture the idealised situation of the gas phase addition of a single water molecule to protonated formaldehyde (first step of Fig. 5) represents the extreme low pH behaviour. 

In many cases, addition or removal of water proceeds sufficiently slowly that some of the physical properties of unstable species (such as hydrated neutral quinazoline or anhydrous 2-hydroxypteridine) can be observed. In these cases, reaction kinetics can also be examined. Addition of water to pteridine is of special interest in relation to studies of the formation and hydrolysis of Schiff bases. The reaction proceeds in two reversible stages, 3 4 5 ... 

All the Schiff s base complexes discussed so far are low-molecular weight complexes. An erbium(lll)-containing methacrylate metallopolymer was prepared by free-radical polymerization of a Schiff s base monomer LllH, followed by complex formation with hydrated erbium nitrate (Haase et al., 1996). By XRD, the presence of a smectic mesophase was shown. A special feature of the polymer is that the complexing group is an Ai-aryl substituted Schiff s base, rather than an Ai-alkyl substituted one. No further structural data were reported. A detailed study of the mesophase behavior of lanthanide complexes of Ai-aryl substituted Schiff s base ligands LllH was reported by Rao et al. (2002, 2010). The complexes were synthesized by reaction between the... 

---