Hydrogen-bonding to azine-nitrogen

Hydrogen bonding to azine-nitrogen has been postulated to be responsible for 2-carboxamidopyridine being more rapidly hydrolyzed than the 4-isomer while the reverse is true in the esters, is Solvent-... 

The catalytic effect of aromatic nitro groups in the substrate and product or in an added inert nitro compoimd (e.g., w-dinitrobenzene in 18) has been observed in the reaction of 2,4-dinitrochlorobenzene with an amine in chloroform. Hydrogen bonding to benzil or to dimethyl sulfone and sulfoxide also provided catalysis. It is clear that the type of catalysis of proton transfer shown in structure 18 will be more effective when hydrogen bonding is to an azine-nitrogen. 

C. Influence of Cationization of the Azine Moiety and of Hydrogen Bonding to the Azine-Nitrogen... 

Even without a cage effect, the entropy effect will be somewhat more favorable for ortho reaction when hydrogen bonding to an azine-nitrogen atom generates the necessary nucleophile. The possibility of proton transfers between the solvent molecules (MeOH) near the reaction site and the more distant MeO is expected to produce a favorable increase (relative to other solvents) in the entropy of activation, which can reinforce the effect of a favorable point of... 

A hydrogen-bonded cyclic transition state can be postulated for a nucleophile like ethanolamine or ethylene glycol anion whose hydrogen bonding to an azine-nitrogen in aprotic solvents can facilitate reaction via a cyclic transition state such as 78, cf. Section II, F. Ethanolamine is uniquely reactive with 2-chloronitrobenzene by virtue of a cyclic solvate (17) of the leaving group, a postulate in line with kinetic evidence. 

The effect of hydrogen bonding to nuclear substituents in transition states is reviewed in Sections I,D, 2,b, and II, E. Relative reactivity at different ring-positions is postulated to be alterable by hydrogen bonding of an azine-nitrogen to the solvent or to the reagent (Section II, B, 3 and III,B). However, there appears to be no kinetic data relevant to this postulate. 

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