Schiff bases from

As has been outlined for the Strecker synthesis, the Ugi reaction also proceeds via initial formation of a Schiff base from an aldehyde and an amine. The imine intermediate is attacked by the isocyanidc, a process which is supported by protonation of the imine by the carboxylic acid component. The resulting a-amino nitrilium intermediate is immediately trapped by the carboxylate to give an 6>-acyl imidiate. All steps up to this stage are reversible. Only the final oxygen to nitrogen acyl shift is irreversible and delivers the A-acyl-a-amino amide as the thermodynamically favored product which contains two amide groups. 

Ligand abbreviations acacjtrien = hexadentate Schiff base from triethylenetetramine (trien) and acetylacetone (acac) Saljtrien = hexadentate Schiff base from triethylenetetramine (trien) and salicylaldehyde. 

L = tridentate Schiff base from pyridine-2-aldehyde and N,N,2,2-tetramethylpropane-l,3-diamine. 

Fig. 6.3 Content of SchifF bases (% from control) in thymocytes (A), EAC (B), and L1210 cells (C) after 1 h of incubation in the presence of photoexcited samples of fullerenes (1 - fullerene CM, 2 - C60-composite-l, 3 - C60-composite-2). P < 0.05 compared to the control...

Structures have been determined for [Fe(gmi)3](BF4)2 (gmi = MeN=CHCF[=NMe), the iron(II) tris-diazabutadiene-cage complex of (79) generated from cyclohexanedione rather than from biacetyl, and [Fe(apmi)3][Fe(CN)5(N0)] 4F[20, where apmi is the Schiff base from 2-acetylpyridine and methylamine. Rate constants for mer fac isomerization of [Fe(apmi)3] " were estimated indirectly from base hydrolysis kinetics, studied for this and other Schiff base complexes in methanol-water mixtures. The attenuation by the —CH2— spacer of substituent effects on rate constants for base hydrolysis of complexes [Fe(sb)3] has been assessed for pairs of Schiff base complexes derived from substituted benzylamines and their aniline analogues. It is generally believed that iron(II) Schiff base complexes are formed by a template mechanism on the Fe " ", but isolation of a precursor in which two molecules of Schiff base and one molecule of 2-acetylpyridine are coordinated to Fe + suggests that Schiff base formation in the presence of this ion probably occurs by attack of the amine at coordinated, and thereby activated, ketone rather than by a true template reaction. ... 

The coordination shell comprises three fac-S and three fac-N-donor atoms in [Fe(6mp)3] [FeClJCl, where 6mp = 6-thiopurine, (189). Both iron(II) and iron(III) complexes are included in a review of transition metal complexes of thiosemicarbazones. " 5,5 -Dimethyl-l,2,3-cyclohex-anetrione-l,2-dioxime-3-thiosemi-carbazone, dcdt (190), acts as an A, A, 5 -donor to Fe +, giving a bis-ligand complex (contrast [Fe(7V,7V -dcdt)3] with Fe " "). The Schiff bases from pyridine 2-carboxaldehyde and thiosemicarbazide or 4-phenyl thiosemicarbazide also act as A, VV, 5 -donors, both to Fe " " and to Fe " ". The antibacterial activity of these complexes was assessed, in... 

Eichhom and his co-workers have thoroughly studied the kinetics of the formation and hydrolysis of polydentate Schiff bases in the presence of various cations (9, 10, 25). The reactions are complicated by a factor not found in the absence of metal ions, i.e, the formation of metal chelate complexes stabilizes the Schiff bases thermodynamically but this factor is determined by, and varies with, the central metal ion involved. In the case of bis(2-thiophenyl)-ethylenediamine, both copper (II) and nickel(II) catalyze the hydrolytic decomposition via complex formation. The nickel (I I) is the more effective catalyst from the viewpoint of the actual rate constants. However, it requires an activation energy cf 12.5 kcal., while the corresponding reaction in the copper(II) case requires only 11.3 kcal. The values for the entropies of activation were found to be —30.0 e.u. for the nickel(II) system and — 34.7 e.u. for the copper(II) system. Studies of the rate of formation of the Schiff bases and their metal complexes (25) showed that prior coordination of one of the reactants slowed down the rate of formation of the Schiff base when the other reactant was added. Although copper (more than nickel) favored the production of the Schiff bases from the viewpoint of the thermodynamics of the overall reaction, the formation reactions were slower with copper than with nickel. The rate of hydrolysis of Schiff bases with or/Zw-aminophenols is so fast that the corresponding metal complexes cannot be isolated from solutions containing water (4). 

The Schiff bases from substituted benzaldehydes and anilines will carbonylate in the presence of cobalt carbonyl, as catalyst at 225°C. producing phthalimidine derivatives, XVI, in good yield (70, 52). This reaction may be explained as an... 

Schiff bases from substituted and unsubstituted benzaldehyde and aliphatic or aromatic amines are more stable [6-8]. Benzaldehyde substituents such as nitro, dialkylamino, hydroxyl, methoxyl, or halo have been used [7a-c]. Benzalaniline, m.p. 51°-52°C, is prepared in 87% yield by adding 1.0 mole of aniline to 1.0 mole of benzaldehyde at room temperature in the absence of a solvent. The crude product is added to alcohol [5] in order to induce crystallization [6]. 

Mononuclear V02+ and heterodinuclear V02+ complexes (with Cu2+, Ni2+, Mn2+, Zn2+, Pd2+ as the second metal ion) have been reviewed.894 In this section, we shall mainly discuss more recent work. Many complexes were with Schiff bases from the reaction of /3-triketones or /3-ketophenols with ethylenediamine. One important aim has been to compare the magnetic, spectral and chemical properties for the metal ion Ma in [Ma(diken)] and Mb in [Mb(diketonato)2], with the corresponding properties in the binuclear complexes formed (see equation 66). 

FIGURE 18-5 Pyridoxal phosphate, the prosthetic group of aminotransferases. (a) Pyridoxal phosphate (PLP) and its aminated form, pyri-doxamine phosphate, are the tightly bound coenzymes of aminotransferases. The functional groups are shaded, (b) Pyridoxal phosphate is bound to the enzyme through noncovalent interactions and a Schiff-base linkage to a Lys residue at the active site. The steps in the formation of a Schiff base from a primary amine and a carbonyl group... 

Ketimine 121,744s. See also Schiff base from pyridoxal phosphate 742 as electron acceptor 746, 747 a-Ketoacid. See 2-Oxoacid Ketoamine 434s Ketodeoxyoctonate. See KDO Ketone(s), acidity of 46 Khorana, H. Gobind 84 Kidney cells, alkaline phosphatase in 645... 

What are the chemical structures of the intermediates in Eq. 23-37, and why are there so many of them The answer to the last question is that the initial photochemical process is very fast. Subsequent conformational rearrangments and movement of protons are slower, occur in distinct steps, and give rise to the observed series of intermediates. To shed light on these processes many experiments have been done with analogs of retinal,502,505 508 often using very rapid spectroscopic techniques.37,508 These studies have shown that the isomerization of the Schiff base from... 

The Schiff base from 2-aminomethylfuran and salicylaldehyde forms a zinc(II) complex which has been shown, by NMR and IR spectroscopy, to contain the ligand in the form (21), rather than as a 2-furylimine (22).330 The complex [Zn2La] (H2L = 23) has been described and considered to have the structure (24).331... 

Besides benzalanilines, also anils derived from heterocyclic aldehydes may be employed (Table VI), as, for example, in the reactions of 4,5-diphenyl-2-(p-tolyl)oxazole (58) with SchifFs bases from p-chloro-aniline and 3-formylpyridine and 2-formylthiophene, leading, respectively, to the /3-(3-pyridyl)-4-(4,5-diphenyloxazol-2-yl)styrene (59) 33 and the /S-(2-thienyI) analog (60).11... 

Thus, with methyl and also ethyl cyanoacetate, the anilinomethylene chromonedione (220) (this form has been found94 to predominate in dimethylsulfoxide solution rather than the tautomeric structure of the formal SchifFs base from 3-formyl-4-hydroxycoumarin and aniline) yields, initially, with potassium hydroxide in DMF, following hydrolysis, the pyronocoumarin 221. 

Aryl amines, 403 4-Aryl-2-aminothiazoles, 178-182 mercuration of, 81 nitration of, 72 reduction of, 86 Schiff bases from, 99 Aryl diazonium salts, reaction with, rho-danine, 419... 

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