Carbodiimide Synthesis via Ti-Catalyzed Nitrene Transfer from Diazenes to Isocyanides

Simple Ti imido halide complexes such as [Br2Ti­(NtBu)­py2]2 are
competent catalysts for the synthesis of unsymmetrical carbodiimides
via Ti-catalyzed nitrene transfer from diazenes or azides to isocyanides.
Both alkyl and aryl isocyanides are compatible with the reaction conditions,
although product inhibition with sterically unencumbered substrates
sometimes limits the yield when diazenes are employed as the oxidant.
The reaction mechanism has been investigated both experimentally and
computationally, wherein a key feature is that the product release
is triggered by electron transfer from an η2-carbodiimide
to a Ti-bound azobenzene. This ligand-to-ligand redox buffering obviates
the need for high-energy formally TiII intermediates and
provides further evidence that substrate and product “redox
noninnocence” can promote unusual Ti redox catalytic transformations.