Imines, Iminium Salts, and Related Compounds

Imination of aldehydes, ketones, and acid chlorides has been 

Relatively stable ketimines were obtained by the base-catalysed 

Chiral imines were prepared from cyclohexane and methoxyamines derived from D-camphor derivatives the imines undergo metalation and alkylation to give 2-alkylcyclohexanones of high enantiomeric 

The di-imine (118), prepared from glyoxal and (m-iodobenzyl)amine, as its nickel(II) complex, catalyses the 

Symmetrical 1,4-di-imines have been prepared by dehydrodimerization 

Classically, imines have been obtained via derivatisation of carbonyl compounds although problems exist in the use of unreactive carbonyl compounds and/or volatile imines. This transformation has nevertheless been achieved in excellent yields in the absence of solvent merely by treating carbonyl compounds dispersed on alumina with amines that have been similarly dispersed on alumina, Trimethylsily1 trifluoromethanesulphonate has been found to catalyse formation of imines from carbonyl compounds and 1,N-bis-(trimethysilyl )amines, these reactions also proceeding in excellent yield.The same research group also discovered that sulphen-imines could be isolated from reactions between tJ-bis (trimethyl- 

Reagents i, [Ph2P(0)0)2 (0 5 equiv.), CHCI3 ii, 1N-H2S0 (aq), MeOH iii, NoOH 

N-Tosylimines have been synthesised in variable yields by a 

Imines may be obtained directly from amines when the latter are treated with tert-butyl hydroperoxide, catalytic amounts of 

Palladium(0)-catalysts, in particular, tetrakis(triphenyl-phosphine)palladium have been found to catalyze 3-aza-Cope 

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ADDITION TO IMINES, IMINIUM SALTS AND RELATED COMPOUNDS... 

H(65)1889, 2005EJO3553>. Starting dihydro[l,2,4]triazolo[3, 4-4]benzo[l,2,4]triazines 482 readily react with aromatic aldehydes to yield iminium salts 483. These salts treated with a base (e.g., triethylamine) are deprotonated to reactive 1,3-dipolar azomethine imines 484. In contrast to related five-membered heterocycles, these compounds are relatively unstable on storage in the solid form and particularly in solution. Fortunately, this obstacle can be easily circumvented by their in situ preparation and subsequent 1,3-dipolar cycloaddition. These compounds can participate in 1,3-dipolar cycloadditions with both symmetric and nonsymmetric dipolarophiles to give the expected 1,3-cycloadducts in stereoselective manner. Selected examples are given in Scheme 82. 

In contrast to cycloaddition reactions of carbonyl compounds and olefins the related reactions of imines and nitriles have been less well documented. Some characteristic examples will be discussed here, further details, as well as the photoreactions of iminium salts, can be found mainly in reviews94,95 and other sources96-99. 

The E-isomer is generally more stable than the Z-isomer due to diminished steric hindrance, so it is assumed that the E-isomer is the major product (shown for 83). Water, a reaction by-product, may be removed to give a better yield of product and azeotropic distillation is used as well as molecular sieves (see Section 18.6.3). Enamines are structurally related to an end (HO—C=C) in that the heteroatom is directly attached to the alkene unit. Enamines are often isolable compounds, whereas enols tautomerize spontaneously to the carbonyl form. Note that when imine 76 is formed from iminium salt 80, there is no enamine product. In fact, the C-H in 80 is much less acidic that the N-H unit, so the product is the imine rather than the enamine. It is noted that there is an equilibrium between an imine and an enamine, known as imine-enamine tautomerism, but it will be ignored in this book. Many different secondary amines can be used in this reaction, including cyclic amines (see Chapter 26, Section 26.4.1) such as pyrrolidine (90), piperidine (91), and morpholine (92). It is important to note that it is generally easier to form an enamine from a ketone than from an aldehyde. 

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