Unexpected formation of 1,2‐dihydro‐2‐azolyl‐and azinylisoquinolines by the reduction of the corresponding isoquinolinium salts with sodium borohydride in methanol

The reduction of different 2‐azolyl‐and azinylisoquinolinium salts with sodium borohydride in methanol was studied. Surprisingly, contrary to what is found in the literature 1,2‐dihydroisoquinoline derivatives were obtained. Their formation was attributed to the electron withdrawing character of the heterocyclic ring in position 2 of the isoquinolinium moiety. This was corroborated by synthesis and reduction of differently substituted 2‐phenyl‐ and 2‐methylisoquinolinium salts.