Reactions of diborane with aromatic heterocycles—21: Reactions with nitrogen-containing heterocycles related to pyridine

The reactions of solutions of B₂H₆ in ethers with a variety of aromatic heterocycles containing one or more six-membered rings with only one nitrogen per ring have been examined. Quinoline and isoquinoline form monoborane adducts which hydroborate in the presence of an excess of B₂H₆. Protonolysis gives the 1,2,3,4-tetrahydro-compounds in both cases. N-Methylquinolinium and N-methylisoquinolinium iodides both hydroborate. Hydroboration and halogen loss occur with 2-, 3- or 4-haloquinolines (chlorine or bromine). Prior to hydroboration 8-hydroxyquinoline forms a borane adduct which rapidly eliminates one equivalent of hydrogen in a probable intramolecular ring closure. Sodium 8-hydroxyquinolinate combines with B₂H₆ in an unsymmetrical cleavage reaction to form B₂H₇^? or BH₄^?, and the same ring closed product formed by 8-hydroxyquinoline. Phenanthridine and N-methylphenanthridinium iodide both readily undergo hydroboration. The products formed by the latter with B₂H₆ are 5-methyl-5,6-dihydrophenanthridine-borane and an iodoborane etherate. 1,8-Naphthyridine forms a transient borane complex which rapidly undergoes hydroboration; protonolysis gives 1,2,3,4-tetrahydro-l,8-naphthyridine. 2,2′-Bipyridyl undergoes a complex reaction with one equivalent of BH₃ giving a mixture of bpy·2BH₃ and hydroborated products, but 4,4′-bipyridyl does not appear to hydroborate. 1,10- and 1,7-Phenanthroline both hydroborate giving a mixture of reduced products on protonolysis. Pyridine-borane and 2-phenylpyridine-borane both decompose in refluxing diglyme, but protonolysis gives no reduced products in either case.