Amidines azomethines

During studies on the use of amidines as azomethine ylide sources, Jones et al. (67-69) reported in a series of papers the application of their general strategy to an asymmetric process. Quatemization of the dihydroimidazole 214 with an a-halo ester followed by DBU-induced ylide formation and subsequent cyclization furnished a range of nitrogen heterocycles in a one pot generation and cyclization protocol (70) (Scheme 3.73). 

A -Silylmethyl-amidines and -thioamides (42) (X=NR or S) undergo alkylation at X with, for example methyl triflate, and then fluorodesilylation to give the azomethine ylides 43 (identical with 38 for the thioamides) (25,26). Cycloaddition followed by elimination of an amine or thiol, respectively, again leads to formal nitrile ylide adducts. These species again showed the opposite regioselectivity in reaction with aldehydes to that of true nitrile ylides. The thioamides were generally thought to be better for use in synthesis than the amidines and this route leads to better yields and less substituent dependence than the water-induced desilylation discussed above. 

Metal chelates with different sized metallocycles and coordination units MN4 have been created (Sec. 2.2.4.2) on the basis of amidines 214, triazenes 215, dioximes 216, azomethines 217-222, (5-aminovinylimines 223, o-aminoazocompounds 224, hydrazoneimines 225, and formazanes 226, 228, 229. In the majority of cases, the syntheses of these complexes have been carried out by interaction of the ligands above and metal salts (mostly metal acetates) in alcohol medium (methanol,... 

Azomethine ylids derived from amidines undergo internal [3 + 2]cyclization reactions with a styrenic double bond present in the starting amidine. This strategy has been applied to the synthesis of physostigmine and erythramine. 

A general route to N-protonated azomethine ylides from Af-(trimethylsilyl-methyl)amidines and thioamides and their cycloadditions to olefins was reported. This is illustrated in a general way in equation 125124. 

Formal replacement of the two hydrogen atoms at the 3-position by a substituted nitrogen atom yields an azomethine (>C=N-) linkage at this position - the ensuing product being conveniently referred to as an azomethine-type isoindolinone derivative. If, however, the two hydrogen atoms are replaced by an appropriately substituted carbon atom to yield a C=C double bond, the resulting semicyclic amidine derivative is classed as a methine-type isoindolinone. 

From a chemical standpoint, the azomethine-type isoindolinones 2 and 4 are derivatives of phthalimide, and may also be viewed as acylated exocyclic amidines with the acylamino group forming a part of the heterocycle. 

Characteristic IR-Absorption Bands of Compounds with C=N Groups (in cm ) (Azomethines, Schiff s Bases, Imines, Amidines, Azines, Imino Carbonates)... 

Lund [28] proposed an empirical rule which, nevertheless, has only been verified for a limited number of examples, namely that compounds with the C = N - Y grouping are reduced under polarographic conditions with consumption of two electrons to form a saturated compound if Y is a carbon atom if Y is nitrogen or oxygen, four electrons are consumed in the reaction, the N - Y bond splits, and the double bond is then saturated. However, as shown by later investigations, the Lund rule is not always valid, and even related compounds can be reduced by both a two-electron and a four-electron mechanism depending on the pH of the medium or on the nature of substituents (amidines for example). It would be better to restrict the use of this relationship to the reduction of azomethines in acidic solutions. 

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