Subject Schiff bases

Much work has been devoted to the study of Schiff base complexes, in particular M(salen), where M = metal, has been the subject of extensive work 114). The early work by Calvin et al. (section 111(A)) suggested that the 2 1 (M O2) dioxygen adduct, type I, formed by Co(salen) in the solid state, contains a peroxo linkage. An X-ray analysis 115, 116) of the complex (Co Salen)202(DMF)2 supports this hypothesis see Fig. 5 for the pertinent results of this study. 

A total synthesis of (+)-55 was performed by Cushman et al. 69) (Scheme 17). It was based on cycloaddition of Schiff base 68 to anhydride 69. The addition product 70, received in the form of a mixture of diaste-reomers, was then subjected to thermal decarboxylation to give rise to diastereomer 71 with the desired trans configuration as the major product. The latter upon methanolysis and selective reduction furnished (+)-55. 

The synthesis of phthalimidines by dicobalt octacarbonyl-catalyzed carbonylation of Schiff bases was first described by Pritchard78 and the scope of the reaction was evaluated by Murahashi et a/.79 Later Rosenthal et al.80-83 subjected a variety of related compounds to carbonylation, and also achieved a phthalimidine synthesis directly from benzonitrile under the conditions of the oxo process.84 An example illustrating the formation of a phthalimidine is shown in Scheme 49 a comprehensive review of the scope and mechanism of reactions of this type is available.85... 

The ligand reaction step may occur either with the template metal still intact or may take place after removal of the metal ion from the ring. As already mentioned, many of the Schiff-base macrocycles are unstable in the absence of a coordinated metal ion. However, for such systems, it has often been possible to hydrogenate the coordinated imine functions directly. The resulting saturated ligands will not be subject to the hydrolytic degradation which occurs for the imine precursors in the absence of their metal ion. 

Activation volumes for aquation of Schiff base complexes [Fe(C5H4NCH=NHR)3] + (R = Me, Et, Pr , Bu ) in 0.1 M aqueous HCl are between - -11 cm mol and - -14cm mol v and thus within the range established earlier " for (substituted) tris-l,10-phenanthroline-iron(II) complexes. These positive values are consistent with dissociative activation, as are those for dissociation of [Fe(5Brphen)3] + and of [Fe(5N02phen)3] " " in the presence of edta. AF and values for aquation of [Fe(5Brphen)3] have the subject of isochoric analysis. " Medium effects on activation volumes have been reviewed for iron-diimine complexes in binary aqueous solvent mixtures. 

The various reactions undergone by coordinated amino acids have been the subject of several reviews28,31,32,438,446 and only a brief discussion will be given here. The reactions which occur can be roughly classified under three headings (a) aldol condensations, (b) reactions of complexes of amino acid Schiff bases, and (c) isotopic exchange and racemization at the a-carbon of the amino acid. 

Secondary amines can be prepared from the primary amine and carbonyl compounds by way of the reduction of the derived Schiff bases, with or without the isolation of these intermediates. This procedure represents one aspect of the general method of reductive alkylation discussed in Section 5.16.3, p. 776. With aromatic primary amines and aromatic aldehydes the Schiff bases are usually readily isolable in the crystalline state and can then be subsequently subjected to a suitable reduction procedure, often by hydrogenation over a Raney nickel catalyst at moderate temperatures and pressures. A convenient procedure, which is illustrated in Expt 6.58, uses sodium borohydride in methanol, a reagent which owing to its selective reducing properties (Section 5.4.1, p. 519) does not affect other reducible functional groups (particularly the nitro group) which may be present in the Schiff base contrast the use of sodium borohydride in the presence of palladium-on-carbon, p. 894. 

Myriad polydentate aza-macrocycles have been reported 41. The extent of the subject forces limitation of this discussion to only macrocycles containing a pyridine or dipyridine subunit. Most of these coronands have been synthesized by a SchifF base condensation of an aldehyde or ketone with a hfc-primary amine in the presence of a metal ion. The metal ion acts as a template, resulting in dramatic increases in yield of the desired cyclic product over linear polymerization products42 46. Lindoy and Busch45 have described this effect in two ways, kinetic and thermodynamic. If the metal ion controls the steric course of a series of stepwise reactions, the template effect is considered to be kinetic. If the metal ion influences an equilibrium in an organic reaction sequence by coordination with one of the reactants, the template effect is termed thermodynamic. It is the kinetic effect that is believed to be operative in most metal ion-assisted (in situ) syntheses of... 

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. 

Benzodiazocine 264 was prepared through a 4-component Ugi reaction including a primary amine tethered to a BOC-protected internal amino nucleophile, followed by a postcondensation base-catalyzed cyclization. Thus, 2 equiv of aldehyde 262 were employed to promote Schiff base formation and a one-pot, double scavenging protocol with immobilized tosylhydrazine and di-isopropylethylamine removed both the excess aldehyde and any unreacted acid 261. The intermediate 263 was then subjected to treatment with TFA, followed by proton scavenging with resin bound morpholine, to promote cyclization to afford the eight-membered ring (Scheme 47) <2001TL4963>. 

The peroxide-quinone dioxime or peroxide-dibenzoyl-quinone dioxime cured EPR is much more heat-stable [151] than those of ovenaging samples because both crosslinking as well as radical trapping is done simultaneously. The aliphatic primary diamine [152] or aromatic primary diamine [153] were subjected to polycondensation separately with a quinone to give a Schiff base polymer of type VI or VII i.e. 

Macrocyclic ligands with all-nitrogen donor sets are much studied and both tin and, in particular, lead are popular subjects in coordination studies of these ligands. Examples of such ligands used to complex tin include (66) and (67), prepared by Schiff-base condensations. The complex (68) was isolated from an attempted template synthesis of a macrocycle in which the condensation of the component parts of the ligand was incomplete. 

The first reports of a reaction of an amine with an aldehyde by Schiff [584] led to the establishment of a large class of ligands called Schiff bases. Among the most important of the Schiff bases are the tetradentate salen ligands (N,N -bis(salicy-laldehydo)ethylenediamine), which were studied extensively by Kochi and coworkers, who observed their high potential in chemoselective catalytic epoxidation reactions [585]. The best known method to epoxidize unfunctionalized olefins enantioselectively is the Jacobsen-Katsuki epoxidation reported independently by these researchers in 1990 [220,221]. In this method [515,586-589], optically active Mn salen) compounds are used as catalysts, with usually PhlO or NaOCl as the terminal oxygen sources, and with a O=Mn (salen) species as the active [590,591] oxidant [586-594]. Despite the undisputed synthetic value of this method, the mechanism by which the reaction occurs is still the subject of considerable research [514,586,591]. The subject has been covered in a recent extensive review [595], which also discusses the less-studied Cr (salen) complexes, which can display different, and thus useful selectivity [596]. Computational and H NMR studies have related observed epoxide enantioselectivities... 

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