A novel reductive dimerization/oxidative dehydrogenation of aldimines mediated by lanthanoid metals.

A lanthanoid metal-mediated novel reductive dimerization/oxidative dehydrogenation of a variety of aldimines has been achieved. Aromatic aldimines (1) were dimerized in the presence of 0.5 mol of ytterbium metal (Yb) and 1-naphthaldehyde (1-NpCHO) to give the corresponding vicinal diimines (2) in good to high yields. Samarium metal (Sm) or samarium(II) diiodide (SmI2) gives unsatisfied yields under the same reaction conditions. As an oxidant, 1-NpCHO gives the best result. In addition, HMPA plays an important role as a cosolvent in this reaction. Reaction of various aldimines with Yb metal is discussed in detail. Moreover, it has been found that the electron-donating substitutents on the benzene ring promote the reaction and that the electron-withdrawing substitutents retard the reaction. The reaction of deuterated N-benzylideneaniline with Yb metal confirmed that transformation of the hydride occurred from aldimines to the oxidant aldehyde. The mechanism of the reaction was discussed.