Fused thienopyridine ring systems

Deuteration in deuteriosulfuric acid of some derivatives of thieno fused diazaborines was studied by NMR spectroscopy. It was found that the pyridinic nitrogen was protonated and that deuteration occurred in the thiophene ring of the protonated molecule. In conformity with the above-mentioned electrophilic substitutions, the thiophenic /3-positions were more reactive than the a-positions. Isosteric thienopyridines showed similar reactivity and it was thus concluded that the systems were related (77JHC893). 

Using their CNDO results Helland and Skancke calculated indices of reactivity (frontier electron density, FED, for electrophilic substitution frontier orbital density, FOD, for nucleophilic substitution frontier radical density, FRD, for radical substitution) for the thienopyridines. It was indicated that the FED index has its highest value for C-3 in the [2,3-]- and 3,2- -fused systems and for C-2 in the [3,4-1-fused isomers. As far as the former group is concerned, the predictions are in agreement with experimental observations (see Section IV,A.). Little experimental evidence is available for the [3,4-]-fused systems, but it seems highly probable that they would have a considerable tendency to undergo addition reactions at the 1,3-positions, since the product would contain a normal rather than a quinoid pyridine ring [Eq. (23)]. 

Factors (a) and (b) together explain the fact that the [cl-fused systems are stronger bases than those that are 161-fused. A further point to note is that isoquinoline is less basic than the isosteric thienopyridines, but the reverse is true for quinoline. The introduction of methyl groups into the pyridine ring has the expected effect of increasing base strength Dressier and Joullie48 list pKt values for a number of methyl derivatives in the [3,2-c]- and [2,3-c]-fused series. 

The IR spectra of thienopyridines are apparently unremarkable, since no detailed discussion of them has appeared. In the [cj-fused systems the two adjacent hydrogen atoms and the isolated hydrogen atom of the pyridine ring give rise to out-of-plane deformation absorptions near 820 and 890 cm-1, respectively,48 and in thieno[3,4-6]pyridine it is reported that the three vicinal hydrogen atoms give bands at 775 and 800 cm-1.40... 

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