Schiff-bases organic solvents

Common impurities found in aldehydes are the corresponding alcohols, aldols and water from selfcondensation, and the corresponding acids formed by autoxidation. Acids can be removed by shaking with aqueous 10% sodium bicarbonate solution. The organic liquid is then washed with water. It is dried with anhydrous sodium sulfate or magnesium sulfate and then fractionally distilled. Water soluble aldehydes must be dissolved in a suitable solvent such as diethyl ether before being washed in this way. Further purification can be effected via the bisulfite derivative (see pp. 57 and 59) or the Schiff base formed with aniline or benzidine. Solid aldehydes can be dissolved in diethyl ether and purified as above. Alternatively, they can be steam distilled, then sublimed and crystallised from toluene or petroleum ether. 

Other aza macrocyclic complexes, such as Schiff bases or tetraazaannulene derivatives, have been also used with some success for the electrocatalytic reduction of C02 in organic solvent in the presence of proton sources.166-168... 

Cobalt(II)-Schiff-base complexes with a nitrogenous base are also well known to bind molecular oxygen in an organic solvent at room temperature. 

On the other hand, the production of desired compounds through reduction of starting material requires the electron donors to be oxidized (reductant). Alcohols are often used not only as a solvent but as the donor to produce useful compounds, e.g., anilines from nitrobenzenes,22) alcohols from aldehydes,23) and secondary amines from the corresponding Schiff bases.24) From the organic synthetic point of view, however, the separation of undesired products, aldehydes or ketones, from the alcohols is necessary unless subsequent reaction processes consume them25,26) or they are easily removed by distillation or other procedures. A recent report has shown that water acts as the electron donor and is converted into 02 in the photocatalytic regio-selective reduction of terpenes mixed with aqueous suspension of Ti02.27,28) It is notable that isolation of the desired product from the reaction mixture is simple in this type of photocatalytic reduction. 

The malonaldehyde-amine reaction has been extensively studied by Tappel and co-workers (2), by Csallany et al. (3), by Privett and co-workers (4), Knook et al. (5), Buttkus and Bose (6 ), as well as many others. As shown in Figure 1, malonaldehyde may react with a primary amine to produce an initial ene-amine Schiff base ( 7 ), then by further condensation with another amine, an amino-imino-propene compound. Heat and acid are required in the usual model reaction. The model compounds (lysine with malonaldehyde, for example) fluoresce between 430 and 470 nm when excited between 350 and 360 nm (8). These are typical of the "aging" pigments, the organic solvent-soluble lipofuscin in oxidizing tissue. 

In 1980 Karampurala et al prepared V02+, Mn2+, Zn2+, Cr3+, and Fe3+-based Schiff base polymers from 5,5 -methylene bis-salicylaldehyde and aniline.4 Similar to the polymers prepared by O Connell, all of these polymers are insoluble in common organic solvents. It is not possible to characterize these insoluble polymers by conventional methods, such as osmometry and visco-metry, and only elemental analyses and IR can be used for characterization. Instead of 5,5 -methylene bis-salicylaldehyde, hydrazinecarboxamide was subsequently used as monomer for the reactions (Fig. 14).30 However, the solubility issue had not been resolved, all of the V02+, Mn2+, Zn2+, Cr3+, and Fe3+ Schiff base polymers are insoluble in common solvents. 

A quantum chemical study of the mechanism of the cis-trans photochemical isomerization in retinal like protonated Schiff bases uses the model compound 1-imino-2.3-pentadiene23. The lowest excited state is B like but the second A like state is particularly labile photochemically. The photoisomerization of all trans-retinal has been studied in a variety of organic solvents and organized media232 The structure of the isomers involved in the photoisomerization of retinoic acid and photoprotective effects in model physiological solutions have been determined233. picosecond time-resolved absorption study of... 

Aromatic and aliphatic aldehydes in the presence of dialkylamines and an equivalent of acid such as hydrochloric, perchloric or p-toluenesulfonic acid give iminium salts, which add cyanide ion to form a-(dialkylamino)nitriles. An alternative preparation involves the reaction of the aldehyde with dialkylamines in the presence of acetone-cyanohydrin, a-(A, -dialkylamino)isobutyronitiiles, diethyl phosphorocyanidate or TMS-CN. Another route to a-aminonitrile starts with an aldehyde, the salt of an amine and KCN in organic solvents under solid-liquid two-phase conditions by combined use of alumina and ultrasound. Chiral a-aminonitriles were prepared by Strecker-type reactions, cyano-silylation of Schiffs bases, amination of a-siloxynitriles or from an A -cyanomethyl-l,3-oxazolidine synthon. Reaction of tertiary amines with CIO2 in the presence of 5.1 mol equiv. of aqueous NaCN as an external nucleophile affords a-aminonitrile. °... 

The 1,2-addition of aniline with quinones or ketones to form imines (Schiff bases) is less favorable in aqueous solution than in organic solvent, because the overall equilibrium favors hydrolysis in aqueous or partially aqueous solvents. In the case of sterically hindered quinones in aqueous solution, however, 1,2-addition by aniline is more favorable, and in some instances becomes the dominant mode of attack. As an illustration, 4-methylaniline was reported to undergo both 1,4- and 1,2-addition to 2,6-dimethyl-p-benzoquinone, resulting in a 3 1 product ratio of anilinoquinone to imine (15). 

The accessibility of the primary amine functions of chitosan has been quantified in organic solvent by formation of a salicylaldimine Schiff base upon treatment with salicylaldehyde. When contacting salicylaldehyde the aerogel beads of chitosan become yellow and their UV-visible diffuse reflectance spectra exhibit the absorption band at 318 nm, characteristic of the Schiff base. Quantitative GC analysis of the remaining salicylaldehyde in the solution allows calculation of the fraction of reacted amino groups, which corresponds to the fraction of accessible sites. In the aerogel formulation, up to 70% of the amine groups are accessible [92]. 

The same process can be used for the synthesis of 100% of L-amino acids by racemizing the Schiff base of the D-amide in a short time using small amounts of base in organic solvents. 

The non-cyclization of these compounds in organic solvents is explained by a fast cw-fra/M-isomerization and the non-planar geometry of these Schiffs bases. 

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