The carbonyl analogy

The electron-withdrawal to the cyclic nitrogen atom can be transmitted to the y-position of pyridine (560) (illustrating the principle of vinylogy). Hence, y-substituents have properties similar to those of a-substituents. 3-Substituents in pyridine are not directly conjugated with the heteroatom usually the reactivity is intermediate between that of the same substituent attached to a benzene ring, and that of an a- or y-substituted pyridine. 

In the diazines, triazines and tetrazines, the effects of the additional nitrogen atom(s) are roughly additive. In Table 4 the positions of substituents in the common azine ring systems are listed in order of increasing reactivity. The limit is reached in 2-, 4- or 6-substituted 1,3,5-triazines for which the reactivity approximates to that in the corresponding carbonyl compound (559). 

Position of substituent Ring system Number of a- or y-N Number of 3-iV 

The influence of additional nitrogen atoms in the azines sometimes allows new reactions. An example of this is that of nucleophilic displacement of a cyano group, as in (561) — (562) this does not normally occur in the pyridine series, but is analogous to a reaction of acyl cyanides (RCOCN). 

Substituents in the 5-position of pyrimidines (563) are the only substituents on diazines which are not a or 3 to a ring nitrogen atom, and these behave similarly to the substituents in the 3-position of pyridines. 

---

Table 8 Reactivity of Substituents The Carbonyl Analogy (B-68Mt4020o)...

Table 3 The Carbonyl Analogy for Reactions of Azine Substituents...

We will survey the reactions of N-linked substituents classified by the atom attached to the cyclic nitrogen. Unlike heterocyclic C-substituents, where the benzene prototype and the carbonyl analogy link much of the typical chemical behavior to familiar compounds, no simple model exists for N-substituents. However, certain trends are clear. The existence of the positive pole in cations of type (922) ensures that nucleophilic attacks (a)-(d) are the most important of the following reaction types, many of which occur for several of the different classes of N-substituents. 

Table 3 The carbonyl analogy for reactions of azine substituents...

Nitrogen often appears attached directly to carbon in Fischer-type carbene complexes. Equations 10.823 and 10.924 provide two examples of N-substituted carbene complex synthesis. The first procedure involves attack by the amide on one of the carbonyls (analogous to equation 10.6) followed by alkylation. 

Carboxylic esters of a-hydroxymethylbenzoin and O-acyl 2-oximinoketones were investigated by photo-CIDNP. These two classes of polymerization initiators were found fo undergo a-cleavage through the triplet state of the carbonyl and the carbonyl-analogous functional group, respectively, as the first step. A variety of ofher radical pairs was identified in these reactions, most of which have to be ascribed to the secondary photochemistry of the products rather than to pair substitution because continuous illumination with a 1000 W high pressure arc lamp was used. 

Although the most widely accepted explanation for the hindered rotation in N—C(=S) bonds employs the foregoing 2a-2b resonance model, this cannot satisfactorily explain the fact that the rotational barrier is greater, typically by 2-3 kcal 1110P1, for X-thiocarbonyls than for the carbonyl analogs.16 It has been suggested that this traditional picture is actually more appropri-... 

---