Reactions at the Nitrogen Atoms

With acids HX, colored mono or colorless di salts are formed, depending on the molecular proportion of the acid e.g., 2-quinolyl-methanes give 23 and 24. The cationic mono salts (23) correspond to 

On quaternization with alkyl halides, 2-quinolyl-4-quinolyl-methanes yield isocyanines and di-(4-quinolyl)methanes the corresponding cyanines. Quinolylamines give azacyanines in an analogous way. Since cyanine dyestuffs are readily prepared by condensation of the A-alkyl quaternary compounds39,40 of quinolylmethanes and -amines, by reactions analogous to those described in Section II, A, 1, the method of preparation involving alkylation of the parent compounds is of little practical use. 

For steric reasons, the dye salts cannot exist in the purely di-cia form according to the energy of the system (depending on the position of the respective dipoles, etc.) they have the linear all-frana or the 

Riester, in Ullmanns Encyclopaedia of Technical Chemistry (W. Foerst, ed.), Vol. 14, p. 310. Chem. Publications, Weinheim, 1963. 

For the cyanine dyestuffs, a characteristic correlation exists between basicity, the sterically induced twisting of parts of the molecule out of the plane, and the electronic excitation energy.42 45 

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The most common methods suitable for the synthesis of different azolium compounds will be discussed here. Two routes are particularly useful for the preparation of the imidazolium salts (1) substitution reactions at the nitrogen atoms of imidazole [25] and (2) multicomponent reactions for the generation of an Af,Af -substituted heterocycle which are particularly useful for the synthesis of imidazolium salts bearing aromatic, very bulky, or particularly reactive N,N -sub-stituents (Fig. 3a,b) [26]. Both methods offer the opportunity to produce unsym-metrically substituted imidazolium salts of type 1 either by stepwise alkylation of imidazole or by the synthesis of an W-arylated imidazole derivative followed by 77 -alkylation [27]. Nevertheless, the method of choice for the preparation of the imidazolium salts 1 is the 77,77 -substitution of imidazole. Several other methods for the preparation of imidazolium salts with previously unattainable substitution patterns have also been described [28, 29]. 

Theoretical properties of A-methoxy-A-thiomethylformamide (237) suggest that the reactivity patterns of A-alkoxy-A-thioalkylamides would be different to those of other anomeric hydroxamic esters described in this chapter. The reduced pyramidality at nitrogen together with the more conventional jt overlap between the nitrogen lone pair and the carbonyl would disfavour reactions at the nitrogen atom, which dominates the reactivity patterns of other anomeric hydroxamic esters. 

Ring-closure reactions that involve a /-amino group on a 1,2-diazine skeleton and a vinyl moiety in its t>/// t>-position occur via the so-called /-amino effect. Although this reaction does not involve reaction at the nitrogen atom itself, it is a very specific reaction that can only occur on /-amines and is therefore incorporated in this section. The reaction... 

Le Baut and co-workers144 have recently studied the alkylation of y-pyronaphthalone (78) in order to increase the regioselectivity of reaction at the nitrogen atom of 79. The results are good examples of increases in yield and selectivity, using PTC as compared to more conventional methods using nonpolar or polar aprotic solvents. 

A steric parameter based on the proton affinity Ah and methyl cation affinity Ach3, calculated by computational chemistry methods, for reaction at the nitrogen atom series of compounds in which the nitrogen atom was unhindered, e.g. pyridines in which the 2- and 6-positions are unsubstituted [Jenkins et al, 1994 Jenkins et al., 1995 Baxter et al, 1996]. Using these affinity values, a reference regression model was found which correlates the methyl cation affinity Achs to the proton affinity Ah for unhindered compounds ... 

Pyridine can also be used as an efficient catalyst for the bromination of benzene (see Vogel, page 860) through reaction at the nitrogen atom 16 while the crystalline reagent pyridinium... 

Reactions at the Nitrogen Atom. The 1,2,4-dithiazoUdine-3,5-dione heterocyclic system can be regarded as a moiety for the protection of two reactive functional groups, the primary amine and the isocyanate, and hence fV alkylated derivatives (6) can be regarded as containing either of these two groups in a masked form. 

The same mechanism is predicted by the calculations [163] also for the substitution reactions at the nitrogen atom of a nitro group. They are less common than Eq. (5.15) and proceed readily only in very few cases. Thus, the nitro-containing compounds do not practically exchange their oxygen atoms when treated with water in acidic or alkaline medium [167]. As a model for studying the MERP peculiarities, a degenerate transformation was taken ... 

Pyridyl lithium gives a small yield of the Michael addition product with benzalacetophenone , as does 4-picolyl sodium with methyl vinyl ketone i2 . In contrast, 2-picolyl sodium forms the carbinol, perhaps because of initial reaction at the nitrogen atom... 

In Chapter 9, we considered alcohols and haloalkanes. The chemistry of these compounds, which includes nucleophilic substitution and ehmination reactions, is governed by a C—O or C—X bond of an sp -hybridized carbon atom. In Chapter 22, we discussed the chemistry of alkylamines. These compounds undergo substitution reactions at the nitrogen atom rather than the carbon atom. They undergo elimination reactions with difficulty. 

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