Metal Hydride Reduction of Isoquinolines and Isoquinolinium Ions

Complex Metal Hydride Reduction of Isoquinolines and Isoquinolinium Ions 

The reduction of isoquinolinium ions has been extensively investigated with borohydride and aluminum hydride ions. The use of boro-hydride ion in a protonic solvent normally leads to the formation of 1,2,3,4-tetrahydroisoquinolines, whereas the reduction with aluminum hydride ion in an aprotic medium generally gives a 1,2-dihydroiso-quinoline. This 1,2-dihydroisoquinoline contains an enamine system and may undergo further reaction on treatment with acid. The 1,2-and 3,4-dihydroisoquinolines as well as isoquinolinium ions are reduced by the borohydride ion in a protonic medium to the 1,2,3,4-tetrahydroisoquinolines. 

Reduction of Isoquinolinium Ions and Dihydro-isoquinolines WITH BOROHYDRIDE 

The reduction of 2-substituted-isoquinolinium salts has been reported by Torossian65 with potassium borohydride in water, by Mirza,68 and by Durmand et al.,87,68 using sodium borohydride in aqueous methanol to yield 1,2,3,4-tetrahydroisoquinolines. The reduction of the second double bond appears to arise from a mechanism similar to that leading to tetrahydropyridines from pyridinium ions (see Section I). Mirza66 (see also Bose60) found that the reduction of berberine (60) with sodium borohydride could be stopped at the 1,2-dihydro-intermediate (61), and Karrer and Brook70 showed that the 1,2-dihydroisoquinoline formed by the lithium aluminum hydride reduction of l-phenyl-2-methylisoquinolinium iodide (62) could be further reduced to the 1,2,3,4-tetrahydroisoquinoline (63) with sodium borohydride in methanol. Awe et al.71,72 and Huffman73 

Durmand, X. Lusinchi, and R. C. Moreau, Bull. Soc. Chim. France 1961, 270. 

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