Comparative study of reactions of 2-benzylisoquinolinium and 3,4-dihydro-2-benzylisoquinolinium salts with carbon disulfide in two base-solvent environments

3,4-Dihydro-2-(p-nitrobenzyl)isoquinoliniburmo mide (9a) reacts with carbon disulfide in both aqueous hydroxide- dbxane and triethylamine-pyridine to afford the expected 5,6-dihydro-3-@-nitrophenyl)thiazolo[ 2,3- a]isoquinolinium-2-thiolate( 12) and a new product identified as 1,2,3,4-tetrahydro-2-(p-nitrobenzyl)isoquinoline (13a). Deuterium-labeling studies reveal that 13a is formed in a process by which adduct 12 is obtained from intermediate 1 la; a different pathway is followed in each base-solvent system. In contrast, 2-@-nitrobenzyl)- isoquinolinium bromide (1, X = p-NOJ gives only the thiolate product 2 (X = p-N02) in both base-solvent environments. Substrates 1 and 9a undergo no hydrogen-deuterium exchange at C, in either base-solvent system; only 1 sustains exchange at C1 and then only in aqueous hydroxide-dioxane. The implications of these results concerning the mechanisms for the 1 to 2 and 9a to 12 transformations are discussed. A convenient direct reduction of 9 with ammonium formate, initiated to confirm the structural assignment of 13, is described.