Functional tautomerism

An account of non-prototropic tautomerism (mainly annular metallotropy) and nonaromatic functional tautomerism will also be included. 

Generally the name of a compound should correspond to the most stable tautomer (76AHCS1, p. 5). This is often problematic when several tautomers have similar stabilities, but is a simple and reasonable rule whose violation could lead to naming phenol as cyclohexadienone. Different types of tautomerism use different types of nomenclature. For instance, in the case of annular tautomers both are named, e.g., 4(5)-methylimidazole, while for functional tautomerism, usually the name of an individual tautomer is used because to name all would be cumbersome. In the case of crystal structures, the name should reflect the tautomer actually found therefore, 3-nitropyrazole should be named as such (97JPOC637) and not as 3(5)-nitropyrazole. 

Pyridones and other six membered compounds (functional tautomerism)... 

Pyridones and other six-membered compounds (functional tautomerism). The pyridone /hydroxypyridine tautomerism (76AHCS1, p. 87), especially 2-pyridone (15a)/2-hydroxypyridine (15b), has received more attention from theoreticians than any other example of tautomerism, probably in part because it is a simple model for biologically important molecules such as thymine, cytosine, and uracil (Scheme 8). 

Pyrazolinones and other five-membered compounds (functional tautomerism). These studies have often used the most simple but rather experimentally neglected pyrazolin-5-one 19. Its four tautomers are called CH 19a, NH 19b, 5-OH 19c, and 3-OH 19c (Scheme 10) [76AHCS1, p. 313]. 

Tliey also described an interesting case of a porphyrin displaying simultaneously annular and functional tautomerism, very common in the azole series but very rare in porphyrins (88JOC1132). Tire only tautomers observed, 77a-77c, have an 18-atom 18-7r-electron structure similar to that of [18]diazaannulene.Tlie relative amounts of tautomers 77a-77c are solvent dependent for instance, in toluene-dg and in CD2CI2 they are 77a (53 and 78%), 77b (29 and 13%), and 77c (18 and 9%). 

The tautomerism of other pyrazoles has also been studied. The examination of the tautomerism, protonation, and E/Z isomerism of a 4-nitroso-5-aminopyrazole and its salts shows that this compound exists as a mixture of two amino/nitroso isomers 50a and 50b the imino/oxime tautomer 50c was not present [66], The salt is a mixture of isomers 51a and 51b resulting from the protonation on the oxygen of the nitroso tautomers. The functional tautomerism of l,5,6,7-tetrahydro-4//-inda-zol-4-ones (a pyrazole derivative) has been studied theoretically and experimentally [67], In the case of imidazoles 2-substituted by Se, there is experimental evidence that the selenone is more stable than the selenol tautomer [68],... 

The crystal molecular structure of /V1-hydroxylophine A -oxide (= I -hydroxy-2,4,5-triphenyl-17/-imidazole 3-oxide) (59) has been determined [72], This compound presents a case of degenerate or autotrope tautomerism (59a and 59b are identical), which is very common in annular tautomerism of NH-azoles but very rare in functional tautomerism [40], In the solid state, the tautomeric proton is in the middle between two consecutive monomers of the catemer [59]n [72],... 

Structural isomers may be divided into five sub-types skeletal, positional, functional, tautomeric and meta. 

To have a reasonable probability to crystallize two tautomers their intrinsic difference in energy should be rather low (1976MI1, 2000 AHC(76)1). This is not generally the case in functional tautomerism (e.g., oxo/hydroxy), but it is common for the annular tautomerism of azoles (1992JOC3938). 

Scheme 13.3 Prototropic tautomerism in organic compounds (a) annular tautomerism in pyrazoles functional tautomerism in (b) pyrazolines [47] and (c) Schiff bases [48],...

---