Imine Schiff base

The mechanistic pathway of the ordinary Friedlander synthesis is not rigorously known. Two steps are formulated. In a first step a condensation reaction, catalyzed by acid or base, takes place, that can lead to formation of two different types of products (a) an imine (Schiff base) 4, or (b) an o ,/3-unsaturated carbonyl compound 5 ... 

HCN adds to imines, Schiff bases, hydrazones, oximes, and similar compounds. The CN ion can be added to iminium ions ... 

Alkylidene sulfenes (75), generally prepared by the dehydrohalogenation of alkylsulfonyl chlorides, add readily to electron-rich multiple bonds. For example, with enamines, the thietane dioxide (e.g., 76) is formed diazoalkanes yield thiirane dioxides (episulfones) and imines (Schiff bases) afford 1,2-thiazetidine 1,1-dioxides. There are available numerous reviews of sulfenes, including cycloaddition reactions.102... 

Doebner-von Miller reaction is a variant of the Skraup quinoline synthesis (page 545). Therefore, the mechanism for the Skraup reaction is also operative for the Doebner-von Miller reaction. An alternative mechanism shown below is based on the fact that the preformed imine (Schiff base) also gives 2-methylquinoline ... 

The reaction proceeds as follows 1) An imine (Schiff base) is formed between a carboxyl group on the bound pyruvate and the amino group of histidine, with the elimination of a water molecule, 2) The bond to the carboxyl group of histidine is cleaved, liberating C02> 3) The second imine intermediate so formed is hydrolyzed to the free enzyme and histamine. 

Glycoproteins, such as horseradish peroxidase, are coupled selectively to other proteins or NH2-groups bearing molecules via oligosaccharide side chain oxidation. The vicinal OH groups of oligosaccharide residues are oxidized by periodate to aldehyde groups, which react with amines to form imines (Schiff bases). 

The structure of opsin is unknown, but its prosthetic group (1 l-c/.v-retinal) is bonded to it through an imine (Schiff base) function formed between the aldehyde group of the retinal and the side-chain amino function of a lysine unit of opsin ... 

Although azides cannot undergo addition to imines (Schiff bases),16,214 when the possibility of imine-enamine tautomerism exists, reaction occurs with the enamine to yield 5-aminotriazolines.35,43,81,180,208,214,223-226 In fact, Alder s N-4 compounds were obtained as shown in Scheme 49. 

In Section 18-15, we saw that amines attack ketones and aldehydes. When this nucleophilic attack is followed by dehydration, an imine (Schiff base) results. The analogous reaction of a hydrazine derivative gives a hydrazone, and the reaction with... 

Carbonyl addition reactions include hydration, reduction and oxidation, the al-dol reaction, formation of hemiacetals and acetals (ketals), cyanohydrins, imines (Schiff bases), and enamines [54]. In all these reactions, some activation of the carbonyl bond is required, despite the polar nature of the C=0 bond. A general feature in hydration and acetal formation in solution is that the reactions have a minimum rate for intermediate values of the pH, and that they are subject to general acid and general base catalysis [121-123]. There has been some discussion on how this should be interpreted mechanistically, but quantum chemical calculations have demonstrated the bifunctional catalytic activity of a chain of water molecules (also including other molecules) in formaldehyde hydration [124-128]. In this picture the idealised situation of the gas phase addition of a single water molecule to protonated formaldehyde (first step of Fig. 5) represents the extreme low pH behaviour. 

The pyrolysis and photolysis of fran5-retinoic acid in aqueous ethanol has been studied/ Mechanisms of thermal equilibration of retinylidene imines (Schiff bases) and their protonated immonium salts have been investigated Electrochemical studies on trans- and cw-retinal have been reported/ ... 

The carbonyl group is a reactive function and, although aromatic aldehydes are somewhat less reactive than their aliphatic counterparts, benzaldehydes have an extensive chemistry. Many reactions replicate those of aliphatic aldehydes, but are mentioned here for completeness. Thus, oxidation of the carbonyl group leads to carboxylic acids and reduction gives alcohols. The aldehyde group reacts with a range of N-nucleophiles (Scheme 6.9). Imines (Schiff bases) are formed with amines and hydrazones with hydrazines. Semicarbazide gives semicarbazones and hydroxylamine forms oximes. 

The first method used in the preparation of oxaziridines was the treatment of imines (Schiff bases) with peracids, especially peracetic acid. Although other methods have been occasionally employed in the preparation of oxaziridines, the reaction of peracids with imines has proven to be quite versatile and, invariably, the method of choice. This method was discovered by three independent groups in 1956-1958. ... 

While organometallic reagents condense with N-substituted imines (Schiff bases) to afford, after hydrolysis, good yields of substituted amines, the reaction with N-unsubstituted imines (203) derived from ammonia (which are easily hydrolyzed and self condense) is not synthetically useful. As a result, the use of masked imines containing labile silicon- or sulfur-Hiitrogen bonds, such as N-trimethylsilyl-imines (204) or N-sulfenimines (205), has been explored. 

Fructose 1,6-bisphosphate An imine (Schiff base) 1 j CHgOPOa - 1... 

As was true in step 4 of glycolysis (Figure 29.4), this aldol reaction actually takes place not on the free ketone but on an imine (Schiff base) formed by reaction of dihydroxyacetone phosphate with a side-chain -NH2 group on the enzyme. Loss of a proton from the neighboring carbon then generates an enamine (Section 19.9), an aldol-like reaction ensues, and the product is hydrolyzed. 

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). 

Primary amines (RNH2) also add reversibly to ketones or aldehydes to give imines (Schiff bases) and related compounds via the intermediate hemiaminals. The position of the equilibrium depends on the structure of the amine and the carbonyl compound. With alkylamines, the equilibrium favors the carbonyl compound, but it can be driven to the imine by removal of H20. With hydrazines (R2NNH2) and hydroxyl-and alkoxylamines (RONH2), the equilibrium greatly favors the hydrazone, oxime, or oxime ether, and it is difficult to drive the reaction in the reverse direction. Secondary amines (R2NH) can form hemiaminals, but they cannot form imines. 

A heteroatom in a C=X bond can use its lone pair to react with a Lewis acid to give a product for which a carbocationic resonance structure can be drawn. Protonation of a carbonyl compound belongs in this category. One of the lone pairs on O coordinates to H+ to give a compound with two major resonance structures, one of which is carbocationic. The carbocationic resonance structure is not the best structure, but it tells the most about the reactivity of the ion. If the carbonyl C has heteroatoms directly attached (e.g., esters, carboxylic acids, and amides), more resonance structures can be drawn. Reactions of imines (Schiff bases) often begin by protonation of N. 

The alkyne complex, (77s—C5H5X775—C5Me5)Ti(diphenylacetylene) reacted with C02 to give a five-membered metallacycle.53 Similarly, nickel complexes reacted with alkynes and C02 to yield the same type of metallacycles.54 The oxidative coupling of C02 with imines (Schiff bases) at electron-rich nickel(O) centers leads to cyclic carbamato complexes.55... 

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