Catalytic C=N bond formation by metal-imide-mediated imine metathesis

Molybdenum bis(imide) complexes of the general formula (DME)Cl2Mo(=NR)(2) catalytically metathesize acyclic imine substrates. This C=N bond-forming reaction has significant implications for the synthesis of small molecules by ring-closing metathesis and polymers by ring-opening metathesis. A series of closely related =NR transfer react:ions were examined in an effort to fully understand the process, (RO)(2)Mo(=CHR')(=NAr) (1a, Ar =2,6-diisopropylphenyl in all cases; R = C(CF3)(2)CH3) underwent alkylidene/imine exchange with a variety of imines in situ to give a mixed bis(imide) product and an olefin. The reactivity decreased if the alkoxide substituents were changed for more electron donating ones, (DME)Cl2Mo(=NR)2 (3a, R = 2,6-diisopropylphenyl; 4, K = t-Bu) reacted with imines in an imide/imine metathesis to give mixed bis(imide) complexes and new imines. The reaction rates depended strongly on the steric demands of the imide NR substituent, Replacement of the chlorides with more electron donating alkoxide ligands inhibited the reactions. Compounds 3a and 4 also exchanged imide ligands with each other (imide/imide metathesis). Finally, compounds 3a and 4 were found to catalytically metathesize mixtures of two imines at 80 degrees C in 105 and 22 h, respectively. It was established that the reaction is mediated by the presence of unique initiation products in catalytic mixtures. A catalytic cycle is proposed and the nature of the mechanism is discussed in terms of the data gathered on all of the metatheses.