Imine bonding

Such stmctures are stabilized by having the imine bond conjugated between two aromatic rings and the possibiUty of tautomerism between two equivalent stmctures ... 

The imine bond —CH=N— is formed during polycondensation of aromatic/aliphatic diamines with aromatic/ aliphatic dialdehydes ... 

In contrast, under similar conditions, 2,7-dialkyl-3//-azepines, e. g. 9, suffer reduction at the imine bond to give 2,3-dihydro-l//-azepines.76... 

Under reflux conditions, 5//-dibenz[c < lazepine (7) adds acetic anhydride across the imine bond to give 6-acetyl-6,7-dihydro-5//-dibenz[c,e]azepin-5-yl acetate (8).5... 

Phenylmagnesium bromide adds to the imine bond of 5//-dibenz[c,< ]azepine 6-oxide to yield 5-phenyl-6,7-dihydro-5//-dibenz[ce]azepin-6-ol (mp 171 —172°C).6... 

The highly strained and reactive 2iT-azirines have been extensively studied for various synthetic purposes, such as ring expansion reactions, cycloaddition reactions, preparation of functionalized amines and substituted aziridines. The older literature on azirines in synthesis has extensively been reviewed [69]. Concerning azirines with defined chirality only scarce information is available. Practically all reactions of azirines take place at the activated imine bond. Reduction with sodium borohydride leads to cz5-substituted aziridines as is shown in Scheme 48 [26,28]. 

Grignard reagents react with the imine bond as expected to give substituted aziridines in 60-90% yield (Scheme 49) [23b]. 

In marked contrast to the other hetero atom multiple bond functions cited in this section the >C=N- imine bond was not found to undergo hydrometalation. Imines with neighboring C-H bonds as in PhCH=NMe do react with 1 via imine/enamine tautomerism, but not by hydrozirconation [197]. 

The reaction of Ni11 perchlorate with (740a) gave the complex (741), in which the imine bond of the proligand had undergone hydrolysis. In the resulting dimeric Ni2 structure with a Ni Ni distance of 3.088 A, both metal ions are in octahedral coordination environment. When (740b) was used,... 

Compounds of types i, 2, and 3 were deduced to have planar, trigonal configurations from kinetic and stereochemical observations71 however, confirmation of this postulate was provided only a few years ago by the X-ray structure analysis of (bistrimethylsilyl)aminobis(trimethylsilylimino)phosphorane8). The P/N(imine) bonds were found to be relatively short, which was attributed to a high rc-bonding component in the planar system of coordinatively unsaturated phosphorus. 

The values of the 15N CP MAS chemical shift of Lys296 nitrogen bonded to retinal via the —C=N bond ( Schiff base) was equal to 155.4 ppm for rhodopsin and 282.8 ppm for metarhodopsin (relative to 5.6 M aqueous NH4C1).70 The results proved the imine bond polarisation, which facilitates Schiff base hydrolysis. The comparison between chemical shifts for metarhodopsin and model compounds suggested that Schiff base linkage of the all-frans retinal chromophore in Metall is in a polar environment. 

Butyne-l,4-diol has been hydrogenated to the 2-butene-diol (97), mesityl oxide to methylisobutylketone (98), and epoxides to alcohols (98a). The rhodium complex and a related solvated complex, RhCl(solvent)(dppb), where dppb = l,4-bis(diphenylphosphino)butane, have been used to hydrogenate the ketone group in pyruvates to give lactates (99) [Eq. (15)], and in situ catalysts formed from rhodium(I) precursors with phosphines can also catalyze the hydrogenation of the imine bond in Schiff bases (100) (see also Section III,A,3). 

Scheme 19 Stoddart has employed the clipping method to prepare [2]rotaxane 42. Reversible imine bonds are used to generate a dynamic mixture containing the threaded and un-threaded components...

Some studies document hydrolysis of an arginine residue to ornithine and/or citrulline [87], The reactions are insufficiently understood, but an attractive hypothesis is that they involve the hydrolysis of the imine bond of the guanidino group (see also Sect. 11.6). 

Fig. 6.4 Reversible interconversion of amino acid and keto acid. Conjugation of the imine bond in the aldimine with the electron sink of the pyridine ring plus protonation of the pyridine nitrogen as well as the metal ion - all this results in weakening of the C-H bond of the amino acid residue. Thus, also catalyzed is a-proton exchange, racemization of a chiral center at the a-carbon atom and decarboxylation of the appropriate amino acid. ...

A reversible Ugation technique was desired that would aUow the use of the mPE system in dynamic combinatorial libraries (DCL) in an effort to identify masterpiece sequences [81]. The imine bond metathesis is known to have an equilibrium constant close to unity, undergoes reactions at reasonable rates at room temperature, and has a geometry that is compatible with the phenylacetylene unit [82] therefore it was chosen as a component for mPEs. 

Incorporation of an imine unit into a mPE began with a Monte Carlo search of dodecamer (58e), which indicated that the oligomer adopted a six-turn helical structure [86]. To verify these results solvent denaturation studies were performed on 58e which showed a helix coil transition with a AG(CH3CN) very similar to that of the native oligomers (3.0 0.2 for 58e vs 3.2 0.1 for 15). The small difference in the AG(CH3CN) indicates that the imine bond has a negligible effect on the stability of the folded state of the oligomer. With this... 

Fig. 36 Plot of the free energy change for metathesis reactions versus the chain length of the ligation product with a central imine bond (filled square) and with an imine bond off-center (filled diamond) for imine-containing mPE oligomers. The magnitude of the equilibrium shifting was referenced to the free energy change for dimer formation...

The metathesis reaction of the imine bond was also able to dynamically select for oligomers that form the most stable host-guest complex [88]. It has been shown that rodlike and dumbbell-shaped guests have a higher affinity and specificity for oligomers of 20-22 repeat units [58,61 ]. Imine starter sequences 60,61a, and 61b, which can potentially form 16- (63a), 22- (63b), and 28-mers (63c) as well as smaller molecular weight (MW) materials (62a and 62b), were chosen as starter sequences for an imine ligation experiment (Fig. 37). When equiUbrated in chloroform the product distribution of 63a-c was close to 1 1 1,... 

The reaction of compound 6 with methoxide in methanol was found to give yet another type of product In this case, the reaction occurred not at the imine bond but at the isoindole carbonyl group The product was thus a (5-oxo-imidazolin-2-yl)... 

Molecular modeling of the reaction predicts attack of the CN" ion on the re face of the iV-allyl benzaldimine carbon to provide an (5)-adduct. The aromatic ring of the imine and the quinuclidine hydrogen bond stabilizes the iminium above the pyridazine, blocking the rear face of the imine bond. Nucleophilic attack by CN is... 

The classical method for preparing isoxazole involves the condensation of 1,3-dicarbonyl compounds with hydroxylamine, a reagent that contains the preformed N—O bond. The regiochemistry of the reactions can usually be rationalized by assuming that the first step involves imine bond formation at the more reactive carbonyl group. Thus, reaction of formyl ketone (44-1) with hydroxylamine gives... 

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