Diimidazolium Salt HBDIM: An Easily Available, Low-Cost, CageCarbene Precursor with Broad Applications in Transition Metal-Catalyzed Reactions.

ABSTRACT

The preparation of diimidazolium salt HBDIM 1, a precursor for a di-NHCs ligand, from cheap and easily available agent hexabenzylhexaazaisowurtzitane (HBIW) is reported. Under basic conditions, HBDIM undergoes facile deprotonation to in situ generate CageCarbene, which could efficiently coordinate to transition-metals, such as, Au, Cu or Pd, to give the corresponding bimetallic complexes 2-4. These complexes were isolated and fully characterized, including X-ray diffraction of their single crystals. It was found that the steric hinderance of CageCarbene is similar to that of SIMes but smaller than that of IPr, and electronically, CageCarbene is a strong σ-donator similar to SIMes and a stronger σ-donator than IPr. Further studies showed that complexes 2-4 were highly reactive to catalyze up to 17 reactions. Control experiments utilizing a N-benzyl-substituted monoimidazolium salt showed much lower catalytic reactivity when it was bound to Au or Cu, but exhibited similar reactivity for the Pd complex. Kinetic studies showed that the low reactivity of the monodentate carbene-ligated Au or Cu complex was due to the low stability of the complex under the reaction conditions.