Reactivity values five-membered heterocycles

P-Values for Side-Chain Reactivities of Five-Membered Heterocycles... 

The reactivity of these five-membered systems with nucleophiles has received note. Zatsepina and Tupitsin and their co-workers recorded the use of deuterium exchange rates in methyl groups of five-membered heterocycles in ethanolic KOEt as a source of o values, with a p value of 4.7.271 Spinelli and his co-workers revealed that even for tri- and tetrasubstituted five-membered heterocycles the Hammett equation employing er values where appropriate gives good correlations. The systems studied are 32-37, in all cases the slow step... 

The previously described reactivity data for the five-membered heterocycles are gathered (in terms of o+ values) in Table 6.12 no data are given for nitration because the rates are encounter controlled and meaningless in terms of electronic effects. Among the other data, those for mercuriation, protiodemercuriation, and protiodeboronation are doubtful, and other qualifying aspects are noted in the table footnotes. The following main features are noteworthy. 

The carbonylation reaction is also applicable to other five-membered N-hetero-cycles, such as thiazoles, oxazoles, and pyrazoles [36], The reactivity of the substrates increases with increasing pfCa values of the conjugate acids of the N-heterocycles according to the series imidazole (pfCj 7.85) > thiazole (pfCa 3.37) > oxazole (pKa 2.91) > pyrazole (pKa 2.09). This indicates that the coordination of the substrates by the sp nitrogen to the ruthenium center is the key step in the carbonylation of C-H bonds in N-heterocycles. 

The ASE values correlate with magnetic susceptibility for the five-membered heteroaromatic compounds. Magnetic and polarizability criteria put the order of aromaticity as thiophene > pyrrole > furan. The other criteria of aromaticity discussed in Section 8.2 are also applicable to heterocyclic compounds. HOMO-LUMO gaps and Fukui functions (see Topic 1.5) have been calculated for compounds such as indole, benzofuran, and benzothiophene and are in accord with the known reactivity patterns of these heterocycles. 

In addition to five- and six-membered N-heteroaromatics, other N-heterocyclic compounds have been found to serve as substrates for C-H carbonylation reactions. Benzoimidazole 13 reacted in the same way with CO and olefins in the presence of Ru3(CO)12, but carbonylation occurred at the C-H bond / to the sp2 nitrogen to give 14 (Table 1). The same relationship between reactivity and the pK, values of conjugate acids of heterocycles was also observed in the /8 carbonylation [4],... 

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