Imine Schiff base and

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/ ... 

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. 

The in situ formed basic cyclopentadienyl complex of titanium, ethylene-bis( f-tetrahydroindenyl)-titanium hydride (EBTHI)TiH) containing a planar element of chirality, is active for the reduction of a C=N double bond of both N-aryl-imines (Schiff bases) and unsubstituted imines (108,109). (EBTHl)TiX2 derivatives are used as precursors of the hydride analogue. The chiral zirconium analogues of the above-mentioned complexes provided a breakthrough in asymmetric polymerization of terminal oleflns (110). 

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... 

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. 

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 ... 

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... 

The interest in the mechanisms of SchifF base hydrolysis stems largely from the fact that the formation and decomposition of SchifF base linkages play an important role in a variety of enzymatic reactions, for example, carbonyl transfers involving pyridoxal phosphate, aldol condensations, /3-decarboxylations and transaminations. The mechanisms for the formation and hydrolysis of biologically important SchifF bases, and imine intermediates, have been discussed by Bruice and Benkovic (1966) and by Jencks (1969). As the consequence of a number of studies (Jencks, 1959 Cordes and Jencks, 1962, 1963 Reeves, 1962 Koehler et al., 1964), the mechanisms for the hydrolysis of comparatively simple SchifF bases are reasonably well understood. From the results of a comprehensive kinetic investigation, the mechanisms for the hydrolysis of m- and p-substituted benzylidine-l,l-dimethylethylamines in the entire pH range (see, for example, the open circles in Fig. 13) have been discussed in terms of equations (23-26) (Cordes and Jencks, 1963) ... 

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. 

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). 

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... 

Addition to the Carbonyl Group — The internal, cyclic hemiacetal formation is one of the illustrations of such addition. The H2N-X nucleophiles, with X being NH2 (hydrazine), NHAr (arylhydrazines), OH (hydroxylamine), NHCONH2 (semi-carbazide), NHCSNH2 (thiosemicarbazide), or alkyl (primary amine), produce hydrazones, arylhydrazones, oximes, semicarbazones, thiosemicarbazones, and alkyl imines (Schiff bases), respectively, following the following path 5.4 + 5.15 — 5.16 ->. .. -> 5.21. 

C.d. curves for the imines (Schiff bases) formed from some keto-steroids and 2-phenylethylamine, tyramine, or hexanamine show n- n bands generally of the same sign as those of the corresponding ketones, but near 235 nm.80 Additional... 

Jacobsen et al. have reported that peptide Schiff bases and an Al-salen complex are valuable for asymmetric hydrocyanation of imines with TBSCN [660] and TMSCN [661], respectively. It is, however, most likely that HCN arising from these cyanosilanes and adventitious water is the reactive nucleophile. Hoveyda et al. also have used TMSCN as a source of HCN in the hydrocyanation of imines catalyzed by a Ti-peptide Schiff base complex [662],... 

Reactions using highly acidic active methylene compounds (pAa = 9-13) comprise nearly all the early examples of imine condensation reactions, some of which date back to the turn of the century. Reviews by Layer and Harada have summarized many of these reactions and include examples using diethyl malonate, ethyl cyanoacetate, ethyl malonamide, acetoacetic acid, benzoylacetic esters and nitroalkanes. Conditions of these reactions vary they have been performed both in protic and aptotic solvents, neat, and with and without catalysts. Elevated temperatures are generally required. Reactions with malonates have useful applications for the synthesis of 3-amino acids. For example, hydrobenzamide (87), a trimeric form of the benzaldehyde-ammonia Schiff base, and malonic acid condense with concomitant decarboxylation to produce p-phenylalanine (88) in high yield (equation 14). This is one of the few examples of a Mannich reaction in which a primary Mannich base is produced in a direct manner but is apparently limited to aromatic imines. 

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