Imines chelates

The first section will consist of the synthesis and reactions of imine chelates, as this is the most widely and consistently used aspect of ligand reactions. The discussion will be subdivided according to the types of ligands involved and will range from bidentate through to multidentate ligands, macrocycles and cage compounds. 

SYNTHESIS AND REACTIONS OF IMINE CHELATES 61.1.2.1 Simple Imines Formed by Thermodynamic Template Reactions... 

The initial reactions of this type were used to produce copper(II) complexes of salicylaldimines, where additional coordination support is provided by the phenolic oxygen atoms (equation l).10 11 Similar use can be made of pyridine-2-carbaldehyde in the formation of imine chelates. For example, the combination of pyridine-2-carbaldehyde and 2-aminobenzenethiol in the presence of metal ions yields tridentate complexes (Scheme l).12 If the reaction is carried out in the absence of the metal ion, the benzothiazolidine (1) is isolated. Thus the metal ion is essential for the formation of the imine product in this case. Metal complexes of the related tridentate ligands (2),13 (3),14 (4)li and (5)16 can be prepared similarly. 

The synthesis of quadridentate imine chelates usually requires the combination of two equivalents of the carbonyl compound and a diamine, as in the formation of complexes of ligands (6),17 18 (7)19 and (8).20,21 In similar fashion, thermodynamic template reactions allow the very effective synthesis of quinquedentate and sexadentate metal complexes of ligands such as (9),22 (10),23 (ll),24 (12)25 and (13).26 Condensation of 1,1,1-tris(aminomethyl)ethane with pyridine-2-carbaldehyde alone yields... 

Pyridine-2,6-dicarbaldehyde and 2,6-diacetylpyridine have been widely used in the template synthesis of imine chelates ranging in complexity from linear tridentates, such as (17),38 39 to macrocyclic structures with a range of ring sizes, such as (18).40-42 The in situ formation of macrocyclic ligands of this type depends upon the ring size and the strength of complexation of the triamine by the metal ion at the pH of the reaction. Related complexes with an additional donor atom attached to R2 have been synthesized also.43 44... 

One of the earliest observations of imine formation on cobalt(III) was that arising from the treatment of pyruvatopentamminecobalt(III) with base to pH > 12 to give the imine chelate (92)... 

The acetylacetone carbanion undergoes addition to the imine carbon atom of complex (92) but subsequent cyclization and deacylation processes occur (Scheme 42).212 The apical ammonia group is the most acidic and consequently is favoured to cyclize on to the carbonyl group. In the bis-(1,2-diaminoethane) complex related to the imine chelate (92), the two apical nitrogen atoms are no longer geometrically equivalent. Thus two products are formed when hydrogen cyanide is added to the complex and subsequent cyclization takes place (Scheme 43).213 However, the cyclization reaction is stereoselective. 

Hydride reduction of imine chelates such as (92) offers a synthetic route to a-amino acids, as demonstrated for cyclopropyl glycine and proline (Scheme 44).214... 

Alkyllithium addition to the (ti -arene) dicarbonylchromium imine chelates has been examined. Treatment of optically pure chelate (41) with methyllithium provides amine (42) with an enantiomeric excess of 94% (Scheme 5). No diastereoselectivity was reported for alkyllithium additions to (arene)tri-carbonylchromium complex (40). In the absence of additional examples, the generality of this chromium chelate methodology for asymmetric amine synthesis cannot be assessed. 

Pyridine-2,6-dicarbaldehyde and 2,6-diacetylpyridine have been widely used in the template synthesis of imine chelates ranging in complexity from linear tridentates, such as to... 

Lee, D.W., Kim, M.K., Kim, I.W., Park, J.H. and No, K.T. (1996). Studies on the Chromatographic Behaviors of Pd(ii)-alpha-isonitroso-beta-diketone Imine Chelates in Reversed-Phase Liquid Chromatography Using Molecular Descriptors. BuU.Kor.Chem.Soc., 17,1158-1161. 

Presumably the activity arises becuase the metal ion imparts some iminium character to the chelate. At the same time the metal prevents protonation of the imine and stabilizes the imine chelate. For example, species of this type are stable in 6 M HC1. Cyanide ion should add in the same manner as the carbanions and we have investigated this reaction using the pyruvato imine bis(ethylene-... 

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