Pyrrolopyridines protonation

The UV spectra of simple pyrrolopyridines have a typical aromatic appearance with several shoulders in addition to the main peaks shown in Table 2. The compounds are with one exception stronger bases than pyridine (pfsTa 5.2), suggesting mesomeric stabilization of the cation (Scheme 3). This is confirmed by the bathochromic shift of band III on protonation. 

The most commonly studied pyrrolopyridine is compound (1). This compound is capable of selfassociation, a phenomenon that has led to much investigation, especially by NMR techniques. NMR data in carbon tetrachloride and benzene show that the chemical shift of the H-l proton moves downfield as the concentration of solute is increased. This type of shift is typical for a proton that undergoes hydrogen bond formation. From the NMR data, the dimerization constant for 1H-pyrrolo[2,3-/>]pyridine (1) has been determined to be 18.9 + 10.3 mol l l <8IJPC3l8l>. 

Bicyclic 5-6 Systems Two Heteroatoms 1 1 Table 17 Proton NMR chemical shift data (ppm) for pyrrolopyridine derivatives. 

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