Regioselective Ring Opening Reactions of Pyridine N-Oxide Analogues by Magnesium Hydride Complexes

The stoichiometric reactions of phosphinimino-amino (PIA)-supported magnesium hydride complex 1, [L1MgH](2) (L-1 = (2,6-Pr-i(2)-C6H3)NC(Me)CHP(Cy-2)N(2,6-Me-2-C6H3)), with pyridine N-oxide and 2-phenylpyridine N-oxide afforded 2,4-pentadiene-1-oximate complex 2 and 5-phenyl-2,4-pentadiene-1-oximate complex 3, respectively. The reaction of 1 with 2-methylpyridine N-oxide showed a unique regioselectivity to produce 2,4-hexadiene-1-oximate 4a in toluene and 3,5-hexadiene-2-oximate 4b in THF, respectively. Treatment of beta-diketiminato (BDI)-supported magnesium hydride complex 5, [L2MgH](2) (L-2 = (2,6-Pr-i(2)-C6H3)NC(Me)CHC(Me)N(2,6-Pr-i(2)-C6H3)), with quinoline N-oxide gave 1,2-dihydroquinoline type product 6, while treatment of complex 5 with 2-methylpyridine N-oxide either in toluene or THF afforded 1-methyl-2,4-pentadiene-1-oximate complex 7 as the only product. All these complexes were fully characterized by NMR spectroscopy and X-ray diffraction analyses, and mechanism researches were conducted to understand the ring-opening reaction of pyridine N-oxide.