Azomethine function

Oxidative ring closure by formation of a nitrogen-nitrogen bond may be obtained, using suitable heterocyclic hydrazones, formazans, and certain other azomethine functions as substrates. 

These experiments clearly demonstrated that two electrons are involved in each of the two polarographic steps, which correspond to the reduction of the amine oxide and the 4,5-azomethine functional group. 

Possible combinations of substrate and reagent 1. Azomethine function as reagent... 

The acid-catalyzed cyclization of the o-azomethine function to a (-aniline was first observed in 1938 by Rudy and Cramer,82 although this reaction was not recognized until recently and has been a subject of considerable research and of erroneous conclusions. The simple anils, exemplified by compound 167, were shown more recently to... 

The first report of enantioselective addition of organometallic reagents to an azomethine function appeared in 1990. Tomioka and co-workers reported the addition of organohthium compounds to J -arylimines in the presence of a stoichiometric amount of a chiral P-amino ether as the asymmetric controller [19a]. This significant contribution resulted from an observation made earlier in the context of a related synthetic project [20aj. Tomioka et al. studied the addition of methylhthium (MeLi) and -butyllithium ( -BuLi) to unsaturated N-(4-methoxyphenyljimines derived from benzaldehyde (2a), cinnamaldehyde (2b),... 

The tremendous success in the catalytic asymmetric addition of organozinc reagents to aldehydes spurred Itsuno and co-workers to examine the reactivity of diethylzinc with silyl imines in the presence of chiral amino alcohols and diols. Unfortunately, this type of azomethine function failed to react [23a]. The use of activated N-acyl- and iV-phosphinoylimines turned out to be crucial as evidenced by the following reports on the alkylation of these functions using di-... 

The first example of enantioselective allylation of an azomethine function was reported in 1995 by Itsuno and co-workers [42a]. These researchers studied the addition of preformed chirally modified allylboranes to N -(trimethylsilyl)ben-zaldehyde imine (5a) (<2 g of imine, ca. 0.27 M). Of the wide range of chirally modified allylboron reagents reported in the literature, the use of chiral allylbo-ronates 42a-c and 5-allyldialkylborane 43 were logical first choices given their utility in the enantioselective addition to carbonyl substrates (Scheme 20). 

Comparative experiments with aryllithium compounds (phenyllithium and para-fluorophenyllithium), however, resulted in low yields of the methyl esters in spite of the fact that a 300 % excess of chloroformic ester had been used. Reactions of C1COOR with lithio derivatives of heterocycles containing an azomethine function (e.g. lithiated thiazole, imidazole, pyridine) cannot succeed, since the excess of chloroformate will react with the basic nitrogen atom. A comparable situation arises if the organolithium intermediate has been generated by means of LDA reaction of C1COOR with the diisopropylamine liberated in the metallation will provide HC1 which will of course inactivate the organolithio compound. 

Whereas nucleophilic addition of alkyl-lithium compounds to the optically pure arene(tricarbonyl)chromium complex (8) proceeds without asymmetric induction, the chelates (9) react to give amines (10), after hydrolysis, with optical purity of up to 94%." Replacement of the phenyl groups on the azomethine function by alkyl groups should provide an efficient route to a large number of chiral amines. 

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