Salicylaldehyde imines

Comforth has reviewed literature reports and independently studied the special cases of reaction of 1 with salicylaldehyde and with 2-acetoxybenzaldehyde. Coumarins (10) are afforded in the condensation of 1 with salicylaldehyde or its imine, whereas when 2-acetoxybenzaldehyde is used, acetoxy oxazolone 12 is the major product. The initial aldol condensation product between the oxazolone and 2-acetoxybenzaldehyde is the 4-(a-hydroxybenzyl)oxazolone 11, in which base-catalyzed intramolecular transacetylation is envisioned. The product 9 (R = Ac) can either be acetylated on the phenolic hydroxy group, before or after loss of acetic acid, to yield the oxazolone 12, or it can rearrange, by a second intramolecular process catalyzed by base and acid, to the hydrocoumarin, which loses acetic acid to yield 10. When salicylaldehyde is the starting material, aldol intermediate 9 (R = H) can rearrange directly to a hydrocoumarin. Comforth also accessed pure 4-(2 -hydroxyphenylmethylene)-2-phenyloxazol-5(4//)-one (13) through hydrolysis of 12 with 88% sulfuric acid. 

A structurally related tetrameric macroheterocycle is compound 13 that is prepared in a one-pot synthesis (yield 64%) from salicylaldehyde and (3-aminophenyl)boronic acid in methanol (Fig. 4). Due to its insolubility it has been characterized only by mass spectrometry. If a substitutent is introduced at the imine function (R = Me, Ph), trimeric structures (14 and 15) are... 

Figure 3.4 Multistage imine formation and metal complexation produces a diverse library of amine-, salicylaldehyde- and salicylaldimine-zinc complexes.

Intramolecular hydrogen-transfer to an excited imine is responsible for the photochromism of derivatives of salicylaldehyde (o-hydroxybenzaldehyde) irradiation converts the colourless or yellow imine into a much more intensely coloured photoproduct 15.9), but removal of the light source allows rapid reversal of the process. Reactions of this kind form the basis of some organic photochromic glass formulations exposure to increasingly bright light alters the proportion of the isomers in favour of the dark-coloured photoproduct. 

It has been verified by electron-spin magnetic-resonance spectroscopy that for copper(II) bis-acetylacetonate26 in mole-percent solution in a crystal of palladium(IJ) bis-acetylacetonate and for copper (II) bis-salicylaldehyde-imine27 in mole-percent solution in a crystal of nickel(II) bis-salicylaldehyde-imine the four coplanar short bonds formed by the copper atom are largely covalent and the two long bonds have very little covalent character. 

Phenylcoumarins are conveniently prepared by the Knoevenagel condensation of salicylaldehyde with a benzyl cyanide, in the presence of base such as sodium hydroxide or piperidine, followed by acid hydrolysis of the resultant imine. A second convenient synthesis is via condensation of 2-methoxybenzaldehyde with benzyl cyanide and cyclization of the 2-methoxy-a-phenylcinnamic nitrile in pyridine (Scheme 12). 

The use of chiral complexes gives rise to enantioselectivity of carbon—carbon bond formation and this phenomenon has also been applied to resolution54 and enantioselective deuteration55 56 of amino acids. Both the kinetic acidity of the a-methylene protons and the enantioselectivity of bond formation are greatly enhanced by the formation of chiral complexes (32) and (33) of imines derived from the amino acid and salicylaldehyde or pyridoxal respectively.57-59 Similar use has been made of inline complexes (34) derived from pyruvic acid and the amino acid.60 61 Very recently, an asymmetric synthesis of threonine has been achieved using the chiral imine complex (35).62... 

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,... 

ALET3-teff-BuTYL Chloride-Cobalt Compound Catalyst. PVC, 20 grams, was suspended in 190 ml chlorobenzene, and the resultant slurry was cooled to 5°C. The addition of 2 grams of butadiene was followed by the successive addition of 0.01 mmole of cobalt (II) bis(salicylaldehyde imine), 2 mmoles of terf-butyl chloride, and 2 mmoles of triethylalumi-num. The reaction mixture was stirred at 8°C for 60 minutes and poured into 1 liter of methanol. The vacuum-dried product weighed 21.9 grams, representing an add-on of 5.1%. 

Et2AlCl could be replaced by the sesquichloride or by a mixture of a trialkylaluminum and a reactive halide such as benzyl chloride or tert-butyl chloride. The effective cobalt compounds were those which are known to yield cis-1,4-polybutadiene—e.g. cobalt stearate, cobalt acetyl-acetonate, cobalt bis(salicylaldehyde imine), cobalt chloride-pyridine, etc. Et2AlCl concentration could be varied within the range 0.3-5% by weight based on PVC, and the cobalt compound concentration was 0.002-0.01 mole per mole of Et2AlCl. 

Figure 4-22. The imine formed from the reaction of 1,2-diaminoethane with salicylaldehyde is stable to hydrolysis.

A mixture of (R)-[N-3-(3-fluoro-4-(4-morpholinylphenyl)-2-oxo-5-oxazolidinyl]methanol 4-nitrobenzenesulfonate ester, isopropanol (149 ml), acetonitrile (245 ml), salicylaldehyde (13.7 ml, 129 mmol) and aqueous ammonia (30%, 257 ml, 4.02 mol), is heated to 40°C and stirred at 39-42°C for 24 hours. The mixture is then cooled to -22°C and the precipitate collected by vacuum filtration, washed with water (10 ml) and dried to give the (S)-[N-3-(3-fluoro-4-(4-morpholinylphenyl)-2-oxo-5-oxazolidinyl]methylamine salicylaldehyde imine. 

Lanthanides form complexes with Schiff bases or imines quite readily. The important ligands are obtained by the condensation of an amine with salicylaldehyde derivative or a /6-diketone. Schiff base complexes have been prepared by the reaction of ligand with a lanthanide 2-propanolate. The solvent medium is benzene and the liberated propanol during the complexation reaction is removed by distillation as alcohol-benzene azeotrope [57,58]. 

One of the best known Schiff base ligands is bis(salicylaldehyde)ethylenedi-imine (salen) ... 

The hydrolysis of salicylaldehyde imines is catalyzed by boric acid, substituted arylboronic acids, and diphenylborinic acid. The effects of different substituted phenyl-boronic acids on the rate of hydrolysis at pH 6.0 has been studied by Rao and Philipp [168]. The second-order rate constants, are higher for phenylboronic... 

Direct Combination of a 1,3-Diketone, an Amine, and a Metal Ion. Direct combination is a potentially useful reaction, and it has been used to prepare metal-salicylaldimine compounds. The procedure consists of mixing (and refluxing if necessary) a stoichiometric mixture of salicylaldehyde with an amine, a metal ion, and a base in dilute aqueous methanol. This procedure has not been applied commonly to the preparation of metal derivatives of jS-keto imines, although the preparation of 4,4 -(ethylene-dinitrilo)di-2-pentanonatocopper(II) from 2,4-pentanedi-one (acetylacetone), ethylenediamine, and copper has been described. Attempts to effect condensation of 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (hexafluoroacetylacetone) and ethylenediamine by direct combination of the reactants in the presence of copper(II) acetate were imsuccessful. ... 

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