Catalytic Formation of Asymmetric Carbodiimides at Mononuclear Chromium(II/IV) Bis(alkoxide) Complexes

Herein
we report the synthesis of Cr imido complexes in bis­(alkoxide)
ligand environments and their nitrene transfer reactivity with isocyanides.
The reaction of Cr2(OR)4 (OR = OCtBu2Ph) with bulky aryl or alkyl azide
results in the formation of the trigonal-planar Cr­(IV) mono­(imido)
complexes Cr­(OR)2(NR1), whereas less bulky aryl
azides form the Cr­(VI) bis­(imido) complexes Cr­(OR)2(NR1)2. Cr­(IV) mono­(imido) complexes undergo facile
reaction with 1 equiv of 2,6-dimethylphenyl isocyanide (CNR2) to form the corresponding carbodiimides R1NCNR2. In contrast, no reaction of Cr­(OR)2(NR1)2 complexes with CNR2 is observed. The reaction
of Cr­(OR)2(NR1) with excess isocyanide leads
to the isolation of the Cr­(II) complex Cr­(OR)2(CNR2)4, along with the observation of the anticipated
carbodimide product. Cr­(OR)2(CNR2)4, which can also be obtained by treating Cr2(OR)4 with 4 equiv of isocyanide, reacts with azides N3R1 (R1 = adamantyl, mesityl) to produce the respective
carbodiimides. Catalytic formation of carbodiimides R1NCNR2 is observed from the mixtures of azides R1N3 (R1 = mesityl, 2,6-diethylphenyl, 2-isopropylphenyl,
adamantyl) and several different aryl isocyanides CNR2 using
2.5 mol % of Cr2(OR)4.