Tautomeric fixation

Compounds 120 can also be oxidized regioselectively to afford aminonitrones 122. The X-ray structural analysis of 122 (Ar =/)-MeC6H4) shows an unexpected tautomeric fixation of the hydrogen atom and indicated that the resonance structures 122 and 122" participate in the stabilization of the nitrone 122. Thus, compounds 122 are unusually stable due to the contributions of anionic as well as cationic delocalized mesomeric structures (Scheme 26) <2000JPR245, 1999H(51)763>. 

A multiply bonded nitrogen atom deactivates carbon atoms a or y to it toward electrophilic attack thus initial substitution in 1,2- and 1,3-dihetero compounds should be as shown in structures (110) and (111). Pyrazoles (110 Z = NH), isoxazoles (110 Z = 0), isothiazoles (110 Z = S), imidazoles (111 Z = NH, tautomerism can make the 4- and 5-positions equivalent) and thiazoles (111 Z = S) do indeed undergo electrophilic substitution as expected. Little is known of the electrophilic substitution reactions of oxazoles (111 Z = O) and compounds containing three or more heteroatoms in one ring. Deactivation of the 4-position in 1,3-dihetero compounds (111) is less effective because of considerable double bond fixation (cf. Sections 4.01.3.2.1 and 4.02.3.1.7), and if the 5-position of imidazoles or thiazoles is blocked, substitution can occur in the 4-position (112). 

Examples of the fixation of very minor prototropic tautomeric forms on complexation are known in the series of complexes of l-(2 -pyridyl)-5-hydroxypyrazoles 383 [76AHC(S1), p. 333 70ZOB1114] and 2-[2 -hydroxy (A-tosylaminophenyl)]benzazoles 385 (Scheme 142) (98ZOB496). 

Exocyclic conjugation causes a small upheld shift of the ring hydrogen resonances, as can be seen in Table 6. The increase in 7r-bond fixation also results in an increase in the 3/4,5 coupling constant to about 6.0 Hz. The use of coupling constants for the investigation of tautomerism is discussed in Section 4.17.5. 

In our rationalization, we have relied heavily on the classical concept of aromaticity with particular emphasis on bond order and bond fixation. These concepts, together with acid-base and tautomeric equilibria and hydrogen bonding, are capable of explaining nearly all of the quantitative... 

Keywords Nucleobase Kaolinite DPT Adsorption Tautomerism Carbon fixation cycle... 

Full details of the formation of 3,4-cyclobuta[l,2]cyclohepten-6-one (58) derivatives from the dichlorocarbene ring-expansion reaction of 1- and 2-methoxybiphenylenes have appeared. Spectral data suggest that there is considerable bond fixation in the seven-membered ring, to minimise cyclo-butadienoid character in the four-membered ring. Similar considerations apply to the fused tropolones (59) and (60), which exist in the tautomeric forms shown and undergo methylation only at the points indicated. 

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