Coordinated chelates, imine formation

Examples of carbanion additions to cobalt-coordinated imines will be considered in the next section (7.4.3.2) as chelate ring formation is a consequence in some cases. 

The most important reaction of this type is the formation of imine bonds and Schiff bases. For example, salicylaldehyde and a variety of primary amines undergo reaction to yield the related imines, which can be used as ligands in the formation of metal complexes. However, it is often more desirable to prepare such metal complexes directly by reaction of the amine and the aldehyde in the presence of the metal ion, rather than preform the imine.113 As shown in Scheme 31, imine formation is a reversible process and isolation of the metal complex results from its stability, which in turn controls the equilibrium. It is possible, and quite likely, that prior coordination of the salicylaldehyde to the metal ion results in activation of the carbonyl carbon to amine nucleophilic attack. But it would be impossible for a precoordinated amine to act as a nucleophile and consequently no kinetic template effect could be involved. Numerous macrocyclic chelate systems have been prepared by means of imine bond formation (see Section 61.1.2.1). In mechanistic terms, the whole multistep process could occur without any geometrical influence on the part of the metal ion, which could merely act to stabilize the macrocycle in complex formation. On the other hand,... 

One of the most spectacular and useful template reactions is the Curtis reaction , in which a new chelate ring is formed as the result of an aldol condensation between a methylene ketone or inline and an imine salt. The initial example of this reaction was the formation of a macrocyclic nickel(II) complex from tris(l,2-diaminoethane)nickel(II) perchlorate and acetone (equation 53).182 The reaction has been developed by Curtis and numerous other workers and has been reviewed.183 In mechanistic terms there is some circumstantial evidence to suggest that the nucleophile is an uncoordinated aoetonyl carbanion which adds to a coordinated imine to yield a coordinated amino ketone (equation 54). If such a mechanism operates then the template effect is largely, if not wholly, thermodynamic in nature, as described for imine formation. Such a view is supported by the fact that the free macrocycle salts can be produced by acid catalysis alone. However, this fact does not... 

Intramolecular imine formation between a coordinated aminate ion and a 2-oxo acid has been utilized to synthesize the two racemic amino acids 2-cyclopropylglycine and proline.463 Thus anation of [Co(NH3)5OH2]3+ by Br(CH2)3C0C02H at pH 5 gives two major products (145) and (146). Both are converted to tetraammineiminocarboxylato chelates by attack of an adjacent deprotonated ammonia. The cyclopropylimine complex can, for example, be reduced by alkaline BH4 to give the (RS)-2-cyclopropylglycine complex. 

Imine formation in coordinated chelates has been known for a very long time but what has not been all that clear until recently is the stability of the coordinated imine when the imine group is essentially exo to the chelate ring. Systems such as... 

The [Co(NH3)s(NH2CH2COCH3)] ion undergoes an intramolecular base-catalyzed cyclization reaction to give a coordinated carbinolamine, which undergoes a slower base-catalyzed dehydration to give a chelated imine. The synthesis, characterization and kinetics of formation of various reaction products are described. Several reactions of the imine product have been carried out, including BH4 reduction and condensation with methyl vinyl ketone. 

In contrast to earlier known imines, those imines derived from a-(methoxymethyl)benzene-ethanamine, which allow formation of a rigid chelate by additional coordination of the lithium with the methoxy group, enabled the preparation of a-alkylated cyclic ketones in very high enantiomeric excesses (90-99% ee)7,8. However, alkylations of imines derived from medium ring ketones were accomplished in 30-82% ee9. The alkylation of acyclic ketones was performed with enantiomeric excesses of more than 75 % and, in the case of the imine derived from 4-heptanone, proceeded with complete asymmetric induction10. 

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 presence of a coordinating ether can effect a- or -chelation, as shown for imine 188. The stereochemical features of the stable six-membered metallocycle from 188 provide for diastereofacial selectivity, as illustrated in the formation of 189 (Scheme 5.2.41).58... 

Mechanism of the arylamine formation is the following. Oxidative addition of aryl halide generates the arylpalladium 6, to which an amine coordinates. Abstraction of proton from the coordinated amine with a base (BM) affords the arylpalladium amide 7, and its reductive elimination yields the arylamine 8. Reductive elimination of 7 is a rate-determining step. A path, competitive to the path from 7 to 8, is jS-H elimination to give the imine 9 and the Pd hydride 10, and the reduced arene 11 is formed by reductive elimination of 10. This is a serious side reaction. The elimination of jS-H may be minimized by the use of chelating ligands. 

The condensation reaction of coordinated amines with formaldehyde and nitro-alkanes is believed [166] to proceed through an imine (or aminol) intermediate sensitive to attack of the carbon acid conjugate base on the imine carbon atom (Scheme 2-11). The intramolecular path shown in this diagram is entropically favourable, so construction of the six-membered chelate cycle becomes possible and its formation proceeds in high yield. It should also be pointed out that the first condensation step involves nucleophilic attack on an HCHO molecule by a coordinated amido ligand arising from deprotonation of the metal-bound primary amine. 

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