Carbodiimide metathesis catalyzed by vanadium oxo and imido complexes via imido transfer

Vanadium oxo and imido complexes: V(NC6H4Me)(O'Bu), (1), V(NC6H4Me)Cl-3 (2), V(O)(O'Bu)(3) (3), V(O)(O'Pr), (4), V(O)(acac)(2) (5), are catalysts for the metathesis of symmetrical carbodiimides (RNCNR, where R = cyclohexyl, isopropyl, and p-tolyl). At 138 degrees C in p-xylene using 5 mol% of complexes 1-5, the above carbodiimides are metathesized to the asymmetrical carbodiimides: N-cyclohexyl,N'-p-tolylcarbudiimide (6), N-isopropyl,N'-p-tolylcarbodiimide (7), N-cyclohexyl,N'-isopropylcarbodiimide (8). An equilibrium mixture of carbodiimides is reached within minutes to hours depending on the catalyst employed. Yields obtained from gas chromatography range from 32 to 70%. Complex 2 is the most efficient catalyst with a turnover frequency greater than 100 h(-1). The oxo complexes (3-5) appear to be precatalysts. A mechanism is proposed for initiation with oxo complexes and a subsequent catalytic cycle involving imido complexes. (C) 1999 Elsevier Science S.A. All rights reserved.